Cobalt-doped zinc manganese oxide porous nanocubes with controlled morphology as positive electrode for hybrid supercapacitors

Chemical Engineering Journal 361 (2019) 1030–1042

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Cobalt-doped zinc manganese oxide porous nanocubes with controlled morphology as positive electrode for hybrid supercapacitors Sk. Khaja Hussain, Jae Su Yu

T

⁎

Department of Electronic Engineering, Institute for Wearable Convergence Electronics, Kyung Hee University, Yongin-si, Gyeonggi-do 17104, Republic of Korea

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

ZnMn O (ZMO) or Co-doped ZMO • porous nanocubes (PNCs) were pre2

4

pared by a solvothermal method.

NH F played a vital role in the growth • process of the nanocubes. ZMO PNCs greatly enhanced • Co-doped the electrochemical performance. value of the ZMO:5Co electrode • isThe4 Ctimes higher compared to the 4

s

pristine ZMO.

ZMO:5Co-based HSC delivered • The higher energy and power densities.

A R T I C LE I N FO

A B S T R A C T

Keywords: Zinc manganese oxide Hybrid supercapacitors Porous nanocubes High-energy density

Rationally designed porous structured electrode materials have attracted significant potential interest in hybrid supercapacitors owing to their more predominate surface area and endow the superior energy storage capability. The synthesis strategy of these multifunctional porous structures is more desirable and still inferior. Herein, we synthesized novel ZnMn2O4 (ZMO) or cobalt (Co)-doped ZMO porous nanocubes (PNCs) by a facile solvothermal method, followed by calcination in air. The NH4F played an important role as a template in the formation of nanocubes morphology and detailed growth process was investigated. Impressively, with the incorporation of different molar concentrations of Co ions into pristine ZMO, the electrochemical performance was enhanced and the capacitance values were significantly increased due to the porosity and multi-metal ions synergistic effect. The pristine ZMO and the optimized Co-doped ZMO PNCs exhibited maximum specific capacitance values of ∼267 and ∼1196 F g−1, respectively, at 1 A g−1 of current density. The optimized ZMO:5Co PNCs electrode exhibited more than 4 times in its specific capacitance value with respect to the pristine ZMO PNCs at a constant current density (1 A g−1), and it also showed excellent cycling performance (∼85.5%) at higher current density (7 A g−1). Furthermore, a hybrid supercapacitor (HSC) device was made by utilizing ZMO:5Co PNCs (positive) and activated carbon (negative) electrodes, exhibiting a maximum specific capacitance of ∼68 F g−1 at 1 A g−1 of current density and a high energy density of 27.38 W h kg−1 at a high power density of 1059 W kg−1 within a potential window of 1.45 V. The HSC device also showed excellent cycling stability with ∼80.5% of capacitance retention after performing the 4000 cycles.

⁎

Corresponding author. E-mail address: [email protected] (J. Su Yu).

https://doi.org/10.1016/j.cej.2018.12.152 Received 19 October 2018; Received in revised form 15 December 2018; Accepted 27 December 2018 Available online 28 December 2018 1385-8947/ © 2018 Elsevier B.V. All rights reserved.

Chemical Engineering Journal 361 (2019) 1030–1042

S. Khaja Hussain, J. Su Yu

1. Introduction

[39–42] and it is also a positive electrode material in supercapacitors to store the energy [43–45]. Furthermore, limited electrochemical reports were found in the field of supercapacitors for the excellent featured ZnMn2O4 composite as an electrode material. Accordingly, we considered the ZnMn2O4 composite as a pristine material to study the electrochemical properties for energy storage applications. Typically, the electrochemical performance of electrode material depends on the physical properties such as surface area and mechanical stability. Furthermore, the performance was improved by tailoring the nano/microstructure morphology and its porosity, and this has been proven by several research studies [46–49]. Especially, the porous nanostructured morphologies often increase the electrochemical activity of the metal oxide electrode materials, resulting in enhanced electrochemical properties with good specific capacitance and better cycling life-time [48]. At higher current densities, these porous nanostructured materials possess volume expansion with good void space, which helps the diffusion of the electrolyte ions easily at the electrode/electrolyte interface in the continuous cycling process, enabling the good structure and cycling stability. In this work, we rationally designed an easy and costeffective way for preparing the porous ZnMn2O4 or Co ions doped ZnMn2O4 nanocubes by a facile solvothermal method. The synthesized ZnMn2O4 or Co ions doped ZnMn2O4 porous nanocubes were further used as an electrode material for HSCs. Furthermore, to evaluate the practical functionality, a HSC device was made by assembling the optimized Co-doped ZnMn2O4 electrode materials as a cathode and carbon electrode as an anode material.

Nowadays, renewable energy sources have been great and increasing concerns for personalized energy technologies owing to the limited sources from fossil fuels in environments [1–7]. The portable and various electronic devices such as smartphones, flashlights, laptops, cameras, memory backup-power systems, etc. are expected to be a clean and new class of devices for renewable energy sources [4,8–15]. High power density and as well as high energy density plays a key role in the rapid growth process of portable electronic devices for the conventional energy technology [4,16,17]. Usually, supercapacitors and batteries are being implemented to increase both the energy and power densities. Supercapacitors possess several benefits such as quick charge/discharge times, easy operation mode, and enhancement of cyclic life-time. However, they still exhibit poor energy density and severely limit their potential for widespread practical applications [18–20]. On the other hand, batteries also produce chemical energy storage owing to their self-discharge and good cyclic performance, but a mismatch problem arises in their excellent performance due to the low specific capacity of anode materials [21,22]. In this regard, development of the electrochemical materials with such high energy and high power densities is an essential and challenging study. In general, hybrid supercapacitors (HSCs) which are a combination of battery-type/pseudocapacitive Faradaic electrode and non-Faradic electric double layered capacitive electrode could achieve the higher energy and power densities due to their hybridization, and further they can enhance the operating voltage to achieve the high-performance hybrid supercapacitors [23,24]. In HSCs, a battery-type Faradaic electrode (positive electrode) has been fabricated from lithium intercalated or transition metal (TM) related compounds. On the other side, the capacitive electrode (negative electrode) materials are based on graphene or activated carbons (ACs) [23,25]. Thus, HSCs are actively pursuing the desirable energy and power densities with assured dual characteristics of the batteries and supercapacitors for practical applications. Generally, TM oxides are the common battery-type or pseudocapacitive materials and have been extensively applied in their respective energy storage fields due to their low cost and multiple oxidation states [26–30]. Recently, mixed or bimetallic TM oxide composites have received a significant attention to show better capacitance performance as compared with the monometallic composites in aqueous electrolyte [21,27]. More importantly, these mixed or bimetallic TM oxide composites serve as the best electrode materials for high power applications. Rational engineering of metal cations doping into the TM ions as an electrode material is very important to improve the electrochemical performance as an anode material in batteries and next-generation supercapacitors [31–34]. Up to date, there have been many research reports on metal ions incorporated into various TM oxide host lattices (Cu-doped Mn2O3, [35] Mn-doped V2O5, [23] cobalt (Co)-doped ZnSnO4 [27] and Mn-doped Zn2GeO4 [22]) with improved electrochemical performance and double or triple times enhanced specific capacities for battery applications. In the above battery studies, the incorporation of metal cations not only enhanced the specific capacities of the TM oxide electrode materials, but also increased the cycle performance and maintained the good rate capability due to the multimetal synergistic effect. Thus, it is believed that the diverse metal cations incorporated into desirable TM oxide composites offer a new pathway to design HSC electrode materials with improved energy density for energy storage device applications. The spinal-like structure of bimetallic TM oxides having the general composite formula of AX2O4 (A, X = Mn, Cu, Co, Fe, Ni, Zn, etc.) has attracted much attention as anode materials for high-performance lithium ion batteries (LIBs) [36–38]. Among the AX2O4 materials, particularly, the Mn-based ZnMn2O4 composite is eco-friendly, non-toxic, low-cost, and naturally abundant compared with other battery-type materials like NiCo2O4. Thus, the advantages of the spinal ZnMn2O4 composite are being a good novel anode electrode material in LIBs

