Noble metal-free hydrogen evolution catalysts for water splitting.

Featuring work from the research group of Dr Yu Zhang, School of Chemistry and Environment, BeiHang University, Beijing, China

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Noble metal-free hydrogen evolution catalysts for water splitting This review highlights the recent research efforts towards the synthesis of noble metal-free electrocatalysts, especially at the nanoscale, and their catalytic properties for the hydrogen evolution reaction (HER).

See Xiaoxin Zou and Yu Zhang, Chem. Soc. Rev., 2015, 44, 5148.

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Noble metal-free hydrogen evolution catalysts for water splitting† Xiaoxin Zouc and Yu Zhang*ab Sustainable hydrogen production is an essential prerequisite of a future hydrogen economy. Water electrolysis driven by renewable resource-derived electricity and direct solar-to-hydrogen conversion based on photochemical and photoelectrochemical water splitting are promising pathways for sustainable hydrogen production. All these techniques require, among many things, highly active noble metal-free hydrogen evolution catalysts to make the water splitting process more energy-efficient and economical. In this review, we highlight the recent research efforts toward the synthesis of noble metal-free electrocatalysts, especially at the nanoscale, and their catalytic properties for the hydrogen evolution reaction (HER). We review several important kinds of heterogeneous non-precious metal electrocatalysts, including metal sulfides, metal selenides, metal carbides, metal nitrides, metal phosphides, and heteroatom-doped nanocarbons. In the

Received 2nd December 2014

discussion, emphasis is given to the synthetic methods of these HER electrocatalysts, the strategies of performance improvement, and the structure/composition-catalytic activity relationship. We also summarize

DOI: 10.1039/c4cs00448e

some important examples showing that non-Pt HER electrocatalysts could serve as efficient cocatalysts for promoting direct solar-to-hydrogen conversion in both photochemical and photoelectrochemical water

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splitting systems, when combined with suitable semiconductor photocatalysts.

1. Introduction Strong dependence on fossil fuels has made our economy susceptible to price spikes, and overuse of fossil fuels is also intensifying air pollution and global warming. Thus, developing a clean, renewable alternative to fossil fuels is a matter of utmost urgency. Among the various alternative energy strategies, constructing an energy infrastructure that uses hydrogen as the primary carrier connecting a host of energy source to diverse end uses may enable a secure and clean energy future. To this end, effective storage and production of hydrogen are key elements of the hydrogen economy. Besides the storage of hydrogen as a compressed gas or as a cryogenic liquid, many kinds of hydrogen storage materials including metal hydrides, nanostructured materials, and chemical storage materials have been widely studied due to their high

a

Key laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, Beijing Key Laboratory of Bio-inspired Energy Materials and Devices, School of Chemistry and Environment, BeiHang University, Beijing, 100191, P. R. China. E-mail: [email protected] b International Research Institute for Multidisciplinary Science, Beihang University, Beijing, P. R. China c State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, International Joint Research Laboratory of Nano-Micro Architecture Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, P. R. China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cs00448e

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hydrogen content.1–23 Very recently, considering the high hydrogen content and easy recharging as a liquid, hydrous hydrazine and formic acid have been generally considered as very promising chemical hydrogen storage materials. Development of a highly selective and efficient catalyst to significantly improve the kinetic properties for the catalytic decomposition of these liquid hydrogen storage materials at room temperature is urgently important for its practical application. Particularly, Zhang et al. firstly found that the Rh–Ni–graphene hybrid material exerts 100% selectively and exceedingly high activity to complete the decomposition reaction of hydrous hydrazine at room temperature owing to the role of graphene as a communicating platform in facilitating the electron transfer and mass transport kinetics during the catalytic reaction process.22 For the formic acid system, Yan’s group reported that CoAuPd/C can lead to the efficient decomposition of formic acid for CO-free hydrogen generation at room temperature.23 This is the first time that a non-noble metal (i.e., Co) has been used as the active part of the catalyst in the formic acid system. This improvement would lead to a new approach to further develop cost-effective and highly efficient solid catalysts for generation of hydrogen from formic acid. On the other hand, unlike oil and natural gas, hydrogen is not energy, but only an energy carrier for storing and transporting energy. Hydrogen does not exist naturally on earth, and thus we have to make it before we can use it. At present, more than 500 billion cubic meters (or 44.5 million tons) are produced

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Fig. 1

Three main pathways for industrial hydrogen production.

annually worldwide.24,25 Most of this hydrogen is used for industrial purposes, such as refining petroleum and producing ammonia for fertilizers and other chemicals. There are currently three main pathways for hydrogen production industrially (Fig. 1), that is, steam methane reforming, coal gasification and water electrolysis. Steam methane reforming and coal gasification make more than 95% of the whole hydrogen, while only 4% of the hydrogen is produced by water electrolysis. Obviously, current primary hydrogen production is still strongly dependent on the fossil fuels—the finite and nonrenewable resource. The hydrogen production techniques based on fossil fuels cannot really solve the pollution and CO2 emission problems. For example, during steam methane reformation, the hightemperature reaction between hydrocarbon and water results in the simultaneous generation of hydrogen and carbon dioxide. CO2 as a greenhouse gas is finally released into the atmosphere. This hydrogen production method, apparently, violates our original intention—reducing air pollution and global warming by the employment of hydrogen power. Among these three main hydrogen production pathways, water electrolysis still provides hope that it is possible to produce hydrogen in a sustainable way, because its feedstock is water—an abundant and renewable hydrogen source.26 But this is still based on a premise that the hydrogen production

Xiaoxin Zou was awarded a PhD in Inorganic Chemistry from Jilin University (China) in 06/2011; and then moved to the University of California, Riverside and Rutgers, The State University of New Jersey, as a Postdoctoral Scholar from 07/2011 to 10/2013. He is currently an associate professor at State Key Laboratory of Inorganic Synthesis and Preparative Chemistry in Jilin University. His research interests focus on the Xiaoxin Zou design and synthesis of noble metal-free, nanostructured and/or nanoporous materials for water splitting and renewable energy applications.


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reaction is driven by solar-, wind- or other renewable resourcesderived electricity. In fact, solar and wind technologies are booming worldwide, especially in China, Europe, the United States and Japan.27 This might lay a good foundation for the water electrolysis technique. On the other side, hydrogen can also solve other issues along the way, such as making best use of wind energy. The trouble with wind energy is its intermittency and unpredictability. At times of low demand such as during the night, excess wind power is wasted. If we can use the excess power to split water to make hydrogen, we will store electricity indirectly as the form of hydrogen. The potential use of hydrogen, instead of petroleum-based fuels, for transportation applications is attracting more and more attention. Hydrogen-powered vehicles (also known as fuel cell electric vehicles, FCEVs) might offer performance similar to that of combustion ones, but better pollution control (zero emissions of CO2 and pollutants). Many of the world’s major car manufactures, such as Ford, Toyota, BMW and Hyundai, have made their sustained efforts to bring FCEVs into clear and realistic future. If hydrogen-powered vehicles are to enter our lives, our demand for hydrogen will increase sharply. At that time, we would have to produce hydrogen on a larger scale in a more environmentally friendly way. Water electrolysis might be such a technique that can meet our requirements (Fig. 2).26 However, the practical widespread application of this technique is constrained by its high cost. Although water electrolysis has a long history, continuous technological improvement and material innovation are still highly desirable to drastically reduce the cost of this process. In particular, while Pt-based materials are the hydrogen evolution catalysts for electrochemical water splitting, developing efficient non-noble metal electrocatalysts, composed of earth-abundant elements, is quite appealing with the aim of providing costcompetitive hydrogen.28–32 Moreover, direct solar-to-hydrogen conversion based on photochemical and photoelectrochemical water splitting is another promising scheme for sustainable hydrogen production (Fig. 2). Currently, direct solar-to-hydrogen

Yu Zhang

Yu Zhang received his doctoral degree in Chemistry from Jilin University in 2007. Then, he worked as a new energy and industrial technology development organization (NEDO) fellow at Hiroshima University, Japan. In March 2013, he joined Beihang University as a ‘‘Zhuoyue’’ program Associate Professor. His interests mainly focus on advanced materials for hydrogen storage/production, lithium/sodium ion battery and fuel cells.

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Fig. 2 Sustainable pathways for hydrogen production from renewable energy, such as solar energy.26 Copyright 2014 with permission from The American Association for the Advancement of Science.

conversion is in the conceptualization stage still. But ongoing technological progress will ultimately make this approach enter the stage of actual application in the future. This also requires, among many things, efficient hydrogen evolution catalysts, preferably based on earth-abundant elements.33–36 Against such a backdrop, researchers have been exploring noble metal-free hydrogen evolution (electro)catalysts with great enthusiasm in the past few years, and their efforts have already led to abundant achievements, especially with the help of nanoscience and nanotechnology.33–36 In this review, we summarize recent developments in the area of noble metal-free electrocatalysts for the hydrogen evolution reaction (HER)—one of the two half reactions of the water splitting reaction. We specially review several important kinds of heterogeneous non-precious metal electrocatalysts, including metal sulfides, metal selenides, metal carbides, metal nitrides, metal phosphides, and heteroatom-doped nanocarbons. In the discussion, particular attention is paid to the synthetic methods of these HER electrocatalysts, the strategies of performance improvement, and the structure/composition–catalytic activity relationship. We also highlight the employment of HER electrocatalysts as non-Pt cocatalysts for promoting solar-to-hydrogen conversion in both photochemical and photoelectrochemical water splitting systems. Based on the results achieved in this area, several future directions are proposed and discussed finally.

2. Electrochemistry of the hydrogen evolution reaction As shown in Fig. 3, an electrolyzer has three component parts: an electrolyte (i.e., H2O), a cathode and an anode. The hydrogen evolution catalyst (HEC) and the oxygen evolution catalyst (OEC) are coated on the cathode and anode, respectively, to speed the water splitting reaction. When driven by an external voltage applied to the electrodes, water molecules are decomposed into hydrogen and oxygen. The hydrogen can be stored for fuel and the oxygen is released into the atmosphere. Thus, the water splitting reaction can be divided into two half-reactions: the water oxidation reaction (or oxygen evolution reaction) and

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Fig. 3

Schematic diagram of an electrolyzer.

the water reduction reaction (or hydrogen evolution reaction). According to different media in which water splitting takes place, the water splitting reaction can be expressed chemically in different ways (see below). Total reaction H2O - H2 + 1/2O2 In acidic solution Cathode 2H+ + 2e - H2 Anode H2O - 2H+ + 1/2O2 + 2e In neutral and alkaline solutions Cathode 2H2O + 2e - H2 + 2OH Anode 2OH - H2O + 1/2O2 + 2e Regardless of the media in which water splitting takes place, the thermodynamic voltage of water splitting is 1.23 V at 25 1C and 1 atm. It is worth noting that the thermodynamic voltage of water splitting is temperature-dependent, and it can be reduced by increasing the electrolytic temperature. However, in fact, we must apply voltages higher than the thermodynamic potential value (i.e., 1.23 V at 25 1C) to achieve electrochemical water splitting. The excess potential (also known as overpotential, Z) is mainly used to overcome the intrinsic activation barriers present on both anode (Za) and cathode (Zc), as well as some other resistances (Zother), such as solution resistance and contact resistance. Thus, the practical operational voltage (Eop) for water splitting can be described as:37 Eop = 1.23 V + Za + Zc + Zother It is clearly seen from this equation that reduction of the overpotentials by suitable methods is the central issue in order to make the water splitting reaction energy-efficient. Indeed, Zother can be reduced by optimizing the design of the electrolytic cell, whereas Za and Zc have to be minimized by highly active oxygen evolution and hydrogen evolution catalysts, respectively. In this context, the development of efficient water splitting catalysts, preferably based on sustainable and earth-abundant


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elements, is highly desirable in order to make the overall watersplitting more economical. Besides electrode materials, the effective active area of electrode is another important factor in determining the reaction overpotential. The improvement of the effective active area of the electrode can be achieved by the optimization of the electrode preparation method (e.g., nanostructuring). In addition, the bubble effect is also an issue that cannot be ignored. During water electrolysis, lots of bubbles are generated on the electrode surface and simultaneously some of them do not get away from the electrode immediately. This directly leads to the loss of the effective active area, and thus the increase of the reaction overpotential. The HER generally involves three possible reaction steps in acidic media, while the HER mechanism in alkaline media is still ambiguous. The first one in acidic media is the so-called Volmer step: H+ + e - Hads. The reaction of an electron with a proton produces an adsorbed hydrogen atom (Hads) on the electrode surface. After generation of Hads, the hydrogen evolution reaction can proceed by the Tafel step (2Hads - H2) or the Heyrovsky step (Hads + H+ + e - H2) or both. Regardless of the routes by which the HER occurs, Hads is always involved in the HER. Thus, the free energy of hydrogen adsorption (DGH) is widely accepted to be a descriptor for a hydrogen-evolving material. For example, DGH is approximately zero for Pt, and correspondingly Pt is the best solid-state hydrogen evolution catalyst. If DGH is large and positive, the Hads is bound strongly with the electrode surface, making the initial Volmer step easy, but the subsequent Tafel or Heyrovsky steps difficult. If DGH is large and negative, Hads has a weak interaction with the electrode surface, resulting in a slow Volmer step that limits the overall turnover rate. Therefore, an optimal non-Pt HER catalyst should also provide appropriate surface properties and have a nearly zero DGH.

