Cobalt oxide-based nanoarchitectures for electrochemical energy applications

Progress in Materials Science 103 (2019) 596–677

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Cobalt oxide-based nanoarchitectures for electrochemical energy applications

T

Jun Mei, Ting Liao, Godwin A. Ayoko, John Bell, Ziqi Sun

⁎

School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, 2 George Street, Brisbane, QLD 4000, Australia

ARTICLE INFO

ABSTRACT

Keywords: Cobalt oxides Co3O4 Graphene Batteries Electrocatalysis Supercapacitors

Cobalt oxide nanostructures have been considered as promising electrode materials for various electrochemical applications, especially for batteries, supercapacitors, and electrocatalysis, owing to their unparalleled advantages of high theoretical capacity, highly-active catalytic properties, and outstanding thermal/chemical stability. If hybridized with property-complementary nanomaterials, such as nanocarbon, CNTs, graphene, metal oxides/sulfides and conductive polymers, their electrochemical properties can be further enhanced in terms of specific reversible capacity/capacitance, rate capability, cycling stability, and catalytic activity. In this review, we first give a comprehensive overview on recent progress in both monolithic cobalt oxide nanostructures and their hybrid nanomaterials for batteries, supercapacitors, and electrocatalysis applications. Then, structure-property relationships of the cobalt oxide-based nanomaterials and current challenges in both nanoarchitectures design and their applications in electrochemical energy devices are proposed, and an outlook on future research of this family of

Abbreviations: 0D, zero-dimensional; 1D, one-dimensional; 2D, two-dimensional; 3D, three-dimensional; AAO, anodic aluminum oxide; AC, activated carbon; AEMFCs, anion exchange membrane fuel cells; AFM, atomic force microscopy; AIMD, ab initio molecular dynamics; APS, 3-aminopropyltrimethoxysilane; ASCs, asymmetric supercapacitors; BA, benzyl alcohol; BDHC, blood powder-derived heteroatom doped carbon; BFSTEM, bright-field scanning transmission electron microscope; BP, blood powder; BT, barberry tannin; CA, carbon aerogel; Ca-AF, calcium alginate fibers; CE, Coulombic efficiency; CNBs, cubic nanoboxes; CNTs, carbon nanotubes; COPs, covalent organic polymers; CP, carbon paper; CST, chitosan; CV, cyclic voltammetry; CVD, chemical vapor deposition; CWs, carbonized wing scales; DETA, diethylenetriamine; DMF, N, N-dimethylformamide; EDLCs, electric double-layer capacitors; EFM, electrostatic force microscopy; EIS, electrochemical impedance spectrum; EPD, electrophoretic deposition; f-CNTs, functionalized CNTs; FFT, fast Fourier transform; FTO, fluorine-doped tin oxide glass; GHCS, graphene-like holey Co3O4 nanosheets; GO, graphene oxide; HER, hydrogen evolution reaction; HR-SEM, high-resolution scanning electron microscope; HRTEM, highresolution transmission electron microscopy; HOR, hydrogen oxidation reaction; HTSs, hierarchical tubular structures; LDH, layered double hydroxide; LIBs, lithium-ion batteries; LSV, linear sweep voltammetry; MFCs, microbial fuel cells; moCNT, mildly oxidized CNTs; MOFs, metal-organic frameworks; MWCNTs, multiwall CNTs; NBAs, nanobelt arrays; NC, N-doped carbon; N-CN, nitrogen-doped carbon networks; NCNT, nitrogendoped CNT; NMEG, nitrogen modified microwave exfoliated graphite oxide; NPs, nanoparticles; NWAs, nanowire arrays; OER, oxygen evolution reaction; ORR, oxygen reduction reaction; ox-CNTs, oxygen-containing CNTs; PAH, poly(allylamine hydrochloride); PAN, poly(acrylonitrile); PANI, polyaniline; PBA, 1-pyrenebutyric acid; p-CNT, pristine CNTs; PEI, polyethyleneimine; Pind, polyindole; PPy, polypyrrole; PS, polystyrene; p-TSA, ptoluenesulfonic acid; PVD, physical vapor deposition; PVP, poly(vinylpyrrolidone); QDs, quantum dots; QTNBs, quadrate tubular nanoboxes; RRDE, rotating ring-disk electrode; rGO, reduced graphene oxide; RNBs, rectangular nanoboxes; RT-SOFCs, reduced-temperature solid oxide fuel cells; TCFP, Teflon-coated carbon fiber paper; TEM, transmission electron microscope; SACNT, super-aligned CNT array; SAED, selected area electronic diffraction; SAXS, synchrotron small angel X-ray scattering; SCF, supercritical fluid; SEI, solid-electrolyte interphase; SEM, scanning electron microscope; SIBs, sodium-ion batteries; S/N, signal-to-noise ratio; SSCs, symmetric supercapacitors; STEM, scanning transmission electron microscopy; SWCNTs, single-wall CNTs; TMOs, transition metal oxides; URFCs, unitized regenerative fuel cells; UV–Vis, ultraviolet–visible; VLS, vapor-liquidsolid; VSS, vapor-solid-solid; XAFS, X-ray absorption fine structure; XANES, X-ray absorption near-edge spectroscopy; XAS, X-ray absorption spectroscopy; XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffraction; ZIF, zeolite imidazolate frameworks ⁎ Corresponding author. E-mail address: [email protected] (Z. Sun). https://doi.org/10.1016/j.pmatsci.2019.03.001 Received 14 February 2018; Received in revised form 1 March 2019; Accepted 4 March 2019 Available online 05 March 2019 0079-6425/ © 2019 Elsevier Ltd. All rights reserved.

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materials in electrochemical energy applications are brought forward. This understanding on the relationships of synthesis-nano/microstructure-property-performance of cobalt oxide-based nanomaterials is expected to lay a good foundation for pushing this promising class of materials to the practical application in energy conversion and storage devices and to provide a good reference for the readers in the fields of materials, chemistry, sustainable energy, and nanotechnology.

1. Introduction Cobalt oxides, possessing the merits of earth abundance, low cost, and good environmental compatibility, have been a focus in many research areas. There are four typical types of cobalt oxides, namely, cobalt(II) oxide (CoO), cobalt(III) oxide (Co2O3), cobalt (IV) oxide (CoO2), and cobalt(II,III) oxide (Co3O4) [1,2], in which Co3O4 and CoO are the most common ones, due to their excellent thermal stability and exceptional physical and chemical properties [3–6]. Fig. 1 presents the crystal structures of the low-valence cobalt oxide, CoO in a face-centered cubic structure, and the mixed-valence Co3O4 in a well-known spinel crystal structure. In the structure of Co3O4, the tetrahedral 8(a) sites and the octahedral 16(d) sites are occupied by Co(II) and Co(III) ions, respectively [7,8]. To date, cobalt oxides with different dimensionalities have been successfully synthesized and exploited for various applications, especially the electrochemical applications in renewable energy technologies, such as batteries, supercapacitors, and electrocatalytic reactions [9–11], primarily due to their high theoretical specific capacity/capacitance, high catalytic activity, and mechanical and chemical stability [12–21]. It is noteworthy that spinel Co3O4 demonstrates a remarkable theoretical capacity as high as 890 mA h g−1 for lithium-ion batteries (LIBs) and an excellent capacitance of over 3000 F g−1 for supercapacitors [22–24]. Moreover, it has been validated that the spinel cobalt oxide is one of the most promising candidates as efficient non-precious electrocatalysts in metal-oxygen batteries [25]. Nevertheless, bulk monolithic cobalt oxides still present some drawbacks in practical applications. As electrode materials, they often suffer from low rate performance and poor cycling stability, attributed to its inherently poor electronic conductivity, slow reaction kinetics, and severe volume expansion during the repeated ion uptake and removal processes of rechargeable batteries, which cause intensive local stress and eventually lead to electrode failure and electrolyte degradation [26]. On the other hand, some electrochemical energy devices involve more than one type of catalytic reactions, for example, unitized regenerative fuel cells (URFCs) includes both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), which demand high-performance catalysts that can be functionalized to different reactions simultaneously [27]. Most cobalt oxides, however, present excellent monofunctional catalytic performances but are limited for only one targeted reaction. For instance, the typical Co3O4 has good OER activity but inferior ORR kinetics [28]. If we want to make this family of materials capable of triggering two or more catalytic reactions simultaneously, elaborate design on their chemical states, surface properties, compositions, crystallinities, morphologies, dimensionalities, etc., has to be approached. One of the general strategies to address the above-mentioned issues is the fabrication of nanometer-sized materials with welldesigned structures, in which the reduced dimensionalities increases significantly the rate of ion insertion/removal and shortens dramatically the distance of ion and charge/carrier transport, the enhanced surface area permits high contact area with electrolyte and allows more active sites for chemical/electrochemical reactions, and the small particle size allows better accommodation of the strain associated with electrochemical reactions [29–32]. Some intrinsic issues of cobalt oxides, such as the low charge/ionic conductivity, however, cannot be conquered via this strategy. Recent research has shown that the formation of cobalt oxide-based hybrid structures by the introduction of one of two property-complementary nanomaterials, such as nano-carbon, graphene and CNTs, is another effective approach to resolve these critical issues. It is believed that the hybrid structures can take the full use of both the high capacity of cobalt oxides and the excellent cycle performance of additive materials [33]. Indeed, the addition of even a very small

Fig. 1. Crystal structures of (a) CoO and (b) Co3O4 (Navy blue, dark green and red balls indicate Co2+, Co3+, and O2− ions, respectively). 597

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amount of carbon nanomaterials into Co3O4 can significantly enhance its electron transfer, accelerate the reaction kinetics, suppress the aggregation of metal oxide nanostructures, and accommodate the volume change of the electrodes, and thus improve the rate and cycling performance of the Co3O4-based anodes. It is argued that, however, the addition of large amount of carbon would sacrifice the capacity of Co3O4 due to the low capacity of carbon materials. Therefore, reliable fabrication of well-designed cobalt oxide-based nanomaterials is yet complex and remains a major challenge, and rational design of cobalt oxide-based nanomaterials for electrochemical energy devices is constantly being pursued. Owing to the promising potential of cobalt oxide-based materials, the research on this type of materials has been constantly increasing in recent years. To date, around 40,000 records have been found on topics related to cobalt oxides by searching the keywords of “Co3O4 OR CoO OR Co2O3 OR cobalt oxide” from the Web of Science™ database (by 23th, January 2018), among which 38,800 records are related to their electrochemical applications, when the keywords were changed “Co3O4 OR CoO OR Co2O3 OR cobalt oxide AND electrochemistry” in the same database. Relevant papers published in the most recent five years accounted for more than one-third of the overall records between 1900 and 2017. As shown in Fig. 2a, the steady increase in publication numbers every year also attests to the significance of this family of materials in the fields of materials and electrochemistry. In addition, among the three types of cobalt oxides, considerable progress has been made on the synthesis and electrochemical applications of Co3O4, which occupies a heavy proportion of the scientific papers published to date, particularly in the past five years. It is interesting that even though a large number of publications and very active researches about this family of materials, based on the knowledges of authors, currently only one review article was published on cobalt oxides, which is on the fabrication of hollow Co3O4 nanostructures appeared in 2012 [7]. Therefore, a comprehensive review on the fabrication, structure, and electrochemical applications of cobalt oxide-based nanomaterials is urgently desired to be a good reference for the readers in the fields of materials, chemistry, sustainable energy, and nanotechnology. This review intends to summarize the emerging studies related to the monolithic nanostructures and the hybrid materials of cobalt oxide semiconductors, especially for Co3O4 and Co3O4-based nanoarchitectures, in electrochemical energy-related applications, as shown in Fig. 2b. Firstly, we try to draw a general profile of cobalt oxide nanomaterials in the applications of different types of electrochemical energy devices together with a brief introduction on the energy storage mechanisms of this family of materials for batteries, supercapacitors, and electrocatalysis. Then, recent progress in the synthesis and electrochemical energy applications of cobalt oxides with different dimensionalities is explored and the effect of morphologies and structures of the nanomaterials on the electrochemical performances is reviewed. Subsequently, the design and construction of the cobalt oxide-based hybrids by combining with various property-complementary nanomaterials, which has been approved to be an important approach to enhance their electrochemical performances in energy-related applications, is comprehensively reviewed. In this section, the fabrication and the electrochemical performances of the cobalt oxide-based nanomaterials are summarized based on the selection of complimentary counterparts and then the dimensionality. Finally, current challenge of cobalt oxide-based materials in energy-related applications and some tangible strategies for improving the electrochemical performances of cobalt oxide-based nanomaterials are proposed and

Fig. 2. (a) Statistic analysis of the scientific publications on cobalt oxides from the Web of Science™ database (by 23th, January 2018). (b) An overview of the structure of this review on cobalt oxide-based nanoarchitectures for electrochemical energy applications. 598

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outlined. Besides, the structure-property relationships of cobalt oxide-based nanomaterials and their impacts on their electrochemical performances are fully discussed, and at last a brief outlook is given to provide a guideline for the further research of this class of materials. 2. Electrochemical energy applications of cobalt oxide-based nanomaterials Electrochemical cells and systems play a key role in a wide range of critical technologies enabling for renewable energy conversion and storage, including the most established technologies such as rechargeable batteries, ion transport membranes and supercapacitors, and electrocatalytic reactors. Cobalt oxide-based materials are one family of the foremost electrode materials in these electrochemical energy devices. In this section, the application of cobalt oxide nanomaterials in those typical electrochemical energy conversion and storage devices together with their energy conversion and storage mechanisms will be summarized and outlined. 2.1. Rechargeable batteries Rechargeable batteries are one of the dominate power sources in a diverse range of applications, from electronic vehicles to microchips. Each battery is composed of a positive and a negative electrode, which are separated by solid/liquid electrolytes, enabling ion transfer between two electrodes [34–36]. When we evaluate the performance of rechargeable batteries, several factors are usually taken into concerns as summarized as follows. (i) Voltage: normally, it is difficult to achieve a thermodynamically equilibrium voltage for a battery, due to the reversibility deviation of the electrode processes. Instead, we often measure an open cell voltage (OCV). (ii) Capacity: the capacity of a battery is the amount of electrical charge which can be drawn from the battery at a specific voltage. This can be calculated based on the t equation: C = 0 I ·dt , where C, I, and t refer to the capacity (Ah), discharge current (A), and discharge time (h), respectively. (iii)

Fig. 3. (a) Voltage-composition profiles for various metal oxide electrodes for LIBs at a rate of C/5 in a voltage range of 0.01–3 V, and (b) capacity fading curves of metal oxide electrodes within 50 cycles (inset: rate capability of CoO electrode) [22] (Copyright 2000, Nature Publishing Group). (c) Potential-composition profiles of Co3O4 and the corresponding in-situ X-ray diffraction (XRD) patterns (x = 2.0) [23] (Copyright 2000, The Electrochemical Society). (d) Division of three distinct regions of a thick Co3O4 film deposited on the two-contact geometry transistor device used for in-situ monitoring the electric change during the insertion and deinsertion of lithium, (e) the origin of inferior conductivity of metallic Co phase owing to the formation of less conductive Li2O on the surface, and (f) the origin of the increase of resistance at the Co3O4 NPs/current collector interface after the lithiation of Co3O4 NPs owing to the formation of Li2O at the interface [69] (Copyright 2014, American Chemical Society). 599

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t

Specific energy and energy density: the whole energy that a battery can deliver is represented by the equation: E = 0 U ·I ·dt , where E, U, I, and t represent the energy content (Wh), voltage (V), discharge current (A), and discharge time (h), respectively. The specific energy (Wh kg−1), or we called gravimetric density, refers to the energy content at a basis of the weight of a battery. The energy density (Wh L−1 or Wh cm−3), an important parameter for designing portable batteries, is the amount of energy stored in a unit volume of a battery. (iv) C-rate and cycle life: the C-rate specifies the speed a battery being charged or discharged relative to its maximum capacity, while the cycle life of a battery is the number of repeated charge/discharge processes before that the capacity decays under a specific percent (e.g. 80%). To date, a wide range of rechargeable batteries have been designed and investigated, such as metal ion (e.g. Li+, Na+, K+, Ca+, Mg2+, Al3+, etc.) batteries, metal-air/O2 batteries (e.g. Li-O2, Li-CO2, Na-O2, Zn-O2 battery, etc.), metal-sulfur batteries (e.g. Li-S, NaS, K-S battery, etc.), metal-selenium batteries (e.g. Li-Se, Na-Se battery, etc.), and some hybrid batteries (e.g. Mg-Li battery). In terms of cobalt oxide-based nanomaterials, the applications are mainly involved in lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), and Li-O2/air batteries. 2.1.1. LIBs LIBs are a member of the family of rechargeable batteries, in which lithium ions move from the anode to the cathode during discharging and backwards when charging [37,38]. Since the first practical application of lithium batteries by Sony in 1991, LIBs have become one of the most popular rechargeable batteries owing to their many outstanding features, including superior energy density, no memory effect, low maintenance, and little self-discharge [39–43]. Since the function of LIBs is based on reversible shuttling of Li+ ions and differences in electrochemical potentials between the cathode and the anode, the inherent properties of the electrode materials are a crucial factor that largely determines the overall performance of batteries. Even to date, one of the most attractive research areas in LIBs is to design elaborate nanostructure of the electrode to solve the grand challenges in LIBs, such as the unsatisfied energy density, slow lithium ion and electron transport, and large volume change of electrode materials during cycling processes [44,45]. As one of the most attractive alternatives to commercial graphite, cobalt oxides have been widely studied as an anode material since their first use in LIBs was released in 2000 by Poizot et al. [22,46–61]. It revealed a conversion mechanism in Li storage for cobalt oxides and some other similar transition metal oxides (TMOs), such as FeO and NiO. This mechanism, involving the reduction and oxidation of metal nanoparticles (NPs) accompanied by the formation and decomposition of Li2O, differs from the previous Li insertion/deinsertion or Li-alloying storage mechanisms [22]. For example, the reversible electrochemical reaction of cobalt oxide electrodes in LIBs can be expressed as: CoO + 2Li ↔ Li2O + Co and Co3O4 + 8Li ↔ 4Li2O + 3Co, for CoO and Co3O4 electrodes, respectively. As shown in Fig. 3a, cobalt oxide electrode exhibits similar charging/discharging behaviors to FeO an NiO with a voltage window ranging from 0.01 to 3 V and a reversible capacities between 600 and 800 mA h g−1. The capacity retention capabilities of these metal oxide electrodes, however, are distinct from each other. Both CoO and Co3O4 electrodes demonstrated more stable capacities over cycling than these of other metal oxide electrodes (Fig. 3b), and a good rate capability with 85% of the remained capacity was found for CoO electrode even at a high rate of 2 C (inset in Fig. 3b). Furthermore, Co3O4, in theoretically, demonstrates a specific capacity as high as 890 mA h g−1 with eight lithium reacting with one cobalt oxide unit in each formula unit, which is almost three times higher than that of the commercially used graphite anode (less than 372 mA h g−1) [62–66]. Surprisingly, it is noteworthy that the experiments concluded that Co3O4 electrode can deliver an initial capacities of over 1200 mA h g−1 [67]. The reaction pathways and products of Co3O4 during charging and discharging are relatively complex. For example, a LixCo3O4 intercalated phase always forms during the reduction of Co3O4 phase [68], whose stability is highly associated with the specific discharge rate, the cycling temperature, and the structural parameters, such as crystallite size and specific surface area, of the Co3O4 material [23]. Fig. 3c shows the composition of Co3O4-based LIBs at different discharge current densities. It is clearly observed that a LixCo3O4 phase exists and remains stable at a relatively high rate but is decomposed into CoO phase at a low rate. Despite the fact that cobalt oxides often exhibit an excellent initial specific capacity as anode materials for LIBs, a dramatic initial capacity loss always appears, leading to a low reversible capacity after several cycles. To make this problem more explicit, Kim et al. systematically explored the origins of the irreversible capacity loss of cobalt oxide by probing the changes of the electronic and structural properties of hollow-structured Co3O4 NPs aroused by the conversion reactions related with the formation/decomposition of Co and Li2O [69]. In this method, the Co3O4 NPs were embedded into the channel of a two-contact transistor device with source and drain electrodes as current collectors to in-situ monitor the insertion and deinsertion of lithium during the lithiation/delithiation process [69]. By this means, the contributions from the Co3O4 film, the Co3O4/electrolyte interface, and the Co3O4/current collector interface can be separated to study the origin of the irreversible capacity loss of Co3O4 in the first cycle. As displayed in Fig. 3d, the thick Co3O4 NP film can be divided into three distinct regions. In “Region 1”, in which the active materials were directly in contact with liquid electrolyte, severe electrical and structural degradation occurred during lithiation/delithiation process, as confirmed by an obvious conductivity decrease monitored by the transistor device and further the structural change characterized by electrostatic force microscopy (EFM) and atomic force microscopy (AFM). In “Region 2”, the electrical and structural degradation was not as severe as those in the front region. In “Region 3”, dramatic drops of the conductivity of Co3O4 NPs and the voltage at the source/drain contacts were observed, which was not favorable for efficient electronic transport. Generally, the conversion reaction accompanying with the formation of metallic Co clusters increases the electrical conductivity in the first lithiation step. In this measurement, however, the increase of conductance was far lower than the production of Co metallic phase. The possible explanation is that the produced Co clusters might be covered by a more electrically resistive Li2O phase (Fig. 3e). In the same manner, the increase of the resistance between Co3O4 NPs and the current collector after the first lithiation step could also be attributed to the reduced physical contacts induced by the formation of Li2O at the interface (Fig. 3f). Therefore, they believed that the increased contact resistance 600

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between the Co3O4 NPs and the current collector and in “Region 3” contributes to the irreversible capacity loss during the initial charging/discharging process for the cobalt oxide-based nanomaterials [69]. Besides the dramatic initial capacity loss, there are other drawbacks for cobalt oxides as electrode materials for LIBs, including low electrical conductivity, serious aggregation, and obvious volume expansion/shrink ratio during the intercalation and de-intercalation processes of Li+ ions, which result in considerable capacity loss at high rates or after long-term cycling. Moreover, the resource for cobalt minerals is relatively limited in most regions of the world. To make cobalt oxide nanomaterials more applicable and cost-effective, extensive studies on optimization of both the structures and the components of cobalt oxide-based composites have been proceeded to overcome the above-described shortcomings. The effective approaches include: (i) the incorporation of cobalt oxides into a conductive matrix, such as nanocarbon, CNTs, graphene, and organic polymers, can greatly improve the overall electrical conductivity and inhibit the initial capacity loss; (ii) the design of core-shell structures, such as core-shell nanowires and nanospheres with cobalt oxide nanostructures as cores, can effectively buffer the stress originated from the insertion and deinsertion of Li+ ions; (iii) the combination of cobalt oxides with other heterogeneous metal-based components, such as metal sulfides, metal alloys, metal NPs, and metal hydroxides, can exhibit a surprising synergetic effect to enhance both the reactive activity of the materials and the cycling stability of the LIBs; (iv) the construction of porous structures can offer more active sites for Li+ anchoring and more diffusion channels for electrolyte penetrating; (v) the introduction of heteroatoms into the cobalt oxide crystals can change the lattice parameters and lead to the formation of defects for better accommodating the Li+ insertion behaviors. It has been validated that these strategies are effective to improve the electrochemical performances of cobalt oxides in LIBs with enhanced electron transfer, accelerated reaction kinetics, suppressed self-aggregation, and accommodated volume change, and thus significantly enhance the overall battery performances of LIBs. 2.1.2. SIBs Owing to the abundant and low-cost sodium resources, SIBs have attracted considerable research attention as a potential alternative to LIBs [70–95]. SIBs share a similar working mechanism to LIBs, however, the Na+ (1.02 Å) has a larger ionic radius than Li+ (0.76 Å), leading to more sluggish diffusion kinetics and more significant volumetric changes during repeated charging/discharging cycles. Many electrode materials that show outstanding electrochemical performances for LIBs are proved to be unsuitable for SIBs. Hence, the current major research target for SIBs is still to explore suitable electrode materials.

Fig. 4. Electrochemical performances and Na+ storage mechanism of cobalt oxides. (a) transmission electron microscope (TEM) image of mesoporous Co3O4 (m-Co3O4) microspheres (inset: schematic illustration of electron and ion transport pathways upon discharging) [106], (b) schematic illustration of the sodiation/de-sodiation cycle of m-Co3O4 electrode [106] (Copyright 2015, Wiley-VCH), (c) rate capabilities of the atomically thin Co3O4 nanosheets grown on stainless steel mesh (ATCSM), the conventional Co3O4 nanostructures (CWM), and the Co3O4 electrode prepared by conventional casting method (ATCSC) (inset: capacity ratios vs. current densities), (d) scanning electron microscope (SEM) image of ATCSM [98] (Copyright 2016, IOP Publishing Ltd). 601

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NiCo2O4 spinel oxide was the first reported metal oxide as anode electrode for SIBs in 2002 [96,97]. It was observed that this spinel showed an initial discharge capacity of ∼618 mA h g−1, which is lower than the theoretical value (890 mA h g−1) based on the reduction reaction to form four Na2O per formula unit (NiCo2O4 + 8Na ↔ Ni + 4Na2O + 2Co). Moreover, a reversible capacity of 200 mA h g−1 was obtained for the resultant NiCo2O4-based SIBs [96]. Inspired by this preliminary study on the spinel structures for SIBs and the outstanding electrochemical performances of cobalt oxide-based nanomaterials for LIBs, a variety of cobalt oxides and their hybrids (e.g. Co3O4/C, Co3O4/CNT, Co3O4/graphene, etc.) have been explored as potential electrode materials for SIBs [98–105]. Yang et al. synthesized highly-ordered mesoporous Co3O4 (m-Co3O4) microspheres (Fig. 4a) by using mesoporous silica (KIT-16) as sacrificial hard template as an electroactive anode material for SIBs [106]. The dual porosity mesoporous ordered structure with enhanced electrode-electrolyte interface facilitated mass transport and Na+ diffusion. Compared to the referential bulk Co3O4 (bCo3O4) and m-Co3O4 without fluoroethylene carbonate (FEC) in the electrolyte, the m-Co3O4 porous structures with FEC additives in organic electrolyte delivered an initial capacity of 782 mA h g−1 at a current density of 30 mA g−1, with 267 mA h g−1 remained at 2430 mA g−1 and 416 mA h g−1 remained at 90 mA g−1 after 100 cycles. Ex situ XRD patterns confirmed the occurrence of reversible conversion reaction during the uptake/extraction of sodium ions. As summarized in Fig. 4b, Na+ ions were inserted into m-Co3O4 by surface reconstruction reactions during the first discharge to form NaxCoyOz, which was then partially converted into CoxO and Na2O based on conversion reactions, accompanied by a surface amorphization process along with further sodiation. The subsequent charge process mainly contributed by de-intercalation reactions of Na+ [106]. Later, Dou et al. reported an atomically thin Co3O4 nanosheetcoated stainless steel mesh (Fig. 4d) with improved capacitive Na+ storage for SIBs, which delivered average capacities of 509.2 mA h g−1 for the initial 20 cycles (50 mA g−1) and 427.0 mA h g−1 at a high rate of 500 mA g−1 (Fig. 4c) [98]. The enhanced electrochemical performance is primarily ascribed to the unique atomic thickness of the obtained cobalt oxide nanosheets and the

Fig. 5. Cobalt oxide for the application in Li-O2/air batteries. (a) Schematic illustration of cobalt oxide nanosheets/carbon paper (Co3O4 NSs/CP) electrode for Li-air batteries [139] (Copyright 2013, Royal Society of Chemistry). (b) (top) schematic illustration of OER mechanism of Li2O2 on Co3O4 surface; (bottom) the variation of catalytic activity of transition-metal (TM)-doped Co3O4 (1 1 1) in OER as a function of ionization potentials of the doped metals [141] (Copyright 2015, American Chemical Society). (c) Schematic illustration of the reaction mechanism of the {1 1 2} faceted Co3O4 platelets enclosed by Co3+ and Co2+ sites for both ORR and OER, and (d) a comparison of cyclic behaviors of the {1 0 0} faceted Co3O4 cubes enclosed by only Co2+ sites and the {1 1 2} faceted Co3O4 platelets enclosed by Co3+ and Co2+ sites [145] (Copyright 2015, American Chemical Society). 602

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direct growth technique on the current collector, which give rise to improved surface redox pseudocapacitance and interfacial double layer capacitance for Na+ storage behaviors. It has been demonstrated that the electrochemical performances of cobalt oxide-based nanomaterials for Na+ storage are much inferior to those for Li+ storage. To classify the distinct Li+ and Na+ storage performances, Xu et al. carried out a comparative study on the possible structural changes of porous cobalt oxide during lithiation and sodiation processes by in operando synchrotron small angel X-ray scattering (SAXS), X-ray diffraction (SXRD), and X-ray absorption fine structure (XAFS) techniques [107]. They found that during the sodiation process, the porous cobalt oxide performed less pore structure changes, local wall structure changes, oxidation state shifting, and crystal structure distortions than these in the lithiation process. Further ab initio molecular dynamics (AIMD) simulations indicated that cobalt oxide possessed a low intrinsic sodiation activity, which led to inferior Na+ storage properties compared to its Li+ storage performance, and thus had less structural and electronic disturbances [107]. Overall, the electrochemical performances of cobalt oxides for SIBs are much under satisfactory and the underlying reaction mechanisms for Na+ storage are still vague. To make cobalt oxide promising for SIBs, it is highly desired to conquer the intrinsic low sodiation activity and increase the reversible specific capacity. In spite of some attempts on cobalt oxide-based hybrid structures for SIBs have been carried out in recent years, very limited progress has been made and only slow improvement on the Na+ storage performances for cobalt oxide-based nanomaterials has been achieved. Therefore, the development of cobalt oxide-based nanomaterials in the application of SIBs is still at an infant stage, and more detailed studies are urgently needed. 2.1.3. Li-O2/air batteries Rechargeable Li-O2 batteries have been receiving a great deal of interest because their theoretical specific energy density far exceeds LIBs [108–111]. In this type of batteries, the basic operation mechanism critically depends on O2 being reduced at the porous cathode surface to O22−, which then combines with Li+ from the electrolyte to form Li2O2 during discharging (O2 + 2Li+ + 2e− ↔ Li2O2), while the reverse reaction during charging is the decomposition of the peroxide [112–114]. If ambient air is used as the oxygen sources for Li-air batteries, it is essential to use high-efficiency oxygen selective membranes to effectively separate O2 from the air and to avoid the negative influences derived from CO2, H2O, and other contaminants in the air [115–117]. Currently, some other challenges, such as the issues of stability, anti-pollution, oxygen solubility and diffusivity of the electrolytes, rational design of architecture, catalytic activity, etc. of the air cathodes, and dendrite, cycling efficiency, etc. of the lithium metal anodes, still need to be urgently addressed to achieve large-scale and practical applications of Li-O2/air batteries [118]. In particular, the sluggish oxygen kinetics for OER during the charge process involving in the decomposition of Li2O2, are one of the most critical issues, which greatly restrict the overall electrochemical performances of this family of batteries. Cobalt oxides have been extensively used in traditional heterogeneous catalysis, which is largely attributed to its appealing surface redox reactivity, high adsorption capacity, and high specific area achieved by optimized design [119]. In 2007, Débart et al. compared the catalytic activities of iron-, copper- and cobalt-based catalysts for rechargeable Li-O2 batteries, and found that Co3O4 exhibited the best compromise between the discharge capacity and the cycling capacity retention with an initial capacity of 2000 mA h·g−1 and a capacity retention of 6.5% per cycle together with the lowest charging voltage of 4 V [120]. It has also been verified that cobalt-based oxides exhibit a high affinity with Li2O2, an intermediate towards LiO2, and promote the electrochemical oxidation of Li2O2, which plays an important role in enhancing the cycling performance [121–123]. These results indicate that cobalt oxide-based nanomaterials have a great potential as efficient cathode catalysts for Li-O2 batteries. To date, cobalt oxides with different structures, including mesoporous NPs [124], nanowires [125], nanosheets [126], nanorods [127], flakes [128], spherical shell [129], mesoporous spheres [122], and their derivatives, such as Co3O4/C [130–132], Co-Mn-O [133], Co3O4/Pd [134], Co3O4/N-graphene [135], CoO/rGO [136], CoO/C [121], etc., have been investigated as promising electrocatalysts at the O2 electrodes of Li-O2/air batteries [137,138]. As shown in Fig. 5a, cobalt oxide nanosheets grown on carbon paper (Co3O4 NSs/CP) were reported as the free-standing cathode for Li-air batteries [139]. In this design, a combination of hierarchically porous and interconnected Co3O4 nanosheets with highly-conductive CP facilitated electron transfer throughout the cathode, and the porous structure of Co3O4 also offered sufficient diffusion channels and abundant active sites to achieve efficient mass transfer and superior catalytic activity. The catalytic mechanism of Co3O4 exposed with different crystal facets for OER in a Li-O2 battery has been studied by firstprinciples calculations based on an interfacial model of Li2O2/Co3O4/O2 [140]. As shown in Fig. 5b (top), the computational results indicated that the O-rich Co3O4 (1 1 1)C with a relatively low surface energy and high oxygen concentration presented an excellent catalytic activity in reducing the overpotential and the O2 desorption barrier, resulted by the electron transfer from Li2O2 to the underlying surfaces. In the meantime, the basic sites of Co3O4 (1 1 0)B surface induced the decomposition of Li2O2 into Li2O and the formation of dangling Co-O bonds, which led to a high charging voltage in following cycles. Furthermore, the calculations on transition-metal (TM)-doped Co3O4 (1 1 1) suggested that the Pd-doped Co3O4 (1 1 1) exhibited a comparable charging overpotential and a better catalytic activity in reducing O2 desorption energy (Fig. 5b, bottom) [140]. Later, the same group found that the oxygen desorption barriers manifested a linear correlation with surface acidity but the Li+ desorption energies presented a volcano-like relationship with surface acidity in OER [141]. It was inferred that this family of materials with an appropriate surface acidity can achieve higher catalytic activity, reduced charging voltage, and decreased activation barrier of the rate-determinant step. Based on this result, it has been validated that CoO has comparable activity with that of Co3O4 for OER [141]. Radin et al. also confirmed the Li-O2 electrode containing a trace of Co can significantly promote the charge transport in Li2O2 through first-principle calculations [142]. For example, if the doping level of Co in the Li-O2 batteries reached 13 ppm, only 10 mV of potential was needed to drive a current density of 1 μA cm−2 through a 100 nm thick film [142]. Besides the theoretical prediction on the effect of different exposed facets on the catalytic activity, the experimental investigations 603

