Recent Trends and Perspectives in Electrochemical Water Splitting

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Review

Recent Trends and Perspectives in Electrochemical Water Splitting with an Emphasis to Sulphide, 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 ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02479 • Publication Date (Web): 19 Oct 2016 Downloaded from http://pubs.acs.org on October 19, 2016

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Recent Trends and Perspectives in Electrochemical Water Splitting with an Emphasis to Sulphide, 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, 2011-2015), 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, TX-77843, USA. * To whom correspondence should be addressed, E-mail: [email protected] and [email protected], Fax: +91 4565-227651; Tel: +91 4565-241487. Abstract: Increasing demand for finding ecofriendly and everlasting energy source is now totally depending on the 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 cheap and more ecofriendly. Although there are more than one type of fuel cells, the hydrogen (H2) and oxygen (O2) fuel cell is the one with zero carbon emission and the byproduct of which is just water. 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 H 2 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 a greater attention than the steam reforming. The reasons are as follows: very high purity of H2 produced, the abundant source, avoids high temperature and pressure reactors and so on. In earlier days, the noble metals such as Pt (cathode), 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 have elaborated how the group VIII 3d metal sulphide, selenide and phosphide nanomaterials have been risen as an abundant and cheaper electrode materials (catalysts) beyond the oxides and hydroxides of the same. We will also highlight the evaluation perspective of such electrocatalysts towards water electrolysis in detail. 1 ACS Paragon Plus Environment

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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, ever-lasting, zero emitting and ecofriendly combined fuelcombustion technology as an alternate energy server very urgently.1 As said above, the fuel cell technology is among the ways of producing energy in a eco-friendly manner than the other existing ones. The fuel cell operating with H2 and O2 will be the choice amongst the available fuel cells as it emits water as the combustion product.2–7 Currently, the steam reforming of fossil fuels and the water electrolysis (H2O (l) → H2 (g) + 1/2O2 (g): ΔG0 = +237200 J/mol, ΔE0 = 1.23 V vs reversible hydrogen electrode (RHE)) are the ways of producing H2 to afford these fuel cells for their fuel need. Between steam reforming and water electrolysis, the first one is not environmentally friendly as it produces CO2 along with H2 and thereby reducing the purity of H2 which will in turn affect the life cycle and the cell efficiency of fuel cells.2–7 Water electrolysis is proceeded via the following two half-cell reactions. The first one is the reduction of H+ ions at cathode (2H+ (aq) + 2e− → H2 (g)) i.e., the hydrogen evolution reaction (HER)8–10 and the second one is the 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 problem less way of H2 production, 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 are needed to be replaced by the available nonnoble metals and their compounds. To do so, recently the attention are diverted towards the nonnoble metals such as Mn,44–47 Fe,48–53 Co,44,49,54–76 Ni16,47,53,76–101 based OER catalysts and Mo58,83,102–111 and W112,113 based HER catalysts. Though the production of purest H2 is the main objective of water electrolysis, we cannot neglect the counter reaction (OER) as it is the sluggish one between them and affects the faradaic efficiency of the electrolytic cell to a greater extent. HER under acidic conditions is facile as the availability of protons is plenty and it proceeds by a multi-step reaction with two possible mechanisms.114,115 The mechanism by which HER is being proceeded is usually revealed by the experimentally obtained Tafel slope value.8–10,116 The first step in the multi-step HER is discharging protons on electrode’s surface to form adsorbed 2 ACS Paragon Plus Environment

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hydrogen (Hads) which is called as the Volmer reaction (H+ (aq) + e− → Hads) and can be expressed as: b1,V = 2.303 RT / αF

(1)

where, b stands for the corresponding Tafel slope, R is ideal gas constant, T the temperature, F the faraday constant and α the symmetry factor of value 0.5. Depending on the coverage of H ads, the second step will either be the electrochemical desorption of H2 known as the Heyrovsky reaction or the 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 preferable 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 which can be expressed as: b2,H = 2.303 RT / (1+α) F

(2)

In case of high Hads coverage, two adjacent Hads will join together chemically and evolve a molecule of H2 which is the Tafel reaction and can be expressed as: b2’,T = 2.303 RT / 2F

(3)

The calculated Tafel slopes of above reactions at standard conditions are 0.118 V/dec, 0.039 V/dec and 0.029 V/dec for Volmer, Heyrovsky and Tafel reactions respectively. For 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. In alkaline conditions, the HER is comparatively more sluggish as it directly depends on the anodic OER which supplies the protons to cathode by the deprotonation of hydroxide ions and affects the HER kinetics. Under such conditions the following reaction takes place during HER at the cathode: 4e− + 4H2O (l) → 4OH− (aq) + 2H2 (g)

(4)

The protons formed by the deprotonation of hydroxides at anode get combined with the abundant OH− ions in alkaline solutions and makes the HER to struggle more to move forward further. The OER on the other hand has a different story. The kinetics of OER in acidic and alkaline media vary depending on the material by which it is being catalyzed. The noble metal catalysts such as Ir and Ru and their compounds catalyze OER in acidic medium with ease than in alkaline conditions. On the other hand the catalysts derived out of VIII group 3d metals (Fe, Co and Ni) 3 ACS Paragon Plus Environment

