Metal-organic framework-derived nanocomposites for electrocatalytic hydrogen evolution reaction

Journal Pre-proofs Metal-organic framework-derived nanocomposites for electrocatalytic hydrogen evolution reaction Ziliang Chen, Huilin Qing, Kun Zhou, Dalin Sun, Renbing Wu PII: DOI: Reference:

S0079-6425(19)30100-8 https://doi.org/10.1016/j.pmatsci.2019.100618 JPMS 100618

To appear in:

Progress in Materials Science

Received Date: Accepted Date:

24 April 2019 12 November 2019

Please cite this article as: Chen, Z., Qing, H., Zhou, K., Sun, D., Wu, R., Metal-organic framework-derived nanocomposites for electrocatalytic hydrogen evolution reaction, Progress in Materials Science (2019), doi: https://doi.org/10.1016/j.pmatsci.2019.100618

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2019 Published by Elsevier Ltd.

Metal-organic framework-derived nanocomposites electrocatalytic hydrogen evolution reaction

for

Ziliang Chena,1, Huilin Qingb,1, Kun Zhouc,, Dalin Suna, Renbing Wua,* a

Department of Materials Science, Fudan University, Shanghai 200433, China

b

Department of Macromolecular Science, Fudan University, Shanghai 200433, China

c

School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore ABSTRACT The rapid development of hydrogen energy is strongly dependent on the economic and efficient production of hydrogen. The electrocatalytic splitting of water to molecular hydrogen via the hydrogen evolution reaction (HER) provides an appealing solution for producing high-purity hydrogen, but low-cost and highly active electrocatalysts are required for HER. Among currently investigated HER electrocatalysts, metal-organic framework (MOF)-derived nanocomposites constructed from transition metals (TMs)/TM compounds (TMCs) and carbon materials offer extremely promising and attractive HER activities because of their unique properties, such as tunable compositions, readily regulated electronic structures, controllable morphologies, and diverse configuration. Herein, this article provides a comprehensive overview of MOFderived nanocomposites as HER electrocatalysts for water splitting. It begins with the introduction of the fundamentals of electrocatalytic HER. Afterwards, various of ingeniously designed strategies for improved MOF-derived HER electrocatalysts are

*Corresponding

authors.

E-mail addresses: [email protected] (K. Zhou); [email protected] (R. Wu) 1Authors

contributed equally to this work. 1

meticulously summarized and discussed, with special emphasis on the component manipulation of the TMs/TMCs, carbon matrix modifications, morphology tuning and electrode configuration engineering. Finally, future perspectives on the development of these nanocomposites as HER electrocatalysts are proposed. Keywords: Metal-organic framework, Transition metal-based compounds, Carbon matrix, Nanocomposites, Electrocatalyst, Hydrogen evolution reaction Contents 1. Introduction.............................................................................................................03 2. Fundamentals of electrocatalytic HER...................................................................07 2.1. Mechanism of HER..........................................................................................07 2.2. Assessment of the HER activity........................................................................08 2.2.1. Overpotential..........................................................................................08 2.2.2. Tafel slope and exchange current density................................................09 2.2.3. Faradaic efficiency..................................................................................10 2.2.4. Stability…………………………….......................................................10 2.3. Factors determining the HER activity..............................................................10 2.3.1. Intrinsic property of the active sites.........................................................10 2.3.2. The number of active sites......................................................................11 2.3.3. Electron transfer ability...........................................................................12 2.3.4. Mass transfer ability................................................................................12 2.4. Factors unveiling the HER activity..................................................................13 2.4.1. Turnover frequency.................................................................................13 2.4.2. Gibbs free energy for H intermediate adsorption...................................13 2.4.3. Charge transfer resistance.......................................................................13 2.4.4. Electrochemical active surface area .......................................................14 2.4.5. Surface structure.....................................................................................14 3. MOF-derived nanocomposites for HER……………...………...............................16 3.1. Component manipulation of TMs/TMCs.........................................................19 3.1.1. Single metal, bimetal and alloy...............................................................19 3.1.2. Metal chalcogenides................................................................................24 2

3.1.3. Metal phosphides....................................................................................30 3.1.4. Metal nitrides..........................................................................................35 3.1.5. Metal carbides.........................................................................................38 3.1.6. Single atom catalyst................................................................................41 3.2. Carbon matrix modification.............................................................................45 3.2.1. Graphitization degree of carbon matrix..................................................46 3.2.2. Heteroatom doping..................................................................................49 3.2.3. Dual carbon confinement........................................................................51 3.3. Morphology tuning..........................................................................................58 3.3.1. Regulation of geometric dimension........................................................59 3.3.2. Construction of hollow structure…........................................................62 3.3.3. Creation of core-shell structure…..........................................................66 3.4. Electrode configuration engineering................................................................70 3.4.1. Rooting MOFs directly onto metal substrates........................................75 3.4.2. Rooting MOFs directly onto carbon substrates......................................77 3.4.3. Pseudomorphic replication......................................................................81 4. Conclusions and perspectives.................................................................................84 Acknowledgements....................................................................................................86 References..................................................................................................................86 1 Introduction Currently, the energy crisis and environmental pollution are global concerns. To ensure a green and sustainable future, employing clean and renewable energy as alternatives to traditional fossil fuels is of great urgency [1‒6]. Hydrogen, which possesses several appealing merits including a high gravimetric energy (~142 MJ kg−1), clean combustion products, and renewable nature, has been regarded as one of the most ideal energy carriers [7,8]. In this respect, tremendous effort has been devoted to the development of hydrogen energy in the past decades [9‒12]. The primary challenge towards the large-scale utilization of hydrogen energy is to seek a green, highly efficient, and low-cost method for hydrogen production [13‒18]. Electrochemical water splitting 3

can not only convert electricity into chemical energy in an eco-friendly manner but also be easily coupled with other intermittent energy sources (e.g., wind and solar), offering a promising solution to high-purity hydrogen production [19‒26]. However, as one of the essential reactions involved in water splitting, the hydrogen evolution reaction (HER) is thermodynamically uphill, requiring a large driving overpotential and resulting in high energy consumption [27‒33]. To address these issues, precious Ptbased materials have been currently developed as effective electrocatalysts to reduce the energy barrier and accelerate the HER process [34]. Unfortunately, the limited global supply on the earth and prohibitive market price severely hamper the large-scale application of precious materials. To search for alternatives to precious metal catalysts, plenty of non-precious 3d transition metals (TMs) (e.g., Fe, Co, Ni, etc.) and TM compounds (TMCs) (e.g., oxides, chalcogenides, nitrides and phosphides) have been intensively investigated in recent years [35‒45]. From the perspective of their electronic structures, TMs species have large numbers of unpaired d-orbital electrons, which can easily facilitate the chemisorption of hydrogen, making these materials able to catalyze HER [46,47]. Nevertheless, TMs or TMCs particles easy aggregate, show low specific surface areas, have low conductivities, can undergo chemical corrosion and the desorption of H intermediates from their active sites can be inefficient, leading to inferior electrocatalytic activities compared to the commercially available precious metal ones [48,49]. To address this challenge, one effective approach is to hybridize TMs/TMCs with carbonaceous materials (e.g., porous carbon, carbon nanotubes and graphene) [50‒53]. The coupled carbon not only alleviates the corrosion of TMs/TMCs from strong acidic/alkaline electrolytes but also forms electronic interactions with TMs/TMCs, facilitating tuning of the ad-/desorption ability of the H intermediate [51]. 4

Furthermore, the incorporation of carbon into TMs/TMCs may effectively increase their specific surface area, which is beneficial to exposing active sites [53]. Therefore, the development of hybrid composites consisting of TMs/TMCs and carbonaceous materials (defined as TMs/TMCs@C) for HER has sparked considerable research interest in the field of hydrogen energy. Several methods, including chemical vapor deposition, high-energy ball milling, and solvothermal reactions followed by thermal treatment, have been explored to prepare the TMs/TMCs@C electrocatalysts [54‒60]. However, these methods usually involve multistep process and complicated fabrication conditions and constructing hybrid composites with strong coupling between the TMs/TMCs and the carbon matrix may be difficult and result in an unsatisfactory catalytic performance. As a novel class of crystalline porous materials, metal-organic frameworks (MOFs) formed by bridging metal ions with organic linkers have attracted much attention because of their diverse, designable and tailorable features [61]. Recently, MOF has been demonstrated as a valuable template/precursor to synthesize TMs/TMCs@C hybrid composite [62‒76]. Specifically, unlike the high energy consumption and harsh operating conditions of the abovementioned synthetic methods, the strategy of synthesizing TMs/TMCs@C hybrid composites via the thermal annealing MOF precursors is very convenient and easily scalable. More importantly, benefiting from the intriguing properties of MOF precursors, the derived TMs/TMCs@C hybrid composites have the advantages of morphological diversity, composition modulation, favorable surface structures and strong coupling effects, resulting in exhibiting remarkable HER activities [77,78].

5

Fig. 1. Design strategies towards MOF-derived TMs/TMCs@C electrocatalysts for hydrogen evolution reaction. Currently, substantial progress has been made in the development of MOF-derived TMs/TMCs@C electrocatalysts, and new findings are continuously being reported [79‒85]. In this regard, a timely summary of the encouraging works in this field is of great significance to guide the development of improved MOF-derived electrocatalysts for HER. Despite several brilliant reviews on MOF-derived functional materials aiming at energy storage and conversion applications having been reported in recent years [86‒90], a systematical review dedicates to the design strategies for MOF-derived 6

TMs/TMCs@C composites for electrocatalytic HER is still lacking. In light of this, herein, a comprehensive review focusing on the design strategies towards MOF-derived HER electrocatalysts is provided in detail, and the component manipulation of TMs/TMCs, carbon matrix modification, morphology tuning and electrode configuration engineering are discussed (Fig. 1). Additionally, this review also introduces the fundamentals of electrocatalytic HER to establish the dependence of the activity on the composition/structure/morphology and highlight the urgent challenges with respect to the future development of MOF-derived nanocomposites for electrocatalytic HER. 2 Fundamentals of electrocatalytic HER 2.1 Mechanism of HER Electrochemical water splitting involves two fundamental half reactions, known as the cathodic HER and anodic OER [91–93], as described by the equations listed in Table 1. As one of the two half reactions in water splitting, the HER process comprises two steps. The first step is the absorption of a hydrogen atom produced by the discharge of H2O or H3O+ on an active site, which is known as the Volmer reaction [94]. The second step is the formation of hydrogen. Of particular note, there are two pathways to accomplish this step [94]. One is the Heyrovsky reaction, by which hydrogen is formed through the electrochemical desorption, and the other is known as the Tafel reaction, by which hydrogen is formed through the chemical desorption (Table 2). On the basis of the above reaction pathways, the exertion mechanism of the HER can be assigned as one of two types, i.e., Volmer-Heyrovsky and Volmer-Tafel. These two mechanisms can be active in both acidic and alkaline solutions, and they depend on the discrepant contents of the ions in the electrolyte [94,95]. 7

Table 1 The process of water splitting in acidic and alkaline media [94] Type

Acidic media

Alkaline media

HER

2H3O+ + 2e‒ → 2H2O + H2

2H2O + 2e‒ → H2 + 2OH‒

OER

2H2O → 4H+ + O2 + 4e‒

4OH‒ → 2H2O + O2 + 4e‒

Overall

2H2O → 2H2 + O2

Table 2 The detailed process of HER in acidic and alkaline media [95] Acidic media

Alkaline media

Volmer reaction

H3O+ + e‒ → H* + H2O

H2O + e‒ → H* + OH‒

Heyrovsky reaction

H3O+ + e ‒ + H* → H2 + H2O

H2O + e‒ + H* → H2 + OH‒

Tafel reaction

H* + H* → H2

H* + H* → H2

2.2 Assessment of the HER activity The overpotential (η), Tafel slope (b), exchange current density (j0), Faradaic efficiency, and cycle durability are often used as the critical indexes to assess HER activity. Because both the principle and the measurement of the above indexes have been elaborately clarified in previously reported literature [96–98], we thus give concise descriptions. 2.2.1 Overpotential The overpotential is an extra applied potential higher than the equilibrium potential that is used to overcome the energy barrier originating from the electron transfer, mass diffusion in the solution, and interactions on the electrode surface in practical electrolysis [99]. As a consequence, the overpotential recorded from an approximately linear sweep polarization (LSV) curve takes into accounts the activity, concentration, and resistance overpotentials. Herein, the activity overpotential and the concentration overpotential are caused by electrochemical polarization and concentration polarization, 8

respectively, while the resistance overpotential results from the resistance generated from the solutions, wires, and contact points. It should be remarked that only the activity overpotential is correlated with the intrinsic properties of the electrocatalysts and thus deemed as a critical parameter for estimating the electrocatalytic performance. To extract the activity overpotential, the concentration overpotential arising from the concentration polarization should be avoided by rotating the working electrode, stirring the electrolyte, and/or increasing the temperature during the test, while the resistance overpotential caused by the solution and contact resistance (Rs) should be eliminated through an IRs compensation, where I is the electric current and the Rs value can be extracted from the real part of the electrochemical impedance spectroscopy at high frequency. Currently, to better elucidate the HER activity, the overpotential at the current density of 10 mA cm–2 (η10) is chosen as the vital reference since a current density of 10 mA cm–2 is coupled with an expected efficiency of 12.3% for solar water splitting devices [100–102]. 2.2.2 Tafel slope and exchange current density The Tafel slope for HER is widely employed to reveal the dominant reaction mechanism and reaction kinetics within a certain potential window, which can be calculated from the simplified Tafel formula (η = b·logj + a, in which η, j, a and b represent the overpotential, current density, adjusted constant and Tafel slope, respectively) [103]. Typically, when the HER process is dominated by Volmer, Heyrovsky, or Tafel mechanisms, the corresponding b values are approximately 116, 38, and 29 mV dec−1, respectively. Furthermore, the smaller b values are associated with faster reaction kinetics. In addition to the Tafel slope, the exchange current density (j0) can also be calculated from the simplified Tafel formula when η is equal to 0. j0 is considered to be 9

an important indicator for evaluating the catalytic efficiency, and it is associated with the electrode surface state and the catalytic surface area. A large j0 value signifies the fast electron transfer and a favorable catalytic surface area [104]. 2.2.3 Faradaic efficiency The Faradaic efficiency represents the utilization efficiency of electrons related to the electrochemical reaction, and it can be calculated as the ratio of the experimental amount of H2 produced to the theoretical value. Specifically, we can use a fluorescence sensor or volumetric method to obtain the experimental amount of H2 produced under a constant oxidation current (I) within a certain time (t). On the other hand, the theoretical amount of H2 production (nH2) can be calculated on the basis of the following equation: nH2 = (I·t)/2F, where F is the Faraday constant. 2.2.4 Stability Generally, there are two methods to evaluate catalytic stability. One is the dependence of the current density on time, i.e., the i–t curve. During this measurement, the imposed current density should be no less than 10 mA cm−2 and the duration should be no less than 12 h. The other method is performing cyclic voltammetry (CV) or LSV cycles with no less than 1000 times. 2.3 Factors determining the HER activity In general, the HER activity is determined by the following four inherent aspects, i.e., the intrinsic properties of the active sites, the number of available active sites, the electron transfer ability and the mass transfer ability. 2.3.1 Intrinsic property of the active sites The active sites during the HER process are the practical functional regions where the H intermediates can be absorbed and transformed into hydrogen. Thus, the intrinsic 10

properties of the active sites determine the electrocatalytic HER activity to a large extent [105]. The intrinsic properties of the active sites are highly dependent on the nature of the catalysts, which can have a defined electronic structure, showing a discrete chemisorbed bond with the H intermediate [106]. For example, TMs species have large numbers of unpaired d-orbital electrons in their atomic structure, resulting in strong hydrogen absorption abilities, while some anionic species (such as C, O, S, and Se) are prone to trapping protons, enabling the strong hydrogen desorption abilities [107]. In addition to the chemical composition, the ligand/strain effects, crystal defects and/or metal-anion covalency have also been demonstrated to greatly influence the electrocatalytic HER activity of active sites [108,109]. Therefore, both the chemical composition and the crystal structure of a catalyst should be considered in order to achieve a high catalytic HER performance. 2.3.2 The number of active sites The number of active sites is also an important factor in determining electrocatalytic HER performance. A larger number of active sites allows a larger the number of absorbed H intermediates. Thus, the amount of hydrogen generated in a unit of time is dependent on the number of active sites, which controls efficient hydrogen production. Building catalytic systems with high specific surface areas through micro/nanostructure engineering is a well-established way of remarkably increasing the number of active sites, [110]. Because the electrocatalysis reaction is a surface process, where only the surface atoms participate, recent reports have highlighted that to more accurately reflect the intrinsic electrocatalytic activity, the current density should be normalized to a quantified parameter related to the catalyst surface (e.g., the number of active sites, or alternatively the surface area of the catalyst) rather than the geometric 11

area of the electrode [111,112]. 2.3.3 Electron transfer ability The HER process involves a two-electron transfer, and the electrocatalytic reaction basically occurs on the surface of the electrocatalyst. Ensuring rapid electron transfer on the surface of the electrocatalyst is quite important for accelerating the HER. Since the electron transfer ability relies on the electrical conductivity of the catalytic system, there is no doubt that developing a catalytic system with high electrical conductivity is the hinge. To reach this goal, two efficient approaches have been proposed. One is to synthesize an electrocatalyst with the exceptional inherent electrical conductivity, such as those of Pt, Au, and transition metal nitrides [113–116]. The other is to load the electrocatalysts (e.g., transition metal oxides, chalcogenides, phosphides and so on) on a special matrix with high electrical conductivity, such as graphene foam, carbon cloth, and nickel foam, which will significantly enhance the electron transfer ability of the catalyst [117–122]. 2.3.4 Mass transfer ability The HER process is not only a simple electron transfer process but also associated with the mass transfer. To be specific, both the reactants and products would reversibly diffuse from the liquid electrolyte to the surface of the solid electrocatalyst during the electrocatalytic reaction, which is accompanied by a physical adsorption and desorption process [123]. Generally, if the electrocatalyst possesses micropores and mesopores, it will have a strong physical effect on the reactants, which will facilitate the subsequent chemical adsorption process. On the other hand, if the electrocatalyst possesses macropores and a hollow structure, it will show a strong mass diffusion ability, which will facilitate chemical desorption process. Otherwise, the reactants or the products will 12

block the pathways between the active sites and the surrounding environment, resulting in the steric effect and decreasing the catalytic rate. 2.4 Factors unveiling the HER activity The turnover frequency (TOF), charge transfer resistance (Rct), Gibbs free energy for H intermediate adsorption (ΔGH*), electrochemical active surface area (ECSA) and surface structure, are often used as the critical parameters to elucidate the HER activity. 2.4.1 Turnover frequency The TOF represents the ability of an electrocatalyst of generate a desired product in per catalytic site as a function of time and is a measure of the intrinsic catalytic activity of each catalytic site [124–129]. The TOF value is usually used to evaluate the HER activity, and it can be determined based on the following equation: TOF = (j·A)/(2F·n), where j is the measured current density at a given overpotential, A is the working electrode area, n is the number of moles of the active material and F is the Faraday constant. 2.4.2 Gibbs free energy for H intermediate adsorption The hydrogen adsorption free energy (ΔGH*) is widely used as a vital descriptor to estimate the intrinsic HER activity [106,130]. To be specific, competitive adsorption and desorption of hydrogen atom occurs during the HER process. Only when ΔGH* is equal to zero, can the best balance between absorption and desorption be reached, and this corresponds to the highest HER activity [106]. The ΔGH* value for the catalyst can be obtained by the density functional theoretical (DFT) calculation method. 2.4.3 Charge transfer resistance The Rct is associated with the interface charge transfer ability of the electrode, which can be obtained by fitting the semicircles in the high frequency region of the 13

electrochemical impedance spectroscopy (EIS) spectra. Lower Rct values indicate faster reaction rates. Higher applied external overpotentials result in smaller Rct values. Notably, EIS spectra should be recorded within the hydrogen evolution voltage (usually at η10). In addition, vigorous turbulence such as drastically rotating or stirring should be avoided during the EIS measurements. 2.4.4 Electrochemical active surface area A larger ECSA manifests the more potential active sites hosted in the catalyst. Because the ECSA is proportional to the electrochemical double-layer capacitance (Cdl), a popular method to evaluate the ECSA of a catalyst is based on the equation ECSA = Cdl/Cs, in which Cs represents the specific capacitance [131]. The Cdl value can be extracted from CV curves at different scan rates. Specifically, current responses within a certain potential window without the faradaic processes are recorded with increasing scan rates usually from 4 to 100 mV s–1. By plotting Δj/2 (Δj represents the difference between the anodic and the cathodic current density at a certain potential) as a function of the scan rate, we can obtain the slope, which corresponds to the Cdl value [132]. Notably, because the nature of the real electrochemical surface area of the electrocatalysts is difficult to accurately determine, the ECSA is sometimes used only as an indirect estimation for comparisons [133,134]. 2.4.5 Surface structure The surface structure of the catalyst usually involves the surface area and porosity, in which surface area is closely related to the accessibility of the active sites, while the porosity, including pore volume and pore size distribution, has a large influence on the mass transport. Physisorption measurements are widely employed to evaluate surface area and porosity. Typically, Brunauer–Emmett–Teller (BET) theory, a multilayer adsorption model, is used for the surface area analysis, and the Barrett–Joyner–Halenda 14

(BJH) model, is most commonly used for the porosity analysis on the basis of the Kelvin equation. With respect to porosity, one of the most important features is the pore size distribution. According to their diameter, pores can be divided into micropores (< 2 nm), mesopores (2~50 nm) and macropores (> 50 nm). All of them can be beneficial to accelerating the mass transfer, e.g., the micropores and mesopores can allow strong physical adsorption of the reactants, while the macropores can greatly promote the mass desorption process [135–138]. Note that because the capillary condensation in micropores is quite complicated, pores less than 2 nm determined by the BJH analysis are not reliable. Concerning of this, Dollimore and Heal (D-H), Horvath–Kawazoe (HK), and the density functional theory (DFT) models have been effectively and rapidly developed for micropore analysis [139].

