Rare Metals2020年第4期

A review and perspective on molybdenum-based electrocatalysts for hydrogen evolution reaction

Wei Hua Huan-Huan Sun Fei Xu Jian-Gan Wang

State Key Laboratory of Solidification Processing,Center for Nano Energy Materials,School of Materials Science and Engineering,Northwestern Polytechnical University

Shaanxi Joint Laboratory of Graphene (NPU)

作者简介：*Jian-Gan Wang e-mail:wangjiangan@nwpu.edu.cn Dr. is currently a professor in the School of Materials Science and Engineering at Northwestern Polytechnical University.He received his Ph.D.degree in Materials Science from Tsinghua University in 2013.Then, he joined in the Center for Nano Energy Materials.He has published>80 peer-reviewed papers.His research interests focus on advanced electrochemical energy materials and devices.;

收稿日期：1 December 2019

基金：financially supported by the National Natural Science Foundation of China (Nos.51772249 and 51821091);the Fundamental Research Funds for the Central Universities (Nos.G2017KY0308 and 3102019JC005);the Natural Science Foundation of Shaanxi Province (Nos.2018JM5092 and 2019JLM-26);the Innovation Program for Talent (No.2019KJXX066);the Post-doctoral Program of Shaanxi Province (No. 2018BSHTDZZ16);

A review and perspective on molybdenum-based electrocatalysts for hydrogen evolution reaction

Wei Hua Huan-Huan Sun Fei Xu Jian-Gan Wang

State Key Laboratory of Solidification Processing,Center for Nano Energy Materials,School of Materials Science and Engineering,Northwestern Polytechnical University

Shaanxi Joint Laboratory of Graphene (NPU)

Abstract：

Water electrolysis has been considered as a sustainable way for producing renewable energy of hydrogen.However,this process requires a low-cost and high-efficient hydrogen evolution reaction(HER) catalyst to improve the overall reaction efficiency.Molybdenum(Mo)-based electrocatalysts are regarded as the promising candidates to replace the benchmark but expensive Ptbased HER catalysts,due to their high activity and stability in a wide pH range.In this review,we present a comprehensive and critical summary on the recent progress in the Mo-based electrodes for HER,including molybdenum alloys,molybdenum sulfides,molybdenum selenides,molybdenum carbides,molybdenum phosphides,molybdenum borides,molybdenum nitrides,and molybdenum oxides.Particular attention is mainly focused on the synthetic methods of Mo-based materials,the strategies for increasing the catalytic activity,and the relationship between structure/composition and electrocatalytic performance.Finally,the future development and perspectives of Mo-based electrocatalysts toward high HER performance are proposed.

Keyword：

Mo-based materials; Electrocatalysts; Hydrogen evolution reaction; Water splitting;

Received： 1 December 2019

1 Introduction

The rapid development of human civilization enables an ever-increasing dependence on various energies.Nowadays,fossil fuels are still the most important energy consumed in the world.However,due to its limited reserves on earth and the increasing consumption,the shortage or even exhaustion of fossil energy will become an inevitable problem.In addition,the burning of fossil fuels has also contributed to a sharp deterioration of the ecological environment.Therefore,developing clean and renewable energy is one of the most challenging issues in the world today [ 1, 2, 3] .Among various energy carriers,hydrogen has attracted extensive attention,because of its extremely high gravimetric energy density,highly clean and renewable ability.However,the main current hydrogen production is still deriving from the fossil energy,which cannot fundamentally solve the problems.By contrast,water electrolysis is considered to be a sustainable way to produce the renewable energy of highpurity hydrogen.However,the sluggish hydrogen evolution reaction (HER) kinetics significantly reduce the production efficiency of water electrolysis,thus resulting in a substantial cost increase [ 4, 5] .Pt-based noble metal materials show prominent electrocatalytic activities for HER,but the scarcity and high cost block its worldwide application.To reduce the usage of Pt-based catalysts,it is of great significance to exploit new low-cost yet high-efficient noble metal-free HER catalysts.

Up to now,various transition-metal-based materials(mainlyⅧand VI B groups) have been widely studied and are expected to be high-performance catalysts for HER [ 6] .In particular,the materials inⅥB group (molybdenum and tungsten) play a very important role as HER catalysts.Among them,molybdenum (Mo)-based materials exhibit better HER properties,thereby attracting massive research attention in recent years.The superior performance of Mobased compounds makes them applicable in a large number of fields,such as molecular probes [ 7] ,electrochemical capacitors [ 8] ,catalysts [ 9, 10] ,and batteries [ 11, 12, 13, 14, 15] .The study of Mo-based materials for electrochemical HER can be traced back to several decades ago.Recently,a variety of Mo-based compounds,including Mo-based alloys,sulfides,selenides,carbides,phosphides,borides,nitrides,and oxides,have been widely investigated as excellent HER catalysts for hydrogen production.It is worth noting that the Mo-based compounds could exhibit high electrocatalytic activity and stability in a wide pH range,such as strong acidic,strong alkaline,and neutral solution.Therefore,it is highly desirable to give a periodical summary of Mo-based electrocatalysts,which may provide a useful guideline for their future exploration.

In this review,we present a comprehensive and critical summary of the most recent developments of the Mo-based electrocatalysts for HER.After a brief introduction of the fundamental mechanism of HER,the synthetic methods of the Mo-based HER catalysts,the strategies for improving the catalytic activity,and the relationship between structure/composition and HER performance are systematically discussed.In the final section,the challenges and perspectives on the future development of Mo-based electrocatalysts toward high-efficient HER are proposed.