2. Experimental procedure 2.1. Materials In a typical synthesis process, zinc acetate dihydrate (Zn (CH3COO)2·2H2O, ≥99.9%), manganese acetate tetrahydrate (Mn (CH3COO)2·4H2O, ≥99.9%), cobalt acetate tetrahydrate ((CH3COO)2Co·4H2O, ≥99.9%), ammonium fluoride (NH4F, ≥99.9%), isopropyl alcohol (IPA, (CH3)2CHOH, ≥99.7%) and hexamethylenetetramine (HMTA, C6H12N4, ≥99.9%) were purchased from Sigma Aldrich Co., South Korea. Nickel (Ni) foam was obtained from MTI Corporation, South Korea. Potassium hydroxide (KOH, ≥85%), N-methyl-2-pyrrolidone (NMP, C5H9NO, ≥99.9%) and polyvinylidene fluoride (PVdF, –(C2H2F2)n–) were purchased from Daejung Chemicals Ltd., South Korea. The above all the chemicals were of analytical grade purity and utilized as received without any further purification except the Ni foam. The resistivity of distilled (DI) water was 18.3 MΩ, produced from a Milli-Q water purifier. 2.2. Synthesis The ZnMn2O4 porous nanocubes (ZMO PNCs) or Co ions doped ZMO porous nanocubes (ZMO:Co PNCs) were prepared by a facile one-pot solvothermal method. Initially, the stoichiometric amounts of Zn (CH3COO)2·2H2O (1 mmol) and Mn(CH3COO)2·4H2O (2 mmol) were added into a 50 mL of DI water. After 10 min, 8 mmol of HMTA and desired molar concentrations (0, 5, 10 and 15 mmol) of NH4F were added to the above aqueous solution and it was ultra-sonicated for 5 min. The reaction solution was stirred at 500 rpm for about 30 min to attain the complete dissolution of all the reactants. To this mixture, 30 mL of IPA was added dropwise and the magnetic stirring was continued at room temperature to the final resultant solution for about 1 h. The homogenously mixed reaction solution was kept into a 100 mL volume Teflon-lined autoclave and then heated at 160 °C for 12 h. After completion of the progressive reaction conditions, the autoclave was cooled to room temperature and the precursor solution was centrifuged at 3000 rpm for 5 min and washed with DI water and ethanol, respectively, to remove the possible residual salts during the preparation process. A transparent hydroxyl form of ZnMn2(OH)6 powder sample 1031

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was obtained after drying the precipitate in a convection oven at 110 °C for 12 h. Later, the prepared ZnMn2(OH)6 powder sample was calcined at 450 °C for 3 h in air (raising temperature of 2 °C min−1) to enhance the crystallinity of the ZMO for further physiochemical and electrochemical characterizations. In the above process, the ZMO PNCs were obtained with 10 mmol NH4F addition. To prepare the ZMO:Co PNCs, the above synthesis procedure was repeated by doping or introducing the stoichiometric amounts of different Co concentrations (x = 1, 3, 5 and 7 mol%) by using (CH3COO)2Co·4H2O as the Co source into the (1x) mmol Zn(CH3COO)2·2H2O in the reaction solution [Zn(1-x)Mn2O4: xCo (x = 1, 3, 5 and 7 mol%)]. The Co-doped (1, 3, 5 and 7 mol%) ZMO samples were designed as ZMO:1Co, ZMO:3Co, ZMO:5Co and ZMO:7Co.

the practical application point of view, a hybrid supercapacitor (HSC) was assembled using an AC as the negative (anode) electrode and ZMO:5Co as the positive (cathode) electrode in aqueous 1 M KOH electrolyte solution. Before the assembly, the both positive and negative electrodes were soaked in alkaline aqueous electrolyte solution for about 1 h. A pouch-type HSC device was fabricated using an aluminum covered film which consists of sufficient amount of KOH electrolyte solution and the positive and negative electrodes were separated by a cellulose filter paper in order to avoid the short-circuit in between them. To achieve the better energy storage performance, the charge balance in between the two electrode materials was optimized. In this manner, the mass ratio of the positive and negative electrodes for HSC device was calculated by the following mass balancing equation: [48,50]

2.3. Physiochemical characterization

m+/ m− = (Cs− × ΔV −)/(Cs+ × ΔV +)

The synthesized ZMO and Co ions doped ZMO powder samples were characterized by using a field-emission scanning electron microscope (FE-SEM; JEOL JSM-6700) and a field-emission transition electron microscope (FE-TEM; JEM-2100F, JEOL). The energy dispersive X-ray spectroscopy (EDS) and elemental mapping analysis of the materials were measured via a high-resolution FE-SEM (HR FE-SEM; CARL ZEISS, Merlin) with the X-ray spectrometer. A Thermo Nicolet – 5700 Fourier transform infrared (FTIR) spectrometer was used to characterize the FTIR spectra with KBr pellet method, to identify the existed functional groups. The X-ray photoelectron microscopy (XPS; Thermo Multi-Lab 2000 System) was employed to identify the chemical composition and oxidation states of the elements. The surface area and the pore size distribution of the samples were analyzed by using the BrunauerEmmett-Teller (BET) and BELSORP-max00131.