Fig. 2. An overview of the great potential to be an advanced HER electrocatalyst for MOF-derived TMs/TMCs@C nanocomposites. In summary, an electrocatalyst with outstanding HER activity should possess the superior intrinsic properties related to their active site, a large number of available active sites, strong electron transfer abilities and suitable mass transfer ability to ensure a high TOF, moderate ΔGH*, low Rct, and large ECSA, as well as a favorable surface area and porosity. In this regard, MOF-derived TMs/TMCs@C nanocomposites, featuring tunable compositions, controllable morphologies, and changeable structures, hold great promise as the advanced HER electrocatalysts (Fig. 2). 15

3 MOF-derived nanocomposites for HER The synthesis of MOF-derived TMs/TMCs@C nanocomposites generally consists of two main procedures, i.e., preparation of the MOF precursor and subsequent thermal treatment under controlled temperature and atmosphere conditions. In 2012, Zhang et al. reported the synthesis of hierarchical Fe2O3 microboxes through the annealing treatment of microcube-like Fe-based MOFs (PBA, Fe4[Fe(CN)6]3) at 650 °C under air [140]. The formation of hierarchical Fe2O3 microboxes was ascribed to the thermally induced decomposition and oxidation of the PBA microcubes. Similar to this strategy, Xu et al. synthesized spindle-like Fe2O3 by the thermal annealing of a typical Fe-based MOF (MIL-88-Fe, Fe3O(H2O)2Cl(BDC)3·nH2O) at 350 °C under air [141]. Unfortunately, both of the above findings only focused on the transformation of transition metal ions but ignored the utilization of the organic linker in the MOF structure during the thermal treatment. In 2013, Yang et al. annealed a typical Zn-based MOF (ZIF-5, Zn4O(HCO2)6) under a N2 atmosphere rather than air so as to avoid the degradation of the organic linker [142]. During the annealing process, Zn species and O species combined and formed ZnO quantum dots while the organic linker was decomposed and carbonized into amorphous carbon, generating the final hybrid structure consisting of ZnO quantum dots uniformly embedded within an amorphous carbon matrix (Fig. 3a). Following this observation, in 2015, Yang et al. used a special bimetallic MOF template without O species (Fe-Co-PBA) to synthesize hybrid composites consisting of FeCo alloys encapsulated within graphitic carbon via thermal treatment at 800 °C under an Ar atmosphere (Fig. 3b) [143]. During the thermalinduced reaction process, the Fe3+ and Co2+ were reduced and alloyed into Fe-Co alloy, while the organic linker was carbonized into porous graphitic carbon. This work provided the first example of a MOF-derived hybrid composites composed of an alloy 16

and carbon. Inspired by the above findings, Wu et al. reported the first approach for the fabrication of cobalt sulfides and porous carbon hybrids through the simultaneous decomposition and sulfidation of a Co-based MOF (ZIF-67) template at 600 °C under Ar (Fig. 3c) [144]. Specifically, during the thermal treatment, Co2+ liberated from ZIF67 reacted with sulfur powders to form cobalt sulfides, while the Co species functioned as the catalyst to promote the decomposition and graphitization of the organic linker, resulting in the hybrids comprising CoSx nanoparticles embedded within a graphitic carbon matrix and carbon nanotubes (CNTs).

Fig. 3. Schematic diagrams for the synthesis of nanocomposites consisting of (a) ZnO 17

QDs coated by porous carbon [142], (b) FeCo alloy encapsulated by N-doped graphitic carbon [143], and (c) CoSx nanoparticles embedded within porous carbon/CNT matrix [144]. Adapted with permission from Ref. [142]. Copyright 2013, American Chemical Society. Adapted with permission from Ref. [143]. Copyright 2015, The Royal Society of Chemistry. Adapted with permission from Ref. [144]. Copyright 2015, Willey-VCH. Guided by the above pioneering works, a large number of MOF-derived TMs/TMCs@C composites have been designed and explored as HER electrocatalysts in the past five years. Since the HER activity of a catalyst is highly dependent on its composition, structure and morphology, the design of MOF-derived TMs/TMCs@C electrocatalysts can be summarized as follows: i) component manipulation of TMs/TMCs; ii) carbon matrix modification; iii) morphology tuning; and iv) electrode configuration engineering. 3.1 Component manipulation of TMs/TMCs The compositions of MOF-derived TMs/TMCs@C nanocomposites mainly involve two types of chemical species. One is an inexpensive transition metal element, such as Fe, Co, Ni, Mn, V, Cu, Mo and W; the other is a nonmetal element, such as B, C, N, S, Se, Te, and P. Because each species shows a certain affinity for the H intermediate, altering the catalyst components to achieve a moderate ΔGH* is thus an effective method for optimizing the HER activity. In view of this, several kinds of TMs/TMCs-based HER catalysts have been developed based on MOF precursors, such as single metal, bimetal, alloys, transition-metal chalcogenides, phosphides, carbides, nitrides as well as single-atom catalysts. 3.1.1 Single metal, bimetal and alloy TM nanoparticles derived from MOF precursors play two pivotal roles in modifying the HER activity. One is interacting with the electronic structure of the 18

carbon matrix and tuning the adsorption ability of the active sites for H*; the other is increasing the electrical conductivity of the catalytic system and accelerating the electron transfer. Single metal-based electrocatalysts can be easily prepared by a straightforward pyrolysis of MOF precursors. For example, Xu et al. uniformly encapsulated Ni nanoparticles within N-doped graphitic carbon (Ni@NC-800) via the one-pot thermal annealing the Ni-based MOF (Ni2(BDC)2(TED), BDC = 1,4benzenedicarboxylic acid, TED = triethylenediamine) at 800 °C under flowing N2 (Fig. 4a–d) [145]. It was found that strong electron interactions occurred between the Ni nanoparticles and the surrounding graphitic carbon, i.e., electron transfer from Ni to carbon. Such an electron transfer effectively weakened the affinity of Ni for H while enhancing the affinity of C for H, thus promoting the HER activity. To be specific, in 1.0 M KOH, the as-prepared Ni@NC-800 required an overpotential of 205 mV to afford a current density of 10 mA cm‒2, and it exhibited a Tafel slope of 160 mV dec‒1 (Fig. 4e and 4f), making it superior to many Ni-based electrocatalysts. In addition, a negligible activity decay was observed even after 1000 cycles (Fig. 4g).

19

Fig. 4. Ni-based MOF-derived Ni@NC-800 nanocomposite: (a) schematic illustration of the synthetic process, (b) FESEM image, (c, d) TEM images, (e) LSV curve, (f) Tafel slope, and (g) cycle durability in 1.0 M KOH [145]. Adapted with permission from Ref. [145]. Copyright 2017, Wiley-VCH. According to the famous “volcano curve” established by Norskov et al., some metal species, such as Fe, Co, Ni, and W, show strong adsorption ability of H*, while other metal species such as Ag, Zn, Cu, and Al, exhibit weak adsorption abilities [106]. Accordingly, one can envision that combining multiple metals with different adsorption abilities for H* should provide a moderate ΔGH*, and thus lead to an optimal HER activity. Kuang et al. recently put forward a strategy that combining the advantages of Co and Cu species [146]. Specifically, ZIF-67 dodecahedra were first in-situ grown on the Cu(OH)2 nanowires as the precursors through a coprecipitation method, and this 20

material was then converted to a hybrid composite composed of Cu and Co bimetal nanoparticles embedded within the N-doped mesoporous carbon framework (CuCo@NC) via thermal annealing at 800 °C under flowing Ar (Fig. 5a–d). Notably, Cu ions liberated at relatively low temperatures (below 450 °C) first penetrated the pores of ZIF-67 to avoid self-aggregation. As the temperature of the thermal treatment was further increased, the Cu species would bind with the N species to form the Cu-N bonds, stabilizing the N content in the carbon frameworks derived from ZIF-67. Benefiting from these phenomena, the CuCo@NC electrocatalyst required 145 mV of overpotential at a current density of 10 mA cm‒2 in 0.5 M H2SO4 (Fig. 5e), which was much lower than those of Cu@NC (> 450 mV) and Co@NC (380 mV). In addition, the catalyst showed a Tafel slope as low as 79 mV dec‒1 (Fig. 5f) and a negligible current decay after ~8 h in 0.5 M H2SO4 (Fig. 5g).

21

Fig. 5. (a) Schematic diagram for the synthesis of CuCo@NC nanocomposite and its (b) FESEM image, (c, d) TEM images, (e) LSV curve, (f) Tafel slope, and (g) cycle durability in 0.5 M H2SO4 [146]. Adapted with permission from Ref. [146]. Copyright 2017, Wiley-VCH. Unlike the multiple metal strategy, in which metal nanoparticles are isolated and embedded within carbon matrix, alloying is a unique strategy in which the metal species can be mixed to afford an atomic-scale solid solution, which can not only imbue a moderate ΔGH* but also generate a synergistic effect to boost the HER activity [147]. 22

For example, Yang et al. prepared a composite consisting of ternary FeCoNi alloys encapsulated within graphitic carbon (FeCoNi@C) by thermal annealing an Fe-Co-NiPB precursor at 600 °C under flowing N2 (Fig. 6a–d) [148]. When investigated as an HER electrocatalyst, the as-prepared composite afforded a current density of 10 mA cm‒2 at the required overpotential of 149 mV in 1.0 M KOH, showing much higher electrocatalytic activity than its single metal counterpart (Fig. 6e). DFT calculations further unraveled that upon increasing the degrees of freedom of the alloys or altering the metal proportions in the FeCoNi ternary alloys, the electronic structures of the nanocomposites could be deliberately tuned by varying the number of electrons transferred between the alloys and the graphitic carbon, thus modifying the HER activity (Fig. 6f and 6g).

23

Fig. 6. Fe-Co-Ni-PB-derived FeCoNi@C nanocomposite: (a) schematic illustration of the synthetic process, (b) FESEM image, (c, d) TEM images, (e) LSV curve in 1.0 M KOH, (f) charge density distribution, and (g) ΔGH* value [148]. Adapted with permission from Ref. [148]. Copyright 2016, American Chemical Society. 3.1.2 Metal chalcogenides Transition metal chalcogenides, including transition metal sulfides, selenides, and tellurides, consist of positive metallic and negative S/Se/Te anions, resulting in strong ionic bonds. On the one hand, the incomplete filling of the d shells endows the transition metal chalcogenides with the ability to adsorb H* [149]. On the other hand, the presence 24

of S/Se/Te anions generates charge polarization, inducing partial negative charges localized at these centers in the corresponding sulfides/selenides/tellurides structure, promoting the attraction of protons to the anion-terminated surfaces, enhancing the HER activity [96]. Among transition metal chalcogenides, most transition metal sulfides are semiconductors, exhibiting inferior electrical conductivities compared to those of their selenide and telluride counterparts, which have lower charge transfer abilities during the HER process [150]. Nevertheless, this issue could be effectively alleviated by constructing transition metal sulfides@C nanocomposites through the sulfidation of MOF precursors. For example, Tian et al. reported an advanced NiS2@C electrocatalyst synthesized by the sulfidation of a Ni–BTC precursor [151]. The HER activity of the NiS2@C electrocatalyst outperforms most nickel sulfide-based electrocatalysts, with an overpotential of only 219 mV to achieve a current density of 10 mA cm–2 in 1.0 M KOH. Since the transition metal species could show several valence states, their sulfides are usually formed in different stoichiometric ratios. For instance, in addition to the NiS2based electrocatalysts, Ni3S2- [152], Ni3S4- [153], NiS- [153], and Ni9S8- [154] based electrocatalysts have also been investigated, and all of them can exhibit superior HER activities. In light of the alloying strategy, recent studies have intentionally prepared MOF-derived bimetal sulfides as HER electrocatalysts [155–157]. For instance, Huang et al. synthesized several Co-based polyhedral bimetallic sulfides (MxCo3−xS4, M = Zn, Ni, and Cu) by the sulfidation of bimetallic ZIFs (designated as M-Co-ZIFs) at 350 °C under flowing N2 (Fig. 7a–c) [155]. Remarkably, all of these MxCo3−xS4 polyhedra exhibited much better electrocatalytic HER activities than that of pristine Co3S4 polyhedra. In particular, to drive a current density of 10 mA cm‒2 in 1.0 M KOH, the required overpotential for Zn0.3Co2.7S4 polyhedra was as low as 85 mV (Fig. 7d), 25

demonstrating the advantages of an alloying strategy. DFT results revealed that the incorporation of the second metal in the Co3S4 lattice not only enhanced the hybridization between the Co 3d orbit and the S 2p orbit and narrowed the band gap but also effectively optimized the Gibbs free energy for H* adsorption (Fig. 7e).

Fig. 7. (a) Schematic diagram for the synthesis of MxCo3–xS4@C nanocomposite and its (b) FESEM image, (c) TEM image, (d) LSV curve in 1.0 M KOH, and (e) ΔGH* value [155]. Adapted with permission from Ref. [155]. Copyright 2016, American Chemical Society. Compared to metal sulfides, the corresponding metal selenides usually exhibit a better HER activity owing to their higher intrinsic electrical conductivity [158]. The preparation of MOF-derived metal selenide@C nanocomposites is similar to that of 26

metal sulfide@C nanocomposites, except that sulfur sources are replaced by selenium derivatives [159]. For instance, Huang et al. prepared a nanocomposite consisting of coral-like NiSe encapsulated within a N-doped carbon framework by reacting a NiMOF with Se vapor at 600 °C under flowing Ar [160]. When applied as an HER electrocatalyst, it only required an overpotential of 123 mV in 0.5 M H2SO4 to afford a current density of 10 mA cm−2. Additionally, this catalyst also possessed a Tafel slope as low as 53 mV dec–1. Similar to the bimetal sulfides, the MOF-derived bimetal selenides also exhibited higher HER activities than their single metal counterparts. For instance, by reacting the Fe3+-etched ZIF-67 with Se vapor, Wu et al. prepared the (Fe, Co)Se2 polyhedra (Fig. 8a–d), which showed HER activity superior to that of pristine CoSe2 polyhedra and even comparable to that of the commercial Pt@C electrocatalyst (Fig. 8e) [161]. DFT calculations indicated that the improved HER activity could be attributed to the favorable adsorption-desorption behavior and the accelerated HER kinetics, which were induced by the local charge polarization between the incorporated Fe and Se atoms (Fig. 8f and 8g).

27

Fig. 8. ZIF-67-derived Fe-CoSe2@C nanocomposite: (a) schematic illustration of the synthetic process, (b–d) TEM images, (d) LSV curve in 1.0 M KOH, (f) density of states and (e) ΔGH* value [161]. Adapted with permission from Ref. [161]. Copyright 2018, American Chemical Society. Metal tellurides are believed to be the most promising HER electrocatalysts in the metal chalcogenides family owing to the following two points. One is that the Te atoms have more metallic character than S and Se atoms, which may be a favorable property for electrocatalysts [162]. The other is that transition metals alloyed with Te have been demonstrated to have improved tolerance to electrolytes with a broad range of pH values [163]. In view of these points, Wang et al. recently synthesized nanocomposites consisting of CoTe2 nanoparticles within a N-doped carbon framework via a straightforward telluridation process with ZIF-67 at 700 °C under a H2/Ar gas mixture [164]. The nanocomposite exhibited a remarkable HER catalytic activity in 1.0 M KOH, 28

with a Tafel slope as low as 58.04 mV dec–1 and an overpotential of 208 mV at a current density of 10 mA cm–2. Moreover, they also showed exceptional cycle durability with negligible decay after 20 h at a current density of 10 mA cm−2. Encouragingly, they also displayed excellent HER activity in acidic solution (0.5 M H2SO4), i.e., a low overpotential of 240 mV at a current density of 10 mA cm−2 and a small Tafel slope of 61.67 mV dec–1. Recently, Wang et al. synthesized and compared the HER activity of several cobalt tellurides with different stoichiometric ratios by reacting the ZIF-67 with Te vapor at controlled temperatures (Fig. 8a–c) [165]. It was found that the HER activity increased in the order of CoTe@C (η10 = 397 mV, b = 115.1 mV dec–1) < CoTe2@C (η10 = 295 mV, b = 97.8 mV dec–1) < CoTe1.1@C (η10 = 178 mV, b = 77.3 mV dec–1) in 1.0 M KOH (Fig. 8d, e). After a continuous reaction for 20 h or cycling for 1000 cycles, CoTe1.1@C also showed a negligible activity decay. EIS analysis suggested that the CoTe1.1@C possessed the strongest electron transfer ability (Fig. 8f). DFT calculations revealed that the ΔGH* value of CoTe1.1 (−0.1202 eV) was the lowest and was very close to the ideal ΔGH* value (0 eV), indicating the optimal H adsorption on its surface (Fig. 8g, h). The lowest ΔGH* value of CoTe1.1 can be attributed to its optimal electric structure.

29

Fig. 9. ZIF-67 derived CoTe1.1@C nanocomposites: (a) FESEM image, (b, c) TEM images, (d) LSV curve, (e) Tafel slope, (f) EIS spectrum in 1.0 M KOH, (g) schematic crystal structure, and (h) ΔGH* value [165]. Adapted with permission from Ref. [165]. Copyright 2019, Elsevier. 3.1.3 Metal phosphides Aside from transition metal chalcogenides, transition metal phosphides have also been demonstrated to be promising as advanced HER electrocatalysts. In general, transition metal phosphides for HER show the following three merits. First, metal phosphides usually consist of triangular prismatic structures, resulting in the formation of large numbers of unsaturated surface atoms, which enhances the intrinsic catalytic activity [166]. Second, both metal and P atoms within transition metal phosphides can function as active sites, which is different from when only metal atoms can serve as the main active sites, as in metal chalcogenides, greatly boosting the HER activity [166]. 30

More importantly, the P atoms can serve as a base and trap positively charged protons during HER due to their high electronegativity and the negative charge feature [167]. Benefiting from these features, various MOF-derived transition metal phosphide@C nanocomposites with superior HER activities have been reported. For instance, You et al. prepared the well-incorporated Co-P/NC nanocomposites by direct calcination of 3D well-performed ZIF-67 dodecahedra (uniform distribution with average size of 800 nm) at 900 °C under flowing Ar followed by a phosphorization treatment at 300 °C [168]. When applied as the HER electrocatalyst, the as-prepared nanocomposites showed remarkable catalytic activities in 1.0 M KOH, driving a current density of 10 mA cm−2 at an overpotential of 154 mV. Wang et al. synthesized porous Ni2P@C nanosheets by the phosphorization of NiO nanosheet/MOF-74 precursors (Fig. 10a–d). The as-synthesized Ni2P@C nanosheet exhibited an excellent electrocatalytic performance towards HER in 1.0 M KOH, with a low overpotential of 168 mV at a current density of 10 mA cm–2 (Fig. 10e), a small Tafel slope of 63 mV dec–1 (Fig. 10f), as well as a negligible current decay after 10 h (Fig. 10g) [169].

31

Fig. 10. Ni-MOF-74-derived porous Ni2P@C nanocomposite: (a) schematic illustration of synthetic process, (b) FESEM image, (c, d) TEM images, (e) LSV curve, (f) Tafel slope, and (g) cycle durability in 1.0 M KOH [169]. Adapted with permission from Ref. [169]. Copyright 2018, the Royal Society of Chemistry. Chen et al. synthesized Co-incorporated FeP nanotubes via heating the Co-Fe MIL88B precursor followed by phosphorization (Fig. 11a–d) [170]. The Co-Fe-P nanotube exhibited an outstanding HER catalytic activity over a wide pH range, requiring small overpotentials of 66, 138, and 86 mV at a current density of 10 mA cm−2 in 0.5 M H2SO4 (Fig. 11e), 1.0 M phosphate buffer solution (PBS) (Fig. 11f), and 1.0 M KOH (Fig. 11g), respectively, and these values are much better than those of pristine FeP nanotubes (Fig. 11h). DFT calculations revealed that the solid solution of Co in the FeP lattice increased the density of states (DOS) near the Fermi level, enhancing the 32

intrinsic electrocatalytic activities of the Co-Fe-P nanotubes (Fig. 11i, j).

Fig. 11. Co-Fe MIL-88B-derived Co-Fe-P nanotubes: (a) schematic illustration of synthetic process, (b) FESEM image, (c, d) TEM images, cycle durability in (e) 1.0 M KOH, (f) 1.0 M PBS, and (g) 0.5 M H2SO4, (h) LSV curve in 1.0 M KOH, (i) density of states, and (j) charge density distribution [170]. Adapted with permission from Ref. [170]. Copyright 2018, Elsevier.

33

Fig. 12. (a) Schematic diagram for the synthesis of Cu0.3Co2.7P/NC nanocomposite and its (b, c) TEM images, corresponding (d) P, (e) Cu, (f) Co and (g) C elemental mappings, (h) LSV curve in 1.0 M KOH, (i) cycle durability in 1.0 M KOH and (j) ΔGH* value [171]. Adapted with permission from Ref. [171]. Copyright 2016, Wiley-VCH. Given that the content of the incorporated second metal substantially influences the HER catalytic activity, Song et al. investigated the HER activity of Cu-incorporated CoPx polyhedra with different Cu/Co ratios (Fig. 12a–g) [171]. It was found that an appropriate Cu doping was the decisive factor for achieving the best electrocatalytic performance. To be more specific, as the amount of Cu dopant increased, the Co-P phase mainly existed in the forms of Co2P, CoP, and then CoP2, respectively. On the one hand, a high atomic percentage of P phase is desired because more negatively charged P can more readily attract the hydrogen protons at low coverage and desorb H2 34

at high coverage, which will be in favor of improving the HER activity. However, on the other hand, the electrical conductivity is usually compromised for metal phosphides with high P contents, lessening the electron transfer ability during the HER process. As a result, Cu-Co-P materials with a Cu/Co ratio of 1:7 could exhibit the best HER activity in 1.0 M KOH, with an overpotential of 220 mV at a current density of 10 mA cm–2 (Fig. 12h) and a small current decay after 1000 cycles (Fig. 12i). Such a result was also in line with the DFT calculation results, in which the ΔGH* (–0.26 eV) value is closest to zero (Fig. 12j). 3.1.4 Metal nitrides Transition metal nitrides are special interstitial compounds, and they possess a high melting point and flexible mechanical property. More importantly, the formation of metal–nitrogen bonds in the density of states of the metal d-band usually generates a smaller deficiency, endowing the metal nitrides with electron donating character [172– 174]. This leads to higher catalytic activities as compared to the corresponding metal counterparts and thereby, an opportunity for using metal nitrides as HER electrocatalyst. In this regard, several MOF-derived metal nitride@porous carbon nanocomposites have been synthesized and investigated as HER electrocatalysts. For instance, Chen et al. synthesized hybrid composites consisting of Co5.47N nanoparticles encapsulated within porous carbon through the nitridation of ZIF-67 polyhedra at 700 °C under flowing NH3 (Fig. 13a–g) [175]. During the nitridation, the N and Co species tended to combine and form Co-N compounds. The resulting hybrid composites exhibited an outstanding HER performance in 1.0 M KOH, affording a current density of 10 mA cm−2 at a low overpotential of 149 mV (Fig. 13h) and a small Tafel slope of 51 mV dec–1 (Fig. 13i). Moreover, the cobalt nitrides are more stable than that of bare Co during the cycle durability tests (Fig. 13j). 35

Fig. 13. ZIF-67-derived Co5.47N NP@N-PC nanocomposite: (a) schematic diagram illustration of synthetic process, (b) FESEM image, (c) TEM image, (d) HAADF image, corresponding (e) Co, (f) N and (g) C elemental mappings, and (h) LSV curve, (i) Tafel slope, (j) cycle durability in 1.0 M KOH [175]. Adapted with permission from Ref. [175]. Copyright 2018, American Chemical Society. In order to avoid using a toxic NH3 during the nitridation, Mahmood et al. developed a new strategy. By calcinating a mixture of the MOF precursors with melamine at 800 °C under an inert atmosphere followed by acid treatment, they successfully fabricated a hybrid catalyst composed of Fe2N nanoparticles decorated on highly porous and defect-rich carbon nanosheets (Fig. 14a–d) [176]. Notably, the 36

melamine can be decomposed into NH3, C3N2+, C3N3+, and C2N2+ upon heating, leading to the conversion of Fe species into active FexNy species. The resulting catalyst showed an outstanding HER activity in 0.5 M H2SO4, with a small Tafel slope of 49 mV dec−1 and a low overpotential of 123 mV at a current density of 10 mA cm−2. To clarify the underlying mechanism for the excellent HER catalytic properties, DFT analysis was performed and the calculation results suggested that the Fe2N could generate a much larger electronic cloud at the edge of the carbon sheets (Fig. 14e), which rendered them much more catalytically active. As expected, the coupling architecture between the Fe2N and carbon matrix showed a ΔGH* value (−0.20 eV) closer to zero than those of pristine Fe2N and a carbon matrix (Fig. 14f), indicating the optimal hydrogen adsorption ability.