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catalyze the OER in alkaline medium more favorably than in acidic medium. This is mainly correlated to the mechanism by which they catalyze it. In 1986, Matsumoto and Sato13 had given a detailed review on various reported OER mechanisms in acidic and alkaline media which includes Krasil’shchkov, Bockris,46,117 Yeager and Wade & Hackerman52 pathways in addition to the most recognized electrochemical oxide pathway and the oxide pathway. In alkaline conditions, all the proposed mechanisms begin with an essential elementary step of hydroxide coordination to the active site and proceeds via different proposed other elementary steps.46,117 The kinetic barriers associated with each elementary step raise the overall overpotential required. An elementary step with the sluggish kinetics is the rate determining step (RDS).11,14 In general, the electrochemical reactions that occur at anode (OER domain) and at cathode (HER domain) in acidic and alkaline media are given as: In acid

In alkali

4e− + 4H+ (aq) → 4H2 (g)

(5)

2H2O (l) → 4H+ (aq) + 4e− + O2 (g)

(6)

4e− + 4H2O (l) → 4OH− (aq) + 2H2 (g)

(4)

4OH− (aq) → 2H2O (l) + 4e− + O2 (g)

(7)

In practice there is no electrolytic cell that performs with 100% faradaic efficiency due to various thermodynamic and kinetic hindrances. As stated above, the best catalyst that catalyze HER is the Pt and the catalysts that catalyze 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 advisable as it increases the cost of H2 production, hinders the magnification of productivity in 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 OER as efficient as Ir and Ru in alkaline conditions. Nevertheless, the discovery is not fruitful as they did not consider the counter reaction’s (HER) kinetics in alkali. Moreover, 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 out an efficient, durable non-noble OER catalysts in acidic medium as an alternate to Ir and Ru based catalysts. However, this could not even rid the expensive Pt from the job of HER. One way to rid the Pt is replacing it with another non-noble metal based catalysts such as sulphide, selenide and 4 ACS Paragon Plus Environment

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phosphides but the resulting electrolytic cell will be the kind of asymmetric one and will lead to other technical problems. The second way overcoming this pitfall is to design a bifunctional catalyst out of non-noble metals which can catalyze both OER and HER at the same time which do not need any separator. Such bifunctional catalysts made from Fe, Co and Ni are being reported frequently with parallel and better activities in some cases for both HER and OER to that of Pt, Ir and Ru. The reported such bifunctional catalysts are almost the phosphides and sulphide, selenide of Fe, Co and Ni as nanostructures in various forms. As an emerging field in energy sector, there are few review reports on many of these materials and their subsequent application to water splitting. However, all of the available review reports up to now had either been focused on one type of catalyst formed out of these three metals viz., Fe, Co and Ni or all three type catalysts formed out of a single metal. Examples for the first kind are the review papers on transition metal phosphides and chalcogenides. Meanwhile examples for the second kind are the review reports available on the heterogeneous electrocatalysis by catalysts derived out of Co, Ni and Fe where by taking a single metal, the catalytic activity trend of its oxides, sulphides, 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 sulphide, selenide and phosphide catalysts formulated out of Fe, Co and Ni would highly be desired. In this review, we will take you through a detailed survey on following topics. The parameters involved in the evaluation of catalysts for both HER and OER. Effects of hetero atoms (S, Se and P) in the material properties of the metal and in subsequent catalytic activities, basic requirements for an efficient bifunctional water electrolytic catalyst, an elaborated overview on the synthetic methodologies of metal (Fe, Co, Ni and their mixed versions) phosphides, sulphides and selenides and their applications of these materials in electrochemical HER and OER and finally the challenges ahead in developing new efficient bifunctional catalysts and opportunities will be stressed. 2. Parameters Used to Evaluate the Catalytic Activity. The following parameters are the widely recognized ones to evaluate and compare the catalytic activity of catalysts. In this part of the review, the merits and demerits associated with each of these parameters is discussed and justified.

2.1. Overpotential (ŋ). 5 ACS Paragon Plus Environment

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There is no electrochemical reaction that occurs at the potential predicted only by thermodynamic considerations excluding the kinetic hindrances experienced in a real system.11,14,114,115 As a consequence of which an additional driving force in terms of an extra potential is needed to effect such electrochemical reactions which is called as the overpotential and denoted with a symbol of ‘ŋ’. For both OER and HER, there are three sources of overpotential viz., the activation overpotential, 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 it will vary from one material to another. Hence, by choosing an efficient catalyst, it can be minimized. The concentration overpotential occurs as soon as the electrode reaction begins as a result of 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 multiplying the resultant current density with the R u. The result of which is a potential (E). In other words, the drop in potential known as IR drop which needs to be subtracted from the experimental potential. In case of HER, the activation overpotential which can be termed as the onset overpotential is important than others as the kinetics of HER is faster than OER.8,9,116,118 This can be calculated from the polarization curve obtained by plotting overpotential vs current density. In contrast, the OER case differs and needs more attention on other parameters too to calculate the overpotential. As we said above, all the proposed mechanisms of OER in acid medium proceeds through the first elementary step of water coordination and an 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. and others have studied the thermodynamics of OER mechanism and proposed the following expression for calculating the theoretical OER overpotential (ŋOER) under ideal conditions and U = 0 vs standard hydrogen electrode (SHE).13,14,117 ŋOER = (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. 6 ACS Paragon Plus Environment

