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Recent Trends and Perspectives in Electrochemical Water Splitting with an Emphasis on Sulfide, Selenide, and Phosphide Catalysts of Fe, Co, and Ni: A Review Sengeni Anantharaj,† Sivasankara Rao Ede,† Kuppan Sakthikumar,† Kannimuthu Karthick,† Soumyaranjan Mishra,†,‡ and Subrata Kundu*,†,§ †

Electrochemical Materials Science (ECMS) Division, CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi-630006, Tamil Nadu, India ‡ Centre for Education (CFE), CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi-630006, Tamil Nadu, India § Department of Materials Science and Mechanical Engineering, Texas A&M University, College Station, Texas 77843, United States ABSTRACT: Increasing demand for finding eco-friendly and everlasting energy sources is now totally depending on fuel cell technology. Though it is an eco-friendly way of producing energy for the urgent requirements, it needs to be improved to make it cheaper and more eco-friendly. Although there are several types of fuel cells, the hydrogen (H2) and oxygen (O2) fuel cell is the one with zero carbon emission and water as the only byproduct. However, supplying fuels in the purest form (at least the H2) is essential to ensure higher life cycles and less decay in cell efficiency. The current large-scale H2 production is largely dependent on steam reforming of fossil fuels, which generates CO2 along with H2 and the source of which is going to be depleted. As an alternate, electrolysis of water has been given greater attention than the steam reforming. The reasons are as follows: the very high purity of the H2 produced, the abundant source, no need for high-temperature, high-pressure reactors, and so on. In earlier days, noble metals such as Pt (cathode) and Ir and Ru (anode) were used for this purpose. However, there are problems in employing these metals, as they are noble and expensive. In this review, we elaborate how the group VIII 3d metal sulfide, selenide, and phosphide nanomaterials have arisen as abundant and cheaper electrode materials (catalysts) beyond the oxides and hydroxides of the same. We also highlight the evaluation perspective of such electrocatalysts toward water electrolysis in detail. KEYWORDS: water splitting, hydrogen evolution, oxygen evolution, overpotential, Tafel analysis, metal chalcogenides, metal phosphides, electrolysis

1. INTRODUCTION An unusually increased rate of depletion of conventional fossil fuels and the environmental hazards associated with the use of these carbon-emitting fuels have triggered the research community to find an abundant, everlasting, zero-emitting, and eco-friendly combined fuel-combustion technology as an alternate energy server very urgently.1 Fuel cell technology is among the ways of producing energy in a more eco-friendly manner than the other existing ones. The fuel cell operating with H2 and O2 will be the choice among the available fuel cells as it emits water as the combustion product.2−7 Currently, steam reforming of fossil fuels and water electrolysis (H2O(l) → H2(g) + 1/2O2(g): ΔG° = +237 200 J/mol, ΔE° = 1.23 V vs reversible hydrogen electrode (RHE)) are the ways of producing H2 to afford fuel for these fuel cells. Between steam reforming and water electrolysis, the first one is not environmentally friendly because it produces CO2 along with H2, thereby reducing the purity of H2, which in turn affects the life cycle and the cell efficiency of fuel cells.2−7 Water © 2016 American Chemical Society

electrolysis proceeds via the following two half-cell reactions: reduction of H+ ions at the cathode (2H+(aq) + 2e− → H2(g)), i.e., the hydrogen evolution reaction (HER),8−10 and oxidation of water (2H2O(l) → O2(g) + 4H+(aq) + 4e−), i.e., the oxygen evolution reaction (OER).11−16 At first look, water electrolysis may pretend to be an easy and problemless way to produce H2, but it is not. Electrolysis of water is an area where the science and technology need to be improved to overcome the issues associated with it. One such issue is that the noble metals and their compounds catalyzing the HER (Pt)17−25 and OER (Ir, Ru)26−43 need to be replaced by the available non-noble metals and their compounds. To do so, recently attention has been diverted toward OER catalysts based on non-noble metals such as Mn,44−47 Fe,48−53 Co,44,49,54−76 and Ni16,47,53,76−101 and HER catalysts based on Mo58,83,102−110 and W.111,112 Though Received: August 29, 2016 Revised: October 14, 2016 Published: October 19, 2016 8069

DOI: 10.1021/acscatal.6b02479 ACS Catal. 2016, 6, 8069−8097

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ACS Catalysis

including the Krasil’shchkov, Bockris,46,116 Yeager, and Wade and Hackerman52 pathways in addition to the most recognized electrochemical oxide pathway and the oxide pathway. Under alkaline conditions, all of the proposed mechanisms begin with an essential elementary step of hydroxide coordination to the active site and proceed via different proposed other elementary steps.46,116 The kinetic barriers associated with each elementary step raise the overall overpotential required. An elementary step with the most sluggish kinetics is the rate-determining step (RDS).11,14 In general, the electrochemical reactions that occur at the anode (OER domain) and at the cathode (HER domain) in acidic and alkaline media are given as follows:

the production of purest H2 is the main objective of water electrolysis, we cannot neglect the counter-reaction (the OER), as it is the sluggish one between them and affects the Faradaic efficiency of the electrolytic cell to a greater extent. The HER under acidic conditions is facile, as plenty of protons are available, and it proceeds by a multistep reaction with two possible mechanisms.113,114 The mechanism by which the HER proceeds is usually revealed by the experimentally obtained Tafel slope value.8−10,115 The first step in the multistep HER is discharging protons on the electrode surface to form adsorbed hydrogen (Hads), which is called as the Volmer reaction (H+(aq) + e− → Hads). The Tafel slope for the Volmer reaction, b1,V, can be expressed as b1,V = 2.303RT /βF

in acid:

(1)

where R is the ideal gas constant, T is the absolute temperature, F is the Faraday constant, and β is the symmetry factor (equal to 0.5). Depending on the coverage of Hads, the second step is either electrochemical desorption of H2, known as the Heyrovsky reaction, or chemical H2 desorption, known as the Tafel reaction. If the coverage of Hads is low and the electrode surface has sufficient active sites near the Hads sites, the adsorbed H atoms will preferably join with a proton and an electron simultaneously to evolve a molecule of H2 (Hads + H+(aq) + e− → H2(g)). This is called the Heyrovsky reaction, and its Tafel slope can be expressed as b2,H = 2.303RT /(1 + β)F

2H 2O(l) → 4H+(aq) + 4e− + O2 (g)

(6)

4e− + 4H 2O(l) → 4OH−(aq) + 2H 2(g)

(4)

4OH−(aq) → 2H 2O(l) + 4e− + O2 (g)

(7)

In practice, there is no electrolytic cell that performs with 100% Faradaic efficiency because of various thermodynamic and kinetic hindrances. As stated above, the best catalyst that catalyzes the HER is Pt, and the catalysts that catalyze the OER are Ir and Ru and their compounds. However, use of such relatively less abundant precious metals for a corrosive and destructive water electrolysis is not advisible as it increases the cost of H2 production, hinders the magnification of productivity on a large scale, and more. Later, people have found that the oxides and hydroxides of group VIII 3d metals, viz., Fe, Co, and Ni alone and with a combination of other metals in the row, can catalyze the OER as efficiently as Ir and Ru under alkaline conditions. Nevertheless, the discovery is not fruitful as they did not consider the kinetics of the counter-reaction (the HER) in alkali. Moreover, the HER is sluggish even with Pt because of the reasons stated above. To overcome this issue, there are two possibilities. The first one is to find efficient, durable, non-noble OER catalysts in acidic media as alternates to Ir- and Ru-based catalysts. However, this could not even eliminate the expensive Pt from the job of the HER. One way to get rid of the Pt would be to replace it with other non-noble metal-based catalysts such as sulfides, selenides, and phosphides, but the resulting electrolytic cell will be the kind of asymmetric one, leading to other technical problems. The second way to overcome this pitfall is to design a bifunctional catalyst out of non-noble metals that can catalyze both the OER and HER at the same time without the need for any separator. Such bifunctional catalysts made from Fe, Co, and Ni are being reported, frequently with parallel or in some cases better activities for both the HER and OER compared with Pt, Ir, and Ru. Such reported bifunctional catalysts are almost always phosphides, sulfides, and selenides of Fe, Co, and Ni as nanostructures in various forms. As an emerging field in the energy sector, there are few reviews of many of these materials and their subsequent applications to water splitting. However, all of the available reviews to date have either been focused on one type of catalyst formed out of these three metals (viz., Fe, Co, and Ni) or all three types of catalysts formed out of a single metal. Examples

(2)

(3)

The calculated Tafel slopes of the above reactions under standard conditions are 0.118 V/dec, 0.039 V/dec, and 0.029 V/dec for the Volmer, Heyrovsky, and Tafel reactions, respectively. An electrocatalytic HER with a Tafel slope of 0.029 V/dec is termed to follow the Tafel−Heyrovsky mechanism, and the rate-limiting step would be the electrochemical desorption step. Under alkaline conditions, the HER is comparatively more sluggish because it directly depends on the anodic OER, which supplies the protons to the cathode by the deprotonation of hydroxide ions and affects the HER kinetics. Under such conditions, the following reaction takes place at the cathode: 4e− + 4H 2O(l) → 4OH−(aq) + 2H 2(g)

(5)

in alkali:

In the case of high Hads coverage, two adjacent Hads will join together chemically and evolve a molecule of H2. This the Tafel reaction, and its Tafel slope can be expressed as b2 ′ ,T = 2.303RT /2F

4e− + 4H+(aq) → 2H 2(g)

(4)

The protons formed by the deprotonation of hydroxides at anode get combined with the abundant OH− ions in alkaline solutions, making the HER struggle more to move forward further. The OER on the other hand has a different story. The kinetics of the OER in acidic and alkaline media vary depending on the material by which it is being catalyzed. Noble metal catalysts such as Ir and Ru and their compounds catalyze the OER more easily in acidic media than under alkaline conditions. On the other hand, the catalysts derived from VIII group 3d metals (Fe, Co, and Ni) catalyze the OER more favorably in alkaline media than in acidic media. This is mainly correlated to the mechanism by which they catalyze it. In 1986, Matsumoto and Sato13 published a detailed review on various reported OER mechanisms in acidic and alkaline media, 8070

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ACS Catalysis for the first kind are the reviews of transition metal phosphides and chalcogenides. Meanwhile, examples of the second kind are the reviews available on heterogeneous electrocatalysis by catalysts derived out of Co, Ni, and Fe, in which for a single metal the catalytic activity trend of its oxides, sulfides, selenides, phosphides, and borides would have coherently been summarized. Hence, to have a better experience in understanding the basics to frontiers of electrochemical water splitting, a complete review on sulfide, selenide, and phosphide catalysts formulated out of Fe, Co, and Ni would highly be desired. In this review, we present a detailed survey of the following topics: the parameters involved in the evaluation of catalysts for both the HER and OER, the effects of the heteroatoms (S, Se, and P) on the material properties of the metal and the subsequent catalytic activities, the basic requirements for an efficient bifunctional water electrolytic catalyst, a detailed overview of the synthetic methodologies of metal (Fe, Co, Ni, and their mixed versions) phosphides, sulfides, and selenides, and the applications of these materials in the electrochemical HER and OER. Finally, the challenges ahead in developing new, efficient bifunctional catalysts and opportunities are stressed.

in acidic solution or hydroxide ion coordination in alkaline solution, which is then followed by a series of other elementary steps. The kinetic barriers associated with each of these steps will contribute to the overall activation overpotential of an OER catalyst. Man et al.14 and others13,116 have studied the thermodynamics of the OER mechanism and proposed the following expression for calculating the theoretical OER overpotential (ηOER) under ideal conditions with U = 0 vs standard hydrogen electrode (SHE): ηOER = (ΔGmax /e) − 1.23 V

(8)

However, the theoretical and experimental values had very large difference in the standard free energy change associated with the elementary step involving the conversion of oxide to peroxide. This clearly says that the kinetic hindrances are not considered in the thermodynamic prediction of the overpotential. Because of the varying kinetics of these elementary steps from material to material, instead of the onset overpotential, the overpotential at a fixed current density (j) such as 10 mA/cm2 (η10) has now widely been accepted as an essential quantitative activity parameter to evaluate an electrocatalyst.118−120 The same is also used for the HER. For materials with a strong redox peak (giving a current density greater than 10 mA/cm2) within the potential window of gas evolution and for high-performance catalysts (giving current densities greater than 500 mA/cm2) such as layered double hydroxides,100,121−124 the overpotentials at higher current densities such as 50 and 100 mA/cm2 are also used as alternate activity parameters. 2.2. Tafel Slope and Exchange Current Density (j0). The Tafel plot of an electrocatalytic process is generally obtained by replotting the polarization curve (e.g., linear sweep voltammogram (LSV)) as a plot of log(j) versus η. The slope of the linear portion of the Tafel plot is defined11 as the dependence between the iR-compensated overpotential and the current density, which is expressed as follows:

2. PARAMETERS USED TO EVALUATE THE CATALYTIC ACTIVITY The following parameters are the widely recognized ones fpr evaluating and comparing the catalytic activities of catalysts. In this part of the review, the merits and demerits associated with each of these parameters are discussed and justified. 2.1. Overpotential (η). There is no electrochemical reaction that occurs at the potential predicted only by thermodynamic considerations excluding the kinetic hindrances experienced in a real system.11,14,113,114 As a consequence of these hindrances, an additional driving force in terms of an extra potential is needed to effect such electrochemical reactions, which is called the overpotential (denoted by the symbol η). For both the OER and HER, there are three sources of overpotential, viz., the activation overpotential, the concentration overpotential, and the overpotential due to the uncompensated resistance (Ru), which is the resistance exerted by the electrochemical interfaces. The activation overpotential is an intrinsic property of the material that catalyzes the electrode reaction and varies from one material to another. Hence, it can be minimized by choosing an efficient catalyst. The concentration overpotential occurs as soon as the electrode reaction begins as a result of the sudden drop in concentration near the interfaces. This can be minimized by stirring the solution. The resistance overpotential can be removed by carrying out Ohmic drop compensation, which is now available in many electrochemical workstations.8 Otherwise, it can also be done manually just by multiplying the resultant current density by Ru, the result of which is a potential (E). This drop in potential, known as the iR drop, needs to be subtracted from the experimental potential. In case of the HER, the activation overpotential, which can be termed as the onset overpotential, is more important than the others, as the kinetics of the HER is faster than that of the OER.8,9,115,117 This can be calculated from the polarization curve obtained by plotting the overpotential versus the current density. In contrast, the OER case differs and needs more attention on other parameters too in order to calculate the overpotential. As we said above, all of the proposed mechanisms of the OER proceed through the first elementary step of water coordination

d log(j) = 2.303RT /αnF dη

(9)

The Tafel slope is inversely proportional to the charge transfer coefficient (α), as the remaining other parameters are constants (viz., the ideal gas constant (R), temperature (T), Faraday constant (F), and the number of electrons transferred (n), which is equal to 4 for the OER and 2 for the HER). This indicates that a catalyst with a high charge transfer ability should possess a small Tafel slope. This is the reason why it is often used as a primary activity parameter in determining the catalytic activity. However, with the above-mentioned path (i.e., converting the LSV into the Tafel plot), there are several issues that can lead to a misinterpretation of the catalytic activity. In general, the LSV obtained with lowest possible scan rate gives the Tafel slope with the least experimental inaccuracy. The scan rate at which the polarization curve used for Tafel plot was determined becomes a serious problem when the catalyst is highly capacitive. This leads to large error while determining the exchange current density because the exchange current density is usually obtained by extrapolating the linear fit toward the corresponding current density on the logarithmic scale at the equilibrium potential (i.e., at zero overpotential). In such a case the catalyst with a high overpotential will have a large exchange current density value, which is not possible because a high 8071