37

Fig. 14. (a) Schematic diagram for the synthesis of Fe2N@C nanocomposite and its (b) FESEM image, (c, d) TEM images, (e) charge density distribution, and (f) ΔGH* value [176]. Adapted with permission from Ref. [176]. Copyright 2018, Wiley-VCH. 3.1.5 Metal carbides As another family of special interstitial compounds, transition metal carbides are of particular interest as HER electrocatalysts because of their electronic structures are similar to that of Pt and exceptional chemical stability resistance to alkaline and acidic corrosion, which enables efficient HER activity over a wide range of pH values [177]. Among the investigated metal carbides electrocatalysts, a Mo-based carbide was 38

demonstrated to exhibit the closest HER activity to that of Pt by both experimental and theoretical analyses [178]. However, it should be noted that although the large family of MOFs covers a wide range of metal species, the reported MOF-derived, well-defined, transition metal-based materials are typically limited to a handful of metal species (e.g., Fe, Co, Ni, Cu, and Zn). Therefore, the preparation of MOF-derived Mo-based carbides electrocatalysts is of great interest. Wu et al. employed a cage-confinement pyrolysis for synthesizing the nanostructured octahedral MoCx octahedral nanoparticles (Fig. 15a–d)

[179].

In

their

reported

method,

a

Cu-based

MOF

(HKUST-1,

Cu3(BTC)2(H2O)3) was used as a host, and then Mo-based Keggin-type POMs (H3PMo12O40) were periodically inserted in the largest pores of the host, resulting in the formation of the Cu-MOF@Mo-POMs precursor. By thermal annealing the precursor at 800 °C under flowing N2 followed by Fe3+ etching treatment, MoCx nanocrystals embedded within the carbon matrix were successfully generated. The asprepared MoCx-based HER electrocatalyst afforded a current density of 10 mA cm–2 at overpotentials of only 142 and 151 mV in 0.5 M H2SO4 (Fig. 15e) and 1.0 M KOH (Fig. 15f), respectively. Motivated by this finding, a series of MoCx-carbon-based HER electrocatalysts were synthesized by using W(CO)6/RHO-[Zn(eim)2] (MAF-6, Heim: 2-ethylimidazolate) [180], (PMo12O40)2·2H2O [Zn(4,4 ′ -bis(imidazolyl)biphenyl)2]3 [181], MoCl5/MIL-53(Al) [182], and PMo12@MIL-100(Fe) [183] as precursors.

39

Fig. 15. (a) Schematic diagram for the synthesis of octahedral MoCx nanoparticle and its (b, c) HRTEM images, (d) corresponding elemental mappings, LSV curves in (e) 0.5 M H2SO4 and (f) 1.0 M KOH [179]. Adapted with permission from Ref. [179]. Copyright 2015, Nature Publishing Group. In addition to the cage confinement strategy, directly loading the metal species within the MOF host in advance provides another possibility to achieve metal carbidesbased composites. Shi et al. attempted to synthesize Mo-based carbides consisting of ultrafine MoC (~3 nm) encapsulated within graphitic carbon via the one-step pyrolysis of Mo3(BTC)2 (BTC = benzene-1,3,5-tricarboxylate) at 700 °C under flowing Ar [184]. 40

The as-prepared nanocomposites exhibited low overpotentials of 77 and 124 mV, small Tafel slopes of 50 and 43 mV dec−1, and high exchange current densities of 0.212 and 0.015 mA cm−2 in 1.0 M KOH and 0.5 M H2SO4, respectively. Even after 3000 LSV cycles, the HER activity almost remained as the original one (Fig. 16d). In addition to Mo-based carbides, Co3ZnC-based HER electrocatalysts have also been reported. For example, by the straightforward carbonization of a Zn-Co-ZIF precursor at 600 °C under a H2/Ar gas mixture, Co3ZnC/Co embedded in a nitrogen-doped carbon nanotube-grafted carbon polyhedra could be prepared, and it exhibited remarkable HER activity [185].

Fig. 16. (a) schematic diagram for the synthesis of MoC@C hybrid and its (b, c) HRTEM images and (d) cycle durability in 1.0 M KOH [184]. Adapted with permission from Ref. [184]. Copyright 2016, the Royal Society of Chemistry. 3.1.6 Single atom catalyst Recent studies have shown that MOFs could function as ideal templates to prepare 41

carbon-supported single-metal atoms, showing exceptional catalytic activities [186– 191]. Theoretically, isolated single-atom catalysts (SACs) could offer remarkably better HER activities as compared to the corresponding bulk and nanosized electrocatalysts owing to their much higher atom utilization and unique low coordination and unsaturation [187]. In 2016, Fan et al. achieved the first atomically isolated Ni species anchored on graphitic carbon (A-Ni-C) by employing a Ni-MOF (Ni(L-asp)(H2O)2·H2O) as a precursor [187]. In their reported method, three main steps were involved in preparation of the A-Ni-C electrocatalysts (Fig. 17a–d). First, the NiMOF was carbonized at 700 °C under flowing N2 to obtain the composites composed of Ni nanoparticles encapsulated within graphitic carbon (Ni@C). Second, the obtained composites were leached by hydrochloric acid (HCl) to remove the excess Ni metal and obtain composites composed of little Ni nanoparticles protected by layers of graphitic carbon (HCl-Ni@C). Finally, the HCl-Ni@C was activated by applying a constant electrochemical potential, generating the final A-Ni-C hybrid. When the A-Ni-C hybrid was used as the HER electrocatalyst, it could drive a current density of 10 mA cm‒2 in 0.5 M H2SO4 at an overpotential of only 34 mV, much lower than that of the Ni@C electrocatalyst.

42

Fig. 17. Ni-MOF-derived porous A-Ni-C nanocomposites: (a) schematic illustration of the synthetic process, (b) TEM image, (c) HRTEM image, (d) high-angle annular darkfield scanning transmission electron microscopy (HAADF) image, (e) LSV curve and (f) Tafel slope in 0.5 M H2SO4 [187]. Adapted with permission from Ref. [187]. Copyright 2016, Nature Publishing Group. The MOF precursor could be directly transformed into a carbon framework to support both intrinsic and extrinsic single metal atoms. For instance, Chen et al. reported a single-atom-W catalyst (W-SAC) supported on a Zr-MOF (UiO-66-NH2)derived carbon framework [188]. The W-SAC was synthesized in three steps (Fig. 18a): i) the tungsten precursor (WCl5) was locked within the framework of UiO-66-NH2 to obtain the WCl5/UiO-66-NH2 composite; ii) the composite was then heated under Ar at 950 °C, during which the uncoordinated -NH2 groups in the UiO-66-NH2 effectively inhibited the aggregation of W species; iii) the pyrolyzed product was washed by hydrofluoric acid (HF) solution to remove the zirconia, generating the final W-SAC 43

(Fig. 18b–d). Notably, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) revealed the atomic dispersion of the W species within the N-doped carbon matrix (Fig. 18e). X-ray absorption fine structure (XAFS) spectroscopy further revealed that the single W atoms were prone to combine the N and C species to form W1N1C3 moieties (Fig. 18f–h), which was believed to be the favored local structure of the W species for HER catalysis. As expected, the W-SAC showed a low overpotential of 85 mV at a current density of 10 mA cm−2 in 1.0 M KOH (Fig. 18i), which was much smaller than that (169 mV) of W nanoparticles encapsulated within a N-doped carbon matrix. DFT calculations indicated that the electron density on the C atoms coordinated to W apparently increased upon combining the W with the C species, which effectively promoted the HER activity. In addition, the formation of W1N1C3 also greatly enhanced the HER activity (Fig. 18j and 18k).

44

Fig. 18. (a) Schematic diagram for the synthesis of W-SAC and its (b, c) TEM images, (d) corresponding elemental mappings, (e) HAADF image, (f) XANES curve, (g) k3weight FT-EXAFS curve, (h) k3-weight FT-EXAFS fitting curve, (i) LSV curve in 1.0 M KOH, (j) charge density distribution, and (k) ΔGH* value [188]. Adapted with permission from Ref. [188]. Copyright 2018, Wiley-VCH. 3.2 Carbon matrix modification In addition to the components of the TMs/TMCs, the carbon matrix derived from MOF precursors also greatly affects the electrocatalytic activity. Generally, a MOFderived carbon matrix could enhance the HER activity of the composites based on the following four aspects: (i) improving the electrical conductivity of the nanocomposites, which is beneficial for the electron transfer and penetration; (ii) inhibiting the aggregation of nanoparticles, which increases the number of available catalytic active 45

sites; (iii) generating a coupling effect between the nanoparticles and the carbon matrix, which synergistically boosts the catalytic activity; (iv) and serving as active sites, which accelerates the hydrogen production. To acquire a favorable MOF-derived carbon matrix, three design strategies, namely, adjusting the graphitization degree of carbon matrix, heteroatom doping, and dual carbon confinement have been well established. 3.2.1 Graphitization degree of carbon matrix Higher graphitization degrees of the carbon matrix result in higher the electrical conductivities; thus, the electron transfer ability is enhanced during the HER process. Thermal treatment temperature is a main factor influencing the graphitization degree of the MOF-derived carbon matrixes [192]. Recently, Chen et al. synthesized a hybrid composites consisting of Co9S8 nanoparticles encapsulated in carbon matrix through simultaneous sulfidation and carbonization of a Co-based PBA at different temperatures [193]. It was found that with the increase of the sulfidation temperature from 500, 600, to 700 °C, the coordinated organic ligands (C≡N) liberated from the PBA were converted into amorphous carbon (AC), graphitic carbon (GC) and carbon nanotubes (CNT), respectively, indicating the increase of the graphitization degree (Fig. 19a). DFT calculations revealed that the CNTs possessed a stronger charge coupling with Co9S8 than those of AC and GC. Currently, to obtain a carbon matrix with high electrical conductivity (e.g., GC and CNT), a high temperature (> 600 °C) is usually employed to treat the MOF precursors. However, particle aggregation and a low specific surface area can occur at a high temperature, lessening the electrocatalytic activity [193,194]. Thus, achieving a balance among the graphitization degree, particle distribution, and specific surface area is crucial for achieving hybrid composites with superior HER activities. In addition to the thermal treating temperature, it was found that the presence of 46

specific metal ions (Co, Fe, and Ni) within MOFs could noticeably promote the graphitization degree of carbon during the annealing process. For instance, Gadipelli et al. demonstrated that the doping of Zn-MOF (ZIF-8) with a Ni species could effectively enhance the graphitization degree of the carbon matrix in products calcinated at the same temperature [195]. Moreover, the presence of these special metals can be functioned as catalysts to catalyze the decomposition of organic linker into highgraphitization CNTs. For instance, Zhou et al. reported the synthesis of composites comprised of embedded CoSe2 nanoparticles within defective carbon nanotubes (CoSe2@D-CNTs) through a carbonization-oxidation-selenization procedure from a ZIF-67 precursor (Fig. 19b–d) [196]. In their reported method, the polyhedral ZIF-67 precursors were first calcinated at 800 °C under flowing Ar to prepare Co nanoparticles embedded within carbon nanotubes (Co@CNTs). Second, the Co@CNTs were oxidized at 350 °C in air to generate the Co3O4@D-CNTs hybrids. Finally, the Co3O4@D-CNTs hybrids were further converted to CoSe2@D-CNTs through a selenization process. Interestingly, the preoxidation treatment could introduce a large number of defects into the carbon nanotubes, which strengthened the coupling between CoSe2 and the defective CNTs, leading to a remarkably improved HER performance compared to that of the CoSe2@C hybrid prepared by the direct selenization of the ZIF67 precursor. As shown in Fig. 19e, the as-prepared CoSe2@D-CNTs showed outstanding HER activity in 0.5 M H2SO4, with an overpotential of 132 mV at a current density of 10 mA cm–2, a small Tafel slope of 82 mV dec–1 as well as a robust durability at a high current density.

47

Fig. 19. (a) Schematic diagrams of Co9S8 encapsulated by AC, GC and CNT matrix [193]; (b) schematic diagram for the synthesis of CoSe2@DC hybrid and its (c, d) TEM images and (e) LSV curve in 0.5 M H2SO4 [196]. Adapted with permission from Ref. [193]. Copyright 2018, the Royal Society of Chemistry. Adapted with permission from Ref. [196]. Copyright 2016, Elsevier. It should be mentioned that in addition to the specific metal ions (Co, Fe, and Ni) within the MOFs, the thermal annealing atmosphere and the particle size of the MOFs 48

may have an important impact on the formation of the CNTs [197–200]. For instance, the H2/Ar gas mixture [197] and the smaller MOF particle sizes [199] are believed to be beneficial for the formation of CNTs during thermal treatment. 3.2.2 Heteroatom doping Heteroatom doping is also an efficient way to modify the carbon matrix since it not only tunes the electronic structure of the carbon atoms but also strengthens the electronic coupling between the metal and carbon atoms, accelerating the electron transfer, altering the ab-/desorption ability of the H intermediate, or even generating new electrocatalytic active sites [201]. Combining the DFT calculations with experimental studies, Qu et al. found that the HER activity was strongly dependent on the heteroatoms doping-induced charge distribution over the carbon matrix, and they confirmed that the HER activity induced by the heteroatoms increased in the order of B < S < P < N [202]. According to the above findings, the doping of heteroatom N into a carbon matrix could be the most popular way to improve the HER activity. Two routes have been developed to realize the heteroatom N doping of MOF-derived carbon matrixes. One is to select a precursor that innately contains N element, the other is to combine the MOF with another N-containing material as the precursor, allowing the injection of the heteroatom N during the post treatment. In the first route, by thermal treating the MOF precursor containing the N species, the N-doped carbon materials could be directly achieved. For instance, Wang et al. synthesized a leaf-like hybrid composites consisting of Co nanoparticles encapsulated within N-doped carbons by annealing a 2D ZnCo-ZIF precursor at 700 °C under flowing Ar/H2 mixture [203]. The as-synthesized composites used as HER electrocatalysts could drive a current density of 10 mA cm‒2 at an overpotential only 213 and 200 mV in 0.5 M H2SO4 and 1.0 M KOH, respectively. Unlike the first route, the second route is relatively complex in the 49

fabrication procedure. For instance, Huang et al. first synthesized Co-MOFs by coordinating the H2NDC (2,6-naphthalenedicarboxylic acid) with Co(acac)2. Afterwards, the as-prepared Co-MOFs were mixed with hexamethylenetetramine (C6H12N4), which has a rich N content [204]. Finally, by heating the above mixture at 650 °C under flowing argon, the hybrid composites composed of Co nanoparticles embedded within N-doped carbon were successfully achieved. As an HER electrocatalyst, it could exhibit an overpotential as low as 103 mV at a current density of 10 mA cm–2 in 1.0 M KOH. Compared to single-heteroatom doping, multiple-heteroatom doping could synergistically boost the HER performance. For instance, Liu et al. reported the hybrid composites consisting of Co nanoparticles encapsulated within a N, B-codoped interconnected graphitic carbon matrix (Co/NBC) by direct carbonization of a cobaltbased boron imidazolate framework (BIF-82-Co, C15H23BCoN8O5) under flowing N2 (Fig. 20a) [205]. The optimized Co/NBC composites exhibited excellent HER catalytic activity with an overpotential of only 117 mV at a current density of 10 mA cm‒2 in 1.0 M KOH. Tabassum et al. synthesized hybrid composites composed of CoP nanoparticles encapsulated within B, N-codoped carbon nanotubes (CoP@B,N-CNTs) through a pyrolysis process followed by a phosphorization (Fig. 20b–e) [206]. Notably, in order to achieve B doping, the intermediates composed of Co nanoparticles encapsulated within B, N-codoped carbon nanotubes (Co@B,N-CNTs) were fabricated by the pyrolysis of a mixture of boric acid, urea, PEG-2000, and ZIF-67 nanocrystals at 900 °C under flowing Ar. The hybrid CoP@B,N-CNTs investigated as an HER electrocatalyst could achieve a current density of 10 mA cm‒2 in 0.5 M H2SO4 at an overpotential of ~100 mV (Fig. 20f), which was much lower than those of the N-doped (257 mV) and B-doped analogs (379 mV), strongly highlighting the synergistic doping 50

effects of B and N for improving the HER activity. Impressively, the hybrid also showed an exceptional HER activity in both 1.0 M KOH (Fig. 20g) and 1.0 M PBS (Fig. 20h). 3.2.3 Dual carbon confinement As another way to modify the carbon matrix, dual carbon confinement, i.e., introducing an additional carbon matrix with high electrical conductivity (such as CNTs or reduced graphene oxide (rGO)) into the MOF-derived TMs/TMCs and porous carbon hybrids, has recently attracted much attention. It was found that dual carbon confinement could not only effectively enhance the electrical conductivity of the carbon matrix but also strengthen the structural stability [207]. Additionally, the introduction of graphene could effectively prevent the particle aggregation at high temperatures due to its strain confinement effect [208]. For example, Hou et al. synthesized a ZIF67@GO precursor and converted it into the final composites consisting of Co nanoparticles uniformly coated by both porous carbon polyhedra and rGO sheets at 900 ºC under flowing Ar [209]. Interestingly, during the carbonization of the organic groups of ZIF-67, N species would be injected into the rGO, resulting in the formation of Ndoped rGO, which could further facilitate the electron transfer. As expected, when using the as-prepared hybrid composites as HER electrocatalysts, they required an overpotential of only 229 mV to drive a current density of 10 mA cm‒2 in 0.5 M H2SO4, much lower than that (~380 mV) of rGO-free Co nanoparticles embedded in porous carbon polyhedra. To further reinforce the coupling effect between the rGO and the MOF-derived composites, in situ combining the MOFs with GO instead of simple mixing is required. Xu et al. achieved the electrostatic adsorption of Ni2+ on the carboxyl group of GO by stirring a mixture of Ni(NO3)2·6H2O and 100 mL of GO solution (Fig. 21a–i) [210]. Following that, K3[Co(CN)6]2 was added to the above 51

solution to allow the in situ nucleation and growth of Ni-Co-PBA nanocrystals on the GO surface. Finally, the formed Ni-Co-PBA anchored on the reduced graphene composite was selenized at 350 °C under flowing Ar to prepare (Ni, Co)Se2 nanocages anchored on the rGO composites ((Ni,Co)Se2-rGO). When investigated as an HER electrocatalyst, the as-prepared (Ni,Co)Se2-rGO could achieve a current density of 10 mA cm‒1 in 1.0 M KOH at an overpotential of only 128 mV (Fig. 21j), which is lower than that of rGO-free (Ni,Co)Se2 nanocages (159 mV). In addition, the as-prepared (Ni,Co)Se2-rGO catalyst showed the Tafel slope as small as 79 mV dec–1 and a ~100% of Faradaic efficiency (Figs. 21k and 21l).

52

Fig. 20. (a) Schematic diagram for the synthesis of Co/NBC nanocomposite [205]; (b) schematic diagram for the synthesis of CoP@BNC nanocomposite and its (c) FESEM image, (d, e) TEM images and LSV curves in (f) 0.5 M H2SO4, (g) 1.0 M KOH and (h) 1 M PBS, respectively [206]. Adapted with permission from Ref. [205]. Copyright 2018, Wiley-VCH. Adapted with permission from Ref. [206]. Copyright 2017, Wiley-VCH.

53

Fig. 21. (a) Schematic diagram for the synthesis of (Ni,Co)Se2-rGO nanocomposite, (b) FESEM image of NiCo PBA nanocubes, (c) FESEM and (d, e) TEM images of NiCo PBA-GO, (f) FESEM, (g, h) TEM, (i) HADDF images and corresponding elemental mappings of (Ni,Co)Se2-rGO. The (j) LSV curve, (k) Tafel slope, and (l) Faradaic efficiency in 1.0 M KOH of (Ni,Co)Se2-rGO nanocomposite as the HER electrocatalyst [210]. Adapted with permission from Ref. [210]. Copyright 2017, American Chemical Society. Motivated by the in situ combination strategy, MOFs could also be directly grown on carbon matrix with carboxylic groups [211–214]. For instance, Wu et al. in situ coupled CoP polyhedra with carbon nanotubes (Fig. 22a–d) [211]. Specifically, Co2+ was first deposited on the surface of modified CNTs through its electrostatic 54

interactions with the carboxylic group. Heterogeneous crystal nucleation and uniform growth of ZIF-67 polyhedra on the surface of the CNTs were then induced by further adding 2-methylimidazolate (2-MeIm), in which 2-MeIm ligands could coordinate with Co2+ at room temperature. Following that, two thermal-induced conversion processes, namely, oxidation and phosphorization, were employed to transform the ZIF67@CNTs precursors into composites consisting of CoP nanoparticles encapsulated within both porous carbon polyhedra and CNTs (CoP@CP/CNTs). To afford a current density of 10 mA cm‒1 in 0.5 M H2SO4, the as-synthesized CoP@CP/CNTs electrocatalyst required an overpotential of only 139 mV (Fig. 22e), which is much lower than that of CNT-free CoP polyhedra (339 mV) and that of a physical mixture of CoP polyhedra and CNT (245 mV). Additionally, the CoP@CP/CNTs electrocatalyst also showed a small Tafel slope (Fig. 22f) and good cycle stability (Fig. 22g). These results strongly highlight the advantages of in situ coupling between MOF-derived composites and a second carbon matrix for electrocatalytic HER processes.

55

Fig. 22. (a) Schematic diagram for the synthesis of CoP-CNTs nanocomposite and its (b) FESEM image, (c, d) TEM images, and (e) LSV curve, (f) Tafel slope, (g) cycle durability in 0.5 M H2SO4 [211]. Adapted with permission from Ref. [211]. Copyright 2017, Wiley-VCH. Based on the above discussions, integrating these merits, including a high graphitization degree, appropriate heteroatom doping, and coupling a second carbon material into the MOF-derived carbon matrix, is expected to synergistically boost the HER activity of the electrocatalysts. Chen et al. reported the synthesis of ultrafine Co nanoparticles encapsulated in N-doped carbon‐nanotube‐grafted graphene sheets (Co@N-CNTs@rGO) as an advanced HER electrocatalyst [215]. Fig. 23a shows the synthetic scheme for this process. First, both sides of the GO sheet were coated with 56

ZIF-8 nanocubes (ZIF-8@GO) through an electrostatic interaction followed by a coordination reaction. After that, sandwich-like precursors consisting of truncated cubic ZIF-67@ZIF-8 crystals with core–shell feature homogeneously wrapped on both sides of the GO sheet (ZIF-67@ZIF-8@GO) were achieved via the heteroepitaxial growth of ZIF-67 based on the ZIF@GO template. Finally, the Co@N-CNTs@rGO hybrid was synthesized by simultaneous thermal-induced carbonization and reduction of the ZIF-67@ZIF-8@GO precursor at 900 °C under flowing Ar (Fig. 23b–l). The synthesis strategy was decent and the resulting composites integrated the advantages of the nitrogen-rich dopant, the uniform distribution of Co nanoparticles, the in situformed highly graphitic N-CNTs@rGO, the large surface area, and the abundant porosity. As a result, the as-fabricated Co@N-CNTs@rGO hybrid showed an exceptional electrocatalytic HER activity in 1.0 M KOH, with an overpotential of 108 mV at a current density of 10 mA cm‒2 (Fig. 23m) and negligible activity decay after 1000 LSV cycles (Fig. 23n). DFT calculation results further promulgated that the Co, N, and C species could be used to synergistically tune the hydrogen absorption affinity, allowing optimization of the ΔGH* value (Fig. 23o–q).