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This clearly says that the kinetic hindrances are not considered in the thermodynamic prediction of overpotential. Due to the varying kinetics of these elementary steps from material to material, instead of the onset overpotential, the overpotential at a fixed current density such as 10 mAcm-2 (ŋ10) has now widely been accepted as an essential quantitative activity parameter to evaluate an electrocatalyst.119–121 The same is also used in HER too. For materials with strong redox peak (giving current density more than 10 mAcm-2) within the potential window of gas evolution and high performance catalysts (giving current more than 500 mAcm-2) such as layered double hydroxides,100,122–125 the overpotentials at higher current densities such as 50 mAcm-2 and 100 mAcm-2 are also used as alternate activity parameters. 2.2 The Tafel Slope and the Exchange Current Density (j0). Tafel plot of an electrocatalytic process is generally obtained by replotting the polarization curves (e.g. linear sweep voltammogram (LSV)) as log current density (j) vs overpotential (ŋ). Slope of the linear portion in the Tafel plot is defined11 as the dependence between the iR compensated overpotential and the current density which is expressed as follows: dlg(j)/d ŋ = 2.303 RT/αnF

(9)

Tafel slope is inversely proportional to the charge transfer coefficient (α) as remaining other parameters are constants viz., ideal gas constant (R), temperature (T), Faraday constant (F) and the number of electrons transferred (n, 4 in case of OER and 2 in case of HER). This is indicating that a catalyst with a high charge transfer ability should possess a low Tafel slope value. 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 LSV into Tafel plot), there are several issues which will lead us to misinterpret the catalytic activity. In general, the LSV obtained with lowest possible scan rate will give Tafel slope with less experimental inaccuracies. The scan rate at which the polarization curve used for Tafel plot becomes a serious problem when the catalyst is highly capacitive. This will lead to large error while determining the exchange current density. Because, the exchange current density is usually obtained by extrapolating the linear fit towards the corresponding current density in logarithmic scale at the equilibrium potential (i.e., at zero overpotential). In such case the catalyst with high overpotential will have large exchange current density value which is not possible. Because, the high exchange current density means that the efficiency of transferring electron across the catalytic interface will be facile and require very small 7 ACS Paragon Plus Environment

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activation energy. Hence, the result would be 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 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 sec. In case of LSV even with a scan rate of 1 mVs-1, the current obtained cannot be the real steady state current. To overcome this issue, a traditional practice can be employed which obtaining the steady state current of the catalyst from the chronoamperometric i–t curves obtained at various overpotential with a regular small intervals (say 5 mV). Similar experimental comparison of Tafel slopes obtained from CV (ran at 5 mVs-1) and from the steady currents obtained by chronoamperometric curves for commercial IrTiO2 electrode is given as Figure 1.11

Figure 1: Tafel curves of a commercial IrTiO2 electrode (Umicore AG & Co.) obtained by a cyclic voltammetry measurement at 5 mV s-1 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.

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The linear portions of the Tafel plots by both the methods are closely agreeing with one another. However, in case of the lower overpotential region the story is different. As we can see from Figure 1 that the Tafel plot obtained from CV is showing larger exchange current density than the one obtained from the steady state current and the exchange current density from the chronoamperometry will be more precise as it eliminated capacitive current. Though the later method seems to be the better one than the method of replotting the polarization curves, it is also not reflecting the intrinsic and exact catalytic activity of a catalyst. Since the potential drop due to the uncompensated resistance cannot be easily excluded in this method as it is usually done in the former method re-plotting polarization curves by IR compensation. To avoid this issue, very recently, Hu et al. proposed a new method for obtaining the more accurate Tafel slope of a catalyst from electrochemical impedance spectroscopy (EIS).126 In this method the Nyquist plot of the catalyst at various overpotential at regular intervals are acquired and the slope of the plot of log charge transfer resistance (Rct) vs the overpotential is obtained which is the exact Tafel slope of a catalyst and depends only on the charge transfer ability of the catalyst as it excludes the Ru from the calculation. Depending on the requirements and conditions any one of these methods can be opted for finding the Tafel slope of an electrocatalyst. Apart from the methods of obtaining Tafel slope, there has been a serious discussion on considering the exchange current density (j0) as an additional activity parameter to that of Tafel slope.11 The exchange current density is more often used as an activity parameter in HER than in OER. As we stated earlier that the kinetics of HER is facile beyond the defined overpotential of a catalyst almost all reported catalyst will have similar kinetics in HER. This means that the exchange current density is directly correlated to the onset overpotential in HER. Hence, it can be used there as an activity parameter. In contrast, the OER case is different and the kinetics differs from one catalyst to another one and they are strongly pH dependent. A classic example of this kind can be the comparison of RuO2/IrO226,38,41,43,127–129 with Ni-Fe98,130–133 system in alkaline medium. In this case the onset overpotential of IrO2/RuO2 will be lower than Ni-Fe but the Tafel slope will be lower for the latter one. This implies that though the driving force to start the reaction on IrO2/RuO2 is low, the rate of charge transfer/ electron transfer is lower than Ni-Fe systems in alkaline medium. This is one of the reason why people have adapted the overpotential at fixed current density (e.g. 10 mAcm-2) as an additional activity parameter than the exchange current 9 ACS Paragon Plus Environment