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ACS Catalysis

the slope of the plot of log(Rct) vs η (where Rct is the charge transfer resistance) is obtained. This is the exact Tafel slope of the catalyst and depends only on the charge transfer ability of the catalyst as Ru is excluded from the calculation. Depending on the requirements and conditions, any one of these methods can be chosen for finding the Tafel slope of an electrocatalyst. Apart from the methods of obtaining Tafel slope, there has been a serious discussion of considering the exchange current density (j0) as an activity parameter in addition to the Tafel slope.11 The exchange current density is more often used as an activity parameter for the HER than for the OER. As we stated earlier, the kinetics of the HER is facile beyond the defined overpotential of the catalyst, so almost all reported catalysts have similar kinetics in the HER. This means that the exchange current density is directly correlated to the onset overpotential in the HER. Hence, it can be used there as an activity parameter. In contrast, the OER case is different, as the kinetics differs from one catalyst to another one and is strongly pHdependent. A classic example of this kind can be the comparison of the RuO2/IrO226,38,41,43,126−128 and Ni− Fe98,129−132 systems in alkaline media. In this case the onset overpotential of IrO2/RuO2 is lower than that of Ni−Fe but the Tafel slope is lower for the latter one. This implies that though the driving force to start the reaction on IrO2/RuO2 is lower, the rate of charge transfer/electron transfer is lower than for the Ni−Fe system in alkaline media. This is one of the reasons why people have adopted the overpotential at fixed current density (e.g., 10 mA/cm2) as an additional activity parameter rather than the exchange current density (j0) or the onset overpotential for the OER.11 Hence, hereby we can conclude that for the HER the Tafel slope and the onset overpotential and/or exchange current density can be taken as activity parameters. For the OER, the Tafel slope and the overpotential at a defined current density can be taken as activity parameters. 2.3. Stability. The stability of an electrocatalyst is usually tested by subjecting it to CV cycling at a higher scan rate, which is otherwise known as the accelerated degradation test, and to chronoamperometric or chronopotentiometric analyses. In the case of the HER, the accelerated degradation test is carried out for several thousands of cycles as the polarization starts from 0 V vs NHE. In the case of the OER, the number of cycles reported in accelerated degradation test ranges from 250 to 1000. Beyond 1000 cycles, it is rare to see such a report on an OER catalyst with extreme stability. After the accelerated degradation test, the shift in onset overpotential (η0) (cathodic for HER and anodic for OER) and the overpotential at a defined current density of 10 mA/cm2 (η10) are measured as indicative parameters of stability, with a smaller increase in overpotential indicating a higher stability. Stability under constant exposure to a fixed potential (chronoamperometry) or a fixed current density (chronopotentiometry) is examined for durations of several minutes to hours. It has now been widely accepted that a stable current density (e.g., 10 mA/cm2) for more than 12 h by chronoamperometry or a negligible increase in overpotential at a current density of 10 mA/cm2 for more than 12 h by chronopotentiometry is enough to recognize an efficient electrocatalyst for both the OER and HER. As highlighted in the overpotential case, for high-performance materials and catalysts with strong redox peaks, other current densities such as 50 or 100 mA/cm2 and their corresponding potentials can also be used to run chronopotentiometric and chronoamperometric analyses for long times.17,27

exchange current density means that transferring electrons across the catalytic interface will be facile and require a very low activation energy. Hence, the result would be a low overpotential. Besides the problem with capacitive currents, the current observed in the Faradaic region is also not the true one, as it is not the steady-state current, and this leads to a considerable experimental inaccuracy in the determined Tafel slope value. This can be explained as follows. In a chronoamperometric analysis (at constant potential), it is common to observe the steady-state current only after few seconds. In the case of linear sweep voltammetry, even at a scan rate of 1 mV/s the current obtained cannot be the real steady-state current. To overcome this issue, a traditional practice can be employed in which the steady-state current of the catalyst is obtained from the chronoamperometric i−t curves obtained at various overpotentials with regular small intervals (say 5 mV). A similar experimental comparison of Tafel slopes obtained from cyclic voltammetry (CV) (run at 5 mV/s) and from the steady-state currents obtained by chronoamperometric curves for a commercial IrTiO2 electrode is given in Figure 1.11 The linear

Figure 1. Tafel curves of a commercial IrTiO2 electrode (Umicore AG & Co.) obtained by a cyclic voltammetry measurement at 5 mV/s and by a chronoamperometry experiment by holding each potential for 30 s in 20% O2-saturated 0.1 M HClO4 at room temperature and a rotation speed of 900 rpm. Reproduced with permission from ref 11. Copyright 2014 Royal Society of Chemistry.

portions of the Tafel plots obtained by the two methods are in close agreement with one another. However, in the loweroverpotential region the story is different. It can be seen from Figure 1 that the Tafel plot obtained from CV shows a larger exchange current density than the one obtained from the steady-state current, and the exchange current density from chronoamperometry will be more precise as it eliminates the capacitive current. Though the latter method seems to be better than the method of replotting the polarization curves, it also does not reflect the intrinsic and exact catalytic activity of a catalyst because the potential drop due to the uncompensated resistance cannot be easily excluded in this method as it is usually done in the former method of replotting polarization curves by iR compensation. To avoid this issue, Hu and co-workers very recently proposed a new method for obtaining a more accurate Tafel slope for a catalyst from electrochemical impedance spectroscopy (EIS).125 In this method, the Nyquist plots of the catalyst at various overpotentials at regular intervals are acquired, and 8072

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ACS Catalysis 2.4. Faradaic Efficiency. Faradaic efficiency is another quantitative parameter used for both the HER and OER. It can be defined as the efficiency of an electrocatalyst to transfer electrons provided by the external circuit across the interface to the electroactive species to effect the electrode reaction (in our case, either the HER or OER). There are two methods to find the Faradaic efficiency.8 The first one is the electrochemical method using a rotating ring disc electrode (RRDE), which is applicable to the OER only. The catalytically active material is coated on the disc of the RRDE without disturbing the ring. The commonly used RRDE is the one with a glassy carbon (GC) disc and Pt ring. Prior to that, the collection efficiency of the RRDE must be known or to be determined experimentally by studying the conventional ferrous/ferric redox system’s response at various rotation rates. The potential at the disk is then swept over the same experimental potential window of the OER, while at the same time the potential at the Pt ring is set at a constant potential for the oxygen reduction reaction (ORR). The ORR potential set at the Pt ring is varied depending on the pH of the electrolyte solution. Equation 10 is used to calculate the Faradaic efficiency (FE) of an OER catalyst:133−135

FE =

IR nD IDnR NCL

TOF =

INA AFn Γ

(11)

where I is the current, NA is the Avogadro constant, A is the geometrical surface area, n is the number of electrons transferred, and Γ is the surface or total concentration of catalyst in terms of number of atoms. There are several methods available to determine the surface or total concentration catalyst in terms of number of atoms. The redox peak in the cyclic voltammogram can be used to find the surface concentration after activation of the catalyst by CV cycling.133−135 By the Avogadro’s number method, the total concentration of atoms can be calculated using the average particle diameter of the catalyst.17 Another method is the assumption of a monolayer.27 If the catalyst surface is flat and smooth or the catalyst has a sheet morphology, this assumption can be made. However, we are aware that each of these methods has its own drawbacks. The first method may cause potential error when there is more than one element in the catalyst or if the catalyst is not fully activated. The second method does not reflect the exact catalytic property of the catalyst as it also includes the atoms in the core of the particle, which actually do not participate in the catalytic cycle. The third method may lead to potential error when the material is not completely flat, is prone to destruction under harsh electrochemical conditions, or does not have a sheet morphology. Hence, it is advisible to adopt a method appropriate for the catalyst and its nature. 2.6. Mass and Specific Activities. The mass and specific activities of an electrocatalyst are two other quantitative active parameters used to define the catalytic activity of an electrocatalyst. The current normalized by the catalyst loading is the mass activity, which is expressed in amperes per gram (A/ g).136 On the other hand, the current normalized by the electrochemical surface area (ECSA) or the Brunauer− Emmett−Teller (BET) surface area is the specific activity. As in the case of the TOF, the mass and specific activities of an electrocatalyst should be given at a defined overpotential. More insights about mass and specific activities are described in detail in section 3.1. Among the six parameters discussed to describe the catalytic activity of an electrocatalyst, the overpotential, Tafel slope, and stability parameters are the mandatory ones.

(10)

where IR and ID are the current at the ring and disc, respectively, nR and nD are the numbers of electrons transferred at the ring and disc, respectively, and NCL is the collection efficiency of the RRDE used. This is an extremely useful technique to find the true activity of an OER catalyst that has the following possibilities of losing its Faradaic efficiency: an OER catalyst with one or more strong redox peaks within the potential window of the OER (almost all Ni-, Co-, and Febased catalysts), an electrocatalyst that can facilitate other unwanted side reactions, and an electrocatalyst that heats up during the electrocatalysis process. The second method of determining the Faradaic efficiency is common for both the HER and OER. In this case, the quantity of gas (H2/O2) evolved is calculated by integration from the chronoamperometric or chronopotentiometric analysis. Then the practically obtained gas (H2/O2) quantity is calculated, for which any one of the following three methods can be employed.8 The first method is the conventional water gas displacement method, and the second is gas chromatography. The third method is a spectroscopic technique applicable only to the OER in which the evolved oxygen is excited from the triplet state to the singlet state and allowed to relax by fluorescence. The intensity of the fluorescence is a direct measure of the quantity of oxygen evolved. However, the method for determining the practically evolved gas quantity can be chosen depending on the nature of the catalyst and the availability of resources to do such studies. The ratio of the quantities of gas determined by the practical method and the theoretical method is the Faradaic efficiency of the catalyst under study.8 The selection from among these methods is also determined mainly by the demands of the catalyst used. 2.5. Turnover Frequency (TOF). The TOF is another quantitative parameter used to evaluate an electrocatalyst at a defined overpotential. The TOF of the catalyst is defined as the number of moles of O2/H2 evolved per unit time. Equation 11 is used to calculate the TOF for an electrocatalytic gas evolution reaction:

3. METHODS OF NORMALIZING THE EXPERIMENTALLY OBSERVED CURRENT AND THE ASSOCIATED MERITS AND DEMERITS Though the geometrical-surface-normalized current density has now been widely used, it has its own merits and demerits in quantifying the activity of an electrocatalyst. A good survey of such normalization methods was briefly discussed by Fabbri et al. in their recent review article on oxide-based OER catalysts.11 3.1. Normalization of the Current by the Geometrical Surface Area. It is not good to have the current density normalized by the geometrical area of the substrate electrode (in units of A/cm2geo), as this does not reflect the intrinsic catalytic property of the material. Moreover, it does not care about the catalyst loading, giving different potentials for the same current density with the varying catalyst loading. In other words, if the coverage of the loaded catalyst is 100% and sufficient to form a monolayer, then the geometrical area of the substrate electrode can be used to normalize the current. In cases where the loading is low (in which there will be many catalytically inactive substrate electrode sites) or very high (in 8073

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ACS Catalysis Table 1. Merits and Demerits of Various Current Normalization Methods Used in Electrocatalysis of Water Splitting normalization method geometrical surface area

ECSA

BET surface area

catalyst loading

merits widely accepted and used method fair comparison with existing literature reports is easy good for planar electrodes such as foils and deposited thin films

demerits does not reflect the intrinsic catalytic property of the catalyst may vary depending on catalyst loading and its optimization ceometrical area of the substrate electrode is not equal to the actual surface area of the catalyst participating in catalysis

can reflect the intrinsic catalytic property of the catalyst loading-sensitive

difficulties in determining the ECSA

ease of determining the BET surface area most suitable for porous materials and catalysts loading-sensitive

does not reflect the intrinsic catalytic property

directly cares about the loading regardless of type of catalyst suitable when same material is used at different loadings

direct comparison with theory and experiment is not feasible

large experimental inaccuracies between one method and other, such as CV and EIS studies comparison with existing reports would be tedious

leads to large errors because not all gas adsorption sites are electrochemically active not suitable for planar and thin film electrodes

does not reflect the intrinsic catalytic property of the material comparison of catalysts of varying particle size (e.g., mono- and polydispersed catalysts), density (e.g., metals and metal aerogels), morphology (e.g., catalysts with spherical, rod, wire, and sheet structures), and topography (e.g., catalysts with smooth and rough surfaces) is not possible comparison with existing reports is also not possible

cm2ECSA) will reflect the intrinsic catalytic property of the catalyst, unlike the one normalized by the geometrical surface area. The current density obtained by this method is often called the specific activity.136 However, the ECSAs determined by cyclic voltammetry and impedance analysis differ from one another significantly, which may lead to potential error. Recently, people have been using the BET surface area of the catalyst. However, this is also lacking in experimental accuracy. This is mainly due to the fact that all of the sites in the BET surface area determined by gas adsorption and desorption need not be electrocatalytically active. The experimental incongruity becomes large when the catalyst is composed of more than one element. 3.3. Normalization of the Current by the Catalyst Loading. Fabbri et al.11 recently justified why the current normalized by the catalyst loading is a more reliable activity parameter than the current normalized in other ways. The loading-normalized current density (in units of A/g) is otherwise known as the mass activity. Though it has lesser experimental inaccuracy than the others, it has more disadvantages than the other two methods explained above: direct comparative evaluation of the activity with the theoretical activity is not possible, the intrinsic catalytic property of the material is not soundly represented by this parameter, and it does not allow a fair comparison of catalysts of varying particle size (e.g., mono- and polydispersed catalysts), density (e.g., metals and metal aerogels), morphology (e.g., catalysts with spherical, rod, wire, and sheet structures), and topography (e.g., catalysts with smooth and rough surfaces). To have a better clarification of the merits and demerits of these methods, we have summarized them in Table 1. Having reviewed the advantages and disadvantages of each of these normalization methods, we can hereby come to a conclusion that depending on the nature of the catalyst, any one of these methods can be chosed for normalization of the current. However, to enable fair comparison with earlier

which only the surface layer participates in the electrode reaction, thereby excluding the catalyst present under the surface layer), the geometrical area of the substrate electrode is not equal to the actual surface area of the catalyst participating in catalysis. In such cases, normalization by the geometrical surface area will lead to large errors while quantifying the catalytic activity of an electrocatalyst. However, this is the conventional and widely accepted method for current normalization. Hence, to get information on the optimum catalyst loading, knowledge about the catalyst morphology (if a nanomaterial) and the wettability of the catalyst are needed. Beyond everything, an experimental study of the effect of loading can also reveal information about the minimum catalyst loading to form a near-monolayer catalytically active surface. The increase in catalyst loading will not affect the potential at a fixed current density beyond a certain amount of loading that is actually the minimum amount of catalyst required to form the monolayer. However, care must be taken while doing such a study because if the increase in catalyst loading from one trial to another is large, then the actual optimum loading may slip in between somewhere. Moreover, if the loading difference is large, the catalyst resistance will drastically increase, which will in turn lead to an increase in the overpotential beyond certain loading limits instead of a steady potential at a fixed current density. This may lead to potential error in determining the optimum catalyst loading to form a monolayer. Having discussed all such difficulties, we can conclude here that the geometrical surface area can be used normalize the experimentally observed current when a planar electrode is used to catalyze the electrode reaction. 3.2. Normalization of the Current by the Electrochemical Surface Area (ECSA) and the Brunauer− Emmett−Teller (BET) Surface Area. It is better to use the ECSA (or roughness factor)-normalized current density, as it is more sensitive to the catalyst loading and will vary with the same. The current normalized by the ECSA (in units of A/ 8074