57

Fig. 23. (a) Schematic diagram for the synthesis of Co@N-CNTs@rGO composite; (b) FESEM image and (c) TEM image of core-shell ZIF-67@ZIF-8@GO precursor; (d, e) FESEM images, (f–h) TEM images, (i) HAADF image, (j–l) corresponding elemental mapping images, and LSV curve, cycle durability in 1.0 M KOH for Co@NCNTs@rGO composite; (o) the comparison of ΔGH* values of Co@N-C and Co@C composites and the corresponding charge density distributions of (p) Co@C–H and (q) Co@N-C–H systems [215]. Adapted with permission from Ref. [215]. Copyright 2018, Wiley-VCH. 3.3 Morphology tuning The surface structure of the MOF-derived TMs/TMCs@C nanocomposites is highly dependent on that of the MOF precursors. To develop a robust nanocomposite 58

with a satisfactory specific surface area and porosity, the careful tuning of the morphology of MOF precursors is indispensable [216–273]. Up to now, the developed strategies including the regulation of geometric dimension and construction of hollow and/or core-shell structure, have been employed to tune the morphologies of MOFderived TMs/TMCs@C nanocomposites. 3.3.1 Regulation of geometric dimension Most previous works basically focused on the three-dimensional (3D) MOFderived electrocatalysts due to the easy preparation of 3D MOF precursors [218]. For instance, Lim et al. prepared well-defined 3D Fe-PBA nanocubes with an average size of 600 nm through a coprecipitation reaction at room temperature [219]. With a further phosphorization at 350 °C under flowing Ar, the precursors could be efficiently converted into the well-inherited 3D FeP@NC nanocomposites, which not only possessed a specific surface area as high as 133.3 m2 g−1 but also featured hierarchical pores with a large number of mesopores and a small number of micropores. When applied as an HER electrocatalyst, the as-prepared FeP@NC showed a remarkable catalytic activity in 1.0 M KOH, driving a current density of 10 mA cm−2 at an overpotential of 180 mV. Relative to 3D MOF-derived electrocatalysts, reports regarding on onedimensional (1D) and two-dimensional (2D) MOF-derived electrocatalysts have been much less frequent during the past few years. This might be attributable to the following two aspects. One is the arduous preparation procedures involved with solvothermal reactions of the 1D/2D MOF precursors; the other is the rapid destruction of the original structure during thermal treatment due to their tendency to undergo self-aggregation and their low surface free energy [220,221]. Nevertheless, in comparison with 3D MOF-derived electrocatalysts, owing to their ultrathin nature and much higher surface 59

area-to-volume ratio, the corresponding 1D and 2D species theoretically not only possess higher electron transfer ability, but also have more exposed electrocatalytic active sites [222]. Recently, Zhao et al. synthesized a 1D Fe-Ni-MOF (MIL-88-Fe/Ni, (Fe,Ni)3O(H2O)2Cl(BDC)3) through a solvothermal reaction at 100 °C for 48 h [223]. Following that, 1D hybrid composites consisting of FeNi3 and NiFe2O4 nanoparticles encapsulated by porous carbon/carbon nanotubes (Fe-Ni@NC-CNTs) were achieved through the carbonization of 1D MIL-88-Fe/Ni at 800 °C under flowing Ar (Fig. 24a– d). The as-prepared 1D Fe-Ni@NC-CNTs electrocatalyst was endowed with high electrical conductivity, a hierarchical pore feature and a robust 1D structure, leading to a rapid electron and mass transfer abilities, abundant potential active sites, and structural stability during electrocatalytic processes. As a consequence, the required overpotential of Fe-Ni@NC-CNTs for driving a current density of 10 mA cm‒2 towards HER in 1.0 M KOH was only 202 mV, showing superior catalytic activity. Anandhababu et al. synthesized ultrathin 2D ZIF-67 nanoplates with an average thickness of 4 nm through a solvothermal reaction [224]. After a subsequent thermal oxidation and phosphorization processes, ultrathin CoPO nanosheets with high porosity, a number of accessible active sites as well as oxygen defects could be obtained (Fig. 24e–g), resulting in superior HER activity. To be specific, the as-obtained 2D porous CoPO nanosheets showed a very low overpotential of 158 mV at a current density of 10 mA cm−2 in 1.0 M KOH towards HER.

60

Fig. 24. Schematic diagram for the synthesis of (a) Fe-Ni@NC-CNTs composite and its (b‒d) TEM images [223]; (e) schematic diagram for the synthesis of CoPO nanosheet and its (f) TEM image and (g) AFM image [224]. Adapted with permission from Refs. [223] and [224]. Copyright 2018, Wiley-VCH. Li et al. prepared 2D ZIF-67 nanoplates by a solvothermal reaction between a 3D ZIF-67 template and a Co(NO3)2 solution [225]. With a further phosphorization treatment at 300 °C, the as-obtained CoP@NC nanocomposites still maintained their 2D morphology. When served as an HER electrocatalyst, CoP@NC nanocomposites showed excellent HER activity in 1.0 M KOH, with a small Tafel slope of 59 mV dec–1 and a low overpotential of 140 mV at a current density of 10 mA cm–2, which was remarkably superior to that of its 3D CoP@C counterpart. Cong et al. recently synthesized a 2D Co@NC HER electrocatalyst by employing a new 2D Co-MOF 61

{[Co3(µ2-OH)4(hpa)2]·2H2O} as the precursor and subjecting it to pyrolysis at 800 °C under flowing N2 [226]. 3.3.2 Construction of hollow structure Compared to their solid counterparts, hollow nanostructures possess larger surface area owing to the extra void spaces, ensuring a greater number of electrochemically active sites and more sufficiently accessible contact interfaces between the electrode and electrolyte for efficient mass transfer [227–237]. Motivated by this circumstance, tremendous effort has been dedicated to fabricating MOF-derived TMs/TMCs@C electrocatalysts with hollow structures. Of particular note, unlike the hard template method for fabricating hollow structures, the MOF-templated method can serve as both morphology-determining scaffold and an essential component within the resultant hollow structures, which not only greatly simplifies the fabrication procedures but also endows the obtained hollow structure with open porous channels [238]. Generally, two strategies have been applied to fabricate MOF-derived electrocatalysts with hollow structures. One is the preparation of hollow MOF precursors followed by a thermal treatment; the other is the direct thermal conversion of solid MOF precursors to their hollow derivates [239–253]. The transformation of solid MOF precursors to their hollow counterparts/derivatives mainly involves four mechanisms, Ostwald ripening, controlled etching, heterogeneous contraction, and the Kirkendall effect [254–259]. Zou et al. fabricated a hollow Ni-MOF precursor through a solvothermal reaction, and the Ostwald ripening process was believed to dominate the formation of this hollow structure (Fig. 25a–d) [239]. Specifically, the Ni2+ species first bridged with the organic groups of 1,3,5-benzene acid to generate amorphous solid spheres in the initial coordination stage. However, these amorphous solid spheres were metastable and they were inclined to become a more thermodynamically stable phase by crystallizing upon 62

further solvothermal reaction. During the crystallization process, the inner part of the solid sphere gradually dissolved and migrated to the surface, resulting in the formation of the hollow architecture. By the direct thermal annealing this hollow Ni-MOF precursor at 600 °C under flowing Ar, the hollow structure could be maintained and a hybrid consisting of ultrathin graphene shells encapsulating Ni nanoparticles (Ni@graphene) was obtained. When the as-prepared hollow Ni@graphene was used as an HER electrocatalyst, it could drive a current density of 10 mA cm‒2 in 1.0 M KOH at an overpotential of 240 mV, making it superior to many solid Ni-based electrocatalysts [240]. Lian et al. prepared the Co0.6Fe0.4P nanocages by the phosphorization of the hollow Co-Fe-PBA (Co3[Fe(CN)6]2) nanocages at 300 °C under N2 flow [241]. Notably, the formation of the hollow Co-Fe-PBA precursor was governed by a controlled ammonia etching (Fig. 25e–h). The presence of the ammonia would induce a redox reaction of Co3[Fe(CN)6]2 and the etching of the CoII-N≡C-FeIII sites, in which CoII-N≡C-FeIII species were mainly distributed in the inner part of the nanocubes. Therefore, as the etching time proceeding, the inner part of the truncated Co-Fe-PBA nanocubes was gradually etched, leaving a hollow cage-like nanostructure. After phosphorization treatment, the resulting Co0.6Fe0.4P nanocages exhibited exceptional HER performance, offering a current density of 10 mA cm−2 at a small overpotential of 133 mV in 1.0 M KOH. Such a performance was superior to that of its solid Co0.6Fe0.4P counterparts, demonstrating the advantage of the hollow structure.

63

Fig. 25. (a) Ostwald ripening process for the hollow Ni-MOF precursor and TEM images of hollow Ni-MOF precursor prepared with reaction time at (b) 3 h, (c) 5 h, (d) 10 h [239]; (e) schematic illustration of the structural evolution from the Co–Fe PBA nanocube to nanocage through etching and (f–h) the corresponding FESEM images [241]. Adapted with permission from Ref. [239]. Copyright 2015, American Chemical Society. Adapted with permission from Ref. [241]. Copyright 2019, the Royal Society of Chemistry. Wang et al. synthesized a hollow hybrid consisting of Co nanoparticles 64

encapsulated within a porous carbon matrix by the direct thermal annealing ZIF-67 at 650 °C under flowing N2 (Fig. 26a–d) [242]. The formation of such a hollow structure obeyed the heterogeneous contraction mechanism. To be specific, the organic species in ZIF-67 polyhedra decompose and release gases during the calcination. Meanwhile, a large temperature gradient along the radial direction of the polyhedral templates initiated a strong adhesive force at the interface of the target shell, inducing an outward shrinkage and resulting in a hollow interior. As an HER electrocatalyst, the obtained hollow composites could drive a current density of 10 mA cm‒2 in 1.0 M KOH at an overpotential of approximately 220 mV, making it superior to many solid Co-based HER electrocatalysts. The hollow structures obtained through a sulfidation or selenization process were related to the Kirkendall effect. For instance, Guan et al. synthesized hollow CoS2 nanotubes through the sulfidation of 1D ZIF-67 nanorods at 350 °C under flowing N2 (Fig. 26e–h) [243]. During the sulfidation, the S and Co species tended to combine and form CoS2 compound. Because the diffusion rate of Co species was faster than that of the S species, a nonequilibrium mutual diffusion process occurred, resulting in the formation of voids in the faster diffusing zone (nanorods). The resulting hollow CoS2 nanotubes exhibited outstanding HER performance in 1.0 M KOH, exhibiting a small Tafel slope of 81 mV dec–1 and a low overpotential of 193 mV at a current density of 10 mA cm−2.

65

Fig. 26. (a) Schematic diagram for the synthesis of hollow Co@NCNPs nanocomposite through hetero-contraction mechanism and its (b) FESEM image and (c, d) TEM images [242]; (e) schematic diagram for the synthesis of hollow CoS2-based composite based on the Kirkendall effect, FESEM images of (f, i) Co-MOF nanoarrays, (g, j) Tube-like arrays obtained by reaction of Co-MOF nanowires with TAA, and (h, k) hollow CoS2-based composite [243]. Adapted with permission from Ref. [242]. Copyright 2017, Elsevier. Adapted with permission from Ref. [243]. Copyright 2017, the Royal Society of Chemistry. 3.3.3 Creation of core-shell structure Creation of MOF precursors with a core-shell structure is another effective way to 66

improve the electrocatalytic activity of their derivates due to the merits of having a multilevel structure. Generally, there are two types of MOF precursors with core-shell structures. The first group are formed by homogrowth between two MOFs, in which the two MOFs have a similar crystal/topological structure [244,245]. As mentioned in Section 3.2.3, Chen et al. reported the core-shell ZIF-8@ZIF-67 with cubic morphology. Recently, Pan et al. also synthesized core-shell ZIF-8@ZIF-67 dodecahedra through the epitaxial growth of ZIF-67 on ZIF-8 seeds (Fig. 27a) [244]. Such a core-shell structure integrated many unique advantages during post thermal treatment, such as plenty of N species from ZIF-8, highly active Co species from ZIF-67 as well as a large surface area with hierarchical pores from both ZIF-8 and ZIF-67. As expected, after pyrolysis, oxidation, and phosphorization of the core-shell ZIF-8@ZIF-67 dodecahedra, the resulting hybrid composites consisting of CoP nanoparticles encapsulated within Ndoped hollow carbon nanotube hollow polyhedra (Fig. 27b–e) exhibited superior HER activity in 1.0 M KOH, with a low overpotential of 115 mV at a current density of 10 mA cm‒2 (Fig. 27f), a Tafel slope as small as 53 mV dec–1 (Fig. 27g), and negligible decay after 1000 cycles or 24 h (Fig. 27h). In addition, DFT analysis also revealed the optimal ΔGH* for the CoP/NCNHP nanocomposite (Fig. 27i). The fabrication of coreshell MOFs through homogrowth between two MOFs can also be applied to other MOFs, such as PBA [148].

67

Fig. 27. (a) Schematic diagram for the synthesis of CoP/NCNHP nanocomposite, (b) HAADF-STEM and elemental mappings of the as-synthesized core-shell ZIF-8@ZIF67 precursor, (c, d) TEM images and (e) corresponding elemental mappings of CoP/NCNHP nanocomposite and its (f) LSV curve, (g) Tafel slope, (h) cycle durability in 1.0 M KOH, and (i) ΔGH* value [244]. Adapted with permission from Ref. [244]. Copyright 2018, American Chemical Society. The other method is via the in situ coordination of one MOF to the surface of another one, in which these two MOFs have entirely different crystal/topological structures, which offers the chance for the anion-exchange reaction between these two MOFs [246–248]. Taking Co-ZIF (ZIF-67) and Co-PBA as examples, the two MOFs have different crystal structures. Wang et al. transformed ZIF-67 nanocubes into ZIF67/Co-Fe PBA yolk-shell nanocubes through an anion-exchange reaction between ZIF67 and [Fe(CN)6]3− ions (Fig. 28a–h) [246]. Notably, the ion-exchange reaction is terminated upon formation of a gap between the ZIF-67 core and the PBA shell. Such hybrid precursors would imbue the obtained TMs/TMCs@C with an internal void space, thin carbon shells, a high porosity and favorable surface areas, ensuring the abundant 68

surface active sites and a high mass diffusion efficiency. Hu et al. also prepared the Fedoped CoP hollow triangle plate arrays (Fe-CoP HTPAs) [247]. In their reported method, ZIF-67 TPAs were first prepared through a coprecipitation reaction in the presence of Co2+ and 2-MeIm in an aqueous solution (Fig. 28i–n). Following that, the as-prepared ZIF-67 TPAs were then dispersed in an aqueous solution of K4[Fe(CN)6] to exert the ligands exchange reaction to generate the Co2[Fe(CN)6] HTPAs precursors. Finally, with a further phosphorization treatment at 300 °C under flowing N2, the precursor was converted into the hollow Fe-CoP HTPAs, which exhibited an outstanding HER electrocatalytic activity with an overpotential of 98 mV at a current density of 10 mA cm‒2 in 1.0 M KOH (Fig. 28o).

69

Fig. 28. (a) Schematic diagram for the synthesis of Co3O4/Co-Fe oxide nanocomposite; (b, c) TEM images of core-shell ZIF-67/Co-Fe PBA precursor; (d, e) FESEM images; (f, g) TEM images and (h) corresponding elemental mappings of Co3O4/Co-Fe oxide hybrid [246]. (i) Schematic diagram for the synthesis of Fe-CoP nanocomposite and its (j–l) FESEM images, (m, n) TEM images and (h) LSV curve in 1.0 M KOH [247]. Adapted with permission from Refs. [246] and [247]. Copyright 2018, Wiley-VCH. 3.4 Electrode configuration engineering The electrodes were generally implemented by casting the active materials onto glassy carbon electrode using Nafion as the binders for electrochemical measurements. However, the usage of the Nafion polymer not only blocked the active sites but also increased the interfacial resistance and the dwelt volume, lessening the mass transfer ability and lowering the utilization rate of the active sites [249–251]. To address this issue, assembling MOF-derived nanocomposites into integrated electrodes with well70

preserved microstructures, i.e., self-supported electrodes, has been demonstrated as an effective approach. The advantages of self-supported electrocatalysts lie in their large electroactive surface areas, easy electrolyte penetration, facile access of reactants to the active sites, efficient electron transfer along the nanostructure, as well as strong adhesion between the active species and substrates [252]. Given these facts, fabricating the self-supported MOF-derived TMs/TMCs@C electrocatalysts has received increasing interest. To better illustrate the advantages of the self-supported electrodes, Table 3 listes the HER performances of a series of MOF-derived TMs/TMCs@C electrocatalysts, and these data show that that almost all self-supported electrodes exhibit outstanding HER activity and stability. Table 3 MOF-derived TMs/TMCs@C nanocomposites for HER

Sample ID

Electrolyte

η10 (mV)

CoP/Co-MOF/CF*

1.0 M PBS

49

Tafel slope (mV dec-1) 63

1.0 M KOH

34

56

0.5 M H2SO4

21

43

0.1 M HClO4

56.9

52.3

1.0 M KOH

115.6

69.3

0.5 M H2SO4

112

70

1.0 M KOH

115

103

Co-Co9S8@SN-CNT

0.1.0 M KOH

120

Co/Co9S8@SNGS

0.1.0 M KOH

CoPS@NPS-C

CoxP/NC-S CoP@NPCS

j0 (mA cm-2)

Catalyst loading (mg cm2) ~5

0.7

Stability

Ref.

2000 cycles

17

3000 cycles

46

and 24 h 0.78

2000 cycles

50

92

0.4

5000 cycles

79

350

96.1

0.305

N/A

81

0.5 M H2SO4

93

63

0.34

0.357

1000 cycles

83

1.0 M KOH

191

106

0.166

Ni2P/rGO

1.0 M KOH

142

58

0.024

0.25

2000 cycles

84

MoP/NF*

1.0 M KOH

114

54.6

0.3

1000 cycles

124

FeCo@NGC

0.5 M H2SO4

262

174

0.285

10000

143

cycles Ni@NC

1.0 M KOH

205

160

~0.31

1000 cycles

145

CuCo@NC

0.5 M H2SO4

145

79

0.182

30000 s

146

FeCo@NGC

1.0 M KOH

211

77

0.32

10000

148

71

cycles NiS2@C

1.0 M KOH

219

157

Zn0.30Co2.70S4@NC

0.5 M H2SO4

80

47.5

0.15

0.210

10 h

151

0.285

1000 cycles

155

and 60 h ZnCoS-NSCNT/NP

1.0 M KOH

152

103

~0.21

1000 cycles

156

Ni-Co-MoS2 @NC

0.5 M H2SO4

155

51

0.286

1000 cycles

157

Co-Ni-Se/C/NF*

1.0 M KOH

148

81

1.5

24h

158

Zn0.1Co0.9Se2@NC

0.5 M H2SO4

140

49.9

0.285

5000 cycles

159

and 30 h NiSe@NC

0.5 M H2SO4

123

53.3

0.35

2000 cycles

160

1.0 M KOH

250

55.3

1.0 M PBS

300

66.2

Fe-CoSe2@NC

0.5 M H2SO4

143

40.9

~0.5

N/A

161

CoTe2@NCNTF

1.0 M KOH

208

58.04

0.285

2000 cycles

164

0.5 M H2SO4

240

61.67

Co1.11Te2/C

1.0 M KOH

178

77.3

N/A

1000 cycles

165

Co-P/NC

1.0 M KOH

154

51

0.283

1000 cycles

168

Ni2P/C

1.0 M KOH

168

63

~0.34

1000 cycles

169

Co-Fe-P/C

1.0 M KOH

86

66

0.285

1000 cycles

170

1.0 M PBS

138

138

0.5 M H2SO4

66

72

Cu0.3Co2.7P/NC

1.0 M KOH

220

122

0.4

1000 cycles

171

Co5.47N NP@N-PC

1.0 M KOH

149

86

0.453

10 h

175

Fe2N/S/N@C

0.5 M H2SO4

123

90

~0.4

30000

176

cycles MoCx

0.5 M H2SO4

142

53

0.023

1.0 M KOH

151

59

0.029

WC@NPC

0.5 M H2SO4

51

49

2.4

Mo2C/C

1.0 M KOH

165

63.6

Fe3C/Mo2C@NPGC

0.5 M H2SO4

98

45.2

0.104

0.8

3000 cycles

179

0.209

5h

180

1

20 h

182

0.14

1000 cycles

183

and 10 h MoC@GS

0.5 M H2SO4

132

46

Co3ZnC/Co-NCCP

1.0 M KOH

188

108

0.0135

0.76

3000 cycles

184

~0.21

1000 cycles

185

and 10 h A-Ni-C

0.5 M H2SO4

34

41

1.2

0.283

4000 cycles

187

and 25 h W-SAC

0.1.0 M KOH

85

53

0.204

10000 cycles

72

188

Co-SAC

0.5 M H2SO4

260

84

CoSe2@DC

0.5 M H2SO4

138

82

ZnCo-NC

0.5 M H2SO4

213

77

1.0 M KOH

200

86

1.0 M KOH

103

Co/N-Carbon

0.5

0.5

N/A

191

0.357

1000 cycles

196

0.463

1000 cycles

203

and 12 h 0.6

2000 cycles

204

and 10 h Co/NBC

1.0 M KOH

117

146

~1.8

5000 cycles

205

CoP@BCN

0.5 M H2SO4

87

32

0.4

2000 cycles

206

1.0 M KOH

215

52

1.0 M PBS

122

59

0.5 M H2SO4

229

126

0.714

5000 cycles

209

(Ni,Co)Se2-GA

0.5 M H2SO4

128

79

2.5-2.8

40 h

210

CoP-CNTs

0.5 M H2SO4

139

52

0.267

2000 cycles

211

N/Co-doped CP//NRGO

and 20 h Co-NC/CNT

1.0 M KOH

203

125

0.306

1000 cycles

212

and 30000 s 1.0 M KOH

108

55

0.5 M H2SO4

87

52

NDCHN-35

1.0 M KOH

201

133.2

0.5

N/A

217

NiCoP@C

1.0 M KOH

124

42

0.283

N/A

218

FePNC

0.5 M H2SO4

180

40.3

0.177

40 h

219

Fe-Ni@NC-CNTs

1.0 M KOH

202

0.5

40000 s

223

CoP-NS/C

0.5 M H2SO4

140

59

~0.14

3000 cycles

225

Co@C-800

0.5 M H2SO4

305

107

0.206

N/A

226

Ni@Graphene

1.0 M KOH

240

120

0.350

10 h

240

Co0.6Fe0.4P-1.125

1.0 M KOH

133

61

0.27

N/A

241

3D-CNTA

1.0 M KOH

185

135

0.16

N/A

242

CoS2 NTA/CC*

1.0 M KOH

193

88

~1.2

20 h

243

CoP/NCNHP

0.5 M H2SO4

140

53

0.39

1000 cycles

244

1.0 M KOH

115

66

Fe-CoP HTPA*

1.0 M KOH

98

69

N/A

50 h

247

NF@Ni/C-600*

1.0 M KOH

37

57

7.0

N/A

260

HP-CoP NA/NF*

1.0 M KOH

70

83.2

N/A

1000 cycles

261

Co@N-CNTs@rGO

0.96

~0.5

1000 cycles

215

and 100h

90

and 12 h CoSe2 NP/CP*

0.5 M H2SO4

137

42.1

2.5-3.0

N/A

262

S-CoWP@S,N-C*

0.5 M H2SO4

190

30

0.75

40 h

266

CoNC@MoS2/CNF*

1.0 M KOH

143

68

0.4

1500 cycles

267

73

NiFeSe@NiSe@CC*

1.0 M KOH

62

48.9

N/A

50 h

268

NHPBAP*

0.5 M H2SO4

28

33

0.27

N/A

270

1.0 M KOH

121

67

1.0 M KOH

190

98

~4

3000 cycles

271

Ni@CoO@CoNC*

and 20h Ni-Fe-P@C

1.0 M KOH

233

92.6

N/A

24 h

274

0.5 M H2SO4

55.7

49

0.18

3000 cycles

275

1.0 M KOH

67.2

66

0.18

and 20 h

FeNiP/NC

1.0 M KOH

177.1

62.1

N/A

10 h

276

Co9S8/CoS1.097/rGO

0.5 M H2SO4

188

96

1.68

1000 cycles

277

FeNiP/C

1.0 M KOH

178

69

N/A

4000 cycles

278

NR@NF* CoP/Mo2C-NC

0.2512

and 20000 s FeP/C

1.0 M KOH

254

88.1

0.1288

N/A

4000 cycles

278

and 20000 s NiP2/C

1.0 M KOH

427

91.2

0.0355

N/A

4000 cycles

278

and 20000 s ZIF-8-C6*

0.5 M H2SO4

155

54.7

0.063

~2.5

2000 cycles

279

Mo2C@NCFP*

0.5 M H2SO4

167

73

0.15

N/A

1000 cycles

280

and 10 h Ni-Co-Se/CFP*

1.0 M KOH

250

72

~0.65

1000 cycles

281

Co3S4@MoS2/C

1.0 M KOH

136

74

0.283

2000 cycles

282

and 10 h AB:CTGU-5(1:4)