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density (j0) or the onset overpotential in OER.11 Hence hereby we can conclude that for HER Tafel slope with onset overpotential and/or exchange current densities can be taken as activity parameters. For OER the Tafel slope with the overpotential at defined current density can be taken as activity parameters. 2.3 The Stability. Stability of an electrocatalyst is usually tested by subjecting it to CV cycling at higher scan rate which is otherwise known as the accelerated degradation test and to chronoamperometric or chronopotentiometric analyses. In case of HER, the accelerated degradation test is carried out for several thousands of cycles as the polarization starts from 0 V vs NHE. In case of OER, the number of cycles reported in accelerated degradation test ranges from 250 to 1000 cycles. 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 (ŋ0) (cathodic for HER and anodic for OER) as well as the overpotential at a defined current density of 10 mAcm-2 (ŋ10) is measured as an indicative parameter of stability. Lesser the increase in overpotential, higher the stability. Stability under constant exposure to a fixed potential (chronoamperometry) or a fixed current density (chronopotentiometry) is examined for duration of several minutes to hours. It has now been widely accepted that a stable current density (e.g. 10 mAcm-2) for more than 12 h by chronoamperometry or a negligible increase in overpotential at a current density of 10 mAcm-2 by chronopotentiometry for more than 12 h is stable enough to recognize an efficient electrocatalyst for both 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 mAcm-2 or 100 mAcm-2 and their corresponding potentials can also be used to run chronopotentiometric and chronoamperometric analysis for long time.17,27

2.4 Faradaic Efficiency. Faradaic efficiency is another quantitative parameter used both in 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 it is either HER or OER). There are two methods to find out the faradaic efficiency.8 The first one is the electrochemical method using the rotating ring disc electrode (RRDE) which is applicable to OER only. The catalytically active material is coated on the disc of RRDE without disturbing the 10 ACS Paragon Plus Environment

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ring. The commonly used RRDE is the one with GC disc and Pt ring. Prior to that the collection efficiency of the RRDE going to be used must be known or to be determined experimentally by studying the conventional ferro-ferri redox system’s response at various rpm. The potential at the disk is then swept in the same experimental potential window of OER and at the same time the potential at the Pt ring is set at a constant potential of ORR. Depending on the pH of the electrolyte solution the ORR potential set at the Pt ring is varied. The following equation (10) is used to calculate faradaic efficiency of an OER catalyst.134–136 FE = IR × nD / ID × nR × NCL

(10)

where IR and ID are the current at the ring and disc, nR and nD are the number of electrons transferred at the ring and disc and NCL is the collection efficiency of the RRDE used. It is an extremely useful technique to find out the true activity of an OER catalyst which have the following possibilities of losing its faradaic efficiency. An OER catalyst with one or more strong redox peaks within the potential window of OER (almost all Ni, Co and Fe based catalysts), an electrocatalyst which could facilitate other unwanted side reactions and an electrocatalyst which gets heated up during the electrocatalysis process. The second method of determining faradaic efficiency is common for both HER and OER. In this case the quantity of gas (H2/O2) evolved is needed to be calculated by the integration of from the chronoamperometric or chronopotentiometric analysis. Then the practically obtained gas (H2/O2) quantity should calculated for which any one of the following three method can be employed.8 The first one is the conventional water gas displacing method. Second comes the gas chromatography and the third one is a spectroscopic technique and applicable only to OER in which the evolved oxygen is excited from doublet to singlet state and allowed it relax by fluorescence. The intensity of the fluorescence light is the direct measure of the quantity of the oxygen evolved. However, the method of the practically evolved gas quantity determination can be chosen depending on the nature of catalysts and availability of resources to do such studies. The ratio of the quantity of gas determined by practical method and theoretical method is the faradaic efficiency of the catalyst under study.8 The selection between these methods is also determined mainly by the demands of the catalysts used. 2.5 Turnover Frequency (TOF).


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TOF is another quantitative parameter used for evaluating an electrocatalyst at defined overpotential. The TOF of catalyst can be defined as the number of moles of O2/H2 evolved per unit time. The following equation (11) is used to calculate the TOF an electrocatalytic gas evolution reaction. TOF = I × NA / A × F × n × Ƭ

(11)

where I is the current, NA is the Avogadro number, A is the geometrical surface area, n is the number of electrons transferred and is the surface/total concentration of catalyst in number of atoms. There are more than one method available for the determination of surface or total concentration catalyst in number of atoms. The redox peak in the CV can be used to find out the surface concentration after activating the catalyst by CV cycling.134–136 Using Avogadro number method the total concentration of atoms can be calculated with use of the average particle diameter of the catalyst.17 Another method is the assumption of monolayer.27 If the catalyst surface is flat and smooth or the catalysts morphology is sheet, we can go with this assumption. However, we are aware that each of this method has its own drawbacks. The first method may cause potential error when we have more than one element in the catalyst or if the catalyst is not fully activated. The second method is not reflecting the exact catalytic property of the catalyst as it includes the atoms in the core of the particle too which actually do not participate in the catalytic cycle. The third method may lead to potential error when the material is not completely flat, prone to destruction under harsh electrochemical conditions and with some other morphologies other than sheets. Hence, it is advisable to adopt a method appropriate for the catalyst and its nature. 2.6 Mass and Specific Activities. 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 and expressed in Ag-1.137 On the other hand, the current normalized by the electrochemical surface area (ECSA) or by the BET surface area is the specific activity. As 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 section 3.1 in detail. Among the six parameters discussed to describe the catalytic activity of an electrocatalyst, the overpotential, Tafel slope, and the stability parameters are the mandatory ones.