ΔV and m are the specific capacitance, In the above equation, potential window and mass of the positive electrode material, respectively. Similarly, Cs−, ΔV− and m− are the specific capacitance, potential window and mass of the negative electrode material, respectively. Using Eq. (1), the mass ratio in between the positive and negative electrode materials was optimized to be 0.73. Cs+,

+

(1) +

3. Results and discussion To derive the growth process of the ZMO PNCs, the NH4F concentration-dependent experiments were carried out and the obtained FE-SEM images are depicted in Fig. S1. The FE-SEM images clearly show a closer inspection of the morphological properties of the prepared hydroxyl form of ZnMn2(OH)6 powder samples at the fixed amount of HMTA and varied molar concentrations of NH4F, i.e., 0, 5, 10 and 15 mmol. The reaming all the solvothermal conditions were maintained constant. Fig. S1(a) and (b) shows the low- and high-magnification FE-SEM images of the sample without addition of NH4F. If there was no addition of NH4F, the prepared ZnMn2(OH)6 exhibited the various nucleus centers with aggregated redouble nanoparticles. It is well known that when HMTA was hydrolyzed, it can produce OH− and NH4+ ions at elevated temperatures [34,51]. The nanoparticles were obtained mainly due to the liberation of NH4+ ions from the HMTA in the reaction media. The revealed nanoparticles consist of higher active sites with nucleation centers, and it is necessary to add another capping agent to attain a well-defined or desirable morphology at controlled experiments. When 5 mmol NH4F was added to the reaction solution, heterogeneous morphologies of nanoparticles and nanocubes with different particle sizes were noticed (Fig. S1(c) and (d)). The added NH4F to reaction system reduced the interfacial energy of nanoparticles, and further the nanoparticles tend to aggregate themselves and later combined each, thus creating ZnMn2(OH)6 nanocubes. The high-magnification FE-SEM image of Fig. S1(d) shows the moderately formed nanocubes with the combination of nanoparticles via Ostwald’s ripening phenomenon. If the molar concentration was increased to 10 mmol, the growth process of nanocubes was elongated, all the nanoparticles were completely converted into uniform shapes of ZnMn2(OH)6 nanocubes due to dominant Ostwald’s ripening process and the average length of the cubes was approximately 500–600 nm. The morphology of the obtained nanocubes can be observed in the low- and high-magnification FE-SEM images of Fig. S1(e) and (f). However, when 15 mmol NH4F was supplied, the edges of nanocubes were truncated, revealing indistinct morphology (Fig. S1(g) and (h)). Thus, the whole growth process of ZnMn2(OH)6 nanocubes involved nucleation, aggregation and Ostwald’s ripping process by tuning the concentration of NH4F under mild solvothermal conditions. In the growth study, the added NH4F in the reaction solution played a vital role. As per the basic literatures and previous reports, [52,53] NH4F can be easily decomposed and produce NH4+ ions and HF in a hot aqueous solution. The possible chemical equations are as follows:

2.4. Preparation of electrode materials and electrochemical measurements The preparation process of working electrodes was carried out by a traditional slurry coating technique. At first, a piece of Ni foam sheet was cleaned using 1 M HCl to remove the oxidation layer from the surface of Ni foam. After cleaning with HCl, the Ni foam was washed with ethanol and DI water several times and it was dried under a flow of N2 gas stream. The dried Ni foam was cut into the size of 1 × 2 cm2 and used as a current collector. In brief, 80 wt% of the ZMO or Zn(1−x)Mn2O4: xCo (x = 1, 3, 5 and 7 mol%) active materials, 10 wt% PVdF binder and 10 wt% conductive carbon black (super P) were grinded well for 15 min with help of the agate mortar and sufficient drops of NMP solvent were added to make a well-dispersed electrode slurry. The electrode slurry was uniformly pasted on the 1 × 1 cm2 active area of Ni foam (1 × 2 cm2) and it was dried in a vacuum oven at 80 °C for 4 h. The electrode slurry coated on the Ni foam current collector was used as a working electrode, and the Ag/AgCl and Pt wire were considered as the reference and counter electrodes, respectively in a traditional three electrode system. The instantly prepared 1 M KOH electrolyte having the pH of ∼13.8 was employed as an electrolyte solution to investigate the electrochemical characteristics. At room temperature, the electrochemical measurements such as cyclic voltammetry (CV), galvanic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) having the frequency range of 0.01 Hz to 100 kHz at 10 mV of amplitude were performed for the above designed ZMO and Co ions doped ZMO positive electrode materials in 1 M KOH electrolyte solution via IviumStat electrochemical workstation (IVIUM Technologies). The masses of the ZMO and Co ions doped ZMO active materials loaded on each electrode were estimated to be 1.2–1.4 mg, to calculate the specific capacitance values in a beaker type three-electrode system. 2.5. Preparation of an aqueous hybrid supercapacitor device To evaluate the possibility of the optimized ZMO:5Co electrode for 1032

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(2)

NH 4 F+ H2 O→ NH3·H2 O+ HF NH3 + H2 O→ NH3·H2 O⇔

NH+4

+ OH−

The proposed mechanism for the formation of ZMO PNCs or CoZMO PNCs can be observed in Scheme 1. Fig. 1(a) and (b) shows the morphological FE-SEM images of the ZMO product after calcination. From the low-magnification images of Fig. 1(a), the ZMO product retained the uniform ZnMn2(OH)6 nanocube morphology with similar sizes. During the calcination, the decomposition of the ZnMn2(OH)6 precursor nanocubes led to the continuous growth of the porous ZMO nanocubes, as can be seen in the high-magnification FE-SEM image (Fig. 1(b)). It is clear that the uniformly obtained porous nature of ZMO NCs after calcination offers larger surface area and provides superior crystallinity for the rapid transportation of electrons and ions. The morphology of the synthesized Co-doped ZMO product was also investigated as depicted in Fig. 1(c) and (d). The low-magnification SEM image (Fig. 1(c)) of the optimized ZMO:5Co sample revealed the wellmaintained nanocube morphology with increased porosity as compared with the pristine ZMO PNCs. The high-magnification FE-SEM image (Fig. 1(d)) clearly displayed the porous nature of the ZMO:5Co single nanocube. The possible reason for the increased porosity is mainly due to the incorporation/addition of the Co ions from the cobalt acetate to the aqueous solution in Eq. (4). After addition of cobalt acetate, the liberation of the acetate acid became enhanced in the system and further complexation with alcohol is also increased at desirable solvothermal conditions, which further accompanies the growth of the hydroxyl Co-doped ZnMn2(OH)6 nanocubes. In the calcination process, there is a gradual conversion of Co-doped ZnMn2(OH)6 nanocubes into more porous form of ZMO:Co nanocubes. Consequently, with the doping of Co ions into the ZMO PNCs, the porosity was enhanced, and it was further confirmed by performing the surface analysis measurement. The crystallinity of the synthesized pristine ZMO and Co incorporated PNCs is the primary assert to confirm their phases. Fig. 1(e) shows the XRD patterns of the pristine ZMO and ZMO:1Co, ZMO:3Co, ZMO:5Co and ZMO:7Co PNCs. As shown in Fig. 1(e), the major XRD diffraction peaks of the ZMO PNCs were well indexed with the tetragonal phase (JCPDS card # 77-0470) with a space group of 141/amd (141)). The predominant diffraction peaks at the 2θ degree positions of 29.3, 31.2, 33.0, 36.3 and 60.7° corresponded to the (1 1 2), (2 0 0),