3. Experimental method for characterizing the electrochemical activity of HER catalysts To elucidate the catalytic activity of a given HER electrocatalyst, there are some important parameters that are required to be measured/calculated carefully. They mainly include total electrode activity, Tafel plot, stability, Faradic efficiency, as well as turnover frequency. (i) Total electrode activity The total electrode activity is generally estimated first by performing cyclic voltammetry (CV) or linear sweep voltammetry (LSV). Because non-Faradaic capacitive current may constitute a certain portion of the total current we observed, especially for those carbon-containing catalysts, we only get a preliminary assessment of the material’s electrocatalytic activity from the CV or LSV results. To more exactly determine the catalytic activity of a material, we need to measure its steadystate currents at various applied voltages with a dwell time of at least 5 min. The obtained currents are usually normalized to the


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superficial geometric electrode area in the literature. In some cases, the currents we measured are also normalized to the sample quality in order to achieve the activity per unit mass. To compare the activities between the samples, two special overpotentials in HER are often provided deliberately. One is the so-called ‘‘onset overpotential’’. The onset overpotential is actually a poorly-defined term. If one wants to use this term in the research paper, a suitable current density value (0.5–2 mA cm 2) should be clearly shown. The other relevant overpotential is that the electrocatalyst needs to yield a current density of 10 mA cm 2 (which is the current density expected for a 12.3% efficient solar water-splitting device). (ii) Tafel plot Tafel plot depicts the dependence of steady-state current densities on a variety of overpotentials. Generally, the overpotential (Z) is logarithmically related to the current density ( j) and the linear portion of the Tafel plot is fit to the Tafel equation: Z = a + b log j; where b is the Tafel slope. From the Tafel equation, we can derive two important parameters, Tafel slop (b) and exchange current density ( j0). b is generally related to the catalytic mechanism of the electrode reaction, whereas j0, which is obtained when Z is assumed to be zero, describes the intrinsic catalytic activity of the electrode material under equilibrium conditions. A catalytic material having a high j0 and a small Tafel slop (b) is desirable actually. (iii) Stability Good structural and catalytic stabilities of a HER catalyst are of crucial importance for a material that has some practical applications potentially, especially considering that the HER catalyst mostly works in a strongly reductive environment at the pH extremes (pH 0 or 14). There are two methods for characterizing the electrocatalytic stability of a HER catalyst. One method is to measure the current variation with time (i.e., the I–t curve). For this measurement, it would be better to set a current density larger than 10 mA cm 2 for a long period of time (410 h). The other method is to conduct the recycling experiment by performing CV or LSV. The number of cycles should be larger than 5000 times to elucidate the stability of a material. (iv) Faradic efficiency Faradic efficiency describes the efficiency with which electrons participate in a desired reaction in an electrochemical system. For the electrochemical hydrogen evolution reaction, Faradic efficiency is defined as the ratio of the experimentally detected H2 amount to the theoretical H2 amount, which can be calculated from the current density based on a 100% Faradaic yield. (v) Turnover frequency Turnover frequency (TOF) is defined as the number of reactant that a catalyst can convert to a desired product per catalytic site per unit of time, which can exhibit the intrinsic activity of each catalytic site. However, it is very difficult to get a precise TOF

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value for many solid-state (heterogeneous) catalytic materials, such as HER nanocatalysts, that generally possess easily accessible surface atoms/catalytic groups as well as some inaccessible internal atoms/catalytic species. In such cases, many researchers try to calculate and report the TOFs based solely on the surface atoms or the easily accessible catalytic sites of the materials, which is of course a reasonable approach. In other cases, the TOFs are calculated based on the total catalytic species present in the materials, no matter whether they are all accessible, not equally accessible, or some even not accessible at all. Although this clearly does not give the exact value (gives an underestimated value actually), it is sometimes done because of unavailability of ways to completely determine all the accessible active sites in the catalytic system. Despite being relatively imprecise though, the latter may still give insights into the comparative catalytic activity or efficiency of two or more materials if carefully executed. For instance, in most cases, one can only get TOFs calculated per unit mole of atoms present in the HER nanocatalyst, despite it is very likely that only the surface atoms, whose number may be hard to figure out precisely, may only be responsible for the catalysis. This will obviously result in TOFs that would be grossly underestimated compared with the real TOF values of the active sites, because it is almost impossible to have every atom in a material like nanoparticles to be catalytically active or equally accessible. Nevertheless, when similar materials are compared, such results could still be relevant and useful.

4. A general view of the elements used for constructing HER electrocatalysts Fig. 4 shows a general view of the elements that are used for constructing HER electrocatalysts. These elements, according to the general physical and chemical properties, roughly fall into three groups: (i) noble metal platinum (Pt)—the stateof-the-art HER electrocatalysts; (ii) transition metals that are used for constructing noble metal-free HER electrocatalysts, mainly including iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), molybdenum (Mo), and tungsten (W); (iii) nonmetals that are used for constructing noble metal-free HER electrocatalysts, mainly including boron (B), carbon (C), nitrogen (N), phosphorus (P), sulfur (S), and selenium (Se). To date, almost all the efficient noble metal-free HER electrocatalysts have been synthesized based on the above twelve non-precious elements. Fig. 5 shows the crustal abundance of the metals constructing HER electrocatalysts in weight percent. The following conclusions can be made by comparing their abundance in the crust. (i) The abundance of Pt is about 3.7 10 6%, which is orders of magnitude smaller than that of other non-precious metals. This is in agreement with the fact that noble metal Pt has a highest cost among them. (ii) The abundance of the six non-noble metals increases in the order of W = Mo o Co o Cu o Ni { Fe. It is worthwhile to note that we should keep in mind the difference in their abundance and potential cost when we attempt to design such non-noble metal HER catalysts. Because iron and nickel

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Fig. 4 Elements that are used for constructing HER electrocatalysts.

Fig. 5 Crustal abundance of metals that are used for constructing HER electrocatalysts.

have the two highest abundance and lowest price among them, developing efficient iron- or nickel-based HER electrocatalysts should be highly desirable in order to make the hydrogen production reaction economically viable. This challenging target is also accordant with the ideas of biomimetic chemistry because hydrogenases, which catalyze the conversion of protons and electrons into dihydrogen, are widespread in nature and their active sites always contain iron and/or nickel ions.

5. Metal sulfides Developing functional bio-inspired catalysts is an important advance to bridge the gap towards large-scale sustainable hydrogen production. Albeit the presence of nitrogenase and hydrogenase in nature that can mediate the HER, it is impossible that enzyme-based devices will make a significant contribution to high-level hydrogen generation.38–43 These elegant biocatalysts are capable of functioning in the natural environment with superb catalytic selectivity, but under extreme conditions (such as strong acidic and alkaline media) they will lose their ability rapidly. Inspired by the structure/composition of nitrogenase and hydrogenase, researchers have exploited a series of metal sulphides as efficient HER electrocatalysts (see below). This is a profound achievement in the field of noble metal-free HER electrocatalysts.


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5.1

Crystalline MoS2

5.1.1 Active sites of MoS2. The study of MoS2 on electrochemical HER can be traced to the 1970s,44 which showed that bulk MoS2 crystals were non-reactive toward HER. As a result, MoS2 has not been considered as a promising hydrogen evolution electrocatalyst for a long time. This tide was totally reversed by Hinnemann et al. in 2005. They found that the (1010) Mo-edge structure in MoS2 had a close resemblance to the active site of nitrogenase.45 Furthermore, they showed that the computational free energy of atomic hydrogen bonding to the MoS2 edge was close to that of Pt (Fig. 6), raising the possibility of MoS2 as a promising HER electrocatalyst in theory. In their study, they also prepared MoS2 nanoparticles supported on graphite, and experimentally verified the catalytic activity of nanoscale MoS2 for HER. This is the first time that the MoS2 edge structure is considered to be the actual active site. To further identify the active sites of MoS2, Jaramillo et al. prepared different sizes of MoS2 nanoparticles with the predominance of the sulfide Mo-edges.46 Electrocatalytic activity measurements showed that the catalytic performance of MoS2 nanoparticles is related to the edge state length, rather than the area coverage, directly establishing the relationship between MoS2’s edges and the catalytic active sites. Homogeneous catalysts with precise molecular structures usually have explicit definition and quantitation of the active sites relative to heterogeneous catalysts. Thus, it is a smart way to employ appropriate molecular catalysts to imitate the possible active sites of heterogeneous catalysts. Keep this in mind, three molecular catalysts, including [Mo3S4]4+, [(PY5Me2)MoS2]2+ and [Mo3S13]2 , which mimic the structure of triangular active edge sites in MoS2, have been prepared by different groups (PY5Me2 = 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine).47–49 The expected electrocatalytic activities of these discrete molecular units verified again the active site of nanoscale MoS2 from a different perspective. 5.1.2 Strategies for improving the catalytic activity of MoS2. Based on the instructional studies on active sites, as stated above, and the semiconductive nature of MoS2, researchers have conducted some rational designs on nanoscale MoS2 catalysts for improving their catalytic activity. The strategies can be roughly divided into the categories ‘‘active site engineering’’ and ‘‘electronic

Fig. 6 Calculated free energy diagram for hydrogen evolution relative to the standard hydrogen electrode at pH 0.45 Reprinted with permission from ref. 45. Copyright 2014 American Chemical Society.


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conductibility engineering’’. The active site engineering mainly includes three aspects: (i) increasing the number of exposed active sites, (ii) enhancing the reactivity of active sites, and (iii) improving the electrical contact to active sites. The electronic conductibility engineering can be achieved by two ways: (i) doping suitable heteroatoms into the lattice of MoS2 and (ii) coupling MoS2 with conductive species, such as carbon nanotubes and graphene. In some cases, the two aforesaid strategies have been integrated in a single material system, either intentionally or unintentionally. In this section, we will summarize these achievements classified by different yet specific tactics.

(i) Constructing active site-rich nanosheets. Compared with bulk materials, nanostructured materials usually have larger specific surface areas and higher density of surface reactive sites. Thus, a convenient method to enhance the catalytic activity of MoS2 towards HER is preparing them in a nanostructured form. With the aim of increasing the active sites of MoS2, some methods have been developed by different groups to prepare its nanostructured counterpart.50–60 Because MoS2 possesses a unique layered crystal structure, it shows a quite strong tendency to form sheet-like nanocrystals. This is also why nanosheet morphology dominates the whole MoS2 nanostructures. Chemical exfoliation of bulk MoS2 by the lithium intercalation method is widely used to obtain single- or few-layered MoS2 nanomaterials. This method typically includes an intercalation process of lithium compounds (e.g., n-butyllithium) between MoS2 layers followed by a furious exfoliation of Liintercalated compounds via reacting with water. Interestingly, chemically-exfoliated MoS2 nanosheets exhibit an unexpected phase transformation from the thermodynamically favored 2H phase to the metastable 1T polymorph partially (Fig. 7).55 The structure of the 2H phase can be described by two S–Mo–S layers composed of edge-shared MoS6 trigonal prisms, whereas the structure of the 1T phase is described by a single S–Mo–S layer built from edge-sharing MoS6 octahedra. In 2013, Jin’s group first demonstrated that the 1T polymorph of MoS2 had higher metallic character and more competitive HER activities than the 2H phase because of the better electrical conductivity of the former.56 Chhowalla’s group further found that the active sites of 1T MoS2, different from the ones limited by the edges of the 2H phase, mainly located in the basal plane.57 Thus, the authors proposed that the enhanced catalytic activity of 1T MoS2 was attributed to both the increase of active sites and the improvement of conductivity. Furthermore, Pumera’s group investigated the effects of lithium intercalation compounds on the degrees of exfoliation and the catalytic activity of exfoliated MoS2 nanomaterials.58 Their results showed that the MoS2 nanosheets, which were produced using n-butyllithium and tert-butyllithium as the intercalator, exhibited higher degrees of exfoliation and better catalytic properties than that obtained with methyllithium. The catalytic activities of the exfoliated MoS2 can be further enhanced by a simple electrochemical pretreatment.59 The electrochemical activation process might lead to the 2H–1T transition, as confirmed by DFT calculations.

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Fig. 8 Schematic representation of the synthesis procedure to obtain MoS2 quantum dots interspersed in MoS2 nanosheets using a liquid exfoliation approach in a 1-methyl-2-pyrrolidone solution.62 Reprinted with permission from ref. 62. Copyright 2014 American Chemical Society.

Fig. 7 Structures of 2H and 1T MoS2.55 Reprinted with permission from ref. 55. Copyright 2014 American Chemical Society.

Besides lithium intercalation, ball-milling and sonication techniques can also be used to acquire highly active MoS2 nanosheets. Wu et al. synthesized MoS2 nanosheets with high active site density via a ball-milling-assisted microdomain reaction method by using MoO3 and S microparticles as the starting materials.60 In addition, Wang et al. successfully converted commercial bulk MoS2 into distorted nanosheets by using the high-energy ball milling technique.61 The resulting MoS2 nanomaterial showed significantly enhanced catalytic activity compared to its commercial counterpart because of the lattice defects and dislocations generated by mechanical milling. Furthermore, Shaijumon’s group reported a sonicationassisted synthetic method to make a hybrid nanostructure of 1 nm-sized quantum dots interspersed in few-layered MoS2 nanosheets (Fig. 8).62 The resulting hybrid nanomaterial showed excellent HER activity with a large exchange current density of 3.2 10 2 mA cm 2, probably due to the enhanced edge to basal plane ratio resulting from the unique morphology of the material. Hydro(solvo)thermal synthesis, one of the bottom-up synthetic methods, has also been proven to be an effective way to prepare MoS2 nanosheets. Xie et al. reported the large scale synthesis of the defect-rich MoS2 nanosheets with a thickness of B5.9 nm in a solvothermal system (Fig. 9).63 The authors found that excess thiourea in the reaction system played a crucial role in the formation of defect-rich nanosheet morphology, and thiourea functioned as a reductant to reduce Mo(VI) to Mo(IV) and as an additive to stabilize the sheet-like morphology. Due to the introduction of defects as additional active sites in the nanosheets, this material displayed an obvious enhancement of catalytic activity compared to defect-free MoS2 sheets. In addition, Yan et al. synthesized MoS2 nanosheets with a thickness of 4–6 nm via simple solvothermal treatment of (NH4)2MoS4 in the mixture solvent of N,N-dimethylformamide

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Fig. 9 (a) Structural models of defect-free and defect-rich structures. (b) As-designed synthetic pathways to obtain the above two structures.63 Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA Weinheim.

and H2O.64 They confirmed the presence of bridging S22 or apical S2 in the nanosheet structure by XPS analyses, and the positive role of these unsaturated sulfur sites on HER performance of MoS2 nanosheets. In another study, Chung et al. prepared edge-exposed MoS2 nanosheet-assembled structures using L-cysteine as the sulfur source and Na2MoO4 as the Mo source in a hydrothermal reaction.65 However, the driving force for the self-assembly of MoS2 nanosheets is still unclear. Moreover, the authors revealed the linear relationship between exchange current density and the number of sulfur edges, demonstrating that the active site for HER was the sulfur edge of MoS2 nanosheets. Very recently, Lu et al. hydrothermally prepared a MoS2 nanostructured film, which was composed of vertically aligned nanoplates.66 More importantly, the authors revealed that the resulting nanostructured film had a ‘‘superaerophobic’’ surface, which significantly improved the removal speed of small gas bubbles during HER. Compared to the above-mentioned methods, the chemical vapor deposition (CVD) technique has shown some unique advantages, especially in terms of creating high-quality uniform MoS2 nanosheets and controlling the thickness of nanosheets. In this regard, Cao’s group successfully grew MoS2 thin films


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a high-quality dendritic monolayer MoS2 material with abundant edges was synthesized on a large scale.69 Furthermore, a centimeter-scale, nearly complete, monolayer MoS2 film was prepared on STO by controlling the growth parameters (such as time and temperature).