0.5 M H2SO4

18

45

0.86

~0.14

96 h

283

MoN-NC

0.5 M H2SO4

62

54

0.778

0.145

3000 cycles

284

and 15 h MoO2@PC-rGO

0.5 M H2SO4

64

41

0.48

0.14

5000 cycles

285

CoP-N-C

0.5 M H2SO4

91

42

0.16

0.424

1000 cycles

286

Co@NC-G-700

0.5 M H2SO4

140

62

~0.21

50000 s

287

N-CoS2@NC/Ti*

0.5 M H2SO4

105

61

N/A

1000 cycles

288

0.19

and 20 h p-CoSe2/CC*

1.0 M KOH

138

83

2.3

1000 cycles

289

and 20 h FeP@GPC

0.5 M H2SO4

72

68

0.871

~0.35

3000 cycles

290

and 20 h Ni2P-NPCM-900-2

1.0 M KOH

125

51

0.124

2000 cycles

291

and 20 h MoP@PC

0.5 M H2SO4

153

66

74

0.41

2000 cycles

292

Co/Co9S8@NSOC

1.0 M KOH

216

10h

293

Co@NC

0.1.0 M KOH

190

126.8

0.210

50 h

294

Ni/C

1.0 M KOH

40

77

0.5

10 h

295

Co-P@NC-800

0.5 M H2SO4

98

74

0.283

12 h

296

Co/CoN/Co2P-NPC

1.0 M KOH

99

51

~0.35

48h

297

PdCo@NC

0.5 M H2SO4

80

31

0.285

10000

298

cycles Co0.17Fe0.79P/NC

1.0 M KOH

139

57

0.285

24h

299

CuxNiyFe4−x−yN/NF*

0.1.0 M KOH

32

103

~0.6

500 cycles

300

MOF-CoSe2

0.5 M H2SO4

42

0.539

2000 cycles

301

and 20 h N-Co3O4@C@NF*

1.0 M KOH

42

56

N/A

60h

302

CoP-350

0.5 M H2SO4

126

64

~0.45

15h

303

CoSe2/CF*

1.0 M KOH

95

52

2.9

5000 cycles

304

and 20 h Co/CoP–HNC

1.0 M KOH

180

105.6

0.19

10 h

305

NGO/Ni7S6

0.1.0 M KOH

380

45

0.21

N/A

306

(Ni,Co)Se2/C-HRD*

1.0 M KOH

87

65

1.5

2000 cycles

307

and 36 h Co@BCN

0.5 M H2SO4

96

63.7

N/A

2000 cycles

308

and 10 h CoSx@MoS2

0.5 M H2SO4

239

103

RuO2/Co3O4

1.0 M KOH

89

91

0.0628

0.285

500 cycles

309

0.285

10000

310

cycles

and

40000 s AB&CTGU-9 (3:4)

0.5 M H2SO4

128

87

~0.0706

21 h

311

Ni QD@NC@rGO

1.0 M KOH

133

64

0.71

24 h

312

Co@N-CS/N-

1.0 M KOH

66

65

~3.2

30 h

313

HCP@CC*

*means the self-supported MOF-derived HER electrocatalysts.

3.4.1 Rooting MOFs directly onto metal substrates 3D metal scaffolds such as nickel foam (NF) are often employed as substrates due to the fact that they can provide high catalyst loading, rapid mass transfer and compact electrode contact in the 3D networks. Sun et al. reported the synthesis of a Ni(BDC) precursor grown on the surface of NF (NF@Ni(BDC)) by a solvothermal reaction between Ni(NO3)2·6H2O and 1,4-benzenedicarboxylic (H2BDC) (Fig. 29a, b) [260]. 75

Through further annealing at 600 °C under a H2/Ar gas mixture, the Ni(BDC) precursor was converted into a hierarchical architecture, in which the Ni nanoparticles were well encapsulated within the sheet-like carbon matrix with a large number of extended CNTs (Ni/C-CNTs) (Fig. 29a, c–g). Such 3D Ni/C-CNTs tightly anchored on the NF exhibited outstanding catalytic activity for HER in 1.0 M KOH (Fig. 29h–j), requiring an overpotential of 37 mV to drive a current density of 10 mA cm‒2. This HER activity even outperformed the commercial Pt@C benchmark catalyst, demonstrating the advantage of 3D NF metal substrates.

Fig. 29. (a) Schematic diagram for the synthesis of Ni@CNTs@NF nanocomposite, FESEM images of (b) Ni(BDC)@NF precursor and (c, d) Ni@CNTs@NF nanocomposite; (e–g) TEM images of Ni@CNTs hybrid and (g) corresponding 76

elemental mappings; (h) LSV curve, (i) Tafel slope, (j) EIS spectrum of Ni@CNTs@NF composite in 1.0 M KOH [260]. Adapted with permission from Refs. [260]. Copyright 2018, the Royal Society of Chemistry. Recently, Wang et al. developed a novel growth-etching-phosphorization strategy to synthesize hierarchical porous CoP nanostructure arrays on an NF substrate (HPCoP NA/NF) as a self-supported HER electrocatalyst [261] (Fig. 30a–g). More specifically, interconnected 2D ZIF-67 arrays were first prepared via a coordination reaction at room temperature, n and thethey were uniformly converted into porous CoP arrays through a Co2+ etching process followed by phosphorization. This HP-CoP NA/NF catalyst exhibited outstanding HER activity in 1.0 M KOH (Fig. 30h–j), with a low overpotential of 70 mV at a current density of 10 mA cm–2. 3.4.2 Rooting MOFs directly onto carbon substrates Carbon fiber paper (CFP) and carbon cloth (CC), 3D networks composed of carbon fibers with diameters of 7~10 μm, are commercially available, or can be homemade by carbonizing the cellulose fiber paper [262–265]. As current collectors with high electrical conductivity and robust mechanical stability, CFP and CC have also been employed as ideal alternatives to replace the metal substrates for the growth of MOFs. Recently, Wang et al. fabricated the precursors composed of CoW-MOF ({[W(CN)8](SCN)3Co3(C13H14N2)6}n) nanowires grown in situ on CC through a facile hydrothermal process [266]. The CoW-MOF nanowires@CC material was further converted into a nanowire-like composite consisting of S-doped CoWP nanoparticles embedded in a S- and N-doped carbon (S-CoWP@(S,N)-C@CC) matrix through calcination at 700 °C followed by phosphorization (Fig. 31a–g). The ΔGH* value for the CoWP@N-C was small and quite close to zero, implying it had a high intrinsic catalytic activity (Fig. 31h). As expected, when used as an HER electrocatalyst, the 77

catalytic activity of the as-prepared S-CoWP@(S,N)-C@CC composite was comparable to that of the commercial Pt@C electrocatalyst, requiring a low overpotential of 35 mV to drive a current density of 10 mA cm‒2 (Fig. 31i) and exhibiting a Tafel slope as small as 35 mV dec–1 (Fig. 31j) in an acidic electrolyte.

Fig. 30. (a) Schematic diagram for the synthesis of CoP arrays@NF composite; the FESEM images of (b, e) Co-MOFs@NF, (c, f) Co(OH)2@NF and (d, g) CoP arrays@NF composite; (h) LSV curve, (i) Tafel slope, and (j) cycle durability of CoP arrays@NF composite in 1.0 M KOH [261]. Adapted with permission from Ref. [261]. Copyright 2018, Elsevier. 78

Fig. 31. (a) Schematic diagram for the synthesis of S-CoWP@S,N-C@CC composite; FESEM images of (b) CoW-MOF, (c) S-CoWP@S,N-C and (d) S-CoWP@S,N-C nanowires; (e–g) TEM images of S-CoWP@S,N-C nanowire; (h) ΔGH* value, (i) LSV curve in 0.5 M H2SO4, and (j) Tafel slope in 0.5 M H2SO4 of S-CoWP@S,N-C@CC composite [266]. Adapted with permission from Ref. [266]. Copyright 2018, American Chemical Society.

79

Fig. 32. (a) Schematic diagram for the synthesis of CoNC@MoS2@CC composite; FESEM images of (b, c) Co-MOF grown on carbon nanofiber and (d) CoNC@MoS2@CC hybrid, (e–g) TEM images of CoNC@MoS2 hybrid, (h) LSV curve, (i) Tafel slope, and (j) cycle durability test in 1.0 M KOH of CoNC@MoS2@CC electrode [267]. Adapted with permission from Ref. [267]. Copyright 2017, the Royal Society of Chemistry. Notably, in order to promote the nucleation and growth of MOFs on the CC, the CC is usually treated with an acidic solution to form the hydroxyl groups in advance. To avoid this tedious pretreatment procedure, Ji et al. found that an electrospun 80

polyaniline (PAN) nanofiber film could directly attract transition metal ions and contribute to the uniform growth of leaf-like Co-ZIF via a facile liquid-phase deposition method (Fig. 32a–c) [267]. The as-prepared Co-ZIF-on-nanofiber film was then heated at 350 °C under flowing N2, generating a hybrid composed of Co–N–C flakes grafted a CNF film (CoNC@CNF). To further boost the electrocatalytic activity, a solvothermal reaction was performed to incorporate MoS2 nanoprisms into the CoNC@CNF (CoNC@MoS2/CNF) (Fig. 32d–g). Owing to the synergistic effects of the CoNC@MoS2 hybrid nanostructures and porous carbon, the resultant CoNC@MoS2/CNFs exhibited high electrocatalytic activity (Fig. 32h–j), offering a current density of 10 mA cm‒2 at an overpotential of 143 mV for HER in 1.0 M KOH. 3.4.3 Pseudomorphic replication In addition to rooting the MOFs directly onto the surface of self-supported substrates (e.g., NF, CC and graphene film), a pseudomorphic replication strategy, which involves building none-MOF phase precursor onto the surface of substrates in advance and then converting the precursor into MOFs has also been developed [268– 274]. Relative to the direct rooting route, pseudomorphic replication is more controllable and can be used more easily achieve oriented growth of MOFs on substrates in a uniform manner. Cai et al. reported the oriented growth of ZIF-67 nanowall arrays on NF through the coordination reaction between sacrificial metal oxide (CoO) nanowires arrays and 2-MeIm at room temperature (Fig. 33a–c) [271]. With a further pyrolysis at 500 °C under flowing N2, the ZIF-67 was converted into the Co-N-C composite, resulting in the formation of a Ni@CoO@CoNC hybrid. Noticeably, the Ni@CoO@CoNC required an overpotential of 190 mV at a current density of 10 mA cm–2 with a Tafel slope of 98 mV dec–1, which is superior to those of CoO@CoNC powder deposited on Ni foam (Ni/CoO@CoNC) and pristine Ni@CoO 81

(Fig. 33d), powerfully demonstrating the advantages of the pseudomorphic replication.

Fig. 33. (a) Schematic diagram for the synthesis of Ni@CoO@CoNC composite; (b) FESEM image and (c) TEM image of CoO@CoNC composite; (d) LSV curve of Ni@CoO@CoNC composite in 1.0 M KOH [271]. Adapted with permission from Ref. [271]. Copyright 2017, Elsevier. Ahn et al. recently reported a hydroxide template-induced phase transformation of MOFs on NF [274]. Specifically, the Ni-Fe hydroxides nanorods grown on NF were first prepared by a solvothermal reaction, and then transformed into the corresponding NiFe-ZIF nanorods via pseudomorphic replication reaction. Finally, these NiFe-ZIF nanorods were converted into the nanocomposites consisting of Ni-Fe phosphides embedded within amorphous carbon through phosphorization reaction at 300 °C under flowing N2 (Fig. 34a–f). When the resulting nanocomposites were used as electrocatalysts, they required an overpotential of only 79 mV for HER to afford 10 mA cm‒2 in 1.0 M KOH, (Fig. 34g–i). Indra et al. recently utilized CC-supported metal carbonate hydroxide nanowires (Co(OH)(CO3)0.5·xH2O) as the initial phase and 82

morphology model. After treatment with K3[Co(CN)6] solution at room temperature, the Prussian blue analogue with a uniform cubic morphology was achieved uniformly [272]. Deng et al. prepared an oriented coating of 2D CoNi-MOFs on CC through the solvothermal reaction of an interlaced 2D metal hydroxide (Co(OH)2) nanosheet template [273]. Despite these findings being encouraging, the investigations using pseudomorphic replication are still in their infancy, possibly due to the lack of appropriate replication templates.

Fig. 34. (a) Schematic diagram for the synthesis of Ni-Fe-P nanorods@NF composites and their (b, c) FESEM images, (d, e) TEM images and (f) corresponding elemental 83

mappings, and (g) LSV curve, (h) Tafel slope, (i) cycle durability in 1.0 M KOH [274]. Adapted with permission from Ref. [274]. Copyright 2017, the Royal Society of Chemistry. 4 Conclusions and perspectives TMs or TMCs usually suffer from low electrical conductivity, particle aggregation, and/or unsatisfactory surface structures, limiting their direct use as HER electrocatalysts. Within this context, tremendous effort has been devoted to designing TMs/TMCs@C composite electrocatalysts through a MOF-driven strategy. Despite the great progress has been made, the development of the MOF-derived TMs/TMCs@C electrocatalysts for improved HER performance remains of great interest. Therefore, in this review, we have elaborately summarized and discussed the design strategies for improved MOF-derived HER electrocatalysts, with special emphasis on component manipulation of the TMs/TMCs, carbon matrix modifications, morphological tuning and electrode configuration engineering. We believe that the implementation of these dedicate design strategies is propelling the development of MOF-mediated water splitting systems. To better explore advanced MOF-derived HER electrocatalysts, the following urgent issues should be rationally considered. (i) Because the surface structure of MOF-derived TMs/TMCs@C nanocomposites is strongly dependent on the MOF precursors, designing the MOF precursors with controlled morphologies will be pivotal. Currently, 1D, 2D and 3D MOF precursors have been individually investigated, while integrating MOF precursors with different dimensions has rarely been reported. It is well known that 1D/2D MOFs can ensure the high utilization of active sites, but they exhibit poor morphological/structural stability and low porosity during thermal treatment. In comparison with 1D/2D MOFs, 3D MOFs possess more stable morphology/structure and higher porosity upon thermal 84

treatment, but they usually exhibit a low utilization of active sites. Thus, integrating the merits of 1D/2D MOFs with those of 3D MOFs to derive nanocomposites with multiple dimensions and complex configurations is highly desired but still remains a grand challenge. In addition to the composite morphologies, new MOF-derived composite systems (e.g., poly-anion composites) should also be explored, as they are expected to exhibit high HER activities. (ii) MOF precursors can easily aggregate, fuse, and/or collapse during high temperature treatment, which greatly decrease the exposure of active sites and mass transfer ability, resulting in poor electrocatalytic activities. To address these issues, a relatively lower temperature treatment may be required. Nevertheless, the carbonization of organic groups at low temperatures can lead to a low graphitization degree of the carbon matrix and a poor electrical conductivity. Therefore, achieving an optimal balance among the graphitization degree, particle distribution and surface structure within the MOF-derived nanocomposites using an appropriate temperature is still challenging. Moreover, how to achieve the large-scale synthesis of MOF-derived electrocatalysts in a low-cost and green way requires further exploration. (iii) Despite a substantial number of studies demonstrating that MOF-derived HER electrocatalysts show low overpotentials, fast reaction kinetics, and favorable Gibbs free energies towards H intermediate, investigations of their long-term stabilities at a high current density (over 500 mA cm-2) are rare, hampering their practical application in water electrolysis devices. In addition, most reports barely discuss their stability under working conditions and ignore the degradation mechanism. Therefore, extensive effort should be devoted to unveiling the fundamentals of the activity degradation and developing highly active MOF-derived HER electrocatalysts with long-term stability at high current densities. 85

(iv) DFT calculations have been widely performed and play a critical role in predicting how the surfaces affect the HER catalysis as well as the adsorption/desorption energy changes of water and proton. To corroborate these theoretical studies, advanced in situ/operando characterization methods, such as in situ synchrotron radiation, should be well developed. On the one hand, these techniques can be employed to gain a deeper insight into the correlation between the composition/structure/morphology of the MOF-derived nanocomposites and their electrocatalytic HER activity, thus providing solid guidance for optimizing their HER activity. On the other hand, some debatable matters regarding MOF-derived electrocatalysts could be better clarified, e.g., the identification and establishment of the electrocatalytic HER mechanism of MOF-derived TMs/TMCs@C nanocomposites in alkaline electrolytes. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51672049, 51871060, 51727801 and 51831009), Research Grant for Talent Introduction of Fudan University, China (Grant No. JJH2021103), the China Postdoctoral Science Foundation (Grant No. 2018M640337), the Recruitment Program of Global Youth Experts and Fudan’s Undergraduate Research Opportunities Program, FDUROP. References [1] Davis SJ, Lewis NS, Shaner M, Aggarwal S, Arent D, Azevedo IL, et al. Net-zero emissions energy systems. Science 2018;360:eaas9793. [2] Ojha K, Saha S, Dagar P, Ganguli AK. Nanocatalysts for hydrogen evolution reactions. Phys Chem Chem Phys 2018;20:6777-99. [3] Zhao G, Rui K, Dou SX, Sun W. Heterostructures for electrochemical hydrogen evolution reaction: a review. Adv Funct Mater 2018;28:1803291. 86

[4] Murthy AP, Madhavan J, Murugan K. Recent advances in hydrogen evolution reaction catalysts on carbon/carbon-based supports in acid media. J Power Sources 2018;398:9-26. [5] Li S-L, Xu Q. Metal-organic frameworks as platforms for clean energy. Energy Environ Sci 2013;6:1656-83. [6] Freire C, Fernandes DM, Nunes M, Abdelkader VK. POM&MOF-based electrocatalysts for energy-related reactions. ChemCatChem 2018;10:1703-30. [7] Pang YP, Liu YF, Gao MX, Ouyang LZ, Liu JW, Wang H, et al. A mechanicalforce-driven physical vapour deposition approach to fabricating complex hydride nanostructures. Nat Commun 2014;5:1-8. [8] Liu YF, Pan HG, Gao MX, Wang QD. Advanced hydrogen storage alloys for Ni/MH rechargeable batteries. J Mater Chem 2011;21:4743-55. [9] Liao B, Lei YQ, Chen LX, Lu GL, Pan HG, Wang QD. Effect of the La/Mg ratio on the structure and electrochemical properties of LaxMg3-xNi9 (x = 1.6-2.2) hydride storage electrode alloys for nickel-metal hydride batteries. J Power sources 2014;129:358-67. [10] Liao B, Lei YQ, Chen LX, Lu GL, Pan HG, Wang QD. A study on the structure and electrochemical properties of La2Mg(Ni0.95M0.05)9 (M = Co, Mn, Fe, Al, Cu, Sn) hydrogen storage electrode alloys. J Alloys Compd 2004;376:186-95. [11] Liu YF, Zhong K, Luo K, Gao MX, Pan HG, Wang QD. Size-dependent kinetic enhancement in hydrogen absorption and desorption of the Li-Mg-N-H system. J Am Chem Soc 2009;131:1862-70. [12] Pan HG, Liu YF, Gao MX, Zhu YF, Lei YQ, Wang QD. An investigation on the structural and electrochemical properties of La0.7Mg0.3(Ni0.85Co0.15)x (x = 3.15~3.80) hydrogen storage electrode alloys. J Alloys Compd 2003;351:228-34. [13] Li X, Hao X, Abudula A, Guan G. Nanostructured catalysts for electrochemical water splitting: current state and prospects. J Mater Chem A 2016;4:11973-2000. [14] Lu J, Wang S, Ding C, Lv W, Zeng Y, Liu N, et al. Metal organic frameworks derived CoSe2@N-doped-carbon-nanorods as highly efficient electrocatalysts for oxygen evolution reaction. J Alloys Compd 2019;778:134-40. [15] Liu J, Zhu D, Guo C, Vasileff A, Qiao SZ. Design strategies toward advanced MOF-derived electrocatalysts for energy-conversion reactions. Adv Energy Mater 2017;7:1700518. [16] Li Y, Niu S, Rakov D, Wang Y, Caban-Acevedo M, Zheng S, et al. Metal organic framework-derived CoPS/N-doped carbon for efficient electrocatalytic hydrogen evolution. Nanoscale 2018;10:7291-7. [17] Liu T, Li P, Yao N, Cheng GZ, Chen SL, Luo W, Yin YD. CoP‐Doped MOF‐based electrocatalyst for pH‐universal hydrogen evolution reaction. Angew Chem Int Ed 2019;58:4679-84. [18] He T, Zhang C, Du A. Single-atom supported on graphene grain boundary as an efficient electrocatalyst for hydrogen evolution reaction. Chem Eng Sci 87