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Figure 2. (A, B) LSVs for the HER on various transition metal phosphide catalysts with current normalization by geometrical surface area and ECSA, respectively. (C) Plots of average TOF vs E for various transition metal phoshides. (D−F) Volcano plots for various transition metal phosphides using different activity parameters such as geometrical-area-normalized current, ECSA-normalized current, and average TOF per surface site. Reproduced with permission from ref 143. Copyright 2015 Royal Society of Chemistry.

experimental and theoretical approach (Figure 2). In this particular report, the HER activities of these catalysts were benchmarked by plotting the free energy of H adsorption against the current normalized by both geometrical surface area and ECSA and also against the TOF. According to this study, an optimal free energy of H adsorption is predicted for Fe0.5Co0.5P when the activity parameters such as the current normalized by ECSA and the TOF were plotted against it. However, the Volcano plot obtained by plotting the current normalized by the geometrical surface area against the H adsorption free energy showed a slightly controversial activity trend and placed Fe0.5Co0.5P just below MoP/S and Fe0.25Co0.75P. This is mainly believed to be an effect of the contribution of non-Faradaic capacitive current to the catalytic current. The same group also revealed the reason behind the exceptional HER activity trends observed with these FeP-based catalysts from their dependence of the free energy of H adsorption on the increasing fraction of monolayer H coverage, as shown in Figure 3. From Figure 3, one can notice that the free energy of H adsorption decreases for FeP and Fe0.5Co0.5P beyond a certain fraction of H coverage on these catalytic surfaces and approaches a negative free energy of H adsorption at high H coverages. This indicates that beyond a certain amount of H adsorbed via the electrochemical discharge of protons, these catalytic surfaces turn into H2 donors simultaneously. The exceptional activity observed with these catalysts is attributed to this particular behavior. Later, a similar combined experimental−theoretical work on revealing the activity trends of CoP catalysts for the HER depending on the Fe dopant content was reported by Tang and co-workers.144 Interestingly, they too ended up with the same conclusion that the Fe0.5Co0.5P is the catalyst with Pt-like HER activity in acid with a low overpotential of 37 mV at a current density of 10 mA/cm2. The scheme of the free energy of H

reports, it is essential to have data on the current density normalized to the geometrical surface area. Otherwise, it is better to provide all of them together, and the same will help us to come to semiquantitative conclusion on the reliability of these methods.

4. BOND ENERGETICS OF INTERMEDIATES: THEORETICAL INSIGHT FROM DENSITY FUNCTIONAL THEORY (DFT) CALCULATIONS For an electrocatalyst to be efficient, the bond strengths of the intermediates should be neither too high nor too low. In case of the HER, the intermediate is the H-adsorbed active site after electrochemical discharge of a proton. Although all of the Ptgroup metals have been predicted to be highly active toward the HER, the actual activity is in the order Pt > Pd > Ni.137−142 This is mainly because of the increased reluctance shown toward desorption of Hads from the catalytic site by Pd and Ni. The high metal−Hads bond strength ultimately results in catalytic poisoning.137−142 As the standard redox potential for hydrogen evolution is zero, the associated standard free energy of hydrogen adsorption is should also be zero. The efficiencies of a set of HER catalysts are usually determined by calculating the standard free energies of H adsorption through DFT calculations and plotting them against a parameter such as the exchange current density (j0), the current density at a defined overpotential, or the TOF (Sabatier volcano plot). The catalysts that are placed at or near the summit of the plot are said to be highly active for HER electrocatalysis. Such an analysis of the activity trends of various transition metal phosphides (viz., Ni2P, FeP, Fe2P, CoP, Co2P, MoP, MoP/S, Fe0.25C0.75P, Fe0.75Co0.25P, and Fe0.5Co0.5P) in comparison with Pt nanoparticles (NPs) and Ti foil was performed by Kibsgaard and co-workers143 using a combined 8075

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theoretical and experimental studies on these catalysts such as the above-said reports are highly desired.143,144 In the case of the OER, the reaction proceeds through adsorption of water/hydroxide on the catalytic site (S−OH/S− OH 2) followed by oxidation of the same along with deprotonation to form the metal oxide (SO). Formation of the hydroperoxide intermediate (S−O−OH) then occurs by coordination of one more water molecule/hydroxide ion at the same site, and this is followed by deprotonation and reductive elimination of O2 from the active site, making it ready for the next catalytic cycle. These are the common elementary steps in all of the proposed pathways for the OER mechanism in acidic and alkaline media.11,13,14,116 We can see from this that the OER is associated with a series of elementary steps and that during each step the chemical environment around the active site is changing. If any one of these intermediates is not favored by its standard free energy of bonding, then the catalyst will not be able to catalyze the OER. On the other hand, if the bond strength is too high for any one of these intermediates, then catalyst poisoning occurs, resulting in a very high overpotential for the OER. In 2007, Rossmeisl et al.145 presented a good theoretical survey of the efficiency of the OER activities of oxides of Ir, Ru, and Ti in which they defined the theoretical OER activity of the same by plotting their oxygen binding energies against −ΔG (the negative of the change in the standard free energy) associated with each of the above-said elementary steps of the OER. That plot also contained a horizontal dashed line indicating the thermodynamic overpotential for the OER (1.23 eV). A material with high activity has to stay on the horizontal line, but it will not be seen becaue of the kinetic hindrances experienced by the catalysts in the real system, which will fetch them an additional potential (the overpotential). Moreover, the plot also says that regardless of the nature of the binding of oxygen to the active site, the activity is always limited by the oxide (SO*) and peroxide (S−O−O*) formation steps. However, catalysts with lower oxygen binding energies will stay closer to the theoretical horizontal line of the OER (where RuOx is placed just above IrOx as it binds more strongly to oxygen than RuOx does). In the report of Man et al.,14 similar interpretations were made for various other binary metal oxides, including the three studied by Rossmeisl and co-workers along with some perovskites as OER catalysts. Here the difference in the standard free energies of two subsequent steps, namely, hydroxide coordination and oxidation of the same with deprotonation (ΔGO* − ΔGHO*) was plotted against the theoretical oxygen overpotential. It should be noted that the standard free energy associated with the formation of hydroperoxide from the oxide intermediate was not included here. This could lead to potential errors in predicting the catalytic activities of some catalysts with slow kinetics of hydroperoxide formation. As far as this work is being considered, the catalysts placed at the top of the plots, such as Co3O4, RuO2, IrO2, NiO, SrCoO3, LaNiO3, and SrNiO3, are the ones with the optimum oxygen bonding energies to catalyze the OER efficiently. From the above discussion, we can now conclude here that the catalytic surface with high oxygen binding energy would have the peroxide formation step as the limiting step and the surface that binds oxygen weakly will have the hydroxide coordination and oxide formation steps as the limiting steps. Therefore, an OER catalyst should neither bond too strongly nor too weakly to oxygen, as we have seen in the case of the HER.

Figure 3. Plots of free energy of H adsorption on various metal phosphide catalysts vs increasing fraction of H monolayer coverage. Reproduced with permission from ref 143. Copyright 2015 Royal Society of Chemistry.

adsorption with respect to reaction coordinate (Figure 4) shows that Fe0.5Co0.5P has an optimal free energy of H adsorption very close to that of Pt.

Figure 4. Scheme of free energy of H adsorption with corresponding reaction coordinate. Reproduced from ref 144. Copyright 2016 American Chemical Society.

Although both of these reports say that Fe0.5Co0.5P is a better nonprecious catalysts with Pt-like HER activity, it is not the bestthere is still room to improve the catalytic behavior. From Figure 3 we can see that only FeP and Fe0.5Co0.5P show such a reduction in the free energy of H adsorption, whereas the other catalysts do not show such a trend on the same. This could lead to the question of what makes them almost equally active as FeP and Fe0.5Co0.5P. The answer lies in the higher negative free energy of H adsorption at lower H coverage and lower positive free energy of H adsorption at higher H coverage on these surfaces. This higher negative free energy of H adsorption at low H coverage shows its readiness to adsorb hydrogen, and the lower positive free energy of H adsorption the ease of H2 delivery from these catalytic surfaces and at the same time, its ability to achieve full Hads coverage as seen with Pt. This is the reason why these FeP based systems have been showing parallel activity to that of Pt in catalyzing HER. Examples of this kind of catalyst are MoP and MoP/S, as shown in Figure 3. This inference leads us to believe that the better transition metal phosphide catalysts are still hidden and that changing the stoichiometric compositions among all of these known metal phosphides could lead to better HER catalysts. To carry out such worthy studies, combined 8076

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co-workers148 on P-terminated MoP surfaces focusing the Gibbs free energy of hydrogen adsorption (ΔG°H) revealed that at lower Hads coverage ΔGH° is sufficiently negative (−0.34 eV) to accelerate the adsorption of hydrogen. However, when the surface becomes fully covered by Hads, ΔG°H becomes positive and accelerates the desorption of hydrogen, as observed earlier in the cases of MoS2 with more S on the edges. This is in agreement with the earlier reports of Kibsgaard et al.143 and Tang et al.144 In the cases of S- and Se-doped metal lattices, a similar mechanism should be responsible for the enhanced HER catalytic property. However, the increased electronegativity of S and the relatively higher strength of the S−H bond (∼360 kJmol−1) do not allow many of the sulfides of Fe, Co, and Ni to act as efficient HER catalysts. However, a few selected sulfides such as Ni3S2, CoS, CoS2, FeS2, and Co9S8 are reported to be good in catalyzing the HER in acid. When the bond strength is considered as an influencing factor in enhancing the hydrogen desorption from the catalytic site, the selenides stand a step ahead of both sulfides and phosphides, as the strengths of E−H bonds (E = S, P, Se) are in the order S−H (363 kJ/mol) > P−H (322 kJ/mol) > Se−H (276 kJ/mol).149 Moreover, like phosphides, both sulfides and selenides can trap protons by acting as bases to promote the discharge step faster. The story on the influences of these heteroatoms in the OER is different from the one we described above for the HER. Unlike the HER, these heteroatoms do not have any direct influence in enhancing the OER kinetics of these materials. In a report by Subbaraman et al.,150 the trends in the electrocatalytic OER activities of these 3d M2+ ions with oxide environments are described, and it is stated that Ni2+ is more active than other divalent cations that appear just before in the series. This is mainly attributed to the increased 3d−2p repulsion between the metal d-band center and the coordinated oxygen’s p-band centers. However, when we have electronegative ligands (S2−, Se2−, and P3−) in the vicinity of catalytically active metal sites, there could be two effects. One of these is that the localized negative charges on these heteroatoms (S and P only) will actually deactivate the catalyst from coordinating with the hydroxide ligand as a result of the increased 3p−2p repulsion. This will increase the overpotential considerably. However, after a tiresome coordination with hydroxide ligands, the oxidation coupled with deprotonation followed by peroxide formation will be supported because the deprotonation will increase the electron population on the metal center and thereby feed these electronegative heteroatoms. Similarly, the formation of peroxide by coordination of an additional hydroxide ligand happens just above the surface layers of these heteroatoms and therefore is not affected during the coordination processes. After formation of the peroxide intermediate, the delivery of the dioxygen molecule is actually accelerated by the enhanced 3p−2p repulsion between the heteroatoms and the peroxide entity. In the case of Se in place of S and P, we think that the coordination step with hydroxide will be less affected because both the metal centers and the ligand centers have 3d subshells as the outermost orbitals. However, because of the negative charge localized on Se sites and the cumulative 3d−2p repulsion between the metal d-band center and Se d-band center, the delivery of the dioxygen molecule may become faster. The facts discussed above on the OER performance of group VIII metal (Fe, Co, and Ni) sulfides, selenides, and phosphides are in resonance with the experimental reports. These are the

5. EFFECT OF HETEROATOMS (S, SE, AND P) IN WATER SPLITTING A brief and nice highlight on the effect of P doping in metal lattices on the electrocatalytic HER has recently been given by Shi and Zhang.8 In this section of the review, we will also elaborate the effect of other heteroatoms such as S and Se not only in the HER but also in the OER. As highlighted by Shi and Zhang,8 the polarization-induced partial negative charges localized on P centers in a metal phosphide structure with a P-terminated surface attracts protons as a base and make their discharge easier, promoting the HER easily. As a consequence of this, it is obvious to expect that metal phosphides with higher P content should show higher HER catalytic activities. Interestingly, among the reported phosphides of Co, Ni, and Fe, the polymorphs with higher P content are reported to be the highly active ones. Between CoP and Co2P, CoP is the more active one (Figure 5a).146 Similarly, among Ni2P, Ni5P4, and Ni12P5, Ni5P4 with the highest P content is the most active one (Figure 5b).147 Besides trapping the protons by acting as a base, these P centers have recently been found to enhance the hydrogen desorption at high Hads coverage. A DFT study carried out by Wang and

Figure 5. (a) P-content-dependent HER activities in NixPy. Reproduced with permission from ref 147. Copyright 2015 Royal Society of Chemistry. (b) P-content-dependent HER activities in CoxP. Reproduced from ref 146. Copyright 2015 American Chemical Society. 8077

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of the procedures with a few necessary modification from one material to another. The simple method of synthesizing metal sulfides is coprecipitation, in which the metal precursor salt is placed in a solvent that dissolves both the metal ion and the sulfide precursors. In this method, the reaction temperature and the pH should be carefully monitored to get reasonable yield. The main disadvantage of this method is that it requires additives such as surfactants, scaffolds, ligands, and chelating agents to control the size and shape of the synthesized metal sulfide nanostructure. Moreover, these additive are hard to remove after the synthesis. There are no reports on the preparation of MxSy (M = Fe, Co, Ni) nanostructures by this coprecipitation method for electrocatalytic water splitting applications. The most common method of all time used to prepare metal sulfide nanostructures alone or with some three-dimensional (3D) matrix support is the solvothermal method. The common sulfide sources used in this method are thiourea (TU, (NH2)2CS) and thioacetamide (TAA, CH3(CS)NH2).160,161 Among the sulfides of Fe, Co, and Ni, the sulfide nanostructures of Co have been relatively reported more than other two metal sulfides. Di Giovanni et al.162 prepared FeS nanoparticles by solvothermal decomposition of Fe2S2(CO)6 at 230 °C for the HER under neutral conditions. Other available reports describe Fe-based sulfides with Co as a dopant. Shen and co-workers reported the formation of Fe0.5Co0.5S@ mesoporous N-doped graphitic carbon as an efficient ORR and OER catalyst under alkaline conditions by a solvothermal route where TU was used as the S source.163 Apart from these, Faber et al.164 recently reported pyrite-type FeS2 for the HER obtained by a two-step procedure that includes the formation of a metallic thin film on a desired substrate by electron beam evaporation followed by thermal sulfidization taking elemental S as the source directly. Co-based sulfides with varying S content such as CoS, CoS2, Co3S4, and Co9S8 have been prepared by solvothermal methods and reported with different morphologies. You et al.165 compared the HER electrocatalytic activities of hollow nanoprisms of CoS prepared by microwave (MW) and solvothermal methods taking TU as the S source. Kornienko et al.166 showed the HER electrocatalytic activity of amorphous CoS with some O content in CoS2-like small amorphous clusters of CoS. Aslan and co-workers prepared CoS nanoparticles and anchored them on carbon nanotubes (CNTs) by a solvothermal method for the HER under alkaline conditions.167 Wang’s group recently published an interesting report on the design of CoS@CTs@CP, an efficient 3D electrocatalyst for overall water splitting that comprises carbon tubes (CTs), carbon paper (CP), and CoS sheets (Figure 6).168 Besides CoS, pyrite-type CoS2 is the next best documented Co-based sulfide nanocatalyst for water splitting. Faber et al.169 synthesized pyrite-type CoS2 by electron beam evaporation of the metal target followed by thermal sulfidization and applied them to the HER. These pyrite CoS2 began to draw attention in mid-2014 and have been reported frequently with modified morphologies and support materials such as graphene oxide (GO), CNTs, N-doped CNTs (NCNTs), etc. The following are some significant reports about them. It was Faber and coworkers who reported the HER activity of CoS2 for the first time.169 Soon after that, Zhang’s group reported pyrite CoS2 prepared by a solvothermal route as a versatile catalyst for the HER over a wide pH range of 0−14.170 In the same year, Peng et al.171 fabricated CoS2 nanosheets on graphene@CNT as a flexible electrode for the HER. Then a similar report by

reasons why the selenides are better OER catalysts than the phosphides and sulfides. In the HER, the selenides and phosphides are better than the sulfides as a result of the strengths of the E−H bonds (E = S, Se, P) described above. Hence, we conclude here by saying that an efficient water splitting catalyst should be designed by considering the bond energies of the intermediates formed and the electronic structure requirements of each element in the catalyst.