(3)

The Zn, Mn, and Co metal precursors in aqueous solution are not stable due to the existing acidic environment. From the above Eq. (3), the generation of the NH4+ ions controlled the pH nature of the reactants and reduced the acidic nature in the growth solution. By increasing the concentration of NH4F, the generation of NH4+ ions became more, the acidic nature was gradually decreased and the alkali environment got more dominant in the growth solution[52], and it is further favorable to grow the uniform ZnMn2(OH)6 nanocubes. When the transition metal acetates were dissolved in aqueous solution, the possible exploratory reactions are as follows:

Zn(CH3 COO)2 + 2Mn(CH3 COO) 2 + 6H2 O→ ZnMn2 (OH)6 + 6CH3 COOH

(4)

In the above proposed chemical equation, zinc acetate and manganese acetate reacted with water molecules, forming ZnMn2(OH)6 nuclei and liberated acetic acid. As per the experimental, the obtained products further reacted with the alcoholic system (IPA) and proceeded the solvothermal conditions. At the supplied facile conditions, a complex formation in between released acetic acid from the aqueous system and alcohol [50] was initiated, which further escorts the formation of ZnMn2(OH)6 nuclei. On the other hand, the generation of the OH− ions from the NH4F becomes more, so it immediately offers the growth of the ZnMn2(OH)6 nanocubes in the dominant basic environment. It is a well-accepted fact that the desirable and distinct nano or micro structured related morphologies are strongly favorable to exhibit better electrochemical properties. In this mode, the uniformly obtained ZnMn2(OH)6 nanocubes at 10 mmol of NH4F were considered as an optimal sample and promoted for the calcination process. Accordingly, the obtained ZnMn2(OH)6 nanocubes were calcined in air atmosphere and were transformed into the crystallized uniform ZMO product:

ZnMn2 (OH)6 +

Δ 1 O2 → ZnMn2 O4 + 3H2 O 6

(5)

Scheme 1. Schematic diagram for the formation process of ZMO and Co-doped ZMO PNCs. 1033

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Fig. 1. Low- and high-magnification FE-SEM images for (a and b) ZMO PNCs, (c and d) ZMO:5Co PNCs. (e) XRD patterns of the pristine ZMO and ZMO:1Co ZMO:3Co ZMO:5Co ZMO:7Co PNCs after calcination. (f) Crystal structure of pristine ZMO PNCs. (h) Nitrogen adsorption-desorption isotherms of the pristine ZMO and optimized ZMO:5Co PNCs. The inset of (h) represents their corresponding pore size distribution.

assigned to the stretching vibrations of the octahedral Mn3+ ions [45]. The O–H stretching and bending vibration bands of the hydroxyl groups at 3365 and 1629 cm−1 are very weak owing to the calcination, which further supports to the good crystallinity of the prepared ZMO and ZMO:5Co PNCs. The specific surface area of the ZMO and optimized Codoped ZMO PNCs and their pore distribution were analyzed by BET and Barrett-Joyner-Halenda (BJH) methods, respectively and the obtained results can be seen in Fig. 1(h). From the BET results, the specific surface area (asBET) for the ZMO and ZMO:5Co samples were 16.34 and 37.54 m2 g−1, respectively. It is clear that the Co-doped ZMO sample exhibited higher BET surface area than the pristine ZMO. This is ascribed to the more complexation of the transition metal ions with the IPA solvent in the composite in a solvothermal system. A type-IV isotherm shape for the pristine and Co-doped samples showed a hysteris loop, which further indicates that the samples were in porous nature. Furthermore, the corresponding average pore volume for the pristine ZMO and ZMO:5Co PNCs were 0.1361 and 0.2118 cm3 g−1 at 5 nm of pore size, respectively, as shown in the inset of Fig. 1(h). Hence, the obtained higher surface area and many existing pores can be favored to enhance the electrochemical capacitances. To gain a better understanding of the existing chemical composition and their surface electronic states, XPS is one of the most significant analyses. In order to perform the XPS measurement, the optimized ZMO:5Co powder sample was dissolved in aqueous solution and further applied on silica substrate. The obtained results after measurement are depicted in Fig. 2. As shown in Fig. 2(a), the XPS survey scan spectrum exhibited the presence of the Zn, Mn and O elements, and their corresponding binding energies values were corrected by referencing to C 1 s (283.5 eV). The detailed existed oxidation states in the chemical

(1 0 3), (2 1 1) and (2 0 2) tetragonal crystal planes, respectively. The XRD patterns of the Co-doped ZMO PNCs are also in good agreement with the standard PCPDF card of the pristine ZMO PNCs and a possible impurity peak at 23.1° was identified with low intensity which is related to the diffraction peak of Mn2O3 (JCPDS # 89-4836). However, the existing diffraction peak of the Mn2O3 was not taken into account due to its low intensity and all the samples exhibited major diffraction peaks of ZMO PNCs tetragonal phase, indicating the good crystal purity of the synthesized samples. It is also noticeable that the diffraction peaks at the 2θ degree positions of 33.0° (1 0 3) and 36.3° (2 1 1) shifted towards lower angle sides with increasing the concentration of Co ions as shown in Fig. S2. The shifted diffraction peak positions confirmed that the Co ions were completely incorporated into ZMO PNCs phase. The shift of diffraction peaks also can be observed in the earlier reports such as Co-doped Zn2GeO4, Mn doped Zn2GeO4 compounds, etc., indicating that the obtained XRD results are in good agreement with the reported studies [31,54]. An ideal crystal structure was drawn for the pristine ZMO PNCs using a Diamond software as shown in Fig. 1(f). The ZMO PNCs crystal structure belonged to the tetragonal system with the corresponding space group of 141/amd (141) and exhibited a bodycentered unit lattice system. From the crystal structure, it is clear that Mn3+ and Zn2+ ions covered the centers of the stacked octahedral and tetrahedral sites and the O2− ions were distributed at the corners of the octahedron (Mn3+) and tetrahedron (Zn2+) [37,55]. It is obvious that the Zn2+ ions replaced the bivalent Mn3+ ions in comparison with the Mn3O4. The structural properties of the ZMO and optimized ZMO:5Co PNCs were further characterized by FTIR analysis as depicted in Fig. 1(g). The recorded FTIR spectra for the pristine and Co-doped ZMO PNCs showed predominant peaks at 517 and 617 cm−1 which are 1034