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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 on such normalization ways was briefly discussed by Fabbri et al. in their recent review article on oxide based OER catalysts.11 3.1 Normalization of Current by Geometrical Surface Area. It is not good to have such a geometrical area of substrate electrode normalized current density (Acm-2geo) as it is not reflecting the intrinsic catalytic property of the material. Moreover, it does not care about the catalyst loading which will give different potential for the same current density with the varying catalyst loading. In other words, if the coverage of 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 case where the loading is low (there will be lot catalytically inactive substrate electrode sites) or very high (the surface layer will only be participating in the electrode reaction and thereby excluding the catalyst present under the surface layers) the geometrical area of the substrate electrode is not equal to the actual surface area of the catalyst participating in catalysis. In such cases the normalization with geometrical surface area will lead to large errors while quantifying the catalytic activity of an electrocatalyst. However, it is the conventional and widely accepted way of current normalization. Hence, to get information on the optimum catalyst loading, knowledge about the catalysts morphology (if nanomaterial) and the wettability of catalyst are needed. Beyond everything, an experimental study on loading effect 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 certain amount of loading which is actually the minimum amount of catalyst required for forming the monolayer. However, care must be taken while doing such study. Because, if the increase in catalyst loading from one trial to another trial is large then we may slip the actual optimum loading in between somewhere. Moreover, if the loading difference is large, the catalyst resistance will drastically increase which will in turn increase the overpotential beyond certain loading limits instead of 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 13 ACS Paragon Plus Environment

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difficulties, we can conclude here that the geometrical surface area can be used normalize the experimentally observed current when the planar electrode is catalyzing the electrode reactions. 3.2 Normalization of current by the Electrochemical Surface Area (ECSA) and BrunauerEmmet-Teller (BET) surface area. It is better to use ECSA or the roughness factor normalized current density as it is more sensitive to the catalyst loading and will vary with the same. The current normalized by ECSA will reflect the intrinsic catalytic property of the catalyst unlike the one normalized by geometrical surface area. The current density obtained by this method is often called as the specific activity (Acm-2ECSA).137 However, the determination of electrochemically active surface area by cyclic voltammetry and by impedance analysis differ from one another significantly and may lead to potential error. Recently, people are using the BET surface area of the catalyst. However, this is also lacking in experimental accuracy. This is mainly due to that the all the sites in the BET surface area determined by gas adsorption and desorption are need not to be electrocatalytically active. The experimental incongruity becomes large when the catalyst is comprised of more than one element. 3.3 Normalization of Current by the Catalyst Loading. Fabbri et al. have recently justified why the current normalized by the catalyst loading is a more reliable activity parameter than the others ways.11 Such loading normalized current density is otherwise known as the mass activity (Ag-1). Though it has lesser experimental inaccuracy than others, it has more disadvantages than other two methods explained above. Direct comparative evaluation of activity with the theoretical activity is not possible, the intrinsic catalytic property of the material is not sound by this parameter and it will not allow us to have fair comparison with the 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 on the merits and demerits of these methods, we have summarized the same here as Table 1. Table 1: Merits and demerits of various current normalization methods utilized so far in electrocatalysis of water splitting. Normalization method

Merits

Demerits


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

Widely accepted and used method.



Does not reflect the intrinsic catalytic property of the catalyst.



Fair comparison with existing literature reports is easy.



May vary depending on catalyst loading and its optimization.



 Good for planar electrodes such as foils and deposited thin films.

Geometrical 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.



Difficulties in determining ECSA.



Large experimental inaccuracies between one method to other such as CV and EIS studies.



Comparison with existing report would be tedious.



Does not reflect the intrinsic catalytic property.



All gas adsorption sites are not electrochemically active sites. Hence will lead to large errors.



Not suitable for planar and thin film electrodes.



Direct comparison with theory and experiment is not feasible.



Does not reflect the intrinsic catalytic property of the material.



Comparison between catalysts of varying

Geometrical Surface Area

ECSA 

BET Surface Area

Loading sensitive.



Ease of determination of BET surface area.



Would be most suitable for porous materials and catalysts.



Loading sensitive.



Regardless of type of catalyst, it directly cares about the loading.



Suitable when same material has been taken in different loading.

Mass of loaded catalyst

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.