6. OTHER REQUIREMENTS FOR A GOOD WATER SPLITTING CATALYST In addition to the key thermodynamic and kinetic requirements of an electrocatalyst discussed above, it is important to highlight some other essential requirements for an electrocatalyst to be economically affordable in large-scale water electrolysis. The first of these is the availability of resources of the catalytic material from which the catalyst can be formulated, as we can no longer rely on noble metals (Pt, Ir, and Ru) to do this simple water electrolysis. The second one concerns health and environmental hazards. The catalyst should at least be less harmful to persons working with it and to the environment in which it is applied. Except for oxides and hydroxides of Fe, Co, and Ni, other compounds are potentially harmful. The third one is the conductivity of the catalyst. Although heteroatom doping of the metal lattice is fruitful in enhancing the catalytic activity, doping beyond certain limits will turn the metal surfaces into semiconducting and even insulating ones, which is not acceptable in electrocatalytic water splitting because it will drastically increase the Ohmic drop. The fourth one is the need for a high surface area of the catalyst, which would allow the amount of catalyst used to be reduced. The wettability is another parameter to be considered while formulating a water splitting catalyst, as higher wettability lowers the Ohmic drop caused by the formation of gas bubbles on the catalyst surface. Another serious problem in dealing with these Fe-, Co-, and Nibased catalysts is the current selectivity, as they tend to have strong redox reactions within the potential window of the OER. In such cases we should be aware of where the applied current is being spent. A good electrocatalyst for water oxidation should have more current selectivity toward the OER rather than its own redox reactions. To know this, the Faradaic efficiency determination by the RRDE experiment can be used. The final thing we should worry about is the corrosion resistance. Since the medium of water electrolysis is going to be either highly acidic or highly basic, the material that is supposed to catalyze the HER and OER should have very high corrosion resistance. With the sulfides, selenides, and phosphides of Fe, Co, and Ni, this problem is well documented in the literature and patents, as the oxides of these heteroatoms (S, Se, and P) are wellknown corrosion inhibitors and resistors.151−159 The above discussion certainly implies the advantageous use of the sulfides, selenides, and phosphides of group VIII 3d metals in water electrolysis. In the following sections, the synthetic strategies applied in designing such catalysts and their subsequent applications in water electrolysis are highlighted in detail. 7. SYNTHETIC STRATEGIES EMPLOYED IN GROUP VIII 3D METAL (FE, CO, AND NI) SULFIDE, SELENIDE, AND PHOSPHIDE NANOSTRUCTURES 7.1. Synthesis of Metal (Fe, Co, and Ni) Sulfides. The synthesis of metal sulfides has been achieved by following one 8078

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graphene by a solvothermal approach using TU as the S source was published by Liu and co-workers in 2013, and they applied this material to the ORR and OER under alkaline conditions.184 Recently Liu et al.185 reported the electrodeposition of Ni− Co−S and applied it to total water splitting. In a couple of recent reports by Sun and co-workers, the sulfides of Ni and Co have been fabricated into nanowire arrays via a solvothermal route for application in total water splitting electrocatalysis.186,187 Ansovini et al.188 recently reported the formulation of a 3D Co9S8−NixSy/Ni foam electrode for the HER by a solvothermal method. The sulfide of Co−Fe alloy on N-doped mesoporous carbon for the ORR and OER was recently reported by Shen et al.163 Wang et al.189 reported the advantages of Co-doped FeS2 nanosheet/CNT arrays for HER application. Similarly, Long et al.190 reported the fabrication of ultrathin Fe−Ni−S nanosheets by a topotactic sulfidization of Fe−Ni layered double hydroxides and examined their HER properties under acidic conditions. Apart from these, the sulfides of these three metals (Fe, Co, Ni) are also often alloyed with other metals to improve their electrocatalytic properties. One such catalyst, Ni−Mo−S on carbon fiber cloth synthesized by a solvothermal method for the HER under neutral conditions, was reported by Miao et al.191 For a better comparative overview of latest synthetic methodologies of these metal sulfides, we have tabulated the materials, precursors, and methods in Table 2. 7.2. Synthesis of Metal (Fe, Co, and Ni) Selenides. The synthesis of metal selenides resembles the synthesis of metal sulfides in many ways. The most common method of forming metal selenide nanostructures is the solvothermal route. The Se sources used for metal selenide synthesis are selenourea (SU), selenium metal powder, sodium hydrogen selenide (NaHSe) derived from the reaction of NaBH4 and crude Se powder, and sodium selenide (Na2Se) in some cases, particularly when aprotic solvents are used as the medium of synthesis.192 Unlike the sulfides, the numbers of reports on the selenides of these three metals are uneven. The most reported one is CoSe2 for both the OER and HER. The lone report on NiSe nanowires (NWs) was published by Tang et al.193 in 2015, where the NiSe NWs were solvothermally grown on Ni foam and applied for water splitting under strongly alkaline conditions (1 M KOH). NaHSe derived from the reaction of crude Se powder and sodium borohydride was used as the Se source during the solvothermal treatment of surface-etched-clean Ni foam. In the case of Fe, there has been no report for either the HER or OER. However, very recently Wang and co-workers reported that the OER activity of NiSe was drastically increased when Fe was introduced into the system, with a decrease in the overpotential of >100 mV.194 This indicates that although the catalysts with Fe alone have not been shown to date to possess any significant OER activity, their incorporation into the lattices of active catalysts composed of Ni and Co may reduce the overpotential and increase the activity. In that report, the Ni to Fe ratio was maintained at 3:1 during the Ni−Fe ultrathin precursor sheet formation and also while it was selenized with NaHSe by a hydrothermal route. Apart from the above two reports, the electrocatalysis by selenides of group VIII 3d transition metals is mainly occupied by CoSe2. Cobalt diselenide has been reported in both amorphous and crystalline forms with various morphologies and supports. Some of the important findings on the synthesis of this CoSe2 are briefly summarized below. Other than these there are two recent interesting reports on Ni3Se2 films and

Figure 6. (a, b) SEM images of CP/CTs/Co−S. (c, d) TEM images, (e) SAED pattern, and (f) elemental mapping of CT/Co−S. Reproduced from ref 168. Copyright 2016 American Chemical Society.

Ganesan et al.172 with S- and N-doped graphene as a support for CoS2 for use in the ORR and OER was published later in 2015. Other than these two sulfides of Co, Co3S4 on N-doped CNTs was prepared by Wang and co-workers by a simple anion exchange method taking Co(OH)2/NCNTs as the starting material, and this material was applied to the OER.173 Feng et al.174 proposed method for the formulation of Co9S8 armored with C that utilizes thiocyanuric acid as the S source and thermal decomposition at 700 °C under an atmosphere of N2. Very recently, Liang et al. published an interesting report on CoS and Zn-doped CoS for total water splitting where these catalysts were grown on a Ti mesh.175 Like the sulfides of Fe and Co, Ni-based sulfides with good electrocatalytic properties toward water splitting have been reported. NiS and Ni3S2 are the most common Ni-based sulfides applied for water splitting. The exceptional OER performance of Ni3S2 grown on Ni foam with low overpotential was first reported by Zhou et al.176 in 2013. They synthesized this material by a simple solvothermal sulfurization of Ni foam taking S directly as the sulfur source. In 2015, Tang et al.177 showed the HER performance of Ni3S2 nanosheets grown on Ni foam by a hydrothermal method for a pH range of 0−14. Then the same Ni3S2 in the form of a nanorod (NR) array on the same Ni foam for use in total water splitting was reported by Ouyang et al. later in 2015.178 In the last two reports of Ni3S2 preparation, TU was used as the S source, and the solvothermal treatment was kept constant in all three methods. In another report by Tang et al.,179 NiS hierarchical arrays grown on carbon cloth (CC) for efficient HER electrocatalysis were obtained adapting a solvothermal route of synthesis. Another interesting report by Yang and co-workers achieved highly efficient and stable water splitting catalysis with Fedoped NiS nanaoarrays prepared by a solvothermal route.180 Similarly, the electrocatalytic properties of NiS for the HER and OER individually and also for bifunctional catalytic activity were reported. Unlike Ni3S2, the bifunctional activity of NiS in 1 M KOH was first reported in 2015 by Zhu et al.181 following similar hydrothermal route of sulfidizing Ni foam. Later in 2015, Li et al. reported the atomic layer deposition of NiS on a silica substrate for the OER under alkaline conditions.182 Very recently, Mabayoje et al.183 comparatively reported the role of the anion in nickel sulfide and selenide toward the OER. Apart from the individual binary metal sulfides, it is common to see reports on sulfides of alloys of Fe, Co, and Ni. The sulfide of Ni−Co alloy is the most often reported alloy sulfide of this kind. The first report of the preparation of NiCo2S4@ 8079

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ACS Catalysis Table 2. Methodologies and Precursors Used To Form the Sulfides of Fe, Co, and Ni s. no.

material and morphology

metal precursor

source of sulfur

1 2

iron sulfide nanoparticles Co0.5Fe0.5S@N-MC

Fe2S2(CO)6 cobalt(II) acetate and iron(III) nitrate

Fe2S2(CO)6 thiourea

3

FeS2, CoS2, NiS2 films

high-purity Fe, Co, and Ni metals

sulfur powder

4 5 6

hollow cobalt sulfide nanoprisms CoSx film CoS@CNT

cobalt acetate hydroxide CoCl2·6H2O CoCl2·6H2O

thioacetamide thiourea thioacetamide

7 8

CoS@CNT@CFP sheets CoS2 micro- and nanowires

Co(NO3)2·6H2O Co(NO3)2·6H2O and (Co(OH)(CO3)0.5·xH2O

thiourea sulfur powder

9

CoS2 pyramids@Ti foil

CoCl2·6H2O

thiourea

10

CoS nanosheets@graphene@CNT

Co(Ac)2·4H2O

CS2 and thiourea

11

Co(TU)4(NO3)2 complex

12

CoS nanoparticles@N- and S-doped graphene oxide Co3S4@NCNTs

13

Co9S8 nanoparticles@C

Co(NO3)2·6H2O

Co(TU)4(NO3)2 complex Na2S and thioacetamide trithiocyanuric acid

14 15

Zn0.76Co0.24 S/CoS2 nanowires@Ti mesh Ni3S2 nanorods@Ni foam

ZnCo2O4 nanowires nickel foam

sulfur powder thioacetamide

16

Ni3S2 nanosheets@Ni foam

Ni(NO3)2·6H2O

thiourea

17

Ni3S2 nanorod arrays@nickel foam

nickel foam

thiourea

18

NiS2 nanosheets@CC

Ni(NO3)2·6H2O

sulfur powder

19

iron-doped nickel disulfide nanoarray@Ti

NiFe LDH

sulfur powder

20

nickel sulfide microsphere film@Ni foam

nickel foam

sulfur powder

21

NiSx films

bis(N,N′-di-tert- butylacetamidinato)nickel(II)

hydrogen sulfide

22 23

NiS film NiCo2S4 nanoparticles@graphene

NiCl2 Ni(CH3COO)2·4H2O and Co(CH3COO)2·4H2O

thiourea thiourea

24 25 26 27

NiCoS nanosheet films CoS nanosheets@Ti mesh NiCo2S4 nanowires@CC Co9S8−NixSy@Ni foam

Ni(CH3COO)2·4H2O and Co(CH3COO)2·4H2O CoCl2·6H2O NiCl2·6H2O and CoCl2·6H2O Co(NO3)2·6H2O and nickel foam

thiourea thiourea sulfur powder thiourea

28 29

Fe1−xCoxS2/CNT iron−nickel sulfide nanosheets

Fe(NO3)3 and Co(Ac)2 FeNi LDH

thioacetamide thioacetamide

30

Ni−Mo−S nanosheets@CC

Na2MoO4·2H2O and NiSO4·6H2O

L-cysteine

CoCl2·6H2O

NiSe2 nanowires fabricated via electrodeposition and solvothermal routes by Sun and co-workers for application in total water splitting.195,196 Like the other two selenides (NiSe NWs and Ni−Fe−Se nanosheets), the synthesis of CoSe2 is also achieved by the solvothermal method. It was Xu and coworkers who in 2013 reported the exceptional HER activity of CoSe2 nanobelts to which Ni/NiO was anchored by electrodeposition on a glassy carbon electrode (GCE).197 Soon after in 2014, the same was reported as CoSe2 NPs grown on carbon fiber paper (CFP) by Kong et al.,198 who utilized thermalpyrolysis-assisted cobalt oxide NP formation on CFP followed by selenization under a Se vapor atmosphere. The HER

method and reaction conditions solvothermal heating at 230 °C self-assembly by F127 surfactant and annealing at 900 °C in Ar atmosphere electron beam evaporation and thermal sulfidation microwave heating for 15 min electrodeposition at pH 7 solvothermal method at 140 °C for 24 h, N2 atmosphere electrodeposition for 8 min thermal sulfidation at 500 °C for 1 h hydrothermal method at 180 °C for 15 h hydrothermal method combined with vacuum filtration solid-state thermolysis at 400, 500, and 600 °C solvothermal method at 160 °C for 15 h annealing at 700 °C in N2 atmosphere for 3 h annealing at 400 °C for 2 h hydrothermal method at 180 °C 4 h hydrothermal method at 120 °C 10 h hydrothermal method at 160 °C for 6 h hydrothermal method at 100 °C for 10 h annealing at 400 °C for 60 min under Ar atmosphere annealing at 300 °C with a rate of 8 °C min−1 under Ar atmosphere vapor-phase atomic layer deposition electrodeposition solvothermal method at 200 °C for 6 h electrodeposition electrodeposition annealing at 300 °C for 2 h annealing of cobalt thiourea complex on Ni foam at 300 °C for 5 min 90 °C in an oil bath for 24 h. hydrothermal method at 120 °C for 6 h hydrothermal method at 200 °C for 24 h

ref 162 163

164 165 166 167 168 169 170 171

172 173 174 175 176 177 178 179 180 181

182 183 184 185 186 187 188

189 190 191

performance shown by this CoSe2 NPs/CFP catalyst was better than the previous report. In 2015, more attention was paid to synthesizing CoSe2 in various forms using various methods with the ultimate aim of total water splitting. In this regard, for the first time Liu et al.199 reported the bifunctional catalytic activity of CoSe2 as electrodeposited amorphous thin films on Ti foils for full water splitting. Before this, Carim and co-workers had already reported the HER activity of electrodeposited CoSe amorphous thin films in 2014.200 Then the attention was deflected toward crystalline CoSe2. Zhang et al.201 reported the HER activity of polymorphic CoSe2 with mixed orthorhombic and cubic phases. Recently, Liao et al.202 reported the OER 8080