2 Fundamental mechanism of HER

There are two widely accepted mechanisms for hydrogen evolution:Volmer-Heyrovsky and Volmer-Tafel mechanisms,as shown in Fig.1.Generally,the HER process can be expressed by the following chemical equations according to the different electrolytes.

In acidic solution:

More specifically,the reaction starts with an adsorption process of a hydrated proton or water molecule (H₃O⁺or H₂O) on the surface of the electrocatalyst via an electrochemical reduction step to generate hydrogen intermediates (H^*)(Volmer step).Subsequently,the hydrogen is produced either though the chemical combination of two H^*(Tafel step),or though the electrochemical bonding of a H₃O⁺(or H₂O) with the adsorbed H^*(Heyrovsky step).It is worth noting that electrocatalytic HER is a two-step process regardless of Volmer-Heyrovsky and Volmer-Tafel mechanisms.Either of the adsorption (Volmer) or the desorption (Heyrovsky or Tafel) step could become the rate-limiting process,revealing that the electrocatalytic HER performance is associated with the adsorption energy of the adsorbed hydrogen (H).Figure 2[16]exhibits a summary of various metals with a volcano-type relationship between exchange current density and Gibbs free energy.The ideal catalyst with the best HER catalytic activity should be situated on the peak point of the volcano curve.From the abovereaction mechanisms,it is obvious that the change of pH values results in different reactants and products involved in the reactions.Specifically,the Volmer step in acidic solution merely corresponds to the electrochemical reduction in a proton into an adsorbed H,while in neutral and alkaline solutions,it is not only determined by the adsorption energy of hydrogen,but also related to the dissociation of water and the desorption of hydroxide ions.

Fig.1 Two mechanisms of HER on surface of electrocatalyst in acidic or neutral and alkaline solutions

Fig.2 Volcano curve of exchange current density (i0) as a function of calculated Gibbs free energy (ΔGH*) of adsorbed atomic hydrogen for pure metals.Reprinted with permission[16].Copyright (2007)The American Association for the Advancement of Science

3 Mo-based electrocatalysts for HER

Molybdenum is a second-row transition metal element with an atomic number of 42 (Fig.3).From the volcano curve in Fig.2,Mo metal has an inappropriate adsorption energy for hydrogen,which is not conducive to the subsequent desorption process and thus limits its catalytic activity.However,the introduction of other metallic or nonmetallic elements to form Mo-based alloys or compounds,respectively,can effectively tune the electronic structure and endow Mo-based materials more suitable for hydrogen adsorption energy.To this end,a large number of Mobased materials,including Mo-based alloys,sulfides,selenides,carbides,phosphides,borides,nitrides,and oxides,have been reported to show excellent catalytic properties in recent years.It is worth noting that most of the Mo-based compounds also exhibit good metallic conductivity,which is one of key factors affecting the electrocatalytic performance.Moreover,these Mo-based materials have good chemical and electrochemical stability,enabling them applicable in a wide range of pH values.

Fig.3 A summary of various Mo-based materials and key factors for determining HER performance

3.1 Mo-based alloys

The catalytic performance of Mo metal for HER can be tailored by alloying other metallic elements with the molybdenum to form Mo-based alloys.The introduction of heteroatoms can tune the adsorption/desorption energy of hydrogen on the surface of Mo atom to promote the catalytic process.In 1984,Brown et al. [ 17] first studied a series of binary alloys as promising HER electrocatalysts.They found that NiMo alloy displayed the highest activity and the performance of Mo-based alloys followed the trend of Ni-Mo>Co-Mo>Fe-Mo.In recent years,Fang et al. [ 18] constructed porous NiMo nano wire arrays on the nickel foam (NF) for HER.The unique hierarchical structure had a larger specific surface area to offer more active sites.Combined with the high catalytic activity of NiMo components,the performance of the catalyst was comparable to that of the state-of-the-art Pt/C.Thereafter,a MoNi₄ alloy supported by MoO₂ cuboids on NF was reported by Zhang et al. [ 19] via regulating the outward diffusion process of nickel atoms in NiMoO₄ precursor during calcination (Fig.4a).Theoretical evidence(Fig.4b,c) revealed that MoNi₄ could lower the energy barrier of water dissociation and accelerate the sluggish kinetics of Volmer step in alkaline solution.As a result,as shown in Fig.4d,e,the MoNi₄ catalyst exhibited an extremely small overpotential of 15 mV at 10 mA·cm^-2and a low Tafel slope of 30 mV·dec^-1 in 1.0 mol·L^-1KOH solution.

Mo-based alloys,especially MoNi4 alloys,possess excellent ability of diminishing the activity energy of water dissociation and thereby exhibit the best HER performance in alkaline solutions,as far as we known.However,the effects of mass loading and support interaction on the performance still need further study.

Fig.4 a Synthetic illustration of MoNi₄ alloy supported by MoO₂ cuboids on Ni foam;b calculated adsorption free energy diagram for Volmer step;c calculated adsorption free energy diagram for Tafel step;d polarization curves and e Tafel plots of four samples.Reprinted with permission [19] .Copyright (2015) Springer Nature

3.2 Mo-based sulfides

Molybdenum disulfide (MoS2),a typical two-dimensional(2D) layered material [ 20] ,has been extensively studied as the HER catalyst.MoS₂ was not considered as a potential candidate for a long time before the twenty-first century,because the bulk MoS₂ was found to be inactive for HER.Until 2005,Hinnemann et al. [ 21] found that the edge sites of MoS₂ had good catalytic activity theoretically through density functional theory (DFT) calculations.In order to experimentally identify the active site of MoS₂,Jaramillo et al. [ 16] quantitatively regulated the number of edge sites and basal sites of MoS₂ nanoparticles.Electrochemical test results revealed that the catalytic performance was linearly correlated with the number of edge sites on MoS₂ catalyst,which indicated that the edge sites of MoS₂ were the active sites for HER.