Having reviewed the advantages and disadvantages of each of these normalization ways, we can hereby come to a conclusion that depending on the nature of the catalyst, any one of this method can be opted for the normalization of current. However, to enable fair comparison with 15 ACS Paragon Plus Environment

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earlier reports, it is essential to have data on the current density normalized geometrical surface area. Otherwise, it is better to provide all of them together and the same will help us to come to semi-quantitative conclusion on the reliability of these methods. 4. Bond Energetics of Intermediates: A Theoretical Insight by Density Functional Theory (DFT) Calcutions. For an electrocatalyst to be efficient, the bond strength of the intermediates should neither be too strong nor too week. In case of HER, the intermediate is the H adsorbed active site after electrochemical discharge of a proton. Though all the Pt group metals have been predicted to be highly active towards HER, the actual activity is in the order of Pt>Pd>Ni.138–143 This is mainly because of the increased reluctance shown towards desorption of Hads from the catalytic site by Pd and Ni. The high metal-Hads bond strength ultimately results in catalytic poisoning.138–143 As the standard redox potential of hydrogen evolution is zero, the associated standard free energy of hydrogen adsorption is should also be zero.

Figure 2: (A-B) LSVs of HER on various transition metal phosphide catalysts with current normalization by geometrical sureface area and ECSA. (C) Plot of average TOF vs E of various transition metal phoshides. (D-F) The Volcano plot of various transition metal phosphides with different activity parameters such as geometrical are normalized current, ECSA normalized current and the average TOF per surface site. Reproduced with permission from ref. 144. Copyright 2015 Royal Society of Chemistry. 16 ACS Paragon Plus Environment

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The efficiency of a set of HER catalysts are usually determined by calculating the standard free energy of H adsorption through DFT calculations and plotting them against any one of the following parameters such as exchange current density (j0), the current density at a defined overpotential and TOF, (Sabatier-Volcano plot). The catalysts that are placed at or near to the summit of the plot are said to be highly active for HER electrocatalysis. Such a worthy report by Kibsgaard and co-workers on 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 NPs and Ti foil was given via a combined experimental and theoretical approach (Figure 2). In this particular report, the HER activity of these catalysts was 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.

Figure 3: Plot of free energy of H adsorption on various metal phosphide catalysts with respect to increasing fraction H coverage monolayer. Reproduced with permission from ref. 144. Copyright 2015 Royal Society of Chemistry.


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According to this study, an optimal free energy H adsorption is predicted for Fe0.5Co0.5P when the activity parameters such as the current normalized by ECSA and TOF were plotted against it. However, the Volcano plot obtained by plotting current normalized by geometrical surface area against H adsorption free energy had shown a slightly controversial activity trend and placed the Fe0.5Co0.5P just below the MoP/S and Fe0.25Co0.75P. This is mainly believed to be an effect of contribution of non-faradaic capacitive current to the catalytic current. The same group have also revealed the reason behind the exceptional HER activity trends observed with these FeP based catalysts from their dependence of free energy of H adsorption with respect to increasing fraction of monolayer H coverage as shown in Figure 3. From Figure 3, one can notice that the free energy H adsorption getting decreased for FeP and Fe0.5Co0.5P beyond certain fraction of H coverage on these catalytic surfaces and approaches towards the –ve free energy of H adsorption at high H coverage. This indicates that beyond certain amount of H is being adsorbed via the electrochemical discharge of protons, these catalytic surfaces turn into H2 donors simultaneously. This particular behavior is attributed the exceptional activity observed with these catalysts.

Figure 4: Scheme of free energy of H adsorption with corresponding reaction coordinate. Reproduced with permission from ref. 145. Copyright 2016 American Chemical Society.


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Later, a similar combined experimental-theoretical work on revealing the activity trends of CoP catalysts in HER depending on the Fe dopant content was reported by Tang and coworkers. Interestingly, they too end up with the same conclusion that the Fe0.5Co0.5P is the catalyst with the Pt-like HER activity in acid with a low overpotential of 37 mV for a current density of 10 mAcm2

. The scheme of free energy of H adsorption with respect to reaction coordinate (Figure 4) shows

that Fe0.5Co0.5P is having an optimal free energy of H adsorption very closer to that of Pt. Though both these reports says that Fe0.5Co0.5P is the better non-precious catalysts with Pt-like HER activity, this is not the best. Still there rooms to improve the catalytic behavior. From Figure 3, we can see that only the FeP and Fe0.5Co0.5P show such a reduction in the free energy of H adsorption but other catalyst do not show such a trend on the same. This could lead to a question that then what makes them almost equally active to that of FeP and Fe0.5Co0.5P? The answer lies in the higher –ve free energy adsorption at lower H coverage and lower +ve free energy of H adsorption at higher H coverage on these surfaces. This higher –ve free energy adsorption at low H coverage shows its readiness to adsorb hydrogen and lower +ve free energy adsorption at high H coverage shows the ease of H2 delivery from these catalytic surfaces. Examples for this kind of catalysts are MoP and MoP/S as shown in Figure 3. This inference lets us to believe that the better transition metal phosphide catalysts are still hidden and the changing the stoichiometric compositions among all these known metal phosphide could lead us to the better HER catalysts possibly. To carry out such worthy studies, combined theoretical and experimental studies on these catalysts such as the above said reports are highly desired and thanks to them.144-145 In case of OER, the reaction proceeds through adsorption of water/hydroxide on the catalytic site (S−OH / S−OH2), then the oxidation of the same along with the deprotonation to metal−oxide (S=O), then formation of hydroperoxide intermediate (S−O−OH) by the coordination one more water molecule/hydroxide ion on the same site followed by deprotonation and the reductive elimination of O2 from the active site and the same is ready for the next catalytic cycle. These are the common elementary steps in all the proposed pathways of OER mechanism in acidic and alkaline media.11,13,14,117 We can see from this that OER is associated with a series of elementary steps and during each step the chemical environment around the active site is changing. If any one of this intermediate is not favored by its standard free energy of bonding, then the catalyst will not be able to catalyze OER. On the other hand, if the bond strength is too high for any one of these intermediates, then catalytic poisoning is effected and the result will be very high 19 ACS Paragon Plus Environment