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ACS Catalysis

source, trioctylphosphine (TOP), is used along with trioctylphosphine oxide (TOPO) as a structure-directing agent in various molar ratios. In the high-temperature solidstate synthesis, the desired metal precursor is reacted with sodium hypophosphite (Na2HPO2), commercially known as hypo, as the source of P under highly oxygen-free conditions. Other than hypo, elemental phosphorus (preferably red phosphorus) and ammonium hydrogen phosphate ((NH4)2HPO4) are also used as P sources.8,10 However, with the latter two P sources, the reaction temperature needs to be 2-fold higher than that required for the solution-phase synthesis using TOP. The inert atmospheres used to date for the synthesis of these metal phosphide nanostructures are Ar, H2, N2, and mixtures of H2 and N2 under either static or flowing conditions. Similarly, in all of the liquid-phase syntheses the common metal precursors used are the acetate or acetylacetonate complexes of the respective metal ions or the nitrates and chlorides of the metals with a phase-transfer catalyst (PTC) such as tetraoctylammonium bromide (TOAB).206,207 In the case of the high-temperature solid−gas-phase reaction, the nitrates are the most common metal precursors used. Unlike the sulfides and selenides, the phosphides of Fe, Co, and Ni have been equally explored in terms of synthesis and their subsequent applications to water splitting. In the case of Fe, the phosphides reported to date are FeP, FeP2, and Fe2P. Among these, FeP is the more frequently reported one. As mentioned in the earlier discussion on the effects of heteroatoms in water splitting, the higher P content in FeP helps it to be more active than Fe2P. Moreover, where bifunctionality is concerned, among the available reports on the phosphides of Fe there is only one report by Yan et al.,208 who grew ZnO NWs on CFP, which were then doped with Fe3+ by hydrolysis followed by phosphidization at 300 °C with hypo in an Ar atmosphere. Similarly, there is a lone report by Xiong et al.209 on the OER catalytic activity of FeP NRs supported on CFP obtained through a set of hydrothermal and gas-phase phosphidization reactions. Almost all of the other reported FeP materials in various morphologies and crystalline forms are devoted only to HER catalysis. Some of their synthesis sequences are briefly summarized below. FeP has been reported as nanowires, nanotubes, nanosheets, thin films, and nanocrystals and as a composite with N-doped CNTs for HER applications. The first report on HER electrocatalysis on FeP was published by Xu and co-workers, who synthesized FeP nanoporous nanosheets by an anion exchange method.210 Du and co-workers proposed an interesting method of preparing FeP NRs by a simple hardtemplate method using anodized aluminum oxide (AAO) as the desired hard template, onto which the Fe3+ precursor was loaded through a sequence of soaking and drying processes followed by phosphidization with hypo in an alumina boat at 350 °C.211 Later, Liang et al.212 obtained nanoarrays (NAs) of FeP NRs on CC by a solvothermal treatment to form the Fe2O3/CC precursor, which was then phosphidized using hypo at relatively lower temperature than other methods (Figure 8). Using a similar synthetic route, Tian and co-workers obtained 3D FeP NP thin films on CC that showed better HER performance than other phosphides in both acidic and neutral electrolytes.213 In a similar report by Yang et al.,214 the same CC was used once again for the formation of rugae-like FeP and FeP2 nanocrystal (NC) arrays, which were used later for HER electrocatalysis in acid. Jiang and co-workers chose a Ti plate instead of CC as the substrate for the FeP NW array.215 The synthesis was a two-step process involving initial

activity of coral-like CoSe2 nanostructures prepared by a solvothermal route. As expected, the CoSe2 was then composited with various other active catalysts such as MoS2 and Ni−Fe LDH and synergistically enhancing compounds and substrates such as CeO2, TiO2, and graphene to improve the catalytic activity of CoSe2. A clever thing done by Hou and coworkers was the synthesis of a CoSe2/Ni−Fe LDH on graphene composite, which resulted in better bifunctional water splitting catalytic activity than any other report.121 Similarly, Gao et al.105 synergistically improved the HER performance of CoSe2/MoS2 by making a composite of them. With a different synthetic route involving microwave irradiation, Ullah et al.203 reported the same CoSe2 but as a composite with graphene and TiO2 with improved HER performance. Zheng and his group members showed the effect of CeO2 on the OER catalysis led by CoSe2 with a nanobelt morphology.204 As a step ahead in the HER catalysis by CoSe2, Liu et al.205 formed a 3D HER catalyst by simple hydrothermal growth of CoSe2 on carbon cloth (Figure 7).

Figure 7. (a) XRD patterns of the precursor and selenized product scratched down from CC. (b, c) SEM images of Co(OH)F NW/CC. (d) TEM image of a Co(OH)F NW. (e, f) SEM images of CoSe2 NW/CC. (g) TEM image of a CoSe2 NW. (h) HRTEM image and (i) SAED pattern of a CoSe2 NW. (j) STEM image and EDX elemental mapping of cobalt and selenium for a CoSe2 NW. (k) SEM image of a CoSe2 MP. Reproduced from ref 205. Copyright 2015 American Chemical Society.

We have summarized the methods and materials used for the synthesis of these metal selenides in Table 3. 7.3. Synthesis of Metal (Fe, Co, and Ni) Phosphides. Although there are two good surveys on the synthesis of transition metal phosphides and their applications to the HER, it would be better to have a consolidated review of the OER activities of Fe, Co, and Ni phosphides in addition to the sulfides and selenides of the same. The synthesis of metal phosphide nanostructures is much more difficult than that of simple sulfides and selenides and is almost always done at high temperature in an inert atmosphere. The metal phosphide syntheses mainly fall into three categories. In the hightemperature liquid-phase synthesis, the most common P 8081

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ACS Catalysis Table 3. Methodologies and Precursors Used for the Synthesis of Selenides of Fe, Co, and Ni s. no.

material and morphology

source of selenium

metal precursor

1

MoS2/CoSe2

Co(OAc)2·H2O and (NH4)2MoS4

Na2SeO3

2

Co(NO3)2·6H2O, Ni(NO3)2·6H2O, and Fe(NO3)3·9H2O

Na2SeO3

3

Co0.85Se/NiFe LDH nanosheets NiSe nanowire film@Ni foam

nickel foam

Se powder

4

(Ni0.75Fe0.25)Se2@CFC

Ni(NO3)2·6H2O and Fe(NO3)3·9H2O

Se powder

5 6 7

Ni3Se2 film@Cu foam NiSe2 nanoparticle film Ni/NiO/CoSe2 nanocomposite

Ni(CH3COO)2·4H2O NiCl2·6H2O Co(Ac)2·H2O and nickel(II) 2,4- pentanedionate

SeO2 SeO2 Na2SeO3

8

CoSe2 Nanoparticles@CFP

Co(NO3)2·6H2O

Se powder

9 10

CoSe film@Ti mesh amorphous cobalt selenide films@Ti foil polymorphic CoSe2 3D coral-like CoSe

Co(Ac)2·4H2O Co(Ac)2·4H2O

SeO2 SeO2

CoCl2 Co(NO3)2·6H2O

SeO2 Se powder

CoCl2

Se powder

14

CoSe2/graphene−TiO2 heterostructure CeO2/CoSe2 nanobelts

Co(Ac)2·H2O and Ce(CH3COO)2

Na2SeO3

15

CoSe2 nanowires

Co(NO3)2·6H2O

Se powder

11 12 13

method and reaction conditions

ref

°C

105

°C

121

°C

193

°C

194

°C

195 196 197

hydrothermal method at 200 for 10 h hydrothermal method at 150 for 48 h hydrothermal method at 140 for 12 h hydrothermal method at 180 for 24 h electrodeposition electrodeposition hydrothermal method at 180 for 16 h thermal selenization in Ar atmosphere electrodeposition electrodeposition

electrodeposition hydrothermal method at 180 °C for 15 h microwave heating for 15 min polyol reduction method at 278 °C for 1 h hydrothermal method at 140 °C for 10 h

198 199 200 201 202 203 204 205

phosphidization under an atmosphere of N2 with hypo at 350 °C and applied these composites to the HER.216 The phosphides of Co have been studied more than those of Fe in both the HER and OER. The phosphides of Co known to date are CoP and Co2P. As seen in the case of FexP, CoP with the higher P content is reported to be a better catalyst for the HER and OER than Co2P. Moreover, the number of reports on CoP is 3-fold higher than that on Co2P. Co2P has been reported as nanoneedles and NWs and as a composite with CNTs and NCNTs for OER and HER applications. The synthesis of Co2P resembles the synthesis of FexP. One such recent report with a significantly different synthetic route was made by Dutta and co-workers, who synthesized Co2P nanoneedles by a liquid-phase alkylamine-assisted synthesis but instead of TOP as the P source, PH3 gas produced ex-situ was purged into the reaction vessel at 230 °C. This method provided the significant advantage of avoiding the strong possibility of explosion by other methods in the case when a little bit of O2 is present inside the reaction chamber.217 In the case of CoP, there is a cluster of reports on their synthesis and subsequent applications to the HER and OER. The synthesis of CoP can be done by any of the above-seen methods for the synthesis of FexP, viz., liquid-phase metal ion reduction and phosphidization, high-temperature solid−gas-phase reaction, or anion exchange reaction. Like the contribution made by Sun and co-workers in the case of sulfide and selenide catalysts of these three metals, they have done an enormous amount of work on cobalt phosphide too by various methodologies and subsequently used them for both HER and OER applications. Among them, fabrication of self-supported nanoporous CoP nanowire arrays,218 CoP nanowire arrays for sensing and photcatalytic HER,219 templated-assisted synthesis of CoP nanotubes,220 CoP nanowire arrays on Ti mesh,221 3D interconnected CoP nanowire arrays,222 and a combined experimental−theoretical study of CoP with varying Fe dopant

Figure 8. (a, b) SEM images of (a) Fe2O3 NAs/CC and (b) FeP NAs/ CC. (c) TEM and (d) HRTEM images of FeP nanorods. (e) SAED pattern recorded from the FeP nanorod. (f) STEM image and EDX elemental mapping of C, P, and Fe for the FeP NAs/CC. Reproduced from ref 212. Copyright 2014 American Chemical Society.

hydrothermal growth of FeOOH on the Ti plate followed by its chemical conversion to an FeP NW array on the Ti plate at low temperature. As expected, Liu and co-workers composited Ndoped CNTs to FeP by growing the Fe2O3/NCNT precursor composite first via a hydrothermal route followed by 8082

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ACS Catalysis contents are the significant works.144 Popczun et al.223 reported the HER activity under acidic conditions of CoP NPs prepared by the first method mentioned above. Subsequently, a number of reports appeared in literature with either a significant improvement in the catalytic activity or an improvement resulting in a relatively easier synthetic route. CoP has been reported with a branched starlike morphology by Popczun et al.,224 as hollow polyhedra by Liu and Li,225 as urchinlike NCs by Yang et al.,226 as porous NR bundles by Niu et al.,227 as nanosheets on a Ti foil substrate by Pu et al.,228 as selfsupported mesoporous NRs by Zhu et al.,229 as surface-oxidized NRs by Chang et al.,230 again as NRs by Huang et al.,231 as nanoporous NWs by Gu et al.,232 with a mixed morphology by Jiang et al.233 and as CoOOH-covered CoP NPs by Ryu et al.234 Apart from various morphologies, as expected CoP was composited with carbonaceous materials such as CNTs by Liu et al.235 and with GO by Ma et al.236 via hydrothermal deposition of CoP on GO. One other important study was reported by Hou et al.,237 who decorated the CNTs with ultrafine CoP NPs and applied them as a highly active bifunctional water splitting catalyst. Beyond this, an interesting report on CoP@C core−shell nanostructures was first made by Wang’s group, who applied them to HER electrocatalysis.238 Like the FeP case, CoP was also grown on CC by Li et al.239 through the two-step universal process of growing metal phosphides on CC. The phosphides of Ni are quite different from those of Fe and Co. Unlike the latter, the phosphides of Ni have been reported beyond the mono- and dimetallic centers. The nickel phosphides reported to date are NiP, Ni2P, Ni5P4, Ni12P5, and NiP2. The P content and its direct influence on the catalytic performance in the HER is well-explained in the very recent review of metal phosphide synthesis for HER applications by Shi and Zhang.8 Moreover, the evolution of nickel phosphides with varying P content led Kucernak and Sundaram240 to make a linear relationship between the P content and the catalytic activity of the metal phosphide toward the HER in acidic media. As observed with Fe and Co, the nickel phosphide with high P content (NiP) was found to be a better HER catalyst than the others. However, such a direct correlation between the P content and the OER activity of metal phosphides cannot be made because the mechanism of the OER is much more complicated than that of the HER. Syntheses of nickel phosphides were also done by any one of the three known methods of phosphidization as explained for Co and Fe phosphides. Nickel phosphides with various P contents were obtained simply by changing the molar ratio of the metal ion precursor to the source of P, as was done by Kucernak and Sundaram.240 Among the known nickel phosphides, Ni2P is the most frequently reported one with various morphologies such as metallic nanosheets by Li and co-workers,241 urchinlike crystals on Ni foam by You and co-workers,242 nanoaggregates by Li and co-workers,243 W-doped Ni2P microspheres by Jin and co-workers,111 nanosheets of Ni foam by Shi and coworkers,244 monodispersed NCs with different phases by Pan and co-workers,147 nanoflakes (NFs) on graphene/Ni foam hybrid electrode for the HER from pH 0 to 14 by Han and coworkers,245 and NRs on Ni foam by Wang and co-workers.246 The second most reported nickel phosphide is NiP. NiP has been reported by Yu et al.247 as carbon-coated porous nanoplates that were applied in the OER, and Wang and coworkers reported the one-step formation of NiP nanosheet arrays on Ni foam for efficient HER in acidic electrolyte.248