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have also demonstrated that the exposed surfaces of cobalt oxides play a critical role in the catalytic performances [29,119,143]. Su et al. explored the effects of the exposed crystal planes of single crystalline Co3O4 on their catalytic properties for Li-O2 batteries [144]. They verified that the correlation of the exposed surfaces of Co3O4 crystal on reducing charge/discharge overpotential is in the order of {1 0 0} < {1 1 0} < {1 1 2} < {1 1 1} [144]. Song et al. further studied the specific catalytic active sites of the {1 0 0} faceted Co3O4 cube enclosed by only Co2+ sites and the {1 1 2} faceted Co3O4 platelets enclosed by Co3+ and Co2+ sites for both ORR and OER in Li-O2 batteries (Fig. 5c) [145]. Electrochemical measurements revealed that the {1 1 2} faceted Co3O4 platelets delivered a slightly lower initial overpotential compared to the {1 0 0} faceted Co3O4 cubes electrode. The discharge capacity of the Co3O4 platelets remained stable over 45 cycles, while that of the Co3O4 cube was stable for only 20 cycles (Fig. 5d). Based on this result, the Co3+ sites can significantly promote the adsorption properties and enable high round-trip efficiency and good cyclic stability [145]. 2.1.4. Other batteries Apart from the above-mentioned batteries, cobalt oxide nanomaterials and their composites are also employed as electrode materials for other types of batteries, such as Zn-O2 batteries [146–153] and Li-S batteries [153,154]. In spite of the high energy density for Li-O2 batteries (3458 Wh kg−1), the metallic lithium used in this type of batteries is expensive and unsafe aroused by its inherently chemical activity with water or air. Zn-O2 batteries are more promising due to the high Zn reservation on earth and the relatively stable chemical properties of Zn metals compared with lithium. It has been calculated that the theoretical specific energy density for Zn-O2 batteries is about 1350 Wh kg−1 [155] Most reported Zn-O2 batteries are operated in aqueous electrolytes, and one of the major challenges for aqueous Zn-O2 batteries is unsatisfying catalytic activity for oxygen-related reactions near the cathode. Thereby, to achieve excellent reversible capacity for electrically rechargeable Zn-O2 battery, bifunctional electrocatalysts for catalyzing the evolution and reduction of oxygen are highly required. Some cobalt oxide-base composites have been proven to be efficient bifunctional catalysts for both OER and ORR, which will be systemically discussed in the following part on electrocatalysis. Li-S batteries are believed to be another promising next-generation high-energy density (2600 Wh kg−1 or 2800 Wh L−1) rechargeable batteries [156–160]. This is largely dependent on the ample sulfur resources in earth’s crust and the high theoretical capacity (1672 mA h g−1) calculated based on the conversion reaction of sulfur to Li2S [161–169]. Cobalt oxide-based nanomaterials have been explored for Li-S batteries with the functions of effective adsorption of the soluble lithium polysulfides during discharging and active catalytic transformation of insoluble Li2S2/Li2S into soluble polysulfides during charging [154]. Chang et al. designed an electrode with bottlebrush-like Co3O4 nanoneedle arrays on flexible carbon cloth fibers (CC@Co3O4) as a multifunctional “super-

Fig. 6. Study on the electrochemical reaction steps of cobalt oxide for the application in Li-S batteries. (a) The first galvanostatic cycle at 0.5 C, (b) Raman spectra in different voltages of Co3O4 nanowires on carbon cloth (CC@Co3O4) [154], (c) TEM images of CC@Co3O4 in fully discharged states at the 100th (c-1), 200th (c-2), 300th (c-3) and 500th (c-4) cycles at 0.5C, and (d) CC@Co3O4 in fully charged states at the 100th (d-1), 200th (d-2), 300th (d-3) and 500th (d-4) cycles at 0.5 C [154] (Copyright 2016, Royal Society of Chemistry). 604

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reservoir” to prolong the cycle life of Li-S batteries [154]. During discharging, soluble lithium polysulfides (Li2Sn, 4 < n < 8) were effectively absorbed on polar Co3O4 nanoneedle arrays and then transformed into insoluble solid Li2S2/Li2S. When charging occurred, Co3O4 nanoneedles catalyzed the electrochemical transformation reactions of Li2S2/Li2S into soluble polysulfides (Fig. 6a and b). Transmission electron microscope (TEM) was employed to monitor the morphology of the “super-reservoir” within CC@Co3O4 over different cycles, as shown in Fig. 6c and d. After 100 cycles, the uniform layer of Li2S2/Li2S formed on the Co3O4 nanoneedle surfaces in the fully discharged state (Fig. 6c-1) nearly disappeared in the fully charged state (Fig. 6d-1). No obvious change in the morphology was observed with the cycling numbers prolonged to 200 (Fig. 6c-2 and d-2) and 300 cycles (Fig. 6c-3 and d-3). Even at the 500th cycle (Fig. 6c-4 and d-4), the reversible transformation of the formation and decomposition of Li2S2/Li2S in the fully discharged and charged states, respectively, were still clearly observed. As a result, the CC@Co3O4 delivered an initial capacity as high as 1231 mA h g−1 at 0.5 C and a low capacity decay of 0.049% per cycle at 2.0 C over 500 cycles [154]. Although some explorations in cobalt oxide-based nanomaterials for different types of batteries have been reported, the electrochemical performances in these batteries are still inferior to these of Li-based batteries. Hence, to fully understand the roles of cobalt oxides in non-Li batteries, more work is still needed to figure out the underlying electrochemical reactions and ion storage mechanisms with the assistance of advanced electrochemical monitoring and characterization techniques. With in-depth investigations on the battery application of cobalt oxides, novel types of battery s may also be innovated based on this family of promising nanomaterials. 2.2. Supercapacitors Supercapacitors or ultracapacitors, can provide high power density, long cycle life (> 100,000 cycles), and high dynamic of charge propagation compared to normal rechargeable batteries [170–176]. Unlike the traditional electric double-layer capacitor (EDLC) using carbon-based electrode materials with only double-layer capacitance, the supercapacitors using metal oxide-based electrode materials have additional pseudocapacitance derived from reversible Faradic redox reactions [177,178]. There are serval key parameters to evaluate the performance of supercapacitors. (i) Capacitance: the capacitance of a super1 1 1 capacitor can be expressed as the equation of C = C + C , where C, C+, and C− are the capacitances of the whole device, the + _ positive electrode, and the negative electrode, respectively. In order to evaluate specific electrode materials for supercapacitors, specific capacitance (F g−1) is usually used via dividing the overall electrode capacitance by the mass of the corresponding electrode materials. (ii) Energy density and power density: the energy density (E, Wh kg−1) is the energy stored in a supercapacitor per unit 1 mass and can be calculated according to the equation of E = 2 CV 2 (V stands for the voltage window). While the power density (P, W kg−1) is the energy delivered per unit time, which can be obtained by the equation of P = 4R V 2 (R is the equivalent inner resistance). It is easily concluded that high energy density and power density of a supercapacitor can be achieved by increasing the electrode 1

Fig. 7. Effect of oxygen vacancies on the supercapacitor performances of cobalt oxide nanowires [222]. (a) Schematic illustration of the formation of oxygen vacancies in Co3O4 nanowires, (b) galvanostatic charge/discharge profiles of the reduced (red) and the pristine (black) Co3O4 nanowires at 2.0 A g−1, (c) cycling stability of the reduced and pristine Co3O4 nanowires at 2.0 A g−1, (d) partial charge density of the reduced Co3O4 nanowires, (e) the calculated conductivity of the reduced and pristine Co3O4 nanowires (Inset: an enlarged plot of the reduced Co3O4 nanowires) (Copyright 2014, Wiley-VCH). 605

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capacitance and voltage and reducing the inner resistance. (iii) Rate capability and cycle life: similar to batteries, the rate capability and cycle life are two critical indexes to evaluate the performance of a supercapacitor. Among the reported pseudocapacitive type electrode materials (e.g. RuO2, Co3O4, MnO2, NiO, V2O5, SnO2, Fe2O3, etc.), Co3O4 is particularly attractive for supercapacitor application, due to its low cost, remarkable redox activity, and extremely high theoretical specific capacitance of around 3560 F g−1 [51,172,179–181]. Many work have reported on the synthesis of various cobalt oxide nanostructures for pseudocapacitor application, because the specific capacitance strongly depends on the morphologies and structures of electrode materials [182–199]. The reported capacitance values of the supercapacitors by using cobalt oxides as electrode, however, are typically lower than 1000 F g−1, and the capacity retention is also under satisfactory. This is largely due to limited ion diffusion pathways and poor electron transport for cobalt oxide semiconductors [200]. To address these issues existing in the pristine cobalt oxides, some cobalt oxide-based hybrids, such as cobalt oxide/carbon, cobalt oxide/CNTs, cobalt oxide/graphene, cobalt oxide/polymers, and cobalt oxide/other metal oxides, have developed to further enhance the capacitance [59,201–210]. The purposes for the introduction of other components to hybridize with cobalt oxides mainly include: (i) improving the specific capacitance, (ii) widening the operation voltage window, (iii) enhancing the reaction reversibility, (iv) boosting the overall electrochemical performances at high rates. Hence, relying on the exquisite design, cobalt oxide-based nanomaterials endowed with more exposed active sites, higher reaction activity, better electrical conductivity, and superior chemical stability have been synthesized to meet the requirements for high-performance supercapacitors. Hierarchically porous structures with the above-mentioned structural merits are one class of the most attractive and potential structures for the applications of catalysis and energy-related devices [211,212]. Kong et al. reported a unique combination of mesoporous Co3O4 nanoneedles with MnO2 nanosheets to form ordered 3D core-shell hierarchical porous nanowire arrays (NWAs) on nickel foam as superior anode for supercapacitors. This 3D hierarchical structure gave rise to outstanding capacitive properties with high specific capacitances of 932.8 F g−1 at a scan rate of 10 mV s−1 and 1693.2 F g−1 at a current density of 1.0 A g−1, and delivered a remarkable long-term cycling stability (only 10.2% capacitance loss after 5000 cycles at 2.0 A g−1) and a high energy density (66.2 Wh kg−1 at a power density of 0.25 kW kg−1), demonstrating a superior performance to that of individual Co3O4 nanoneedles [201]. Oxygen vacancies play a significant role in influencing the physical properties and chemical reactivity of metal oxides [213], such as TiO2 [214,215], WO3 [216], MoO3 [217–219], and MnO2 [220,221]. It has been proved that the properties of cobalt oxide-based metal oxides, such as the electronic structure, charge transport, and surface properties, are closely related to oxygen vacancies. Wang

Fig. 8. Phosphate ion functionalized cobalt oxide nanosheets for pseudocapacitors [223]. (a) Schematic illustration of the preparation of phosphate ion functionalized Co3O4 (PCO) nanosheets, (b) SEM image of PCO, (c) CV curves of the asymmetric supercapacitor (PCO//3DPG) device assembled by a PCO cathode and 3D porous graphene gel (3DPG) anode at different scan rates, (d) Ragone plots of various asymmetric supercapacitor devices, (e) schematic illustration of the surface reactivity and electrode kinetics on PCO surface (Copyright 2016, Wiley-VCH). 606

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et al. applied NaBH4 to treat mesoporous Co3O4 nanowires to obtain reduced Co3O4 nanowires with oxygen vacancies for improving their catalytic and electrochemical performances (Fig. 7a) [222]. X-ray photoelectron spectroscopy (XPS) characterization confirmed that two new satellite peaks centered at 786.2 and 802.7 eV appeared in the Co 2p peaks of the reduced Co3O4, corresponding to a chemical state of Co2+, which suggest that the Co3+ ions were partly reduced into Co2+ ions accompanying with the formation of new oxygen vacancies. As electrode materials for supercapacitors, the galvanostatic charge/discharge profiles (Fig. 7b) indicated that the NaBH4-reduced Co3O4 nanowires delivered a high specific capacitance of 978 F g−1, more than three-fold enhancement than that of the pristine Co3O4 nanowires (288 F g−1). After 2000 charge/discharge cycles, the capacitance of the reduced Co3O4 nanowires could remain 883 F g−1 (Fig. 7c). The density functional theory (DFT) calculation revealed that a high electron delocalization occurred surrounding the positive charged oxygen vacancies (Vo2+) and the partial reduction of the neighboring Co3+ into Co2+ (Fig. 7d), resulting in much higher conductivity and electrocatalytic activity than that of the pristine Co3O4 [222]. Recently, phosphate ion functionalized Co3O4 (PCO) nanosheets with enhanced surface reactivity were fabricated by Zhai et al. as electrode materials for high-performance pseudocapacitors [223]. As displayed in Fig. 8a and b, the PCO was prepared via thermal treatment of the Co3O4 nanosheets grown on a carbon cloth in Ar atmosphere with the presence of NaH2PO2·H2O as phosphate ion sources for pseudocapacitors. The as-prepared PCO electrode delivered a large specific capacitance of 1716 F g−1 at 5 mV s−1 (∼4.5 A g−1) in 6.0 M KOH electrolyte with excellent capacitance retentions of 97% after 2000 cyclic voltammetry (CV) cycles and 85% after 10,000 cycles. Furthermore, the asymmetric supercapacitor (PCO//3DPG) device assembled by a PCO cathode and 3D porous graphene gel (3DPG) anode retained remarkable pseudocapacitive behavior with fast and reversible charge storage capability in the voltage window of 0–1.5 V (Fig. 8c), which achieved an energy density of 71.58 Wh kg−1 at an average power density of 1500 W kg−1 (Fig. 8d). This work demonstrated that the introduction of phosphate ion (H2PO4− and PO3−) onto Co3O4 nanosheets significantly enhances the surface reactivity and electrode kinetics to increase the active reaction sites and reduces the transfer resistance (Fig. 8e) [223], and thus indicated that heteroatom doping should be an effective way to enhance the electrochemical performances of cobalt oxide-based nanomaterials. In summary, even though significant advantages of low cost, excellent redox reactivity, and high theoretical specific capacitance endorse the promising application of cobalt oxides in supercapacitors, the conspicuous shortcomings of poor capacity retention and rate capacity hinder the practical utilization of this family of materials. Besides nanostructure engineering to enlarge the exposed surface area and the contact interfaces to electrolyte and to shorten the diffusion and mass transport length of ions and charges/ carriers, it has been demonstrated that the design of heterostructures, the introduction of vacancies and defects, and the implantation of heteroatoms are feasible strategies to further enhance the electrochemical performances of cobalt oxides for supercapacitor applications. 2.3. Electrocatalysis Electrocatalysis plays a key role in clean energy conversion and storage applications, such as water splitting, fuel cells, and metalO2 batteries. The primary goal for electrocatalysis process is to achieve high-efficiency conversion of common molecules in the atmosphere (e.g·H2O, O2, CO2, etc.) into highly value-added products (e.g. H2, CmHn, NH3, etc.) [224,225]. In the past decade, significant progress has been made in the exploration of some crucial electrochemical transformation reactions, especially those involving CO2, H2O, H2, and O2, such as hydrogen oxidation reaction (HOR), hydrogen evolution reaction (HER), CO2 reduction reaction (CRR), OER and ORR. Other emerging electrocatalytic reactions, such as H2O2 production and CO2/N2 reduction reactions, are also attracting more and more attentions [224]. Several factors in charge of the electrocatalysis performance are summarized as follows: (i) Onset potential: It is generally accepted that the potential value at a current density of 10 mA cm−2 (Ej = 10) can be used to compare the onset potentials of different catalysts. (ii) Overpotential: the overpotential in electrocatalysis means the extra energy over the thermodynamically expected energy needed to drive the electrocatalytic reactions, which is usually measured in the unit of mV. (iii) Tafel slope: Tafel slop is another crucial parameter to evaluate the activity of electrocatalysts, and it can be sued to evaluate the rate-determining steps of the electrocatalytic reactions, based on the equation of = b·log(j/ j0 ), where η, b, j, and j0 represent the overpotential, Tafel slope, current density, and exchange current density, respectively. (iv) Stability: in spite of many explorations have been performed to evaluate the stability of electrocatalysts, such as extended reaction time, varying reaction systems (e.g. acidic and alkaline electrolytes), and morphology and structure examination changes, until now, no well-defined parameter is utilized to judge the stability of a catalyst. (v) Turnover frequency (TOF) and mass activity: The TOF value can be calculated according to the following equation. TOF = (j ·S)/(4·F·n), where j, S, F and n refer to the current density, electrode area, Faraday constant (96,500 C mol−1) and the moles of the active materials, respectively. Besides, the mass activity is evaluated by j/m, in which j is measured current density and m is the mass loading of the working electrode. For electrocatalysis reactions, the selection of catalysts is very critical especially in practical applications, as we need to make a balance between cost and effectiveness. Great efforts have been made on the design of efficient but low-cost electrocatalysts. Cobalt oxides, as one of semiconductor-type catalysts with ample natural resources, are low cost and high catalytic efficiency, and expected to be one promising cost-efficient catalyst in electrocatalytic reactions to replace the low-abundant and noble metals (e.g. Pd, Ir, Pt, Au, etc.) and their alloys (e.g. Pt/Ir, Pt/Au, Pd/Au, etc.), if we can increase the active site numbers and enhance the activity of each site of the oxides. Cobalt oxide-based nanomaterials have been reported for various electrocatalytic reactions, including the widespread OER, ORR, HER, and CRR, as well as the traditional oxidation of methanol [226–230]. In this section, we mainly focus on the first four representative types of gaseous electrocatalytic reactions. Considering the fact that two or more reactions are often involved in the 607

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practical applications, such as fuel cells, metal-O2 batteries, water splitting, and other related fields, bifunctional and multi-functional cobalt oxide-based catalysts are particularly stressed with a separated focus. 2.3.1. OER The reaction process of OER mainly includes a four proton-coupled electron transfer step and formation of oxygeneoxygen (OeO) bond. In acidic conditions, the reaction happens accompanied by the oxidation of water molecules to give one oxygen molecule and four protons (2H2O → 4H+ + O2 + 4e−), while the oxidation of hydroxyl groups to produce H2O and O2 (4OH− → 2H2O + O2 + 4e−) is involved in basic conditions [231–233]. Compared to RuO2 and IrO2, which are regarded as the state-of-the-art catalysts for OER with low overpotential and Tafel slope, the less active Co3O4 is still attractive due to its easy access in nature, low input-output ratio for industrialization, and good environmental compatibility [234–239]. In spite of the geometry-site-dependent performances are suggested for OER, the specific active sites of Co3O4 remain elusive. Some research indicated that Co3+ species on the surface of Co3O4 crystal should be active sites for OER catalysis [240–243], however, other studies revealed that the presence of Co2+ is also helpful for boosting OER performances, which are responsible for the formation of cobalt oxyhydroxide (CoOOH) [244–246]. Nevertheless, the sluggish kinetics and the relatively considerable overpotential are major challenges for the application of cobalt oxides in OER [247]. To meet the requirements of practical application of this family of promising materials, the electrocatalytic performances of cobalt oxides must be further enhanced by executing some effective strategies, such as the introduction of porous structures [185,248,249], the hybridization with carbon matrix [250–253], and the doping of some metal atoms [254–258], which help increase the contact interfaces, the electric conductivity of the system, and the number of the active sites of the materials. To increase the accessible surface area and the active sites on the surfaces of the electrocatalyst, 2D graphene-like holey Co3O4 nanosheets (GHCS) (inset in Fig. 9a and b) were fabricated by a bottom-up self-assembly approach and were utilized as an electrocatalyst for OER in 0.1 M KOH [248]. As shown in Fig. 9a, two peaks located at around 0.285 and 0.575 V in the anodic scan were assigned to the oxidation of Co2+ to Co3+ and Co3+ to Co4+, respectively. The linear sweep voltammetry (LSV) curves revealed that

Fig. 9. OER performance of 2D graphene-like holey cobalt oxide nanosheets [248]. (a) CV curves in 0.1 M KOH at a scan rate of 30 mV s−1, (b) linear sweep voltammetry (LSV) curves at 10 mV s−1, (c) Tafel plots, and (d) cycling stabilities of graphene-like holey Co3O4 nanosheets (GHCS) (Inset in a shows the schematic illustration of GHCS; inset in b shows SEM image; HCF represents holey Co3O4 nanoflowers; BCC represents bulk Co3O4 nanocubes) (Copyright 2016, Elsevier). 608

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a low onset potential of 0.617 V (0.25 mA cm−2) was displayed for GHCS, which is almost comparable to commercial IrO2 (0.611 V) (Fig. 9b). Specifically, at a potential of 0.8 V, the obtained GHCS gave an OER current density of 12.26 mA cm−2, much higher than holey Co3O4 nanoflowers (HCF, 9.46 mA cm−2), bulk Co3O4 nanocubes (BCC, 2.45 mA cm−1), and referential IrO2 (3.42 mA cm−2). Moreover, the GHCS possessed a Tafel slope of 66 mV dec−1, which is lower than other referential samples (Fig. 9c). Long-term stability tests confirmed that the GHCS exhibited only 2.0% loss after 2000 cycles, greatly outperforming HCF (4.9%) and BCC (16.5%), respectively (Fig. 9d) [248]. Therefore, it is concluded that the design of graphene-like holey structures is very helpful to enhance the electrocatalytic performances of the materials, owing to significantly improved active surfaces and decreased energy barriers for mass conversion and transfer. The second strategy to enhance the electrocatalytic performance of cobalt oxides is to increase the intrinsically-inferior electrical conductivity. Ma et al. reported that carbon coating on the surface of cobalt oxide nanowires arrays could increase the electrical conductivity of each single wire, and thus significantly enhance their overall electrochemical properties. In this study, Co3O4-carbon porous NWAs (Co3O4C-NA) were prepared by carbonization of Co-based MOF precursor grown on Cu foil under an inert condition as free-standing and binder-free electrode for OER (Fig. 10a and b) [251]. When operated in alkaline solutions (0.1 M KOH), the electrode afforded a low onset potential of 1.47 V (Fig. 10c) and a stable current density of 10.0 mA cm−2 at 1.52 V for at least 30 h, accompanied by a high Faradaic efficiency of 99.3%. Moreover, the Co3O4C-NA hybrid electrode exhibited a Tafel slope value of 70 mV dec−1 (Fig. 10d), which is much lower than those of Co-based MOF (142 mV dec−1) and IrO2/C (97 mV dec−1). As displayed in the electrochemical impedance spectrum (EIS) in Fig. 10e, Co3O4C-NA exhibited a smaller contact and charge transfer impedance than the one without carbon coating. Furthermore, the porous structure of NWAs and the direct growth of active materials on Cu foil brought the electrode with enlarged active surface area and excellent structural stability for long-term cycles [251]. The significant enhancement of the electrocatalytic performances of the carbon-coated Co3O4 suggests the effectiveness of the strategy on increasing the electrical conductivity of the materials. 2.3.2. ORR Generally, there are two reaction pathways available for ORR. In acidic solutions, a four proton-electron transfer step with the reduction of O2 to H2O (O2 + 4H+ + 4e− → 2H2O) and a two-proton-electron one with reducing O2 to H2O2 (O2 + 2H+ + 2e− → H2O2) occur. The former is the preferred pathway for ORR and is also a desirable pathway for the operation of fuel cells. The latter, however, is detrimental to the ORR reaction efficiency albeit attractive for H2O2 production [259]. If operated in alkaline

Fig. 10. OER performance of carbon coated cobalt oxide porous nanowire arrays (NWAs). (a) Schematic illustration of the fabrication of Co3O4carbon porous NWAs (Co3O4C-NA), (b) SEM image of Co3O4C-NA [251], (c) polarization curves of cobalt oxides for OER (Inset: optical image of the generation of O2 bubbles on the electrode surface by using Co3O4C-NA as catalyst at 1.70 V), and (d) Tafel plots of the electrocatalytic reactions in an O2-saturated 0.1 M KOH solution at a scan rate of 0.5 mV s−1 [251], (e) electrochemical impedance spectrum (EIS) of the cobalt oxide electrode during reaction recorded at 1.60 V (Inset: the corresponding equivalent circuit diagram) [251] (Copyright 2014, American Chemical Society). 609

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environment, O2 can be reduced by either the four-proton-electron pathway to form OH− (O2 + 2H2O + 4e− → 4OH−), or the two-proton-electron pathway to produce HO2− and then OH− (O2 + H2O + 2e− → HO2− + OH−; HO2− + H2O + 2e− → 3OH−) [260]. Pt metal has been considered as the best ORR catalyst. The limited resources and high cost, however, make the Pt-based catalysts less attractive for its commercial applications [261]. Cobalt oxides have been explored as an alternative ORR catalyst to replace the expensive Pt catalyst for fuel cells [262], such as microbial fuel cells (MFCs) [263], anion exchange membrane fuel cells (AEMFCs) [264], and the reduced-temperature solid oxide fuel cells (RT-SOFCs) [265]. It has also been proven that the active sites in Co3O4 for ORR are the surface Co2+ ions, and thus cobalt oxides exposed with Co2+ enriched {1 1 1} crystal planes show the best electrocatalytic activity compared to the cobalt oxides exposed with {1 1 0} and {1 0 0} faces nanorods (NR), nanocubes (NC) and nanooctahedrons (OC) with the different exposed nanocrystalline surfaces ({1 1 0}, {1 0 0}, and {1 1 1}), uniformly anchored on graphene sheets, which has allowed us to investigate the effects of the surface structure on the ORR activity. Results show that the catalytically active sites for ORR should be the surface Co2+ ions, whereas the surface Co3+ [266,267]. Unfortunately, as it appears in other electrocatalytic reactions, the intrinsically low electrical conductivity of cobalt oxides is a big barrier to achieve satisfying electrocatalytic performance in ORR. Hence, cobalt oxide-based nanomaterials with doping or the composites by hybridizing with highly conductive materials have been developed to address the critical issue to improve their electrochemical properties [28,268–276]. Among the cobalt oxide-based nanostructures, the formation of ternary mixed valence oxides with a spinel structure are an important class of materials to provide outstanding ORR catalytic activity in alkaline conditions. Liang et al. fabricated MnCo2O4/Ndoped graphene hybrids (MnCo2O4/N-rmGO) as high-performance ORR electrocatalysts (Fig. 11a), by taking advantage of the higher catalytic activity of MnCo2O4 than pristine Co3O4 and the strong coupling with N-doped graphene [268]. This hybrid showed more positive onset (0.95 V) and peak (0.88 V) potentials than these of the Co3O4/N-doped graphene (Co3O4/N-rmGO) hybrids (0.93 and 0.86 V, respectively), and presented only 20 mV of the half-wave potential difference to the Pt/C catalyst at the same mass loading (Fig. 11b). Moreover, rotating ring-disk electrode (RRDE) measurements in 1.0 M KOH solution confirmed that the yield of peroxide species was less than 10% for the hybrid, much lower than that of the corresponding physical mixture between MnCo2O4 NPs and Ndoped graphene nanosheets (about 15%). It was also found that Mn substitution mediates the NP size and phase, and provides further potential to enhance the catalytic performances [268]. Therefore, it is pretty effective to enhance the ORR performance of cobalt oxide to improve the electrical conductivity by incorporating with highly conductive species, such as nanocarbon and graphene. It is also demonstrated that doping is another useful way to further improve the catalytic activity of cobalt oxides. 2.3.3. HER HER is another important electrocatalytic reaction, which can generates H2 fuel via the reduction of water (2H+ + 2e− → H2 or 2H2O + 2e− → H2 + 2OH−). HER has also gained considerable attention primarily due to its potential applications in hydrogenbased fuel cells, alkaline water electrolysis, petroleum refining, NH3 synthesis, etc. [277–285]. The general catalytic mechanisms of HER in both acidic and alkaline media have been fully understood [286–288], however, the produce of other by-products is also possible in some cases when use some specific electrolytes [289]. In the electrocatalysts for HER, the well-known and best catalysts at present is still Pt and Pt-group metals because of their fast kinetics and nearly zero overpotential [290–292]. Cobalt oxides have been explored as potential catalysts for HER, but the activities in their pristine form are very poor [293]. It has been reported that the hybridization with highly conductive or chemically active components, such as nanocarbon, metal sulfides and metals, can greatly increase their electrochemical properties in HER application [293–296]. Yan et al. reported that an excellent HER activity could be achieved by the design of 3D core-shell Co/Co3O4 nanosheets, in which the metallic cobalt was core and the amorphous cobalt oxide was shell [293]. The 3D metallic framework was beneficial for

Fig. 11. ORR performance of MnCo2O4/N-doped graphene hybrids (MnCo2O4/N-rmGO) [268]. (a) Schematic illustration of ORR on MnCo2O4/NrmGO, (b) rotating-disk electrode (RDE) voltammograms in O2-saturated 1.0 M KOH at a sweep rate of 5 mV s−1 at 1600 rpm (Copyright 2012, American Chemical Society). 610

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increasing electrical conductivity, and the thin amorphous Co3O4 surface layer (2–5 nm in thickness) enriched with hydroxyl groups could effectively improve the HER activity. This 3D Co/Co3O4 core-shell structure exhibited a very small onset potential of 30 mV and a current density of 10 mA cm−2 at an overpotential of ∼90 mV in 1.0 M KOH [293], providing a proof-of-concept on the ideas of the design of novel low-cost electrocatalysts toward high HER performances. 2.3.4. CRR To deal with the limited fossil feedstocks on earth and the continuously increasing CO2 emissions, the electroreduction of CO2 into fuels have become a very attractive topic. Depending on this reaction, a large quantity of useful products, such as CO, methane, formate, formic acid, methanol, formaldehyde, and C2+ hydrocarbons and oxygenates, can be formed via various multi-electron transfer pathways [297–312]. Electrocatalytic CO2 reduction mainly incorporates the following four steps: (i) adsorption of CO2 on active sites, (ii) activation of CO2 to CO2− or HCO2− or other intermediate, (iii) dissociation of CeO bonds accompanying with the transfer of protons and electrons, (iv) desorption of the products from active sites. In these steps, the activation step is the critical bottleneck in the reduction of CO2. Recent studies have indicated that the reduction reactions by using cobalt oxide and their derivations as electrocatalysts can be triggered at low overpotentials [313–315]. With a combination of theoretical and experimental study, Gao et al. found that oxygen-deficient cobalt oxide ultrathin nanosheets and partially oxidized atomic cobalt nanosheets had higher intrinsic activity and selectivity towards formate production, compared to the normal surface cobalt atoms on bulk samples [313,314]. Gao et al. compared the electrochemical performance of the Vo-rich Co3O4 nanosheets in CO2 reduction reaction (Fig. 12a). The presence of O(II) vacancy could facilitate CO2 adsorption and spontaneous HCOO− desorption, and thus significantly enhanced the catalytic performance of the Co3O4 nanosheets. The Vo-rich Co3O4 nanosheets exhibited a current density of 2.7 mA cm−2 at the potential of −0.87 V, almost two times larger than that of the Vopoor Co3O4 layers (Fig. 12b), and a decreased onset potential (0.78 vs. 0.81 V) as well as a lower Tafel slope (37 vs. 48 mV dec−1). At a given potential of −0.87 V, the Vo-rich Co3O4 nanosheets achieved a maximum faradaic efficiency of 87.6% after 4 h for producing formate, much higher than 67.3% of the Vo-poor layers (Fig. 12c), and presented negligible decay with a Faradaic efficiency of over 85% in 40 h (Fig. 12d). This study proved that the proton transfer in the CO2 reduction on Co3O4 surface is a rate-limiting step. The oxygen(II) vacancies in the surface layers decreased the activation barrier of the proton transfer from 0.51 to 0.40 eV and had the function of stabilizing the formate anion radical intermediate (HCOO−*) [313]. Partial oxidation of atomic cobalt layers or four-atom-thick nanosheets with coexistence of cobalt metal and cobalt oxide domains generated a current density of 10.59 mA cm−2 at −0.85 V (Fig. 13a) with the highest Faradaic efficiency (90.1%) at an overpotential to 0.24 V in the production of formate (Fig. 13b). Over 40 h, the partially oxidized layers did not show obvious decay in current density with approximately 90% formate selectivity [314]. The superior performance of the cobalt or cobalt oxide nanosheets can be attributed to their atomic thickness endowing Co3O4 with ample active sites for a large number of adsorption of CO2 and the enhanced electronic conductivity. Gao et al. also studied the effects of the thickness of nanosheets on the catalytic activity, based on two types of Co3O4 nanosheets in thicknesses of 1.72 and 3.51 nm, respectively [315]. It was found that the thinner the nanosheets, the better electrocatalytic performances were achieved. The Tafel slopes were 56 mV dec−1 for the nanosheets in 1.72 nm and 79 mV dec−1 for those in 3.51 nm, while it reached 97 mV dec−1 for bulk samples. The 1.72 nm Co3O4 layers delivered a current density of

Fig. 12. Effect of oxygen vacancies on CO2 electroreduction reaction of cobalt oxide layers [313]. (a) Schematic illustration of the formation of Vorich and Vo-poor Co3O4 layers, (b) LSV curves in CO2-saturated (solid line) and N2-saturated (dashed line) 0.1 M KHCO3 solutions, (c) Faradaic efficiencies of formate production, and (d) chronoamperometry results at −0.87 V of Vo-rich and Vo-poor Co3O4 layers (Copyright 2017, Nature Publishing Group). 611

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Fig. 13. Electrochemical properties of CO2 reduction reaction of cobalt oxides. (a) LSV curves in a CO2-saturated (solid line) and N2-saturated (dashed line) in 0.1 M Na2SO4, and (b) the Faradaic efficiencies for formate using different Co-based catalysts at each given potential for 4 h [314] (Copyright 2016, Nature Publishing Group). (c) LSV curves in a CO2-saturated (solid line) and N2-saturated (dashed line) in 0.1 M KHCO3 solutions, and (d) the Faradaic efficiencies for formate using the different thickness of Co3O4 catalysts with at each given potential for 4 h [315] (Copyright 2016, Wiley-VCH).