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overpotential of OER. In 2007, Rossmeisl et al. have presented a good theoretical survey on the efficiency of OER activity of oxides of Ir, Ru and Ti where they have defined the theoretical OER activity of the same by plotting their oxygen binding energy against the –ΔG (the negative change in the standard free energy) associated with each of the above said elementary steps of OER. The horizontal dashed line indicates (as appeared in original publication) the thermodynamic overpotential (1.23 eV) for OER.146 A material with high activity has to stay on the horizontal line but it will not be seen due to the kinetic hindrances experienced by the catalysts in real system which will fetch them additional potential known as overpotential. Moreover, the plot also says that regardless of the nature of 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, the catalyst with weaker oxygen binding energy will stay closer to the theoretical horizontal line of OER (where RuOx is placed just above IrOx as it binds more strongly to oxygen than RuOx). In the report of Man et al., similar interpretations were made on various other binary metal oxides including the three studied by Rossmeisl and co-workers along with some perovskites as OER catalysts.14 Here the change in the standard free energies of two subsequent steps namely the hydroxide coordination and the oxidation of the same with deprotonation (ΔGO* − ΔGHO*) against their theoretical oxygen overpotential. Note here that the standard free energy associated with the formation of hydroperoxide from the oxide intermediate has not been included here. This could lead to potential error in predicting the catalytic activity of some catalysts with the slow kinetics in hydroperoxide formation. As far as this work is being considered, the catalysts placed on 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 OER efficiently. From the above discussion we can now conclude here that the catalytic surface with strong oxygen binding energy would have the peroxide formation step as the limiting step and the surface that binds oxygen weakly will have hydroxide coordination and oxide formation steps as the limiting step. Therefore, an OER catalyst should neither bond too strongly nor too weakly to oxygen as we have seen in the case of HER. 5. Effect of Hetero Atoms (S, Se and P) in Water Splitting. A brief and nice highlight on the effect of P doping in metal lattices on electrocatalytic HER has been recently given by Shi and Zhang.8 In this section of this review we will also elaborate the effect of other hetero atoms such as S and Se not only in HER but also in OER.


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a

b

Figure 5: (a) P content dependent HER activities in NixPy. Reproduced with permission from ref. 148, Copyright 2015 Royal Society of Chemistry. (b) P content dependent HER activities in CoxP. Reproduced with permission from ref. 147, Copyright 2015 American Chemical Society.


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As highlighted by Shi and Zhang, the polarization induced partial negative charged localized on P centers in a metal phosphide structure with a P terminated surfaces attracts proton as a base and make their discharge easier and promotes the HER easily. As a consequence of which it is obvious to expect that metal phosphides with higher P content should show higher HER catalytic activity. Interestingly, amongst the reported phosphides of Co, Ni and Fe, the polymorphs with more P content are reported to be the highly active ones. Between CoP and Co2P, the CoP is the more active one.147 Similarly, among Ni2P, Ni5P4 and Ni12P5, the Ni5P4 with higher P content is the more active one (Figure 5, a-b).148 Besides trapping the protons by acting as a base, these P centers have recently found to be enhancing the hydrogen desorption at high Hads coverage. A DFT study carried out by Wang and his co-workers on P terminated MoP surfaces focusing the Gibbs free energy of hydrogen adsorption (ΔG0H) have revealed that at lower Hads coverage, the ΔG0H was sufficiently negative (-0.34 eV) to accelerate the adsorption of hydrogen. However, when the surface becomes fully covered by Hads, the ΔG0H turns into positive and accelerate the hydrogen desorption. As observed earlier in cases of MoS2 with more S on the edges.149 This is in agreement with the earlier reports of Kibsgaard et al.144 and Tang et al.145 In cases of S and Se doped metal lattices, similar mechanism should be operating behind the enhanced HER catalytic property. However, the increased electronegativity of S and the relatively stronger bond strength of S–H (~360 kJmol-1), does not allow many of the sulphides of Fe, Co and Ni to act as an efficient HER catalyst. However, Few selective sulphides such as Ni3S2, CoS, CoS2, FeS2 Co9S8 are reported to be good in catalyzing 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 than both sulphides and phosphides as the strength of E–H (E = S, P and Se) bonds is in the order of S– H (363 kJmol-1) > P–H (322 kJmol-1) > Se–H (276 kJmol-1).150 Moreover, like phosphides, both sulphides and selenides can trap the protons to promote the discharge step faster by acting as bases. The story on the influences of these hetero atoms in OER is different from the one we described above for HER. Unlike HER, these heteroatoms does not have any direct influence in enhancing the OER kinetics of these materials. In a report of Subbaraman et al. the trends in the electrocatalytic OER activity of these 3d M2+ ions with oxide environment is described and stated that the Ni2+ is more active than other divalent cations whichever appear just before in the series.151 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− 22 ACS Paragon Plus Environment