Fig. 10 Hopping of electrons in the vertical direction of MoS2 layers. The right side illustrates the potential distribution in the multilayer film and the hopping of electrons through the potential barrier in the interlayer gap.67 Reprinted with permission from ref. 67. Copyright 2014 American Chemical Society.

with the precise control of layer number on glassy carbon substrates by a CVD method.67 Their electrocatalytic results showed that the exchange current densities decreased by a factor of B4.47 for the addition of every one more layer. The authors believed that the electrochemical HER only occurred at the outmost layer of the MoS2 film, and electrons had to overcome the interlayer potential barrier to reach the surface of the catalyst to drive the HER. Hence, this layer-dependent catalytic property of MoS2 was related to the layer-dependent efficiency of electron’s interlayered hopping (Fig. 10). The greatest contribution of this work lies in first pointing out that the hopping efficiency of electrons in the vertical is an important parameter to determine the electrocatalytic performance of MoS2. In another study, Liu’s group reported the scalable synthesis of monolayer MoS2 on Au foils via the low-pressure CVD (Fig. 11).68 These mono-layered MoS2 with a uniform morphology can be transformed easily onto other substrates by a chemical wet etching method, and the coverage of MoS2 on the substrate can be well controlled by simply tuning the distance between the MoO3 precursor and the Au substrate. In addition, when the Au substrate was replaced by a SrTiO3(STO) substrate,

Fig. 11 SEM images of monolayer MoS2 grown on Au foils with different precursor substrate distance (Dss) and different temperature. (a) A schematic illustration of the coverage or flake size dependence on Dss. (b–d) SEM images of monolayer MoS2 samples with coverage of B70%, 50%, and 10% synthesized under the same conditions (grown at 530 1C) but with different Dss of B10.0, 11.0, and 12.0 cm, respectively. (e) Raman spectra of the monolayer MoS2 flakes shown in (b–d).68 Reprinted with permission from ref. 68. Copyright 2014 American Chemical Society.


(ii) Creating porous structure. A traditional, yet effective way to enhance the performance of heterogeneous catalysts is increasing their specific surface areas by creating a porous structure in them. For the sake of maximizing the number of highly energetic exposed active sites, MoS2 with a nanoporous structure is actively pursued.70–74 Apart from a larger surface area, the porous structure might provide the catalysts with other advantages, such as a greater contact area with reactants and sufficient transport of reactants and products. Jaramillo’s group successfully synthesized a highly ordered double-gyroid MoS2 bicontinuous network with nanopores (B3 nm) via electrodepositing Mo onto a silica template, followed by sulphidization with H2S.70 After etching the silica template, they obtained a mesoporous MoS2 thin film with a negative morphology of silica (Fig. 12). The layer-to-layer spacing (6.6 Å) of the mesoporous MoS2 material was found to be slightly larger than that (6.15 Å) of bulk MoS2, which was attributed to the curvature of the MoS2 structure resulting from the constraint of the double-gyroid morphology. For a similar reason, this mesoporous MoS2 thin film engendered more exposed edge sites than core–shell MoO3–MoS2 nanowires (which were also synthesized by the same group), while nanowires preferentially grew inactive basal planes of MoS2 parallel to the nanowire axis.71 Moreover, Tan et al. produced a monolayer MoS2 film using a curved internal surface of 3D nanoporous gold (NPG) as the substrate (Fig. 13).72 The MoS2 film completely inherited the 3D curvature of NPG substrates, which bent MoS2 lattices with large out-of-plane strains. Further DFT calculations demonstrated that this out-of-plane lattice bending directly resulted in the continuous changes of S–Mo–S bonding angles, and thereby a local semiconductor-to-metal transition. They also confirmed the similarity between the charge density of S atoms at the bent regions and that of edge S atoms with dangling bonds, indicating that the structural curvature could generate catalytically active sites analogous to edge sites of MoS2.

Fig. 12 Synthesis procedure and structural model for mesoporous MoS2 with a double-gyroid morphology.70 Copyright 2014 with permission from Nature Publishing Group.

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Fig. 13 Monolayer MoS2@NPG towards catalytic HER. (a) Schematic diagram of the fabrication process of monolayer MoS2@NPG hybrid materials by a nanoporous metal-based CVD approach. (b) Schematic HER catalyzed by the monolayer MoS2@NPG hybrid material.72 Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA Weinheim.

Besides leveraging various templates to confine the porous morphologies, Lu et al. reported a simple hydrothermal reaction to prepare MoS2 porous thin films.73 In their synthetic system, thiourea was used as the sulfur source and the Mo substrate functioned as both the Mo source and the substrate/current collector. The thickness of the films could be adjusted from 400 nm to 1.3 mm by controlling the reaction time. The Tafel slopes of all the samples were found to be in the range of 41–45 mV dec 1 with an exchange current density of 2.5 10 7 mA cm 2, revealing the excellent performance of the resulting porous materials. In addition, Tour’s group successfully prepared edge-oriented MoS2 nanoporous films with a thickness of B1 mm and a pore size ranging from 5 to 10 nm.74 The synthetic procedure mainly consisted of two steps: (i) the electrochemical anodization of Mo metal; and (ii) gas-solid reaction with sulfur vapor. The obtained sponge-like film also showed a larger layer-to-layer spacing (6.5 Å) with regard to bulk MoS2 due to the curvature of the surface structure. More importantly, this scalable synthetic method could easily make nanoporous, flexible and conformal electrodes for HER (Fig. 14). (iii) Doping heteroatoms. Doping is one of the effective methods used to tune the structure and HER activity of MoS2.

Fig. 14 Schematic of the fabrication process and photographs of the flexible electrodes. (a) Schematic of the fabrication process and (b) photographs of the flexible electrodes. The left photograph shows the visible difference between the Mo oxide (dark color) and MoS2 (gray color). The right photograph shows the flexibility of the edge-oriented MoS2 film.74 Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA Weinheim.

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The deliberate introduction of a foreign metal or nonmetal element in the MoS2 lattice affords the opportunity to engineer the electronic and/or surface structures of the host material for improving the HER performances. To date, metal elements including Co, Ni, V and Li have been successfully doped into the MoS2 crystal structure.75–79 And importantly, these dopants provide some favorable effects on the properties of MoS2. For example, Co-doped MoS2 nanomaterials have been studied by different groups.75,76 The cobalt dopants were found to prefer to locate at the S-edge of MoS2, leading to the reduction of the free energy of hydrogen adsorption at the Co-promoted S-edge. Thus, the promotion effect of Co for MoS2 was the increase in the number of active sites.75,76 A similar promotion effect was also observed for Ni-doped MoS2.77 Different with Co and Ni dopants, V dopants in MoS2 did not increase the active site, but obviously improved the conductivity of MoS2.78 In particular, Xie’s group successfully prepared ultrathin V-doped MoS2 nanosheets, which were proven to be superior hydrogen evolution catalysts relative to pristine MoS2. In contrast to Co, Ni and V dopants that were introduced into the intralayer of MoS2, Li ions were intercalated electrochemically into the interlayer of MoS2 (Fig. 15).79 Cui’s group systematically studied the structure and catalytic properties towards HER of Li-intercalated MoS2. They found that Li-doping created multiple effects on the structure and properties of MoS2, including the variation of the oxidation state of Mo, the transition of the 2H to 1T phase and the expansion of the van der Waals gap. Because of these positive variations, the Li-intercalated MoS2 exhibited an obvious enhancement in HER activity compared to the pristine MoS2. Besides metal-doping, nonmetal-doping was also used to enhance the catalytic activity of MoS2.80,81 Xie’s group reported the synthesis of oxygen-doped MoS2 ultrathin nanosheets in a hydrothermal system at a temperature range from 140 to 200 1C.80 Because of the relatively low synthesis temperature, the final MoS2 nanosheets inherited a small amount of Mo–O bonds in the molybdate precursor. First-principle calculations demonstrated that the oxygen-doped MoS2 had a narrower bandgap (1.30 eV) than that of the pristine MoS2 (1.75 eV). This indicated that oxygen-doping could result in a higher intrinsic conductivity for MoS2 (Fig. 16). The reason behind the reduced bandgap is the oxygen dopants-induced enhanced hybridization between the Mo d-orbital and the S p-orbital. In another study, Zhou et al. prepared nitrogen-doped MoS2 nanosheets, which gave an enhanced and stable electrocatalytic activity towards HER.81 N-doping was proposed to have a similar role to oxygen-doping in enhancing the electronic conductivity of MoS2. (iv) Coupling conductive substrates. Coupling MoS2 with conductive species is a direct and viable method to improve its electronic conductibility, and thereby enhance the HER activity. In this context, a pioneering work was conducted by Dai’s group.82 In 2011, Dai et al. presented a solvothermal synthesis of MoS2 nanosheets on graphene (also known as reduced graphene oxide) for the first time (Fig. 17). The resulting


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Fig. 17 Solvothermal synthesis of a MoS2–graphene nanocomposite. Reprinted with permission from ref. 82. Copyright 2014 American Chemical Society.

Fig. 15 Schematics and galvanostatic discharge curve of Li electrochemical intercalation into MoS2 nanofilms. (A) Crystal structure of 2H MoS2. (B) Schematic of the battery testing system. The cathode is a MoS2 nanofilm with molecular layers perpendicular to the substrate, where the green and yellow colors represent the edge sites and the terrace sites, respectively. The anode is the Li foil. (C) Galvanostatic discharge curve representing the lithiation process. Li intercalates into the van der Waals gaps of MoS2 to donate electrons to the slabs and expand the layer spacing. The voltage monotonically drops to 1.2 V vs. Li+/Li to reach a Li content of 0.28, after which the system undergoes a 2H to 1T MoS2 firstorder phase transition. The atomic structure is changed from trigonal prismatic to octahedral, along with the electronic semiconducting to metallic transition.79 Copyright 2014 with permission from PNAS.

Fig. 16 (A) Calculated density of states (DOS) of the oxygen-incorporated MoS2 slab (top) and the pristine 2H–MoS2 slab (bottom). The orange shading clearly indicates the decrease of bandgap after oxygen incorporation. (B) The charge density distributions of valence band (left) and conduction band (right) near the oxygen atom in the oxygen-incorporated MoS2 ultrathin nanosheets, respectively. Black lines represent the contour lines of the charge density.80 Reprinted with permission from ref. 80. Copyright 2014 American Chemical Society.

MoS2/graphene nanocomposite exhibited extremely high catalytic activity (Tafel slope, 41 mV decade 1) and durability toward HER. There were two proposed roles of graphene, electrical coupling and chemical coupling. On one hand, the conducting graphene network provided the internal electron transport channels from the less-conducting MoS2 nanosheets to the electrodes. On the other hand, strong chemical interactions between MoS2


and graphene afforded the site-confined growth of MoS2 nanoparticles free of aggregation. In addition, Zheng et al. leveraged the confinement effect within the graphene layers to solvothermally fabricate MoS2 nanosheets/graphene composite HER catalysts in a controllable manner.83 And Firmiano et al. developed a microwave-assisted synthetic approach to rapidly make MoS2/graphene HER catalysts.84 Nanosized tungsten carbide was further discovered to be an effective cocatalyst for further improving the catalytic activity og MoS2/graphene due to the promoted electron transfer rate.85 Besides two-dimensional graphene, one-dimensional nanocarbons (e.g., carbon nanotube and carbon nanofiber) were also employed to synthesize MoS2-based hybrid materials with enhanced catalytic activity.86–89 Generally, one-dimensional nanocarbons had similar functions to graphene in promoting the electroactivity of MoS2. Compared to other carbon materials as mentioned above, porous carbons can not only provide a conductive framework, but also a high specific surface area as well as an abundant mass transfer channel of the reductant and product. Liu’s group successfully grew highly dispersed and ultrafine MoS2 nanoparticles onto a mesoporous graphene foam (MGF).90 This MGF, the 3D structure of which prevented graphene sheets from restacking, had a high surface area of 819 m2 g 1 and a pore size of 25 nm. Because of the excellent properties of MGF, the MoS2–MGF nanocomposite served as an efficient hydrogen evolution catalyst having low overpotential and high stability. The same group also used mesoporous carbon spheres as the support material for preparing MoS2/porous carbon composite catalysts.91 In another study, graphene-based aerogels, which have a highly porous carbon framework, large open pores, and good electrical conductivity, have been proven to be another outstanding carbon support for MoS2 by Hou et al.92 They prepared hydrothermally a 3D hybrid of MoS2 nanosheets/ nitrogen-doped graphene aerogels, which was used for hydrogen evolution in microbial electrolysis cells. The catalytic performance of this composite material was higher than those of MoS2 nanosheets and nitrogen-doped graphene, and was comparable to that of the Pt/C catalyst. For powdered HER catalysts, their usage must be fixed first on a conductive support electrode (e.g., glassy carbon electrode or ITO glass) using Nafion as the binder. To avoid the use of expensive Nafion and to ensure the electronic contact between HER catalysts with the support electrode, carbon-based selfsupported MoS2 electrodes were developed recently.93–95 Ma et al. developed a solvothermal method to grow MoS2 nanoflowers on graphene paper (Fig. 18).93 The resulting MoS2–graphene paper

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Fig. 19 Hydrogen evolution on amorphous MoSx.96b Reprinted with permission from ref. 96b. Copyright 2014 American Chemical Society. Fig. 18 SEM images of a MoS2–graphene paper composite.93 Reproduced by permission of The Royal Society of Chemistry.

directly served as a free-standing and flexible working electrode for HER. Apart from graphene paper, two commercialized carbon supports, carbon cloth and carbon fiber paper, were also used to form self-supported MoS2 electrodes.94,95 Carbon cloth and carbon fiber paper are composed of carbon fibers and is highly conductive and flexible. Note that carbon cloth is much cheaper than carbon fiber paper. Very recently, Wang’s group developed a simple solvothermal method to fabricate MoS2/carbon cloth electrodes with vertically oriented MoS2 nanosheet layers.94 The nanostructured MoS2/carbon cloth electrode exhibited excellent HER activity and electrocatalytic stability. The authors proposed some possible reasons for explaining the high performance of the MoS2/carbon cloth electrode. They mainly included (i) the intimate contact of MoS2 nanosheets with carbon cloth; (ii) the rapid release of H2 bubbles from the nanostructured electrode surfaces; (iii) the easy diffusion of the electrolyte into the active sites due to the 3D nanostructure; and (iv) the abundant exposed active sites in the vertically oriented S-rich MoS2 nanosheet arrays. 5.2

Amorphous MoSx

Owing to the structural indeterminacy, amorphous materials are often ignored unconsciously by the researchers in the field of heterogeneous catalysis. The same situation occurs in the study of amorphous molybdenum sulfide. The discovery of this type of materials can be traced back to 1980, while their excellent catalytic activities towards HER were confirmed in 2011.96 Hu’s group reported that electrochemically-deposited amorphous molybdenum sulfide films were efficient HER catalysts for the first time (Fig. 19).96 Their significant geometric current densities (15 mA cm 2 at Z = 200 mV), together with the fast, mild and scalable synthetic methods, opened up a new way to achieve economical hydrogen production. Due to the uncertain structure and atomic-scale heterogeneity, it is difficult to identify the catalytic active sites on amorphous molybdenum sulfide surfaces, and thereby to reveal the precise catalytic mechanism. The unsaturated S atoms were widely accepted to be catalytically active sites, and they were often identified as terminal S22 and S2 via XPS analysis. In fact, the unsaturated S atoms in amorphous MoSx are similar to