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Fig. 2. (a) XPS survey scan spectrum of the ZMO:5Co PNCs and high-magnification spectra of (b) Zn 2p (c) Mn 2p (d) O 1 s and (e) Co 2p.

calcination and it clearly confirmed the regular sizes of the nanocubes and the absence of porosity. Fig. 3(a) shows the FE-TEM image of the single ZMO nanocube particle and it can be identified that the surface of the obtained ZMO nanocubes exhibited porous nature which is further shown in the high-magnification FE-TEM image (Fig. 3(b)). The lattice distance (0.1 nm) was calculated by taking the edge part of the corresponding single ZMO PNC as shown in the high-resolution TEM (HRTEM) image (Fig. 3(c)). The latticed distance of 0.1 nm was well matched with the (5 1 2) lattice plane of the ZMO tetragonal phase. The typical FE-TEM image of the ZMO:5Co PNCs was depicted in Fig. S3(b) (Supporting Information). From the low-magnification FE-TEM images (Fig. S3(b)), the pristine ZMO PNC morphology was retained with increased porosity by the incorporation of Co ions. These FE-TEM images are more consistent with the obtained ZMO:5Co FE-SEM results. The high-magnification FE-TEM image of the single ZMO:5Co PNC (Fig. 3(d)) clearly showed the more porous nature and the encircled drawn area on the FE-TEM image of the nanocube, further demonstrating the increased porosity as compared with the pristine ZMO PNCs (Fig. 3(a)). These results agree with the measured specific surface area from the BET surface analysis. Fig. 3(e) shows the HRTEM image of the

composition were mainly identified from the high-resolution XPS spectra of the Zn 2p, Mn 2P, O 1 s and Co 2p peaks. From Fig. 2(b), the bands originated at 1019.6 and 1042.6 eV (energy difference = 23 eV) belong to the Zn 2p3/2 and Zn 2p1/2 core levels, respectively, [31,37,54] suggesting the element Zn in 2+ oxidation state. The peaks at 640.3 and 652.1 eV with an energy separation of 11.8 eV in Fig. 2(c) are assigned to the Mn 2p3/2 and Mn 2p1/2 levels, respectively, which strongly confirms the element Mn in 3+ state [37]. From Fig. 2(d), the O 1S peaks displayed two peaks: one is at 528.4 eV, corresponding to the typical metal-oxygen bond and the other one exists at 530.7 eV which is related to the lattice oxygen. In addition, the high-resolution spectrum (Fig. 2(e)) showed low intensity of Co 2p peaks due to the small amount addition of Co ions and the obtained maximum binding energies were located at 778.9 eV (Co 2p3/2) and 794.5 eV (Co 2p1/2), indicating that the establishment of Co ions in 2 + oxidation state is consistent with the Co-doped compounds [34,54,56]. To determine the complete morphological features and sizes of the synthesized pristine ZMO and Co-doped ZMO PNCs, the FE-TEM analysis was further accomplished. Fig. S3(a) in Supporting Information shows the FE-TEM image of the ZnMn2(OH)6 nanocubes before 1035

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Fig. 3. (a) Low- (b) high-magnification FE-TEM images. (c) HRTEM images of the single ZMO PNC. (d) FE-TEM image, (e) HRTEM image and (f) SAED pattern of the optimized single ZMO:5Co PNC. (g) EDS spectrum and (h–k) elemental mapping images of the ZMO:5Co PNCs.

pseudocapacitive property of the prepared electrode material. This is in good agreement with the obtained redox peaks in CV results (Fig. 4(a)). In order to enhance the redox performance and discharge time of the ZMO PNCs electrode for charge storage applicability, different concentrations of conductive Co ions were incorporated into ZMO PNC electrode and their electrochemical tests were conducted at room temperature. Fig. 4(c) and (d) shows the comparative CV and GCD curves of the pristine ZMO, ZMO:1Co, ZMO:3Co, ZMO:5Co and ZMO:7Co electrode materials at a constant scan rate (20 mV s−1) and current density (1 A g−1), respectively. It is obvious that all the electrode materials under the constant scan rate in the applied potential window of 0.0–0.45 V (vs. Ag/AgCl) showed a battery-type CV behavior with a couple of strong redox peaks due to the Faradic redox reactions in between the Mn2+/Mn3+ and Co2+/Co3+. Interestingly, when the Co ion concentrations increased, the redox peaks in the CV curves shifted towards positive and negative poles within the applied potential window as compared with the pristine ZMO PNCs. The shift of the redox peaks is mainly ascribed to the possible synergistic behavior of the introduced conducive transition metal (Co) ions which can change the polarization of the electrode materials [58,59]. Thus, the shift of the redox peaks indicates that an increase of the ion diffusion rate can contribute to the favorable Faradaic redox reactions in basic electrolyte solution. The Co-doped ZMO PNCs exhibited the higher integral area under the CV curves with higher current background than the pristine ZMO PNCs, which signifies the enhancement of the electrochemical activity, and further it could be expected higher pseudocapacitive nature from all the electrode materials. The involved possible electrochemical reactions from the more conductive transition metal atoms of Mn and Co during the electrochemical measurements for the pristine ZMO and Co-doped ZMO PNCs can be described as follows:

edge of the single ZMO:5Co PNC, exhibiting the interplanar spacing of 0.16 nm, exposed with the (3 1 2) crystal facet of ZMO tetragonal phase (JCPDS card # 77-0470). The corresponding selected area electron diffraction (SAED) pattern from Fig. 3(f) revealed that a number of bright spots were in a circular manner which implies that the Co-doped ZMO PNCs were in a polycrystalline nature. The EDS spectrum of the optimized ZMO:5Co was predicted as shown in Fig. 3(g), revealing the establishment of the Zn, Mn, O and Co elements, and their corresponding weight percentages can be seen in the inset. Fig. 3(h–k) shows the EDS color mapping for the EDS layered FE-SEM image (inset of Fig. 3(g)), demonstrating the homogenous distribution of the Mn, Zn, O and Co elements in the composite. By integrating the XRD, XPS, FESEM, FE-TEM and EDS analyses, one can conclude that the Co2+ ions were successfully incorporated into the prepared ZMO PNCs and could replace the Zn2+ ions owing to near ionic radius of Co2+ and Zn2+ ions [54,56,57]. The obtained ZMO and Co ions doped ZMO PNCs were coated on Ni foam to explore the charge storage properties by a traditional three electrode configuration in 1 M KOH electrolyte (Ag/AgCl reference electrode). The CV curves at different scan rates of 5, 10, 20, 30, 40, 50, 60, 70 and 80 mV s−1 were recorded for the pristine ZMO PNCs electrode in the range of 0.0 to 0.45 V potential (Fig. 4(a)). The obtained CV curves at various scan rates exhibited a couple of redox (oxidation and reduction) peaks with non-rectangular shapes, indicating the typical battery-like behavior owing to the possible reversible Faradic redox reactions of Mn2+/Mn3+ transitions. At higher scan rates, all the CV curves well maintained the redox peaks with increased current, revealing the superb electrochemical reversibility and capacitive nature of the ZMO PNCs electrode. The GCD curves for the ZMO PNCs were measured at different current densities (1, 1.5, 2, 3, 4, 6, 8, 10 and 12 A g−1) and the obtained results are presented in Fig. 4(b). The charge/discharge curves comprehensively showed the typical asymmetric nature and good reversibility, which indicates the

ZnMn2 O4 + OH− + H2 O↔ ZnOOH + 2MnOOH + e−

MnOOH + OH− ↔ MnO2 + H2 O+ e− 1036

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Fig. 4. (a) CV and (b) GCD curves of the pristine ZMO PNCs at various scan rates and current densities. Comparative (c) CV curves at 20 mV s−1 of scan rate and (d) GCD curves at 1 A g−1 of current density for the pristine ZMO, ZMO:1Co, ZMO:3Co, ZMO:5Co and ZMO:7Co PNCs electrodes. (e) CV curves and (f) GCD curves of the optimized ZMO:5Co PNCs at various scan rates and current densities. (g) Specific capacitance at various current densities and (h) EIS spectra of the ZMO, ZMO:1Co, ZMO:3Co, ZMO:5Co and ZMO:7Co PNCs electrodes. (i) Comparative areal and specific capacitances of the ZMO:5Co PNCs. (j) Cycling stability of the pristine ZMO and ZMO:5Co PNCs electrodes and (k) schematic for diffusion of electrolyte ions for single ZMO:5Co PNC.

CoO ↔ CoOOH + e−

electrode, but it was decreased for the ZMO:7Co electrode. At higher doping concentration of Co2+ ions (7 mol%), the ZMO PNCs lattice structure was distorted more due to the overfeeding of Co content. As a result, the discharge time could be decreased for the ZMO:7Co electrode. From the obtained comparative GCD analysis, the discharge time was evidently higher for the ZMO:5Co PNCs electrode, immediately authenticating the high specific capacitance performance, which further gives a consistency with the comparative CV results in Fig. 4(c). The enhancement of the discharge time in the electrode materials is mainly ascribed to the conductive nature of incorporated Co species which can stimulate the electroactive binary electrode system and the

CoOOH + OH− ↔ CoO2 + H2 O+ e− It is also noteworthy to check the discharge time for the ZMO:1Co, ZMO:3Co, ZMO:5Co and ZMO:7Co electrodes with respect to the pristine ZMO PNCs electrode. All the GCD curves showed very similar shapes and their discharge profiles deviated mainly from the high linearity, confirming that the above suggested redox chemical reactions ruled the energy storage capacity of the electrodes. Interestingly, the discharged time was continuously increased for the ZMO:1Co, ZMO:3Co and ZMO:5Co electrodes with respect to the pristine ZMO 1037

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electrodes exhibited semicircles in the high-frequency region and an extended sloped line (Warburg resistance) with ∼45° of angle in the low-frequency region. It is well known that the smaller diameter from semicircles indicates the lower charge transfer resistance (Rct) value among the electrodes. Accordingly, the calculated Rct values from the obtained semicircles in the higher frequency regions for the pristine ZMO, ZMO:1Co, ZMO:3Co, ZMO:5Co and ZMO:7Co PNCs electrodes were ∼6.51, 1.85, 1.34, 1.21 and 1.23 Ω, respectively. The equivalent circuit for the Nyquist plot of the optimized ZMO:5Co PNCs is shown in the inset of Fig. 4(h). It was noticed that the Rct values were greatly reduced after incorporation of Co ions into the pristine ZMO PNCs. This further confirms that the incorporated Co ions greatly enhanced the redox electrochemical properties in basic electrolyte media. For the optimized electrode, the areal capacitance values were 1575, 1310, 1261, 1164, 1083, 964, 894, 848 and 827 mF cm−2 at the applied current densities of 1–12 A g−1, respectively and these were also compared with the obtained specific capacitance values as shown in Fig. 4(i). The cycling performance was investigated for the pristine and optimized electrodes by repeated charge/discharge tests (Fig. 4(j)). From the Fig. 4(j), it is clear that, after performing 2000 cycles, the pristine ZMO and ZMO:5Co PNCs electrodes retained the ∼76 and 85.5% in Cs values with respect to their initial cycle at high current density of 7 A g−1, indicating the excellent electrochemical stability of the electrodes. From the Fig. 4(j), it is also observable that the retention capacity was also increased after the addition of Co content. The excellent electrochemical properties, rate performance and cycling stability of the ZMO:5Co PNCs electrode are very promising to get the high energy and power densities. The superior performance is further schematically shown in Fig. 4(k). As shown in Fig. 4(k), the better porous nature of the ZMO:5Co nanocubes improves the active sites to avoid the aggregations for fast penetration of electrolyte ions and provides the accessibility to build up many channels in irreversible redox reactions, which enables the better rate performance during cycling process. Along with the porous surface morphology, the excellent conductive nature of the Co ions accelerates the reaction kinetics, and thus the cycling stability is enhanced. After performing the 2000 cycles, the morphology of ZMO:5Co PNCs electrode material with the mixed PVdF binder is shown in the FE-SEM image of Fig. S5(a). The EDS analysis was carried out for the retained structure of the ZMO:5Co PNCs electrode material, which clearly indicates the existence of the Zn, Mn, O and Co elements with different atomic and mass percentage ratios with respect to the stoichiometric ratio of the ZMO:5Co electrode composite (Fig. S5(b)). The obtained results from the EDS spectrum demonstrate the change of the active area of the electrode materials in the continuous cycling test. Consequently, from the obtained good electrochemical results, the battery-type features make the ZMO:5Co electrode to be a promising positive electrode material for the fabrication of a HSC device. Fig. 5(a) shows a schematic representation of a HSC device by assembling the ZMO:5Co PNCs as a positive electrode with the commercial AC coated on Ni foam as a negative electrode (AC-Ni). The negative electrode preparation process and its electrochemical features are described in Sections 1 and 2 of the Supporting Information. Also, the procedure for the fabrication process of a HSC device and their mass loadings are provided in Section 2.5. Fig. 5(b) represents the measured CV curves of the AC-Ni electrode (−1.0 to 0.0 V) and ZMO:5Co PNCs electrode (0.0–0.45 V) at a scan rate of 20 mV s−1, respectively, which signifies the total possible potential window for the device. The complete CV and GCD analyses were accomplished for the fabricated pouchtype HSC device in different potential stages to illustrate its consistent characteristic properties at high voltage levels. Consequently, the CV curves were measured under different potential windows (0–0.8 V to 0–1.45 V) at a constant scan rate of 50 mV s−1. With increasing the potential from 0 to 1.45 V, there were no hydrogen or oxygen evolutions and the redox humps were observed, suggesting that 1.45 V is used as an optimum voltage to further investigate the performance of