The identification of the active sites greatly promoted the development of MoS₂.However,as the typical 2D layer materials,the preferentially exposed planes of MoS₂are thermodynamically more stable basal planes instead of the active edge planes.Furthermore,the semiconductor nature also limits the HER activity of MoS₂.Based on the above disadvantages,there are three key factors that affect the catalytic performance of MoS₂:(1) the number of active sites,(2) the intrinsic catalytic activity,and (3) the electrical conductivity.

The first way to improve the performance of MoS₂ is to increase the number of active sites.Owing to the active sites of MoS₂ at the edges,the first thing that comes to mind is to expose more edge sites relative to basal plane sites.Kibsgaard et al. [ 22] successfully prepared a highly ordered porous double-gyroid MoS₂ bicontinuous network.The mesoporous structure with a nanoscale radius of curvature exposed more edge sites relative to basal plane sites via the geometric constraints,leading to an enhanced HER performance.Xie et al. [ 23] demonstrated that the introduction of defects on the basal planes could expose more active edge sites.It is found that during the synthesis process,excessive thiourea would adsorb on the surface of the initially formed nanosheet,preventing the further growth of the crystal,generating quasiperiodic defects on the inert basal plans,resulting in more exposed edge sites.Since the basal plane of MoS₂ is more stable and has more exposed sites,more active sites can be obtained to endow the basal plane sites with activity.2H phase of MoS₂ can be described by two S-Mo-S layers composed of edge-sharing MoS₆ trigonal prisms,while the metallic 1T phase is described by a single S-Mo-S layer built from edge-sharing MoS₆ octahedra.The most common method for converting 2H phase to 1T involved an exfoliation process with lithium compounds as intercalators.In 2013,Lukowski et al. [ 24] prepared metallic nanosheets of 1T-MoS₂ via this method.As a result,the density of active sites in the 1T phase was 10times higher than that of the origin sample.In contrast,the 2H phase MoS₂ obtained by exfoliation exhibited only slightly enhanced catalytic activity,indicating that the number of edge sites was not the most important reason for the significantly improved properties of 1T phase MoS₂ nanosheets.Voiry et al. [ 25] further confirmed that when the edge sites of the 1T phase and the 2H phaseMoS2 were oxidized,the performance of the 2H phase declined significantly,while that of the 1T phase was not affected.The results demonstrated that the edge sites of the 1T phase were not the main active sites,and the basal plane showed higher catalytic activity.

The second activation method seeks to increase the intrinsic catalytic activity.The main strategy to improve the intrinsic catalytic activity of MoS₂ is cation doping.In2008,Bonde et al. [ 26] first studied the Co-doped MoS₂.They found that Co species was preferentially located at the S-edges of MoS₂.DFT calculations revealed that Co doping optimized the adsorption energy of hydrogen of the S-edge,leading to the increased intrinsic catalytic activity for HER.Subsequently,Tsai et al. [ 27] studied the effects of various transition metals doping on the properties of MoS₂ by DFT.DFT calculations indicated that doping metals with higher activity at the S-edge would result in weaker H binding,while metals with lower activity would lead to stronger H binding.To date,several of these metal dopants have been experimentally shown to increase the HER activity of MoS₂.

The third activation strategy is to improve the electronic conductivity of MoS₂.The 2H phase of MoS₂ has poor conductivity due to its natural semiconductor structure,thus limiting its catalytic activity toward HER.Sun et al. [ 28] successfully controlled the intrinsic electrical properties of the original MoS₂ by doping vanadium (V) atoms into the MoS₂ intralayer for the first time and realized the semimetal MoS₂ with adjustable conductivity and high carrier concentration.Owing to the increased conductivity,V-doped MoS₂ showed better catalytic performance with respect to that of the original MoS₂,including a lower onset overpotential of 130 mV and a smaller Tafel slope of69 mV·dec^-1.Besides metal element doping,nonmetal element doping can also improve the conductivity of MoS₂.Xie et al. [ 29] studied the effect of oxygen incorporation on the conductivity of MoS₂ catalyst.Density of states results indicated that the band gap of oxygen-incorporated MoS₂was only 1.30 eV due to the enhanced hybridization between Mo d orbital and S p orbital,which was much narrower than that of the pure MoS₂ (1.75 eV),suggesting that the incorporation of oxygen resulted in enhanced intrinsic electronic conductivity.In addition to element doping,coupling with conductive substrates (mainly carbon-based materials) is another common and facile way to increase the conductivity of MoS₂.So far,one-dimensional carbon nanotube (CNT) and carbon nanofiber,two-dimensional graphene,and three-dimensional porous carbons were all employed as the substrate to improve the conductivity of MoS₂ [ 30, 31, 32, 33] .Moreover,graphene paper,carbon cloth,and carbon fiber paper coupled with MoS₂can be served as a free-standing working electrode without the expensive binder [ 34, 35, 36] .