0.68 mA cm−2 at −0.88 V, which was more than 1.5 and 20 times higher than these of Co3O4 layers 3.51 nm in thickness and the bulk counterpart, respectively (Fig. 13c), and the maximum faradaic efficiency of the 1.72 nm thick Co3O4 layers (over 60% for 20 h) was much higher than that of the 3.51 nm thick Co3O4 layers (51.2%) and the bulk counterpart (Fig. 13d) [315]. These investigations demonstrated that the manipulation of the chemical states of surface atoms and vacancies are very critical to increase the active sites and to lower the activation barrier of CO2 reduction reactions. 2.3.5. Bifunctional cobalt oxide-based catalysts In contrast to mono-functional catalysts which exhibit excellent activity towards one specific electrochemical reaction type, some catalysts are capable of catalyzing two different types of reactions simultaneously. The discovery of bifunctional catalysts is attracting considerable attention in the recent years due to the fact that there usually is more than one reaction in a certain practical application. For example, an entire water splitting process involves two half-reactions, HER and OER, for the evolution of hydrogen and oxygen, respectively [258,316–318]. In URFCs, the electrochemically conversion of H2 into electrical energy and the splitting of water into H2 and O2 occur simultaneously [319–322]. Compared to the mono-functional catalysts, the bi-functional catalysts are less common and more challenging to achieve high efficiencies at the same time for catalyzing two different reactions. As mentioned above, cobalt oxides in pristine state have good OER-favorable activity but inferior activities for ORR and HER. Hence, to make cobalt oxides applicable as bi-functional catalysts, one of the simplest approaches is to hybridize cobalt oxides with the materials with complementary functions. For example, the cobalt oxide-based bifunctional catalyst system can be easily constructed by integrating various cobalt oxide structures with functional carbonaceous materials, especially heteroatom-doped carbon, which has been considered as one of the most promising ORR-favorable catalysts [323]. Until now, many pioneering work has been reported using cobalt oxide-based nanomaterials as bifunctional catalysts for two pairs of reactions, HER/OER [294,295,324] and 612

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OER/ORR [325,326], which are essential for the applications for water splitting, fuel cells, and metal-O2 batteries. Menezes et al. found that the Co3O4 nanochains exhibited superior catalytic performances toward OER than the solvothermally synthesized Co3O4 nanostructures and commercial Co3O4 and CoO in both alkaline (PH = 13) and neutral (PH = 7) solutions [325]. To enhance its ORR activity in alkaline media, the carbon-supported Co3O4 nanochains was fabricated by mixing with carbon black and it was observed that the resulting Co3O4/C hybrid outperformed some precious metal catalysts in terms of catalytic activity and chronoamperometric stability [325]. Besides carbon black, heteroatom-doped carbon was also introduced to couple with cobalt oxides improve the ORR and OER performances. Mao et al. reported a CoO NPs/N-doped graphene (N-CG-CoO) hybrid synthesized by the growth of CoO NPs on 3D hollow crumpled N-doped graphene as a bi-functional catalyst for both OER and ORR in alkaline solutions [326]. Compared to the CoO NPs/graphene (CG-CoO) hybrid without nitrogen doped, the N-CG-CoO hybrid exhibited enhanced ORR catalytic activity with a more positive onset potential of about 0.90 V, a more positive half-wave potential of 0.81 V at 1600 rpm, and a smaller Tafel slop of around 48 mV dec−1, etc. Moreover, the onset potential of N-CG-CoO was close to that of the commercial Pt/C (Fig. 14a). The durability of N-doped hybrid, with 12% decay after 20,000 s at 0.75 V, was superior to that of the Pt/ C (41%) in 1.0 M KOH electrolyte, as indicated in Fig. 14b. The OER study showed that the N-CG-CoO catalyst had a small overpotential of about 0.34 V (Fig. 14c) and a small Tafel slope of 71 mV dec−1 (Fig. 14d). These excellent catalytic properties suggested that the N-CG-CoO catalyst is a promising bifunctional electrocatalyst for water splitting and fuel cell applications [326]. In summary, many explorations on cobalt oxides and functional carbon hybrids has proven to be efficient bifunctional electrocatalysts, however, these performances are still far away from the requirements for the practical devices, such as metal-O2 batteries. Hence, further optimization in cobalt oxide/carbon hybrids is required in terms of the optimum ratio of the components, the sources of carbon, the dimensionality of cobalt oxides, the approaches of hybridization, and the avoidance of possible side reaction. Moreover, an in-depth exploration is needed for the formation of heterostructures with other components to simultaneously achieve high OER and ORR activities. 2.3.6. Multi-functional cobalt oxide-based catalysts Based on current progress, individual cobalt oxides can hardly be used for multi-functional electrocatalysts. Cobalt oxide-based hybrids, however, have great potential to achieve this goal via an ingenious combination of individual merits derived from each component to present an attractive synergetic effect. Nevertheless, it is challenging to design suitable structures with desired functions for simultaneously catalyzing different types of reactions. The reported multi-functional catalysts currently are largely focusing on the combination of cobalt oxide structures with functional graphene nanosheets [327,328].

Fig. 14. ORR and OER performances of of CoO NPs/N-doped graphene (N-CG-CoO) hybrid [326]. (a) ORR polarization curves in an O2-saturated 1.0 M KOH electrolyte at a sweep rate of 5 mV s−1 with a rotation speed of 1600 rpm, (b) chronoamperometric responses at 0.75 V in an O2saturated 1.0 M KOH electrolyte, (c) Oxygen evolution currents in an Ar-saturated 1.0 M KOH electrolyte at 20 mV s−1 (Inset: the oxidation peak of CoO), (d) Tafel plots of OER currents of N-CG-CoO (Copyright 2013, Royal Society of Chemistry). 613

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Liu et al. systematically studied a cobalt/cobalt oxides/nitrogen-doped reduced graphene oxide (Co-CoO/N-rGO) hybrid composed of metallic cobalt, cobalt monoxide, and graphene as catalysts for OER, ORR, and HER [327]. As shown in Fig. 15a–c, Co/CoO nanorods with sub-micrometer lengths were anchored on N-rGO sheets. Among their synthesized cobalt-based materials, including CoO/N-rGO, Co-CoO/N-rGO, and Co-CoO + N-rGO, CoO/N-rGO showed the highest activity for OER (Fig. 15d) with much smaller charge transfer resistance (Rct) (∼20 Ω) than CoO/N-rGO (∼30 Ω), while Co-CoO/N-rGO exhibited the highest HER activity (Fig. 15e), superior to individual N-rGO, CoO/N-rGO hybrid, and Co-CoO + N-rGO physical mixture. As for ORR, as indicated in Fig. 15f, a higher activity was achieved by Co-CoO/N-rGO hybrid than CoO/N-rGO with an onset potential of ∼0.88 V and a halfwave potential of 0.78 V. Notably, similar Tafel slopes between Co-CoO/N-rGO (40 mV dec−1) and Pt/C (38 mV dec−1) were observed, especially in low current densities. Furthermore, stable current densities of −2.0 mA cm−2 at 0.75 V for ORR in 0.1 M KOH and −10 mA cm−2 at −0.15 V for HER in 1.0 M KOH were well-maintained in Co-CoO/N-rGO catalyst. Although the Co-CoO/N-rGO catalyst exhibited lower activity than Pt/C in the rotating-disk electrode (RDE) tests (Fig. 15f), it presented improved performance approaching to Pt/C + IrO2 cathode in the zinc-air battery tests [327]. Moreover, Co3O4 NPs anchored on N-doped rGO were also used as an effective multifunctional catalyst for H2O2 reduction, oxygen reduction and evolution reaction [328]. The resultant Co3O4/N-rGO composites demonstrated excellent electrocatalytic activities with a direct reduction to H2O2 at −0.6 V and a detection limit of 0.1 mM towards H2O2 sensor, a peak potential of −0.26 V (vs. Ag/AgCl) for ORR, and an onset potential of 1.54 V and a smaller Tafel slope (204 mV dec−1) compared with 20% Pt/C (308 mV dec−1) for OER [328]. 3. Fabrication and electrochemical performance of cobalt oxide nanomaterials In principle, nanomaterials represent the materials with at least one dimension less than 100 nm [329]. Nanomaterials can be generally classified into four categories based on their dimensionality. Materials with all three dimensions confined into 1–100 nm scales are considered as zero-dimensional (0D) nanomaterials, such as nano-sized particles, clusters, and quantum dots (QDs). Onedimensional (1D) nanomaterials possess two dimensions less than 100 nm and one ultra-long dimension along the radial direction, include nanorods, nanowires, nanoneedles, nanotubes, nanofibers, etc. Two-dimensional (2D) nanomaterials consist of two infinite dimensions in length and width and one nanoscale thickness dimension, and often present a paper-like morphology, involving nanosheets, nanodiscs, nanofoils and nanomeshes. One of the most typical examples of 2D nanomaterials is the isolated single-atomic carbon layer, graphene, which was first exfoliated from bulk graphite in 2004. Compared with other dimensional nanostructures, three-dimensional (3D) nanoarchitectures are assembled from different types of low-dimensional nanoarchitectures and their overall size in three dimensions is often beyond nanometer scales. The representative 3D nanomaterials include micro-/nanospheres, porous/ hollow structures, dendritic structures, hierarchical structures, heterogeneous networks, etc., assembled from 0D, 1D, 2D, or multiscale nanocomponents. In this section, we focus on the recent progress in the synthesis and fabrication of cobalt oxides in various dimensionalities,

Fig. 15. OER, HER, and ORR performances of cobalt oxide/graphene hybrids [327]. (a) SEM image, (b) low-magnification and (c) high-magnification TEM images of cobalt/cobalt oxides/nitrogen-doped reduced graphene oxide (Co-CoO/N-rGO) hybrid, (d) OER, (e) HER and (f) ORR polarization curves at a sweet rate of 10 mV s−1 with a rotating rate of 1600 rpm (Copyright 2015, Wiley-VCH). 614

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including Co3O4, CoO, and mix-valenced CoxOy with different constitutional nanostructures, for typical electrochemical energy applications. 3.1. 0D cobalt oxide nanostructures 0D nanostructures, are usually defined as the overall size ranging from 1 to 100 nm in three dimensions, such as NPs and QDs. Owing to the unique confinement effect derived from electrons, excitons, holes, phonons, and/or plasmons in all dimensions, the physical and chemical properties of 0D nanomaterials usually are isotropic [330]. To date, cobalt oxide NPs have been synthesized via various methods, e.g. microwave irradiation [331], pulsed-laser ablation [332–334], mechanochemical reactions [335], laser ablation in liquid [336], bio-templating route [337], ionic liquid-assisted method [338], surfactant-assisted method [339], hydrothermal method [340], etc., for a wide range of applications, such as heterogeneous catalysts, batteries and supercapacitors [341–345]. As for the electrochemical reactions, the effect originating from the reduced sizes and the defined shapes of these NPs include increased surface area, more active crystal defects, and exposed highenergy crystal facets. The controlled synthesis of cobalt oxide NPs with tunable sizes and shapes thus has been attracting considerable interest. One of the simple strategies is to tune the size and shape by adjusting the synthesis parameters, such as the precursor concentration, the reaction temperature, and the reaction time [346–352]. For example, in 2005, the wurtzite-type hexagonal cobalt monoxide (CoO) nanocrystals with 11 ± 1.7 nm in width and 40 ± 7.3 nm in length were synthesized via thermal decomposition of cobalt(III) acetylacetonate precursor in oleylamine at 200 °C [347]. By change the molar ratio of the solutions and the reaction temperatures, hexagonal pyramid-shaped nanocrystals and cubic nanocrystals have been obtained [347]. It also has found that solid cubic CoO nanoparallelepipeds can be transformed into metallic cobalt hollow structures by thermolysis the solid cubic crystals in oleylamine at a temperature in 270–290 °C [348]. Later, Varón found that the presence of water in reaction solutions can facilitate the formation of a CoO hollow structure by a Kirkendal effect at the room temperature, otherwise Co-CoO core-shell NPs were obtained without water. These hollow CoO NPs, however, were only an intermediate state, which finally underwent disintegration to form minor CoO NPs of 2–3 nm in sizes [352]. As another important type of 0D nanostructures, QDs, have attracted much attention for their well-defined structure in terms of both size and shape, and thus exhibit extraordinary chemical and physical properties, such as the unique quantum confinement effect [353,354]. There are two basic features of this appealing material, e.g. small size and narrow size distribution. The size of QDs is generally not more than twice the exciton Bohr radius of the corresponding semiconductors in all three dimensions, and QDs are mostly in spherical or quasi-spherical morphologies with a diameter in the range of 2–20 nm. Hence, the accurate control of nucleation and growth steps during the synthesis is essential to produce well-defined QDs. In 2014, Zhang and his co-workers synthesized mono-dispersed Co3O4 QDs via a facile reverse micelle strategy assisted with

Fig. 16. Morphologies and performances of 0D cobalt oxide QDs. (a, b) TEM, magnified TEM (Inset: selected area electronic diffraction (SAED) pattern), and (c) high-resolution transmission electron microscopy (HRTEM) images of Co3O4 QDs [356] (Copyright 2014, Royal Society of Chemistry). (d) Comparison of OER activities in terms of overpotentials (η) at 10 mA cm−2 and Tafel slopes for cobalt-based catalysts [357] (Copyright 2017, American Chemical Society). 615

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microwave-promoted solvothermal treatment [355]. In the synthesis, the introduction of microwaves contributed to accelerating the particle nucleation rate, by which the reaction time was easily controlled to effectively avoid the overgrowth of NPs. The specific size of Co3O4 QDs was around 3–4 nm, which is close to the exciton Bohr radius. Moreover, the ultraviolet–visible (UV–Vis) spectra revealed that the absorption edge of the obtained QDs exhibited a clear blue shift compared with that of the NPs synthesized by a solid-state reaction (Co3O4-SSR), which is consistence with the obvious change of exterior color from black (Co3O4-SSR) to yellow (Co3O4-QDs). Benefit from the confinement effect, the conduction band shifted to a higher level than the H+ reduction potential occurred, which makes these QDs attractive for overall water splitting under visible-light irradiation [355]. Subsequently, a one-step, surfactant-free, oil-bath-assisted method by using benzyl alcohol (BA) together with ammonium hydroxide has been employed to fabricate monodispersed, water-dispersible Co3O4 QDs (BA-Co3O4) with sizes of approximately 4.5 nm [356]. As shown in Fig. 16, the as-synthesized QDs presented uniform size (∼4.5 nm) and cube-like shape (Fig. 16a and b), and without obvious aggregation. Furthermore, these QDs showed well-defined Co3O4 spinel crystal structure as evidenced by the observed lattice spacing of 0.200 nm (Fig. 16c), which corresponds to the distance between the {4 0 0} planes. Without negative effect derived from agglomeration, the resultant product exhibited excellent visible light-driven oxygen evolution activities under mild pH conditions [356]. Quite recently, Zhang et al. reported the fabrication of Co3O4−δ QDs with a smaller crystallite size of about 2 nm through multicycle lithiation/ delithiation of mesoporous Co3O4 nanosheets [357]. Meanwhile, the concentration of oxygen vacancies and the population of Co2+ over Co3+ could be tuned. When applied for an OER electrocatalyst for water splitting, as shown in Fig. 16d, Co3O4−δ QDs prepared with 20 lithiation/delithiation cycles (Co3O4−δ QDs-20) exhibited an overpotential of only ∼270 mV at a current density of 10 mA cm−2, which is superior to many previously reported Co-based catalysts and the state-of-the-art IrO2, demonstrating the superior electrochemical reactivity of the 0D Co3O4 QDs [357]. Overall, 0D cobalt oxide nanostructures have been widely investigated for electrochemical applications. Two major issues, however, exist in the high-yield synthesis of highly homogeneous cobalt oxide QDs. The first challenge is the strong tendency of QDs to aggregate into larger aggregates during the drying process and electrode fabrication step. To address this problem, the strategy to hybridize with other supporting materials (e.g. graphene) is an effective solution, which will be discussed in the following sections. Based on this method, the homogeneous dispersion of cobalt oxide QDs can be achieved and the active surfaces and edges area can be maximally utilized. Another issue is that the controllable synthesis of uniform cobalt oxides NPs or QDs in high yields is still very difficult. To find simpler and more efficient preparation approaches, further efforts on the nucleation-and-growth mechanisms of QDs have to be further made.

Fig. 17. Morphologies and performances of 1D cobalt oxide nanostructures. SEM image of (a) nanorods [377] (Copyright 2005, Wiley-VCH), (b) needle-like nanotubes [379] (Copyright 2008, Wiley-VCH), (c, d) nanoneedles [380] (Copyright 2008, Royal Society of Chemistry). (e) Cycling stability for Li+ storage of three nanoneedle samples prepared by annealing the precursor β-Co(OH)2 nano-needles in air at 200 °C (A200), 300 °C (A300), and 400 °C (A400) at a current density of 150 mA g−1 [380] (Copyright 2008, Royal Society of Chemistry). 616

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3.2. 1D cobalt oxide nanostructures The typical 1D nanostructures include nanowires, nanorods, nanofibers, nanoneedles, nanotubes, and nanobelts, and have presented great potential for the applications in electrochemical devices [354,358–374]. Among them, the aspect ratio (length/diameter) of the rod-like nanostructures is often less than 10, while for nanowire/fibers it is usually over 10 [375]. Versatile approaches have been developed to synthesize 1D nanostructures, such as template-assisted wet-chemical routes, chemical vapor deposition (CVD), physical vapor deposition (PVD), thermal decomposition, wet-chemical synthesis, and electrodeposition [177,376]. It is generally accepted that 1D nanostructures can provide a good platform to study the dependence of anisotropic electrical, thermal, and mechanical properties of nano-sized materials [375]. Li et al. fabricated various Co3O4 nanostructures, including nanotubes, nanorods and NPs [377]. Co3O4 nanotubes were fabricated by porous alumina template-assisted chemical decomposition of Co(NO3)2·6H2O precursor. Scanning electron microscope (SEM) confirmed that the length of Co3O4 nanotubes was between ca. 20 and 40 μm and the wall thickness was approximately 20–30 nm. The Co3O4 nanorods with uniform diameter in the range of 100–120 nm (Fig. 17a), were synthesized from inverse microemulsions [377,378]. The Co3O4 NPs with a typical size of around 100 nm were prepared by ball-milling and thermal decomposition of Co (NO3)2·6H2O. In terms of the electrochemical performance as anode materials for LIBs, the Co3O4 nanotube electrode exhibited an initial discharge capacity of 850 mA h g−1 with a wide discharge voltage window and a long plateau, which was higher than that of nanorod (815 mA h g−1) and NP (830 mA h g−1) electrodes. After 100 cycles at a current density of 50 mA g−1, the reversible capacities of these cobalt oxide nanostructures still maintained at 500 (nanotubes), 480 (nanorods), and 450 mA h g−1 (NPs), respectively, corresponding to approximately 58.8%, 57.8%, and 55.2% of the initial capacity [377]. The high performance of the cobalt oxide nanotubes was resulted by the decreased particle size, the increased proportion of the surface atoms, and the hollow structures, making the structural more active for lithium-involved reaction and easier for lithium ion diffusion through 1D nanostructures. Lou et al. reported a self-supported topotactic transformation method for the synthesis of needle-like Co3O4 nanotubes by transforming from β-Co(OH)2 nanorods. As shown in Fig. 17b, it can be clearly observed that these Co3O4 cylindrical nanotubes were constructed from building-blocks of less than 100 nm. These nanotubes delivered an ultrahigh initial discharge capacity of more than 2000 mA h g−1 [379]. Similarly, through controlled thermal oxidative decomposition and re-crystallization of β-Co(OH)2 nanoneedles, mesoporous single-crystalline Co3O4 nanoneedles (Fig. 17c and d) have also been synthesized [380]. When used as an electrode material for LIBs, these nanoneedles exhibited a high reversible capacity of 1079 mA h g−1 after 50 cycles at a current density of 150 mA h g−1 with a voltage window between 3 V and 10 mV (Fig. 17e), indicating their promising application in LIBs

Fig. 18. Morphologies and electrochemical performances of 1D cobalt oxide nanowires/nanobelts. SEM image of (a) Co3O4 nanowires on Ti foil [392] (Copyright 2014, Elsevier), (b) Co3O4 nanowires on Ni foam [394] (Copyright 2016, Elsevier), (c) Co3O4 nanobelt array on Ti foil [395] (Copyright 2010, American Chemical Society). (d) Cycling performance of LIBs based on Co3O4 nanowires on Ti foil at a current density of 500 mA g−1 [392] (Copyright 2014, Elsevier), and (e) Co3O4 nanobelt array on Ti foil at different rates [395] (Copyright 2010, American Chemical Society). (f) SEM image of Ni-doped Co3O4 nanowires arrays for electrocatalysis [397] (Copyright 2015, Royal Society of Chemistry). 617

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[380]. Apart from the application in the common LIBs, 1D Co3O4 nanorods were also used as non-precious electrocatalysts for ORR in AEMFCs [264] and MFCs [263]. As a typical type of 1D nanostructures, nanowires have been widely explored for electronic, optoelectronic, sensors, and electrochemical device applications [365,381–391]. Nanowires are primarily grown via vapor-liquid-solid (VLS) growth, vapor-solidsolid (VSS) growth, wet-chemical synthesis, electrospinning, and electrochemical anodization, etc. [354]. It is worth noting that Co3O4 NWAs or nanobelt arrays (NBAs), which can be grown directly on some conductive current collectors, such as Ti foil (Fig. 18a and c) and Ni foam (Fig. 18b), are good candidates as binder-free electrodes for energy conversion and storage devices with enhanced storage kinetics and structural stability [392–399]. In this structure, each wire or belt is electrically connected to the current collector, enabling effective ion/charge transfer for ion insertion/extraction. Furthermore, the good contact between the active materials and the current collectors is of significance for providing better accommodation of the large volume changes during charging/ discharging cycles. These advantages make these arrays attractive for improving the rate capability and the cycling stability with a reduced overvoltage. For example, grass-like Co3O4 NWAs on Ti foil presented a remarkable Li+ capacity of 662 mA h g−1 even at a high current density of 5 A g−1 and maintained a stable capacity of 1031 mA h g−1 at a rate of 500 mA g−1 after 100 cycles (Fig. 18d) [392], and Co3O4 NBAs on Ti foil were capable of retaining the Li+ capacity of 770 mA h g−1 at 177 mA g−1 over 25 cycles and 330 mA h g−1 at 3350 mA g−1 beyond 30 cycles (Fig. 18e) [395]. Furthermore, via direct doping of Ni into Co3O4 NWAs (Fig. 18f) with the purpose to increase the electroactive sites and to enhance the overall conductivity of electrode, efficient ion/electron transport and low charge transfer resistance for electrocatalytic reactions have been achieved [397]. In short, the biggest advantage for 1D cobalt oxide nanostructures, especially the arrays grown in-situ on current collectors providing good binding force between the active materials and the conductive substrates, can greatly enhance the cycling stability and prolong the catalyst life. The possible influence of the structures, such as the length and diameter, the aspect ratio, the density, and the morphology of the wires/fibers/rods/needles/belts on their electrochemical properties, unfortunately, currently absent systematic investigation. Moreover, the serious expansion of 1D cobalt oxides during repeated cycling processes can destroy the original ordered structures and lead to a higher irreversible capacity/capacitance and a weaker catalytic activity. 3.3. 2D cobalt oxide nanostructures 2D nanostructures, or called ultrathin nanosheets, whose thickness can go downwards only a single or a few atomic layers, have high surface-to-volume ratios as well as unusual physical, chemical, or electronic properties compared to their bulk counterparts [33,354,400–405]. For example, some typical characteristics for graphene includes tunable band gap, high flexibility, an ambipolar electric field effect along with ballistic conductivity of charge carriers, a quantum Hall effect at room temperature, etc. [406]. These advantages make graphene has been widely applied in various fields, such as materials, chemistry, physics, biology, and medicine [407,408]. Furthermore, the research on graphene has also initiated a boom in the development of graphene-like nanomaterials

Fig. 19. Morphologies and performances of 2D cobalt oxide nanosheets. (a, b) SEM images of self-stacked Co3O4 nanosheets, (c) cycling stability of the self-stacked Co3O4 nanosheets compared to commercial Co3O4 products at a current density of 178 mA g−1 [421] (Copyright 2011, Royal Society of Chemistry). (d, e) SEM images of (d) hexagonal Co(OH)2 precursors and (e) hexagonal Co3O4 nanosheets, (f) rate capabilities of hexagonal Co3O4 nanosheets at various rates [422] (Copyright 2016, Wiley-VCH). 618

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[409–411]. To date, the family of 2D nanomaterials encompasses a broad selection of various compositions including almost all the elements in the periodic table [412,413]. Three common approaches are employed for the fabrication of 2D metal oxide nanosheets, namely, CVD growth, mechanical/liquid exfoliation of layered host materials, and wet-chemical self-assembly routes [354,414,415]. We recently have comprehensively reviewed the applications of 2D metal oxide nanostructures in next-generation rechargeable batteries and proposed the strategies for how to further improve the storage performance of typical 2D nanomaterials [33,67,405], thus will not repeat here. For Co3O4 nanosheets, wet-chemical synthesis strategies are the dominant fabrication approaches [416–420]. In this section, three typical Co3O4 nanomaterials with 2D forms, namely, plate-like nanosheets, hexagonal nanosheets, and ultrathin nanosheets, are presented in detail. Wang et al. reported the synthesis of self-stacked Co3O4 nanosheets separated by carbon layers by a solvothermal approach in the

Fig. 20. Synthesis, characterization, and LIB applications of ultrathin 2D cobalt oxide nanosheets. (a) Schematic illustration of wet-chemical molecular self-assembly of metal oxide nanosheets, and (b) the corresponding SEM image of self-assembled ultrathin Co3O4 nanosheets [428] (Copyright 2014, Nature Publishing Group). (c) TEM image, (d) HRTEM image, and (e) the corresponding fast Fourier transform (FFT) pattern of atomically thick Co3O4 nanosheets fabricated by topochemical transformation [429] (Copyright 2013, Royal Society of Chemistry). (f) Schematic illustration of working mechanisms using nanosheets and their bulk counterpart as anode materials for LIBs [429] (Copyright 2013, Royal Society of Chemistry). 619

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presence of poly(vinylpyrrolidone) (PVP) and water followed by a calcination process [421]. As shown in Fig. 19a, the as-obtained microstacks were built from vertically stacked nanoplates approximately 4 μm in diameter and about 40 nm thickness (Fig. 19b). Interestingly, they also found both PVP and water could affect the final morphology of the products. Flower-like particles and hollow spheres were obtained with the existence of only water and only PVP, respectively. The self-stacked nanosheets were used as electrode material for LIBs, and the discharge capacity reached 1644 mA h g−1 in the first cycle and maintained 1070 mA h g−1 after 50 cycles at a current density of 178 mA g−1 (Fig. 19c). Actually, the excellent electrochemical performances were largely ascribed to the carbon layers between the Co3O4 nanoplates, which provide enhanced electrical conductivity and act as buffer spaces between the Co3O4 sheets to accommodate the volume changes during charging/discharging [421]. Besides the self-stacked nanosheets, isolated ultrathin hexagonal Co3O4 nanosheets have also been synthesized via hydrothermal methods [422–427]. Ultrathin Co3O4 hexagonal nanosheets with exposed {1 1 1} facets, which are the reactive high-energy facets, were prepared via thermal treatment of hexagonal Co(OH)2 sheet-like precursors (Fig. 19d) synthesized by a general polyethyleneimine (PEI)-mediated hydrothermal reaction [422]. As indicated in Fig. 19e, the typical Co3O4 hexagonal nanosheets were approximately 100 nm in side length and 15 nm in thickness. It has also been found that pores from several to tens of nanometers simultaneously formed inside the Co3O4 hexagonal nanosheets, aroused by the dehydration of polymer precursors and the crystalline phase transformation during heat treatment. When tested as anode materials for LIBs, this unique Co3O4 nanostructure presented a remarkable capacity of 1007 and 858 mA h g−1 at 100 and 500 mA g−1, respectively (Fig. 19f), together with an excellent capacity retention of 96–99% [422]. Recently, Co3O4 nanosheets with the thickness at an atomic level were developed for electrochemical applications. Sun et al. proposed a generalized approach for molecular self-assembly synthesis of ultrathin 2D transition metal oxide nanosheets (e.g. TiO2, ZnO, Co3O4, WO3, Fe3O4, MnO2, etc.) by rationally employing lamellar reverse micelles, as shown in Fig. 20a [428]. Compared with the exfoliation and CVD methods, this route is suitable for metal oxides with non-layered host materials, and uniform products can be easily obtained in large quantities. Moreover, this process does not involve high temperature and/or pressures, so the cost is relatively low. Taking Co3O4 as an example, the thickness of the as-obtained Co3O4 ultrathin nanosheets was about 1.6 nm, corresponding to two unit-cell layers, but the lateral size can be extended to 10 μm (Fig. 20b). These nanosheets possess high surface area and chemical reactivity, which make them appealing in environment protection and energy conversion and storage applications [410,428]. Freestanding atomically thick Co3O4 nanosheets with exposed {1 1 1} facets have also been synthesized by topochemical transformation from few-layered α-Co(OH)2 precursors [429]. The resulting nanosheets were ultrathin with a thickness of only 1.5 nm and lateral sizes of about 400 nm (Fig. 20c). The crystal structure of spinel Co3O4 and the preferential crystal orientation of {1 1 1} facets were confirmed by a high-resolution transmission electron microscope (HRTEM) (Fig. 20d) and the corresponding fast Fourier