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the edge sites of crystalline MoS2 in terms of the catalytic function. Different from MoS2 nanocrystals, amorphous MoSx has short-range ordered atomic arrangements and significant structural disorder, which probably can also offer some catalytic active ‘‘defect sites’’. While MoS2 nanocrystals are always prepared at elevated temperatures, amorphous MoSx can form at a relatively low temperature, and even at room temperature. Electrodeposition is a simple and effective method to prepare MoSx. It can be simply achieved by consecutive cyclic voltammetry measurements in an aqueous solution of (NH4)2[MoS4] at room temperature. Hu’s group found that by tuning the deposition parameters, the composition (MoSx, x = 2–3) and thickness (40–150 nm) of MoSx can be controlled.96 However, all the films exhibited similar catalytic activities regardless of the deposition methods. To understand the formation mechanism of the MoSx film, the same group investigated the growth and activation of these films using a electrochemical quartz crystal microbalance and by XPS analysis.97 The results showed that the formation of the films mainly included three processes: oxidative deposition, reductive corrosion and reductive deposition. Moreover, the catalytically active phases in all films were the MoS2+x species. In another study, Murugesan et al. reported a nonaqueousbased electrodeposition method for the preparation of a MoSx film, which exhibited a nanoflower morphology, indicating the importance of the electrolyte.98 Apart from electrodeposition, wet chemical synthesis is another easy way to prepare an amorphous MoSx material. Hu’s group reported that amorphous MoS3 could be obtained upon acidification of an aqueous mixture of MoO3 and Na2S.99 In addition, Benck et al. successfully prepared an amorphous molybdenum sulfide material through addition of ammonium heptamolybdate to sulfuric acid, followed by mixing with a sodium sulfide solution.100 Moreover, the authors observed a composition variation from MoS3 to defective MoS2 during HER and a proportional relevance between the current densities and the transformation.101 Like crystalline MoS2, amorphous MoSx could also afford a higher catalytic activity when suitably doped with heteroatoms. Hu’s group investigated the promotion effect of first-row transition metal ions (Mn, Fe, Ni, Co, Cu and Zn) on the HER activity of MoSx.102 The results showed that Fe, Co, and Ni ions


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were effective promoters. At pH 7, Co2+ as the best promoter gave MoSx a 5-fold increase in the current density at Z = 200 mV. At pH 0, the best promoter was Fe2+, which gave a 2-fold increase in the current density at the same overpotential. This group also found that Fe, Co, and Ni ions promoted the growth of the MoSx films, resulting in larger surface area, higher catalyst loading and thus better catalytic activity. To further enhance the performance of MoSx, coupling it with conductive substrates is a viable method.103–108 To date, conducting nanocarbons, nanoporous gold and Ni foam were reported as effective supports to improve the activity of amorphous molybdenum sulfide. For example, Li’s group successfully grew MoSx (x Z 2) materials on highly conductive 3D graphene/Ni foam by thermolysis of ammonium thiomolybdates at elevated temperatures (100–300 1C).109 The resulting film gave a hydrogen evolution rate of 302 mL g 1 cm 2 h 1 at an overpotential of 0.2 V. Using a similar method, this group also used porous 3D sponges or carbon cloth to support MoSx materials.110,111 In the case of carbon cloth-supported MoSx, the authors found that the nitrogen dopants in carbon cloth could enhance the amount of Mo5+ and S22 in the MoSx, and reduce the size of the MoSx particles. Correspondingly, the materials exhibited a very high hydrogen evolution rate up to 6408 mL g 1 cm 2 h 1 at an overpotential of 0.2 V. Distinct from the synthetic methods stated above, Wang et al. fabricated untra-small amorphous molybdenum sulfide nanoparticles with a diameter of B1.47 nm from bulk MoS2 by the highenergy ultrasonication method.112 Moreover, the nanoparticles self-assembled on a Au surface to form a highly active film towards HER. The authors believed that the Au film preferably adsorbed those nanoparticles containing high exposed surface S sites during the assembling process, as confirmed by the fact that the final film had a Mo to S ratio of 1 : 5. This selective enrichment of S edge-rich molybdenum sulfide might be the fundamental cause of the great enhancement of the HER catalytic activity (0.92 mA cm 2 at Z = 0.15 V with an ultra-low loading of 1.03 mg cm 2). In another notable example, Wang et al. fabricated an extremely active polypyrrole–MoSx hybrid structure via the electrochemical copolymerization method (polypyrrole (PPY) is a conductive polymer).113 This hybrid film

Owing to its structural and electronic similarities to MoS2, WS2 has also drawn substantial interest in recent years. To realize its potential application in HER, nanostructured WS2 with wellcontrolled structures and tunable properties are required. Recent research studies in this area have already led to some successful examples of such WS2 nanomaterials.114–119 One of the most notable examples is the strained chemically exfoliated WS2 nanosheets (Fig. 21).114 Chhowalla’s group reported the synthesis of mono-layered nanosheets of chemically exfoliated WS2 by the lithium intercalation method in 2013. The as-exfoliated nanosheets had a high concentration of the strained metallic 1T phase, which was proven to be an important factor in enhancing the catalytic activity of WS2 nanosheets. In addition, the straininduced local lattice distortion was also believed to facilitate HER. In another study, Chhowalla’s group synthesized WS2 nanosheets in a simple hydrothermal reaction system for the first time.115 The selection of the starting materials (tungsten chloride and thioacetamide) was crucial for the formation of sheet-like WS2. When graphene oxide was added in the reaction system, WS2–graphene composite nanosheets with enhanced HER activity were synthesized. Similarly, Cheng et al. prepared WS2 nanosheets with monolayer thickness using WCl6 and S as the precursors by a high-temperature solution-phase method.116 The resulting nanomaterial had efficient and durable activity for HER. The authors also claimed that the catalytic activity of nanosized WS2 matched that of widely-studied MoS2 materials. Moreover, the sonochemical exfoliation method was used to convert WS2 nanotubes into WS2 nanoflakes,117 which possessed higher edge sites and catalytic activity than the mother material. Ball-milling and electrochemical techniques were also effective in assisting the formation of WS2 nanostructured catalysts.118 Recently, cobalt- or nickel-containing WSx films were prepared by an electrodeposition process using [M(WS4)2]2 (M = Co or Ni) as the precursor.119 The obtained cobalt–tungsten–sulfide (CoWSx) and nickel–tungsten–sulfide (NiWSx) had WS2-like layered structures

Fig. 20 Polarization curves of electrodes modified with: (a) PPy, (b) MoSx, (c) (NH4)2MoS4, (d) PPy/MoSx, and (e) Pt/C.113 Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA Weinheim.

Fig. 21 (a) Typical AFM image of exfoliated nanosheets of WS2. Scale bar, 500 nm. (b) High-resolution STEM images of an as-exfoliated WS2 monolayer showing regions exhibiting the 1T superlattice. The inset in b shows the strain tensor map generated from the STEM-HAADF image using peak pair analysis. Light (yellow) and dark (black) colours correspond to regions where the strain is in tension and compression, respectively. Scale bars, 1 nm.114 Copyright 2014 with permission from Nature Publishing Group.


exhibited a small Tafel slope of 29 mV dec 1, and comparable activity to that of the commercial Pt/C catalyst (Fig. 20). 5.3

Tungsten disulfide

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with MII–S–WIV (M = Co or Ni) bimetallic sulfide centers. A further electrocatalysis study gave two important conclusions: (i) CoWSx had a higher catalytic activity than NiWSx. However, the reason behind this phenomenon is still unclear. (ii) The primary catalytic activity of these M–S–W clusters was believed to locate within the MS centers. This work might provide some new insights into the mechanism of heteroatom-doped metal sulfide catalysts.


5.4

Fe, Co, Ni sulfides

Hydrogenases are a class of bio-enzymes that can catalyze protons and electrons into molecular hydrogen at a low overpotential that is close to the thermodynamic potential.38–40 In fact, their catalytic activity is more effective than that of platinum, which is the best known catalyst for HER. Regardless of the type of hydrogenase, metal atoms comprising its active site are only Fe and/or Ni. Specially speaking, the active sites of [NiFe] and [FeFe] hydrogenases—two widely studied bioenzymes— are made of metal-sulfur clusters that are buried in proteins. Inspired by the composition and structure of hydrogenases, researchers have been exploring synthetic hydrogen-evolving catalysts based on iron or nickel for energy conversion processes.120–122 In this context, Giovanni et al. studied FeS (pyrrhotite) nanoparticles as a bioinspired catalyst for electrochemical hydrogen evolution in neutral water.120 Although this material had a relatively low catalytic activity, it exhibited attractive stability without structural decomposition or activity decrease for at least 6 days. Kong et al. found that both FeS2 and NiS2 (pyrite, Fig. 22) were active non-precious HER catalysts in an acidic electrolyte.121 In their study, FeS2 and NiS2 had a comparable catalytic activity towards HER, and FeS2 had a higher stability than NiS2 in acidic solution. Moreover, FeS2 exhibited a higher catalytic activity than FeS that was reported by Giovanni et al.120 In another study, the NiS2 nanosheet array was grown on carbon cloth to serve as a binder-free cathode for efficient HER in a neutral solution.122a Owing to the unique nanostructure, this electrode could achieve a current density at a low overpotential of 243 mV. To sum up, despite some progress, as outlined above, Fe-/Ni-based sulfide catalysts still have a markedly lower catalytic activity than that of other nonprecious HER catalysts, such as MoS2. In view of their natural abundance, it should be of significance to search for highperformance Fe-/Ni-based sulfides for HER in the future. If so, this might, in turn, help us to develop insights into hydrogenases. Cobalt sulfides are now emerging as an attractive HER catalysts, although cobalt has less abundance than Fe or Ni and no biological relevance to HER. Recent studies demonstrated that CoS2 was superior to FeS2 and NiS2 for HER in both acidic and neutral media.121,123 Moreover, CoS2 materials, especially those with fine nanostructures, have become very strong competitors among noble metal-free HER catalysts. There are two important examples in this regard. Jin’s group synthesized metallic CoS2 materials with three different morphologies—film, microwire and nanowire.124 The authors systematically studied their structures, activities and stabilities, and further established the structure–performance relationship (Fig. 23). They came to two important conclusions: (i) CoS2 micro- and nanostructuring could increase the effective electrode

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Fig. 22 Crystal structure of FeS2 in (a) pyrite or (b) macarsite phase. Fe and S are displayed in orange and yellow respectively. (c) Side-view of the stable, nonpolar pyrite (100) surface as an example of the low-index surface with under-coordinated metal cations. The (100) surface is terminated in the sequence [S–Fe–S].121 Reproduced by permission of The Royal Society of Chemistry.

surface area, and improved the HER catalytic activity. In other words, the morphology played a crucial role in determining the overall catalytic efficacy of CoS2. (ii) CoS2 micro- and nanostructuring could promote the release of evolved gas bubbles from the electrode surface, and thereby the increase of its operational stability. In another study, Peng et al. synthesized a CoS2 nanosheet–graphene–carbon nanotube ternary composite material as a freestanding electrode for efficient HER.125 The synthesis of this material was based on a two-step process: the growth of CoS2 nanosheets on graphene, and then the combination with carbon nanotubes by vacuum filtration (Fig. 24). This nanohybrid film was one of the most active non-precious metal hydrogen evolution catalysts, owing to the integration of large specific surface area, high electrical conductivity and nanoporous structure in one material system.

Fig. 23 Electrochemical characterization of CoS2 film, microwire array, and nanowire array electrodes for HER; and scheme illustrating enhanced hydrogen gas bubble release from CoS2 nano- and microstructures.124 Reprinted with permission from ref. 124. Copyright 2014 American Chemical Society.


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Fig. 24 Fabrication procedure and hydrogen evolution process of the freestanding CoS2/graphene/carbon nanotube hybrid electrode.125 Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA Weinheim.

Besides crystalline cobalt sulfides, amorphous cobalt sulfide was also discovered recently to be highly active for HER. This amorphous cobalt sulfide material was simply synthesized on conductive substrates via an electrochemical deposition method at room temperature.126 The most important features of this amorphous material are high activity and stability in neutral media. It was among the most active HER catalysts at pH 7, and it was shown to behave as a durable catalyst for electrochemical for HER at pH 7 with an operating stability of 440 h. It is worth noting that this amorphous material was unstable in acidic media and relatively inactive in alkaline media.

6. Metal selenides Both selenium (Se) and sulfur (S) are nearby group VIA in the periodic table, in which Se and S can be found in the fourth period and third period, respectively. Thus these two elements not only share some similarities but also differences. For similar properties, they all have 6 electrons in the outermost shells, and similar oxidation numbers. The outermost electron configuration of the elements often determines the chemical properties of the compound formed by these elements, implying that metal selenides may also show similar activities for HER compared with metal sulfides. Followed by the intensive research of metal sulfide materials for HER as discussed above, the investigation of various metal selenide materials for HER has received a lot of attention as well. On the other hand, being located at different periods in the periodic table for Se and S gives them several distinct characters: (i) the metallic property of Se is more obvious than S, suggesting the better conductivity; (ii) the radius of Se is larger than that of S; (iii) the ionization energy for Se is smaller than that of S. For these reasons, the metal selenides might possess some unique activities for HER as compared with metal sulfides. 6.1

Mo and W selenides

Lately, research on Mo or W selenides has attracted considerable attention and there are some interesting results in this area.127–134 Yang’s group reported a simple and fast bottom-up route to make hierarchical MoSe2 x (x E 0.47) nanosheets


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with only 2–5 Se–Mo–Se atomic layers via the reaction of MoO2(acac)2 with dibenzyl diselenide under mild conditions (Fig. 25).127 The resulting material was found to be rich in Mo in the composition, and thus lack Se, which might produce more active sites and increase the conductivity for HER. Their method had the potential to serve as a general pathway for making various metal selenides, such as WSe2, SnSe, and PbSe. In addition, Lewis’ group developed a two-step strategy to make catalytically active MoSe2 films by first synthesizing a mixture of molybdenum triselenide (MoSe3), molybdenum trioxide (MoO3) with small amounts of Se, followed with drop-casting the mixture on glassy carbon for HER.128 They found that the catalyst was not good for HER at the beginning, however, the activity increased a lot during the subsequent voltammetric cycles, as observed by the overpotential shift from 400 mV to 250 mV for 10 mA cm 2. During the electrocatalysis, the initial mixture was reduced into a porous MoSe2 film, attributed to the significant enhanced activity. This kind of synthesis was described as operand since the MoSe2 was formed under the conditions of the reaction that it accelerated. Aside from the unsupported Mo selenides, some researchers are making hybrid composites to place metal selenides onto various supports (carbon fiber paper, graphene or tungsten foil) and take the advantages of the support material to increase the HER activity. MoSe2 and WSe2 share similar crystal structures with MoS2 (Fig. 26), the single layers stack one by one via the van der Waals force and expose mostly the basal plane with a small portion of edge sites, which is not favorable for electrochemical HER. In order to obtain a material with maximum amount of edge sites for catalytic application, Cui’s group prepared a vertically arranged MoSe2 film on a flat oxidized silicon substrate to obtain the final material with the whole surface covered with edge sites of MoSe2.129 This reaction was done by the selenization of a Mo film covered oxidized silicon and the vertical growth of MoSe2 was proposed as the relatively

Fig. 25 (a) Crystal structure of the layered hexagonal MoSe2 phase. The Mo atom and Se atom are displayed in grey and yellow, respectively. (b) Reaction equation for hierarchical ultrathin MoSe2 x nanosheets. (c) Schematic illustration of the formation process of hierarltrahin MoSe2 x nanosheets by a bottom-up colloidal synthetic route.127 Reproduced by permission of The Royal Society of Chemistry.