Very recently, Zhuo et al.249 reported the effect of Se doping onto the NCs of NiP and NiP2 on their HER activities. As an interesting advancement in the nickel phosphide research, Jiang and co-workers reported the exceptional HER performance of NiP2 under both acidic and alkaline conditions.250 The higher P content in NiP2 fetches an additional corrosion resistance under harsh alkaline conditions and improves the HER performance of the same. Similarly, the Ni5P4 NCs were found to be highly stable while catalyzing the HER under both acidic and alkaline conditions because of the increased P content, as reported by Laursen and co-workers.251 As a step ahead and above, the same Ni5P4 was later applied to full water splitting by Ledendecker and co-workers because it holds better stability in alkali than other phosphides of Ni.252 Huang and co-workers showed the catalytic and photocatalytic HER performance of Ni12P5 NPs.253 The methodologies and materials applied to the synthesis of the phosphides of Fe, Co, and Ni are summarized in Table 4. Beyond the monometallic phosphides, studies of phosphides of mixed metals and their electrocatalytic performance toward water splitting have recently also started to appear in the literature. The first such mixed metallic phosphide of Ni and Co was reported by Feng and co-workers, who prepared quasihollow Ni−Co−P nanocubes and applied them to the HER in alkaline solution.254 A little later, a ternary array of Ni−Co−P nanosheets was synthesized by Li and co-workers, who applied it to total water splitting under alkaline conditions.255 The advantage of making such bimetallic phosphides is that it helps to achieve better durability performance during both the HER and OER as reported by Li et al.255 Having briefly discussed the synthetic strategies for the sulfides, selenides, and phosphides of Fe, Co, and Ni, we can come to the conclusion that there is still room to improve the synthetic routes of these sulfides, selenides, and phosphides to reduce the associated experimental, environmental, and health hazards by minimizing the utilization of more toxic precursors, avoiding high-temperature reactions and explosive reaction conditions, and minimizing the waste from solution-based syntheses. We have also seen that the phosphides of Fe, Co, and Ni have been found to be better catalysts than the sulfides and selenides of the same. Now it is time to find simplified, easier, one-step, quick synthetic strategies to make the efficient metal phosphides affordable for industrial-level large-scale H2 production more conveniently. Beyond the extensive contributions made by Sun and his group members to this particular field of electrocatalysis of water splitting using sulfides, selenides, and phosphides of Fe, Co, and Ni, they have newly reported two other catalysts, namely, a Ni−Mo hollow nanorod array for total water splitting256 and amorphous NiB alloy NPs grown on Ni foam for total water splitting as well.257 These reports have basically made us aware that there is still room to improve the catalytic efficiencies of these catalysts. From the above discussion, it is quite obvious to expect a review summary on Co-catalyst-based materials for the electrocatalysis of water splitting from Sun and his group members, and the same has appeared very recently in the literature.258 This particular review nicely corroborates the activity trends of various Co catalysts such as its oxides, sulfides, selenides, and phosphides for both the HER and OER in acidic, basic, and neutral environments. However, it should be noted here that Sun’s review is centered around only the Co-based catalysts and does not cover Ni- and Fe-based ones completely. This has been 8083

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8084

CoP nanorod arrays@Ti CoP nanosheet arrays@Ti plate CoP nanorod arrays@Ni foam surface-oxidized CoP nanorods Co2P nanorods

CoP nanoparticles@C CNTs decorated with CoP nanocrystals CoP nanoparticles@RGO CoP nanoparticles@CNT CoP@C core−shell nanocables nickel phosphide Ni2P nanosheets urchinlike Ni2P@Ni foam NixPy

21 22 23 24 25

27 28

40

36 37 38 39

Ni2P nanosheets@Ni foam Ni2P nanoflakes@graphene/Ni foam Ni5P4−Ni2P nanosheet array carbon-coated nickel phosphide nanoplates Ni2P nanorods@Ni foam

CoP hollow polyhedra urchinlike CoP nanocrystals

19 20

29 30 31 32 33 34 35

FeP nanosheets FeP nanorods FeP nanorod arrays FeP nanoparticle film@CC rugae-like FeP nanocrystal@CC 3D FeP nanowires FeP@NCNT surface-oxidized Co2P nanoneedles 3D CoP nanowires@CC CoP nanowires CoP nanotubes CoP@Ti mesh CoP nanoparticles@CC CoP nanoparticles branched CoP nanostructures

3 4 5 6 7 8 9 10 11 12 13 14 15 17 18

material and morphology

iron phosphide nanotubes FeP nanorods@CP

1 2

s. no.

NaH2PO2 NaH2PO2 NaH2PO2 NaH2PO2·H2O trioctylphosphine

Co(NO3)2·6H2O Co(NO3)2·6H2O CoCl2 CoCl2·6H2O Co(Ac)2·4H2O

trioctylphosphine KH2PO4 red phosphorus NaH2PO2 red phosphorus

Ni foam

NaH2PO2 NaH2PO2 triphenylphosphine NaH2PO2 NaH2PO2 NaH2PO2·H2O NaH2PO2·H2O

Co(Ac)2·4H2O Co(Ac)2 cobalt(II) acetylacetonate NiSO4 NiCl2·6H2O Ni foam Ni(NO3)2·6H2O Ni foam Ni foam Ni foam potassium tetracyanidonickelate(II) and NiCl2·xH2O

trioctylphosphine NaH2PO2

cobalt(II) acetylacetonate Co(Ac)2

Co(NO3)2·6H2O Co(NO3)2·6H2O

trioctylphosphine NaH2PO2 NaH2PO2 NaH2PO2 NaH2PO2 NaH2PO2 NaH2PO2 Ca3P2 NaH2PO2 NaH2PO2 NaH2PO2 NaH2PO2 NaH2PO2 trioctylphosphine trioctylphosphine and trioctylphosphine oxide NaH2PO2·H2O NaH2PO2

Fe18S25−TETAH FeCl2 FeCl3·6H2O Fe(NO3)3·9H2O FeSO4 FeCl3 FeCl3·6H2O CoCl2·6H2O Co(NO3)2·6H2O Co(Ac)2 CoCl2·6H2O Co(NO3)2·6H2O CoCl2·6H2O Co2(CO)8 cobalt(II) acetylacetonate

source of phosphorus NaH2PO2 red phosphorus

Fe(NO3)3·9H2O FeCl3·6H2O

metal precursor

Table 4. Methodologies and Precursors Used in the Synthesis of Metal (Fe, Co, and Ni) Phosphides method and reaction conditions

ref

244 245 246 247 248

hydrothermal method at 200 °C for 48 h

236 237 238 240 241 242 243

234 235

227 228 229 230 231

225 226

210 211 212 213 214 215 216 217 218 219 220 221 222 223 224

208 209

heated at 300 °C for 2 h heated at 300 °C for 2 h in Ar flow heated in a sealed tube at 400 °C for 100 min electrodeposition heated at 380 °C for 15 h heated at 400 °C for 2 h in a static Ar atmosphere heated at 275−475 °C for 2 h in a static Ar atmosphere Solvothermal method at 320 °C for 2 h Chemical vapor deposition process heated at 500 °C for 6 h in N2 flow heated at 300 °C for 2 h in a static Ar atmosphere

heated at 250 °C for 2 h in a static N2 atmosphere heated at 300 °C for 150 min in a static N2 atmosphere heated at 300 °C for 2 h in a static Ar atmosphere heated at 300 °C for 1 h in a static Ar atmosphere electrodeposition heated at 300 °C for 2 h in a static N2 atmosphere solvothermal method at 120 °C for 30 min in N2 flow solvothermal method at 300 °C heated at 300 °C for 2 h

heated at 300 °C for 30 min under Ar flow phosphorization treatment in P vapor at 500 °C for 30 min anion exchange reaction by diffusion at >300 °C heated at 350 °C for 2 h under Ar flow heated at 300 °C for 2 h under Ar flow heated at 300 °C for 2 h under Ar flow phosphidation at 500 °C heated at 300 °C for 2 h in a static Ar atmosphere Calcined at 350 °C under N2 flow for 2 h solvothermal method at 230 °C for 30 min heated at 300 °C for 1 h in a static Ar atmosphere heated at 300 °C for 2 h heated at 300 °C for 2 h in a static Ar atmosphere heated at 300 °C for 2 h in a static Ar atmosphere heated at 300 °C for 2 h in Ar flow solvothermal method at 320 °C for 1 h heated at 120 °C for 1 h under vacuum

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rectified in the present review, which includes updated literature reports.

111 144

ref

249 250 251 252 253

method and reaction conditions

Na2PO2 NaH2PO2

8. APPLICATIONS OF GROUP VIII 3D METAL (FE, CO, AND NI) SULFIDE, SELENIDE, AND PHOSPHIDE NANOSTRUCTURES IN WATER SPLITTING Before the discussion of the applications of sulfide, selenide, and phosphide nanostructures of Fe, Co, and Ni, it would be useful to elaborate the most common problems that are faced during the application of nanostructured catalysts in water splitting and some recent trends of fabricating electrodes using these nanostructured catalysts for water splitting. 8.1. Common Problems Encountered with Nanostructured Catalysts in Water Splitting. One of the major problems encountered with nanostructured catalysts is achieving reasonable stability without compromising the catalytic activity. The sources of stability failure can include the binder used to tether the nanostructured catalysts to the substrate/current collector, the pH at which electrolysis is carried out, high surface energy of the nanomaterials, a decrease in conductivity due to overoxidation of the catalyst (in the case of the OER), and the effect of loading on the conductivity. 8.1.1. Binders Used To Tether the Nanostructured Catalysts and Associated Demerits in Water Splitting. In general, the binder is chosen on the basis of the medium in which the electrolysis is carried out. Since the most reactive HER and OER catalysts are found to catalyze these electrochemical reactions at either extremely low pH (strongly acidic) or extremely high pH (strongly alkaline), the most obvious choice is to go with a binder that can offer excellent proton transfer in acidic solutions and good hydroxide conductivity in alkaline conditions. The common commercial binder used for this purpose is Nafion, which is actually a perfluorinated alkyl sulfonate ionomer that is available at various concentrations ranging from 5% to 40% as a solution in water or a methanol/propanol mixture. The combination of the very high fluorine content in the carbon backbone and its high hydrophobicity with the sufficiently high polarity and hydrophilicity of the sulfonate functional group helps the protons to get transferred from the aqueous solutions to the hydrophobic electrode surfaces easily. As a consequence of these advantages, Nafion has been widely used as a binder for the HER and OER under acidic conditions. However, when the electrolysis is performed under alkaline conditions, this proton transfer ionomer fails to deliver the same efficiency as it did in the acidic medium. As a result, the resistance of the catalyst− electrolyte interface is considerably increased, which leads to a drastic degradation in the catalytic activity and affects the stability of the nanostructured-catalyst-modified electrodes. However, the same 5 wt % Nafion solution with additives such as propyl alcohol or isopropyl alcohol in water in the volume ratio of 0.5:2.0:7.5 is being used as a binder for many nanostructured catalysts in alkaline water electrolysis. In this case, the hydroxide conductivity is believed to occur via the water channels available on the thin film formed while drying this binder solution with the desired catalyst on the substrate electrode surface. Although this modification makes the Nafion applicable in alkaline water electrolysis, the serious problem associated with it is the hydration of the thin films formed during drying upon prolonged exposure to strongly acidic and alkaline conditions. This results in a gradual increase in the thickness of the catalyst layer on the substrate electrode, leading to increased catalyst resistance, which in turn decreases the

W-doped NixP microspheres FexCo1−xP nanowire array 46 47

NiSO4 FeCl3·6H2O, Fe(NO3)3·9H2O, CoCl2·6H2O, and Co(NO3)2·6H2O

source of phosphorus material and morphology

Se-doped NiP2 Ni2P NP films@Ti nanocrystalline Ni5P4 Ni5P4 Films Ni12P5 nanoparticles 41 42 43 44 45

s. no.

Table 4. continued

Ni(NO3)2·6H2O Ni(NO3)2·6H2O Ni(acac)2·xH2O Ni foil Ni(Ac)2·4H2O

metal precursor

red phosphorus NaH2PO2 trioctylphosphine oxide red phosphorus triphenylphosphine

heated at 500 °C for 30 min in Ar flow atmosphere heated at 300 °C for 1 h in a static Ar atmosphere solvothermal method at 390 °C for 1·5 h heated at 550 °C for 1 h in inert atmosphere solvothermal method at 390 °C for for 30 min under N2 solvothermal method at 80−90 °C for 2 h heated at 300 °C for 2 h under Ar flow

ACS Catalysis

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ACS Catalysis

their counterparts, and when they are exposed to such a high applied electric potential field, there is a fair chance of agglomeration with nearby particles. This will ultimately reduce the overall number of available active sites compared with the initial stage of the catalytic process. Besides agglomeration, there are chances for overoxidation of the nanomaterial surface due to these higher surface energies in cases where the agglomeration of nearby particles is restricted by high dilution of the catalyst concentration. As a consequence of overoxidation of the catalyst surface, the resistance associated with the catalyst will also increase, which will ultimately result in reduced catalytic activity. Efforts should be made to avoid all of the above problems associated with nanostructured catalysts by optimizing the experimental conditions with care. 8.2. Recent Trends in Electrode Fabrication with Nanostructured Catalysts for Prolonged Water Electrolysis. All of the stability problems elaborated in the previous section can be overcome by simple and brilliant electrode fabrication methods. The most stable nanostructured-catalystmodified interfaces that have recently been reported make use of one of the following two methods: electrochemical deposition or hydrothermal/solvothermal growth of nanostructured catalysts on the desired substrate materials. 8.2.1. Electrochemical-Deposition-Assisted Improvement in the Stability of Nanostructured Catalysts. Obviously, it is more advantageous to support the catalysts on the desired current collectors by electrochemical deposition than to prepare the same by other methods and depend on suitable binders. However, this method is not always suitable for all kinds of materials. It is most reported for metal-, metal oxide-, and metal hydroxide-based nanostructures. Additionally, there are reports for sulfides and selenides too. The common substrates employed are metal foils such as Ti and Cu, metals such as Ni in the form of Ni mesh and Ni foam, indium-doped tin oxide (ITO), fluorine-doped tin oxide (FTO), carbon fiber paper, and carbon cloth. Depending on the material, any of the above substrates can be used for electrodeposition. For example, in photoelectrocatalytic water splitting, ITO and FTO are the most obvious choices of substrates. The major advantages of this method are the ability to monitor the quantity of deposited catalyst and the control over the catalyst film thickness. The disadvantages are the inefficiency in producing nanomaterials with various other morphologies and the requirement for sophisticated instruments such as electrochemical workstations. 8.2.2. Hydrothermal- or Solvothermal-Growth-Assisted Improvement in the Stability of Nanostructured Catalysts. This method has recently been explored by materials scientists, and it is suitable for almost all sort of materials. In this method, the substrate on which the catalysts must be grown is taken along in the hydrothermal/solvothermal vessel after necessary pretreatments. The most common substrates are Ti foil, Ni foam, carbon fiber paper, and carbon cloth. These methods offer the advantage of preparing catalysts with various morphologies of a hierarchical nature, which enables some special studies such as morphology- and shape-selective catalytic studies. The main disadvantage of this method is the lack of control over the quantity of the catalyst grown on the desired substrate. Both methods are efficient in fabricating highly stable 3D catalyst electrodes and should be chosen according to the material of interest. 8.3. Sulfides of Fe, Co, and Ni in Water Splitting. Having discussed the methods of evaluating an electrocatalyst

catalytic activity and affects stability of the catalyst directly. To overcome this problem, attention has now been diverted toward some anion exchange ionomers made up of some indole-based polymers and ionic liquids. However, these are quite expensive, and the efforts that have been made to reduce the overall expenses associated with water electrolysis by making these nanostructured catalysts would become meaningless if we must rely on a costly anion exchange ionomer to attain reasonable stability with our catalysts. Other than these, it is quite common to see reports with some other binders such as poly(tetrafluoroethylene) (PTFE), poly(vinyl pyrrolidine) (PVP), poly(vinyl alcohol) (PVA), N-methylpyrrolidone (NMP), and dimethylformamide (DMF). However, except for DMF the other binders listed above are nonconductive and require a conductive additive such as carbon black in equal proportion to the catalyst material. This could lead to wrong interpretation in assessing the catalytic activity of the desired catalyst by either partially masking the active sites in the catalyst or additionally contributing to the overall activity. Though DMF does not require any such conductive additive, the stability of the catalyst-modified surface is poor compared with other binders, particularly when it is subjected to the accelerated degradation test and long-term galvanostatic or potentiostatic electrolysis. Very recently, in this concern of overcoming the problems associated with the binders, we have exploited the use of DNA molecular self-assemblies, which are cheaper than these anion and proton exchange ionomers, by anchoring of the ultrasmall Pt NPs for the HER under acidic conditions and IrO2 NPs for the OER under alkaline conditions.17,259 These Pt NP-anchored DNA molecular selfassemblies have shown extreme stability and increased catalytic performance over a commercial Pt/C catalyst under identical experimental conditions.17 The mechanism of binding with DNA molecular self-assemblies is mainly dependent on the electrostatic interactions between the DNA molecules and various charged and polarized entities during polarization of the electrode in both the anodic and cathodic directions; apart from these, the catalyst resistance is not increased by the conductivity associated with the DNA as a consequence of free electron movements by resonance among the purine and pyrimidine bases in their backboned. This study now has opened up pathways utilizing other biomolecules with charged and polarized entities, such as amino acids, peptides, proteins, and polysaccharides, as binders for other materials in the future. 8.1.2. pH of the Medium of Water Electrolysis. The medium of water electrolysis plays a crucial role in water electrolysis, as elaborated earlier under various categories. Here the effect of the pH of the water electrolysis solution on the stability of the nanostructured catalyst is discussed. We have seen that at under strongly acidic or alkaline conditions, nonnoble-metal catalysts such as the ones considered in this review are highly prone to corrosion, which affects the stability directly. To overcome this issue, many catalysts have now been designed to be active at neutral pH. In this regard, the metal phosphides, sulfides, and selenides have more advantages than the metals, metal oxides, and metal hydroxides, as the oxides of S, Se, and P are highly resistive toward corrosion. Hence, these materials find very limited stability problems due to the solution pH of water electrolysis. 8.1.3. High Surface Energy and Overoxidation of the Catalyst Surface. These are two other serious problems that reduce the stability of catalysts for water electrolysis. It is known that the nanomaterials have higher surface energies than 8086