Fig. 21. Morphologies and performances of 3D cobalt oxide micro-/nano-spheres. SEM images of Co3O4 in morphology of (a) hollow-sphere monolayer arrays [453] (Copyright 2011, Royal Society of Chemistry), (b) single-shelled hollow spheres [456] (Copyright 2010, Wiley-VCH), (c) double-shelled hollow spheres [456] (Copyright 2010, Wiley-VCH), and (d) mesoporous hollow spheres [452] (Copyright 2014, Nature Publishing Group). (e) Comparison of rate capabilities for Co3O4 mesoporous hollow spheres before (blue squares) and after (red circles) lithiation-induced reactivation (scale bar, 100 nm), and (f) the capacity and coulombic efficiency (CE) at a rate of 5.62 C for the following 500 cycles after 1,000 cycles [452] (Copyright 2014, Nature Publishing Group). 620

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transform (FFT) patterns (Fig. 20e). Compared with the bulk Co3O4, these atomically thin nanosheets are superior for the lithiation/ delithiation behaviors for LIBs (Fig. 20f). The synchrotron radiation X-ray absorption near-edge spectroscopy (XANES) has also identified that the proportion of Co2+ atoms is higher and the charge redistributed on the {1 1 1}-oriented surfaces, which facilitate the storage of lithium ions [429]. Besides the isolated nanomaterials, some highly-oriented 2D arrays directly grown on the substrates have also been fabricated. For example, highly-ordered 2D CoO nanosheets arrays vertically aligned to porous Ni foam substrate have been grown via a galvanostatic electrodeposition technique [430]. This structure was directly used as the anode for LIBs and exhibited excellent cyclability (retained 1000 mA h g−1 after 100 cycles at 1 A g−1) and rate capability (520 mA h g−1 at 10 A g−1) [430]. 2D cobalt oxide nanostructures with confined thickness and high surface-to-volume ratios have shown great potential as electrode materials for ion storage devices. Nevertheless, in practical applications, these ultrathin nanosheets are easily restacked into thick closely packed structures, which severely hamper the insertion and diffusion of the ions and the infiltration of electrolytes into the inside of active materials, especially at high current densities/sweeping rates. Therefore, the direct use of 2D cobalt oxide nanostructures for electrochemical devices is yet full of challenges. Some possible solutions are still under exploration from the perspective of material design and electrode assembly. 3.4. 3D cobalt oxide nanostructures Micro/nanospheres, solid/porous polyhedrons, core-shell structures, mesoporous structures, hollow structures, hierarchical/ heterogeneous structures assembled from low-dimensional nanoarchitectures, etc., with an overall size over 100 nm, are considered as 3D nanostructures. Compared with the lower dimensional nanostructures, 3D nanomaterials are by far possessing more advantageous for their high surface area and hierarchically porous structure, which lead to increased numbers of exposed active sites, good contact with electrolytes, and enhanced ions or electron transport and diffusion in the practical electrochemical applications. As for these 3D nanostructures assembled from 0D, 1D and/or 2D nanostructures, they not only possess the intrinsic properties of those constitutional low-dimensional structures, but also provide many advanced functions with improved performance for a wide variety of applications [182,431–443]. For example, through self-induced assembly of 2D ultrathin nanosheets into 3D hierarchical architectures via a solution-based reaction or a thermal treatment step, porous structures can form in the materials, and the self-aggregation between the 2D nanosheets can be effectively alleviated by this way [444–450]. Hence, 3D nanomaterials are promising for plenty of applications, such as sensors, solar energy harvesting devices, molecular separation membranes, nanoreactors, energy conversion and storage devices, pharmaceutical delivery, and advanced optical devices [435,443,451]. Some representative 3D sphere-like Co3O4 nanomaterials with designed pores or channels inside the shells, such as mesoporous, hollow, double-shelled, and multi-shelled micro-/nano-spheres/quasi-spheres, have been widely studied for electrochemical devices [452–463]. As we can see from Fig. 21a–d, the diameters of these spherical structures were in the range from several hundred nanometers to a few micrometers. These unique 3D spherical structures together with nanopores and nanochannels result in remarkable ion storage performance, due to the improved surface-to-volume ratio, the reduced mass/charge transport lengths, and the low-energy transport paths. Moreover, the hollow structures and the pores as well as channels can provide enough free space to mitigate the unfavorable volume changes caused by the repeated ion intercalation and de-intercalation processes. In addition, the limited outer surface brought by the relative large overall size is unfavorable for the formation of a solid-electrolyte interphase (SEI) layer, which is the main reason to consume lithium ions by forming irreversible compounds [464]. It is for sure that there are some drawbacks for those 3D nanostructures. For an instance, one of the biggest drawbacks for 3D hollow nanomaterials is poor mechanical stability. Degradation occurs when 3D nanomaterials suffer high mechanical stress originating from repetitive volume expansion/contraction cycles. Sun et al. reported that the mechanical degradation of the metal oxide hollow spheres during cycling could be deliberately controlled to hierarchically mesoporous structure together with the formation of a stable SEI layer by a highrate lithiation-induced reactivation process [452]. It is obviously demonstrated from Fig. 21e that Co3O4 mesoporous hollow spheres showed a significantly improved rate capability after lithiation-induced reactivation and an excellent cycling stability for lithium storage at a higher rate of 5.62 C (5.0 A g−1) for the subsequent 500 cycles with a stable reversible capacity of 335 mA h g−1 (Fig. 21f), which is almost 300% higher than these electrodes without reactivation. Moreover, the Co3O4 hollow mesospheres reactivated with a 400-cycle mixed charging and discharging rate scheme (MC-rate: a discharge rate of 2.81 C and a charge rate of 1.69 C in each cycle) exhibited a remarkable cycling stability with no obvious capacity fading after 7000 cycles at a high rate of 5.62 C [452]. The second typical type of 3D Co3O4 structures for electrochemical devices is cube-like/octahedron-like/dodecahedron-like architectures [465–471]. For understanding the structure-property relationships of the 3D Co3O4 polyhedrons, the relationship between the particle size and the electrochemical performance has been investigated. Xu et al. synthesized nano-sized (nO-Co3O4, 387 nm) and micron-sized (mO-Co3O4, 6.65 μm) solid Co3O4 octahedra as electrode materials for LIBs [467]. Electrochemical results showed that the nO-Co3O4 delivered a high charge capacity up to 955.5 mA h g−1 over 200 cycles with no obvious capacity decay at a current density of 0.1 A g−1 (ca. 0.11 C), while the mO-Co3O4 only maintained 288.5 mA h g−1 at the same conditions. This noticeable difference is ascribed to the size effect on volume expansion/extraction. The mO-Co3O4 with large particle sizes suffers larger volume changes during repeated cycling, leading to serious pulverization of electrode materials and subsequent appearance of an electric isolation layer. Interestingly, the nO-Co3O4 maintains the initial octahedral shape after over 300 cycles. On the contrary, the mOCo3O4 broke into small particles with an average size of 1–2 μm and lost the octahedral shape after only 42 cycles [467]. Morphology is another crucial parameter affecting electrochemical properties. Chen et al. explored cobalt-based nanostructures, including cubes, discs, and flowers, for LIBs and concluded that cube-like structure gave the lowest capacity while nanoflowers 621

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exhibited the best performance [468]. Similarly, Song et al. synthesized three types of Co3O4 hollow nanoarchitectures, including quadrate tubular nanoboxes (QTNBs), rectangular nanoboxes (RNBs), and cubic nanoboxes (CNBs), via controlled mild annealing of cobalt coordination polymer nano-solids [472]. Electrochemical measurements demonstrated that QTNBs exhibited outstanding properties, such as a superior capacity of 1200 mA h g−1 at 0.2 A g−1 and a remarkable retention of 625 mA h g−1 at a high current density of 10 A g−1. CNBs, however, delivered the worst performance, such as higher irreversible capacity (344 mA h g−1) and higher charge transfer resistance (Rct) (∼250 Ω) than that of QTNBs (222 mA h g−1, ∼130 Ω) and RNBs (252 mA h g−1, ∼230 Ω), respectively [472]. To improve the performance of Co3O4 cubes or boxes, heterostructure and hierarchical structure were introduced. Huang et al. produced regular Co3O4 microcubes with 2.37 μm in length on average, assembled by many irregular Co3O4 NPs (20–200 nm in diameter and 30–40 nm in thickness, Fig. 22a) [473]. Remarkably, these mono-dispersed micro-/nanostructured cubes displayed discharge capacities as high as 1298 mA h g−1 at 0.1 C and 1041 mA h g−1 at 1 C, respectively [473]. Afterwards, hierarchically porous Co3O4 nanoboxes with well-defined interior voids and functional shells (Fig. 22b) were synthesized via an ion

Fig. 22. Morphologies and performances of 3D cobalt oxide polyhedrons. SEM images of Co3O4 in morphology of (a) mesoporous cubes [473] (Copyright 2014, American Chemical Society), (b) hierarchically porous boxes [474] (Copyright 2016, Royal Society of Chemistry), (c) hollow octahedra [475] (Copyright 2009, American Chemical Society), (d) solid cubes [476], (e) solid octahedra [476], (f) truncated octahedra [476] (Copyright 2012, Wiley-VCH). (g) Discharging and charging curves of Co3O4 octahedra at a current density of 1000 mA g−1, and (h) comparison of cycling stability of Co3O4 cubes, octahedra and truncated octahedra [476] (Copyright 2012, Wiley-VCH). 622

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exchange reaction using Prussian blue analogue as precursor at a low temperature (60 °C) [474]. An extremely large surface area of about 272.5 m2 g−1 was achieved and it provided more active sites to promote the ORR and OER as a Li-O2 battery cathode [474]. Apart from cubes, single-crystalline Co3O4 octahedral hollow cages (Fig. 22c) with tunable surface aperture were also synthesized through a carbon-assisted carbothermal approach and applied for Li+ storage [475]. The hollow quasi-octahedra presented the highest initial discharge capacity (1470.2 mA g h−1) and the best cycling performance (maintained 670 mA g h−1 after 50 cycles at 178 mA g−1) among the fabricated different morphologies of quasi-octahedra, hollow octahedral, and hollow spheres [475]. In addition to the effect of particle size and morphology, it should be noted that exposed crystal facets also have a significant influence on the electrochemical performance. Xiao et al. synthesized three well-defined Co3O4 morphologies with different exposed crystal planes via a simple control of the ratio of Co(NO3)2 6H2O and NaOH [476]. Co3O4 cubes (Fig. 22d) with a particle size of about 500 nm and six exposed {0 0 1} planes, Co3O4 octahedra (Fig. 22e) with a size of 600–700 nm and eight exposed {1 1 1} planes, and Co3O4 truncated octahedral (Fig. 22f) with a size of around 600 nm and eight exposed {1 1 1} and six {0 0 1} planes. Based on those samples, the effect of exposed crystal planes on the electrochemical properties such as lithium ion storage has been systematically investigated. It has confirmed that the performances of these three types of Co3O4 nanostructures ranked as “octahedra > truncated octahedra > cubes” and the Co3O4 octahedra with exposed {1 1 1} planes delivered both the highest charge/discharge specific capacity and the best rate capability (Fig. 22g and h). In fact, the electrochemical performances of the cobalt oxides with different exposed facets depend largely on the redox reaction of Com+/Co0. Surface atomic configurations of Co3O4 unit cell (Fig. 23a) shows that the {1 1 1} plane have more Co2+ (3.75 Co2+) than the {0 0 1} plane (2 Co2+), leading to a faster reaction rate [476]. Subsequently, they found that the Co3O4 octahedra with exposed {1 1 1} planes showed superior electrocatalytic performance for Li-O2 battery than the Co3O4 cubes with exposed {0 0 1} facets (Fig. 23b and c). Based on theoretical calculations and experimental results, the facet-dependent electrocatalytic mechanism of the Co3O4 nano-polyhedrons with different crystal planes have been proposed, as shown in Fig. 23d and e. Compared to the Co3O4 with exposed {0 0 1} planes, Co3O4 with {1 1 1} surfaces has more active Co2+ sites exposed on the surface, which have a stronger absorption ability for Li2O2 in ORR and a lower activation barrier for O2 desorption in OER [477]. A similar study on the facet-dependent activity and stability of Co3O4 nanostructures towards OER has also been reported thereafter [478]. 3D hierarchical architectures, constructed by 1D, 2D, and 3D building blocks, are another important class of nanomaterials with exceptional physical and chemical properties for energy conversion and storage devices [443,479–484]. Generally, wet-chemical synthesis is the most common strategy for the preparation of these types of 3D nanostructures, including one-step self-assembly and two-step self-assembly approaches [443,485,486]. In particular, the versatile two-step synthetic route allows an effective combination with a variety of other techniques (e.g. lithography, hydrolysis, carbonization, etc.) to produce multifunctional nanostructures [487]. In 2012, Wang et al. reported the growth of symmetric Co3O4 hexapods assembled from numerous porous nanorods on copper foil by a facile hydrothermal method followed by a thermal annealing step in nitrogen environment (Fig. 24a) [488]. In this synthesis, NH4F plays a key role for the formation of nanorod-assembled Co3O4 hexapods. Without the addition of NH4F, only bunch-like products were obtained. The specific surface area of Co3O4 hexapods was 134.84 m2 g−1. Most importantly, this nanostructure grown directly on a conductive copper substrate could be used as binder-free anodes for LIBs, which exhibited a high reversible capacity of 800 mA h g−1 and 440 mA h g−1 at current densities of 100 and 500 mA g−1, respectively, after 40 cycles. Moreover, the lithium storage performance has been further enhanced via carbon coating and reached an improved reversible capacity of 1001 mA h g−1 after 40 cycles [488]. Flower-like Co3O4 nanomaterials have also been fabricated through a solvothermal reaction together with a subsequent postcalcination process [489,490]. As indicated in Fig. 24b, Co3O4 nanoflowers grown on Ni foam were consisted of many 2D nanosheets with jagged edges. These nanosheets, with a thickness of between 60 and 110 nm, possessed smooth surface [489]. Afterwards, 3D

Fig. 23. Effect of exposed crystal planes of cobalt oxide structures on electrocatalytic performances. (a) schematic illustration of atomic configurations for Co3O4 structures with different exposed planes [477], (b, c) cycling stability of the Li-O2 batteries based on the cathode catalysts of Co3O4 (b) cubes and (c) octahedrons at a current density of 100 mA h·g−1 with a limited capacity of 1000 mA h·g−1 [477], and (d, e) schematic illustration of facet-dependent electrocatalytic mechanisms of ORR and OER for Co3O4 (d) cubes and (e) octahedrons [477] (Copyright 2015, American Chemical Society). 623

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Fig. 24. Morphologies and performances of 3D dendritic cobalt oxide microspheres. SEM images of Co3O4 in morphology of (a) hexapods [488] (Copyright 2012, Royal Society of Chemistry), (b) nanoflowers [489] (Copyright 2011, Elsevier), (c) red-headed calliandra-like nanoflowers [490] (Copyright 2015, American Chemical Society), (d) 3D pompon-like porous spheres [492]. (e) Comparison on rate capability of 3D pompon-like Co3O4 spheres, Co3O4 nanowires (NWs), and Co3O4 nanoparticles (NPs) at various current densities [492], (f) schematic illustration of possible Li+ transfer pathways for the pompon-like Co3O4 anode [492] (Copyright 2014, Royal Society of Chemistry).

red-headed calliandra-like (Fig. 24c) and dandelion-flower-like hierarchical mesoporous Co3O4 were prepared by calcining the solvothermal synthesized cobalt carbonate hydroxide precursors at 400 °C in air, in which the morphology of the precursors was control by adjusting the proportion of solvent, the amount of urea, and the reaction time [490]. When tested as anode materials for LIBs, the dandelion-flower-like cobalt oxide showed a high initial specific capacity of 1298 mA h g−1 and a stable reversible capacity of 1204 mA h g−1 after 20 cycles, while those of the red-headed calliandra-like Co3O4 exhibited capacities of 1340 and 833 mA h g−1, respectively, at a current density of 100 mA g−1 with a potential window ranging from 0.01 to 3.0 V [490]. Other hierarchical structures, such as star-like, pompon-like, shale-like, and conch-like structures, have been reported for energy storage applications [491–494]. In terms of ion storage, these hierarchical structures have advantages for achieving high-efficiency ion transport and accommodating volume strain during intercalation and de-intercalation processes. For example, the star-like Co3O4 hierarchical structure, synthesized through thermal decomposition of self-assembled Co(OH)F precursors., delivered an initial specific charge capacity of 1036 mA h g−1 at a current density of 50 mA g−1 as anode materials for LIBs [491]; the pompon-like Co3O4 hierarchical porous spheres, as shown in Fig. 24d, synthesized via a combination of hydrothermal method and calcination treatment, presented considerable capacities of about 730 and 650 mA h g−1 at high current densities of 400 and 1200 mA g−1, respectively (Fig. 24e) [492]. This superior electrochemical performance is believed to be resulted from the unique hierarchical structures, the short ion transport lengths, and the multiple transport directions, as illustrated in Fig. 24f. Besides, Co3O4 hexagonal plates assembled by Co3O4 nanocubes [495] and Co3O4 thin films formed from Co3O4 nanoflakes [496] were demonstrated with enhanced electrochemical properties. Recently, a general method for producing 3D mesoporous Co3O4 networks from ultrathin-nanosheet as anode materials for LIBs has been proposed by Zhu et al. [497]. During the fabrication, 3D nitrogen-doped carbon networks (N-CN) (Fig. 25a) were first synthesized via a salt template-assisted method. Cobalt ions were then adsorbed onto the N-CN via a dipping process in cobalt nitrate solution. After the removal of carbon template by heating at 500 °C in air, 3D mesoporous Co3O4 networks (3D-MN Co3O4) were finally obtained, as shown in Fig. 25b and c. The obtained interconnected 3D-MN Co3O4 structures exhibited a large surface area as high as 106.6 m2 g−1 and a pore size distribution ranging from 2 to 10 nm. The presence of mesoporous architecture, which provides increased charge transport pathways (Fig. 25e), demonstrated a superior performance as an anode material for LIB with a high capacity of 1033 mA h g−1 at the rate of 0.1 A g−1 and long-life stability (700 cycles at 5 A g−1) in the potential range from 5 mV to 3 V (Fig. 25d). Moreover, this strategy has been shown to be effective for the synthesis of other 3D mesoporous TMOs, such as Fe2O3, ZnO, Mn3O4, NiCo2O4, and CoFe2O4 [497]. To sum up, 3D cobalt oxides have much complex morphologies and porous structures compared to other types of structures. Generally speaking, the 3D cobalt oxide nanostructures with the existence of porous architectures, such as those with hollow structures and hierarchical structures, demonstrate better electrochemical performances than those solid particles. More attractively, 624

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Fig. 25. Synthesis, characterization, and application of 3D cobalt oxide networks (3D-MN Co3O4) [497]. (a) Schematic illustration of a general synthesis route for 3D cobalt oxide network, (b) the corresponding SEM image, (c) TEM image, and (d) cycling stability of Li+ storage at a current density of 5 A g−1 [497], (e) schematic illustration of electron/ion transport pathways of 3D mesoporous Co3O4 networks (Copyright 2017, WileyVCH).

the 3D nanostructures assembled from low dimensional constituents are promising for practical applications, because they not only can take the full advantage of the merits of low dimensional structures, but also possess extra benefits from their 3D configurations, such as preventing the aggregation of the low-dimensional structures, improving the contact area with electrolytes, and accommodating the volume expansion/contraction during cycles. It is worth noting that the adhesion force between these 3D nanostructures and current collectors during the preparation of electrodes for batteries is always weak, which result in the delamination of active materials and rapid fading of the electrochemical performance during cycling. Therefore, further optimization on the electrode preparation of these 3D nanostructures is needed. 4. Fabrication and electrochemical performance of cobalt oxide-based nanocomposites To conquer the intrinsic issues of cobalt oxides, such as the low electric conductivity and the significant volume variation during charging/discharging, the construction of cobalt oxide-based nanocomposites is one of the most feasible strategies, owing to its appealing advantages, such as simple for fabrication, effective for enhancing the performance, and generalized for most other metal oxides. Via this method, a wide range of cobalt oxide nanocomposites has been produced with the configuration of 0D-1D, 0D-2D, 1D2D, and 2D-2D, etc., by using the techniques of anchoring, depositing, encapsulation, and coating, etc. In this section, we aim to summarize the recent achievements in the rational design of cobalt oxide-based nanocomposites for electrochemical energy applications. 4.1. Cobalt oxide/carbon nanocomposites Hybridizing carbon with cobalt oxide nanomaterials is a simple and effective way to overcome the intrinsic low conductivity of 625

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cobalt oxides to achieve rapid ion insertion/disinsertion and promoted catalytic activity [498–511]. Two-step strategy is often employed for the fabrication of metal oxide nanostructures with complex configurations [354,432–434,484,485,487]. This strategy has also been used for the fabrication of cobalt oxide-based nanocomposites, in which a phase transformation process from the assynthesized cobalt-containing solid precursors (e.g. cobalt hydroxide, cobalt nitrate hydroxide, etc.) to the final pure cobalt oxides is usually performed by thermal treatment in air or in solvents at elevated temperatures [512]. In this section, we classified the cobalt oxides/carbon composites into six typical types based on the dimensionality of cobalt oxide nanostructures, namely 0D cobalt oxide/carbon hybrids, 1D cobalt oxide/carbon hybrids, 2D cobalt oxide/carbon hybrids, 3D cobalt oxide/carbon hybrids, cobalt oxide/heteroatom-doped carbon hybrids, and cobalt oxide/carbon/others hybrids. It should be noted what the carbon refers here is the amorphous carbon coatings or nanocarbon in the forms of fiber-like, sphere-like, or other morphologies, but except for CNTs and graphene, which will be separately summarized in the following sections. 4.1.1. 0D cobalt oxide/carbon hybrids The combination of 0D cobalt oxide NPs with carbon can be mainly classified into two main strategies, namely, carbon surface supported 0D cobalt oxides, and carbon encapsulated 0D cobalt oxides. For the carbon surface supported 0D cobalt oxides, the cobalt oxide NPs can be dispersed, anchored, or deposited on the carbon surfaces. This hybrid is easily to be fabricated by some simple techniques, such as direct mixing of cobalt oxide NPs and carbon in a solution. A uniform dispersion of NPs on the carbon surface with a strong binding force, however, is still a big challenge. In the case of carbon encapsulated 0D cobalt oxides, the cobalt oxide NPs are encapsulated or surrounded by carbon layers, or the NPs are embedded into a carbon matrix. There are some advantages in the carbon encapsulated 0D cobalt oxide hybrids. First, the carbon layers or matrix can help avoid the aggregation of cobalt oxide NPs. Second, the carbon coating on the surfaces of NPs can act as excellent conductive networks for the overall nanocomposites. Moreover, the carbon layers or matrix can contribute to ease the volume expansion of 0D cobalt oxide NPs during the repeated ion insertion/ extraction cycles. For the carbon supported 0D cobalt oxide, the most common configuration is the 0D NPs anchoring on carbon surfaces. Depends on the adsorption of 0D NPs on the outer surface or the inner surface of the carbon supporter, some different morphologies of the hybrids have been reported [513–519]. For example, as shown in Fig. 26a, cobalt oxide NPs (20–40 nm in size) were anchored on the pore walls of amorphous carbon [514]; while as shown in Fig. 26b, cobalt oxide NPs (< 10 nm in size) were adsorbed on the outer surface of carbon spheres (∼100–300 nm in size) [515]. It is clearly observed that the electrochemical performances of the cobalt oxide/carbon hybrids are superior to these of the individual cobalt oxides and carbon when these were used as electrode materials for LIBs [514] or cathode catalyst for Li-O2 batteries (Fig. 26c) [515]. Hao et al. reported a facile hydrothermal and sol-gel polymerization route for the large-scale fabrication of interconnected hybrids, where well-defined Co3O4 NPs (∼5 nm) were anchored on

Fig. 26. Morphologies and performances of 0D cobalt oxide NPs absorbed on carbon matrix hybrid materials. (a) TEM image of the hybrid material of 0D Co3O4 NPs anchored on the inner surface of porous carbon (Co3O4/PC) [514] (Copyright 2015, Elsevier). (b) The hybrid material of 0D Co3O4 adsorbed on the outer surfaces of carbon spheres [515], (c) cycling performance of the Li-O2 batteries with different air electrodes under a limited capacity of 1000 mA h gelectrode−1 and a stable charge voltage of 4.35 V [515] (Copyright 2013, Elsevier). (d) Cycling stability of individual carbon aerogel (CA), Co3O4, and Co3O4/CA hybrids at a current density of 50 mA g−1 [520], (e) schematic illustration of Li insertion/extraction process of the hybrid material of 0D Co3O4 NPs interconnected with CA hybrid nanospheres [520] (Copyright 2013, American Chemical Society). 626

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the surfaces of carbon aerogel (CA) nanospheres to form interconnected networks [520]. Such an interconnected Co3O4/CA hybrid materials presented some unbeatable merits, such as large surface area, good conductivity, fast diffusion channels contributed by their mesoporous structure, mechanical flexibility, chemical stability, and short transport length of Li+ within CA matrix. The incorporation of Co3O4 NPs into the interconnected CA effectively reduced the irreversible reaction between Li and carbon matrix, and thus greatly increased the reversible specific capacity and the initial Coulombic efficiency (CE). The Co3O4/CA hybrid material with 25 wt% Co3O4 loading amount displayed an excellent storage performance, which a CE up to 99.5% was retained, and a high discharge capacity of 779 mA h g−1 after 50 cycles, corresponding to 10.1 and 1.6 times larger than that of Co3O4 (73 mA h g−1) and CA (478 mA h g−1), respectively, was observed (Fig. 26d) [520]. As illustrated in Fig. 26e, the improved Li+ storage performance of the mesoporous Co3O4/CA hierarchical hybrids are mainly due to an intimate contact and a synergistic effect between the Co3O4 NPs and CA matrices. Wang et al. reported the fabrication of hybrid architectures through anchoring of 0D Co3O4 NPs around 5 nm in diameter on 3D arrays of carbon nanosheets (Co3O4/CNS) [521]. The combination of the nanoscale 0D cobalt oxide NPs and the 3D interconnected sheet-like carbon provided some advantages for improving the electrochemical properties, such as the high surface area and ample porosity of conductive carbon nanosheets ensuring fast electrolyte transport. When used as anode materials for LIBs, high lithium storage over 1200 mA h g−1 was recorded, together with outstanding rate capability (e.g. ∼390 mA h g−1 at 10 A g−1). Furthermore, the Co3O4/CNS hybrid material displayed a reversible capacity of approximately 970 mA h g−1 at current density of 1 A g−1 after 500 cycles. The greatly enhanced capacity and initial CE were attributed to the synergistic effect between the nanosized cobalt oxide and the sheet-like interconnected carbon nanosheets of the hybrid material [521]. Cobalt oxide NPs with a size ranging 10–50 nm derived from cobalt-containing zeolite imidazolate frameworks (ZIF) have also deposited on a electrospinning fiber-like carbon matrix (Co3O4/CNF) for non-aqueous Li-air batteries [522]. As a self-standing, binder-free electrode, the Co3O4/CNF composite exhibited a high initial discharge capacity over 760 mA h g−1, improved cycling properties, and a lower charge overpotential at different current densities [522]. These were mainly attributed to enhanced catalytic activity of Co3O4 NPs and stable contact between the homogeneously distributed Co3O4 NPs and the carbon nanofibers. Encapsulation of cobalt oxide NPs into carbon matrix is often achieved via confined growth of NPs inside carbon tubes [523], spheres [524], and fibers [525–527]. Leng et al. reported a type of porous graphic carbon (PGC) encapsulated carbon-coated Co3O4 core-shell NPs (Co3O4@C@PGC) hybrid material. In the synthesis, Co3O4@C core-shell nanostructures, in which Co3O4 NPs were homogeneously encapsulated by a thin carbon shell, were further homogeneously embedded into porous graphitic carbon nanosheets to form the final Co3O4@C@PGC hybrid material [528]. As illustrated in Fig. 27a, when the hybrid is used as electrode for LIBs, the unique structure is helpful to reduce the pulverization and aggregation of cobalt oxide NPs. Furthermore, owing to the presence of abundant micropores and mesopores in PGC nanosheets, the stress originated from the volume changes of cobalt oxides it during the insertion and extraction process of Li+ ions can be easily accommodated. As a result, the resulting Co3O4@C@PGC hybrid electrodes exhibited a much larger reversible capacity at various rates compared with other Co3O4-based electrodes (Fig. 27b) [528]. Song et al. synthesized porous carbon/Co3O4 composites by heat treatment of metal-organic frameworks (MOFs) as cathode materials in rechargeable Li-O2 batteries [529]. During the thermal decomposition of cobalt-containing MOFs (Co-MOFs), an interconnected porous structure with uniform distributed Co3O4 NPs in the carbonaceous matrix was obtained (Fig. 27c). The as-synthesized porous carbon/ Co3O4 hybrids exhibited superior electrochemical performance, and a high initial capacity of around 9850 mA h g−1 at a current density of 100 mA g−1 was achieved (Fig. 27d) [529].