It should be mentioned that the above studies on MoS₂were conducted in acidic electrolytes.Owing to the lack of low-cost earth abundant oxygen evolution reaction (OER)catalyst under acidic condition,the HER catalyst in alkaline solution has attracted more attention in recent years.However,MoS₂ displayed extremely poor catalytic performance for HER in KOH solution,because of the sluggish water dissociation step.Zhang et al. [ 37] reported a novel strategy to improve the kinetics of water dissociation process of MoS₂ via doping nickel atoms in MoS₂nanosheets.Theoretical calculation and experimental results (Fig.5a,b) indicated that the introduced Ni sites could effectively decrease the kinetic energy barrier of Volmer step and promote the formation and desorption of OH^-,leading to an extremely enhanced HER activity with a lower HER overpotential of 98 mV at 10 mA·cm^-2 with respect to that of the pristine MoS₂ (more than 220 mV at10 mA·cm^-2).Zang et al. [ 38] endowed MoS₂ with alkaline HER properties via carbon-incorporated orbital modulation.Fine structural characterization suggested that the electronic structure of MoS₂ has been tuned with carbon introduction.In addition,theoretical calculation (Fig.5c)indicated that carbon incorporation could generate empty2p orbitals perpendicular to the basal plane,leading to significantly optimized water adsorption and dissociation process.The carbon-doped MoS₂ (C-MoS₂) exhibited the optimum alkaline HER performance among the reported MoS₂ catalysts with an overpotential of 45 mV at10 mA·cm^-2 (Fig.5d).For alkaline hydrogen production,the construction of heterogeneous interfaces is considered to be an effective approach to facilitate water dissociation,especially the synergistic interface between the active species and Ni(OH)2.Zhang et al. [ 39] demonstrated that Ni(OH)2 could also enhance the HER activity of MoS₂ in1 mol·L^-1 KOH solution.Three-dimensional (3D)Ni(OH)₂/MoS₂ hybrid catalysts were prepared by a simple hydrothermal and electrodeposition approach.DFT calculation showed that Ni(OH)2 could promote the water dissociation and the nearby MoS₂ acted as the active materials of the adsorption/desorption of hydrogen intermediates.Therefore,this hybrid sample exhibited an outstanding electrocatalytic HER performance in a basic media,including an onset potential of 20 mV and a low overpotential of 80 mV at 10 mA·cm^-2.Furthermore,Zhang et al. [ 40] found that Ni₃S₂ also had the similar functions to Ni(OH)₂,and the MoS₂/Ni₃S₂ heterostructures exhibited prominent catalytic performance in alkaline media.

MoS2 is one of the most thoroughly studied catalysts toward HER,and the catalytic activity is mainly subjected to the limited number of edge active sites and intrinsic semiconductor nature.The researchers proposed three effective strategies to optimize the performance,including increasing the number of active site,enhancing the intrinsic catalytic activity,and improving the electronic conductivity.Based on these strategies,the catalytic activity of MoS₂has shown great improvement in acidic solutions.However,the performance in alkaline solutions is still poor due to the sluggish water dissociation process,which can be to some extent accelerated by metal or nonmetal element doping and/or construction of heterogeneous interface.

Fig.5 a Results of DFT calculations and corresponding mechanisms of HER on surfaces of different electrocatalysts in alkaline media;b linear sweep voltammetry (LSV) curves of MoS₂,different metallic elements-doped MoS₂,and commercial Pt/C electrocatalysts.Reprinted with permission [37] .Copyright (2016) The Royal Society of Chemistry.c Structural analysis and catalytic pathway of C-MoS₂;d polarization curves of carbon cloth (CC),Mo₂C,C-MoS₂,MoS₂ and Pt/C.Reprinted with permission [38] .Copyright (2019) Springer Nature

3.3 Mo-based selenides

Since S and Se are in the same group (VI A) of periodic table,molybdenum selenides (MoSe₂) show many similar properties to the MoS₂,suggesting that MoSe₂ may also have outstanding catalytic performance toward HER.Following the in-depth investigation of MoS₂ mentioned above,the research on MoSe₂ has also attracted extensive attention.

In 2014,Tsai et al. [ 41] studied the origin of the electrocatalytic performance on MoSe₂ for HER by DFT calculation.The result revealed that both Mo-edge and Seedge of MoSe₂ were the major active sites.Compared with the inert basal plane,the enhanced activity of the edge sites came from the localized edge state.Therefore,reasonable design to expose more edge sites could improve the catalytic activity of MoSe₂,which was similar to MoS₂.A MoSe₂ film with vertically aligned layers was prepared by Kong et al. [ 42] via the rapid selenization on molybdenum foil.This edge-terminated structures had highdensity edge sites,which displayed high HER performance comparable to that of MoS₂ with similar structure.Xu et al. [ 43] reported the preparation of S-doped MoSe₂nanosheets with abundant exposed edge sites by using a surfactant to reduce the surface energy of the edges(Fig.6a,b).Moreover,the as-synthesized S-doped MoSe₂nano sheets not only were rich in unsaturated edge sites,but also had large amount of structural defects in the bas al planes (Fig.6c).In addition,as shown in Fig.6d,S doping could also improve the electronic conductivity of MoSe₂.Therefore,the S-doped MoSe₂ displayed prominent electrocatalysis activity with a low overpotential of 156 mV at30 mA·cm^-2 and a small Tafel slope of 58 mV·dec^-1(Fig.6e,f).In addition to S doping,Gong et al. [ 44] demonstrated that MoS_2(1-x) Se_2x alloy also could improve the HER performance.They prepared MoS_2(1-x)Se_2x,(0≤x≤1) alloy nanoflakes with monolayer or few-layer thickness.Remarkably,the d-band electronic structure of Mo could be continuously regulated by the change of relative content of Se and S.Among them,MoSSe showed better performance than MoS₂ and MoSe₂.Because these materials had similar morphology,the authors suggested that MoSSe had higher intrinsic catalytic activity and more suitable adsorption energy of hydrogen.Yin et al. [ 45] studied the effects of synergistic phase and disorder in MoSe₂ nanosheets for HER.They found that the 1T phase formed due to a large amount of excess NaBH₄ reductant in the synthesis process effectively improved the intrinsic activity and conductivity,while the disordered structure formed at a lower reaction temperature provided abundant unsaturated defects as the active sites.