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ACS Catalysis for water splitting and the synthesis of the catalyst materials, we now proceed to a discussion of benchmarking all of the reported sulfides of Fe, Co, and Ni with respect to the overpotential at 10 mA/cm2 (η10) and Tafel slope values under two categories, viz., metal sulfides for the HER and metal sulfides for the OER. The activity parameters for total water splitting catalysts are split up and included along with other catalysts used for the HER and OER. The same trend will be followed for the selenides and phosphides too. As far as total water splitting is concerned, no Fe-based sulfides have been reported to date. There are only few reports on total water splitting by these metal (Fe, Co, and Ni) sulfides. The report by Wang et al.168 on CoS supported on CTs that in turn were grown on CC is the only one on the total water splitting activity of a Co-based sulfide. However, the literature says that the Nibased sulfides are better bifunctional catalysts than the Fe- and Co-based ones. NiS microspheres grown on Ni foam and Ni3S2 nanosheets grown on Ni foam are good in catalyzing both the HER and OER under alkaline conditions.171 However, a significant advancement in the bifunctional activity of these sulfides is achieved when they are present as bimetallic sulfides. The electrodeposited Ni−Co−S reported by Liu et al.185 and the Ni-promoted formation of CoS2 NW arrays on CC as reported by Fang et al.186 are examples of this kind. 8.3.1. Activity Trends of Metal (Fe, Co, Ni) Sulfides in the Electrocatalytic HER. The available reports on the sulfides of Fe, Co, and Ni clearly reveals that they have been employed more for the HER than for the OER and total water splitting. It has been highlighted in the synthesis section that to date there have been no reports on Fe-based sulfides except the one on FeS for the HER by Di Giovanni et al.162 and the one on pyritetype FeS2 for the HER by Faber et al.164 On the other hand, literature reports on Co-based sulfides are more abundant than for both Fe and Ni. Interestingly, almost 80% of the available reports on Co-based sulfides were studied for the HER, and the reports for the OER and total water splitting are limited. Among the various Co-based sulfides such as CoS, CoS2, Co3S4, and Co9S8, CoS and pyrite-type CoS2 are the ones frequently reported for the HER. In the case of Ni-based sulfides, there are only two reports for the HER, namely, the report on NiS2 by Faber et al.164 and the report on Ni3S2 by Tang et al.187 Apart from these, there are reports on the HER activities of bimetallic sulfides such as Co−Fe−S, Co−Ni−S, Fe−Ni−S, and Ni−Co− S. To have a comparative view of the catalytic activities of these sulfides toward the HER, they have been benchmarked with respect to their overpotential at 10 mA/cm2 and Tafel slope, as shown in Figure 9. More information on the electrochemical conditions under which these data were acquired and others are provided in Table 5. From Figure 9 it is clear that the FeNiS catalyst reported by Long and co-workers is the best catalyst with the lowest HER η10 of 105 mV. In Table 5, other catalysts are listed in order of increasing η10. Since there had been a serious discussion of the selection of activity parameters to evaluate an electrocatalyst’s efficiency for water splitting, it is fruitful to arrive at a conclusion whether the use of η10 with the Tafel slope as the primary activity parameter is good for evaluating catalysts. Figure 9 clearly reveals that the overpotential has nothing to do with the kinetics of the electrocatalytic water splitting process, as we can see that some catalysts with low η10 have larger Tafel slopes than some catalysts with relatively larger η10, such as CoS|P/CNT reported by Liu and co-workers,260 which showed an overpotential of 480 mV at 10 mA/cm2 with a Tafel slope of 55 mV/dec. This

Figure 9. Benchmarking the metal (Fe, Co, Ni) sulfides with respect to the HER overpotential at 10 mA/cm2 and the corresponding trend in the Tafel slopes.

primarily indicates that the thermodynamic activity parameter (i.e., the overpotential) does not affect or only slightly affects the kinetic activity parameter (i.e., the Tafel slope). Similar trends are also observed with selenide- and phosphide-based catalysts for both the HER and OER, as discussed in the subsequent sections. 8.3.2. Activity Trends of Metal (Fe, Co, Ni) Sulfides in the Electrocatalytic OER. Although the sulfides of Fe, Co, and Ni have been devoted more toward the HER, there are also reports available on the OER catalytic performance of these sulfides, particularly the sulfides of Co and Ni. There is no data on the OER and total water splitting activities of Fe-based sulfides alone. However, Fe has been reported by Shen and coworkers163 as a bimetallic sulfide with Co as Co−Fe−S on Ndoped mesoporous carbon for the OER along with the ORR. The story with Co-based sulfides is different from that of Febased sulfides. Though Co-based sulfides have also been much devoted to the HER, there are reports on their OER and total water splitting activities. Another thing to be noticed here is that wherever a Co-based sulfide is applied to the OER or total water splitting, it is always applied with a substrate material such as a carbon nanostructure or Ti foil. Among the reported catalysts are Co3S4 on N-doped CNT by Wang et al.,173 CoS nanosheets grown on Ti by Liu et al.,262 and CoS2 grown on Nand S-codoped GO by Ganesan et al.172 In the case of Ni, there is a lone report on Ni3S2 nanosheets grown on Ni foam that showed a low overpotential for the OER under alkaline conditions. Other than these, bimetallic NiCo2S4@graphene has been reported by Liu et al.184 for the OER and ORR under alkaline conditions. As for the HER, as a comparative measure the plots of η10 and the Tafel slope are shown in Figure 10, and the data are listed in Table 6. As we have seen for the HER, the activity trends of these metal sulfide catalysts benchmarked with respect to η10 and the Tafel slope follow an irregular trend in terms of kinetics. This again implies the same conclusion that the Tafel slope (kinetics) has no direct relation to the overpotential (thermodynamics) of a water splitting catalyst. 8.4. Selenides of Fe, Co, and Ni in Water Splitting. The irregularities seen in the trends of selenides of Fe, Co, and Ni are more vigorous for both the OER and HER than those observed with the sulfides. This indicates the strong influence of a ligand with an ionic radius comparable to that of the central metal atom, which belongs to the 3d series of the periodic table with 4s shells. However, the best correlations among these metal selenides are given for a comparative evaluation of the HER and OER activity trends via the benchmarking plots of η10 8087

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ACS Catalysis Table 5. Benchmarking the Metal (Fe, Co, Ni) Sulfides with Respect to the HER Overpotential at 10 mA/cm2 (η10)

a

s. no.

catalyst

electrolyte

η10 (mV)

Tafel slope (mV/dec)a

ref

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

A-FeNiS B-FeNiS CoS2 Ni2.3%CoS2/CC Co3S4/NCNTs NiCoS/CC NSs CoS2 NS/RGO/CNT (Fe0.48Co0.52)S2 NiS/Ni Foam Ni2.3%CoS2/CC Fe0.9Co0.1S2/CNT Co3S4 Co9S8−NixSy/NiF (Co0.59Ni0.41)S2 CoS2/Ti (pH 0.3) NiCo2S4 NA/CC Ni3S2@Ni Fe0.07Ni0.91S2 NiMoS/C 1:1 Zn0.76Co0.24S/CoS on Ti mesh B-NiS FeS2 NiS2 NixSy/NiF MW-CoS (nanoprism) ST-CoS (nanoprism) NiS on CC CoS2/Ti (pH 13.37) Fe0.1NiS2 NA/Ti Ni3S2−Ni Co9S8/C CoS2 NS/RGO 3D G/CoSx Co9S8 (all pH) FeS (pH 7) Co9S8-700 CoS|P/CNT

0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 1 M KOH 0.5 M H2SO4 1 M KOH 0.5 M H2SO4 0.5 M H2SO4 1 M KOH 1 M KOH 0.5 M H2SO4 0.5 M H2SO4 1 M KOH 0.5 M H2SO4 0.5 M H2SO4 1 M KOH 1 M KOH 0.5 M H2SO4 pH 7 1 M KOH 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 1 M KOH 0.5 M H2SO4 0.5 M H2SO4 pH 7 0.5 M H2SO4 1 M KOH 1 M KOH phosphate buffer 0.5 M H2SO4 1 M PBS phosphate buffer 0.1 M phosphate buffer phosphate buffer 0.5 M H2SO4

105 117 128 136 140 140 142 143 150 150 160 160 163 170 190 ∼190 195 196 200 ∼200 202 217 230 230 230 240 243 244 ∼250 270 280 280 330 340 350 370 480

40 48 52 106 70 96 51 47.5 83 106 46 79 88 50.4 72 141 107 58.7 85.3 164 − 56.4 48.8 87 76 90 69 133 108 141 − 82 93 − 150 − 55

190 190 192 235 189 177 171 164 181 186 190 173 188 170 170 187 178 164 191 175 190 164 164 188 175 175 179 183 180 178 174 171 176 174 162 174 260

Dashes indicate that the corresponding data are not available in the respective reports cited here.

selenides for the HER. However, there are some bifunctional water splitting bi- and trimetallic selenides. On the other hand, the most reported Co-based selenide, CoSe2 with various morphologies and substrate materials, has itself shown different trends in the HER catalytic performance. The same can be seen when they are benchmarked with respect to η10 and Tafel slope, as shown in Figure 11. As all of them are the same material, it is hard to make a conclusion about the trend of the HER activity of 3d group VIII metal (Fe, Co, and Ni) selenides. However, the Co0.13Ni0.87Se2/ Ti is the one with the lowest overpotential of 64 mV at 10 mA/ cm2 for the HER among all other selenide-based HER catalysts, as reported by Liu and co-workers.262 The selenides of the group VIII 3d metals that have been reported to date are listed in Table 7 in increasing order of η10 along with the experimental conditions under which the evaluations were made. As observed with the sulfides, the selenides show similar trends in the correlation between the thermodynamic activity parameter (the overpotential) and the kinetic activity parameter (the Tafel slope). This once again emphasizes the independ-

Figure 10. Benchmarking the metal (Fe, Co, Ni) sulfides with respect to the OER overpotential at 10 mA/cm2 and the corresponding trend in the Tafel slopes.

and Tafel slope in Figures 11 and 12 and the data in Tables 7 and 8, respectively. 8.4.1. Activity Trends of Metal (Fe, Co, Ni) Selenides in the Electrocatalytic HER. As we highlighted in the synthetic methodologies part, there are no reports on Ni- and Fe-based 8088

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ACS Catalysis Table 6. Benchmarking the Metal (Fe, Co, Ni) Sulfides with Respect to the OER Overpotential at 10 mA/cm2 (η10)

a

s. no.

catalyst

electrolyte

η10 (mV)

Tafel slope (mV/dec)a

ref

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Fe0.1NiS2 NA/Ti Ni2.3%CoS2/CC NiS/Ni foam Zn0.76Co0.24S/CoS on Ti mesh Ni2.3%CoS2/CC Ni3S2@Ni NiCoS/CC NSs NiCo2S4 NA/CC NiCo2S4@N/S-RGO CoS2/N−S GO ALD NiSx Co3S4 Ni3S2−Ni CP/CNT/CoS Ni3S4

1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 0.1 M KOH 1 M KOH 1 M KOH 0.1 M KOH 1 M KOH 1 M KOH 0.1 M KOH 0.1 M KOH 1 M KOH 0.1 M KOH

∼205 297 300 ∼300 ∼300 330 330 340 355 370 372 375 410 450 555

43 117 89 79 119 163 109 89 − − 41 − 330 − −

180 262 179 175 186 178 177 187 184 172 182 184 178 168 184

Dashes indicate that the corresponding data are not available in the respective reports cited here.

expected, as similar irregular trend in the Tafel slopes is seen when these selenide-based catalysts are queued according to increasing overpotential. This too emphasizes again the independence of the overpotential from the nature of the HER kinetics and vice versa. 8.5. The Phosphides of Fe, Co, and Ni in Water Splitting. Similar to the sulfides and selenides, the Activity trends of the phosphides of Fe, Co, and Ni are comparative evaluated by plotting the overpotential against the phosphide catalysts in the increasing overpotential order as shown in Figure 13 and Figure 14 and the experimental conditions under which the evaluation were made are listed in Table 9 and Table 10 for HER and OER respectively. 8.5.1. Activity Trends of Metal (Fe, Co, Ni) Phosphides in the Electrocatalytic HER. Unlike the selenides, the phosphides of Fe, Co, and Ni have been equally reported for the HER. Where the HER is concerned, the maximum number of reports (∼40) have been published using the phosphides. The sulfides come second, and the number of selenide-based HER catalysts is minor compared with the sulfides and phosphides. The plots of η10 and the Tafel slope for the metal phosphides for the HER are shown in Figure 13, and Table 9 provides the experimental details under which the measurements of the HER activities of these phosphide catalysts were made. From Figure 13 and Table 9 an interesting observation can be made: lower HER overpotentials at 10 mA/cm2 were shown predominantly by the Fe-based phosphides compared with the Co- and Ni-based phosphides. This is in sharp contrast to the HER activity trends of the sulfides and selenides of Fe, Co, and Ni. For the sulfides and selenides, either Co or Ni or a mixture of Co and Ni occupied the top position in the table of HER and OER activity trends, in accordance with the activity trends of 3d metal oxides and hydroxides predicted by Subbaraman and co-workers earlier.150 However, there are some specific phosphides of Ni and Co with specific P contents, such as Ni5P4, NiP2, and CoP, that showed good HER performance comparable to that of the Febased phosphides. As observed with the sulfides and selenides, the phosphide catalysts ranked by the overpotential at 10 mA/ cm2 show an irregular trend in the Tafel slopes, as catalysts with low overpotentials were found to show higher Tafel slopes and vice versa. This indicates to us that the phosphides are not exceptions and that they too have independent thermody-

Figure 11. Benchmarking the metal (Fe, Co, Ni) selenides with respect to the HER overpotential at 10 mA/cm 2 and the corresponding trend in the Tafel slopes.