Fig. 27. Hybrid materials of 0D cobalt oxide NPs encapsulated into a carbon matrix. (a) Schematic illustration of the Li+ extraction/insertion behaviors of 0D Co3O4@C core-shell NPs encapsulated into porous graphitic carbon nanosheets (Co3O4@C@PGC), (b) comparison of specific capacity for different Co3O4-based electrodes at various rates [528] (Copyright 2015, Nature Publishing Group). (c) Schematic illustration of the fabrication and (d) initial discharge/charge profile of MOF-derived carbon/Co3O4 composites [529] (Copyright 2017, Elsevier). 627

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Recently, biomass-derived carbon materials have attracted an increasing attention for their low cost and abundance, which have also been used as the conductive matrix to hybridize with cobalt oxide NPs for renewable energy applications. Li et al. prepared a Co3O4/carbon fiber hybrid structure (Co3O4-CF) by pyrolysis of wet-spun cobalt alginate fibers, in which Co3O4 NPs (∼30–50 nm) were embedded in a micro-sized carbonaceous fibrous matrix (∼5–8 μm) [530]. In the synthesis, cobalt alginate fibers (Co-AF) were first obtained by a metal ion exchange of calcium alginate fibers (Ca-AF), and then the Co-AF was carbonized into final Co3O4-CF. When used for LIBs, the Co3O4-CF hybrid exhibited a reversible capacity up to 780 mA h g−1 at the current density of 89 mA g−1 after 100 cycles [530]. Similarly, they also prepared nano/micro hierarchical Co3O4/cellulose fibers composites (Co3O4/CF), where Co3O4 NPs (∼50–100 nm) were adsorbed on the surfaces of carbonized cellulose microfibers (CFs). This Co3O4/CF hybrid structure showed a reversible capacity of 730 mA h g−1 under the same test conditions with the Co3O4-CF [531]. The excellent electrochemical performances of these biomass derived Co3O4/C hybrid nanocomposites were mainly attributed to the confinement of the biomass substrates to retard the growth of Co3O4 NPs, and the bi-functionality of the carbon fiber matrix, which served simultaneously as scaffold and conducting network and helped improve the overall conductivity and release the mechanical strain induced during the long-time charging/discharging processes. Furthermore, the porous structure of the hybrid materials also greatly enhances the contact between electrolytes and active materials and the effective diffusion of electrolyte through it. A peapod-like cobalt oxide/carbon nanostructure, where the cobalt oxide NPs encapsulated uniformly in a tube-like carbon matrix, as shown in Fig. 28a, were synthesized by Wang et al. and used as electrode materials for batteries [532]. In the synthesis of the peapod-like Co3O4@carbon hierarchical nanostructure, cobalt carbonate hydroxide (Co(CO3)0.5(OH) 0.11H2O) nanobelts were used as templates to be coated with polymeric layers by a hydrothermal reaction, and then were calcined in Ar atmosphere at 700 °C for 200 min, followed with annealing in air at 250 °C for 200 min to obtain the final product. Scanning transmission electron microscopy (STEM) image (Fig. 28a) indicated that Co3O4 NPs 20–30 nm in diameter were encapsulated in tubular carbon fibers with regular intervals. When tested as anode materials for LIBs, the peapod-like nanostructure demonstrated a high specific capacity of about 1000 mA h g−1 at the rate of 100 mA g−1 and wonderful cycling stability with 80% retention when cycled back from very high rate of 1 A g−1 [532]. Later, Gu et al. reported mesoporous peapod-like Co3O4@carbon nanotube arrays (Co3O4@CNT) through a controllable nano-casting process by using an ordered mesoporous carbon (CMK-5) as substrates [533]. SEM (Fig. 28b) and TEM (Fig. 28c) images revealed that the product had a rod-like morphology with a particle size around 0.8–1.0 μm and no cobalt oxide NPs were found on the external surface of the carbon substrate. To analyze the distribution of NPs in the carbon matrix, high-resolution SEM (HR-SEM) and bright-field scanning transmission electron microscope (BF-STEM) images analysis confirmed that Co3O4 NPs were uniformly embedded into the tubular mesopores of the carbon matrix. Furthermore, it was clearly demonstrated in the TEM image that Co3O4 NPs were highly dispersed and homogeneously confined in the mesoporous channels with uniform sizes of 4–5 nm and distances of approximately 3 nm. They also found that the wall thickness of the tube-like carbon in the products could be easily tuned by adjusting the initial loading precursor amount in the SBA-15 template. By varying the thickness of the carbon layer, the sizes of Co3O4 NPs could also be controlled in the range of 3–7 nm and their loading amount could be varied from 45 to 70 wt%. This

Fig. 28. Morphologies and performances of peapod-like cobalt oxide/carbon hybrid nanostructures. (a) Scanning transmission electron microscopy (STEM) image of peapod-like Co3O4@carbon composite [532] (Copyright 2010, American Chemical Society). (b) SEM and (c) TEM images of mesoporous peapod-like Co3O4@carbon nanotube arrays (Co3O4@CNT) [533], (d) Discharge/charge curves, (e) cycling stability at 0.1 A g−1, and (f) rate capability of the peapod-like Co3O4@carbon nanotube arrays (inset in d: schematic illustration of the peapod-like Co3O4@carbon nanotube arrays) [533] (Copyright 2015, Wiley-VCH). 628

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unique mesoporous structure facilitates the insertion/extraction behaviors of lithium ion for LIBs. The Co3O4@CNT mesoporous peapod-like structure exhibited a high specific capacity of 780 mA h g−1 (Fig. 28d) or volumetric capacity of about 1370 mA h cm−3 at a current density of 100 mA g−1, an outstanding cycling performance (700 mA h g−1 over 60 cycles at 0.1 A g−1) (Fig. 28e), and an excellent rate capacity (453 and 408 mA h g−1 at 1.0 and 5.0 A g−1, respectively) (Fig. 28f) [533]. According to the above selected examples, we can know that 0D cobalt oxides dispersed onto or encapsulated into a carbon matrix can effectively enhance the overall conductivity and thus improve the electrochemical properties. However, there are several major shortcomings of the physical dispersion of cobalt oxide NPs on carbon surfaces, including weak binding force between cobalt oxide NPs and carbon matrix, and poor buffer capacity accommodating volume changes. The encapsulation of these NPs into carbon can greatly suppress the serious aggregation of cobalt oxide NPs into bulks, but the slight aggregation inside the carbon may still exist. Meanwhile, confining cobalt oxide NPs within carbon matrix results in indirect contact between active materials and electrolytes because the electrolyte ions need to penetrate through the outer carbon layers. Hence, the design of cobalt oxide NPs uniformly encapsulated within thin carbon layers to form peapod-like analogous structures is a potential solution for improving the electrochemical performance of 0D cobalt oxide NPs, however, more further work on enhancing the diffusion of electrolyte through the carbon matrix, by decreasing the thickness of carbon layers, for example, is needed. 4.1.2. 1D cobalt oxide/carbon hybrids Currently, there are two main strategies used for the synthesis of 1D cobalt oxide/carbon hybrids. In the first strategy, 1D cobalt oxide nanostructures are firstly prepared and a following carbon coating is achieved via physical sputtering or chemical deposition. Via this method, a carbon layer can be coated onto the cobalt oxide nanowires/fibers to produce core-shell structures. The outer carbon layer in the structure is an effective way to overcome the low conductivity, weak reaction kinetics, and obvious volume expansion of cobalt oxides during electrochemical applications. It has reported that this carbon coated 1D cobalt oxide hybrids demonstrated an excellent capacity of more than 1000 mA h g−1 for LIBs and a remarkable cycling stability up to 8000 cycles for supercapacitors [534,535]. However, the homogeneous coating and the accurate control on the thickness of carbon layer on 1D cobalt oxide nanostructures remain challenging. The second strategy on preparing 1D carbon@cobalt oxide composites is the template-assisted route, in which cobalt precursors/cobalt oxides are coated on fiber-like carbonaceous template and followed with a thermal treatment process. Using this strategy, porous tubular structures can be easily obtained. The presence of cavities and porous structures within 1D nanostructures offer more available space to accommodate the mechanical strain aroused by ion insertion/ deinsertion, and is helpful to shorten ion diffusion distance, promote electrochemical reaction kinetics, and improve electrochemical properties. Co3O4-C core-shell NWAs were fabricated by combining hydrothermal synthesis and direct current magnetron sputtering by Chen et al. [535]. As shown in Fig. 29a and b, compared with Co3O4 NWAs, the surface of the carbon-coated nanowires became a little coarser. TEM analysis revealed that the core nanowires had many nanopores with diameters in the range from 2 to 4 nm, and the amorphous carbon layer was homogeneously coated on the porous nanowire surface, with a thickness of 18 nm (Fig. 29c). This array delivered an initial discharge capacity as high as 1330.8 mA h g−1 at 0.5 C and maintained a high reversible capacity of

Fig. 29. Morphologies and performances of cobalt oxide NWAs and cobalt oxide/carbon core-shell NWAs. SEM images of (a) Co3O4 NWAs and (b) Co3O4-C core-shell NWAs [535], (c) TEM image of a Co3O4-C core-shell NWA [535] (Copyright 2012, Royal Society of Chemistry). (d) Cycling stability at 200 mA g−1 [537], and (e) rate capability of carbon-doped Co3O4 hollow nanofibers (C-doped Co3O4 HNFs) and undoped Co3O4 HNFs [537] (Copyright 2016, Wiley-VCH). 629

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989.0 mA h g−1 over 50 cycles, much higher than the unmodified Co3O4 nanowire array (490.5 mA h g−1) [535]. Later, similar structures have also been reported for asymmetric supercapacitors (ASCs) with cobalt oxide nanowires as positive electrode and activated carbon (AC) as negative electrode [534]. These arrays exhibited a high specific capacity of 116 mA h g−1 at 4 A g−1 and a remarkable cycle life with around 92% retention after 8000 cycles [534]. Tan et al. [536] and Yan et al. [537] fabricated carbon-doped fiber-like Co3O4 nanomaterials by using human hair and poly (acrylonitrile) nanofibers (PAN NFs) as templates, respectively [536,537]. In the work of Tian et al., by using human hairs as templates, 1D hierarchical porous Co3O4 nanofibers with {2 2 0} exposed facets, 20–30 nm in width and 3–5 μm in length on carbon matrix (H2@Co3O4), were prepared by solvothermal synthesis and followed calcination. In this structure, well crystallized Co3O4 particles 8–12 nm in diameter, were closely aggregated together inside the nanofibers [536]. Electrochemical tests of H2@Co3O4 for LIBs displayed a high initial discharge capacity of 1368 mA h g−1 and maintained 916 mA h g−1 after 100 cycles at the current density of 0.1 A g−1. Also, 659 and 573 mA h g−1 could be still achieved at high current densities of 1 and 2 A g−1, respectively [536]. In the study of Yan et al., carbon-doped Co3O4 hollow nanofibers (C-doped Co3O4 HNFs) were synthesized by combining electrospinning technique and hydrothermal method, in which PAN NFs acted as both templates and carbon source [537]. This unique configuration possessed many outstanding advantages, such as more Co2+ ions for faster Co2+/Co0 redox reaction, high surface-to-volume ratio, and a large number of oxygen vacancies, plenty of free space for accommodating volumetric expansion, and short diffusion pathway for electron or Li+ ion, which resulted in a high reversible capacity of 1121 mA h g−1 over 100 cycles at the current density of 200 mA g−1, much higher than that of the undoped Co3O4 HNFs (663 mA h g−1) (Fig. 29d). Moreover, reversible specific capacities ranged from 1173 to 607 mA h g−1 have been achieved, which could be recovered to around 1280 mA g h−1 with the current density increased from 50 to 3000 mA g−1 for every 10 successive cycles and subsequently back to 50 mA g−1 for 20 cycles (Fig. 29e) [537]. Currently, there are two main strategies used for the synthesis of 1D cobalt oxide/carbon hybrids. In the first strategy, 1D cobalt oxide nanostructures are firstly prepared and the following carbon coating is achieved via physical sputtering or chemical deposition. Via this method, a carbon layer can be coated onto the cobalt oxide nanowires/fibers to produce core-shell structures. The outer carbon layer in the structure is an effective way to overcome the low conductivity, weak reaction kinetics, and obvious volume expansion of cobalt oxides during electrochemical applications. It has reported that this carbon coated 1D cobalt oxide hybrids demonstrated an excellent capacity of more than 1000 mA h g−1 for LIBs and a remarkable cycling stability up to 8000 cycles for supercapacitors. However, the homogeneous coating and the accurate control on the thickness of carbon layer on 1D cobalt oxide nanostructures remain challenging. The second strategy on preparing 1D carbon@cobalt oxide composites is the template-assisted route, in which cobalt precursors/cobalt oxides are coated on fiber-like carbonaceous template and followed with a thermal treatment process. Using this strategy, porous tubular structures can be easily obtained. The presence of cavities and porous structures

Fig. 30. Morphology and performances of 2D cobalt oxide/carbon hybrids. (a–c) FESEM images of Co3O4/C nanoplates [538] (Copyright 2013, Royal Society of Chemistry). (d) SEM images of carbon-coated Co3O4 nanosheets on carbon paper (CP) (Co3O4@C/CP) [250], (e) polarization curves in 0.5 M H2SO4 electrolytes with a scan rate of 5 mV s−1 [250], (f) galvanostatic tests for OER at a current density of 100 mA cm−2 with Co3O4 and RuO2-based catalysts [250] (Copyright 2016, Elsevier). 630

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within 1D nanostructures offer more available space to accommodate the mechanical strain aroused by ion insertion/deinsertion, and are helpful to shorten ion diffusion distance, promote electrochemical reaction kinetics, and improve electrochemical properties. 4.1.3. 2D cobalt oxide/carbon hybrids Coating carbon on 2D cobalt oxide nanosheets has attracted much attention for electrochemical devices for the purpose of taking the full advantages of the merits and overcoming the shortcomings of the 2D sheet-like nanostructures. The presence of carbon coating not only strengthens the structure stability, but also prevents the nanosheets from agglomeration during long-time catalytic cycles or repeated charging/discharging processes. Most importantly, the wider interspaces between the carbon coated nanosheets allow good electrolyte penetration and offer elastic buffering spaces to accommodate possible volume changes, which is very helpful to enhance the electrochemical performance of the materials. Sun et al. fabricated hierarchically structured Co3O4/C nanoplates on copper foil from nanosheet-assembled cobalt(II)-cobalt(III) layered double hydroxide (CoII-CoIII-LDH) precursors, which were synthesized from oil-droplet templates [538]. As shown in Fig. 30a–c, the surface of smooth Co3O4 nanoplates, with a thickness of about 30 nm, was covered by a thin amorphous carbon layer derived from the carbonation of the surfactant. When employed as binder-free electrodes for LIBs, this Co3O4/C composite exhibited initial discharge and reversible charge capacities of 1254 mA h g−1 and 1035 mA h g−1, respectively, corresponding to an initial CE as high as about 85%, as well as a remarkable reversible capacity of 1079 mA h g−1 over 50 cycles at a current density of 0.1 A g−1, which were better than Co3O4 NPs. Even at high rates, such as 3.0, 5.6 and 7.0 A g−1, the electrode still maintained specific capacities of 837, 732 and 540 mA h g−1, respectively [538]. Moreover, Ni foam has also been used as conductive substrate for the growth of Co3O4/C nanosheet arrays as free-standing electrode materials with improved rate capability, cycling stability, and ion storage performance [539,540]. During electrocatalytic reactions, apart from the activity of catalysts, the interface between catalysts and substrates is also of significance and the selection of both high-efficient catalysts and suitable substrates is crucial. Poor adhesion between the active material and the substrate usually causes the delamination of targeted catalysts, especially in corrosive solutions. Yang et al. grew cobalt oxide nanosheets on CP though a combination of electrodeposition and two-step calcination process [250]. In this experiment, the first electrodeposition of Co-species on pre-treated CP was carried in a standard three-electrode system at the current density of 10 mA cm−2 with 0.1 M Co(NO3)2 solution as electrolyte. It should be noted that a hydrophilic treatment of CP with an acid was essential before coating Co-species. After being heated in air over night, the Co-species/CP sample was treated at 350 °C first under oxygen-deficient condition for one hour, where the electroplated Co-species were converted to CoO (CoO/CP), and then under ambient condition for five hours further to make CoO nanosheets transform into Co3O4 nanosheets (Co3O4/CP). Moreover, carboncoated Co3O4 nanosheets on CP (Co3O4@C/CP) (Fig. 30d) were obtained without changes on morphology, if the Co-species/CP immersed in a glucose solution before the second treatment step. Further characterization by HRTEM revealed that the thickness of the glucose-derived carbon layer was 3.6 ± 0.5 nm. When used as catalyst for OER, the Co3O4@C/CP provided low over-potentials

Fig. 31. Morphologies and performances of 3D cobalt oxide/carbon spheres. (a, b) TEM images of (a) hollow Co3O4-C spheres [541], and (b) coreshell Co3O4-C spheres [541], (c) the discharge/charge curves in the first cycle [541], and (d) cycling performances at a current rate of 178 mA g−1 of the Co3O4-C core-shell and hollow spheres (insets show the structural changes during the insertion/extraction of Li+) [541] (Copyright 2011, Royal Society of Chemistry). (e) TEM image of Co3O4/carbon (C@Co3O4) spheres [542] (Copyright 2011, Elsevier). (f) TEM image of Co3O4/C hollow quasi-nanosphere [543] (Copyright 2012 Wiley-VCH). 631

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of 370 mV in acid (0.5 M H2SO4) solution (Fig. 30e) and 310 mV in alkaline (1.0 M KOH) solution to achieve a current density of 10 mA cm−2. Furthermore, these electrodes maintained stable as high as 86.8 h (Fig. 30f) and 413.8 h (at 100 mA cm−2) in acid and alkaline conditions, respectively [250]. Generally, 2D cobalt oxide/carbon hybrids can be synthesized by coating carbon on cobalt oxide nanosheets or growing cobalt oxide nanosheets on carbon matrix. In-situ growth of cobalt oxide nanosheets on carbon-based substrates as binder-free electrode for electrocatalysis can take the unique merits of 2D nanostructures and provide enhanced binding force between the active materials and the current collectors. 4.1.4. 3D cobalt oxide/carbon hybrids Co3O4/carbon spheres, the most typical example of 3D hybrid structures, have always been the focus of research, due to the attractive synergetic effects of carbon coating and cobalt oxide nanomaterials. A uniformly distributed carbon layer can greatly enhance the surface conductivity, decrease the overall electrical resistance, improve the electrical contact over the electrode, and from uniform and thin SEI films. In particularly, for hollow spheres, the volume change during cycles can be partly buffered by the void spaces and the porous structures, which also can provide large contact areas between the active materials and the electrolyte. In electrocatalysis applications, the introduction of a carbon layer into the 3D cobalt oxide nanostructures is helpful to reduce overpotential, allow better reaction kinetics, and thus accelerate ion and electron transport rates. As shown in Fig. 31a, Co3O4-C hybrid hollow spheres, as reported by Zhong et al. by using a ligand exchange etching method, possessed a diameter of around 0.5 μm and the thickness of the shells was estimated to be about 50–100 nm [541]. If the reaction time of hydrothermal synthesis decreased from 2 h to 16 h, core-shell Co3O4-C spheres (Fig. 31b) were obtained. As anode materials for LIBs, both the core-shell and hollow Co3O4-C spheres exhibited higher initial discharge capacities (1002 mA h g−1 and 1485 mA h g−1, respectively) than the theoretical values of Co3O4 (890 mA h g−1) (Fig. 31c). Moreover, as shown in Fig. 31d, the reversible specific capacity of the Co3O4-C core-shell spheres reached 825 mA h g−1 at a rate of 178 mA g−1 over 40 cycles, higher than that of hollow spheres (670 mA h g−1) [541]. The superior performances for the core-shell structure to hollow structure are largely attributed to the higher void-space utilizing rate and the larger contact surface with electrolyte. Later, Jayaprakash et al. synthesized Co3O4/carbon (C@Co3O4) nanostructured spheres via high-temperature carbonization of the glucose coated cobalt oxide

Fig. 32. Synthesis, morphology, and performances of cobalt oxide/N-doped carbon (NC) hybrids. (a) Schematic illustration of the synthesis of interconnected NC framework with Co/Co3O4 NPs (Co/Co3O4/C-N) hybrid nanostructure, and (b) the corresponding STEM image [273] (Copyright 2014, Elsevier). (c) Schematic illustration of the synthetic procedure of Co3O4/N-doped porous carbon (Co3O4/N-PC) hybrid nanostructure, and (d) the corresponding TEM images [551], (e) rate capability of Co3O4/N-PC electrode for Li+ storage [551], (f) a comparison on Tafel plots of the individual Co3O4 and N-PC, Co3O4/N-PC hybrids, and Ir/C at 5 mV s−1 in 0.1 M KOH solution [551]. (Copyright 2014, Elsevier). 632

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particles. As presented in Fig. 31e, a 22 nm-thick carbon layer was coated uniformly on the surface of Co3O4 particles approximately 400–500 nm in diameter. Electrochemical measurement confirmed that the Co3O4-C spheres exhibited a high lithium capacity of 567 mA h g−1 after 107 cycles at a current density of 440 mA g−1 [542]. Liu et al. prepared carbon-coated Co3O4 hollow quasi-nanospheres (Co3O4/C) by an in-situ carbon-coating and Ostwald ripening solvothermal process [543]. The products had a uniform quasi-spherical morphology of porous carbon shells coated on Co3O4 hollow particles (Fig. 31f). The diameter of the hybrid structures was around 250 nm and the thickness of the uniform carbon layer was about 20 nm. These carbon-coated Co3O4/C quasi-nanospheres exhibited a stable reversible capacity of about 1150 mA h g−1 over 20 cycles as anode of LIBs, whereas the bare Co3O4 hollow NPs showed only 70% retention of the initial capacity after 20 cycles. Furthermore, the rate capability of Co3O4/C remarkably maintained 700 and 150 mA h g−1 at 2 C and even 15 C, respectively [543]. Besides the hollow spherical morphologies, hierarchical and mesoporous cobalt oxide/carbon composites have also been reported as electrode materials for high-performance energy storage devices [503,544]. Lan et al. prepared mesoporous CoO nanocubes on natural rose-derived 3D porous carbon skeleton for the electrode of pseudocapacitor [545]. The obtained hybrid exhibited high capacitances of 1672 F g−1 at 1 A g−1 and 521 F g−1 at 40 A g−1. Notably, about 82% of the capacitance was maintained after 3000 cycles at 5 A g−1, and only 40% capacitance loss was observed after 1500 cycles at a relatively high rate of 10 A g−1 [545]. Wu et al. adopted an interface-modulated calcination strategy to obtain hierarchical yolk-shell Co3O4/C dodecahedrons derived from ZIF-67 frameworks [100]. An interface separation between the ZIF-67 core and the carbon-based shell occurred during the pyrolysis process. As an anode material for LIBs, the yolk-shell Co3O4/C dodecahedrons exhibited a high initial specific capacity (1209 mA h·g−1 at 200 mA·g−1) and remarkable cycling stability (1100 mA h·g−1 after 120 cycles). This type of composites was also explored for sodium ion storage and it delivered an excellent rate capability with average discharge capacities of 307 and 269 mA h·g−1 at 1.0 and 2.0 A·g−1, respectively [100]. 4.1.5. Cobalt oxide/heteroatom-doped carbon hybrids To further improve the electrochemical performance, some heteroatoms, especially nitrogen atoms, have been introduced into carbon skeletons during the hybridizing process to form cobalt oxides/N-doped carbon (NC) nanostructures for electrochemical devices [546–550]. The implantation of a large number of nitrogen atoms and N-containing functional groups can offer abundant both binding sites to anchor with cobalt oxide nanomaterials and active sites to improve ion storage performances or catalytic activities for HER, OER, and ORR. Wu et al. combined an interconnected NC framework with Co/Co3O4 NPs to form a Co/Co3O4/C-N nanostructure by using chitosan (CST) as carbon and nitrogen sources [273]. As demonstrated in Fig. 32a, Co/CST hydrogel was formed via a gelification

Fig. 33. MOF-derived cobalt oxide/NC hybrid for supercapacitors. (a) Schematic illustration of the preparation process of nanoporous carbon and Co3O4 from ZIF-67 precursor [552], (b) schematic illustration of asymmetric supercapacitors (ASCs) with Co3O4 as the positive electrode and carbon as the negative electrode [552], (c) Ragone plots of SSCs (carbon//carbon and Co3O4//Co3O4) and ASCs (Co3O4//carbon) [552], (d) comparison of the ASC cell (Co3O4//carbon) with some previously reported ACS cells [552] (Copyright 2015, American Chemical Society). 633

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step, which was achieved by dissolving Co(NO3)2 6H2O and CST in acetic acid solution (5%), and then treated under ammonia environment. After filtration and drying processes, final product of Co/Co3O4/C-N hybrid was obtained after annealing the aerogel in N2 flow at 750 °C. The corresponding SEM image and STEM (Fig. 32b) image revealed the production of an interconnected carbon framework anchored with NPs in an average size of about 45 nm. The presence of nitrogen was confirmed by elemental mapping analysis. HRTEM images suggested the coexistence of Co and Co3O4 NPs according to the lattice fringes of Co {1 1 1} and Co3O4 {3 1 1} planes with d-spacing of 2.17 Å and 2.56 Å, respectively. Also, the product manifested a high BET surface area of 320.5 m2 g−1. In an ORR application, this hybrid nanomaterial showed remarkable catalytic activity (i.e.. an onset potential of 64.0 mV and an estimated electron transfer number of 3.4–3.9) and stability (i.e. only 18.0 mV shift for half-wave potential after 5000 continuous cycles), as well as excellent tolerance to methanol poisoning effects in an alkaline media [273]. Later, Hou et al. reported the synthesis of Co3O4/N-doped porous carbon (Co3O4/N-PC) hybrid with dodecahedrons structure via thermal transformation of cobaltbased ZIF-67 precursors [551], as illustrated in Fig. 32c. Fig. 32d shows the morphology of Co3O4 NPs with a size of 15–30 nm embedded within N-PC networks. XRD pattern and HRTEM images with the corresponding selected area electronic diffraction (SAED) pattern confirmed the presence of graphitic carbon and cubic Co3O4 NPs in the structure. Benefiting from the unique hybridized structure, compared with pure Co3O4 nanomaterial, the Co3O4/N-PC nanomaterial presented superior Li+ storage performance, including a large discharge capacity of 1730 mA h g−1 at 100 mA g−1, excellent rate capability of 560 mA h g−1 at a current density of 10 A g−1 (Fig. 32e), and remarkable cycling stability with 91.7% capacity retention (1117 mA h g−1) of the second cycle after 110 cycles. Moreover, this hybrid also showed enhanced catalytic activity for OER for water splitting, such as a low onset potential of 1.52 V, high current density, low Tafel slope of 72 mV/decade (Fig. 32f), and only 4.4% decay after 6000 s at 1.7 V [551]. With the successful synthesis of Co3O4/NC hybrid nanomaterials, this type of nanomaterials has attracted strong interest in the application as electrode materials for electrochemical capacitors. Salunkhe et al. synthesized either nanoporous carbon or nanoporous Co3O4 from a single MOF (ZIF-67) by optimizing annealing conditions [552]. Actually, MOFs, with ordered metal cluster coordinated by organic linkers, are suitable and promising candidates to produce uniform and small-sized metal oxide nanostructures. MOF-derived Co3O4 materials, including plate-like/rod-like structure [553], hollow parallelepipeds [554], and mesoporous structure [555], were utilized as electrode materials for lithium ion storage in previous reports. Meanwhile, MOF-derived carbon has also caused much attention in energy and environment-related applications [556,557]. In Salunkhe’s work, as illustrated in Fig. 33a, the carbonization of ZIF-67 at 800 °C in nitrogen produced porous carbon after purified by a followed acid treatment to remove the remained Co NPs, while nanoporous Co3O4 polyhedra can be obtained from the same ZIF-67 by heating in nitrogen at 500 °C for 30 min and then 350 °C in air. The chemical composition of the nanoporous carbon was dominant C together with 1.4 at% Co, 2.7 at% and 5.1 at% N, while the nanoporous Co3O4 was 1.7 at% C and 4 at% N, besides 36.9 at% Co and 57.4 at% O. The specific surface areas of the resulted ZIF-derived nanoporous carbon and Co3O4 were 350 and 148 m2 g−1, respectively. When utilized as electrode materials for supercapacitor, the MOF-derived nanoporous carbon presented a high capacitance value of 272 F g−1, and the Co3O4 showed a high value of 504 F g−1 at a scan rate of 5 mV s−1. Subsequently, symmetric supercapacitors (SSCs) and ASCs were also fabricated to evaluate their electrochemical properties with the as-synthesized carbon and Co3O4 as electrodes [552]. They found that the enhanced capacitance performance for the ASCs (Co3O4//carbon) has been achieved, compared with the SSCs based on carbon (carbon//carbon) and Co3O4 materials (Co3O4//Co3O4). As demonstrated in Fig. 33b, the Co3O4 positive electrode of the ASCs

Fig. 34. Morphologies and performances of the carbonized butterfly wing templates (CWs, hierarchical porous NC frameworks) and the corresponding cobalt oxide/CWs hybrids. (a-d) SEM images of carbonized wing scales with (a) type-I and (b) type-II wing scale structures, and the corresponding Co3O4 nanostructure on CWs composites (CWs-Co3O4) from the (c) type-I and (d) type-II scale frameworks, (e) CV and (f) galvanostatic discharge curves of the CWs-Co3O4 hybrids [558] (Copyright 2015, Elsevier). 634

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offered easy access to a large amount of ions to penetrate into the interior of the porous material. On the other side, nanoporous carbon could play an effective role by providing rapid charge transfer process. Thus, this designed ASC with an optimal mass loading could be operated within a wide potential window between 0 and 1.6 V, leading to an excellent specific energy of 36 Wh kg−1. Compared with the carbon//carbon SSC cell, the Co3O4//Co3O4 SSC cell (Fig. 33c), and the previously reported ASC cells (Fig. 33d), this ASC (Co3O4//carbon) exhibited superior rate capability with the highest specific power of 8000 W kg−1 at a specific energy of 15 Wh kg−1, accompanied by a long-time stability for up to 2000 cycles [552]. Later, multi-channeled hierarchical N-doped porous carbon incorporated Co3O4 nanopillar arrays made from nature Morpho butterfly wings were also employed as binder-free electrodes for high-performance supercapacitors (Fig. 34) [558]. In the experiments, two types of wing scales of natural Morpho butterfly, the basal scale (type I, Fig. 34a) and the cover scale (type II, Fig. 34b), were first carbonized into hierarchical porous NC nanostructures, then the corresponding Co3O4 on carbonized wing scales (CWs) composites (CWs-Co3O4) were synthesized by direct deposition of Co3O4 NPs on the hierarchical porous carbonized wing templates in aqueous Co(Ac)2 solution. As shown in Fig. 34, the carbonized type-I CWs presented a dorsal surface consisting of parallel longitudinal ridge networks with rectangular windows in the interior part (Fig. 34a), and the carbonized type II structure illustrated a scale surface consisting of ridge lamellae with serrated edges (Fig. 34b). After the deposition of Co3O4 nanomaterials, a uniform and close-packed layer of Co3O4 NPs with a diameter of about 20 nm have been coated on the type-I CWs skeleton (Fig. 34c). On the typeII CWs, however, Co3O4 nanopillar arrays with a diameter of about 20 nm and a height of about 150 nm were deposited (Fig. 34d). These 3D hierarchically CWs-Co3O4 composites have demonstrated improved capacitance when used as the electrodes of supercapacitors. Stable redox peaks at different scan rates (Fig. 34e), have been identified. A maximum specific capacity of 978.9 F g−1 at the rate of 0.5 A g−1 (Fig. 34f), 94.5% of capacity retention after 2000 cycles, and a maximum energy density of 99.11 Wh kg−1 have been achieved [558]. Jin et al. have explored cobalt-cobalt oxide/NC hybrids (CoOx@CN) composed of Co0, CoO, and Co3O4 as bifunctional electrocatalysts for simultaneously generating H2 and O2 via both HER and OER in water splitting [295]. The CoOx@CN hybrid in this work was synthesized from inexpensive starting materials (e.g. melamine, CoNO3·6H2O, and D(+)-glucosamine hydrochloride, etc.) and a simple thermal treatment technique (reaction conditions: 800 °C in N2). The formed CoOx NPs had an average size of 12.3 nm and were uniformly dispersed on CN sheets. In electrocatalysis applications, the CoOx@CN hybrid exhibited a small onset potential of

Fig. 35. (a) Schematic illustration of the synthesis of hybrid material of CoO nanocrystals spatially confined in holey N-doped carbon nanowires (CoO/NCWs) [559], (b) HAADF-STEM image and the corresponding elemental mapping of CoO/NCWs [559], (c) the kinetic current (JK) and the electron transfer number (n) at 0.7 V of Co-based and Pt-based catalysts [559] (Copyright 2015, Royal Society of Chemistry). (d) LSV curves of Co3O4 decorated blood powder-derived heteroatom doped porous carbon (BDHC) hybrid (Co3O4@BDHC2, Co3O4: 27 wt%), Pt/C, IrO2 in 0.1 M KOH at a sweep rate of 10 mV s−1 (inset shows the formation of Co3O4@BDHC hybrid nanomaterials) [564] (Copyright 2014, Wiley-VCH). 635