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MoSe2 and the Se-edge on MoSe2 and WSe2 played an important role in the HER. Moreover, the (0001) basal planes were not active for the HER, the activity was mainly from the exposed edge sites. Their results confirmed the significance of edge sites for HER.


6.2

Fig. 26 Nanostructures of layered MoS2 and MoSe2. (a) Layered crystal structure of molybdenum chalcogenide with individual S–Mo–S (or Se–Mo–Se) layers stacked along the c-axis by weak van der Waals interaction. The highly anisotropic crystal structure is the origin of anisotropic electrical and chemical properties. (b) Schematics of MoS2 nanoparticles with platelet-like morphology distributed on a substrate (left), and nanotubes and fullerene-like nanotubes of MoS2 and MoSe2 (right). (c) Idealized structure of edge-terminated molybdenum chalcogenide films with the layers aligned perpendicular to the substrate, maximally exposing the edges of the layers.129 Reprinted with permission from ref. 129. Copyright 2014 American Chemical Society.

easy diffusion of selenium through the van der Waals gaps. Subsequently, Cui’s group reported another supported material by vertically growing MoSe2 and WSe2 on carbon fiber paper.130 This kind of design could expose the maximum amount of edge sites, namely the HER active locations, for a dramatic increase in the activity. The carbon fiber paper was chosen in their experiment because it has a curved and rough surface, which could increase the surface area of the final material and help to display more edge sites. The high stability of this hybrid material for HER was ascribed to the strong binding affinity between the metal selenide layers and the carbon fiber paper substrate. Similarly, Chen’s group designed and prepared perpendicularly oriented MoSe2–graphene nanosheets, which could also expose a lot of active sites for HER.131 Moreover, Lewis’ group reported the deposition of WSe2 thin films on a conductive tungsten foil via a chemical-vapor-transport method.132 The resulting WSe2 platelets were also arranged perpendicularly to the substrate. Furthermore, Wang’s group demonstrated a simple electrochemical method to make the composite of MoSe2–reduced graphene oxide/polyimide.133 The resulting composite film showed good photoelectrocatalytic activities for HER, demonstrating a large cathode current. The advantage of such photoelectric active materials is the application in solar-driven HER, which also utilizes the energy from the sun and realizes a clean energy conversion technique. Except the above experimental results, the DFT calculation method was also employed to explore the origin of HER activity of MoSe2 and WSe2.134 It has been found that Mo-edge on

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Fe, Co, Ni selenides

In 2012, the first row transition metal selenide based HER catalyst (NiSe) was prepared by Yu’s group.135 The reaction was performed in a ternary solvent with various ratios of diethylenetriamine, hydrazine hydrate and deionized water, and the ratios played an important role in controlling the phase and morphology of the final material with the best activity obtained for sea urchin-like NiSe. Recently, Cui’s group prepared a series of first-row transition metal dichalcogenide films (FeSe2, CoSe2, NiSe2) on a mirror-polished glassy carbon substrate to investigate the HER activities and CoSe2 was found to be the best among them.121 These metal dichalcogenides are with cubic pyrite-type or orthorhombic macarsite-type structures, in which metal atoms are octahedrally connected with the Se atom. They believe that the partially filled eg band of CoSe2 might be linked to its excellent activity. A CoSe2 nanoparticle was also prepared using a commercial carbon black nanoparticle as the template to compare the HER activity of the CoSe2 films. It is not surprising that the nanoparticle catalyst outperforms the film one due to the exposed catalytic sites. After that, Cui’s group reported another facile preparation method to synthesize CoSe2 nanoparticles on high surface area carbon fiber paper by the selenization of cobalt oxide modified carbon microfiber paper under Se vapor conditions (Fig. 27).136 Such an approach can be easily extended to make other metal chalcogenides, such as NiSe2. And the resulting 3D electrode functionalized with CoSe2 generated a high cathodic current (100 mA cm 2) at a low overpotential (180 mV) in acidic medium for HER. Besides these, Lewis’s group reported an amorphous film composed of CoSe in a polymeric Se matrix via electrodeposition from an aqueous solution of Co(C2H3O2)2 and SeO2 onto a titanium foil under ambient conditions.137 The prepared cobalt amorphous films only needed ca. 135 mV overpotential to reach a current density of 10 mA cm 2 for HER in 0.5 M H2SO4. Sun’s group reported the synthesis of a CoSe2 nanowire array on carbon cloth (CC) through hydrothermal selenization of a Co(OH)F nanowire array. This CoSe2/CC electrode exhibited excellent catalytic activity (overpotential of 130 mV to get 10 mA cm 2) and durability (448 h) in acidic media.138 In addition, another hybrid material Ni/NiO/CoSe2 with Ni/NiO core–shell nanoparticles decorated on CoSe2 nanobelts was made by Yu’s group.139 The presence of CoSe2 was crucial for the nucleation and growth of Ni because bigger Ni/NiO particles were obtained without the CoSe2. The activity of Ni/NiO/ CoSe2 turned out to be better than either Ni/NiO or CoSe2 due to some reasons: (i) more active sites for HER was provided in the modified CoSe2, (ii) the Ni core was conductive and thus decreases the resistance, (iii) the thin NiO which served as Lewis acid can help to dissociate the water molecule and this


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materials could have the potential to control the activity and thus provide better catalysts for HER.


7. Metal carbides

Fig. 27 (a) Crystal structure of CoSe2 in the cubic pyrite-type phase (left) and the orthorhombic macarsite-type phase (right), in which Co and Se are displayed in orange and yellow, respectively. (b) Photograph of the asprepared CoSe2 catalyst on a piece of 1.5 cm 10 cm carbon fiber paper. (c) SEM image of a layer of CoSe2 catalyst grown on carbon fiber paper. (d) High-resolution SEM image revealing the structure of CoSe2 coating, consisting of nanoparticles in dimension of tens of nanometers.136 Reprinted with permission from ref. 136. Copyright 2014 American Chemical Society.

In 1973, R. B. Levy and M. Boudart discovered that tungsten carbide possessed some platinum-like catalytic behaviors because of its similar d-band electronic density states to platinum.143,144 This pioneering work immediately evoked great interest among chemical researchers, and simultaneously great enthusiasm was devoted to the research of metal carbides with the aim of replacing expensive noble metal catalysts. This has led to many promising carbide-based applications, ranging from heterogeneous catalysis (e.g., decomposition of alcohols and activation of oxygen) to electrolysis in fuel cells and electrolyzers.32,34,36 In this section, we specially highlight the recent research efforts toward developing nanosized carbide materials as the noble metal-free hydrogen evolution electrocatalysts. In the discussion, particular attention is paid to the synthesis of metal carbide nanomaterials with an emphasis on molybdenum and tungsten carbides, and the structural effects on the properties of the materials.145–167 It is worth noting that some carbide-supported Pt catalysts are beyond the scope of this review.168–182 7.1

was further enhanced by the chemical synergistic effect of NiO and CoSe2. 6.3

Sulfur doped metal selenides

Due to the similarities of S and Se, one of them could be doped into the compound formed by the other one with metal, as a result, the hybrid materials may show different properties because of the differences between S and Se. Sampath’s group reported the few-layered alloy material of MoS2(1 x)Se2x with various compositions and the highest HER activity was found when the molar ratio of S and Se was 1 : 1 in the resulting material, which was even better than the pristine few-layered MoS2 and MoSe2.140 Their results indicated that the electrocatalytic activity could be easily tuned by changing the composition of the target materials, that is, incorporation of Se into the MoS2 lattice structure or vice versa. He’s group prepared the WS2(1 x)Se2x nanotubes with controllable S and Se amounts on carbon fiber via the chemical vapor deposition method.141 The as-grown material can serve as an electrocatalyst for HER with small overpotential, high exchange current density and conductivity. Recently, Yan’s group reported the preparation of S-doped MoSe2 nanosheets by reacting Mo-oleylamine and Se-oleylamine-dodecanethiol mixtures, in which the surfactant was used to reduce the surface energy of edge sites and the incorporation of S can create some defects on the basal plane.142 Therefore, more active edge sites were yielded in the final ultrathin nanosheet material. Based on these results, research on other S and Se co-exited metal dichalcogenide may still continue in the future and more appealing outcomes would show up since the tunable ratio of S and Se in the


Molybdenum carbide

In 2012, Hu’s group made the first observation of molybdenum carbide, actually b-Mo2C, showing high activity for HER.145 In this study, Hu et al. studied the electrocatalytic performance of commercial b-Mo2C microparticles, which exhibited unexpectedly high activity and durability toward HER in both acidic and basic media. In addition, Leonard’s group synthesized four phases of Mo–C (Fig. 28), including a-MoC1 x, Z-MoC, g-MoC and b-Mo2C, and compared their catalytic activity in acidic solution.146 The results revealed that the catalytic activities of the four phases towards HER increased in the order of a-MoC1 x o Z-MoC o g-MoC o b-Mo2C. Inspired by these pioneering studies, fine structural optimization of molybdenum carbide catalysts, especially at the nanoscale, has actively been pursued. Liao et al. synthesized nanoparticle-assembled Mo2C nanowires by simple thermal treatment of a MoOx–amine hybrid precursor under an inert atmosphere.147 The Mo2C nanowires were composed of nanocrystallites of about 10–15 nm in size, and had a large surface area of 63.9 m2 g 1. The excellent catalytic activity of this nanowire material was attributed to its large surface areas, nanosized crystallites and porous structure. A similar synthetic strategy was adopted by Wang’s group to prepare porous Mo2C nanorods using anilinium molybdate as the precursor.148 This resulting material possessed high electrical conductivity and catalytic activity in an acid solution, and its catalytic activity could be further improved by loading Ni nanoparticles. Recently some studies demonstrated that carbon nanotube and graphene could function as both a carbon source for the formation of Mo2C, and an effective support material to anchor as-formed MoC2 nanoclusters. This finally led to high-performance MoC2–nanocarbon composites for HER. Sasaki’s group prepared a

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Fig. 28 X-ray diffraction (XRD) patterns of (a) a-MoC1 x, (b) Z-MoC, (c) g-MoC, and (d) b-Mo2C. The insets show the corresponding crystal structures.146 Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA Weinheim.

MoC2–carbon nanotube composite by in situ carburization of ammonium molybdate on carbon nanotubes without using other carbon sources.149 The as-prepared composite catalyst showed superior electrocatalytic activity and stability toward HER compared to the bulk Mo2C. In addition, MoC2–graphene and MoC2–nanocarbon–nanotubes were also prepared by two different groups.150,151 It is worth noting that in the MoC2– nanocarbon–nanotubes ternary composite material, the carbon nanotube–graphene hybrid support was believed to play multiple roles in avoiding aggregation of nanocrystals, providing a large contact area with the electrolyte, facilitating the electron transfer, and thus enhancing the catalytic activity of Mo2C. Furthermore, graphitic carbon nitride was used as a highly active carbon source for the synthesis of the Mo2C/carbon HER catalyst.152 In particular, Alhajri et al. prepared this material via a thermally-driven reaction of a molybdenum precursor with mesoporous C3N4 under an inert atmosphere. The confinement effect of mesopores gave rise to the final material with a small particle size (8 nm) and a large surface area (308 m2 g 1). Biomass, such as soybean and sodium alginate, was recently considered as ‘‘green’’ chemical agents for the preparation of Mo2C-based nanocatalysts. Chen et al. reported the synthesis of b-Mo2C and g-Mo2N nanocomposite catalysts via the solid-state reaction between soybeans and ammonium molybdate (Fig. 29).153 Their experimental results demonstrated that b-Mo2C was the catalytically active phase and g-Mo2N was the acid-proof phase. The hybridization of b-Mo2C and g-Mo2N resulted in the high catalytic activity and durability of the material. The activity of the composite catalyst could be further enhanced by coupling graphene. In the other study, Cui et al. presented the synthesis of Mo2C nanoparticles on graphitic carbon sheets using a biopolymer, sodium alginate, as the carbon source.154 The biomass-assisted synthesis provides us a novel methodology for low-cost, scaled-up synthesis of noble metal-free water splitting catalysts. 7.2

Tungsten carbide

Tungsten carbide, one of the most important metal carbides, was first found to have Pt-like catalytic properties. While tungsten

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Fig. 29 The synthesis from soybeans and ammonium molybdate of a solid catalyst suitable for electrochemical hydrogen production. A solidstate reaction between soybeans and the molybdate leads to their reductive carbonization and nitridation, so as to form b-Mo2C and g-Mo2N crystals.153 Reproduced by permission of The Royal Society of Chemistry.

carbide was demonstrated to be an excellent substrate to support noble metal catalysts (e.g., Pt), the development of pristine tungsten carbide as a high-performance catalyst is more appealing due to the avoidance of noble metals. Wirth et al. systematically investigated a series of commercial IVB–VIB transition metal carbides for HER in an acid solution, and among them, tungsten carbide exhibited the best catalytic activity, even better than molybdenum carbide (Fig. 30).155 This is really a good news for tungsten carbide. However, the performance of tungsten carbide still needs to be further optimized in terms of both activity and stability. Tungsten carbide is typically synthesized in a carbon-rich reaction system. This often leads to the as-obtained tungsten carbide material having some additional surface carbons. These surface carbons usually have adverse effects on the catalytic properties of materials because they can block the direct interaction between the reactants and the WC surface. Chen’s group recently reported an atomic oxygen pretreatment method to remove the carbon on the surface of WC to a certain extent.156 The atomic oxygens were generated in an oxygen plasma source. This pretreatment was proven to improve the catalytic activity for HER. In another study, Hunt et al. presented a very smart method for the preparation of surface-exposed tungsten carbide nanocatalysts 1–4 nm in size (Fig. 31).157 The key steps for the synthesis to be successful are: (i) the synthesis of WOx nanoparticles encapsulated within the SiO2 matrix; (ii) hightemperature carburization treatment; (iii) removal of the SiO2 shell and anchoring tungsten carbide nanoparticles on a highsurface-area carbon support. Very recently, Garcia-Esparza et al. reported the synthesis of carbon-coated tungsten carbide nanocrystals (B5 nm) through the reaction of tungsten precursors with mesoporous graphitic C3N4 as the reactive template.158 Unexpectedly, this material exhibited very high catalytic activity and stability for HER over a wide pH range. It is worth noting that tungsten carbide, generally, has a poor stability in neutral and basic media.159–162 Thus, this result might indicate the importance of additional carbon on the surface of tungsten carbide. In addition, the most incomprehensible thing is how a carbon-coated material could


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Fig. 32 The synthesis of graphene nanoplatelet-supported W2C and WN nanoparticles from ammonium paratungstate, aniline, and graphene nanoplatelets.164 Reproduced by permission of The Royal Society of Chemistry.