2019;194:58-63. [19] Mahmood A, Guo W, Tabassum H, Zou R. Metal-organic framework-based nanomaterials for electrocatalysis. Adv Energy Mater 2016;6:1600423. [20] Seh ZW, Kibsgaard J, Dickens CF, Chorkendorff I, Norskov JK, Jaramillo TF. Combining theory and experiment in electrocatalysis: insights into materials design. Science 2017;355:eaad4998. [21] Song F, Li W, Sun Y. Metal-organic frameworks and their derivatives for photocatalytic water splitting. Inorganics 2017;5:40. [22] Tan Y, Luo M, Liu P, Cheng C, Han J, Watanabe K, et al. 3D Nanoporous Co9S4P4 pentlandite as a bifunctional electrocatalyst for overall neutral water splitting. ACS Appl Mater Interfaces 2019;11:3880-8. [23] Humagain G, MacDougal K, Maclnnis J, Lowe JM, Coridan RH, MacQuarrie S, et al. Highly efficient, biochar-derived molybdenum carbide hydrogen evolution electrocatalyst. Adv Energy Mater 2018;8:1801461. [24] Zhou H, Yu F, Zhu Q, Sun J, Qin F, Yu L, et al. Water splitting by electrolysis at high current densities under 1.6 volts. Energy Environ Sci 2018;11:2858-64. [25] Zhang FS, Wang JW, Luo J, Liu RR, Zhang ZM, He CT, et al. Extraction of nickel from NiFe-LDH into Ni2P@NiFe hydroxide as a bifunctional electrocatalyst for efficient overall water splitting. Chem Sci 2018;9:1375-84. [26] Chen Z, Liu M, Wu R. Strongly coupling of Co9S8/Zn-Co-S heterostructures rooted in carbon nanocages towards efficient oxygen evolution reaction. J Catal 2018;361:322-30. [27] Lu J, Zhou W, Wang L, Jia J, Ke Y, Yang L, et al. Core-shell nanocomposites based on gold nanoparticle@zinc-iron-embedded porous carbons derived from metal-organic frameworks as efficient dual catalysts for oxygen reduction and hydrogen evolution reactions. ACS Catal 2016;6:1045-53. [28] Tang J, Salunkhe RR, Liu J, Torad NL, Imura M, Furukawa S, et al. Thermal conversion of core-shell metal-organic frameworks: a new method for selectively functionalized nanoporous hybrid carbon. J Am Chem Soc 2015;137:1572-80. [29] Wu Y, Liu Y, Li G-D, Zou X, Lian X, Wang D, et al. Efficient electrocatalysis of overall water splitting by ultrasmall NixCo3-xS4 coupled Ni3S2 nanosheet arrays. Nano Energy 2017;35:161-70. [30] Mahmood N, Yao Y, Zhang JW, Pan L, Zhang X, Zou JJ. Electrocatalysts for hydrogen evolution in alkaline electrolytes: mechanisms, challenges, and prospective solutions. Adv Sci 2018;5:1700464. [31] Jia Q, Liu E, Jiao L, Li J, Mukerjee S. Current understandings of the sluggish kinetics of the hydrogen evolution and oxidation reactions in base. Curr Opin Electrochem 2018;12:209-17. [32] Gong M, Wang D-Y, Chen C-C, Hwang B-J, Dai H. A mini review on nickelbased electrocatalysts for alkaline hydrogen evolution reaction. Nano Res 2015;9:28-46. 88

[33] Hao W, Wu R, Zhang R, Ha Y, Chen Z, Wang L, et al. Electroless plating of highly efficient bifunctional boride-based electrodes toward practical overall water splitting. Adv Energy Mater 2018;8:1801372. [34] Bai S, Wang C, Deng M, Gong M, Bai Y, Jiang J, et al. Surface polarization matters: enhancing the hydrogen-evolution reaction by shrinking Pt shells in Pt-Pdgraphene stack structures. Angew Chem Int Ed 2014;53:12120-4. [35] Yin C, Deng J, Fang L, Wang Y, Yang X, Gu X, et al. Hierarchical urchin-like peapoded core-shell-structured NiCo2@Ni1/3Co2/3S2@C catalyst with synergistically high-efficiency electrocatalytic properties toward hydrogen evolution reaction. J Catal 2018;365:351-8. [36] Zhang MD, Dai QB, Zheng HG, Chen MD, Dai LM. Novel MOF-derived Co@NC bifunctional catalysts for highly efficient Zn-air batteries and water splitting. Adv Mater 2018;30:1705431. [37] Vij V, Sultan S, Harzandi AM, Meena A, Tiwari JN, Lee W-G, et al. Nickel-based electrocatalysts for energy-related applications: oxygen reduction, oxygen evolution, and hydrogen evolution reactions. ACS Catal 2017;7:7196-225. [38] Li Y, Polakovic T, Curtis J, Shumlas SL, Chatterjee S, Intikhab S, et al. Tuning the activity/stability balance of anion doped CoSxSe2-x dichalcogenides. J Catal 2018;366:50-60. [39] You B, Sun Y. Innovative strategies for electrocatalytic water splitting. Acc Chem Res 2018;51:1571-80. [40] Wu R, Xue Y, Liu B, Zhou K, Wei J, Chan SH. Cobalt diselenide nanoparticles embedded within porous carbon polyhedra as advanced electrocatalyst for oxygen reduction reaction. J Power Sources 2016;330:132-9. [41] Chen Z, Wu R, Wang H, Jiang Y, Jin L, Guo Y, et al. Construction of hybrid hollow architectures by in-situ rooting ultrafine ZnS nanorods within porous carbon polyhedra for enhanced lithium storage properties. Chem Eng J 2017;326:680-90. [42] Jin J, Zheng Y, Huang S-Z, Sun P-P, Srikanth N, Kong LB, et al. Directly anchoring 2D NiCo metal-organic frameworks on few-layer black phosphorus for advanced lithium-ion batteries. J Mater Chem A 2019;7:783-90. [43] Xu H, Wu R. Porous hollow composites assembled by NixCo1-xSe2 nanosheets rooted on carbon polyhedra for superior lithium storage capability. J Colloid Interface Sci 2019;536:673-80. [44] Wu R, Wang DP, Zhou K, Srikanth N, Wei J, Chen Z. Porous cobalt phosphide/graphitic carbon polyhedral hybrid composites for efficient oxygen evolution reactions. J Mater Chem A 2016;4:13742-5. [45] Wu R, Wang DP, Kumar V, Zhou K, Law AW, Lee PS, et al. MOFs-derived copper sulfides embedded within porous carbon octahedra for electrochemical capacitor applications. Chem Commun 2015;51:3109-12. [46] Man H-W, Tsang C-S, Li MM-J, Mo J, Huang B, Lee LYS, et al. Transition metal89

doped nickel phosphide nanoparticles as electro- and photocatalysts for hydrogen generation reactions. Appl Catal B: Environ 2019;242:186-93. [47] Gao Q, Zhang W, Shi Z, Yang L, Tang Y. Structural design and electronic modulation of transition-metal-carbide electrocatalysts toward efficient hydrogen evolution. Adv Mater 2019;31:1802880. [48] Gu W, Hu L, Shang C, Li J, Wang E. Enhancement of the hydrogen evolution performance by finely tuning the morphology of Co-based catalyst without changing chemical composition. Nano Res 2018;12:191-6. [49] Sun YQ, Xu K, Wei ZX, Li HL, Zhang T, Li XY, et al. Strong electronic interaction in dual-cation-incorporated NiSe2 nanosheets with lattice distortion for highly efficient overall water splitting. Adv Mater 2018;30:1802121. [50] Wu K, Chen Z, Cheong WC, Liu S, Zhu W, Cao X, et al. Toward bifunctional overall water splitting electrocatalyst: general preparation of transition metal phosphide nanoparticles decorated N-doped porous carbon spheres. ACS Appl Mater Interfaces 2018;10:44201-8. [51] Liu K, Zhong H, Meng F, Zhang X, Yan J, Jiang Q. Recent advances in metalnitrogen-carbon catalysts for electrochemical water splitting. Mater Chem Front 2017;1:2155-73. [52] Gao F, Zhao Y, Zhang L, Wang B, Wang Y, Huang X, et al. Well dispersed MoC quantum dots in ultrathin carbon films as efficient co-catalysts for photocatalytic H2 evolution. J Mater Chem A 2018;6:18979-86. [53] Zheng J, Chen X, Zhong X, Li S, Liu T, Zhuang G, et al. Hierarchical porous NC@CuCo nitride nanosheet networks: highly efficient bifunctional electrocatalyst for overall water splitting and selective electrooxidation of benzyl alcohol. Adv Funct Mater 2017;27:1704169. [54] Deng J, Ren P, Deng D, Yu L, Yang F, Bao X. Highly active and durable nonprecious-metal catalysts encapsulated in carbon nanotubes for hydrogen evolution reaction. Energy Environ Sci 2014;7:1919-23. [55] Wang W, Babu DD, Huang Y, Lv J, Wang Y, Wu M. Atomic dispersion of Fe/Co/N on graphene by ball-milling for efficient oxygen evolution reaction. Int J Hydrogen Energy 2018;43:10351-8. [56] Su Y, Zhu Y, Jiang H, Shen J, Yang X, Zou W, et al. Cobalt nanoparticles embedded in N-doped carbon as an efficient bifunctional electrocatalyst for oxygen reduction and evolution reactions. Nanoscale 2014;6:15080-9. [57] Zhou X, Lin SH, Yang X, Li H, Hedhili MN, Li LJ, et al. MoSx-coated NbS2 nanoflakes grown on glass carbon: an advanced electrocatalyst for the hydrogen evolution reaction. Nanoscale 2018;10:3444-50. [58] Wang H, Wang W, Xu YY, Asif M, Liu H, Xia BY. Ball-milling synthesis of Co2P nanoparticles encapsulated in nitrogen doped hollow carbon rods as efficient electrocatalysts. J Mater Chem A 2017;5:17563-9. [59] Huang Y, Ma Z, Hu Y, Chai D, Qiu Y, Gao G, et al. An efficient 90

WSe2/Co0.85Se/graphene hybrid catalyst for electrochemical hydrogen evolution reaction. RSC Adv 2016;6:51725-31. [60] Wu Z, Guo J, Wang J, Liu R, Xiao W, Xuan C, et al. Hierarchically porous electrocatalyst with vertically aligned defect-rich CoMoS nanosheets for the hydrogen evolution reaction in an alkaline medium. ACS Appl Mater Interfaces 2017;9:5288-94. [61] Chen YZ, Wang C, Wu ZY, Xiong Y, Xu Q, Yu SH, et al. From bimetallic metalorganic framework to porous carbon: high surface area and multicomponent active dopants for excellent electrocatalysis. Adv Mater 2015;27:5010-6. [62] Su J, Xia G, Li R, Yang Y, Chen J, Shi R, et al. Co3ZnC/Co nano heterojunctions encapsulated in N-doped graphene layers derived from PBAs as highly efficient bi-functional OER and ORR electrocatalysts. J Mater Chem A 2016;4:9204-12. [63] Liang Q, Jin H, Wang Z, Xiong Y, Yuan S, Zeng X, et al. Metal-organic frameworks derived reverse-encapsulation Co-NC@Mo2C complex for efficient overall water splittin. Nano Energy 2019;57:746-52. [64] Zhang W, Li R, Zhao X, Chen Z, Law AW, Zhou K. A cobalt-based metal-organic framework as cocatalyst on BiVO4 Photoanode for enhanced photoelectrochemical water oxidation. ChemSusChem 2018;11:2710-6. [65] Wu R, Qian X, Zhou K, Liu H, Yadian B, Wei J, et al. Highly dispersed Au nanoparticles immobilized on Zr-based metal-organic frameworks as heterostructured catalyst for CO oxidation. J Mater Chem A 2013;1:14294-9. [66] Wu R, Qian X, Law AW-K, Zhou K. Coordination polymer-derived mesoporous Co3O4 hollow nanospheres for high-performance lithium-ions batteries. RSC Adv 2016;6:50846-50. [67] Wang H, Xu H, Jia K, Wu R. ZIF-8-Templated hollow cube-like Si/SiO2@C nanocomposites for superior lithium storage performance. ACS Appl Energy Mater 2018;2:531-8. [68] Li B, Wang R, Chen Z, Sun D, Fang F, Wu R. Embedding heterostructured MnS/Co1-xS nanoparticles in porous carbon/graphene for superior lithium storage. J Mater Chem A 2019;7:1260-6. [69] Chen Z, Wu R, Wang H, Zhang KHL, Song Y, Wu F, et al. Embedding ZnSe nanodots in nitrogen-doped hollow carbon architectures for superior lithium storage. Nano Res 2017;11:966-78. [70] Wu R, Qian X, Zhou K, Wei J, Lou J, Ajayan PM. Porous spinel ZnxCo3-xO4 hollow polyhedra templated for high-rate lithium-ion batteries. ACS Nano 2014;8:6297303. [71] Wang H, Qian X, Wu H, Zhang R, Wu R, MOF-derived rod-like composites consisting of iron sulfides embedded in nitrogen-rich carbon as high-performance lithium-ion battery anodes. Appl Surf Sci 2019; 481:33-9. [72] Yang Z, Lv H, Wu R. Rational construction of graphene oxide with MOF derived porous NiFe@C nanocubes for high-performance microwave attenuation. Nano 91

Res 2016;9(12):3671-82. [73] Li R, Zhang W, Zhou K. Metal-organic-framework-based catalysts for photoreduction of CO2. Adv Mater 2018;30:1705512. [74] Qian X, Yadian B, Wu R, Long Y, Zhou K, Zhu B, et al. Structure stability of metal-organic framework MIL-53 (Al) in aqueous solutions. Int J Hydrogen Energy 2013;38:16710-5. [75] Wu R, Qian X, Yu F, Liu H, Zhou K, Wei J, et al. MOF-templated formation of porous CuO hollow octahedra for lithium-ion battery anode materials. J Mater Chem A 2013;1:11126-9. [76] Wang DP, Fu M, Ha Y, Wang H, Wu R. Metal-organic framework-derived mesoporous octahedral copper oxide/titania composites for high-performance lithium-ion batteries. J Colloid Interf Sci 2018;529:265-72. [77] Liu M, Zheng W, Ran S, Boles ST, Lee LYS. Overall water-splitting electrocatalysts based on 2D CoNi-metal-organic frameworks and its derivative. Adv Mater Interfaces 2018;5:1800849. [78] Xu J, Qi Y, Wang C, Wang L. NH2-MIL-101(Fe)/Ni(OH)2-derived C,N-codoped Fe2P/Ni2P cocatalyst modified g-C3N4 for enhanced photocatalytic hydrogen evolution from water splitting. Appl Catal B: Environ 2019;241:178-86. [79] Han H, Bai Z, Zhang T, Wang X, Yang X, Ma X, et al. Hierarchical design and development of nanostructured trifunctional catalysts for electrochemical oxygen and hydrogen reactions. Nano Energy 2019;56:724-32. [80] Liang Y, Liu Q, Luo Y, Sun X, He Y, Asiri AM. Zn0.76Co0.24S/CoS2 nanowires array for efficient electrochemical splitting of water. Electrochim Acta 2016;190:360-4. [81] Zhang X, Liu S, Zang Y, Liu R, Liu G, Wang G, et al. Co/Co9S8@S,N-doped porous graphene sheets derived from S, N dual organic ligands assembled CoMOFs as superior electrocatalysts for full water splitting in alkaline media. Nano Energy 2016;30:93-102. [82] Zheng Y, Qiao SZ. Direct growth of well-aligned MOF arrays onto various substrates. Chem 2017;2:751-2. [83] Hu Y, Li F, Long Y, Yang H, Gao L, Long X, et al. Ultrafine CoPS nanoparticles encapsulated in N, P, and S tri-doped porous carbon as an efficient bifunctional water splitting electrocatalyst in both acid and alkaline solutions. J Mater Chem A 2018;6:10433-40. [84] Yan L, Jiang H, Xing Y, Wang Y, Liu D, Gu X, et al. Nickel metal-organic framework implanted on graphene and incubated to be ultrasmall nickel phosphide nanocrystals acts as a highly efficient water splitting electrocatalyst. J Mater Chem A 2018;6:1682-91. [85] Zhang T, Du J, Xi P, Xu C. Hybrids of cobalt/iron phosphides derived from bimetal-organic frameworks as highly efficient electrocatalysts for oxygen evolution reaction. ACS Appl Mater Interfaces 2017;9:362-70. 92

[86] Downes CA, Marinescu SC. Electrocatalytic metal-organic frameworks for energy applications. ChemSusChem 2017;10:4374-92. [87] Zhang H, Liu X, Wu Y, Guan C, Cheetham AK, Wang J. MOF-derived nanohybrids for electrocatalysis and energy storage: current status and perspectives. Chem Commun 2018;54:5268-88. [88] Wang H, Zhu Q-L, Zou R, Xu Q. Metal-organic frameworks for energy applications. Chem 2017;2:52-80. [89] Wang C, Kaneti YV, Bando Y, Lin J, Liu C, Li J, et al. Metal-organic frameworkderived one-dimensional porous or hollow carbon-based nanofibers for energy storage and conversion. Mater Horiz 2018;5:394-407. [90] Cheng N, Ren L, Xu X, Du Y, Dou SX. Recent development of zeolitic imidazolate frameworks (ZIFs) derived porous carbon based materials as electrocatalysts. Adv Energy Mater 2018;8:1801257. [91] Zhang W, Lai W, Cao R. Energy-related small molecule activation reactions: oxygen reduction and hydrogen and oxygen evolution reactions catalyzed by porphyrin- and corrole-based systems. Chem Rev 2017;117:3717-97. [92] Wang W, Xu X, Zhou W, Shao Z. Recent progress in metal-organic frameworks for applications in electrocatalytic and photocatalytic water splitting. Adv Sci 2017;4:1600371. [93] Yan Y, Xia BY, Zhao B, Wang X. A review on noble-metal-free bifunctional heterogeneous catalysts for overall electrochemical water splitting. J Mater Chem A 2016;4:17587-603. [94] Zhang L, Xiao J, Wang H, Shao M. Carbon-based electrocatalysts for hydrogen and oxygen evolution reactions. ACS Catal 2017;7:7855-65. [95] Morales-Guio CG, Stern LA, Hu X. Nanostructured hydrotreating catalysts for electrochemical hydrogen evolution. Chem Soc Rev 2014;43:6555-69. [96] Anantharaj S, Ede SR, Sakthikumar K, Karthick K, Mishra S, Kundu S. Recent trends and perspectives in electrochemical water splitting with an emphasis on sulfide, selenide, and phosphide catalysts of Fe, Co, and Ni: a review. ACS Catal 2016;6:8069-97. [97] Callejas JF, Read CG, Roske CW, Lewis NS, Schaak RE. Synthesis, characterization, and properties of metal phosphide catalysts for the hydrogenevolution reaction. Chem Mater 2016;28:6017-44. [98] Wang J, Xu F, Jin H, Chen Y, Wang Y. Non-noble metal-based carbon composites in hydrogen evolution reaction: fundamentals to applications. Adv Mater 2017;29:1605838. [99] Lu J, Yin S, Shen PK. Carbon-encapsulated electrocatalysts for the hydrogen evolution reaction. Electrochem Energy Rev 2019;2:105-27. [100] Qi J, Zhang W, Cao R. Solar-to-hydrogen energy conversion based on water splitting. Adv Energy Mater 2018;8:1701620. 93

[101] Wang H, Gao L. Recent developments in electrochemical hydrogen evolution reaction. Curr Opin Electrochem 2018;7:7-14. [102] Walter MG, Warren EL, McKone JR, Boettcher SW, Mi Q, Santori EA, et al. Solar water splitting cells. Chem Rev 2010;110:6446-73. [103] Artero V, Chavarot-Kerlidou M, Fontecave M. Splitting water with cobalt. Angew Chem Int Ed 2011;50:7238-66. [104] Nørskov JK, Bligaard T, Logadóttir A, Kitchin JR, Chen JG, Pandelov S, et al. Trends in the exchange current for hydrogen evolution. J Electrochem Soc 2005;152:J23-6. [105] Zeradjanin AR, Grote J-P, Polymeros G, Mayrhofer KJJ. A critical review on hydrogen evolution electrocatalysis: re-exploring the volcano-relationship. Electroanalysis 2016;28:2256-69. [106] Greeley J, Jaramillo TF, Bonde J, Chorkendorff IB, Norskov JK. Computational high-throughput screening of electrocatalytic materials for hydrogen evolution. Nat Mater 2006;5:909-13. [107] Zeng M, Li Y. Recent advances in heterogeneous electrocatalysts for the hydrogen evolution reaction. J Mater Chem A 2015;3:14942-62. [108] Wu W, Niu C, Wei C, Jia Y, Li C, Xu Q. Activation of MoS2 basal planes for hydrogen evolution by zinc. Angew Chem Int Ed 2019;58:1-6. [109] Huang G, Xiao Z, Chen R, Wang S. Defect Engineering of cobalt-based materials for electrocatalytic water splitting. ACS Sustainable Chem Eng 2018;6:1595469. [110] Chen Z, Lu J, Ai Y, Ji Y, Adschiri T, Wan L. Ruthenium/graphene-like layered carbon composite as an efficient hydrogen evolution reaction electrocatalyst. ACS Appl Mater Interfaces 2016;8:35132-7. [111] Sun S, Li H, Xu ZJ. Impact of surface area in evaluation of catalyst activity. Joule 2018;2(6):1024-7. [112] Voiry D, Chhowalla M, Gogotsi Y, Kotov NA, Li Y, Penner RM, et al. Best practices for reporting electrocatalytic performance of nanomaterials. ACS Nano 2018;12:9635-8. [113] Wang Y, Wang J, Han G, Du C, Deng Q, Gao Y, et al. Pt decorated Ti3C2 MXene for enhanced methanol oxidation reaction. Ceram Int 2019;45:2411-7. [114] Wang Y, Chen L, Yu X, Wang Y, Zheng G. Superb alkaline hydrogen evolution and simultaneous electricity generation by Pt-decorated Ni3N nanosheets. Adv Energy Mater 2017;7:1601390. [115] Zhang J, Wang T, Liu L, Du K, Liu W, Zhu Z, et al. Molybdenum disulfide and Au ultrasmall nanohybrids as highly active electrocatalysts for hydrogen evolution reaction. J Mater Chem A 2017;5:4122-8. [116] Jorge AB, Dedigama I, Miller TS, Shearing P, Brett DJL, McMillan PF. Carbon nitride materials as efficient catalyst supports for proton exchange membrane 94

water electrolyzers. Nanomaterials 2018;8:432. [117] Ouyang Y, Li Q, Shi L, Ling C, Wang J. Molybdenum sulfide clusters immobilized on defective graphene: a stable catalyst for the hydrogen evolution reaction. J Mater Chem A 2018;6:2289-94. [118] Li K, Zhang F, Wu H, Wang Y, Du J, Lu Y, et al. Facile synthesis of sheet-shaped Co2P grown on carbon cloth as a high-performance electrocatalyst for the hydrogen evolution reaction. J Solid State Electr 2018;22:3977-83. [119] Ding Y, Li H, Hou Y. Phosphorus-doped nickel sulfides/nickel foam as electrode materials for electrocatalytic water splitting. Int J Hydrogen Energy 2018;43:19002-9. [120] Dou S, Wu J, Tao L, Shen A, Huo J, Wang S. Carbon-coated MoS2 nanosheets as highly efficient electrocatalysts for the hydrogen evolution reaction. Nanotechnology 2016;27:045402. [121] Jin J, Zheng Y, Kong LB, Srikanth N, Yan Q, Zhou K. Tuning ZnSe/CoSe in MOF-derived N-doped porous carbon/CNTs for high-performance lithium storage. J Mater Chem A 2018;6:15710-7. [122] Wang H, Chen Z, Liu Y, Xu H, Cao L, Qing H, et al. Hierarchically porousstructured ZnxCo1-xS@C–CNT nanocomposites with high-rate cycling performance for lithium-ion batteries. J Mater Chem A 2017;5:23221-7. [123] Zou X, Zhang Y. Noble metal-free hydrogen evolution catalysts for water splitting. Chem Soc Rev 2015;44:5148-80. [124] Jiang Y, Lu Y, Lin J, Wang X, Shen Z. A hierarchical MoP nanoflake array supported on Ni foam: a bifunctional electrocatalyst for overall water splitting. Small Methods 2018;2:1700369. [125] Yang L, Liu P, Li J, Xiang B. Two-dimensional material molybdenum disulfides as electrocatalysts for hydrogen evolution. Catalysts 2017;7:285. [126] Voiry D, Yang J, Chhowalla M. Recent strategies for improving the catalytic activity of 2D TMD nanosheets toward the hydrogen evolution reaction. Adv Mater 2016;28:6197-206. [127] Benck JD, Hellstern TR, Kibsgaard J, Chakthranont P, Jaramillo TF. Catalyzing the hydrogen evolution reaction (HER) with molybdenum sulfide nanomaterials. ACS Catal 2014;4:3957-71. [128] Shin S, Kwon DH, Bose R, Min Y-S. High turnover frequency of hydrogen evolution reaction on amorphous MoS2 thin film directly grown by atomic layer deposition. Langmuir 2015;31:1196-202. [129] Han Y, Wu Y, Lai W, Cao R. Electrocatalytic water oxidation by a water-soluble nickel porphyrin complex at neutral pH with low overpotential. Inorg Chem 2015;54:5604-13. [130] Jaksic MM. Electrocatalysis of hydrogen evolution in the light of the brewerengel theory for bonding in metals and intermetallic phases. Electrochim Acta 1984;29:1539-50. 95