occurrence of faster reversible Faradic redox reactions among the Mn2+/Mn3+ and Co2+/Co3+ transitions in alkaline electrolyte media. On the other hand, the existing higher surface area in Co-doped ZMO PNCs can effectively facilitate the penetration of alkaline electrolyte ions through the inner pores of the nanocubes which shorten the transportation of the electronic paths and improve the kinetics of the electrodes in the electrochemical measurement, ensuring the longer discharge time. In the case of the pristine ZMO, there was no addition of conductive Co ionic species and also the nanocube structures maintained less active pores as compared with the ZMO:5Co. Accordingly, the diffusion of alkaline electrolyte ions takes place mostly from the outer parts of the nanocubes. As a result, the pristine ZMO PNCs exhibited the lower discharge time. Additionally, to have a consistency in the electrochemical characteristics and to find out the specific capacitance values for the Co-doped ZMO PNCs, we further studied CV and GCD curves at different scan rates and current densities. It is important to notice that the symmetries of the CV curves at different scan rates (5–35 mV s−1) well maintained the redox peaks in spite of their potentials shifts towards the cathode and anode directions (Fig. 4(e)) with linearly increased current values, suggesting the good electrochemical reversibility and accessibility of the ions on the surface of the ZMO:5Co electrode. Likewise, the GCD behavior was noticed at different current densities (1–12 A g−1) as depicted in Fig. 4(f). The obtained GCD results revealed charge/discharge profiles, further confirming the pseudocapacitive nature of the ZMO:5Co electrode. Based on the discharging times in all the GCD curves of the pristine ZMO and Co-doped ZMO PNCs, the specific capacitances (Cs) were calculated using the following traditional formula [60]:

Cs =

I Δt m ΔV

Here, I is the applied current (A), m is the mass of the active material coated on Ni foam (g), Δt is the discharge time (s) and ΔV is the voltage window (V). Fig. 4(g) illustrates the calculated Cs values of the pristine ZMO, ZMO:1Co, ZMO:3Co, ZMO:5Co and ZMO:7Co PNCs electrodes at different current densities of 1, 1.5, 2, 3, 4, 6, 8, 10 and 12 A g−1. The obtained Cs values for the pristine ZMO PNCs electrode at the above current densities (1–12 A g−1) were ∼267, 257, 244, 229, 212, 199, 196, 183 and 175 F g−1, respectively. The calculated Cs values for the pristine and Co-doped ZMO PNCs were in the order of ZMO:5Co > ZMO:7Co > ZMO:3Co > ZMO:1Co > ZMO. From the Fig. 4(g), the Cs values for the optimized ZMO:5Co electrode were ∼1196, 1021, 972, 898, 824, 741, 707, 655 and 617 F g−1 at 1, 1.5, 2, 3, 4, 6, 8, 10 and 12 A g−1, respectively. The Cs value of the ZMO:5Co PNCs electrode at 1 A g−1 of current density (∼1196 F g−1) was 4 times higher when compared with the pristine ZMO PNCs (∼267 F g−1). The obtained Cs values for ZMO:5Co are not only higher than the ZMO:7Co, ZMO:3Co, ZMO:1Co and ZMO PNCs, but also superior to those of the previously reported manganese and cobalt oxide-based materials. The comparative electrochemical characteristics are provided in Table S1. When Co ions are induced in the porous nature of the pristine ZMO nanocubes, the conductive Co species reduce the charge resistance and accelerate the electron transfer rate, so the kinetics of the electrode are enhanced [34,61]. In addition, the excellent interfacial contact of the ions occurred in electrolyte media owing to the existing surface area. It is reasonable to believe that these splendid characteristics played a significant role in the electrochemical performance of the ZMO:Co PNCs electrode material. The achieved maximum Cs value of the ZMO:5Co PNCs electrode further retained 51.5% of its initial Cs value when the current density value was increased to 12 A g−1, indicating good rate capability of the electrode. Furthermore, to investigate the electrochemical behavior and charge transfer resistance of the ZMO:Co PNCs electrodes, the EIS analysis was executed in 1 M KOH electrolyte media. Fig. 4(h) shows typical Nyquist plots in the frequency range of 0.01 Hz to 100 kHz at 10 mV of amplitude. The illustrated Nyquist plots of the pristine ZMO and ZMO:1Co, ZMO:3Co, ZMO:5Co and ZMO:7Co PNCs 1038

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Fig. 5. (a) Schematic illustration of the assembled HSC device with ZMO:5Co PNCs and AC electrodes. (b) CV curves of the ZMO:5Co PNCs and AC electrodes. (c) CV and (d) GCD curves of the HSC device monitored at different voltages. (e) CV and (f) GCD curves of the HSC device measured at different scan rates and current densities under the fixed potential window of 0–1.45 V. (g) Specific capacitance of HSC device at different current densities. (h) Comparative Ragone plot and (i) EIS spectrum of the HSC device.