Fig. 30 Comparison of the electrocatalytic HER of selected electrocatalysts by cyclic voltammetry. The catalyst samples were mechanically immobilized on a graphite electrode and immersed in 0.1 M H2SO4.155 Copyright 2014 with permission from Elsevier.

show good, even excellent catalytic activity. This somewhat violates the original concept (see above), that is, achievement of high catalytic activity over carbon-free tungsten carbide. Thus, this calls for further mechanistic studies on pristine tungsten carbide and tungsten carbide–carbon composite materials. A nitrogen-doping strategy was proposed by Zhao et al. to enhance the catalytic activity of tungsten carbide.163 They prepared tungsten carbonitride nanoparticles with polydiaminopyridine as both carbon and nitrogen sources as well as Na2WO4 as the tungsten source. A small amount of iron as a graphitization catalyst was necessary to realize the synthesis of a N-rich tungsten carbonitride material. The authors believed that the N species in the material substantially decreased the electron density of the W atoms, and N-bound W species were related to the HER activity of Fe-WCN materials. However, the possible role of iron in the material was not discussed, although the

Fig. 31 Synthesis of ultrasmall tungsten carbide material. (a) STEM image of silica-encapsulated tungsten oxide nanoparticles. (b) STEM image of silica-encapsulated WC nanoparticles. (c) STEM image of WC nanoparticles supported on carbon black.157 Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA Weinheim.


amount of iron in the material is very low. In another study, Chen et al. reported graphene nanoplatelet-supported W2C–WN nanocomposites via an in situ solid-state reaction between aniline and tungstate (Fig. 32).164 This nanocomposite catalyzed the HER with very low overpotential and was stable for at least 300 h under harsh acidic conditions. The authors proposed that the presence of WN moderated the M–H binding strength on the material surface, inhibited the formation of tungsten oxide, and thus enhanced the HER activity.

8. Metal nitrides Transition metal nitrides (TMNs) prossess unique physical and chemical properties.183,184 On the one hand, the inclusion of nitrogen atoms modifies the nature of the d-band of the parent metal, resulting in a contraction of the metal d-band. This makes the electronic structure of TMNs more similar to noble metals (e.g., Pd and Pt).185 On the other hand, nitrogen can nest in the interstices of the lattices due to the small atomic radius, so that the arrangement of metal atoms always maintains close-packed or near close-packed. This endows TMNs with an attractive electronic conductivity.186 These promising features, coupled with their high resistance against corrosion, make this kind of materials more reliable relative to metal or metal alloys. Chen et al. demonstrated the catalytic activity of molybdenum nitride (MoN) towards HER in 2012.187 To improve the catalytic activity, the authors introduced Ni into MoN to form Ni–Mo bimetallic nitrides for the purpose of moderating Mo–H binding strength. The as-obtained Ni–Mo nitride with a nanosheet morphology contained a majority of g-Mo2N and Ni2Mo3N phases. The thickness of the sheets ranged from 4 to 15 nm, and the gap between two single sheets was found to be about 2 nm. According to EXAFS studies, the authors found that Ni species segregated to the surface of NiMoNx, forming a Ni-rich domain. This segregation of the Ni phase during the nitridation process was proposed to be the reason behind the formation of nanosheets. It is self-evident that this sheet nanostructure can afford plenty of highly accessible reactive sites to enhance the HER activity. In addition, the EXAFS results also revealed an increase of Ni–Ni distance and a decrease of Ni–Mo distance during nitride formation, further indicating

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the strong interaction between Ni and Mo in the bimetallic nitride structure. Accordingly, this NiMoNx catalyst exhibited an excellent activity for the HER with a small overpotential of 78 mV and a Tafel slope as small as 35 mV dec 1. And the material could maintain the structure and catalytic activity in acidic media, affording strong evidence for the stabilizing effect of nitridation. In another significant work, Cao et al. successfully prepared a cobalt molybdenum nitride (Co0.6Mo1.4N2) via annealing a Co3Mo3N precursor under flowing NH3 at 400 1C for 1 h.188 This bimetallic nitride possessed a four-layered mixed closed packed structure with alternating layers of octahedral sites and trigonal prismatic sites (Fig. 33). In this structure, Mo ions preferred the trigonal prismatic sites, while a Co–Mo mixture occupies the ochtahedral site. N ions were found in close packed layers with a repeating AABB staking sequence. The authors proposed that this alternative substitution on octahedral sites allowed Co to tune the electronic states of Mo, and potentially enhance the HER activity of MoN. The HER measurement verified this hypothesis. A current density of 10 mA cm 2 at Z = 0.20 V could be achieved by this catalyst, and the activity of ternary Co0.6Mo1.4N2 was much higher than that of binary d-MoN in both acidic and alkaline media. Besides these, Xie et al. recently reported the fabrication of metallic MoN nanosheets with atomic thickness (B1.3 nm) by liquid exfoliation of MoN bulk materials.189 DFT calculations on the density of states (DOS) and the charge density distribution showed that MoN nanosheets had higher electric conductivity and charge density than its bulk counterpart. This means that MoN nanosheets can effectively facilitate electron transport during the catalytic process. Moreover, the authors

Fig. 33 (a) Lab X-ray powder diffraction patterns of Co3Mo3N, CoMoN2, and d-MoN. Asterisk marks the impurity peak of cobalt metal. (b) Fourlayered crystal structure of CoMoN2. (c) Rietveld refinements of neutron diffraction for CoMoN2 showing observed data (black line), calculated pattern (red line) and difference curve (bottom line). Lab X-ray diffraction data (blue line) in the same Q (=2p/d) range between 2 and 7 Å 1 do not clearly show superstructure peaks such as the 013 and 015 reflections which are intense in neutron diffraction data.188 Reprinted with permission from ref. 188. Copyright 2014 American Chemical Society.

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observed that the surfaces of the atomically-thin MoN nanosheets were wholly comprised of apical Mo atoms, and the surface Mo atoms could act as the active sites for catalyzing protons into hydrogen. Therefore, the rich active surface sites, together with the better conductivity, made the MoN nanosheets achieve a high current density of 38.5 mA cm 2 at Z = 300 mV. In another study, a tungsten nitride nanorod array was first synthesized by Sun et al., and it could serve as an all-pH efficient and durable HER catalyst with an overpotential of 198 mV to achieve a current density of 10 mA cm 2.190

9. Metal phosphides Metal phosphide is another kind of promising non-Pt HER catalysts. Like metal sulfides, the conjecture about their possible application in this field also originates from the imitation of hydrogenase. In 2005, Rodriguez’s group first brought forward a standpoint that Ni2P might be the best practical catalyst for the HER according to their DFT calculation, which indicated the excellent catalytic behavior of Ni2P(001) toward the HER.191 In their study, they found that in Ni2P the Ni concentration was diluted by the introduction of P elements, which made Ni2P(001) behave somewhat like the hydrogenase rather than the pure metal surface. The negatively charged nonmetal atoms and isolated metal atoms functioned as proton-acceptor sites and hydride-acceptor sites, respectively. In other words, the proton-acceptor sites and hydride-acceptor sites co-exist on the surface of Ni2P(001), and this so-called ‘‘ensemble effect’’ would facilitate the HER. This bore a resemblance to the catalytic mechanism of [NiFe] hydrogenase and its analogues. Furthermore, they found that during the HER progress hydrogen strongly bound with the Ni hollow sites, but with the assistance of P, the bonded hydrogen could be easily removed from the Ni2P(001) surface. This important theoretical prediction stimulates enormously the investigation on metal phosphides as HER catalysts. Basically, metal phosphides have similar physical properties to ordinary metallic compounds such as carbides, nitrides, borides and silicides. They have relatively high mechanical strength, electrical conductivity and chemical stability. Different from carbides and nitrides with relatively simple crystal structures (e.g., face-centered cubic, hexagonal close-packed or simple hexagonal), the crystal structure of phosphides is based on trigonal prisms due to the large radius of phosphorus atoms (0.109 nm). These prismatic building blocks in phosphides are similar to those in sulfides. But metal phosphides tend to form a more isotropic crystal structure (Fig. 34), rather than a layered structure that is observed in metal sulfides.192 This structural difference possibly results in metal phosphides having a greater number of coordinatively unsaturated surface atoms than metal sulfides. Thus, metal phosphides might have an intrinsically higher catalytic activity than metal sulfides. To sum up, since the past two years metal phosphides have attracted particular attention in the field of HER, and numerous research studies are ongoing.


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Fig. 34 Crystal structures of some transition metal phosphides.192 Copyright 2014 with permission from Elsevier.

9.1

Nickel phosphide

Popczun et al. first confirmed experimentally the HER efficiency of Ni2P nanoparticles (Fig. 35).193 In their study, they synthesized Ni2P nanoparticles (21 nm) through themolysis of nickel(II) acetylacetonate in 1-octadecene, oleylamine and tri-noctylphosphine (TOP). It is worth noting that this reaction is highly corrosive and flammable, and thus must be carried out under rigorously air-free conditions. The resulting Ni2P nanoparticles were hollow and faceted to expose the (001) planes of Ni2P. As for HER measurement, Ni2P nanoparticles were deposited onto a Ti foil substrate followed by annealing treatment. The as-prepared Ni2P/Ti electrode exhibited excellent HER activity with an exchange current density of 3.3 10 5 mA cm 2 and a Tafel slope of B46 mV decade 1. However, the stability of the Ni2P/Ti electrode was not satisfactory. To optimize the synthesis conditions and to improve the stability of Ni2P, Hu’s group developed a simple and scalable solid-state reaction route to prepare Ni2P nanoparticles with a diameter of 10–50 nm via a thermal-driven reaction between NaH2PO2 and NiCl26H2O at 250 1C.194 The resulting Ni2P

Fig. 35 Crystal structures of some transition metal phosphides.193 Reprinted with permission from ref. 193. Copyright 2014 American Chemical Society.


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nanoparticles showed a enhanced stability without activity loss at Z = 170 mV in 1 M H2SO4 for 48 hours. In addition, Pu et al. successfully prepared Ni2P nanoparticle films supported on a Ti plate via electro-deposition of nickel hydroxide nanoparticle films on a Ti plate followed by phosphidation using NaH2PO2.195 The as-obtained binder-free electrode afforded current densities of 20 and 100 mA cm 2 at small overpotentials of 138 and 188 mV, respectively. Kucernak et al. subsequently investigated the effect of phosphorus content on hydrogen evolution activity and corrosion resistance in acidic medium.196 In their study, the three samples with different phosphorous content, nickel– phosphorus alloy (8 at% P), Ni12P5 (29 at% P) and Ni2P (33 at% P). The results showed that the materials with higher phosphorus content were more corrosion-resistant and more HER-active. Besides Fe2P-type Ni2P, other phases of nickel phosphide with different stoichiometry and crystal structure (e.g., NiP2 and amorphous nickel phosphide) are also investigated for HER recently.197–199 For example, Jin et al. reported a grain-mediated electroless method to prepare amorphous undoped, tungstendoped nickel phosphide microspheres supported on Ni foam.199 This is the first time that the HER performance of amorphous nickel phosphide was investigated. In addition, tungsten-doped nickel phosphide exhibited enhanced activity than undoped nickel phosphide, and the former could achieve a high current density of 20 mA cm 2 at Z = 110 mV. However, the role of W-doping is still unclear. 9.2

Cobalt phosphide

Cobalt phosphide is another metal phosphide which has recently been confirmed to show great catalytic activities towards HER. Different from Ni2P with a Fe2P-type structure, CoP has a B31 MnP-type structure type. In the CoP structure, phosphorus atoms are surrounded by six metal atoms at the corners of a highly distorted triangular prism, further forming a zig-zag chain extending in the b direction with a P–P distance of 2.70 Å. Popczun et al. first reported in 2014 that CoP is a highly active and acid-stable HER catalyst (Fig. 36).200 They synthesized multi-faceted, hollow CoP nanoparticles via reaction of Co nanoparticles with thioctylphosphine (TOP). Then these nanoparticles were deposited on a Titanium (Ti) support, followed by annealing treatment, to construct CoP/Ti working electrodes. The as-prepared electrodes produced a cathodic current density of 20 mA cm 2 at an overpotential of 85 mV, and showed great stability in 0.5 M H2SO4 for 24 h. In addition, Sun’s group has made the greatest contribution to this area, especially pioneering the low-temperature toptactic transformation preparation of metal phosphide nanostructures and their arrays. This group reported the synthesis of CoP nanocrystals decorated on carbon nanotubes (CoP/CNT), which showed a notable HER catalytic activity with a current density of 10 mA cm 2 at an overpotential of 122 mV.201 The CoP/CNT was prepared by a low-temperature phosphidation of a Co3O4/ CNT precursor with NaH2PO2. Using a similar method, this group successfully grew nanoporous cobalt phosphide nanowire arrays on carbon cloth (CC) (Fig. 37).202 The authors also found that the obtained CoP/CC was quite stable to sustain

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Fig. 37 (a) XRD patterns of the Co(OH)2 precursor and CoP. (b) Low- and (inset) high-magnification SEM images of Co(OH)2/CC. (c) Low- and (inset) high-magnification SEM images of CoP/CC. TEM images of (d) Co(OH)2 and (e) CoP nanowire. (f) HRTEM image of the CoP nanowire. (g) STEM image and EDX elemental mapping of P and Co for the CoP nanowire.202 Reprinted with permission from ref. 202. Copyright 2014 American Chemical Society.