Fig.6 a High-magnification transmission electron microscope (TEM) image and a high resolution transmission electron microscopy (HRTEM)image of S-incorporated MoSe₂;c scheme of unsaturated edges of nanodomains on nano sheet (100) basal plane;d ultraviolet-visible (UV-Vis)diffuse reflectance spectra;e LSV curves and f Tafel plots of electrodes.Reprinted with permission [43] .Copyright (2014) The Royal Society of Chemistry

In alkaline solution,MoSe₂ can also show good HER catalytic activity via some tuning strategies.Zhao et al. [ 46] demonstrated that metallic atom doping (Ni and Co)could not only improve the water adsorption kinetics of MoSe₂,but also optimize the binding energy of hydrogen.Remarkably,the Ni_0.15Mo_0.85Se₂ nanosheets displayed the highest HER activity in both alkaline and acidic media.Subsequently,they designed and prepared CoSe₂/MoSe₂heterostructures to enhance the performance of MoSe₂ in basic media with the introduction of water dissociation favored CoSe₂ [ 47] .

Owing to S and Se in the same group of periodic table,MoSe₂ and M0S₂ show numerous similar properties and therefore the tuning strategies of MoSe₂ toward HER in both acidic and alkaline media can refer to that of MoS₂.

3.4 Mo-based carbides

Molybdenum carbide (Mo_xC) has received increasing research interest as an effective noble metal-free catalyst toward HER due to its Pt-like electron structure,high electrical conductivity,and good chemical stability in a wide pH range.In 2012,Vrubel and Hu [ 48] studied the HER activity of commercial Mo₂C in detail.They found that Mo₂C was active toward HER in both acidic and basic media.However,the commercial Mo_xC had relatively small specific surface area and the limited number of active sites.Moreover,the traditional synthesis method of temperature-programmed reaction easily led to the seriously agglomeration of particles.Therefore,developing a new strategy to synthesize highly dispersed Mo_xC can enhance the catalytic activity due to the more exposed active sites.

With the development of nanomaterials and nanotechnology,many novel Mo_xC nanostructures have been synthesized by a new organic-inorganic hybrids strategy,which was mainly based on the simple pyrolysis process of Mo species and organic carbon precursors.In 2014,Liao et al. [ 49] prepared the nanoporous molybdenum carbide nano wires (np-Mo₂C NWs) via a facile carburization step of a MoO_x/amine hybrid.The sub-nanosized periodic contact between MoO_x and amine molecules in the hybrid produced a homogeneous reaction environment and accelerated the generation of abundant nanoporosity.The nanoporous nanowires composed of highly dispersed Mo₂C nanocrystals exposed a large number of active sites,leading to the superior HER activity with respect to commercial Mo₂C microparticles.Subsequently,pomegranate-like Mo₂C@C nanospheres were synthesized by Chen et al. [ 50] via a pyrolysis of phosphomolybdic acid and polypyrrole hybrid.Huang et al. [ 51] developed a simple twostep way for uniform～3-nm Mo₂C nanoparticles dispersed on carbon microflowers through the self-polymerization of dopamine containing Mo species.Moreover,Wu et al. [ 52] developed a novel MOFs-assisted method for preparing porous molybdenum carbide nano-octahedrons via introducing guest polyoxometalates into the porous MOFs host as co-precursor,thus facilitating the in situ and homogeneous pyrolysis process to generate small carbide nanocrystallites (Fig.7a-d).As shown in Fig.7e,f,the MoC_x displayed prominent catalytic performance with low overpotentials of 142 and 151 mV at 10 mA·cm^-2 in0.5 mol·L^-1 H₂SO₄ and 1.0 mol·L^-1 KOH,respectively.

A great deal of efforts has been devoted to preparing various Mo₂C nanostructures to acquire more active sites.Meanwhile,it is also important to identify and enhance the intrinsic catalytic activity of molybdenum carbide.Wan et al. [ 53] first compared HER catalytic activity of four molybdenum carbide phases (α-Mo_C1-x,β-Mo₂C,η-MoC,and y-MoC).The electrochemical testing results indicated that the activity trend of four phases followedβ-Mo₂C>γ-MoC>η-MoC>α-MoC_1-x.Althoughβ-Mo₂C displayed the best performance among the four phases,the strong adsorption energy of hydrogen limited the desorption process.As a result,the HER catalytic activity could be further improved via optimizing the hydrogen-binding energy.Lin et al. [ 54] demonstrated that cobalt doping could effectively modulate the electron structures around Fermi level of Mo₂C,achieving better HER activities in both acidic and alkaline media.Moreover,they further developed a novel MoC-Mo₂C heteronanowires as excellent HER catalysts though controlling the pyrolysis process [ 55] (Fig.7g).Owing to the optimized electron density on the surface,MoC-Mo₂C heteronanowires showed a low overpotential (126 and 120 mV at 10 mA·cm^-2) and a small Tafel slope (43 and 42 mV·dec^-1) in acidic and basic solutions,respectively (Fig.7h,k).