[131] McCrory CC, Jung S, Ferrer IM, Chatman SM, Peters JC, Jaramillo TF. Benchmarking hydrogen evolving reaction and oxygen evolving reaction electrocatalysts for solar water splitting devices. J Am Chem Soc 2015;137:434757. [132] Wei c, Xu ZCJ. The Comprehensive understanding of 10 mA cm2geo as an evaluation parameter for electrochemical water splitting. Small Methods 2018;2:1800168. [133] Zheng Y-R, Wu P, Gao M-R, Zhang X-L, Gao F-Y, Ju H-X, et al. Dopinginduced structural phase transition in cobalt diselenide enables enhanced hydrogen evolution catalysis. Nat Commun 2018;9:2533. [134] Zhang Y, Shao Q, Pi YC, Guo J, Huang XQ. A cost-efficient bifunctional ultrathin nanosheets array for electrochemical overall water splitting. Small 2017;13:1700355. [135] Hamngren Blomqvist C, Abrahamsson C, Gebäck T, Altskär A, Hermansson AM, Nydén M, et al. Pore size effects on convective flow and diffusion through nanoporous silica gels. Colloid Surface A 2015;484:288-96. [136] Jang D, Idrobo JC, Laoui T, Karnik R. Water and solute transport governed by tunable pore size distributions in nanoporous graphene membranes. ACS Nano 2017;11:10042-52. [137] Su J, Zhao Y, Fang C. Understanding the role of pore size homogeneity in the water transport through graphene layers. Nanotechnology 2018;29:225706. [138] Xu J, Wu K, Li R, Li Z, Li J, Xu Q, et al. Real gas transport in shale matrix with fractal structures. Fuel 2018;219:353-63. [139] Ramesh K, Reddy KS, Rashmi I, Biswas AK. Porosity distribution, surface area, and morphology of synthetic potassium zeolites: a SEM and N2 adsorption study. Commun Soil Sci Plant Anal 2014;45:2171-2181. [140] Zhang L, Wu HB, Madhavi S, Hng HH, Lou XW. Formation of Fe2O3 microboxes with hierarchical shell structures from metal-organic frameworks and their lithium storage properties. J Am Chem Soc 2012;134:17388-91. [141] Xu X, Cao R, Jeong S, Cho J. Spindle-like mesoporous α-Fe2O3 anode material prepared from MOF template for high-rate lithium batteries. Nano Lett 2012;12:4988-91. [142] Yang SJ, Nam S, Kim T, Im JH, Jung H, Kang JH, et al. Preparation and exceptional lithium anodic performance of porous carbon-coated ZnO quantum dots derived from a metal-organic framework. J Am Chem Soc 2013;135:73947. [143] Yang Y, Lun Z, Xia G, Zheng F, He M, Chen Q. Non-precious alloy encapsulated in nitrogen-doped graphene layers derived from MOFs as an active and durable hydrogen evolution reaction catalyst. Energy Environ Sci 2015;8:3563-71. [144] Wu R, Wang DP, Rui X, Liu B, Zhou K, Law AWK, et al. In-situ formation of hollow hybrids composed of cobalt sulfides embedded within porous carbon 96

polyhedra/carbon nanotubes for high-performance lithium-ion batteries. Adv Mater 2015;27:3038-44. [145] Xu Y, Tu W, Zhang B, Yin S, Huang Y, Kraft M, et al. Nickel nanoparticles encapsulated in few-layer nitrogen-doped graphene derived from metal-organic frameworks as efficient bifunctional electrocatalysts for overall water splitting. Adv Mater 2017;29:1605957. [146] Kuang M, Wang Q, Han P, Zheng G. Cu, Co-embedded N-enriched mesoporous carbon for efficient oxygen reduction and hydrogen evolution reactions. Adv Energy Mater 2017;7:1700193. [147] Zhang T, Liu X, Cui X, Chen M, Liu S, Geng B. Colloidal synthesis of Mo-Ni alloy nanoparticles as bifunctional electrocatalysts for efficient overall water splitting. Adv Mater Interfaces 2018;5:1800359. [148] Yang Y, Lin Z, Gao S, Su J, Lun Z, Xia G, et al. Tuning electronic structures of nonprecious ternary alloys encapsulated in graphene layers for optimizing overall water splitting activity. ACS Catal 2016;7:469-79. [149] Gao MR, Xu YF, Jiang J, Yu SH. Nanostructured metal chalcogenides: synthesis, modification, and applications in energy conversion and storage devices. Chem Soc Rev 2013;42:2986-3017. [150] Roger I, Shipman MA, Symes MD. Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting. Nature Rev Chem 2017;1. [151] Tian T, Huang L, Ai L, Jiang J. Surface anion-rich NiS2 hollow microspheres derived from metal-organic frameworks as a robust electrocatalyst for the hydrogen evolution reaction. J Mater Chem A 2017;5:20985-92. [152] Ho TA, Bae C, Nam H, Kim E, Lee SY, Park JH, et al. Metallic Ni3S2 films grown by atomic layer deposition as an efficient and stable electrocatalyst for overall water splitting. ACS Appl Mater Interfaces 2018;10:12807-15. [153] Qin Z, Chen Y, Huang Z, Su J, Diao Z, Guo L. Composition-dependent catalytic activities of noble-metal-free NiS/Ni3S4 for hydrogen evolution reaction. J Phys Chem C 2016;120:14581-9. [154] Wu Y, Chen W, Xiang Q, Ma Y, Zhu H, Tao P, et al. Coupling interface constructions of MoS2/Fe5Ni4S8 heterostructures for efficient electrochemical water splitting. Adv Mater 2018;30: 1803151. [155] Huang ZF, Song J, Li K, Tahir M, Wang YT, Pan L, et al. Hollow cobalt-based bimetallic sulfide polyhedra for efficient all-pH-value electrochemical and photocatalytic hydrogen evolution. J Am Chem Soc 2016;138:1359-65. [156] Yu Z, Bai Y, Zhang S, Liu Y, Zhang N, Sun K. Metal-organic framework-derived Zn0.975Co0.025S/CoS2 embedded in N,S-codoped carbon nanotube/nanopolyhedra as an efficient electrocatalyst for overall water splitting. J Mater Chem A 2018;6:10441-6. [157] Yu X-Y, Feng Y, Jeon Y, Guan B, Wen X, Lou XW, et al. Formation of Ni-CoMoS2 nanoboxes with enhanced electrocatalytic activity for hydrogen evolution. 97

Adv Mater 2016;28:9006-11. [158] Ming F, Liang H, Shi H, Xu X, Mei G, Wang Z. MOF-derived Co-doped nickel selenide/C electrocatalysts supported on Ni foam for overall water splitting. J Mater Chem A 2016;4:15148-55. [159] Wang X, Li F, Li W, Gao W, Tang Y, Li R. Hollow bimetallic cobalt-based selenide polyhedrons derived from metal-organic framework: an efficient bifunctional electrocatalyst for overall water splitting. J Mater Chem A 2017;5:17982-9. [160] Huang Z, Liu J, Xiao Z, Fu H, Fan W, Xu B, et al. A MOF-derived coral-like NiSe@NC nanohybrid: an efficient electrocatalyst for the hydrogen evolution reaction at all pH values. Nanoscale 2018;10:22758-65. [161] Wu X, Han S, He D, Yu C, Lei C, Liu W, et al. Metal organic framework derived Fe-doped CoSe2 incorporated in nitrogen-doped carbon hybrid for efficient hydrogen evolution. ACS Sustainable Chem Eng 2018;6:8672-8. [162] Hara Y, Minami N, Itagaki H. Electrocatalytic properties of ruthenium modified with Te metal for the oxygen reduction reaction. Appl Catal A: Gen 2008;340:5966. [163] Wu G, Cui GF, Li DY, Shen PK, Li N. Carbon-supported Co1.67Te2 nanoparticles as electrocatalysts for oxygen reduction reaction in alkaline electrolyte. J Mater Chem 2009;19:6581-9. [164] Wang X, Huang X, Gao W, Tang Y, Jiang P, Lan K, et al. Metal-organic framework derived CoTe2 encapsulated in nitrogen-doped carbon nanotube frameworks: a high-efficiency bifunctional electrocatalyst for overall water splitting. J Mater Chem A 2018;6:3684-91. [165] Wang H, Wang Y, Tan L, Fang L, Yang X, Huang Z, et al. Componentcontrollable cobalt telluride nanoparticles encapsulated in nitrogen-doped carbon frameworks for efficient hydrogen evolution in alkaline conditions. Appl Catal B: Environ 2019;244:568-75. [166] Wang MQ, Ye C, Liu H, Xu M, Bao SJ. Nanosized metal phosphides embedded in nitrogen-doped porous carbon nanofibers for enhanced hydrogen evolution at all pH values. Angew Chem Int Ed 2018;57:1963-7. [167] Buss JA, Hirahara M, Ueda Y, Agapie T. Molecular mimics of heterogeneous metal phosphides: thermochemistry, hydrid-proton isomerism, and HER reactivity. Angew Chem Int Ed 2018;57:16329-33. [168] You B, Jiang N, Sheng M, Gul S, Yano J, Sun Y. High-performance overall water splitting electrocatalysts derived from cobalt-based metal-organic frameworks. Chem Mater 2015;27:7636-42. [169] Wang Q, Liu Z, Zhao H, Huang H, Jiao H, Du Y. MOF-derived porous Ni2P nanosheets as novel bifunctional electrocatalysts for the hydrogen and oxygen evolution reactions. J Mater Chem A 2018;6:18720-7. [170] Chen J, Liu J, Xie J-Q, Ye H, Fu X-Z, Sun R, et al. Co-Fe-P nanotubes 98

electrocatalysts derived from metal-organic frameworks for efficient hydrogen evolution reaction under wide pH range. Nano Energy 2019;56:225-33. [171] Song J, Zhu C, Xu BZ, Fu S, Engelhard MH, Ye R, et al. Bimetallic cobalt-based phosphide zeolitic imidazolate framework: CoPx phase-dependent electrical conductivity and hydrogen atom adsorption energy for efficient overall water splitting. Adv Energy Mater 2017;7:1601555. [172] Balogun M-S, Huang Y, Qiu W, Yang H, Ji H, Tong Y. Updates on the development of nanostructured transition metal nitrides for electrochemical energy storage and water splitting. Mater Today 2017;20:425-51. [173] Zhang Y, Ouyang B, Xu J, Chen S, Rawat RS, Fan HJ. 3D porous hierarchical nickel-molybdenum nitrides synthesized by RF plasma as highly active and stable hydrogen-evolution-reaction electrocatalysts. Adv Energy Mater 2016;6:1600221. [174] Xie J, Xie Y. Transition metal nitrides for electrocatalytic energy conversion: opportunities and challenges. Chem Eur J 2016;22:3588-98. [175] Chen Z, Ha Y, Liu Y, Wang H, Yang H, Xu H, et al. In situ formation of cobalt nitrides/graphitic carbon composites as efficient bifunctional electrocatalysts for overall water splitting. ACS Appl Mater Interfaces 2018;10:7134-44. [176] Mahmood A, Tabassum H, Zhao R, Guo W, Aftab W, Liang Z, et al. Fe2N/S/N codecorated hierarchical porous carbon nanosheets for trifunctional electrocatalysis. Small 2018;14:1803500. [177] Gao Q, Zhang W, Shi Z, Yang L, Tang Y. Structural design and electronic modulation of transition metal-carbide electrocatalysts toward efficient hydrogen evolution. Adv Mater 2019;31: 1802880. [178] Liu Y, Yu G, Li D-G, Sun Y, Asefa T, Chen W, et al. Coupling Mo2C with nitrogen-rich nanocarbon leads to efficient hydrogen-evolution electrocatalytic sites. Angew Chem Int Ed 2015;54:10752-7. [179] Wu HB, Xia BY, Yu L, Yu XY, Lou XW. Porous molybdenum carbide nanooctahedrons synthesized via confined carburization in metal-organic frameworks for efficient hydrogen production. Nat Commun 2015;6:6512. [180] Xu YT, Xiao X, Ye ZM, Zhao S, Shen R, He CT, et al. Cage-confinement pyrolysis route to ultrasmall tungsten carbide nanoparticles for efficient electrocatalytic hydrogen evolution. J Am Chem Soc 2017;139:5285-8. [181] Yang X, Feng X, Tan H, Zang H, Wang X, Wang Y, et al. N-doped graphenecoated molybdenum carbide nanoparticles as highly efficient electrocatalysts for the hydrogen evolution reaction. J Mater Chem A 2016;4:3947-54. [182] Qamar M, Adam A, Merzougui B, Helal A, Abdulhamid O, Siddiqui MN. Metalorganic framework-guided growth of Mo2C embedded in mesoporous carbon as a high-performance and stable electrocatalyst for the hydrogen evolution reaction. J Mater Chem A 2016;4:16225-32. [183] Li J-S, Tang Y-J, Liu C-H, Li S-L, Li R-H, Dong L-Z, et al. Polyoxometalate99

based metal-organic framework-derived hybrid electrocatalysts for highly efficient hydrogen evolution reaction. J Mater Chem A 2016;4:1202-7. [184] Shi Z, Wang Y, Lin H, Zhang H, Shen M, Xie S, et al. Porous nanoMoC@graphite shell derived from a MOFs-directed strategy: an efficient electrocatalyst for the hydrogen evolution reaction. J Mater Chem A 2016;4:6006-13. [185] Yu Z, Bai Y, Zhang S, Liu Y, Zhang N, Wang G, et al. Metal-organic frameworkderived Co3ZnC/Co embedded in nitrogen-doped carbon nanotube-grafted carbon polyhedra as a high-performance electrocatalyst for water splitting. ACS Appl Mater Interfaces 2018;10:6245-52. [186] Tao L, Lin C-Y, Dou S, Feng S, Chen D, Liu D, et al. Creating coordinatively unsaturated metal sites in metal-organic-frameworks as efficient electrocatalysts for the oxygen evolution reaction: insights into the active centers. Nano Energy 2017;41:417-25. [187] Fan L, Liu PF, Yan X, Gu L, Yang ZZ, Yang HG, et al. Atomically isolated nickel species anchored on graphitized carbon for efficient hydrogen evolution electrocatalysis. Nat Commun 2016;7:10667. [188] Chen W, Pei J, He C-T, Wan J, Ren H, Wang Y, et al. Single tungsten atoms supported on MOF-derived N-doped carbon for robust electrochemical hydrogen evolution. Adv Mater 2018;30:1800396. [189] Liang Z, Qu C, Xia D, Zou R, Xu Q. Atomically dispersed metal sites in MOFbased materials for electrocatalytic and photocatalytic energy conversion. Angew Chem Int Ed 2018;57:9604-33. [190] Qu YT, Li ZJ, Chen WX, Lin Y, Yuan TW, Yang ZK, et al. Direct transformation of bulk copper into copper single sites via emitting and trapping of atoms. Nature Catal 2018;1:781-6. [191] Zhao W, Wan G, Peng C, Sheng H, Wen J, Chen H. Key single-atom electrocatalysis in metal-organic framework (MOF)-derived bifunctional catalysts. ChemSusChem 2018;11:3473-9. [192] Zhou T, Du Y, Wang D, Yin S, Tu W, Chen Z, et al. Phosphonate-based metalorganic framework derived Co-P-C hybrid as an efficient electrocatalyst for oxygen evolution reaction. ACS Catal 2017;7:6000-7. [193] Chen Z, Wu R, Liu M, Liu Y, Xu S, Ha Y, et al. Tunable electronic coupling of cobalt sulfide/carbon composites for optimizing oxygen evolution reaction activity. J Mater Chem A 2018;6:10304-12. [194] Zhao X, Yang H, Jing P, Shi W, Yang G, Cheng P. A metal-organic framework approach toward highly nitrogen-doped graphitic carbon as a metal-free photocatalyst for hydrogen evolution. Small 2017;13:1603279. [195] Gadipelli S, Li Z, Zhao T, Yang Y, Yildirim T, Guo Z. Graphitic nanostructures in a porous carbon framework significantly enhance electrocatalytic oxygen evolution. J Mater Chem A 2017;5:24686-94. 100

[196] Zhou W, Lu J, Zhou K, Yang L, Ke Y, Tang Z, et al. CoSe2 nanoparticles embedded defective carbon nanotubes derived from MOFs as efficient electrocatalyst for hydrogen evolution reaction. Nano Energy 2016;28:143-50. [197] Xia BY, Yan Y, Li N, Wu HB, Lou XW, Wang X. A metal-organic frameworkderived bifunctional oxygen electrocatalyst. Nat Energy 2016;1:15006. [198] Zhang W, Jiang X, Wang X, Kaneti YV, Chen Y, Liu J, et al. Spontaneous weaving of graphitic carbon networks synthesized by pyrolysis of ZIF-67 crystals. Angew Chem Int Ed 2017;56:8435-40. [199] Bu F, Chen W, Gu J, Agboola PO, Al-Khalli NF, Shakir I, et al. Microwaveassisted CVD-like synthesis of dispersed monolayer/few-layer N-doped graphene encapsulated metal nanocrystals for efficient electrocatalytic oxygen evolution. Chem Sci 2018;9:7009-16. [200] Sun S, Luo J, Qian Y, Jin Y, Liu Y, Qiu Y, et al. Metal-organic framework derived honeycomb Co9S8@C composites for high-performance supercapacitors. Adv Energy Mater 2018;8:1801080. [201] Wu G, Santandreu A, Kellogg W, Gupta S, Ogoke O, Zhang H, et al. Carbon nanocomposite catalysts for oxygen reduction and evolution reactions: from nitrogen doping to transition-metal addition. Nano Energy 2016;29:83-110. [202] Qu K, Zheng Y, Zhang X, Davey K, Dai S, Qiao SZ. Promotion of electrocatalytic hydrogen evolution reaction on nitrogen-doped carbon nanosheets with secondary heteroatoms. ACS Nano 2017;11:7293-300. [203] Wang R, Yuan Q, Sun P, Nie R, Wang X. Tuning the active sites in the cobaltbased nitrogen-doped carbon by zinc for enhancing hydrogen evolution reaction. J Alloys Compd 2019; 789: 789 (2019) 100-7 [204] Huang T, Chen Y, Lee J-M. Two-dimensional cobalt/N-doped carbon hybrid structure derived from metal-organic frameworks as efficient electrocatalysts for hydrogen evolution. ACS Sustainable Chem Eng 2017;5:5646-50. [205] Liu M-R, Hong Q-L, Li Q-H, Du Y, Zhang H-X, Chen S, et al. Cobalt boron imidazolate framework derived cobalt nanoparticles encapsulated in B/N codoped nanocarbon as efficient bifunctional electrocatalysts for overall water splitting. Adv Funct Mater 2018;28:1801136. [206] Tabassum H, Guo W, Meng W, Mahmood A, Zhao R, Wang Q, et al. Metalorganic frameworks derived cobalt phosphide architecture encapsulated into B/N Co-doped graphene nanotubes for all pH Value electrochemical hydrogen evolution. Adv Energy Mater 2017;7:1601671. [207] Han L, Dong S, Wang E. Transition-metal (Co, Ni, and Fe)-based electrocatalysts for the water oxidation reaction. Adv Mater 2016;28:9266-91. [208] Chen Z, Wu R, Liu M, Wang H, Xu H, Guo Y, et al. General synthesis of dual carbon-confined metal sulfides quantum dots toward high-performance anodes for sodium-ion batteries. Adv Funct Mater 2017;27:1702046. [209] Hou Y, Wen Z, Cui S, Ci S, Mao S, Chen J. An advanced nitrogen-doped 101

graphene/cobalt-embedded porous carbon polyhedron hybrid for efficient catalysis of oxygen reduction and water splitting. Adv Funct Mater 2015;25:87282. [210] Xu X, Liang H, Ming F, Qi Z, Xie Y, Wang Z. Prussian blue analogues derived penroseite (Ni,Co)Se2 nanocages anchored on 3D graphene aerogel for efficient water splitting. ACS Catal 2017;7:6394-9. [211] Wu C, Yang Y, Dong D, Zhang Y, Li J. In situ coupling of CoP polyhedrons and carbon nanotubes as highly efficient hydrogen evolution reaction electrocatalyst. Small 2017;13:1602873. [212] Yang F, Zhao P, Hua X, Luo W, Cheng G, Xing W, et al. A cobalt-based hybrid electrocatalyst derived from a carbon nanotube inserted metal-organic framework for efficient water-splitting. J Mater Chem A 2016;4:16057-63. [213] Niu W, Yang Y. Amorphous MOF introduced N-doped graphene: an efficient and versatile electrocatalyst for zinc-air battery and water splitting. ACS Appl Energy Mater 2018;1:2440-5. [214] Wang X, Ma Z, Chai L, Xu L, Zhu Z, Hu Y, et al. MOF derived N-doped carbon coated CoP particle/carbon nanotube composite for efficient oxygen evolution reaction. Carbon 2019;141:643-51. [215] Chen Z, Wu R, Liu Y, Ha Y, Guo Y, Sun D, et al. Ultrafine Co nanoparticles encapsulated in carbon-nanotubes-grafted graphene sheets as advanced electrocatalysts for the hydrogen evolution reaction. Adv Mater 2018;30:1802011. [216] Yang H, Wang X. Secondary-component incorporated hollow MOFs and derivatives for catalytic and energy-related applications. Adv Mater 2018;30:1800743. [217] Zhang L, Hu J-S, Huang X-H, Song J, Lu S-Y. Particle-in-box nanostructured materials created via spatially confined pyrolysis as high performance bifunctional catalysts for electrochemical overall water splitting. Nano Energy 2018;48:489-99. [218] Li Y, Liu J, Chen C, Zhang X, Chen J. Preparation of NiCoP hollow quasipolyhedra and their electrocatalytic properties for hydrogen evolution in alkaline solution. ACS Appl Mater Interfaces 2017;9:5982-91. [219] Von Lim Y, Huang S, Zhang Y, Kong D, Wang Y, Guo L, et al. Bifunctional porous iron phosphide/carbon nanostructure enabled high-performance sodiumion battery and hydrogen evolution reaction. Energy Storage Mater 2018;15:98107. [220] Salunkhe RR, Tang J, Kamachi Y, Nakato T, Kim JH, Yamauchi Y. Asymmetric supercapacitors using 3D nanoporous carbon and cobalt oxide electrodes synthesized from a single metal-organic framework. ACS Nano 2015;9:6288-96. [221] Zhao J, Wang F, Su P, Li M, Chen J, Yang Q, et al. Spinel ZnMn2O4 nanoplate assemblies fabricated via "escape-by-crafty-scheme" strategy. J Mater Chem 2012;22:13328-33. 102