Fig. 36 (a, b) TEM images, (c) SAED pattern, and (d) HRTEM image of CoP nanoparticles. (e) Two views of the MnP-type crystal structure of CoP.200 Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA Weinheim.

hydrogen production over 80 000 s at all-pH values. The most contribution of this work to the related community is that the authors proposed a novel concept for constructing binder-free 3D metal phosphide arrays as efficient hydrogen evolution cathodes. Besides these two efforts stated above, this group further grew CoP nanoparticles, nanosheet arrays and nanowires on carbon cloth or Ti plate, and all these binder-free electrodes showed high catalytic performances.203–205 Sun’s group also investigated the effects of morphology on the hydrogen evolution activity of CoP.206,207 The authors reported a template-assisting synthesis to make CoP nanotubes through low-temperature phosphidation of Co salt inside a porous anodic aluminium oxide template followed by dilute HF etching. The catalytic activity and durability of CoP nanotubes were superior to nanoparticle counterparts.206 This group further compared the electrocatalytic performance of CoP nanowires, nanosheets and nanoparticles.207 They found that CoP nanowires exhibited the highest catalytic activity and stability compared to CoP materials with other morphologies. Besides CoP, the catalytic activity of Co2P was also explored by different groups. Huang et al. successfully synthesized Co2P nanomateials using cobalt acetate and thiphenylphosphine as the starting materials in oleylamine.208 The product has a rod-like morphology with 9.8 1.3 nm diameter and 110.0 11.8 nm length. In addition, the Co2P exhibited a comparable catalytic activity to CoP. Lu et al. further compared the HER activities and stabilities of Co2P, Co1.33Ni0.67P and Ni2P nanorods, and their results showed that the Co2P nanorods showed the highest activity and durability among them.209

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In another important study, Saadi et al. prepared an amorphous cobalt phosphide film on a copper substrate by cathodic depositon from a boric acid solution of Co2+ and H2PO2 .210 The as-deposited thin film with a Co : P ratio of 20 : 1 included multiple constituents, such as metallic cobalt, phosphide, and amorphous-oxide forms. After the HER measurement, the Co : P ratio in the film decreased to 1 : 1, indicating that high-valent Co and P species were removed under HER conditions. This in situ film purification yielded a highly active electrocatalyst actually, which showed a current density of 10 mA cm 2 at an overvoltage of 85 mV. 9.3

Iron and copper phosphides

Compared with Co and Ni, more abundant metal elements (Fe and Cu) have been introduced into the growing family of metal phosphide catalysts. Zhang et al. first reported the electrocatalytic hydrogen evolution activity of nanoporous FeP nanosheets in acidic medium.211 This material was prepared by an anion exchange reaction of Fe18S25–TETAH (TETAH = protonated triethylenetetramine) nanosheets with phosphorous ions. In addition, other approaches were also exploited to prepare FeP nanomaterials. Callejas et al. synthesized MnP-type FeP nanoparticles (B11 nm) via decomposing Fe(CO)5 in a mixture of oleylamine and 1-octadecene at 190 1C, followed by reaction with TOP at 340 1C for 1 h (Fig. 38).212 Moreover, a low-temperature phosphidation reaction was carried out with NaH2PO2 as the phosphorous source to grow FeP nanowire arrays on both Ti plate and carbon cloth, FeP–graphene hybrid nanosheets and FeP–carbon nanotube materials.213–216 Specially, the FeP nanoparticles-modified carbon cloth exhibits ultrahigh catalytic activity comparable to that of commercial Pt/C and good stability in acidic media.216 Copper phosphide was demonstrated to be HER-active by Sun’s group.217 They prepared self-supported Cu3P nanowire arrays on commercial porous copper foam. This material


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Fig. 39 Schematic graph to show the structural evolution upon phosphorization.218 Reproduced by permission of The Royal Society of Chemistry. Fig. 38 TEM image of FeP nanoparticles.212 Reprinted with permission from ref. 212. Copyright 2014 American Chemical Society.

showed a high catalytic performance in acidic solution, and it could achieve a current density of 10 mA cm 2 at a low overpotential of 143 mV. 9.4

Molybdenum and tungsten phosphides

Molybdenum phosphide (MoP) as a well-known hydrodesulfurization (HDS) catalyst. Although HDS and HER are different catalytic processes, there are still some similarities in both reactions, such as the reversible binding and dissociation of H2. In addition, its rhodium- and palladium-like properties were also revealed by DFT calculation. These positive results prompted some researchers to explore the HER activity of MoP. Wang’s group first confirmed the efficient HER catalytic activity of MoP in 2014.218 In their study, they prepared bulk MoP and Mo3P by tuning the reaction parameters such as reaction temperature (Fig. 39). Comparison of the HER performances of Mo, Mo3P and MoP revealed that phosphorization could modify the properties of the metal, and degrees of phosphorization also had an important effect on the HER activities and stabilities. Different from Mo and Mo3P suffering from severe performance degradation in both acidic and alkaline media, bulk MoP exhibited high activity and stability under both conditions. Sun’s group prepared a closely interconnected network of MoP nanoparticles with a high specific surface area of 143.3 m2 g 1 through a temperature-programmed reduction method.219 The network-like MoP nanomaterial gave a high catalytic activity with an onset overpotential of 40 mV and a Tafel slope of 54 mV dec 1 in acidic solution, and maintained its catalytic activity for 24 h. The authors believed that the closely interconnected network of MoP nanoparticles was the key to realize the high HER activity. This group also prepared MoP nanosheets supported on biomass-derived carbon flake via a solidstate reaction, in which (NH4)6Mo7O244H2O, NaH2PO42H2O and sodium alginate were the source of Mo, P and C.220 The thickness of one MoP nanosheet was measured to be around 1.6 nm, and only bulk MoP could be obtained without the addition of sodium alginate. The electrochemical measurements


showed that the material was active in both acid and neutral electrolytes, and the carbon flake enhanced the HER activity of MoP effectively. Kibsgaard et al. introduced sulfur (S) into the surface of MoP, producing a molybdenum phosphosulfide (MoP/S) catalyst with superb activity and stability for the HER in an acidic environment.221 Sulfur-doping was achieved by a post-sulfidation treatment of MoP in an H2S atmosphere. According to the XRD measurement, the short sulfidation treatment did not alter the bulk crystal of MoP. The obtained MoP/S only required an overpotential of 86 mV to reach 10 mA cm 2, whilst MoP with the same morphology reached 10 mA cm 2 at an overpotential of 117 mV. The incorporation of sulfur into the surface apparently was believed to mitigate surface oxidation of the phosphide. Apart from crystalline MoP, Schaak’s group successfully prepared discrete, uniform, and amorphous MoP nanoparticles with diameters of approximately 4 nm through heating Mo(CO)6 and trioctylphosphine in squalane at 320 1C, followed by thermal treatment at 450 1C in H2(5%)/Ar(95%).222 The group further synthesized amorphous tungsten phosphide (WP) nanoparticles with an average diameter of 3 nm through the same method (Fig. 40).223a This material exhibited a very high catalytic performance with a current density of 10 mA cm 2 at an overpotential of 120 mV in acidic solution. Recently, crystalline tungsten phosphide micro–nano structures were successfully prepared

Fig. 40 (a) TEM image and (b) SAED pattern of as-synthesized, amorphous WP nanoparticles. (c) HRTEM image of a single amorphous WP nanoparticle.223a Reproduced by permission of The Royal Society of Chemistry.

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by Sun’s group, and they exhibited similar catalytic activity with the amorphous counterpart.223b,c


10. Heteroatom-doped nanocarbons As seen from the above sections, nanocarbons (e.g., graphene and carbon nanotube) are often selected to support HER nanocatalysts for promoting the catalytic performances (activity and stability) of the latter. Generally, nanocarbons could provide either favorable structural supports (e.g., large surface area) or cooperative electrical effects (e.g., enhanced electron transfer and transport) or both in those cases. However, these nanocarbon materials are HER-inert intrinsically. In this section, we will introduce a new class of HER catalysts, that is, heteroatomdoped nanocarbons.224–229 The fundamental difference between nanocarbon catalysts and conventional metal-based catalysts (e.g., MoS2) is that the catalytically active sites of nanocarbon catalysts do not involve metal ions. Even if some nanocarbon catalysts contain a small amount of metals in them, these metals are well encapsulated inside the carbon shells, and they are not accessible to reactants.227–229 The charms of nanocarbons as ‘‘non-conventional’’ HER catalysts might mainly originate from the cheapness and excellent physicochemical properties (e.g., high chemical stability) of carbon materials. Intentional introduction of some heteroatoms is necessary to make the catalytically inactive pristine nanocarbon materials highly active. This might lead to the creation of defect sites that can modulate the physical and chemical properties of nanocarbon, and more importantly, the addition of reactive sites that mediate the conversion of atomic/molecular species to the desired products. This is particularly well-demonstrated by heteroatom-doped graphene-based HER catalysts. Sathe et al. reported a B-substituted graphene with an enhanced activity for HER. This material was synthesized by doping defective graphene with B atoms using borane tetrahydrofuran (BH3-THF) as the borylating agent.224 In electrochemical water reduction, the material exhibited a lower overpotential, by B100 mV, than its undoped counterpart. To explore the effects of various dopants (N, B, O, S, P, F) in graphene toward HER activity, Qiao’s group conducted DFT calculations to study the electronic properties of differently doped graphene models (Fig. 41).225 The DFT calculation provided us some important information: (i) N and O acted as electron acceptors for the adjacent C, while F, S, B, and P served as electron donors. (ii) N and P codoped graphene had the most favorable H* adsorption–desorption property among several doped graphene models, indicating the best HER catalytic activity. (iii) The different H* adsorption behavior on graphene correlated with graphene’s valence orbital energy. Based on theoretical predictions, the authors prepared N,P-coped graphene that showed a much lower HER overpotential than those of other pure and single-doped graphene samples, and comparable performance to some of the traditional metalcontaining catalysts. In another study, Qiao’s group found that coupling graphitic carbon nitride (g-C3N4) with nitrogen-doped graphene could lead to a highly active composite HER catalyst.226

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Fig. 41 (a) NBO population analysis of six different nonmetallic heteroatoms in a graphene matrix. pN and gN represent pyridinic and graphitic type of N, respectively. Inset shows the proposed doping sites for different elements, sites 1 and 2 are the edge and center in-plane sites, respectively, and site 3 is an out-of-plane center site in graphene. (b) The calculated free energy (DGH*) diagram for HER at the equilibrium potential (URHE = 0 V) for N- and/or P-doped graphene models. (c) Relationship between DGH* and Ediff for various models.225 Reprinted with permission from ref. 225. Copyright 2014 American Chemical Society.

The excellent catalytic performance of the resulting hybrid material originated from the synergistic effect of g-C3N4 and N-doped graphene. g-C3N4 provided highly active hydrogen adsorption sites, and N-doped graphene facilitated the electron-transfer process for proton reduction. Besides graphene-based catalysts, heteroatom-doped carbon nanotubes have also been discovered to be efficient HER catalysts. Zou et al. prepared cobalt-embedded, nitrogen-rich carbon nanotubes by a simple, easily scalable route that involved thermal treatment of the Co2+-embedded graphitic carbon nitride precursor.227a The resulting heteroatom-doped carbon nanotubes have been proven to be highly efficient, non-noble metal electrocatalysts, which can catalyze the HER with activities close to that of 1 wt% Pt. Because cobalt nanoparticles were well encapsulated by carbon nanotubes with high chemical stability, this carbon nanotube material was demonstrated to be catalytically stable enough at all pH values (pH 0–14). This characteristic of carbon nanotube HER catalysts is, usually, not observed in metallic catalysts. In addition, the same author synthesized another Co,N-codoped carbon nanotube material (Fig. 42) using cheaper urea as the starting material instead of carbon nitride.227b This material had the ability to operate stably at not only all pH values (pH 0–14), but also in unpurified seawater (pH 7). The excellent durability was attributed to the tolerance of nanocarbon materials toward the impurities in seawater. This work also provided an attractive possibility of direct usage of seawater for generating H2 to avoid the cost of seawater desalination and purification. In order to reveal the origin of high catalytic activity, Deng et al. conducted density functional theory (DFT) calculations on heteroatom-doped carbon


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claimed that the HER activity of carbon nanotubes was correlated with the amount of surface acidic groups (–COOH), which could act as proton relays. The nanocarbon catalyst reported here provides an illuminating example for creating low-cost, metal-free HER catalysts.

11. Photochemical water splitting with HER electrocatalysts as cocatalysts

Fig. 42 (A) SEM, (B) TEM and (C) HRTEM images of Co,N-codoped carbon nanotubes. Reproduced by permission of The Royal Society of Chemistry.227b

nanotubes (Fig. 43).228 Their results showed that the introduction of metal and nitrogen dopants synergistically optimized the electronic structure of carbon nanotubes, and the adsorption free energy of H atoms on carbon nanotubes. Furthermore, they suggested that the predominant route of HER in this catalytic system was based on the Volmer–Heyrovsky mechanism. In another important work,229 Cui et al. found that HERinert pristine carbon nanotubes could be activated by acidic oxidation to become an active HER electrocatalyst, and the catalytic performance of this catalyst could be further enhanced significantly by cathodic pretreatment. The authors also

Fig. 43 (a) Comparison of projected density of states (DOS) of H(1s) and its bonded C(2p) when H is adsorbed on the surface of pristine CNTs, Fe@CNTs, and Fe@NCNTs. The dashed lines present the center of the occupied band. (b) The free energy profiles of Tafel and Heyrovsky routes for Fe@CNTs. (c) The free energy profiles of the Heyrovsky route for pristine CNTs, Fe@CNTs and Fe@NCNTs. (d) A schematic representation of the HER process on the surface of Fe@NCNTs. The gray balls represent C atoms, yellow for Fe, blue for N, red for O and white for H.228 Reproduced by permission of The Royal Society of Chemistry.