Fig.7 a Preparation process of porous MoC_x nano-octahedrons;b scanning electron microscope (SEM) image (inset:digital photo),c TEM image (inset:selected area electron diffraction (SAED) pattern) and d magnified TEM image of porous MoC_x nano-octahedrons;polarization curves of MoC_x and Pt/C for HER in e 0.5 mol·L^-1 H₂SO₄ and f 1.0 mol·L^-1 KOH.Reprinted with permission [52] .Copyright (2015) Springer Nature,g Schematic illustration of formation of MoC_x HNWs;h,j polarization curves and i,k Tafel plots of (Ⅰ) MoC-Mo₂C-3 1.4,(Ⅱ) Mo₂C,(Ⅲ) MoC-Mo₂C-68.1,(IV) MoC-Mo₂C-30 (mixed),(V) MoC,and (VI) commercial Pt/C for HER in h,i 0.5 mol·L^-1 H₂SO₄ and j,k 1.0 mol·L^-1 KOH,respectively.Reprinted with permission [55] .Copyright (2016) The Royal Society of Chemistry

The new organic-inorganic hybrids synthesis method was exploited to design and construct Mo_xC nanostructures,which was demonstrated as effective tool for increasing the number of active sites.Moreover,phase engineering and doping were two typical strategies for optimizing the binding energy of hydrogen of Mo_xC.Furthermore,there is an interesting phenomenon about Mo_xC toward HER,that is,the overpotential in basic media is smaller than that in acidic media as reported in many literatures,which need to be further studied.

3.5 Mo-based phosphides

In recent years,transition metal phosphides have shown prominent HER performance with high activity and stability in wide pH media.Molybdenum phosphide (MoP) is a well-known catalyst toward hydrodesulfurization (HDS)reaction [ 56] .Since both HDS and HER depend on the reversible combination of catalyst and hydrogen,MoP may display excellent HER catalytic performance.In 2014,Xiao et al. [ 57] first reported MoP as an outstanding HER catalyst in both acid and alkaline electrolytes.They compared and analyzed the properties of Mo,Mo₃P,and MoP catalysts,and the results showed that the catalysts with higher phosphorus content in MoP had better catalytic activity than that with lower phosphorus content or lack in Mo₃P and Mo,respectively (Fig.8a,b).

Although MoP showed high HER activity in bulk form,it could be further improved by exposing more active sites through the construction of nanostructures.Xing et al. [ 58] successfully synthesized a tightly interconnected MoP nanoparticle network with a large amount of exposed active sites via a temperature-programmed reduction process with (NH₄)2HPO₄ as the phosphorus source.Notably,this unique network structure and the high activity of MoP allowed it to exhibit excellent performance for producing hydrogen in acidic solution with low overpotentials of 125and 200 mV at 10 and 100 mA·cm^-2,respectively.Moreover,the preparation of MoP/carbon composites was an effective strategy to increase the specific surface area of catalysts and thus exposed more active sites.A series of Mo-based materials/porous nano-structured carbon composites (i.e.,MoO₂@PC,Mo₂C@PC,MoN@PC,and MoP@PC) were prepared by Yang et al. [ 59] by using UIO-66 as the carbon precursor and different treatments(Fig.8c-e).Among these composites and bulk MoP,

MoP@PC displayed the best catalytic performance toward HER in acidic electrolyte.It is worth noting that the identical morphology of these composites indicated that MoP had the highest intrinsic catalytic activity and MoP@PC performed better than the bulk,indicating that it had more exposed active sites.In addition to porous carbon derived from MOF,building composites on certain carbon materials could also improve the dispersion to achieve better performance.Zhang et al. [ 60] reported various Mobased (MoX_n,MoX_n=MoP,MoS₂,Mo₂C,MoN,and MoO₂) nanowire arrays (NWAs) on carbon fiber paper(CFP) via general topotactic transformations.The results also proved that MoP NWAs/CFP exhibited the best electrocatalytic performance in pH-universal media,which only needed low overpotentials of 56,84,and 52 mV in0.5 mol·L^-1 H₂SO₄,1.0 mol·L^-1 PBS,and 1.0 mol·L^-1KOH to reach the current density of 10 mA·cm^-2,respectively.

For molybdenum phosphide,the increasing P content could effectively enhance the corresponding electrocatalytic performance.Remarkably,MoP showed the best performance among several Mo-based compounds with the identical morphology,indicating that it had the highest intrinsic catalytic activity.However,the synthesis of MoP is still a big challenge,and new phosphorus sources and preparation methods need to be developed.

3.6 Mo-based borides

In 2012,Vrubel and Hu [ 48] first studied the HER properties of molybdenum boride (α-MoB).Subsequently,Park et al. [ 61] successfully synthesized four different crystalline phases of molybdenum borides (Mo₂B,α-MoB,β-MoB,and MoB₂) via arc-melting (Fig.9a).The results(Fig.9b) revealed that the activity ofβ-MoB and MoB₂was similar with that ofα-MoB reported in Ref [ 48] .and the Mo-rich Mo₂B exhibited the lowest HER catalytic activity.In general,the HER performance of molybdenum borides increased with the content of boron element increasing.Based on the above research,they then prepared the single-phase Mo₂B₄ by the tin-flux method,which possessed the same boron content as MoB₂ [ 62] .Remarkably,this phase showed the best performance toward HER in the bulk catalysts under acidic condition.The DFT calculations found that the graphene-like B layer was more active,compared with the phosphorene-like B layer.In addition to the phase engineering strategy,Zhuang et al. [ 63] proposed a novel strategy to improve the catalytic performance of metallic MoB via constructing a Schottky junction with n-type semiconductive g-C₃N₄.Multiple spectroscopic techniques demonstrated that a large amount of charge was transferred to the surface of MoB,which increased the local charge density (Fig.9c-e).Therefore,the MoB/g-C₃N₄ Schottky catalyst displayed excellent HER properties,involving a low Tafel slope of46 mV·dec^-1 and a high exchange current density of17μA·cm^-2.