[222] Salunkhe RR, Kaneti YV, Yamauchi Y. Metal-organic framework-derived nanoporous metal oxides toward supercapacitor applications: progress and prospects. ACS Nano 2017;11:5293-308. [223] Zhao X, Pachfule P, Li S, Simke JRJ, Schmidt J, Thomas A. Bifunctional electrocatalysts for overall water splitting from an iron/nickel-based bimetallic metal-organic framework/dicyandiamide composite. Angew Chem Int Ed 2018;57:8921-6. [224] Anandhababu G, Huang Y, Babu DD, Wu M, Wang Y. Oriented growth of ZIF67 to derive 2D porous CoPO nanosheets for electrochemical-/ photovoltagedriven overall water splitting. Adv Funct Mater 2018;28:1706120. [225] Li H, Ke F, Zhu J. MOF-derived ultrathin cobalt phosphide nanosheets as efficient bifunctional hydrogen evolution reaction and oxygen evolution reaction electrocatalysts. Nanomaterials 2018;8:89. [226] Cong J, Xu H, Lu M, Wu Y, Li Y, He P, et al. Two-dimensional Co@N-carbon nanocomposites facilely derived from metal-organic framework nanosheets for efficient bifunctional electrocatalysis. Chem Asian J 2018;13:1485-91. [227] Yu L, Yu XY, Lou XW. The design and synthesis of hollow micro/nanostructures: present and future trends. Adv Mater 2018;30:1800939. [228] Nai J, Lou XW. Hollow structures based on Prussian blue and its analogs for electrochemical energy storage and conversion. Adv Mater 2018; 30:1706825. [229] Zhu W, Chen Z, Pan Y, Dai R, Wu Y, Zhuang Z, et al. Functionalization of hollow nanomaterials for catalytic applications: nanoreactor construction. Adv Mater 2018;30:1800426. [230] Yu X-Y, Yu L, Lou XWD. Hollow nanostructures of molybdenum sulfides for electrochemical energy storage and conversion. Small Methods 2017;1:1600020. [231] Jinlong L, Wenli G, Tongxiang L, Ken S, Hideo M. Improving hydrogen evolution reaction for MoS2 hollow spheres. J Electroanal Chem 2017;799:3047. [232] Guo X, Feng Z, Lv Z, Bu Y, Liu Q, Zhao L, et al. Formation of uniform FeP hollow microspheres assembled by nanosheets for efficient hydrogen evolution reaction. ChemElectroChem 2017;4:2052-8. [233] Li HH, Yu SH. Recent advances on controlled synthesis and engineering of hollow alloyed nanotubes for electrocatalysis. Adv Mater 2019;31:1803503. [234] Liu D, Xie M, Wang C, Liao L, Qiu L, Ma J, et al. Pd-Ag alloy hollow nanostructures with interatomic charge polarization for enhanced electrocatalytic formic acid oxidation. Nano Res 2016;9:1590-9. [235] Liu H, Ma FX, Xu CY, Yang L, Du Y, Wang PP, et al. Sulfurizing-induced hollowing of Co9S8 microplates with nanosheet units for highly efficient water oxidation. ACS Appl Mater Interfaces 2017;9:11634-41. [236] Ren X, Lyu F, Yang J, Wang F, Xue L, Wang L, et al. Homogeneous cobalt and iron oxide hollow nanocages derived from ZIF-67 etched by Fe species for 103

enhanced water oxidation. Electrochim Acta 2019;296:418-26. [237] Wu R, Wang DP, Han J, Liu H, Zhou K, Huang Y, et al. A general approach towards multi-faceted hollow oxide composites using zeolitic imidazolate frameworks. Nanoscale 2015;7:965-74. [238] Yu L, Wu HB, Lou XW. Self-templated formation of hollow structures for electrochemical energy applications. Acc Chem Res 2017;50:293-301. [239] Zou F, Chen YM, Liu K, Yu Z, Liang W, Bhaway SM, et al. Metal organic frameworks derived hierarchical hollow NiO/Ni/graphene composites for lithium and sodium storage. ACS Nano 2016;10:377-86. [240] Ai L, Tian T, Jiang J. Ultrathin graphene layers encapsulating nickel nanoparticles derived metal-organic frameworks for highly efficient electrocatalytic hydrogen and oxygen evolution reactions. ACS Sustainable Chem Eng 2017;5:4771-7. [241] Lian Y, Sun H, Wang X, Qi P, Mu Q, Chen Y, et al. Carved nanoframes of cobaltiron bimetal phosphide as a bifunctional electrocatalyst for efficient overall water splitting. Chem Sci 2019;10:464-74. [242] Wang S, Qin J, Meng T, Cao M. Metal-organic framework-induced construction of actiniae-like carbon nanotube assembly as advanced multifunctional electrocatalysts for overall water splitting and Zn-air batteries. Nano Energy 2017;39:626-38. [243] Guan C, Liu X, Elshahawy AM, Zhang H, Wu H, Pennycook SJ, et al. Metalorganic framework derived hollow CoS2 nanotube arrays: an efficient bifunctional electrocatalyst for overall water splitting. Nanoscale Horiz 2017;2:342-8. [244] Pan Y, Sun K, Liu S, Cao X, Wu K, Cheong WC, et al. Core-shell ZIF-8@ZIF67-derived CoP nanoparticle-embedded N-doped carbon nanotube hollow polyhedron for efficient overall water splitting. J Am Chem Soc 2018;140:26108. [245] Zhang L, Wu HB, Lou XW. Metal-organic-frameworks-derived general formation of hollow structures with high complexity. J Am Chem Soc 2013;135:10664-72. [246] Wang X, Yu L, Guan BY, Song S, Lou XWD. Metal-organic framework hybridassisted formation of Co3O4/Co-Fe oxide double-shelled nanoboxes for enhanced oxygen evolution. Adv Mater 2018;30:1801211. [247] Hu E, Ning J, Zhao D, Xu C, Lin Y, Zhong Y, et al. A room-temperature postsynthetic ligand exchange strategy to construct mesoporous Fe-Doped CoP hollow triangle plate arrays for efficient electrocatalytic water splitting. Small 2018;14:1704233. [248] Guo C, Guo J, Zhang Y, Wang D, Zhang L, Guo Y, et al. Synthesis of core-shell ZIF-67@Co-MOF-74 catalyst with controllable shell thickness and enhanced photocatalytic activity for visible light-driven water oxidation. CrystEngComm 2018;20:7659-65. 104

[249] Zhang W, Yao X, Zhou S, Li X, Li L, Yu Z, et al. ZIF-8/ZIF-67-derived Co-Nxembedded 1D porous carbon nanofibers with graphitic carbon-encased Co nanoparticles as an efficient bifunctional electrocatalyst. Small 2018;14:1800423. [250] Zhu B, Xia D, Zou R. Metal-organic frameworks and their derivatives as bifunctional electrocatalysts. Coordin Chem Rev 2018;376:430-48. [251] Zhang W, Wu ZY, Jiang HL, Yu SH. Nanowire-directed templating synthesis of metal-organic framework nanofibers and their derived porous doped carbon nanofibers for enhanced electrocatalysis. J Am Chem Soc 2014;136:14385-8. [252] Ma TY, Dai S, Qiao SZ. Self-supported electrocatalysts for advanced energy conversion processes. Mater Today 2016;19:265-73. [253] Ma TY, Dai S, Jaroniec M, Qiao SZ. Metal-organic framework derived hybrid Co3O4-carbon porous nanowire arrays as reversible oxygen evolution electrodes. J Am Chem Soc 2014;136:13925-31. [254] Zhang H, Zhai C, Wu J, Ma X, Yang D. Cobalt ferrite nanorings: Ostwald ripening dictated synthesis and magnetic properties. Chem Commun 2008:564850. [255] Mao B, Guo D, Qin J, Meng T, Wang X, Cao M. Solubility-parameter-guided solvent selection to initiate Ostwald ripening for interior space-tunable structures with architecture-dependent electrochemical performance. Angew Chem Int Ed 2018;57:446-50. [256] Wang Z, Wang H, Wang L, Pan L. One-pot synthesis of single-crystalline Cu2O hollow nanocubes. J Phys Chem Solids 2009;70:719-22. [257] Hu M, Furukawa S, Ohtani R, Sukegawa H, Nemoto Y, Reboul J, et al. Synthesis of Prussian blue nanoparticles with a hollow interior by controlled chemical etching. Angew Chem Int Ed 2012;51:984-8. [258] Guan J, Mou F, Sun Z, Shi W. Preparation of hollow spheres with controllable interior structures by heterogeneous contraction. Chem Commun 2010;46:66057. [259] Liang J, Li X, Cheng Q, Hou Z, Fan L, Zhu Y, et al. High yield fabrication of hollow vesica-like silicon based on the Kirkendall effect and its application to energy storage. Nanoscale 2015;7:3440-4. [260] Sun H, Lian Y, Yang C, Xiong L, Qi P, Mu Q, et al. A hierarchical nickel-carbon structure templated by metal-organic frameworks for efficient overall water splitting. Energy Environ Sci 2018;11:2363-71. [261] Wang H, Li Y, Wang R, He B, Gong Y. Metal-organic-framework templatederived hierarchical porous CoP arrays for energy-saving overall water splitting. Electrochim Acta 2018;284:504-12. [262] Kong D, Wang H, Lu Z, Cui Y. CoSe2 nanoparticles grown on carbon fiber paper: an efficient and stable electrocatalyst for hydrogen evolution reaction. J Am Chem Soc 2014;136:4897-900. [263] Ma TY, Ran J, Dai S, Jaroniec M, Qiao SZ. Phosphorus-doped graphitic carbon 105

nitrides grown in situ on carbon-fiber paper: flexible and reversible oxygen electrodes. Angew Chem Int Ed 2015;54:4646-50. [264] Wei Z, Zhu W, Li Y, Ma Y, Wang J, Hu N, et al. Conductive leaflike cobalt metal-organic framework nanoarray on carbon cloth as a flexible and versatile anode toward both electrocatalytic glucose and water oxidation. Inorg Chem 2018;57:8422-8. [265] Liu L, Niu Z, Zhang L, Zhou W, Chen X, Xie S. Nanostructured graphene composite papers for highly flexible and foldable supercapacitors. Adv Mater 2014;26:4855-62. [266] Weng B, Grice CR, Meng W, Guan L, Xu F, Yu Y, et al. Metal-organic framework-derived CoWP@C composite nanowire electrocatalyst for efficient water splitting. ACS Energy Lett 2018;3:1434-42. [267] Ji D, Peng S, Fan L, Li L, Qin X, Ramakrishna S. Thin MoS2 nanosheets grafted MOFs-derived porous Co-N-C flakes grown on electrospun carbon nanofibers as self-supported bifunctional catalysts for overall water splitting. J Mater Chem A 2017;5:23898-908. [268] Yilmaz G, Tan CF, Lim Y-F, Ho GW. Pseudomorphic transformation of interpenetrated Prussian blue analogs into defective nickel iron selenides for enhanced electrochemical and photo-electrochemical water splitting. Adv Energy Mater 2019;9:1802983. [269] Zhou J, Dou Y, Zhou A, Guo R-M, Zhao M-J, Li J-R. MOF template-directed fabrication of hierarchically structured electrocatalysts for efficient oxygen evolution reaction. Adv Energy Mater 2017;7:1602643. [270] Ge Y, Dong P, Craig SR, Ajayan PM, Ye M, Shen J. Transforming nickel hydroxide into 3D Prussian blue analogue array to obtain Ni2P/Fe2P for efficient hydrogen evolution reaction. Adv Energy Mater 2018;8:1800484. [271] Cai G, Zhang W, Jiao S, Yu S-H, Jiang H-L. Template-directed growth of wellaligned MOF arrays and derived self-supporting electrodes for water splitting. Chem 2017;2:791-802. [272] Indra A, Paik U, Song T. Boosting electrochemical water oxidation with metal hydroxide carbonate templated Prussian blue analogues. Angew Chem Int Ed 2018;57:1241-5. [273] Deng T, Lu Y, Zhang W, Sui M, Shi X, Wang D, et al. Inverted design for highperformance supercapacitor via Co(OH)2-derived highly oriented MOF electrodes. Adv Energy Mater 2018;8:1702294. [274] Ahn SH, Manthiram A. Direct growth of ternary Ni-Fe-P porous nanorods onto nickel foam as a highly active, robust bi-functional electrocatalyst for overall water splitting. J Mater Chem A 2017;5:2496-503. [275] Luo X, Zhou Q, Du S, Li J, Zhang L, Lin K, et al. One-dimensional porous hybrid structure of Mo2C-CoP encapsulated in N-doped carbon derived from MOF as an efficient electrocatalyst for hydrogen evolution reaction over the entire pH range. ACS Appl Mater Interfaces 2018;10:42335-47. 106

[276] Du Y, Han Y, Huai X, Liu Y, Wu C, Yang Y, et al. N-doped carbon coated FeNiP nanoparticles based hollow microboxes for overall water splitting in alkaline medium. Int J Hydrogen Energy 2018;43:22226-34. [277] Sun X, Huang H, Wang C, Liu Y, Hu T-L, Bu X-H. Effective CoxSy HER Electrocatalysts fabricated by in-situ sulfuration of a metal-organic framework. ChemElectroChem 2018;5:3639-44. [278] Du Y, Li Z, Liu Y, Yang Y, Wang L. Nickel-iron phosphides nanorods derived from bimetallic-organic frameworks for hydrogen evolution reaction. Appl Surf Sci 2018;457:1081-6. [279] Lei Y, Wei L, Zhai S, Wang Y, Karahan HE, Chen X, et al. Metal-free bifunctional carbon electrocatalysts derived from zeolitic imidazolate frameworks for efficient water splitting. Mater Chem Front 2018;2:102-11. [280] Chen J, Yang H, Xu X, Su Z, Guo Y, Wang Q. Mo2C based electrocatalyst with filter paper derived N-doped mesoporous carbon as matrix for H2 production. Appl Surf Sci 2018;455:187-94. [281] Ao K, Dong J, Fan C, Wang D, Cai Y, Li D, et al. Formation of yolk-shelled nickel-cobalt selenide dodecahedral nanocages from metal-organic frameworks for efficient hydrogen and oxygen evolution. ACS Sustainable Chem Eng 2018;6:10952-9. [282] Guo Y, Tang J, Wang Z, Kang Y-M, Bando Y, Yamauchi Y. Elaborately assembled core-shell structured metal sulfides as a bifunctional catalyst for highly efficient electrochemical overall water splitting. Nano Energy 2018;47:494-502. [283] Wu Y-P, Zhou W, Dong W-W, Lan Y-Q, Li D-S, Sun C, et al. Surfactant-assisted phase-selective synthesis of new cobalt MOFs and their efficient electrocatalytic hydrogen evolution reaction. Angew Chem Int Ed 2017;56:13001-5. [284] Zhu Y, Chen G, Xu X, Yang G, Liu M, Shao Z. Enhancing electrocatalytic activity for hydrogen evolution by strongly coupled molybdenum nitride@nitrogen-doped carbon porous nano-octahedrons. ACS Catal 2017;7:3540-7. [285] Tang Y, Gao M, Liu C, Li S, Jiang H, Lan Y, et al. Porous molybdenum-based hybrid catalysts for highly efficient hydrogen evolution. Angew Chem Int Ed 2015;54:12928-32. [286] Zhang Z, Hao J, Yang W, Tang J. Defect-rich CoP/nitrogen-doped carbon composites derived from a metal-organic framework: high-performance electrocatalysts for the hydrogen evolution reaction. ChemCatChem 2015;7:1920-5. [287] Wen X, Yang X, Li M, Bai L, Guan J. Co/CoOx nanoparticles inlaid onto nitrogen-doped carbon-graphene as a trifunctional electrocatalyst. Electrochim Acta 2019;296:830-41. [288] Wang H, Li Y, Li Y, He B, Wang R, Gong Y. MOFs-derived hybrid nanosheet arrays of nitrogen-rich CoS2 and nitrogen-doped carbon for efficient hydrogen 107

evolution in both alkaline and acidic media. Int J Hydrogen Energy 2018;43:23319-26. [289] Wan S, Jin W, Guo X, Mao J, Zheng L, Zhao J, et al. Self-templating construction of porous CoSe2 nanosheet arrays as efficient bifunctional electrocatalysts for overall water splitting. ACS Sustainable Chem Eng 2018;6:15374-82. [290] Yao Y, Mahmood N, Pan L, Shen G, Zhang R, Gao R, et al. Iron phosphide encapsulated in P-doped graphitic carbon as efficient and stable electrocatalyst for hydrogen and oxygen evolution reactions. Nanoscale 2018;10:21327-34. [291] Yan L, Jiang H, Wang Y, Li L, Gu X, Dai P, et al. One-step and scalable synthesis of Ni2P nanocrystals encapsulated in N,P-codoped hierarchically porous carbon matrix using a bipyridine and phosphonate linked nickel metal-organic framework as highly efficient electrocatalysts for overall water splitting. Electrochim Acta 2019;297:755-66. [292] Yang J, Zhang F, Wang X, He D, Wu G, Yang Q, et al. Porous molybdenum phosphide nano-octahedrons derived from confined phosphorization in UIO-66 for efficient hydrogen evolution. Angew Chem Int Ed 2016;55:12854-8. [293] Jiao D, Wang R, Lv YR, Wei YL, Wang SQ. One-step MOF-derived Co/Co9S8 nanoparticle embedded in nitrogen, sulfur and oxygen ternary-doped porous carbon: an efficient electrocatalyst for overall water splitting. Chem Commun 2019;55:3203-6. [294] Aijaz A, Masa J, Rosler C, Xia W, Weide P, Fischer RA, et al. Metal-organic framework derived carbon nanotube grafted cobalt/carbon polyhedra grown on nickel foam: an efficient 3d electrode for full water splitting. ChemElectroChem 2017;4:188-93. [295] Wang L, Ren L, Wang X, Feng X, Zhou J, Wang B. Multivariate MOF-templated pomegranate-like Ni/C as efficient bifunctional electrocatalyst for hydrogen evolution and urea oxidation. ACS Appl Mater Interfaces 2018;10:4750-6. [296] Zhai M, Wang F, Du H. Transition-metal phosphide-carbon nanosheet composites derived from two-dimensional metal-organic frameworks for highly efficient electrocatalytic water-splitting. ACS Appl Mater Interfaces 2017;9:40171-9. [297] Hu L, Hu YW, Liu R, Mao YC, Balogun M-S, Tong YX. Co-based MOF-derived Co/CoN/Co2P ternary composite embedded in N- and P-doped carbon as bifunctional nanocatalysts for efficient overall water splitting. Int J Hydrogen Energy 2019; https://doi.org/10.1016/j.ijhydene.2019.03.157. [298] Chen J, Xia G, Jiang P, Yang Y, Li R, Shi R, et al. Active and durable hydrogen evolution reaction catalyst derived from Pd-doped metal-organic frameworks. ACS Appl Mater Interfaces 2016;8:13378-83. [299] Chen J, Zhang Y, Ye H, Xie J, Li Y, Yan C, et al. Meta-organic frameworkderived CoxFe1-xP nanoparticles encapsulated in N-doped carbon as efficient bifunctional electrocatalysts for overall water splitting. ACS Appl Energy Mater 2019;2:2734-42 108

[300] Gan L, Hu L, An H, Fang J, Lai Y, Li J. Sequential engineering of ternary CuFeNi with a vertically layered structure for efficient and bifunctional catalysis of the oxygen and hydrogen evolution reactions. ACS Appl Mater Interfaces 2018;10:41465-70. [301] Lin J, Qi F, Zheng B, Wang X, Yu B, Zhou K, et al. In-situ selenization of Cobased metal-organic frameworks as a highly efficient electrocatalyst for hydrogen evolution reaction. Electrochim Acta 2017;247:258-64. [302] Ha Y, Shi L, Chen Z, Wu R. Phase-transited lysozyme-driven formation of selfsupported Co3O4@C nanomeshes for overall water splitting. Adv Sci 2019; 6:1900272. [303] Gao L, Chen S, Zhang H, Zou Y, She X, Yang D, et al. Porous CoP nanostructure electrocatalyst derived from DUT-58 for hydrogen evolution reaction. Int J Hydrogen Energy 2018;43:13904-10. [304] Sun C, Dong Q, Yang J, Dai Z, Lin J, Chen P, et al. Metal-organic framework derived CoSe2 nanoparticles anchored on carbon fibers as bifunctional electrocatalysts for efficient overall water splitting. Nano Res 2016;9(8):2234-43. [305] Hao Y, Xu Y, Liu W, Sun X. Co/CoP embedded in a hairy nitrogen-doped carbon polyhedron as an advanced tri-functional electrocatalyst. Mater Horiz 2018;5:108-15. [306] Jayaramulu K, Masa J, Tomanec O, Peeters D, Ranc V, Schneemann A, et al. Nanoporous nitrogen-doped graphene oxide/nickel sulfide composite sheets derived from a metal-organic framework as an efficient electrocatalyst for hydrogen and oxygen evolution. Adv Funct Mater 2017;27:1700451. [307] Ming F, Liang H, Shi H, Mei Q, Xu X, Wang Z. Hierarchical (Ni,Co)Se2/carbon hollow rhombic dodecahedra derived from metal-organic frameworks for efficient water-splitting electrocatalysis. Electrochim Acta 2017;250:167-73. [308] Zhang H, Ma Z, Duan J, Liu H, Liu G, Wang T, et al. Active sites implanted carbon cages in core-shell architecture: highly active and durable electrocatalyst for hydrogen evolution reaction. ACS Nano 2016;10:684-94. [309] Yang L, Zhang L, Xu G, Ma X, Wang X, Wang W, et al. Metal-organicframework-derived hollow CoSx@MoS2 microcubes as superior bifunctional electrocatalysts for hydrogen evolution and oxygen evolution reactions. ACS Sustainable Chem Eng 2018;6:12961-8. [310] Liu H, Xia G, Zhang R, Jiang P, Chen J, Chen Q. MOF-derived RuO2/Co3O4 heterojunctions as highly efficient bifunctional electrocatalysts for HER and OER in alkaline solutions. RSC Adv 2017;7:3686-94. [311] Zhou W, Wu YP, Wang X, Tian JW, Huang DD, Zhao J, et al. Improved conductivity of a new Co(II)-MOF by assembled acetylene black for efficient hydrogen evolution reaction. CrysEngComm 2018;20:4804-9. [312] Chen Z, Xu H, Ha Y, Li X, Liu M, Wu R. Two-dimensional dual carbon-coupled defective nickel quantum dots towards highly efficient overall water splitting. Appl Catal B: Environ 2019;250:213-23. 109

[313] Chen Z, Ha Y, Jia H, Yan X, Chen M, Liu M, et al. Oriented transformation of Co-LDH into 2D/3D ZIF-67 to achieve Co-N-C hybrids for efficient overall water splitting. Adv Energy Mater 2019;9:1803918.

110