In 1972, Fujishima and Honda discovered the photo-induced water splitting phenomenon on TiO2 electrodes. Since then, photocatalysis over semiconductor materials has long been deemed as a promising approach for direct solar-to-hydrogen conversion.230,231 The photocatalytic hydrogen evolution reaction generally takes place on the surface of semiconductor materials. When an incident light possessing energy greater than the band gap hits the material, electrons in the valence band are excited to the conduction band, concomitantly leaving holes in the valence band. The photo-generated electrons could reduce the protons or water molecules on the material’s surface into molecular hydrogen. Typically, a hydrogen-evolution semiconductor photocatalyst must have two basic qualities (i.e., thermodynamic factors): a suitable bandgap to harvest photons, and a more negative bottom level of the conduction band than the redox potential of H+/H2. These thermodynamic factors are strongly dependent on the structural and electronic properties of the semiconductor photocatalyst. Besides the thermodynamic factors, some kinetic processes related to photogenerated charge separation and the surface hydrogen evolution reaction also play important roles in determining the efficiency of photocatalytic water splitting. Based on the kinetic considerations, cocatalysts are often loaded on the surface of semiconductor photocatalysts to promote the charge separation, reduce the activation energy barrier, and thereby improve the catalytic activity of photocatalysts.230a In the photochemical hydrogen production system, the most frequently used cocatalyst is Pt nanoparticles, which are also the best HER catalyst in the electrochemical hydrogen production system. Recent studies demonstrated that non-Pt HER electrocatalysts could also deliver their catalytic abilities to the photocatalytic system as non-Pt cocatalysts, when combined with suitable semiconductor photocatalysts. The methods for characterizing the catalytic activities of the HER photocatalysts have been summarized by Kudo et al. in a recent review paper.231 The pioneering work in this context was conducted by Li’s group.232–234 The authors reported that MoS2 nanoclusters served as an excellent cocatalyst for the CdS photocatalyst in 2008.232 After loading by MoS2, the photocatalytic activity of CdS increased substantially with an optimal MoS2 loading amount of 0.2 wt%. The activity was increased 36 times after loading 0.2 wt% MoS2 on CdS. More importantly, the photocatalytic activity of MoS2/CdS was even higher than that of Pt/CdS under the same reaction conditions (Fig. 44). This unexpectedly high activity of MoS2/CdS was attributed to the formation of a nanojunction between CdS and MoS2. Upon light irradiation, the electron–hole pairs were photogenerated

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Fig. 44 (a) The rate of H2 evolution on CdS loaded with 0.2 wt% of different cocatalysts. (b) The HRTEM image of the MoS2/CdS catalyst.230 Reprinted with permission from ref. 232. Copyright 2014 American Chemical Society.

in CdS under visible light irradiation. The photogenerated electrons would transfer from CdS to MoS2 at their interface, while the photogenerated holes remained in CdS. As a result, the lifetime of photogenerated charges in the MoS2/CdS photocatalyst increased significantly, finally leading to the enhancement of photocatalytic hydrogen evolution on CdS. In addition, MoS2 was considered by the authors to be the reactive sites in the composite catalyst. The same group further verified that WS2 had a similar function to MoS2 for enhancing the photocatalytic activity of CdS.234 After Li’s work, there were several reports on new methods to synthesize MoS2/CdS nanostructures with different morphologies,235–240 as well as extension of MoS2 cocatalysts to other semiconductor photocatalysis systems, ZnS,241 ZnxCd1 xS,242,243 CdSe,244 ZnIn2S4,245–247 TiO2,248 Cu2O,249 niobate250 and g-C3N4.251,252 In another important work,253 amorphous MoS3 clusters were explored for enhancing the photocatalytic activity of semiconductor materials. Tang et al. grew amorphous MoS3 clusters on a CdSe-seeded CdS (CdSe/CdS) nanorod material via a microwave heating reaction. The CdSe/CdS heterostructure was prepared for the purpose of controlling the separation of photogenerated holes and electrons. This unique nanostructure could ensure that the hole preferred to localize in the CdSe seed, whilst the electron was largely delocalized. These CdSe/CdS nanorods showed negligible rates of photocatalytic H2 production, whereas deposition of amorphous MoS3 resulted in a photochemically active system for hydrogen generation. In the MoS3-containing photocatalytic system, a hydrogen production rate of 100 mmol h 1 g 1 of was obtained, with an apparent quantum efficiency of 10% at l = 450 nm. Interestingly, the author found that all the samples underwent a 30–50 min induction period before hydrogen generation started, indicating that activation of amorphous MoS3 species occurred during the photocatalytic progress. Based on XPS, XANES, and EXAFS results, the authors claimed that the initial MoS3 precatalyst was photoreduced to form an under-coordinated species structurally similar to MoS3 during the induction period, and this reduced form of MoS3 was an active phase for hydrogen generation. This transformation was similar to the eletrocatalytic reduction of amorphous MoS3, which also in situ transformed MoS2+x species as the real active catalyst. Ni, Co sulfides, which are good electrocatalysts for HER, have also been used to establish Pt-free photocatalytic hydrogen

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Fig. 45 Activity comparison of different photocatalysts. NiS/CdS**: reaction was conducted in Na2S (0.25 M) and Na2SO3 (0.35 M) solution, and NiS + CdS*: physical mixture of CdS and NiS.254 Reproduced by permission of The Royal Society of Chemistry.

production systems.254–263 For example, Xu’s group demonstrated the positive effect of the NiS cocatalyst on the CdS photocatalyst for the first time.254 They prepared NiS/CdS photocatalysts via a hydrothermal loading method by precipitating nickel acetate in the presence of thiourea, and then studied their photocatalytic activities using lactic acid as the sacrificial reagent under visible light. The NiS/CdS photocatalyst with the optimized NiS amount of 1.2 mol% gave a hydrogen production rate of 2.18 mmol h 1, which was about 35 times as high as that obtained on pristine CdS (see Fig. 45). The quantum efficiency was as high as 51.3% at 420 nm. Subsequently, the authors extended the NiS cocatalyst into the g-C3N4 system.255 g-C3N4 is an organic metalfree semiconductor material, and Pt as the cocatalyst was previously used to enhance its catalytic activity. Xu et al. found that the optimal loading amount of NiS on g-C3N4 is about 1.1 wt%, and this NiS/g-C3N4 material gave a hydrogen evolution rate of 48.2 mmol h 1 under visible irradiation in aqueous triethanolamine solution. The optimal catalytic activity was about 250 times higher than that of native g-C3N4. Apart from metal sulfides, metal carbide and phosphide electrocatalysts have been proven to be effective cocatalysts for photocatalytic hydrogen evolution.158,212 For example, tungsten carbide (WC) nanoparticles were demonstrated by Garcia-Esparza et al. to function a cocatalyst for Na-doped SrTiO3 photocatalyst (STO:Na).158 WC-loaded STO:Na gave H2 and O2 in a stoichiometric ratio from water splitting (i.e., overall water splitting), whereas STO:Na only afforded a low hydrogen evolution rate without the O2 product. Based on the authors’ studies, the roles of the WC cocatalyst can be summarized as follow: (i) improving charge separation, (ii) serving as the hydrogen-evolution active site, (iii) preventing the back reaction, H2 + O2 - H2O. In addition, Ni2P and FeP nanoparticles have been used for enhancing the photocatalytic activities of CdS and TiO2, respectively.212,262

12. Photoelectrochemical water splitting with HER electrocatalysts as cocatalysts Photoelectrochemical water splitting is a main research area in so-called artificial photosynthesis, and has been suggested as a


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promising way to store solar energy as hydrogen fuel.24,33,264–274 The photoelectrochemical water splitting system can be considered to be an integration of a light harvesting system and a water splitting system into a single monolithic device. Correspondingly, the most key component in this system is the photoelectrode (i.e., photocathode or photoanode). In particular, a photocathode, where the HER occurs, is generally composed of a light absorber (a semiconductor material) and a HER electrocatalyst. Silicon is a widely-studied light-harvesting material because of its high abundance, strong visible-light absorption, and well-established semiconductor in photovoltaic devices. And Pt is the best HER electrocatalyst. With the aim of replacing noble metal Pt, MoS2 or MoSx has been investigated to be combined with a silicon electrode for the HER.33,266–270 Tran et al. deposited a MoS2 cocatalyst on a Si nanowire electrode by a photo-assisted method.266 When the Si nanowire electrode was interfaced with an aqueous solution containing [MoS4]2 , upon visible light irradiation, [MoS4]2 would be reduced by the photoexcited electrons in Si. This made as-formed MoS2 directly load on the surface of Si nanowires. The authors next employed the MoS2/Si nanowire electrode for hydrogen evolution. They found that MoS2 acted as an efficient cocatalyst that led to a decrease of the overpotential for HER by 200 mV. To load a MoS2 or MoSx cocatalyst on a Si electrode, other groups also explored some methods.33,267–269 For example, an electrochemical deposition method was used by Chorkendorff’s group to load MoSx on the TiOx-protected n+p-silicon photocathode.267 TiOx protection was demonstrated to be necessary because the Si electrode was susceptible to oxidation. In addition, Jin’s group directly dropcasted 1T–MoS2 nanosheets on a Si photoelectrode, resulting in an improved catalytic activity.269 Furthermore, Huang et al. fabricated a MoS3/silicon nanowire photocathode by the spin-coating of (NH4)2MoS4–methanol onto Si nanowires, followed by calcination at 260 1C.33 This electrode exhibited excellent catalytic activity, which was comparable to that of the Pt-modified Si electrode. The authors also used the same method to prepare a WS3 modified Si nanowire electrode.270 Besides Si, InP and Cu2O are attractive photocathodes for hydrogen evolution,271–273 and their activities can also be increased by modifying MoSx cocatalysts. Gao et al. coated vertically aligned p-type InP nanowire arrays with MoS3 nanoparticles.271 The resulting MoS3/InP material served as a cathode for photoelectrochemical hydrogen production from water. A high photocathode efficiency of 6.4% under Air Mass 1.5G illumination was obtained. In contrast, in the literature the solar conversion efficiencies for hydrogen evolution are normally limited to 2.5% even upon using noble metals as cocatalysts. In addition, Morales-Guio et al. electrochemically deposited MoS2+x films as cocatalysts on TiO2-protected copper(I) oxide photocathodes,272 which are the state-of-the-art p-type oxide for photoelectrochemical hydrogen evolution. The composite photocathode produced a photocurrent as high as 5.7 mA cm 2 at 0 V versus RHE under simulated AM 1.5 solar illumination. This photocathode also exhibited excellent stability (410 h) in acidic


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Fig. 46 LSV scans (dark and white light) for p-Si and p-Si/W2C photocathodes with and without Pt nanoparticles deposited on the surface. Current controlled pulsed deposition was used to deposit the Pt nanoparticles (total deposition times shown in parentheses). Before PEC measurements and Pt nanoparticle deposition, the p-Si/W2C photocathode was annealed in Ar at 450 1C. The LSV scan for a dense Pt film on FTO was included. Measurements were conducted in N2 purged 1 M H2SO4 using a scan rate of 25 mV s 1 and a white light intensity of 100 mW cm 2.274 Reprinted with permission from ref. 274. Copyright 2014 American Chemical Society.

environments, whereas the photocathode with Pt nanoparticles as cocatalysts deactivated rapidly under identical conditions. Very recently, Berglund et al. reported the synthesis of W2C/p-Si photocathodes with W2C as the cocatalyst for p-Si.274 They fabricated this electrode by evaporating tungsten metal in an atmosphere of ethylene gas to form a W2C thin film on top of a p-Si substrate. The p-Si/W2C photocathode gave a cathodic photocurrent at potentials more positive than 0.0 V vs. RHE, while bare p-Si photocathodes did not (see Fig. 46). Furthermore, the authors demonstrated that the W2C film could function as a support for Pt nanoparticles, leading to a p-Si/W2C/Pt photocathode. This p-Si/W2C/Pt photocathode afforded comparable photocurrent onset potentials and limiting photocurrent densities to p-Si/Pt photocathodes, but the former had a much lower Pt loading than the latter. This work provides an important example for creating low-cost photocathodes with high efficiency.

13. Conclusions and further outlook The future hydrogen economy is a proposed system of storing and transporting energy using hydrogen. As shown in Fig. 47, there are three main processes during the hydrogen energy cycle: harvesting renewable energy (e.g., solar and wind), splitting water into hydrogen and oxygen using renewable energy, and re-releasing usable energy by reacting H2 with O2. In this review, we specially highlight the advances on hydrogenevolving electrocatalysts for efficient water splitting (i.e., process II). Particular attention is paid to one of the major goals in this field—replacing expensive and rare noble metal electrocatalysts with inexpensive and earth-abundant ones. Despite the enormous strides and many achievements we have made, as outlined in this review, there is still a long way ahead before

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a benchmarking study of the existing catalysts by an impartial third party should be strongly encouraged. Objective comparisons of electrocatalyst performance using standard methods under identical conditions are helpful to evaluate the viability of existing electrocatalysts as well as to inform the development of new catalytic systems. In this regard, McCrory et al. at California Institute of Technology have done some very meaningful studies.275 (iii) Exploration of new materials

Fig. 47

The future hydrogen energy cycle.

water splitting can find a wide range of commercial uses for sustainable hydrogen production with both economic and environmental benefit. In order to realize such an ambition, or even come close, we will have a lot of things to do in the future, with regard to the hydrogen-evolving electrocatalysts. (i) Mechanism investigation Investigation on the mechanism of the known HER electrocatalysts is not only of scientific importance, but can also offer rational guidance to optimization of materials’ performances. This has been confirmed by the fact that the relatively clear catalytic mechanism of MoS2 in acidic media has significantly boosted the rapid development of MoS2-related materials. Nevertheless, for most of the known HER electrocatalysts, especially these composite catalysts, there is still lack of in-depth mechanistic investigation on an atomic level. Moreover, the HER mechanism in alkaline media for all the materials is very ambiguous at present. This, therefore, might call for an integration of theoretical simulation and in situ characterization techniques to clarify the catalytic mechanism. (ii) Standardized testing Establishing standardized testing will be useful to compare different materials obtained by different researchers, and to screen the optimal HER electrocatalysts. It is difficult to directly compare various materials due to different mass loadings of catalysts on the electrode, different preparation methods of the electrode, and different reaction solutions (i.e., the electrolytes) that are used finally. For example, in most of the studies the measured currents are only normalized to the superficial geometric electrode area, but the loading amount of the catalyst is often ignored. Because the overall currents we measured are also related to the amount of the catalyst, the normalization only by the electrode area might lead to an unfair performance evaluation among different materials. Thus, the researchers should provide as much information about electrode activity as possible so that the researchers can easily compare the results with each other. The important performance parameters mainly include Tafel slop, exchange current density, loading amount of catalyst, catalytic activities normalized by both mass and electrode area, Faradic efficiency and stability. In addition,

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Exploring new HER electrocatalysts will be one of our core research goals in the next few years. An ideal non-Pt HER electrocatalyst should meet several standards: (1) high efficiency similar to that of Pt; (2) good durability over a period of at least several years; (3) high chemical/catalytic stability over a wide pH range, even at all pH values; (4) low cost to ensure inexpensive hydrogen production; and (5) scalability to ensure a wide range of commercial uses. However, to date, none of the known HER electrocatalysts possess all the above merits. Therefore, a viable method is to choose the right HER catalyst for the particular application. As for chemical composition, transition metal Fe and Ni compounds (e.g., FeP) would be the first choice for future HER electrocatalysts due to their natural abundance and potential catalytic activity. In addition, the HER catalysts with high tolerance against seawater should be actively pursued because they might make the direct use of seawater possible. (iv) Integration of HER electrocatalysts with oxygen evolution electrocatalysts and/or semiconductor photocatalysts On one hand, the efficiency of electrochemical water splitting is determined by not only the HER electrocatalyst and oxygen evolution electrocatalyst themselves,276 but also their compatibility. On the other hand, the efficiency of photocatalytic or photoelectrochemical water splitting is also strongly influenced by the suitable combination of HER electrocatalysts and semiconductor materials. Thus, a final evaluation for a HER electrocatalyst is necessary to put it in a real electrochemical, or photo(electro)chemical water splitting system.

Acknowledgements This work was financially supported by the fundamental research funds for the central universities, startup funds from Jilin University, the National Natural Science Foundation of China (Grant no. 51372007, 21301014 and 21401066), Jilin province science and technology development projects (20150520003JH). X. Zou and Y. Zhang deeply appreciates the assistances of Prof. Guo-Dong Li and his group members (Jilin University), Prof. Xin-Bo Zhang (Changchun Institute of Applied Chemistry), Xiaoxi Huang (Rutgers University), Prof. Guang-Sheng Wang (BeiHang University), Prof. Huan Liu (BeiHang University), Yang Li (BeiHang University) and Wang Li (BeiHang University).


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Nanostructured hydrotreating catalysts for electrochemical hydrogen evolution.

Graphene supported plasmonic photocatalyst for hydrogen evolution in photocatalytic water splitting.

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