Fig.8 a Schematic graph to show structural evolution after phosphorization;b LSV curves of Mo,Mo₃P and MoP in 0.5 mol·L^-1 H₂SO₄.Reprinted with permission [57] .Copyright (2014) The Royal Society of Chemistry.c Schematic illustration of fabrication of MoP@PC nano-octahedrons;d TEM image and e HRTEM image of MoP@PC.Reprinted with permission [59] .Copyright (2015) Wiley-VCH

Similar to the activity trend of molybdenum phosphide,the HER performance of molybdenum boride increases with the increase in boron content.So far,the research on molybdenum boride mainly focused on acidic solution,but the properties of molybdenum boride in basic media have rarely been reported.

3.7 Mo-based nitrides

Transition metal nitrides (TMNs) are a kind of interstitial compounds in which nitrogen atoms are integrated into the interstices between the metal atoms.The insertion of nitrogen atoms can change the structure of the original metal,leading to the d-band contraction.As a result,TMNs have an electronic structure similar to the precious metal Pt.Molybdenum nitride (MoN) is a typical kind of transition metal nitride.In 2012,Chen et al. [ 64] demonstrated the electrocatalytic HER properties of MoN.Then,the atomic MoN nanosheets with metallic behavior were fabricated by Xie et al. [ 65] via liquid exfoliation.The surface of the atomically thin nanosheets was occupied by Mo atoms,and these Mo atoms on the surface were the active sites toward HER.Based on the above research,Xiong et al. [ 66] prepared 2D defect-rich large-sized MoN nanosheets (dr-MoN) through a NaCI template strategy(Fig.10a-c).This catalyst displayed efficient catalytic activity toward HER in both acidic and alkaline media,including the overpotentials of 125 and 139 mV at10 mA·cm^-2 in 0.5 mol·L^-1 H₂SO₄ and 1.0 mol.L^-1KOH,respectively.

Alloying with additional metal elements in monophase MoN is an efficient strategy to enhance HER catalytic activity.A bimetallic cobalt molybdenum nitride was successfully synthesized by Cao et al.[67]through treating the precursor under flowing ammonia.This ternary nitride(Co0.6Mo1.4N2) displayed a four-layered mixed closed packed structure,which was alternating between octahedral sites (a mixture of Co2+and Mo3+) and trigonal prismatic sites (Mo3+<Moδ+≤Mo4+).The authors suggested that the layered nature of this structure made Co tailor the electronic states of Mo,leading to better catalytic activity than MoN.In addition,Chang et al.[68]successfully prepared NiMoN nanowires with preferentially exposed(100) facet by the in situ exchange of N and O.The DFTcalculation (Fig.10d-h) indicated that the HER performance of this bimetallic nitride on the (100) facet wasmuch better than that on the (001) facet or (101) facet,because of the lower adsorption energy of hydrogen on(100) facet.The facet tuned NiMoN exhibited outstanding properties toward HER,which required extremely low overpotentials of 16 and 47 mV to reach the current density(j) of 10 mA·cm^-2 in 0.5 mol·L^-1 H₂SO₄ and 1.0 mol·L^-1KOH,respectively.

Fig.9 a X-ray diffraction (XRD) patterns and phase structures of Mo2B,α-MoB,β-MoB,and MoB2;b LSV curves for amorphous B,Mo,Mo2B.α-MoB,β-MoB,and MoB2 in 0.5 mol·L-1 H2SO4 (Evac,vacuum energy;Ec,conduction band;Ev,valence band;EF,Fermi level;Φ,vacuum electrostatic potential;X,vacuum ionization energy;V,electric potential;e,electric charge;W,depletion region).Reprinted with permission[61].Copyright (2017) Wiley-VCH.c Energy band diagrams of metallic MoB and n-type semiconductive g-C3N4 before and after Schottky contact.d Periodic slab model of MoB (112)/g-C3N4 Schottky catalyst and its spatial charge density difference isosurfaces,where blue and red isosurfaces represent electron accumulation and electron depletion,respectively;e energy profiles of HER on pure MoB and MoB/g-C3N4 Schottky catalyst.Reprinted with permission[63].Copyright (2017) Wiley-VCH

Alloying with other transition metal (especially Ni) is the most efficient tuning strategy for MoN.Bimetallic NiMoN showed excellent HER electrocatalytic activity,which was compared to that of commercial Pt/C.

3.8 Mo-based oxides

Very few transition metal oxides exhibit excellent catalytic performance of HER,mainly because of the poor electrical conductivity and strong binding energy of oxygen species.Molybdenum dioxide (MoO₂),however,is a potential catalyst for HER due to its excellent electrical conductivity,chemical stability,and more edge active sites (Mo-edge and O-edge).However,the severe agglomeration leads to few exposed active sites,which limits the HER performance.A variety of MoO₂ nanostructures with larger surface areas and more exploded active sites such as nanoparticles [ 69] ,nanobelts [ 70] ,nanoflowers [ 71] ,nanosheets [ 72] ,and nanowires [ 73] have been reported by different synthesis strategies and exhibited much higher HER activity compared with the bulk.In addition to increasing the number of exposed active sites,it is also important to improve the catalytic activity of each site.Doping with metallic or nonmetallic elements has become an effective method for preparing catalysts with excellent properties.Xie et al. [ 74] synthesized phosphorous-doped molybdenum dioxide nanoparticles on Mo foil (MoO₂P_x)(Fig.11a,b).Owing to the strong interaction between P and MoO₂,the asprepared materials showed enhanced catalytic performance with a small overpotential (η) of 135 mV at10 mA·cm^-2 and a low Tafel slope of 62 mV·dec^-1 in0.5 mol·L^-1 H₂SO₄ (Fig.11c,d).The metallic Co/Ni codoped MoO₂ composite was reported by Xu et al. [ 75] via a facile one-step synthetic strategy (Fig.11e,f).Benefiting from the optimized absorption energy of hydrogen,the electrode only required a low overpotential of103 mV to drive the current density of 10 mA·cm^-2 and the corresponding Tafel slope was 75 mV·dec^-1 in1.0 mol·L^-1 KOH (Fig.11g,h).