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85 mV and a low charge-transfer resistance of 41 Ω for HER, and a low overpotential of 0.26 V for a current density of 10 mA cm−2 for OER. Most importantly, as an electrolysis catalyst for water splitting, a current density of 20 mA cm−2 at 1.55 V was achieved with CoOx@CN as cathode and anode catalysts [295]. The remarkable catalytic performance could be possibly attributed by a unique synergistic effect from the metal Co and cobalt oxides, the electron-rich nitrogen, and the highly conductive and high large surface area carbon matrix. CoO nanocrystals spatially confined in holey NC nanowires (CoO/NCWs) fabricated via CVD and colloidal assembly to achieve enlarged interfacial area for improved electrochemical performance was reported by Xu et al. [559]. As shown in Fig. 35a, holey NC nanowires (NCWs) were first synthesized from Fe-Co metal oxides decorated anodic aluminum oxide (AAO) template via pyrimidine vapor deposition and followed HF etching, and then CoO/NCWs hybrid was obtained via colloidal assembly of CoOx precursors performed by a solvent refluxing method and mild heat treatment under Ar atmosphere. As demonstrated in the STEM-HAADF image and the corresponding element mapping illustrated in Fig. 35b, ultrafine CoO NPs with a mean size of 5 nm were uniformly deposited on the 1D NCWs. The rough surfaces, enriched nanoholes, and defects of the holey NCWs could be favorable for the immobilization of cobalt oxide NPs with uniform distribution. The as-obtained catalyst LAO manifested an enlarged electrochemically accessible interfacial area for the stabilization of CoO at a low valence [559]. As a result, the CoO/NCW electrode delivered a remarkable catalytic activity and superior stability for ORR with an electron-transfer number (∼4.0 V) close to that of Pt/C catalyst (Fig. 35c). Furthermore, cobalt oxide NPs coupled with O-and NC nanoweb (Co3O4/ON-CNW) have also proved to be a high-efficient ORR catalyst for hybrid Li-air batteries [560]. Recently, CoO/NC hybrid (NC-CoO/C) was synthesized by embedded CoO NPs into NC layers via an one-step thermal conversion reaction [561]. In this fabrication, polypyrrole-coated Co3O4 NPs supported on carbon were heat-treated in Ar at 900 °C to make the transfer of polypyrrole (PPy) into NC shell and Co3O4 phase into CoO. TEM characterization confirmed the successful transfer of Co3O4 into cubic CoO. With this strategy, high-contact areas between the CoO NPs and the NC layers were realized, and the carbon layer also effectively prevented the aggregation of CoO NPs. This NC-CoO/C catalyst provided a dual-site synergistic activity towards ORR, in which the NC could initiate reaction by reducing oxygen to the peroxide ion (HO2−) and further reduced to hydroxide ions (OH−) at metal oxide sites [562]. Moreover, the introduction of electronegative nitrogen species into carbon induced an electron-deficient environment for cobalt atoms and lead to enhanced OER kinetics [561]. Similarly, Huang et al. reported an effective cathodic catalyst (CoO@N-AC) for ORR in MFCs, where cubic CoO nanosheets exposed with {2 0 0} plane were in-situ grown on nitrogen-doped AC [563]. Strikingly, this nanocatalyst possessed a large surface area of 1577.2 m2 g−1 and a low total resistance of 9.26 Ω. When utilized as air-cathode catalysts for MFCs, a maximum power density of 1650.1 ± 36.2 mW m−2 has been achieved [563]. Besides the most used N-doping, other heteroatoms were also implanted into the carbon matrix to enhance the performance of cobalt oxides/doped carbon hybrid nanomaterials. Zhang et al. have synthesized foam-like porous nitrogen, phosphorus, and sulfur ternary-doped doped carbon/Co3O4 NPs hybrid material (inset in Fig. 35d) [564]. Low-cost commercial CaCO3 NPs with 23 nm in diameter were applied as template and activating agent and blood powders (BP) were used as a carbon source. In this synthesis, Co3O4 NPs were grown on the blood powder-derived heteroatom doped carbon (BDHC) by a thermal deposition technique. XPS results revealed that the atomic percentages of carbon, oxygen, cobalt, nitrogen, phosphorous, and sulfur in the final product were estimated as 66%, 20%, 5%, 3.4%, 3.4%, 1%, respectively. The Co3O4 NPs with an average particle size of 3 ± 1 nm were highly crystallized and well-dispersed in the amorphous carbon matrix, leading to a highest surface area of 1288 m2 g−1. As an efficient bifunctional electrochemical catalyst, the blood powder-derived heteroatom doped porous carbon and Co3O4 hybrid (Co3O4/BDHC) catalysts delivered an outstanding activity and performance towards both OER and ORR in alkaline conditions, which showed a characteristic overpotential of only 380 mV (10 mA cm−2) for OER and a half wave potential of 0.83 V (vs RHE) for ORR. It was concluded that the heterojunction could facilitate electron transfer by a reduced accumulation of electron density within Co3O4 NPs, and suggested that this type of hybrid nanomaterials can give comparable performances to commercial IrO2 catalyst for OER and Pt/C catalyst for ORR (Fig. 35d) [564]. In general, heteroatoms-doped carbon matrix has been widely used to hybridize with cobalt oxides with different dimensionalities for electrochemical applications. Among them, nitrogen atoms are the most common dopant, which is usually introduced into the carbon matrix synchronously in the carbonization process by using a raw material containing both carbon and nitrogen elements. With the introduction of heterogeneous atoms, the carbon skeletons usually become much more affinity to cobalt oxide nanostructures and help to form uniform and strongly anchored cobalt oxide nanostructures. The doped carbon and cobalt oxide hybrids have been verified to exhibit better electrochemical properties than these undoped composites. Some problems, such as the accurate management of doping amounts, the control of doping states, and the specific doping sites, still remains out of control. Most importantly, a quantitative structure-property relationship between the doped structures and the electrochemical properties are still very vague and more scientific research on this relationship is urgently demanded. 4.1.6. Cobalt oxide/carbon/others hybrids In order to meet the requirement of different electrochemical reactions for various applications, a third heterogeneous component or more is often introduced into the cobalt oxide/carbon systems to further optimize some properties, such as mechanical, electrical, physical, and chemical properties [565–572]. For example, the thickness of the carbon layer on Co3O4 NP surface has great influence on the overall electrochemical performance for LIBs. A thicker carbon layer benefits the cycling stability of materials. At the same time, however, it is also a barrier to increase the Li+ diffusion distance, which leads to lower discharge capacity and poor rate capacity. A thinner carbon layer, as a contradiction, will be easily destroyed during the repeated lithiation processes and thus loss its functionality. With the purpose to increase its rigidness and robustness of the carbon coating, Wang et al. proposed to incorporate an analogue of Al oxide, COAl, into 636

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carbon coated Co3O4 nanostructures to form a sandwich-structured composite nanofiber bundle (C@Co3O4@COAl) by using natural collagen fibers (CF) as template, and barberry tannin (BT) as carbon source and bridge reagent to link with COAl, as shown in Fig. 36a [573]. After calcination, the inner CF@Co2+ was converted into core-shell structured C@Co3O4 nanofiber bundle, and simultaneously the outer AlOC complex was transformed into COAl covered on C@Co3O4. SEM image of C@Co3O4@COAl (Fig. 36b) and the corresponding element mapping of Al, Co, C, and O indicated that the final nanofiber bundle exhibited a typical sandwich structure. Apart from the benefits from a continuous conductive pathway provided by the inner carbon nanofiber, the hybrid coating with metal oxides and carbon plays an important role to buffer the volumetric change of Co3O4 nanomaterials and to facilitate fast electron and Li+ transports. As a consequence, the as-obtained composites exhibited significantly improved rate capability (778 and 229 mA h g−1 at the rates of 0.2 and 15.0 A g−1, respectively) and cycling stability (4000 cycles at 5 A g−1) (Fig. 36c), superior to the referred sample with double carbon layers (C@Co3O4@C) [573]. 4.2. Cobalt oxide/CNTs nanocomposites CNTs, including single-wall CNTs (SWCNTs), or multiwall CNTs (MWCNTs), feature their inherent extraordinary electrical and mechanical properties [574–577]. The walls of CNTs, however, are often chemically inert and highly hydrophobic, which bring negative effects on the uniform growth and distribution of metal oxide NPs on their surface. Moreover, the inert feature of the walls and surfaces for CNTs usually results in weak interaction between CNTs and decoration NPs. Some functionalization strategies, such as covalent attachment, noncovalent adsorption/wrapping, and the endohedral filling, are often undertaken to improve the chemical reactivity of CNTs [578,579]. For example, pre-oxidation of CNTs is an effective approach to provide strong coupling with metal oxide NPs. For the case of cobalt oxide nanostructures hybridizing with CNTs, the 0D cobalt oxide NPs can be either deposited on the surface or encapsulated in the inner space of the tube-like structure of CNTs, and 2D/3D cobalt oxide nanostructures can be interconnected or networked by CNT fibers. Besides the two-phase cobalt oxide/CNTs systems, other components can also be introduced to form ternary or multi-species composites. All these hybrid cobalt oxides/CNTs composites have been validated to be very effective to improve the electrochemical properties of this family of materials. 4.2.1. 0D cobalt oxide/CNTs hybrids To date, the assembly of 0D cobalt oxide NPs with CNTs to form 0D-1D hybrids has been paid much attention especially for their applications in electrochemical devices, which can be further classified into two subtypes, namely, cobalt oxide NPs coated on the surface of CNTs and cobalt oxide NPs encapsulated inside CNTs.

Fig. 36. Synthesis, morphology, and performance of sandwich-structured cobalt oxide-based composite nanofiber bundle (C@Co3O4@COAl) [573]. (a) Schematic illustration of the preparation of C@Co3O4@COAl, (b) SEM image of an individual C@Co3O4@COAl fiber, (c) cycling stability of C@Co3O4@COAl at a current density of 5.0 A g−1 (Copyright 2016, Elsevier). 637

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4.2.1.1. 0D cobalt oxide NPs coated on CNTs. The most common configuration of the 0D cobalt oxide/CNTs system is the decoration of cobalt oxide NPs on the surface of CNTs. Owing to the highly hydrophobic property of the pristine nanotubes, the 0D cobalt oxide/ CNTs composites are often fabricated by solid ball-milling, solution-based mixing under strong sonication/magnetic stirring, or supercritical CO2 system, etc. [580–583]. In 2007, a general self-assembly approach was proposed by Li et al. and six different kinds of CNTs-based nanocomposites, including Co3O4/CNTs, TiO2/CNTs, Au/CNTs, TiO2/Co3O4/CNTs, Au/TiO2/CNTs, and Co/CoO/ Co3O4/CNTs, were successfully synthesized [584]. Among them, the Co3O4/CNTs was synthesized by directly mixing Co3O4 NPs with CNTs suspension in maleic acid-ethanol solution. This structure of Co3O4 NPs (4–5 nm in size) on CNTs (Fig. 37a and b) were stable and maintained largely intact even after sonicating treatment [584]. Super-aligned CNT array (SACNT), a special type of vertically aligned CNT arrays possessing the capability of being transformed into continuous films and yarns [585–589], were explored as binder-free current collector for in-situ growth of Co3O4 NPs [590]. In the synthetic procedure, SACNTs were first assembled into continuous CNT films as a flexible and highly conductive scaffold to host Co3O4 NPs (Fig. 37c), and then, Co3O4 NPs were grown on the SACNTs by the pyrolysis of Co(NO3)2 absorbed on the SACNTs. After synthesis, as shown in Fig. 37d, the ordered structure of the film was largely retained and no obvious aggregation of Co3O4 NPs was observed. The resulting Co3O4/SACNT composite anodes for LIBs exhibited an initial discharge capacity of 1200 mA h g−1 (Fig. 37e), a reversible capacity of 910 mA h g−1 and a CE of 97% after 50 cycles (Fig. 37f), and outstanding rate capability of about 820 mA h g−1 at 1 C [590]. Even though the successful loading of cobalt oxide NPs on the surfaces of CNTs, the chemically inert and hydrophobic walls of the pristine CNTs (P-CNTs) result in a weak binding between them, if the CNTs and the Co3O4 NPs are directly mixed without prior functionalization. Surface modification is often used to promote the chemical activity and the wettability. Zhuo et al. compared the electrochemical properties of two types of CNTs, p-CNTs and functionalized CNTs (f-CNTs), with cobalt oxide particles coating [591]. The p-CNTs were purified with nitric acid in an ultrasonic bath and showed a smooth surface with a diameter of 40–60 nm and a length of several micrometers. The f-CNTs presented a rough surface with the existence of holes and functional groups obtained through a steaming treatment in acid vapor [592], and the length reduced to hundreds of nanometers. Then, Co3O4 were thermally decomposed on these CNTs. As shown in Fig. 38a and d, Co3O4 with a diameter of about 150–300 nm were anchored uniformly and strongly on the external surfaces along the axis of the f-CNTs. In contrast, in the Co3O4-p-CNT composite, Co3O4 particles with a larger diameter of 300–500 nm did not present close contact with the p-CNTs, but were dispersed freely (Fig. 38c and d). It is clear that the f-CNTs that have ample functional groups and lateral defects on the walls offer more nucleation sites for the growth of Co3O4, leading to better dispersing and anchoring of Co3O4 as well as a higher mass loading of 87%. When used as anode materials for lithium ion storage devices, the Co3O4-f-CNT composites delivered high discharge capacities (e.g. 719 mA h g−1 at the 2nd cycle) and excellent cycling performance (776 mA h g−1 after 100 cycle) at a current density of 200 mA g−1. Even at a high current density of 1

Fig. 37. Morphology and performance of cobalt oxide NPs/CNT hybrids. (a, b) TEM images of Co3O4/CNTs composite [584] (Copyright 2007, American Chemical Society), (c) SEM images of a pristine super-aligned CNT film (SACNT) [590], (d) SEM image of a Co3O4/SACNT composite electrode [590], (e) galvanostatic charge/discharge curves, and (f) cycling performance of Co3O4/SACNT composite at 0.1 C [590] (Copyright 2013, Royal Society of Chemistry). 638

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Fig. 38. Morphology and performance of cobalt oxide NPs/CNT hybrids. (a) SEM and (b) TEM images of Co3O4/functionalized CNTs hybrid (Co3O4f-CNT) [591], (c) SEM and (d) TEM images of Co3O4/pristine CNTs hybrid (Co3O4-p-CNT) [591], (e) a comparison on the rate capability of Co3O4-fCNT and Co3O4-p-CNT hybrid electrodes [591] (Copyright 2012, Royal Society of Chemistry). (f) TEM of the beaded necklace-like Co3O4/MWCNTs structures [596] (Copyright 2005, Wiley-VCH). (g, h) SEM images of Co3O4-CNT heterostructure with beaded “necklace” morphology [597] (Copyright 2013, Royal Society of Chemistry).

A g−1, the capacity of Co3O4-f-CNT still maintained about 600 mA h g−1, higher than that of Co3O4-p-CNT composites (350 mA h g−1) under the same testing conditions (Fig. 38e) [591]. Other synthetic approaches, such as electrophoretic deposition (EPD) and chemical co-precipitation, were also exploited to grow cobalt oxide NPs anchored on f-CNTs [593–595]. It is noteworthy that the necklace-like cobalt oxide NPs/CNTs structures have been studied for LIBs. Fu et al. fabricated an ordered beaded necklace-like Co3O4/MWCNTs architecture, in which spherical Co3O4 nanocrystallines with a diameter around 90 nm were

Fig. 39. Morphology, characterization and electrocatalytic performance of cobalt oxide NPs on CNTs hybrid nanomaterials. (a) SEM image, (b) TEM image (Inset: electron diffraction pattern), and (c) HRTEM image of CoO/nitrogen-doped CNT (NCNT) hybrid nanostructure (CoO/NCNT) [274]. (d) Oxygen reduction polarization curves (Inset: a scheme of CoO/NCNT for ORR catalysis) [274] (Copyright 2012, American Chemical Society). (e) Oxygen activities of the Co3O4-based and noble metal-based catalysts for bifunctional OER and ORR catalysis [601] (Copyright 2016, The Electrochemical Society). 639

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connected stepwise by MWCNTs to form a beaded “necklace” (Fig. 38f) via the simple treatment of cobalt precursor and MWCNTs under mixed supercritical fluid (SCF) containing CO2 and ethanol in a high-pressure vessel [596]. Xu et al. also reported a Co3O4-CNT heterostructure with a similar bead-on-string architecture by using alkylcarboxyl group-decorated CNTs [597]. The introduction of carboxyl groups onto the CNTs surface makes the CNTs easily dispersed in water before the in-situ growth of Co3O4 spheres. As shown in Fig. 38g and h, The Co3O4 spheres with an average size of ca. 100 nm were threaded with CNTs to form a bead-on-string architecture. Owing to the fact that the CNTs passed through the inner Co3O4 spheres to provide intimate contact and a continuous electron transport channel, the necklace-like nanoarchitectures exhibited superior mechanical stability and electrochemical performance. As anode materials for LIB applications, the composites delivered a high initial discharge capacity of 1248 mA h g−1 and high capacity retention of 758 mA h g−1 after 30 cycles. Even at a current density as high as 6000 mA g−1, this composites offered a high capacity up to ∼400 mA h g−1, while the referenced material, aggregated Co3O4 spheres, showed no performance at such high rates [597]. Apart from the lithium ion storage, this type of cobalt oxide NPs on CNTs hybrids has also been used as an electrocatalyst for both OER and ORR [598–600]. Liang et al. developed CoO/CNT covalent hybrid as highly efficient ORR catalyst by directly growing CoO nanocrystals on mildly KMnO4-oxidized MWCNTs (moCNT), including a mild solution-phase synthesis followed by annealing at 400 °C in an NH3 atmosphere [274]. The coated CoO NPs on CNTs had a mean size of around 5 nm (Fig. 39a and b) and a cubic CoO rock-salt structure (Inset in Fig. 39b and c). The resultant CoO/nitrogen-doped CNT (NCNT) hybrid exhibited much higher ORR current density than that of CoO, NCNT and their physical mixture loaded on Teflon-coated carbon fiber paper (TCFP) (Fig. 39d). Furthermore, the CoO/CNTs hybrid was found to be advantageous to the one hybridized with graphene because of more active sites and much rapid charge transport provided by CNTs. Electrochemical and XANES spectroscopy measurements confirmed the strong coupling between NPs and CNTs, which favor high-efficient charge transport in ORR catalysis. The oxidation degree of CNTs was of significance to the activity of the hybrids, likely due to the balance between the electrical conductivity and the density of functional groups on CNTs. Compared with thermal annealing in Ar, the reduction in NH3 environment gave rise to improved electrochemical properties. It is interesting that this strongly coupled CoO/CNTs hybrid even presented high ORR activity and stability under a corrosive alkaline condition (10 M NaOH) [274]. Further research on the influence of the length of CNTs on the catalytic activity of Co3O4/CNTs has been carried out by So et al. [601]. Three types of CNTs (CM150, CM250, and CM280) with different lengths (30, 70, and 200 μm) were hybridized with Co3O4 and tested for both OER and ORR performances. The results indicated that the Co3O4/ CM150 CNTs with the highest surface area of 170.46 m2 g−1 delivered the highest OER activity, while the CM280 CNTs with the longest length coupled with Co3O4 presented the highest ORR performances. It has demonstrated that, to achieve high-efficient bifunctional performance, the oxygen activity of Co3O4/CM150 CNTs can be further enhanced by using oxygen-containing CNTs (oxCNTs) (Fig. 39e), for example, by chemical functionalization in acidic KMnO4 solution [601].

Fig. 40. Synthesis, morphology, and performance of 1D cobalt oxide/CNTs hybrid nanostructure. (a) Schematic illustration of the formation of hierarchical CNT/Co3O4 hybrid microtubes [606], (b) the corresponding SEM image of CNT/Co3O4 hybrid microtubes [606], (c) cycling performance and CE of CNT/Co3O4 hybrid microtubes [606] (Copyright 2016, Wiley-VCH). (d) TEM image of the Co3O4@MWCNT hybrid, [604], (e) schematic illustration of the synergetic effect of Co3O4 nanorod and MWCNT for water oxidation [604] (Copyright 2014, Royal Society of Chemistry).

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4.2.1.2. 0D cobalt oxide NPs encapsulated inside CNTs. The second type of 0D cobalt oxide/CNTs hybrid materials is to encapsulate/ embed/fill cobalt oxide NPs into CNTs. Park et al. fabricated vertically aligned mesoporous carbon nanotubes filled with Co3O4 NPs (Co3O4/MCT) on a copper current collector through a dual template method, where an AAO membrane was applied as hard template and a block copolymer surfactant F127 was employed as soft template [602]. Co3O4 NPs (10–20 in diameter) were dispersed uniformly on the inside surface of MCT. As an anode material for LIBs, this electrode displayed a high reversible capacity of 627 mA h g−1 over 50 discharge/charge cycles [602]. Overall, for the 0D cobalt/CNT hybrid nanostructures, the deposition of cobalt oxide NPs on the surfaces of CNTs is easy to be achieved, but a uniform dispersion is still very difficult due to the inactive CNT walls. The functionalization of CNTs before the deposition of cobalt oxide nanostructures provides a good solution to address this challenge. Surface modification changes the surface groups, surface roughness, and the wettability of CNTs and provides much higher affinity to the precursors of cobalt oxides, which leads to much stronger contact between cobalt oxide NPs and CNTs and thus superior electrochemical performances. The fabrication of cobalt oxide NPs embedded or encapsulated inside CNTs is more challenging and further study on this interesting type of hybrids is still on the way. 4.2.2. 1D cobalt oxide/CNTs hybrids 1D cobalt oxide nanostructures, with a high aspect ratio, are also integrated with CNTs to improve their electrochemical performances [603–605]. Hierarchical tubular structures (HTSs) composed of Co3O4 hollow NPs and CNTs were designed and fabricated via a multi-step route, as illustrated in Fig. 40a [606]. This hybrid nanostructure, different to the above-mentioned configuration of Co3O4 NPs on the surface of CNT, consist of Co3O4 tubes and vertically aligned CNTs from the walls (Fig. 40b). In this work, electrospun PAN-cobalt acetate (Co(Ac)2) composite nanofibers were selected as the templates to provide cobalt source for the following growth of ZIF-67. Then, owing to a strong coordination of 2-methylimidazole to Co2+ within the fibers, uniform shells of ZIF-67 nanocrystals grew on these nanofibers. After being dispersed in N, N-dimethylformamide (DMF) for removing PAN-Co(Ac)2 core, the obtained ZIF-67 tubulars were further converted into CNT/Co-carbon hybrids by thermal treated in Ar/H2. Finally, Co NPs were oxidized into Co3O4 hollow NPs via annealing in air to generate the hierarchical CNT/Co3O4 microtube hybrid nanostructures. The inner diameters of the Co3O4 hollow NPs and the CNTs were approximately 10 ± 5 nm and 3 ± 2 nm, respectively. These HTSs constructed by hollow NPs offered ample active interfacial sites, and effectively alleviated volume changes during electrochemical reactions. As a result, the as-prepared hierarchical CNT/Co3O4 microtubes delivered a high initial discharge capacity of 1840 mA h g−1 at 0.1 A g−1, an exceptional rate capability (515 mA h g−1 at 6.0 A g−1) and long cycle life over 200 cycles (782 mA h g−1 at 1.0 A g−1) (Fig. 40c) as an anode material for LIBs [606]. Zhang et al. synthesized Co3O4 nanorods anchored on MWCNTs (Co3O4@MWCNT) via a diethylenetriamine (DETA)-assisted

Fig. 41. Synthesis of 2D cobalt oxide nanoplates/2D CNT fishing-net-like layers for LIBs. (a) Schematic illustration of the preparation of layer-bylayer 2D Co3O4 nanoplates/2D CNT nets structure [611], (b) TEM image of the 2D-2D hybrid structure [611], (c) discharge profiles and (d) discharge capacity at different current densities of the 2D Co3O4 nanoplate/2D CNT hybrid [611] (Copyright 2015, American Chemical Society). 641

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solution-phase synthesis method [604]. It is presented in Fig. 40d and e that Co3O4 nanorods were intimately wrapped on the CNTs to form a 1D/1D heterostructure, which is favorable for electron transport and surface electrochemical reactions. In this 1D/1D heterostructure, the Co3O4 nanorods exposed with {1 1 0} facets. As an efficient catalyst for OER, this hybrid exhibited an onset potential of 474 mV (vs. Ag/AgCl) and an overpotential of 310 mV to achieve a current density of 10 mA cm−2 in alkaline electrolytes for water oxidation [604]. In summary, 1D cobalt oxide/CNTs hybrid nanostructures are advantageous for providing high surface-to-volume ratio, remarkable conductivity, and excellent surface activities. The electrochemical properties, however, are quite unsatisfying. The possible reasons include the long-distance electron transport in the 1D geometry and the non-uniform and non-continuous hybridization forms of 1D-1D configuration. It is still relatively difficult to achieve the uniform growth of 1D cobalt oxide nanowires or nanorods on the surfaces of CNTs. Furthermore, the related exploration of 1D/1D hybrid structures on electrochemical energy applications remains very limited. Most reported 1D/1D heterostructures are mainly related to photocatalysis, largely attributed to the significantly enhanced light absorption and scattering contributed by the high length-to-diameter ratio [607–610]. Hence, much more investigated are needed to make this type of materials appealing in electrochemical energy related applications. 4.2.3. 2D cobalt oxide/CNTs hybrids Compared to 1D cobalt oxides, 2D cobalt oxide nanostructures are much easier to form hybrid structures with 1D CNTs. It has been reported that Co3O4 nanoplates coupled with capillary-like MWCNT nets to form a 3D layer-by-layer structure demonstrated both high-rate and long-term cycling stability, when it was employed as LIB anode material [611]. As shown in Fig. 41a, 2D hexagonal α-Co(OH)2 nanoplates and 2D CNT fishing-net-like layer were transferred layer-by-layer to copper foil, and then α-Co(OH)2 nanoplates were converted into Co3O4 nanoplates by thermal treatment at 300 °C for 2 h in air. TEM characterization revealed that the MWCNT nets enveloped the surfaces of each α-Co(OH)2 nanoplate (Fig. 41b). Electrochemical results confirmed that this 2D Co3O4 nanoplates/2D CNT network multilayer structure presented an initial discharge capacity of about 1550 mA h g−1. The discharge capacity still remained at 710 mA h g−1 at a current density as high as 50 A g−1 (Fig. 41c), which was fully recovered when the rate was returned to 2 A g−1 after 75 cycles. In this work, differential capacities of the hybrid at current densities from 0.1 to 50 A g−1 were also examined, as shown in Fig. 41d. Two broad peaks in the 1.5–1.0 V and 1.0–0.5 V represented the intercalation of Li+ ions into the Co3O4 to form a LixCo3O4 intermediate phase, and the full reduction of the intermediate phase to cobalt metal, respectively. It was observed that both broad peaks shifted to the negative direction and the intensity of the peaks associated with the conversion reaction gradually decreased with the increasing current density. These results indicated that the capacity loss at high rates was primarily due to the slow kinetics of the conversion reaction accompanying with a phase transformation process. Meanwhile, no obvious capacity was lost aroused by the formation of the intermediate phase, which was ascribed to the more stable phase state of LixCo3O4 even at high rates [611]. Besides, the fully recovered capacity of the Co3O4 nanoplates/CNT network multilayer structure proved that Co3O4 nanoplates were in good electrical contact with the electrically conductive CNT nets, contributing to the stable long-term cycling performances [611].

Fig. 42. Morphology and performance of MOF-derived cobalt oxide polyhedral hybridized with CNTs nanostructures (MWCNTs/Co3O4) [619]. (a) SEM and (b) TEM images of MWCNTs/ZIF-67, (c) SEM and (d) TEM images of MWCNTs/Co3O4, (e) cycling stability (100 mA g−1) and (f) rate capability of the MWCNTs/Co3O4 (Copyright 2015, American Chemical Society). 642

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4.2.4. 3D cobalt oxide/CNTs hybrids 3D cobalt oxide nanostructures, such as hollow, flake-like, or sphere-like structures with an overall size over 100 nm, have been widely studied to hybridize with CNTs to improve the electrochemical properties of cobalt oxides [612–616]. Some advantages can be brought by 3D nanostructures for the applications in electrochemical devices. For example, hollow structures, which usually associate with low mass density, high porosity, large reaction surface area, and good strain accommodation, high specific surface areas, have been demonstrated to be effective to accommodate the large volume change during cycling, provide extra space for ion storage, facilitate lithium diffusion, and allow better reaction kinetics when this type of nanostructures are used in rechargeable batteries [617,618]. Venugopal et al. reported a template-free soft chemical method to fabricate hollow mesoporous Co3O4 and Co3O4-CNT composites by using acid-treated f-CNTs [612]. It was observed that each Co3O4 hollow particle had a double-layered nanowall 50–100 nm in thickness, and the f-CNTs were penetrated inside the hollow particle. When applied as negative electrodes for LIBs, the Co3O4-CNT hybrid materials showed an initial discharge specific capacity of 1420 mA h g−1, higher than that of Co3O4 (1283 mA h g−1) and the physically mixed Co3O4 + CNT (1380 mA h g−1). What is more, the Co3O4-CNT composites presented superior rate capability with capacity retention at 620 mA h g−1 even at a high rate of 1000 mA g−1 [612]. The excellent performances of the hollow Co3O4/f-CNT hybrid material resulted from the unique hollow and mesoporous structure of Co3O4 NPs and the hybridization with CNTs, which is helpful to mitigate the mechanical stress and enhance the kinetics of the electrochemical reactions. 3D Co3O4 polyhedra/CNTs hybrids have been reported as anode materials for ion storage [619,620]. Huang et al. synthesized MOF-derived Co3O4 polyhedra/MWCNTs hybrid composites by thermal treatment of the in-situ synthesized MWCNTs/ZIF-67 [619]. The MWCNTs/ZIF-67 composite, synthesized by co-precipitation of cobalt ions and 2-methylimidazolate together with the presence of MWCNTs, composed of uniform MWCNT-inserted polyhedral with smooth surfaces and a size of about 500 nm (Fig. 42a and b). After calcination at 400 °C, the MWCNTs/ZIF-67 nanocomposites converted to MWCNTs/MOF-Co3O4 hybrid nanocomposites with well-maintained size distribution and morphology. The most significant difference between the ZIF-67 and the Co3O4 polyhedra is the formation of mesoporous structure, owing to the release of CO2 and H2O from the ZIF-67 polyhedra during calcination (Fig. 42c and d). TEM characterization further confirmed the existence of hierarchical pores, and the intertwined and inserted MWCNTs within the Co3O4 polyhedra. As anode materials for LIBs, the MWCNTs/MOF-Co3O4 composites displayed excellent cycling stability (813 mA h g−1 at 100 mA g−1 after 100 cycles) (Fig. 42e) and good rate capability (514 mA h g−1 at 1000 mA g−1) (Fig. 42f). It is worth noting that this protocol can also be used to prepare MWCNTs/ZnCo2O4 composites by using hetero-bimetallic MWCNTs/ZIF67 (Zn, Co) as precursors [619].