the HSC device. In addition, GCD curves were measured under different potential windows (at current density of 1 A g−1) as shown in Fig. 5(d). Clearly, the optimization voltage is 1.45 V from the CV results. The CV curves at various scan rates (5–90 mV s−1) for the fabricated device can be seen in Fig. 5(e). As shown in Fig. 5(e), the device revealed triangular CV shapes and exhibited a typical hybrid capacitor type performance, and it also well maintained the CV shapes under high scan rates, indicating its good reversibility. To further explore the capacitive performance of the HSC device, the GCD measurements were carried out at different current densities of 1–9 A g−1 as shown in Fig. 5(f). The obtained GCD curves at applied current densities were almost symmetrical, indicating the good electrochemical reversibility and also exhibited higher columbic efficiency of the device. It is important to notice that the obtained potential window (1.45 V) is much higher when compared to the AC-based conventional symmetric capacitors. The HSC delivered the capacitance values of ∼68, 65, 63, 53, 50, 43, 42 and 40 F g−1 at different current densities of 1, 1.5, 2, 2.5, 3, 5, 7 and 9 A g−1, respectively as shown in Fig. 5(g). From the capacitance graph, the device showed ∼59% retention in capacitance value at high current density of 9 A g−1, which implies the good rate capability of the fabricated device. The energy storage of HSC device can be accomplished by calculating the energy density (Ed) and power density (Pd) values using the following formulas [50,62]:

Ed =

Cs × (ΔV)2 2

Pd =

Ed Δt

The calculated energy density and power density values are shown in Ragone plot (Fig. 5(h)). Under 1 A g−1 of current density, the device showed maximum Ed of 27.38 W h kg−1 at a Pd of 1059 W kg−1 for the HSC device. Also, the device well maintained the Ed of 16.1 W h kg−1 at a Pd of 11499 W kg−1 under a high current density of 9 A g−1. However, the obtained higher Ed and Pd for HSC are superior to those of the previously reported hybrid supercapacitors such as NiCo2O4//AC (Ed = 9.1 W h kg−1 and Pd = 5625 W kg−1) [63], Mn3O4//graphene (Ed = 9.6 W h kg−1 and Pd = 41.6 W kg−1) [64], NiCo2S4 on Ni foam// porous carbon (Ed = 22.8 W h kg−1 and Pd = 160 W kg−1) [65], Ni0.9Co1.92Se4//AC (Ed = 26.29 W h kg−1 and Pd = 265 W kg−1) [66], MnO2//FMCNTs (Ed = 30.4 W h kg−1 and Pd = 251 W kg−1) [67] and Co0.85Se//AC (Ed = 21.1 W h kg−1 and Pd = 400 W kg−1) [68]. The charge transfer resistance of the device was further analyzed by EIS analysis (Fig. 5(i)). As shown in Fig. 5(i), the Nyquist plot demonstrates the indistinct semicircle in the higher frequency region and the low Rct from the Nyquist plot can be expected, which further offers the better electrochemical conductivity of the device. A cycling test was performed over 4000 cycles at a current density 1039

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Fig. 6. (a) Cycling stability of the HSC device. The inset of (a) shows the GCD curves for the first 10 cycles. (b) and (c) Digital photographic images of the two series connected HSC devices for glowing the red and green LEDs, respectively. (b) (i–iii) and (c) (i–iii) Digital photographic images of the glowing red and green LEDs at different times, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

of 2 A g−1, to check the stability of the obtained higher density HSC device (Fig. 6(a)). Fig. 6(a) shows the long-term cycling stability with the corresponding coulombic efficiency of the device. During the first 250 cycles, the capacitance was increased, which is probably ascribed to the activation of the both electrode pores. This allows for the bulk electrolyte ion diffusion and further enables more electrochemical active sites. However, after 4000 cycles, the HSC device delivered a capacitance retention of ∼80.5%, which immediately offers the excellent cyclic stability of the device. From the Fig. 6(a), it is also noticeable that the HSC device maintained ∼90.8% coulombic efficiency during the continuous charge-discharge cycling test. The inset of Fig. 6(a) shows the GCD curves for the first 10 cycles, which indicates the high coulombic efficiency of the HSC device. From the obtained excellent cycling stability, high energy and power densities of the HSC device have promising potential applications in the field of energy storage system and are also suitable to drive the low-voltage devices. On the other hand, another HSC device was made with similar electrochemical behavior and connected together in series to attain the higher operating voltage for practical applicability. The connected two pouch-type HSC devices easily illuminated 2 V red and 3 V green light-emitting diodes (LEDs) for about 2 min in a day light as shown in Fig. 6(a) and (b), respectively. The photographs of Fig. 6(a) and (b) describe the glowing of the red and green LEDs after charging in a short time, respectively and the corresponding magnified images show their intense glowing images. Therefore, the solitary structure of the ZMO:Co PNCs with improved energy storage capacities suggests a new pathway to the HSCs.

found that with the incorporation of Co into ZMO PNCs, the porosity of the materials was increased and the electrochemical properties were greatly enhanced. The optimized ZMO:5Co PNCs electrode showed good reversible Faradaic redox reactions in 1 M KOH electrolyte and delivered a high specific capacitance value of ∼1196 F g−1, which is 4 times higher as compared with the pristine ZMO PNCs (∼267 F g−1) at a current density of 1 A g−1. The enhancement of specific capacitance is mainly attributed to the synergetic effect of the multi-metal ions and the increased porosity of the nanocubes. The Co incorporation into the ZMO PNCs can effectively reduce the charge resistance and improve the charge transport rate with increased reaction kinetics. Consequently, the ZMO:5Co PNCs electrode showed superior electrochemical properties in terms of high Cs value, good rate capability and excellent cycling life. Additionally, the high energy density (27.38 W h kg−1) and power density (1059 W kg−1) were achieved from the ZMO:5Co PNCsbased HSC device. From the impressive high energy density of the pouch-type HSC device, the red and green LEDs operated with a potential window of 1.45 V. From the obtained results, the Co-doped ZMO PNCs electrode is expected to be a promising energy storage material in the field of HSC.

4. Conclusions

Appendix A. Supplementary data

In summary, the spinal-like structured ZMO and Co ions doped ZMO PNCs were successfully prepared by a facile solvothermal route followed by calcination at 450 °C for 3 h in air. The growth of the hydroxyl nanocubes was investigated by tuning the amount of NH4F. It was

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2018.12.152.

Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2017R1A2B4011998 and No. 2018R1A6A1A03025708).

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