Fig.10 a Preparation process of defect-rich MoN catalysts.;b TEM image and c HRTEM image of defect-rich MoN catalysts.Reprinted with permission [66] .Copyright (2017) The Royal Society of Chemistry.Active adsorption sites of H^*on d NiMoN (100) facet and e Pt (110) and(111) facets;DFT-calculated adsorption energies of H^*on surfaces of f NiMoN (100),Pt (110),Pt (111) and g NiMoN (100),(001) and (101);h DFT-calculated density of states of NiMoN and Pt.Reprinted with permission [67] .Copyright (2018) Wiley-VCH

Fig.11 a Preparation process of MoO₂P_x/Mo foil electrode;b SEM image of MoO₂P_x on Mo foil;c po1arization curves and d Tafel slope of electrodes.Reprinted with permission [74] .Copyright (2016) The Roya1 Society of Chemistry.e SEM images and f elemental mapping images of Co/Ni-MoO₂;g LSV curves and h Tafe1 p1ots of electrodes.Reprinted with permission [75] .Copyright (2018) Elsevier Ltd

In addition to MoO₂,other Mo-based oxides have also been reported as promising HER catalysts.Ou et al. [ 76] reported a bimetallic Co₂Mo₃O₈ suboxides coupled with conductive cobalt nanowires material as an efficient HER catalyst.This reasonably designed hierarchical structure offered abundant active sites,and the cobalt metal nanowires effectively improved the electrical conductivity.Furthermore,the heterogeneous structure of amorphous MoO_x and Ni²⁺species was constructed by Hua et al. [ 77] via a simple hydrothermal step and the subsequent phosphorization process.Ni²⁺species was conductive to the dissociation of water,while amorphous MoO_x offered active site for the adsorption/desorption of hydrogen.The synergistic interaction between the two components greatly improved the efficiency of hydrogen production in basic media.

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Table 1 Comparison of HER performance for Mo-based catalysts

Similar with Mo_xC,nanostructure engineering is an efficient way to improve the catalytic activity of MoO₂.Moreover,the regulation strategies of doping and alloying can effectively enhance the intrinsic catalytic activity of MoO₂ and significantly lower the overpotential.As discussed in the part of MoP (Sect.3.5),the catalytic activity of MoO₂/carbon compounds was much lower than that of MoP,but MoO₂/NF displayed excellent electrocatalytic performance (Table 1).Therefore,the effects of the supports should be further identified.

4 Conclusive perspectives

Mo-based materials are considered to be the promising candidates as the replacement of the benchmark but expensive Pt-based catalysts due to their excellent HER catalytic activity and stability in a wide range of pH values.In this review,the recent development of the Mo-based electrodes,including molybdenum alloys,molybdenum sulfides,molybdenum selenides,molybdenum carbides,molybdenum phosphides,molybdenum borides,molybdenum nitrides,and molybdenum oxides,for HER has been scrutinized.Moreover,the preparation methods,the strategies for increasing the catalytic activity and the relationship between structure/composition and electrocatalytic performance have been specially highlighted.It is worth pointing out that an ideal catalyst should meet the critical requirements of suitable hydrogen adsorption energy,good electrical conductivity,and sufficient active sites.At present,the enormous research activities enable almost all Mo-based electrocatalysts to exhibit excellent HER performance in acidic electrolytes.As for the alkaline electrolysis,another key point is the water dissociation kinetics,which should be highly active to accomplish the Volmer step.In this regard,MoNi₄ is the best catalyst candidate up to now.Although great progress has been made,there is still a long road ahead for the widely practical application of these Mo-based materials for hydrogen production.In order to achieve this practically viable goal,the scientists and technicians need to make more efforts in the following aspects in the future.

First and the foremost,more attention should be paid to investigate the mechanism of the Mo-based materials as HER catalysts.For example,the catalytic mechanism of MoS₂ in acidic media has been well investigated,which greatly accelerated the development of Mo-based sulfides.However,for the other Mo-based materials,a deeper understanding of their mechanism is still lacking.Therefore,advanced tools,such as in situ spectroscopic measurement and DFT calculations,could be fully utilized to investigate the mechanism at an atomic scale,including the identification of active sites,the effect of pH values on the reaction,and the key factors of the hydrogen-binding energy.

Secondly,the electrode needs to meet the requirements of large-scale commercial applications.In this regard,the material synthesis method should be simple and easy to be scalable for mass production.At present,the new nanotechnology techniques endow the successful preparation of different Mo-based materials with various structures;however,most of these methods are complex and not possible to be implemented at the industrial level.In addition,it is worth noting that only a few studies have obtained high current densities during hydrogen production so far.How to achieve a high current density>500 mA·cm^-2 with a low overpotential is still a substantial challenge.Moreover,the electrocatalytic stability of the reported materials is usually evaluated within tens of hours.In industry,the catalysts need to remain the activity for several years,which goes far beyond the current studies.Therefore,the stability of the as-stated outstanding electrocatalysts requires more critical examinations when they come to a real practice.

Finally,the half-reaction of HER should be integrated with oxygen evolution reaction (OER) to accomplish a full water electrolysis.Hence,the efficiency of electrolytic water would be influenced by OER process and their compatibility.In consideration of OER catalysts in acidic media still depend on precious metals and the catalysts in alkaline solution have made great progress,it is equally necessary to pay more attention to the development of HER catalysts under alkaline condition while exploring low-cost and high-activity OER catalysts in acidic media.

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