Fig. 43. In-situ electrochemical XAS analyses at an open circuit potential (OCV) and (a) OER potential of 1.8 V and (b) ORR potential of 0.6 V, (c) the effect of NP size on OER and ORR performances [623] (Copyright 2016, American Chemical Society). (d, e) TEM images of MOF-derived Co3O4 nanocrystals embedded in N-doped mesoporous carbon and MWCNT hybrid structures (Co3O4@C-MWCNTs) [624] (Copyright 2015, Royal Society of Chemistry). (f) Schematic illustration of assembled two-electrode rechargeable Zn-air battery [625], (g) charge and discharge polarization curves of the assembled Zn-air battery based on Co-embedded CNT/porous carbon (Co-CNT/PC) and Pt/C + RuO2 mixture cathode catalysts [625] (Copyright 2016, Royal Society of Chemistry). 643

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4.2.5. Cobalt oxide/CNTs/others hybrids Heterogeneous cobalt and its oxide nanocomposites, such as Co/CoO and Co/Co3O4, have been hybridized with CNTs to promote the electrocatalytic efficiency for oxygen catalysis or Li-air batteries [621,622]. In the design of cobalt oxide-based catalysts, it was found that the nanoscale particle size and the valence states of cobalt and cobalt oxide significantly influence the electrocatalytic reactions. Seo et al. studied the size-dependent catalytic activity of bifunctional cobalt oxide (CoOx) NP/CNTs catalysts for both OER and ORR [623]. In-situ X-ray absorption spectroscopy (XAS) revealed that the majority of the CoOx NPs during both OER (Fig. 43a) and ORR (Fig. 43b) consisted of Co3O4 and CoOOH phases regardless of their particle sizes. As shown in Fig. 43c, the OER activity of the CoOx/CNT catalysts increased as the decrease of the CoOx NP size, which is correlated to the increased amount of Co(III) species and the larger surface area of CoOx NPs, whereas the ORR activity of the CoOx/CNT catalysts was nearly independent on the CoOx NP size [623]. Cobalt oxide/carbon/CNTs hybrid is another important ternary composite for electrocatalysis. MOF-derived Co3O4 nanocrystals (10–25 nm in diameter) in-situ embedded in N-doped mesoporous graphitic carbon layer and MWCNT hybrid structures (Co3O4@CMWCNTs) (Fig. 43d and e) were synthesized by a facile carbonization and a subsequent oxidation of cobalt-based MOF precursor, ZIF-9, with the presence of MWCNTs [624]. The as-obtained hybrid material showed remarkable electrochemical performances for OER and ORR. As OER catalysts, an onset potential of 1.50 V and an overpotential of 320 mV achieved for a stable current density of 10 mA cm−2 for at least 25 h. Compared to the commercial 20 wt% Pt/C catalyst, the hybrid exhibited superior stability for ORR [624]. Later, Co-embedded CNT/porous carbon (Co-CNT/PC) was synthesized by pyrolysis of ZIF-67 encapsulated Co3O4 NPs, and used as bifunctional electrocatalysts towards ORR and OER in rechargeable Zn-air batteries (Fig. 43f), which showed a similar performance to the assembled battery using the state-of-the-art Pt/C and RuO2 mixture as oxygen cathode electrocatalysts (Fig. 43g) [625]. Besides the above-mentioned nanocarbon, other active components, such as MnO2 and polydopamine, were also introduced into the cobalt-CNTs hybrids as a ternary component for improving the electrochemical performances of the materials [626,627]. In spite of increasing reports on the cobalt oxide/CNT hybrid for electrochemical applications, the fabrication of uniform composites consisting of cobalt oxide and CNTs is a grand challenge, largely owing to the slender tubular structure and the intrinsic chemically inert and highly hydrophobic surfaces of CNTs. Moreover, the high cost of CNTs is still a major barrier for the applications

Fig. 44. Synthesis and performance of 0D cobalt oxide/graphene hybrids. (a) Schematic illustration of the fabrication of 0D Co3O4/graphene composite [648], (b) the corresponding TEM image [648], (c) charging/discharging curves of the 0D Co3O4/graphene composite at a current density of 50 mA g−1 [648], (d) comparison of the cycling stability of Co3O4, graphene, and the Co3O4/graphene composite [648] (Copyright 2010, American Chemical Society). (e) AFM image of Co3O4 NPs/graphene composite with smaller Co3O4 NPs [649] (Copyright 2010, Elsevier). (f) Low magnification and (g) high magnification SEM images of CoO QD/graphene (CQD/GN) composites [655] (Copyright 2012, American Chemical Society). 644

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of CNT-based materials. 4.3. Cobalt oxide/graphene hybrids Graphene, an emerging 2D nanomaterial, have attracted considerable attention for the potential applications in energy conversion and storage devices. Graphene possesses some distinct advantages, including striking electrical conductivity, ultrahigh surface area, together with excellent mechanical/chemical stability. The salient properties of graphene offer promising potentials to create incredible synergic effects to the materials, when it is used as supportive materials to combine with other substances or materials. By integrating with cobalt oxide nanostructures, the graphene-metal oxide composites have presented enhanced performances as electrocatalysts or electrode materials, where graphene mainly acts as conductive supports to reduce the inner resistance. Various architectures composed of graphene nanosheets and cobalt oxide nanomaterials with various morphologies and dimensions have been synthesized in the past decades, such as 0D-2D hybrids, 1D-2D hybrids, 2D-2D hybrids, and 3D-2D hybrids, in which cobalt oxide nanostructures can be deposited/anchored/grown on the surface of graphene nanosheets, or encapsulated/ wrapped by the sheet-like graphene, or assembled into layered/sandwich-like structures in a layer-on-layer or sheet-on-sheet manner. In the 0D-2D configuration, NPs-on-graphene (0D-2D) can effectively improve the dispersion degree of the cobalt oxide NPs and avoid serious aggregation. The nanosheets-encapsulated/wrapped-NPs (2D-0D) configuration is beneficial to accommodate volume changes of cobalt oxide nanostructures during repeated cycles. The nanosheets-on-graphene (2D-2D) stacks can take the full advantage of the merits of geometrical compatibility to enhance the interface force between cobalt oxide layers and graphene layers. 4.3.1. 0D cobalt oxide/graphene hybrids The integration of 0D cobalt oxide NPs with graphene has been widely studied and can be easily achieved via directly mixing dispersants of Co3O4 NPs and graphene/graphene oxide (GO)/reduced graphene oxide (rGO), or in-situ growth of cobalt oxide nanomaterials on graphene surface [628–647]. Wu et al. fabricated Co3O4 NPs/graphene composite via thermal treatment of the Co(OH)2/graphene precursor at 450 °C, as illustrated in Fig. 44a [648]. The resulting Co3O4 NPs, with a size of around 10–30 nm, were homogeneously anchored on the surfaces of graphene nanosheets (Fig. 44b). Even after a long time of sonication, the Co3O4 NPs were still strongly anchored on the graphene surface with a high density, indicating a strong interaction between them. As a result, the obtained nanocomposites exhibited an initial discharge capacity of 1097 mA h g−1 (Fig. 44c) at 50 mA g−1, a reversible capacity of 935 mA h g−1 after 30 cycles at 50 mA g−1 (Fig. 44d), and capacity retention of 484 mA h g−1 after 40 cycles at 500 mA g−1 [648]. Later, Co3O4/graphene hybrid composed of smaller Co3O4 particle sizes (∼5 nm) (Fig. 44e) was reported by Kim et al. for highly reversible anode for LIBs [649]. This anode delivered a high reversible capacity of over 800 mA h g−1 at a current density of 200 mA g−1 in the voltage window between 3.0 and 0.001 V. Remarkably, more than 550 mA h g−1 was retained even at a high rate of 1000 mA g−1 [649]. 3D Porous graphene assembled from 2D graphene sheets can provide a larger surface area with abundant pores and more exposed active sites for electrochemical reactions. Hence, cobalt oxide species deposited on the defective surfaces of the 3D graphene, or cobalt oxide NPs confined inside 3D porous structure, will be helpful for the access of the electrolyte ions, and thus greatly reduce the irreversible capacity and improve the CE as electrode materials or electrocatalysts for electrochemical applications [105,650–653]. For example, Choi et al. fabricated 3D hierarchical porous Co3O4/rGO composite films by the deposition of Co3O4 NPs (5.87 ± 2.1 nm in diameter) onto the surfaces of the 3D porous graphene [651]. The porous structure of graphene was controlled by using polystyrene (PS) spheres as structural guiding templates. When the porous Co3O4/rGO heterogeneous films were directly applied as anode materials without binders and current collectors, they exhibited a good cycling performance with 90.6% retention during 50 cycles. Moreover, an excellent rate capability of 71% retention was maintained at the current rate as high as 1000 mA g−1, which was much better than these of physically mixed Co3O4/rGO (48%) and barely Co3O4 (10%) [651]. Besides the composition with Co3O4 NPs, 3D hollow crumpled graphene were also employed to hybridize with CoO NPs to form CoO/graphene hybrid for the applications of electrochemical reactions, which presented excellent catalytic activity and durability to become promising nonprecious metal-based bi-functional catalysts for both ORR and OER [326,654]. Apart from NPs, cobalt oxide QDs were also hybridized with graphene to form QDs/graphene composites by using Co4(CO)12 as precursor [655]. With the assistance of sonication at ambient temperature, the precursor clusters on graphene nanosheets were decomposed into metallic Co clusters, which were subsequently oxidized into CoO by solution-dissolved O2. As illustrated in Fig. 44f and g, CoO QDs with small sizes of 3–8 nm were firmly anchored and uniformly distributed on the graphene nanosheets. Such CoO QD/graphene nanosheet (CQD/GN) composites with metal oxide QDs on conductive graphene support are advantageous to inhibiting aggregation, buffering volume change of cobalt oxides during cycling, and offering lots of active sites and short ion-diffusion pathways. As expected, as an anode material for LIBs, the composites delivered an outstanding reversible lithium storage capacity of 1592 mA h g−1 at a current density of 50 mA g−1 after 50 cycles, and excellent rate capability (e.g. 1008 mA h g−1 at 1000 mA g−1) [655]. Owing to the excellent performances of 0D cobalt oxide NPs/graphene hybrid nanostructures, extensive research has been reported to fabricate this type of nanocomposites for the applications in electrochemical devices. In most cases, to improve and distribution of cobalt oxide NPs on graphene surfaces, chemically active GO is often employed initially to absorb or couple with cobalt-based precursors, and then to be reduced into rGO with the assistance of thermal treatment and/or reducing agents. After the reduction of GO, however, some functional groups may still remain and some defects may form on the surface, which can result in decreased conductivity compared to the pristine graphene. Therefore, the use of oxidized graphene and the selection of reduction methods are important to achieve a uniform deposition of cobalt oxide nanostructures on the surfaces of graphene but without much 645

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loss of its conductivity. 4.3.2. 1D cobalt oxide/graphene hybrids 1D cobalt oxide nanostructures, including needle-like, wire-like, and fiber-like structures, anchored on 2D graphene nanosheets, or grown on 3D graphene aerogel/foams have been investigated as electrode materials for batteries, capacitors, and catalysts for oxygen electrocatalysis [656–664]. Ryu et al. proposed a bifunctional composite catalyst composed of 1D Co3O4 nanofibers immobilized on 2D nonoxidized graphene nanoflakes (Co3O4 NF/GNF) for an oxygen electrode in Li-O2 batteries [660]. Fig. 45a showed the whole fabrication procedure. First, 1D polycrystalline Co3O4 nanofibers were prepared via an electrospinning strategy to obtain 1D fiber-like network. Then, the Co3O4 NFs/GNFs composites (Fig. 45b) with NFs assembled on both sides of GNFs were synthesized by simply mixing Co3O4 nanofibers with graphene dispersion, where the nonoxidized graphene nanosheets were noncovalently functionalized by 1-pyrenebutyric acid (PBA). When employed as a bifunctional catalyst for the oxygen electrode of Li-O2 batteries, the composite delivered an excellent initial discharge capacity as high as 10,500 mA h g−1 and maintained a limited capacity of 1000 mA h g−1 for 80 cycles (Fig. 45c). The remarkable electrochemical properties were mainly attributed to the improved electrocatalytic activity for both OER and ORR, the fast electron transport, and the easy oxygen diffusion of Co3O4 NF/GNF composites [660]. Besides the layer-by-layer assembly of 1D Co3O4 nanofibers with graphene, vertically aligned CoO nanowires directly grown on 3D graphene networks via a wet chemistry process, as shown in Fig. 45d, were reported for lithium storage without extra addition of any binders or conductive additives, which exhibited a high capacity of 857 mA h g−1, an excellent rate capability, and good cycle performance [661]. Co3O4 nanowire/3D graphene composite was synthesized as free-standing electrode for supercapacitor and for enzymeless electrochemical detection of glucose [662]. In the synthesis, Co3O4 nanowires were in-situ hydrothermally grown on the CVD-

Fig. 45. Synthesis, morphology, and performance of 1D cobalt oxide/graphene hybrids. (a) Schematic illustration of the synthesis of Co3O4 nanofibers networks assembled on both sides of nonoxidized graphene nanoflakes (Co3O4 NF/GNF) [660], (b) TEM image of Co3O4 NF/GNF composite [660], (c) a comparison on cycling performance for Co3O4 NPs, Co3O4 NFs, Co3O4 NF/RGO hybrid, and Co3O4 NFs/GNF hybrid composite at a limited capacity of 1000 mA h g−1 at a current density of 200 mA g−1 [660] (Copyright 2013, American Chemical Society). (d) FESEM image of vertical 1D CoO nanowire grown on 3D graphene network [661] (Copyright 2014, IOP Publishing). (e) SEM images of 1D Co3O4 nanowire grown on 3D graphene form [662], (f) CV curves of 3D graphene/Co3O4 composite electrode at different scan rates [662], (c) galvanostatic charge and discharge curves at different current densities [662], (e) charge and discharge profile of the 3D graphene/Co3O4 composite electrode at 10.0 A g−1 [662] (Copyright 2012, American Chemical Society). 646

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prepared graphene foam. As shown in Fig. 45e, Co3O4 nanowire networks fully covered the graphene skeleton with uniform distribution. Each nanowire was around 200–300 nm in diameter and several micrometers in length. As a free-standing electrode for supercapacitors, the obtained composites were capable of delivering a pseudocapacitive behavior (Fig. 45f) with a specific capacitance of 768 F g−1 at a current density of 10 A g−1 (Fig. 45g), and a long cycling time of over 25,000 s (Fig. 45h). Moreover, 1D Co3O4/3D graphene had great potential for electrochemical sensing. For example, this hybrid nanostructure has been used for nonenzymatic detection of glucose and presented an extraordinary sensitivity of 3.39 mA mM−1 cm−2, and a detection limit lower than 25 nM with a signal-to-noise ratio (S/N) of 8.5 [662]. In general, directly mixing 1D cobalt oxides with graphene benefits to handicap the self-agglomeration of graphene sheets. The binding between the graphene and the cobalt oxide nanostructures, however, is relatively weak through physical contact, and cobalt oxide nanowires are easy to aggregate into random bundles between the graphene layers. 1D cobalt oxide nanostructures grown on graphene foams via a much stronger chemical contact can effectively prevent the restack of graphene, enlarge the specific surface area, and enhance the overall conductivity of the composites. More promisingly, these hybrids can be applied as free-standing electrode for electrochemical devices without any additional binders or current collectors. 4.3.3. 2D cobalt oxide/graphene hybrids The hybridization of 2D cobalt oxide nanomaterials with graphene can be divided into three types, namely, horizontal growth on graphene, vertical growth on graphene, and layer-by-layer sandwiched 2D-2D nanostructures [33,665–667]. Among them, the sheeton-sheet 2D-2D structure is appealing for electrochemical applications due to its enhanced binding force between layers and improved structural stability without serious aggregation of graphene [668–670]. Free-standing 2D Co3O4/graphene hybrid films were synthesized by vacuum filtration followed by a thermal treatment (Fig. 46a), in which the sheet-like Co3O4 (∼20 nm in thickness) and graphene were assembled into a lamellar hierarchical structure, as shown in Fig. 46b, by means of electrostatic interactions [671]. This layered morphology with strong interfacial interactions significantly promotes the interfacial electron and Li+ transport. As binder-free and free-standing electrodes for LIBs, the hybrid films delivered a high specific capacity of ∼1400 mA h g−1 at a current density of 100 mA g−1, improved rate capability, and superior cyclic stability with 1200 mA h g−1 at the 100th cycle at 200 mA g−1 [671].

Fig. 46. Synthesis, morphology, and performance of 2D cobalt oxide/graphene hybrids. (a) Schematic illustration of the fabrication process of Co3O4/graphene nanosheets hybrid films, (b) the corresponding SEM image [671] (Copyright 2013, Royal Society of Chemistry). (c, d) STEM images of atomically thin mesoporous 2D Co3O4 nanosheets/graphene (ATMCNs-GE) composite [672], (e) a comparison on cycling performances of ATMCNs, graphene and ATMCNs-GE at a rate of 2.25 C [672] (Copyright 2016, Wiley-VCH). 647

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Dou et al. fabricated atomically thin mesoporous Co3O4 nanosheets/graphene (ATMCNs-GE) composite under stirring (Fig. 46c and d) [672]. Benefiting from the unique structure of the atomic-level thickness and mesoporous structure of the Co3O4 nanosheets and the conductive graphene, this hybrid exhibited extraordinary electrochemical performance, which delivered high discharge capacities of 2014.7 and 1134.4 mA h g−1 at 0.11 and 2.25 C, respectively, and an outstanding capacity retention of 92.1% at 2.25 C after 2000 cycles (Fig. 46e). These performances significantly outperformed those of the bare ATMCNs and the previously reported Co3O4/C composites [672]. Recently, 2D-2D Co3O4/rGO composites consisting of ultrathin 2D Co3O4 nanosheets and rGO synthesized through a one-pot hydrothermal strategy have been reported as highly-efficient OER catalyst [246]. The obtained 2D-2D hybrid exhibited a low overpotential of 290 mV at 10 mA cm−2 with a small Tafel slope of 68 mA dec−1 in 1.0 M KOH, much smaller than that of bare Co3O4 catalyst. In this work, 0D carbon spheres and 1D CNTs were also employed to replace 2D rGO to be hybridized with Co3O4 nanosheets to form 2D Co3O4/0D carbon spheres and 2D Co3O4/1D CNTs hybrids. Interestingly, they found that the overpotentials decreased with the increased dimensionality numbers of the carbon nanostructures and they suggested that the electrochemical activity of the Co3O4/nanocarbon composites is very closely related to the surface area of the carbon substrates [246]. Overall, the layer-by-layer stacked 2D cobalt oxides and graphene nanosheets 2D-2D hybrid structures have the best geometric compatibility and usually provide superior structural stability for electrochemical applications. In this type of 2D-2D structures, the agglomeration of both cobalt oxide nanosheets and graphene can be largely inhibited. In the fabrication, particularly for wetchemistry synthesis of this type of hybrid nanostructures, we need to pay close attention to the surface potential of both metal oxide and graphene nanosheets. 4.3.4. 3D cobalt oxide/graphene hybrids Various 3D cobalt oxide nanostructures (e.g. spheres, cubes, polyhedrons, etc.) have been reported to be coupled with graphene nanosheets by two primary strategies, namely, anchored on graphene and or encapsulated by graphene [673–678]. The “anchoredon-graphene” strategy is very simple for synthesis and has the potential for large-scale production. One of the biggest drawbacks for this strategy, however, is that there is a weak interface between the metal oxide nanostructures and graphene, which usually leads to strong aggregation of metal oxide nanostructures during electrochemical reactions and then loss of the synergic effect of the hybrids. On the contrary, 3D cobalt oxide/graphene nanocomposites fabricated via the “encapsulated-by-graphene” strategy possess enhanced binding force between the cobalt oxide nanostructures and the graphene. According to the reported studies on the application of the “encapsulated-by-graphene” type nanocomposites, the conductive graphene coating on Co3O4 nanomaterials provides much improved charge and ion transfer and avoids the formation of unstable SEI. Furthermore, the void space between the metal oxide nanomaterials and graphene layer contributes to buffer the volume expansion during long-time cycling life. Graphene-encapsulated 3D cobalt oxide nanostructures are often achieved by self-assembly of the negatively charged GO and the positively charged oxide nanostructures. In 2010, Yang et al. synthesized graphene-encapsulated solid Co3O4 nanospheres (NSs) (GECo3O4) with high electrochemical activity by employing the electrostatic interactions of differently charged Co3O4 nanospheres and graphene [675]. In this experiment, Co3O4 NSs were first modified with 3-aminopropyltrimethoxysilane (APS) to render a positively

Fig. 47. Synthesis, morphologies, and performances of graphene-encapsulated cobalt oxide nanostructures. (a) Schematic illustration of the fabrication of graphene-encapsulated mesoporous Co3O4 microspheres (G-Co3O4) [677], (b) the corresponding FESEM image of G-Co3O4 (Inset: FESEM image of graphene) [677] (Copyright 2012, Royal Society of Chemistry). (c) SEM image of graphene-wrapped mesoporous Co3O4 hollow spheres core-shell (Co3O4@G) nanostructures [676], (d) cycling performance and CE of Co3O4@G electrode at 1.0 A g−1 (inset shows the fabrication process of the core-shell composites) [676] (Copyright 2014, American Chemical Society). 648

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charged surface, and then were encapsulated by negatively charged GO. After chemical reduction of GO, the GE-Co3O4 composites were finally obtained. Electrochemical measurements indicated that the composites exhibited excellent cycle performance with a reversible capacity of ∼1100 mA h g−1 in the first 10 cycles, and 1000 mA h g−1 after 130 cycles [675]. Besides the solid Co3O4 nanospheres, mesoporous Co3O4 spheres featuring high surface area and highly-ordered inner channels were used to be encapsulated by graphene nanosheets to produce Co3O4/graphene hybrid (G-Co3O4) (Fig. 47a) [677]. SEM images revealed that the mesoporous Co3O4 spheres with a diameter of about 700 nm and micropores of 7.2 nm were fully wrapped by graphene shells (Fig. 47b). The obtained composites as anode materials for LIBs exhibited an initial discharge capacity as high as 1533 mA h g−1 at a current density of 100 mA g−1 and remained at a reversible capacity up to 820 mA h g−1 at 200 mA g−1 [677]. Later, a similar principle has also applied to fabricate graphene-wrapped mesoporous Co3O4 hollow spheres to form core-shell (Co3O4@G) nanostructures by using poly(allylamine hydrochloride) (PAH) as the surface modifier of Co3O4 spheres [676]. As shown in Fig. 47c, Co3O4 hollow spheres with a diameter of ∼270 nm were covered by graphene nanosheets. As anode materials for LIBs, this core-shell structure delivered a high reversible capacity of 1076 mA h g−1 over 10 cycles at a current density of 0.1 A g−1 and displayed an excellent cyclic stability (over 600 mA h g−1 after 500 cycles at a high rate of 1.0 A g−1) (Fig. 47d) [676]. Likewise, graphene-encapsulated porous Co3O4 cubes (Co3O4@G) were also synthesized by using uniform Co3[Co(CN)6]2 cubes as the precursor [679]. This structure was expected to be advantageous to fast transport of Li+/electron and alleviation of the volume change during long-term cycles, and thereby a remarkable cycling and rate performances. When evaluated as anode materials, this cubic Co3O4@G exhibited a reversible discharge capacity of 980 mA h g−1 after 80 cycles at 200 mA g−1 and 500 mA h g−1 at high rate of 2 A g−1 [679]. Generally, the encapsulation of 3D cobalt oxide structures by graphene is usually achieved by electrostatic attraction between cobalt oxides and graphene with different surface charges. However, it is unlikely to ensure the cobalt oxides to be completely covered by the graphene nanosheets. The encapsulation of metal oxide by graphene can give rise to superior electric conductivity on the one hand. On the other hand, the diffusion of electrolyte and the transport of ions through the graphene shells would be largely retarded. Further design on the shell structure of this type of hybrid nanostructure is still demanded to reach expected electrochemical performances. 4.3.5. Cobalt oxide/heteroatom-doped graphene hybrids Similar to the heteroatom-doped nanocarbon and CNTs, some heteroatoms have also introduced into the graphene skeleton with the purpose to increase its chemical reactivity to modify its electronic structure, or to enhance its chemical affinity to metal oxides [272,678,680–682]. Hence, compared to the cobalt oxide/pristine graphene composites, the hybrids of cobalt oxide nanostructures with heteroatom-doped graphene have many incomparable advantages for their potential applications in electrochemical devices. For example, the introduction of nitrogen atoms in graphene can offer stronger coupling between cobalt oxides and graphene and be favorable for the nucleation of cobalt oxide at the N-doped sites during synthesis. Furthermore, Co-Nx species in which cobalt is coordinated to or strongly interacts with nitrogen are believed to be potential active sites for ORR [683–685]. Liang et al. reported Co3O4 nanocrystals grown on N-doped RGO nanosheets as a high-performance bi-functional catalyst for ORR

Fig. 48. Morphology and performance of cobalt oxide NPs on N-doped graphene [28]. (a) SEM, (b) TEM, and (c) HRTEM images of the Co3O4 NPs/ nitrogen-doped reduced mildly oxidized GO hybrid (Co3O4/N-rmGO), (d) oxygen electrode activities of Co3O4, Co3O4/N-rmGO and Pt/C catalysts in 0.1 M KOH (catalyst loading ∼0:24 mg cm−2), (e) oxygen evolution currents and (f) Tafel plots of Co3O4, Co3O4/rmGO and Co3O4/N-rmGO (catalyst loading: ∼1 mg cm−2) in 1 M KOH [28] (Copyright 2011, Nature Publishing Group). 649

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and OER [28]. Based on their synthesis route, well-crystallization Co3O4 NPs were grown on graphene sheets by hydrolysis and oxidation of cobalt acetate. When NH4OH was added into this system, the Co3O4 NPs/N-doped reduced moGO hybrid (Co3O4/NrmGO) was obtained. SEM (Fig. 48a) and TEM (Fig. 48b and c) images revealed that well-crystallized Co3O4 NPs (4–8 nm) in spinel structure were anchored on graphene nanosheets. When used as catalyst for electrocatalysis, the Co3O4/N-rmGO hybrid showed remarkable ORR activities than Co3O4 or GO alone in alkaline solutions. As shown in Fig. 48d, the Co3O4/N-rmGO electrode affords higher OER currents compared with Co3O4 nanocrystals and Pt/C. Moreover, when the catalyst loading on the substrate was increased into ∼1 mg cm−2, a small overpotential of 0:31 V affording a current density of 10 mA cm−2 (Fig. 48e) and a small Tafel slope of 67 mV dec−1 (Fig. 48f) were achieved. Besides, it is obviously observed that the Co3O4/N-rmGO hybrid delivered only slightly higher OER activity than the Co3O4/rmGO hybrid, indicating that the N-doped graphene has no effect on OER activity [28]. The integration of nitrogen-doped graphene with cobalt oxide nanomaterials also delivers more appealing electrochemical properties for rechargeable batteries. Lai et al. synthesized Co3O4 NPs grown on nitrogen modified microwave exfoliated graphite oxide (Co3O4/NMEG) and Co3O4 on thermally rGO (Co3O4/tRG-O) with Co3O4 loading ratio of 10–70% [686]. Electrochemical measurements revealed that the 70% Co3O4/NMEG composite delivered an initial reversible capacity of 799 mA h g−1 and retained 910 mA h g−1 after 100 cycles, and the 70% Co3O4/tRG-O had a reversible capacity of 750 ± 20 mA h g−1 but the irreversible capacity loss during the first cycle was 700 ± 20 mA h g−1. More interestingly, Co3O4/NMEG composites with different Co3O4 loading weight ratios from 10% to 70% showed outstanding cycling behaviors with little capacity decay over 100 cycles at a current density of 100 mA g−1. The advantages brought by N-doping becomes much significant at high charge/discharge rate. The enhancement of the electrochemical performances of the Co3O4/NMEG composites was supposed to be from the nitrogen functional groups in NMEG, especially the pyridinic (C]N) and the pyrrolic (C^N) N, which are advantageous for the growth of Co3O4 nanomaterials, the reduced oxygen content of graphene, finally the amelioration of the first cycle efficiency [686]. Then, Yao et al. incorporated 3D scaffold of graphene aerogel with synchronous chemical-doping of nitrogen atoms and 1D mesoporous Co3O4 nanowires to produce N-doped graphene/Co3O4 nanowires composites (NGA-Co3O4) [687]. As illustrated in Fig. 49a and b, the Co (CO3)0.5(OH)·0.11H2O precursor was first synthesized within a self-assembled 3D graphene network, and then transformed into Co3O4 nanowires after cryodesiccation and thermal decomposition. For LIB application, a remarkably high capacity of 1229 mA h g−1 was achieved at a current density of 100 mA g−1 over 100 cycles, which was four folds higher than that of Co3O4 nanowires (314 mA h g−1) and NGA (419 mA h g−1) (Fig. 49c). Moreover, 812 mA h g−1 was still retained after 230 cycles at a rate of 1000 mA g−1 (Fig. 49d) [687]. Other heteroatoms, such as sulfur [688] and boron [689], have also been incorporated into graphene framework for enhancing its electrochemical performances. Recently, CoOx NPs with rich oxygen vacancies strongly coupled with B, N-decorated graphene (CoOx NPs/BNG) via Co-N-C bridging bonds have been reported by Tong et al. [689]. This synthesis was achieved by in-situ NH3 treatment of a Co-Bi/G precursor (Fig. 50a). TEM (Fig. 50b and c) characterizations showed that small CoOx NPs grown uniformly on the graphene sheets, and the existence of CoO was confirmed by the exposed {2 0 0} lattice planes. In the material, abundant oxygen vacancies, Co-N and C-B-N bonds bonds, and strong Co-N-C bridging bonds formed at the pyridinic-N and pyrrolic-N sites were identified by XPS and XANES. It has been reported that N atoms can induce positive polarization of C atoms, while B atoms with a low electronegativity can be positively polarized with the existence of N atoms [690], and these positively polarized C-B and N-C bonds favorer the capture of O2 molecules during ORR. As an efficient bifunctional electrocatalyst for OER/ORR, the CoOx NPs/BNG hybrid

Fig. 49. Synthesis, morphology, and performance of 1D cobalt oxide nanowires on N-doped graphene aerogel [687]. (a) Schematic illustration of the fabrication of nitrogen-doped 3D graphene aerogel/1D Co3O4 nanowires (NGA-Co3O4) composites [687], (b) SEM image of the NGA-Co3O4 hybrids, (c) cycling performance of Co3O4 nanowires, NGA, and NGA-Co3O4 hybrids at a current density of 100 mA g−1, (d) cycling stability of NGACo3O4 hybrid electrode at a high rate of 1000 mA g−1 (Copyright 2016, Wiley-VCH). 650

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Fig. 50. Preparation, morphology, and performance of cobalt oxide/B, N-doped graphene hybrids [689]. (a) Schematic illustration of the preparation of CoOx NPs/B, N-decorated graphene composites (CoOx NPs/BNG), (b) TEM and (c) HRTEM images of CoOx NPs/BNG hybrids, (d) polarization curves in the both ORR and OER regions in 0.1 M KOH solution (Copyright 2017, Wiley-VCH).

was highly efficient for OER with a low overpotential (295 mV at 10 mA cm−1) and Tafel slope (57 mV dec−1), and also active for ORR with a positive half-wave potential (805 mV) and high limiting current density (∼5.67 mV cm−2 at 0.2 V) in alkaline medium, which is accompanied by a low potential gap of 0.72 V (Fig. 50d) [689]. In spite of some major progress have been achieved in the doped graphene/cobalt oxide composites, similar to the heteroatomdoped nanocarbon/cobalt oxide hybrids, further effort on low-cost and large-scale preparation with controllable doping levels is still urgently needed to further enhance their electrochemical performances.