Figure 12. Benchmarking the metal (Fe, Co, Ni) selenides with respect to the OER overpotential at 10 mA/cm 2 and the corresponding trend in the Tafel slopes.

ence of the overpotential from the nature of the HER kinetics and vice versa. 8.4.2. Activity Trends of Metal (Fe, Co, Ni) Selenides in the Electrocatalytic OER. As with the HER catalysis, the OER catalysis by these metal selenides is mainly occupied by CoSe2 and other Co-based sulfides. Some of the significant works include the CoSe oxohalides reported by Rabbani et al.,263 the coral-like 3D CoSe2 crystals reported by Liao et al.,202 and the CeO2-doped CoSe2 reported by Zheng et al.204 For NiSe2 there is a report by Tang et al.193 As we did for the HER, benchmarking plots of the activity parameters (i.e., η10 and the Tafel slope) of these selenides are provided in Figure 12, and other relevant data on the same are included in Table 8. As 8089

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ACS Catalysis

Table 7. Benchmarking the Metal (Fe, Co, Ni) Selenides with Respect to the HER Overpotential at 10 mA/cm2 (η10)

a

s. no.

catalyst

electrolyte

η10 (mV)

Tafel slope (mV/dec)

ref

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Co0.13Ni0.87Se2/Ti MoS2/CoSe2 electrodeposited Ni3Se2 NiSe/NF Ni3Se2/CF Ni/NiO/CoSe2 NiPSe (1.93:0.07) CoSe2−NW/CC CoSe2/CP NiPSe(0.09:1.91) NiSe2 CoSe2-MP/CC EG/CoSe-NiFe-LDH Co0.13Ni0.87Se2/Ti CoPSe/MWCNT/GCE

1 M KOH 0.5 M H2SO4 1 M KOH 1 M KOH 1 M KOH 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 1 M KOH 1 M KOH 0.5 M H2SO4

64 68 70 96 100 100 100 130 137 170 175 193 260 270 370

63 36 82 120 98 39 43 32 40 44 33 50 −a 94 46

262 105 195 193 196 197 249 205 198 249 249 205 121 262 117

Dash indicates that the corresponding data are not available in the respective reports cited here.

Table 8. Benchmarking the Metal (Fe, Co, Ni) Selenides with Respect to the OER Overpotential at 10 mA/cm2 (η10)

a

s. no.

catalyst

electrolyte

η10 (mV)

Tafel slope (mV/dec)

ref

1 2 3 4 5 6 7 8 9

EG/CoSe-NiFe-LDH coral-like CoSe2 electrodeposited Ni3Se2 CoSe2/Ti mesh Ni3Se2/CF CeO2/CoSe2 CoSe2 NPs NiSe/NF Mn3O4−CoSe2

1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 0.1 M KOH

250 290 290 292 ∼300 310 350 400 450

57 40 82 69 80 44 49 −a 49

121 172 195 199 196 204 172 193 185

Dash indicates that the corresponding data are not available in the respective reports cited here.

namics and kinetics of water splitting. However, the shape of the Tafel slope line in Figure 13 is showing merely a linear relation between the overpotential and the Tafel slope values, unlike the sulfides and selenides. 8.5.2. Activity Trends of Metal (Fe, Co, Ni) Phosphides in the Electrocatalytic OER. Application of phosphides of Fe, Co, and Ni to the OER is new, and there are relatively few reports compared with the number of reports available on the same for HER applications. However, here we have made similar comparative evaluation of the available reports on the metal phosphides for OER applications via the benchmarking plots of η10 and the Tafel slope shown in Figure 14 along with Table 10 carrying information on the experimental conditions under which the results on the OER activities of these metal phosphides were acquired. As expected, there is an irregular trend in the Tafel slopes when these catalysts are queued in increasing order of their overpotentials at 10 mA/cm2. Hence, the same conclusion of the independence of the kinetics of an electrocatalyst from the thermodynamics of the catalyst can be drawn. 8.6. Activity Trends of Metal (Fe, Co, Ni) Phosphosulfides and Phosphoselenides in Electrocatalytic HER and OER. Beyond the sulfides, selenides, and phosphides alone as electrocatalysts, people have now moved forward in combining the effects of two different ligands such as P and S and P and Se together with a single metal among Fe, Co, and Ni. The reports where such kinds of materials have been employed for the HER, OER, and total water splitting to date are summarized here. Pyrite-structured CoPS for the HER was

Figure 13. Benchmarking the metal (Fe, Co, Ni) phosphides with respect to the HER overpotential at 10 mA/cm 2 and the corresponding trend in the Tafel slopes.

Figure 14. Benchmarking the metal (Fe, Co, Ni) phosphides with respect to the OER overpotential at 10 mA/cm 2 and the corresponding trend in the Tafel slopes. 8090

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ACS Catalysis

Table 9. Benchmarking the Metal (Fe, Co, Ni) Phosphides with Respect to the HER Overpotential at 10 mA/cm2 (η10)

a

s. no.

catalyst

electrolyte

η10 (mV)

Tafel slope (mV/dec)a

ref

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Ni5P4 FeP/CC nanocrystal (300 °C) Fe0.5Co0.5P/CC NixPy CoP NPs/CC Ni5P4 CoP MNA Ni2P−G@NiF FeP NAS/CC nanoporous CoP CoP/Ti mesh NiP2−NS/CC 3D CoP NWs on Ti CoP-NBAs/Ti CoP/NCNT CoP branched NS on Ti Foil Ni2P/Ni/NF Ni2P−G@NiF CoP NWs FeP/NCNT NiP2−NS/CC CoP@NPC Ni5P4−Ni2P NS CoP/CNT CoP/C CoP NTs Ni2P-NRs-Ni Fe2P/NGR NiCoP hollow NCs Co2P/NCNT Ni2P/NF CoP hollow polyhedra Ni2P−G@NiF CoP NSs CoP/CNT CoP CoP@C Co2P/CNT CoP NPs CoP/RGO (0.30) CoP Co2P

0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 1 M NaOH 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 pH 0−14 1 M KOH 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 H2 sat. 1 M KOH 1 M KOH 0.5 M H2SO4 0.5 M H2SO4 1 M KOH 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.1 M KOH 0.5 M H2SO4 N2 sat. 0.5 M H2SO4 0.5 M H2SO4 1 M KOH 0.5 M H2SO4 H2 sat. 1 M KOH 0.5 M H2SO4 1 M KPi 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 Ar sat 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4

24 34 39 40 48 50 54 55 58 67 72 75 ∼80 90 92 98 98 100 110 113 110 120 120 123 130 130 130 138 145 148 150 159 160 164 165 168 170 195 221 240 383 406

27 29.2 30 46.1 70 14 51 30 45 51 65 51 65 40 49 − 72 30 54 59 64 69 79.1 54 − 60 106.1 − 60.6 62 93 59 40 61 68 46 61 74 87 104.8 90 101

251 214 222 213 221 251 229 245 212 218 221 250 222 227 236 224 242 245 233 162 250 115 246 140 234 220 248 211 254 236 242 225 245 233 236 226 238 236 233 236 236 236

Dashes indicate that the corresponding data are not available in the respective reports cited here.

Table 10. Benchmarking the Metal (Fe, Co, Ni) Phosphides with Respect to the OER Overpotential at 10 mA/cm2 (η10)

a

s. no.

catalyst

electrolyte

η10 (mV)

Tafel slope (mV/dec)a

ref.

1 2 3 4 5 6 7 8 9 10

Ni2P/Ni/NF Ni2P/NF Ni2P Co2P NPs CoP/Ti mesh CoP MNA Ni5P4 CoP/C CP@FeP CoP hollow polyhedra

O2 sat. 1 M KOH O2 sat. 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 1 M KOH 0.1 M KOH 1 M KOH 1 M KOH

205 220 300 310 310 310 330 358 365 400

− − 64 50 87 65 − 66 63.6 57

242 242 248 222 221 229 252 234 209 225

Dashes indicate that the corresponding data are not available in the respective reports cited here.

8091

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ACS Catalysis very recently reported by Liu and co-workers.261 Similarly, Xiao and co-workers reported exceptional HER performance of Co− P−Se NPs.117 Another interesting report was recently published by Zhuo and co-workers on the HER activity of Se-doped pyrite-type NiP2.250 These catalysts are included with the sulfide and selenide benchmarking plots (Figures 9 and 11) for comparative evaluation, and the respective experimental details are included in Tables 5 and 7, respectively. These kinds of materials are quite promising and can offer new pathways to increase the catalytic activities of these sulfides and selenides. Having seen a detailed comparative evaluation by means benchmarking plots and the trends observed with the Tafel slopes, it is now essential to compare the best HER catalysts among the sulfides, selenides, and phosphides as well as the best OER catalysts among the sulfides, selenides, and phosphides. Such comparative plots are provided in Figure 15, from which we can conclude that the phosphides of Fe, Co,

of the lower values of Tafel slopes (approximately up to 55 mV/dec) were seen when the studies were carried in acidic solution. This again indicates that although there are potential catalysts being developed for the HER in alkaline media, there is no catalyst with better kinetics than observed in acidic HER electrocatalysis. One more thing that needs to be emphasized here in the benchmarking of catalysts is that the lowest overpotential is shown by fabricated electrodes such as nanoarrays on various supports such as Ti mesh, CC, CF, Ni foam, and carbon paper. In these cases the catalyst loading is normally 5−10-fold higher than for the normal nanocatalysts used in other studies, which leads us to misunderstand that they are better than the other studies when actually they are not. This again emphasizes the conclusion that the mass-normalized current density would be better than the geometrical-area- or ECSA-normalized current density for a fair comparison of different catalysts. Having reviewed intensively the current perspectives in electrochemical water splitting catalysis, we recommend that the following activity parameters be compulsorily incorporated in future reports of any such electrocatalysts for the OER and HER: overpotential at a defined current density normalized by the geometrical area and the mass of loaded catalyst, the Tafel slope, the exchange current density (for the HER), the mass activity, and the loading.

9. SUMMARY AND OUTLOOK The increasing pressure due to the faster depletion of conventional fossil fuels and the negative impacts of the same on the environment have urged the research community to find alternative and viable sources of energy to enlighten the decendants of humankind in the near future. As per the recent studies of energy consumption and the availability of sources, by 2015 the world will require 30 TW of power from new and nonconventional energy sources.1 Though energies harvested from solar, wind, tide, and other similar natural and endless resources seem to be the most promising future ways of energy generation, the major problem associated with those resources is that they are seasonal and hence require large-scale energy storage systems beyond the incapable batteries and supercapacitors. One such indirect way of large-scale energy storage is water splitting. As we are aware that conventional combustion engines running on conventional carbon fuels are badly contaminating the environment, which is considered as the most serious threat for humankind in the near future, people have now diverted their attention to new zero-carbon-emitting and/or less-carbonemitting engines such as fuel cells. Fuel cell technology is a greener available way of generating electrical energy from various sources such as hydrogen, oxygen, methanol, glycerol, borohydride, formic acid, and more as fuels. Among them, the fuel cell that uses H2 and O2 as fuel is the greenest way to produce electrical energy via a simple electrochemical redox reaction between them that produces water as the byproduct. The efficiency of these fuel cells can be as high as possible if the H2 supplied is the purest. The conventional coal reforming technology for H2 production is certainly not the one that gives the purest H2. The H2 obtained by coal reformation contains a considerable quantity of CO which is a serious threat to the cathodes of fuel cells, as it will poison them by making strong carbonyl coordination compounds with Pt atoms on the catalyst’s surface, thereby reducing the number of active sites available for the desired electrochemical reaction. Great

Figure 15. Benchmarking plots of the best-ever metal (Fe, Co, Ni) sulfide, selenide, and phosphide (a) HER and (b) OER catalysts according to their overpotentials at 10 mA/cm2.

and Ni are better catalysts than the sulfides and selenides of the same for both the HER and OER. This offers us new pathway to modify the catalytic properties of these materials by tuning the heteroatom (S, Se, P) content and/or mixing them with a single metal atom among Fe, Co, and Ni or with a bi- or trimetallic alloy of the same. With the given comparative view, the knowledge of the activity trends of the Fe-, Co-, and Nibased sulfide, selenide, and phosphide catalysts is now clearly exposed. Hence, we can recommend that beyond only the overpotential and the Tafel slope, other activity parameters such as mass activity, specific activity and TOF are to be determined to elucidate the real catalytic efficiency of an electrocatalyst for water splitting. Careful investigation of Tables 5, 7, and 9 reveals that the anomaly observed in the Tafel slope values with respect to the overpotentials is mainly due to the pH of the medium of electrolysis. It is clear that all 8092

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attention has now been given to water electrolysis for the production of H2 in the purest form. However, there are several thermodynamic and kinetic barriers in splitting water electrochemically, and they cause a huge energy loss in this process, which increases the overpotentials of the HER and OER at the cathode and anode further. Hence, both the HER and OER need catalysts. In earlier days, for HER the Pt electrode was used as the state-of-the-art electrocatalyst, and it is still so used. Similarly, for the OER either Ir or Ru and their compounds are used as the state-of-the-art catalysts. However, we know that they are noble, expensive, and less abundant metals. Hence, we cannot afford such expensive materials for simple water electrolysis, during which there are potential chances for losing the materials by corrosion as the OER is the process that can occur at very high anodic overpotential compared with the HER. To avoid such pitfalls, people have started looking at the 3d transition metal oxides and hydroxides for OER electrocatalysis and the chalcogenides of W, Mo, Ta, and Ti for HER electrocatalysis. Very recently, people have found that the sulfides, selenides, and phosphides of 3d transition metals, particularly the group VIII metals (Fe, Co, Ni), can catalyze both the HER and OER at all pH with almost the same kinetics. The advantages of these 3d-transition-metal-based catalysts are that they highly abundant, cheap, efficient, and easy to design in a desired shape and structure depending on the needs for electrolysis. Hence, to make use of these advantages, we should be aware of the activity trends and mechanisms of the OER and HER on the surfaces of these group VIII 3d metal (Fe, Co, Ni)-based sulfide, selenide, and phosphide catalysts. In this review, we have made such a comparative measurement of group VIII 3d metal (Fe, Co, Ni)-based sulfide, selenide, and phosphide catalysts to enable those who are working on increasing the efficiencies of these catalysts for H2 production by water electrolysis with little loss in electrical energy via designing catalysts with low overpotentials for both the HER and OER. This comparative review enables researchers to acquire knowledge of the trends in the activities of these catalysts toward the HER and OER and helps them to formulate new highly efficient water splitting catalysts for the sake of our global future energy requirements.

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Review

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] and subrata_kundu2004@yahoo. co.in. Fax: +91 4565-227651. Tel: +91 4565-241487. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Dr. Vijayamohanan K. Pillai, Director, CSIRCECRI, Karaikudi, India, for his continuous support and encouragement. S.A., S.R.E., and K.S. acknowledge CSIR, New Delhi, and K.K. acknowledges UGC, New Delhi, for the afforded funding through the Senior and Junior Research Fellowship (SRF and JRF) schemes. We also acknowledge The Royal Society of Chemistry and The American Chemical Society for the licenses given to reproduce figures from their publications. 8093

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ACS Catalysis

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ACS Catalysis

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DOI: 10.1021/acscatal.6b02479 ACS Catal. 2016, 6, 8069−8097

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