Fig. 51. Morphology of Co3O4-Mn3O4/GO hybrid and the electrocatalytic pathways [700]. (a) TEM and (b, c) STEM dark field images of Co3O4Mn3O4/GO hybrid, (d) STEM-EDS elemental mapping (green and red points show Co and Mn elements, respectively) of Co3O4-Mn3O4/GO composite, schematic illustration of (e) the interfacial structures and (f) the ORR pathway catalyzed by Co3O4-Mn3O4/GO composite (Copyright 2016, Elsevier). 651

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4.3.6. Cobalt oxide/graphene/others hybrids Carbon (black) [691–694], CNTs [695–697], polymers (e.g. PPy, etc.) [698], and other metal oxides (e.g. MnO2, Mn3O4, CoFe2O4, etc.) [271,699–701] have also hybridized with cobalt oxide nanostructures and graphene networks to further enhance the ion storage performance and catalytic activity of the materials, and to trigger more appealing synergistic effects to meet the requirements for the practical application of cobalt oxide based nanomaterials. Dai et al. synthesized 3D stacked-up Co3O4-Mn3O4/GO ternary composites via coherent nucleation and growth of Mn3O4 on GO supported Co3O4 based on a two-step aqueous synthesis [700]. It can be easily found from Fig. 51a that these oxide NPs, with a mean size of about 13.5 nm (Fig. 51b), were uniformly dispersed on the graphene surfaces. Furthermore, as shown in Fig. 51c and d, the smaller Mn3O4 nanocrystals had been successfully dispersed onto the surfaces of Co3O4 NPs to form an oxide-on-oxide nanostructure on the surface of graphene. In an alkaline environment, compared to individual Mn3O4/GO and Co3O4/GO catalysts, the Co3O4Mn3O4/GO composite exhibited much better electrocatalytic activity (e.g. a half-wave potential of −0.2235 V, etc.) towards ORR. This might be largely attributed to the enhanced covalent electron transfer at the interfaces between these two metal oxides and the GO matrix, resulting from their interphase ligands and synergetic effects (Fig. 51e). During ORR, as depicted in Fig. 51f, the Co3O4 phase on the graphene serves as the main catalyst for catalyzing ORR with the formation of a large amount of OH− via nearly fourelectron pathway and a small amount of H2O2 intermediates via two-electron pathway. Subsequently, with the presence of Mn3O4 phase on the surface of Co3O4 phase, the resultant H2O2 undergoes a chemical disproportionation reaction to produce OH− and O2 specie, which is repeatedly utilized as reactants for the further ORR. The oxide-on-oxide structure in this hybrid brings about a small diffusion distance, which is beneficial to promote the decomposition of H2O2 and the further utilization of O2. Moreover, defects and slight lattice strain were found in the spinel Mn3O4 phase of the composite, which also contribute to the decomposition of H2O2 [700]. Recently, flexible films/papers composed of 1D CNTs and 2D graphene have been paid much attention for energy conversion and storage devices [702–707]. This class of unique combination of 1D/2D hybrid film can effectively prevent the self-stack tendency of graphene nanosheets and possess excellent electronic conductivity and electrochemical performances. What is more, these flexible and conductive films are outstanding substrates as free-standing electrodes for sustainable applications when depositing with active materials [270,696,708–710]. Yuan et al. reported a facile and efficient route to fabricate flexible and free-standing Co3O4/rGO/ CNTs hybrid paper, in which Co3O4 monolayer microsphere arrays were directly grown on the rGO/CNTs film by a simple one-step hydrothermal deposition [696]. As displayed in Fig. 52a, the hybrid paper possessed a layered structure with an average thickness of about 50 μm. As showed in Fig. 52b, the hybrid paper demonstrated good flexibility. The enlarged SEM images (Fig. 52c) revealed that interconnected Co3O4 microsphere array grew on the surface of the rGO/CNTs film with 1 μm in thickness. Electrochemical evaluation for its application as electrochemical capacitors demonstrated that this flexible paper electrode delivered specific capacitance of 378 and 297 F g−1 at 2 and 8 A g−1, respectively in 3.0 M KOH solution with a voltage window between −0.2 to 0.45 V

Fig. 52. Morphology of cobalt oxide/graphene/CNTs hybrid paper. (a) FE-SEM image of Co3O4/rGO/CNTs hybrid paper (inset shows the enlarged part) - [696], (b) photograph showing the flexibility of the hybrid paper [696], (c) the enlarged cross-sectional view of the hybrid paper [696] (Copyright 2012, Wiley-VCH). (d) SEM image of the cross-section of N-doped graphene/CNT/Co3O4 (NG/CNT/Co3O4) paper (inset shows a photograph of the flexible paper) [270] (Copyright 2014, Royal Society of Chemistry). 652

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[696]. In 2014, flexible N-doped graphene/CNT/Co3O4 (NG/CNT/Co3O4) paper (Fig. 52d) was synthesized in ammonia solution with in-situ formation of Co3O4 NPs, reduction of GO into rGO, and doping of nitrogen species [270]. This paper has a thickness of around 4 μm, with CNTs and Co3O4 NPs (∼25 nm in size) existed between the graphene nanosheets. As working electrode for ORR, it delivered an enhanced catalytic performance with high current density and positive onset potential. Furthermore, this as-prepared paper exhibited good cycling stability and tolerance to methanol poisoning [270]. 4.4. Cobalt oxide/metal hybrids The implantation of heterogeneous transition metal atoms/ions/NPs, such as Fe, Ni, Ru, Pd and Pt into cobalt oxide nanostructures, greatly promotes the electrochemical performances of cobalt oxide-based nanomaterials, particularly for electrocatalytic reactions in related fuel cells and metal-air batteries [269,711–719]. Cao et al. reported a binder/carbon-free air electrode with tips-bundled Pt/Co3O4 nanowires grown directly on Ni foam substrate via a hydrothermal route [718]. As shown in Fig. 53a-c, the obtained Pt/Co3O4 nanowires with sharp tips were uniformly arranged on the Ni foam substrate, in which Pt nanocrystals (< 5 nm in size) were homogeneously distributed within Co3O4 nanowires. Interestingly, if without the presence of Pt nanocrystals, only a random arrangement of Co3O4 nanowires was found on the Ni foam substrate, indicating the presence of Pt plays a crucial role on the formation of the ordered tips-bundled structure. Consequently, a LiO2 battery by using this tips-bundled Pt/Co3O4 nanostructures as catalysts (Fig. 53d) showed a stabilized discharge plateau (> 2.50 V) (Fig. 53e) and low charge end voltages (< 4.0 V) up to 55 cycles. In contrast, the cycling of the battery by only using Co3O4 as catalysts maintained only 10 cycles with high charge terminal voltages (> 4.0 V) and low discharge end voltages (< 2.5 V). Further characterization of these electrodes after discharge confirmed that Pt also affected the crystallization habit of the discharge product Li2O2. With Pt nanocrystals, a fluffy and thin Li2O2 layer was uniformly deposited only on the periphery of Pt/Co3O4 nanowires (Fig. 53d), which makes the Li2O2 nanocrystals easy to decompose at low potentials with reduced side reactions. Otherwise, large-sized flower-like particles and plate aggregates of Li2O2 randomly distributed on the bare Co3O4 nanowires and resulted in inferior electrochemical performances [718]. 4.5. Cobalt oxide/metal oxides heterogeneous hybrids The combination of different metal oxide nanostructures into heterogeneous structures has demonstrated to be an effective way to design novel electronic correlation materials and catalytic materials owing to its extraordinary interfacial coupling effects, regulated charge/carrier transport behaviors, synergetic catalytic activities, etc. Lots of cobalt oxide/metal oxide heterogeneous hybrids composed of cobalt oxides and other metal oxides, including Fe3O4/Co3O4 [720,721], Co3O4/RuO2 [722–724], Co3O4/ZnO [725], MnO2/Co3O4 [726–731], Mn3O4/Co3O4 [732], NiO/Co3O4 [324,733–735], Co3O4/CeO2 [736], NiCo2O4/Co3O4 [737], Co3O4/ NiMoO4 [738], Co3O4/CoMoO4 [739], ZnO/NiO/Co3O4 [740], etc., have been designed and synthesized in various architectures, such as core-shell, shuttle-like, flake-like, NPs-on-fiber, NPs-on-wire, rod-on-sheet, horn-on-sheet structures, etc. [741–743]. These hybrids have been extensively studied for the potential applications in batteries, capacitors, and electrocatalysis. Xia et al. reported a two-step solution-based approach to synthesize TMO core/shell nanoarrays on a variety of substrates including fluorine-doped tin oxide glass (FTO), nickel foam, and nickel foil [733]. In this synthesis, the Co3O4 or ZnO NWAs, synthesized by hydrothermal reactions in the first step, served as the backbone for the following chemical bath deposition of NiO nanoflakes. The morphologies of Co3O4/NiO core/shell structures with Co3O4 core nanowire and NiO shell nanoflake were confirmed by SEM and TEM. As shown in Fig. 54a–c, the highly porous Co3O4 nanowires had an average diameter of 70 nm and the thickness of

Fig. 53. Morphology and performance of tips-bundled Pt/Co3O4 nanostructures for Li-O2 battery [718]. (a) schematic illustration of the Pt/Co3O4 cathode architecture, (b) SEM and (c) TEM images of the tips-bundled Pt/Co3O4 nanostructures on Ni foam substrates, (d) schematic illustration of the Pt/Co3O4 cathode architecture and the working mechanism of Li-O2 battery, (e) voltage profiles of Li-O2 batteries at 100 mA g−1 with Pt/Co3O4 cathode (Copyright 2015, American Chemical Society). 653

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Fig. 54. Morphology and performance of Co3O4/NiO core/shell NWAs for supercapacitor [733]. (a, b) SEM and (c) TEM images of Co3O4/NiO core/ shell NWAs on fluorine-doped tin oxide glass (FTO) substrate, (d) a comparison of specific capacitances of different electrodes at various current densities, (e) a comparison on cycling performances of different electrodes at 2 A g−1 (Copyright 2012, American Chemical Society).

NiO nanoflakes was about 10 nm. Benefited from the porous wire core and the flake shell, the Co3O4/NiO NWAs exhibited a quite large surface area as high as 415 m2 g. As electrode material for supercapacitor, the Co3O4/NiO NWAs on nickel foam delivered excellent rate capability (e.g. 452 F g−1 at 2 A g−1 and 384 F g−1 at 40 A g−1) (Fig. 54d), and remarkable cycling stability with an areal capacitance of 2.56 F cm−2 after 6000 cycles, much higher than those of the individual Co3O4 NWAs (1.36 F cm−2) and NiO nanoflake arrays (0.16 F cm−2) (Fig. 54e) [733]. Kim et al. designed a sectionalized MnO2-Co3O4 electrode for metal-air batteries via an agarose gel-mediated strategy, in which the selective electrodeposition of MnO2 and Co3O4 were achieved on 3D porous nickel foam [726]. At the first stage, semi-flexible hydrogel was produced by mixing molten agarose and cobalt precursor ions in an aqueous solution. After an electric current was applied to the gel attached to the nickel foam, cobalt ions were reduced to Co(OH)2, which was further converted into Co3O4 by heat treatment. Finally, MnO2 was incorporated into the opposite side of the substrate by using a similar procedure to form the final sectionalized MnO2-Co3O4 electrode on Ni foam (MnO2-Co3O4/NiF), as illustrated in Fig. 55a. This hybrid structure effectively enhanced catalytic activities for both ORR and OER. The sectionalized electrode showed outstanding cycling performance and longterm stability (e.g. 400 h of operation time) in rechargeable Zn-air batteries (Fig. 55b) [726].

Fig. 55. (a) Schematic illustration of sectionalized bifunctional MnO2-Co3O4 on Ni foam electrode (MnO2-Co3O4/NiF) [726], (b) cycling stability of the assembled rechargeable Zn-air battery using MnO2-Co3O4/NiF electrode at a current density of 1 mA cm−2 [726] (Copyright 2016, Elsevier). 654

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4.6. Cobalt oxide/polymers hybrids Two types of polymers, including porous network polymers (e.g. covalent organic polymers (COPs), etc.) and conductive polymers (e.g. PPy, polyaniline (PANI), polyindole (Pind), etc.), are commonly used to hybridize with cobalt oxide nanostructures to improve its electric conductivity, enhance adhering force, and facilitate rapid charge transfer for electrochemical applications [200,744–750]. The storage mechanism of the conductive polymers depends on the redox reaction process. During the oxidation step, ions are transferred to the backbone of the polymer, which will be released again into the electrolyte when the reduction reaction occurs. Hence, the combination of cobalt oxides with conductive polymers can greatly promote the reaction rate and enhance the ionstorage performances. Zhou et al. developed a novel supercapacitor electrode composed of well-aligned CoO nanowire arrays grown vertically on 3D nickel foam in the presence of conductive PPy to boost the pseudocapacitive performance [200]. As illustrated in Fig. 56a, CoO NWAs were first grown on commercial nickel foam via a combination of hydrothermal and post-annealing steps, and PPy was subsequently immobilized onto these arrays based on a chemical polymerization reaction by using ammonium persulphate as the oxidant and ptoluenesulfonic acid (p-TSA) as the doping agent. After the integration of PPy, the CoO arrays remained their original ordered structures (Fig. 56b). Further TEM analysis confirmed that each nanowire surface was coated by a thin PPy layer with a nanosized thickness and some PPy was attached firmly to the nanowires in particulate forms. This electrode architecture could take full advantage of the high electrochemical activities derived from both CoO and PPy, the high electronic conductivity of PPy polymer, and the short ion diffusion pathway within these ordered mesoporous nanowires. These structural advantages as well as the synergy effects of the hybrid electrode for systematic supercapacitor led to a high specific capacitance of 2223 F g−1 at 1.0 mA cm−1, good rate capability (∼647 F g−1 at 50 mA cm−1), and excellent cycling stability (∼99.8% capacitance retention after 2000 cycles). Furthermore, an aqueous asymmetric supercapacitor device was assembled by using the CoO@PPy as positive electrode and AC film on nickel foam as negative electrode, in 3 M NaOH electrolyte, as depicted in Fig. 56c. The cell voltage was as large as 1.8 V and the device delivered a high energy density of ∼43.5 Wh kg−1 at 87.5 W kg−1, high power density of ∼5500 W kg−1 at 11.8 Wh kg−1 (Fig. 56d), and outstanding cycling life up to ∼20,000 times. This asymmetric supercapacitor thus had the potential to bridge the performance gap between thin-film Li-ion batteries and EDLCs (Fig. 56d and e) [200]. The integration of organic conductive polymers and cobalt oxide nanostructures to form cobalt oxide/polymers hybrids is an effective strategy to contribute to a wide voltage window and a remarkable storage capacity for energy storage devices, especially for the widely studied supercapacitors. Moreover, with the assistance of these polymers, the resulting hybrids feature adjustable redox activities because the functionalization of these organic polymers is easy to be realized via different organic chemical modification techniques. Furthermore, the redox reactions for these conductive polymers occur in the entire polymer skeleton instead of preferentially on the surface for inorganic metal oxides such as cobalt oxides. In this regard, a uniform distribution of metal oxide component in cobalt oxide/polymers hybrids, such as the above-mentioned PPy-coated CoO nanowires, can bring forward an integral conductive network over the whole hybrid frameworks. Another advantage for the introduction of organic polymers is that no phase change behavior for the polymers exists during the charging/discharging processes, which theoretically makes the redox reactions reversible and thus leads to an excellent cycling stability. In practical application, however, these polymers may mechanically

Fig. 56. Preparation, characterization, and electrochemical performances of cobalt oxide/polymer (CoO/PPy) hybrid [200]. (a) Schematic illustration of the synthesis of 3D hybrid nanowire electrode, (b) SEM image of the CoO/PPy hybrid, (c) schematic illustration of the configuration of CoO/PPy and carbon asymmetric supercapacitor, (d) Ragone plot of the assembled supercapacitor device and the referenced EDLC and Li-ion battery, (e) a comparison on volumetric energy and powder densities of the asymmetric supercapacitor (Copyright 2013, American Chemical Society). 655

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degraded aroused by swelling and shrinking, and some side reactions may also appear under different types of electrolyte systems, resulting in a fast capacity decay over cycling. Considering these side effects, optimized design for enhancing the structural stability is still needed to achieve excellent electrochemical properties for this type of materials. 4.7. Other cobalt oxide-based hybrids Other cobalt oxide-based hybrids, including ion-doped cobalt oxides [751–753], cobalt oxides/metal sulfides/selenides [296,754,755], cobalt oxides/CxNy [294,756], cobalt oxides/metal hydroxides [757–759], and some complex cobalt oxide-based hybrids with three or more components [760–763], have been reported as electrode materials or catalysts for electrochemical applications [712,764–768]. Recently, Zhou et al. developed a MOF template-directed fabrication of hierarchical Co3O4@X (X = Co3O4, CoS, C, and CoP) nanostructures, which were derived from cobalt carbonatehydroxide@ZIF-67 (CCH@ZIF-67) [769]. As shown in Fig. 57a, in-situ growth of ZIF-67 on well-aligned CCH nanorods supported on Ni foil was first achieved by hydrothermal synthesis, and then various products (e.g. Co3O4@Co3O4, Co3O4@CoS, Co3O4@C, or Co3O4@CoP) were obtained by different post-treatment methods, including oxidation, sulfurization, carbonization, or phosphorization. This unique hierarchical structure gave rise to ample exposed active sites, shortened ion diffusion pathway, and enhanced conductivity. As electrocatalysts for OER, these derivatives revealed high efficiency catalytic properties, which were much superior to cobalt-based catalysts and the Ir/C catalyst [769]. In particular, Co3O4@CoP exhibited the highest catalytic capability among the Co3O4@X nanostructures with an overpotential of 0.238 V at 10 mA cm−2 (Fig. 57b), and a current density of 45 mA cm−2 at the overpotential of 0.300 V. Moreover, these hybrids also demonstrated improved catalytic performances to other small molecules (e.g., glycerol, methanol, or ethanol) towards electro-oxidation reaction [769]. Cobalt oxide-based composites with other heterogeneous components have been fully explored with the purpose to achieve the best combination of both enhanced structural stability and improved electrochemical properties. Compared with the cobalt oxidebased hybrids with carbon, CNTs, and graphene, the properties are less attractive for these types of nanomaterials. For example, the integration of emerging active metal sulfides with cobalt oxides can result in enhanced activities of the related redox reactions for energy conversion and storage devices, however, the conductivity is still difficult to be alleviated. Moreover, multiple components of the active materials can bring about more complex reactions and the chance of the appearance of some possible side reactions is greatly increased. Moreover, how to identify the specific contribution of each component for the overall electrochemical properties is still a technical issue. Therefore, it is highly demanded that in-depth studies on the underlying reaction mechanisms in the hybrids or nanocomposites with multiple components should be paid more attention. 5. Conclusion and outlook Rapid progress has been made in the research of cobalt oxides and their hybrids for electrochemical applications, particularly in the recent five years. Cobalt oxide nanostructures in different dimensionalities have been fabricated for various electrochemical energy applications, including rechargeable batteries, supercapacitors, and electrocatalysis, for their appealing features of low cost and abundance in earth, high theoretical capacity/capacitance and electrochemical activity, in electrochemical energy storage, and excellent chemical and mechanical stability, etc., compared to other metal oxides. It is out of question that cobalt oxides will be one of the most promising materials as an alternative electrode material for next-generation electrochemical energy devices. The electrochemical performances of cobalt oxides in their pristine forms, as we reviewed in this article, however, are still far from their practical applications, owing to some intrinsic defects, such as poor electrical conductivity, slow reaction kinetics, several morphology and volume changes during electrochemical reactions, etc., which usually result in fast fading in electrochemical performance, poor structural stability and cycling performance, and slow activation process. To address these issues, cobalt oxide-based hybrids with unique merits of each individual component and synergetic effects can

Fig. 57. (a) Schematic illustration of the fabrication of hierarchically Co3O4@X (X = Co3O4, CoS, C, and CoP) structures [769], (b) LSV curves of the Co3O4@X structures [769] (Copyright 2017, Wiley-VCH). 656

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effectively improve the electronic conductivity, enhance the reaction kinetics, and buffer the volume changes during repeated charging and discharging processes. Compared with the pristine cobalt oxide nanostructures, these hybrids exhibit improved rate capability and cycling stability, decreased overpotential, and overall promoted electrochemical properties for energy-related devices. Despite the fact that much progress has been achieved on the investigation of various types of cobalt oxide-based composites, some challenges still exist in their practical electrochemical energy applications In this section, we will give a detailed analysis on the current challenges of the development of cobalt oxide-based materials for electrochemical energy applications. Then, some effective strategies for enhancing the electrochemical properties of cobalt oxidebased nanomaterials will be summarized. To make a good understanding, we also emphasis the structure-property relationships between the cobalt oxide-based structures and the electrochemical properties in energy applications. Finally, an outlook will be given to offer some reference to the further research on this topic. 5.1. Current challenges In spite of applausive progress on this amazing family of materials has been achieved in recent years, some challenges still need to be addressed to meet the high-performance electrochemical applications. As summarized in this review, cobalt oxide-based nanomaterials share some common challenges to other metal oxide-based materials in electrochemical energy applications, such as that the serious agglomeration of the low-dimensional cobalt oxide nanostructures will lead to the loss of effective active sites and accelerate the pulverization or crack of electrode during cycling processes. Below are some typical challenges in the application of cobalt oxide nanomaterials. The first challenge is from the synthesis of the property-on-demand cobalt oxide nanostructures. Even though lots of work on the synthesis and fabrication of cobalt oxide nanostructures have been reported, it is still a grand challenge to achieve a balance between the homogeneity and quality of the morphology/size and the productivity of the nanomaterials. Moreover, the design of some complex nanostructures possessing some extraordinary properties to achieve higher performances, such as 3D multiscale-ordered nanostructures, 3D hierarchical networks and porous structures, highly-dispersed 3D nanocomposites and nanohybrids, and the materials with unusual crystal structures or exposed facets, always brings extra difficulties in materials synthesis. Usually, the finer the nanostructures, the smaller the productivity is. Therefore, the large-scale production of property-on-demand nanostructures with highly-controlled morphology and narrow-size distribution is the next target for possible commercial application of this promising class of materials. The second challenge is from the optimization of cobalt oxide-based nanocomposites or nanohybrids. As reviewed in this paper, a wide range of cobalt oxide-based composites have been reported to date, however, it is still difficult to find an optimum design, including dimensionality match, component ratios, and configurations, etc., for this type of materials. First, as for the dimensionality match, cobalt oxide nanostructures with different dimensionalities (0D, 1D, 2D, and 3D) can be hybridized with the other heterogeneous components with different dimensional structures as well (0D, 1D, 2D, and 3D). There are theoretically sixteen types of combinations for binary cobalt oxide-based hybrids, and even more combined forms for ternary hybrids. Based on the recent progress, it is obviously found that each type has their own advantages and disadvantages. For example, (i) 0D/0D or 1D/1D heterogeneous hybridization can produce good interfacial contacts, however, the self-aggregation issue is the most serious; (ii) 2D/2D heterogeneous hybridization have the best geometric compatibility and can effective prevent the self-aggregation of 2D nanosheets, however, the precise control on the layer-by-layer assembly is relatively difficult; (iii) 0D/2D, 1D/2D, or 3D/2D heterogeneous hybridization can achieve an homogeneous dispersion of different dimensional nanostructures on 2D conductive substrates, however, the binding force between these nanostructures and 2D sheet-like substrates is often weak. Second, in terms of components, quite a few selections (e.g. carbon, CNTs, graphene, metal oxides, polymers, metal NPs, etc.) are available to hybridize with cobalt oxide nanostructures. People have been persistently trying to find whether there is the most suitable heterogeneous component to achieve the most promising cobalt oxide-based material for electrochemical applications, but so far no great progress is approached. Finally, the formation of a suitable configuration of the nanocomposites to give suitable mass transport channels and exposed active sites, such as core-shell, layer-on-layer, and hierarchical porous structures, is also recognized as another grand challenge. The currently designed configurations for cobalt oxide-based nanomaterials all present their own pros and cons. For example, the coating of carbon species onto cobalt oxides or the encapsulation of cobalt oxide into carbon matrices, which can effectively enhance the electrical conductivity and accommodate the volume expansion/extraction of cobalt oxide nanostructures, however, significantly resist the infiltration the wetting of electrolytes inside the cobalt oxide nanostructures. Therefore, more systematically investigations are needed to achieve an optimum design for cobalt oxide-based hybrids to meet the various requirements to achieve outstanding electrochemical properties for energy-related devices. The third challenge is from the side reactions in the complex practical conditions of the electrochemical energy devices. Much theoretical effort is still urgently needed to reveal the overall reaction pathways of cobalt oxides for ion storage and electrocatalysis. In the practical devices, the electrochemical reactions are much more complicated and can be affected by various factors, including the influences from the electrodes, electrolytes, and separators, and some side reactions. For example, the electrochemical reaction pathways in the electrolytes with different components, PH values, additives, etc. are strongly varied. It is therefore highly recommended that the electrochemical performances of the materials for practical applications should be evaluated in entire devices instead of those exanimated in half cells with sole working electrode in laboratory. Furthermore, the formation of cobalt oxide-based hybrids will make the reactions more complex. Further studies focusing on the actual mechanisms in practical devices are highly demanded to get a clear idea on some specific contributions from the constituent components and the interfaces of the electrochemical energy devices. 657

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Besides the above major challenges, there are some other concerns in regards of the practical application of this family of materials, such as abundance, toxicity, and possible environment issues. The abundance of cobalt in the crust ranks around 32nd among all elements, which is very close to lithium, yttrium, neodymium, etc. The overall reservation of cobalt is therefore actually not too rare in the Earth's crust. The distribution, however, is quite uneven. Over half of the world’s cobalt reserves are in Democratic Republic of Congo, which greatly impede the development of cobalt-related industries in other countries. In recent years, driven by the increasing demands in electrochemical energy devices, the price of cobalt in the international metal markets maintains an upward trend. The introduction of cheap metals or metal oxides to form hybrid nanostructures with even superior electrochemical performance should be an effective way to reduce the consumption of cobalt. On the other hands, cobalt oxide and their composites have some negative effects on humans and environment. IARC has classified cobalt and cobalt compounds as Group 2B carcinogens: possibly carcinogenic to humans. ACGIH has classified cobalt and cobalt compounds as Group A3 carcinogens: proven carcinogenic to animals. It has been reported that the maximum tolerated dose (MTD) of CoO is higher than 2000 mg kg−1, which can be classified as a low toxic chemical substance. A high concentration of cobalt oxides, however, has the risk to cause an oxidative stress in human cells, inflammatory response, and even DNA damage, as evidenced by biological toxicity tests. Therefore, during the utilization of cobalt oxide-based materials, the disposal and recycling of cobalt-containing waste should be considered properly to avoid any potential biological and environmental risks. 5.2. Strategies for improving electrochemical properties For addressing the encountered challenges of cobalt oxide nanomaterials for electrochemical energy applications, we intend to propose some effective strategies based on the recent publications. The first strategy is nanostructure engineering, by which at least one of the dimensionalities is less than 100 nm, to enlarge the exposed surfaces, increase the mass transport paths, and maximize the active sites. In this way, the active surface of the oxides, the contact interfaces in the devices, and the mass diffusion and charge/carrier transport via the thinnest dimension can be dramatically improved, and thus improve the performance of the materials. Via proper nanostructure design, it has been found that the volume changes, restacking or agglomerations, and structural stability can be accommodated during electrochemical applications, and thus prolong the lifetime and provide better cycling performances. Some high energy facets with superior chemical and catalytic activities that cannot be obtained on common bulk materials have also been achieved by nanostructure engineering, which are also a novel pathway to enhance the electrochemical performance of cobalt oxides. Moreover, nanostructure engineering increases the chemical and structural affinity of cobalt oxides to combine with other active materials to form nanocomposites or nanohybrids, which usually exhibited a synergetic effect and present superior performance to their physical mixtures. The second strategy is to address the biggest shortcoming of cobalt oxides, the low electrical conductivity, by incorporating with some highly conductive and/or chemical active components to form nanocomposites or nanohybrids. Via the hybridization with amorphous carbon, CNTs, graphene, metal NPs, conductive polymers, and heterogeneous doping, and multiple components. As demonstrated in the examples summarized in this review, the electrical properties of the cobalt oxide nanocomposites and nanohybrids presented significant improvement, which facilitate charge/carrier transport through the materials, and as a result, considerably enhance the specific reversible capacity/capacitance, rate capability, cycling stability, and catalytic activity in the electrochemical energy applications. The introduction of the secondary active materials has also the effect to separate some easily agglomerated cobalt oxide nanostructures and increase the dispersibility of both the oxide nanostructures and the active materials, and thus accommodate the volume variation and boost the structural stability during operation, which can dramatically contribute to the rate and cycling performances of the energy devices. The nanocomposites and nanohybrids have provided better catalytic activities for their increased active sites and internal stresses. Owing to that the interfacial coupling and the contact between cobalt oxides and the secondary active materials are very critical for the electrochemical performance of the nanocomposites, rational design of the morphology, the interfacial chemical bonds, and the functional groups on their surfaces is an important method to further enhance the performance of the materials. 5.3. Structure-property relationships of cobalt oxide nanomaterials Structure-property relationships are a central theme for material research. Different types of structures may bring about some distinctive properties, such as the differences between graphite and graphene. Hence, understanding on the structure-property relationships is critical to get better use of materials. In this review, it is concluded that some factors, such as crystal structure, dimensionality, and composition, performed significant impact on the electrochemical properties of cobalt oxides and cobalt oxidebased hybrid materials. In this section, a summary of three primary categories of structure-property relationships is given in terms of the effects of crystal structure, dimensionality, and composition of cobalt oxide-based nanomaterials on their electrochemical properties. 5.3.1. Relationship between crystal structure and electrochemical performances of cobalt oxides Co3O4 has been verified as a promising candidate as high-performance electrode material or active electrocatalysts for electrochemical energy applications. It is demonstrated that the appealing electrochemical properties are highly related to the unique crystal structure of Co3O4, which results in distinct mechanisms or pathways for electrochemical reactions. In a typical Co3O4 spinel structure, there are 64 tetrahedral sites, one-eighth (8a) of which are occupied by the Co2+ cations, and 32 octahedral sites, one-half (16d) of which are occupied by the Co3+ cations, accompanied by the cubic-close-packed O2− anions (32e). In the interstitial space, 658

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there are 56 empty tetrahedral sites (8b and 48f) and 16 empty octahedral sites (16c). It should be noted that every tetrahedron shares faces with octahedron in the cubic close packing group. As electrode materials for rechargeable batteries, taking the common Li+ storage as example, there are two competing reaction pathways. Either the initial reduction of Co3O4 into CoO/Li2O composite intermediate or the formation of LixCo3O4 intermediate has been observed before the full decomposition of both intermediates into the final Co/Li2O composites upon further reduction, which is quite different from the storage mechanisms for other metal oxides, such as the insertion/deinsertion and the alloying mechanisms. As catalyst for electrochemical reactions, in spite of some controversial conclusions on the specific active sites of Co3O4 for different types of electrocatalytic reactions, the mixed-valence Co3O4 spinel has demonstrated superior electrochemical properties compared with other single-valence metal oxides. It has revealed that the catalytic activity of Co3O4 strongly depends on the exposed facets in an order of {1 0 0} < {1 1 0} < {1 1 2} < {1 1 1} and the {1 1 2} facets enclosed by Co3+ and Co2+ sites presented much superior activity to the {1 0 0} facets enclosed by only Co2+ sites for both ORR and OER. Moreover, it has been demonstrated that the electrochemical properties can be further enhanced by targeted metal substitution of Co2+ with other guest cations, such as Ni and Mn. 5.3.2. Relationship of dimensionality-electrochemical properties of cobalt oxides Based on the review on cobalt oxides with different dimensionalities, it can be concluded that the dimensionality has distinct influences on their electrochemical properties for energy-related devices. A summary is given as follows on the pros and cons of different dimensionality on the electrochemical properties. (i) 0D nanostructures feature unique confinement effects in three directions, but they are easy to restack into large aggregates or bulks during the sample drying process and the electrode assembly step, which greatly reduces the effective active sites and compromises the merits of 0D nanostructures. (ii) 1D nanostructures, especially these arrays grown on the conductive substrates, can offer efficient transport pathways as binder-free electrode materials. The structural deformation, however, usually occurs easily during the cycling processes. (iii) The highly exposed surface area of 2D nanostructures makes them appealing for electrochemical reactions, whereas the strong restacking towards thick layered structures is a major barrier. Moreover, 2D nanosheets with large lateral size are not favorite for the diffusion and mass transport through their thickness dimension and lower the activity of the materials. (v) 3D structures with hierarchical and porous morphologies are attractive, due to the excellent structural stability, high surface area for the good contact with electrolytes, and outstanding buffering capacity for volume changes during the long-time cycles. Nevertheless, the synthetic procedures are always complex and the interfaces between the active materials and the current collectors need further optimization. 5.3.3. Relationship of composition-electrochemical properties of cobalt oxide-based hybrids Compared with cobalt oxide nanostructures, cobalt oxide-based nanocomposites with heterogeneous structure-complementary components have been verified as excellent electrode materials with enhanced electrochemical properties. By the introduction of different types of components, some issues for the pristine cobalt oxide nanostructures can be effective addressed. A summary on the advantages and disadvantages of each cobalt oxide-based category is given as follows: (i) cobalt oxide/carbon hybrids are the most widely studied hybrids. This type of hybridization is easily achievable for its versatile cobalt precursors and carbon sources. An optimum quantitative ratio of the cobalt oxides and the carbon species, however, has not confirmed yet. (ii) Cobalt oxide/CNTs hybrids, especially the common 0D cobalt oxides hybridized with 1D CNTs, can possess enhanced overall electrical conductivity and partly prevent the self-aggregation of cobalt oxide NPs. However, the chemically inert feature of the p-CNTs is a major barrier for a uniform dispersion of cobalt oxide NPs on the CNTs. (iii) Cobalt oxide/graphene hybrids have received a great deal of attention in recent years, and the heteroatom-doped graphene has been confirmed as an excellent conductive candidate to improve the electrochemical properties of cobalt oxide nanostructures with different dimensionalities. However, the current high-cost preparation technique for graphene will greatly hinder the practical application of cobalt oxide/graphene hybrids. (iv) Cobalt oxide/metal hybrids can take the full use of the remarkable electrochemical activities of metal NPs. The inferior cycling stability and the possible use of noble metals, unfortunately, make this type of hybrids less attractive. (v) Cobalt oxide/metal oxides heterogeneous hybrids usually exhibit some synergetic effects. This type of hybrids, however, share the common disadvantage of low electrical conductivity of metal oxides. (vi) Cobalt oxide/conductive polymers hybrids can bring forward more active redox reactions and good conductive networks. The mechanical degradation of polymers is always an issue and often leads to a quick decay of electrochemical performances upon repeated cycling. 5.4. Future outlooks Great progress has been achieved for the design and synthesis of cobalt oxides as well as their hybrids for electrochemical energy devices, especially for rechargeable batteries, supercapacitors, and electrocatalysis. In spite of some major challenges, this family of metal oxides is still unbeatable for the applications in electrochemical energy devices for the salient chemical and physical properties. It is believed that, with further study on the underlying electrochemical reaction mechanisms, some effective solutions to address the current challenges will be found and the way towards practical application of this family of materials will be paved together with the innovation of novel electrochemical techniques and advanced energy devices. Meanwhile, theoretical calculations and simulations will also assist the design of novel cobalt oxide-based materials to meet the practical requirements for electrochemical applications. The reveal of the structure-property relationships of cobalt oxide-based materials for electrochemical applications will significantly promote the development of material science, nanoscience and nanotechnology, and energy-related technology. 659

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Acknowledgements This work was partly supported by an Australian Research Council (ARC) Future Fellowship project (FT180100387) for ZS, an ARC Future Fellowship project (FT160100281) for TL, and an ARC Discovery Project (DP160102627). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47]

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Dr. Ziqi Sun is an Associate Professor at the School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Australia. He received his PhD in Materials Science and Engineering in 2009 at Institute of Metal Research, Chinese Academy of Sciences. After finishing his NIMS postdoctoral fellowship (Japan) that he held for one year on solid oxide fuel cells, he joined the University of Wollongong (UOW) and then moved to Queensland University of Technology (QUT) in 2015. He was awarded with some prestigious awards and fellowships, including ARC Future Fellowship (2018), JSPS Invitational Fellow by Japan Society for the Promotion of Science (2019), TMS Young Leaders Award from the Minerals, Metals and Materials Society, USA (2015), Discovery Early Career Research Award from Australian Research Council (2015), Australian Postdoctoral Fellowship from Australian Research Council (2010), Alexander von Humboldt Fellowship from AvH Foundation Germany (2009), the Vice-Chancellor’s Research Fellowship from University of Wollongong (2013), etc. He also received some honorary positions, such as Chair of TMS Energy Committee, Editor of Sustainable Materials & Technologies (Elsevier), Principal Editor of Journal of Materials Research (SCI, MRS), Associate Editor of Surface Innovations (SCI, ICE), Editorial Board Member of Journal of Materials Science and Technology (SCI, Elsevier) and Scientific Reports (SCI, Nature Group). He held the roles as symposium organizers and session chairs in the prestigious conferences such as AM&ST18, TMS conference, AcerS annual conferences, etc.

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