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and P3−) in the vicinity of catalytically active metal sites, there could be two effects. One of which is the localized negative charges on these heteroatoms (S and P only) and will actually deactivate the catalyst from coordinating with hydroxide ligand due to 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 electron population in metal center and there by feeds these electronegative heteroatoms. Similarly the peroxide formation by an additional hydroxide ligand coordination is happening just above the surface layers of these heteroatoms and therefore not affected during the coordination processes. After forming the peroxide intermediate the delivery dioxygen molecule is actually accelerated by the enhanced 3p–2p repulsion between the heteroatoms and the peroxide entity. In case when we have Se in place of S and P, we think that the coordination step with hydroxide will be less affected as both metal centers and the ligand centers have 3p-bands as outermost orbitals. However, due to 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 dioxygen molecule may become faster. The facts discussed above on the OER performance of group VIII metal (Fe, Co and Ni) sulphides, selenides and phosphides are in resonance with the experimental reports. These are the reasons why the selenides are better OER catalysts than the phosphides and sulphides. In HER, the selenides and phosphides are better than the sulphides, due to the reasons based on the E–H bond (E = S, Se and P) strengths described above. Hence, it is concluded here by saying that an efficient water splitting catalyst should be designed by considering the bond energetics of the intermediates formed and the electronic structure requirements of each elements in the catalyst. 6. Other Requirements for a Good Water Splitting Catalyst. Having discussed the key thermodynamic and kinetic requirements of an electrocatalyst, it is now important to highlight some other but essential requirements for an electrocatalyst to be economically affordable in large scale water electrolysis. Here comes the first which is the availability of resources of catalytic material from which the catalyst can be formulated. Because, we can no more rely on noble metals (Pt, Ir and Ru) to do this simple water electrolysis. The second one is concerned about the health and environmental hazardless. The catalyst should at least be less harming to persons working with and to the environment applying with. In such cases, except oxides and hydroxides of Fe, Co and Ni, other 23 ACS Paragon Plus Environment

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compounds are potentially harmful. The third one is the conductivity of the catalyst. Though these heteroatom doping to the metal lattices are fruitful in enhancing the catalytic activity, doping beyond certain limits will turn the metal surfaces into semiconducting and even insulating which is not appreciable in electrocatalytic water splitting as it will drastically increase the ohmic drop. The fourth one is the high surface area of the catalyst. Hence the amount of catalyst used can be reduced down. The wettability is another parameter to be considered while formulating a water splitting catalyst. Higher the wettability lower the ohmic drop caused by the formation of gas bubbles on the catalyst surface. Another serious problem to deal with these Fe, Co and Ni based catalysts is the current selectivity as they tend to have strong redox reactions within the potential window of OER. In such cases we should be aware where the applied current is being spent. A good electrocatalyst for water oxidation should have more current selectivity towards OER rather than the redox reaction of its own. To know this, the faradaic efficiency determination by RRDE experiment can be opted. The last thing we should worry about is the corrosion resistance. Since the medium of water electrolysis is either going to be highly acidic or highly basic, the material supposed to catalyze the HER and OER should be having very high corrosion resistance. With the sulphide, selenide and phosphides of Fe, Co and Ni, this problem is well documented in literatures and patents as the oxides of these heteroatoms (S, Se and P) are the well-known corrosion inhibitors and resistors.152-160 The above discussion have certainly implies about the advantageous use of the sulphide, selenide 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 is highlighted in detail. 7. Synthetic Strategies Employed in Group VIII 3d Metal (Fe, Co and Ni) Sulphide, selenide and Phosphide Nanostructures. 7.1 Synthesis of Metal (Fe, Co and Ni) Sulphide, selenide. Synthesis of metal sulphide, selenide have been briefed mainly the method by which the materials are prepared under two subsection focusing the sulphides and the selenides as different subsections.

7.1.1 Synthesis of Metal (Fe, Co and Ni) Sulphides. 24 ACS Paragon Plus Environment

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The synthesis of metal sulphides have been achieved by following one of the procedures with few necessary modification from one material to another. The simple method of synthesizing metal sulphides is the co-precipitation in which the metal precursor salt is taken in a solvent that dissolves both metal ion and the sulphide 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 sulphide nanostructure. Moreover, these additive are hardly removed after the synthesis. However, there is no report on the preparation of MxSy (M = Fe, Co, Ni) nanostructures by this co-precipitation method for electrocatalytic water splitting applications. The most common method of all time used to prepare metal sulphide nanostructure alone or with some 3D matrix support is the solvothermal method. The common sulphide sources used in this methods are thiourea (TU, (NH2)2CS) and thioacetamide (TAA, CH3(CS)NH2).161,162 Among the sulphides of Fe, Co and Ni, the sulphide nanostructures of Co have been relatively reported more than other two metal sulphides. Giovanni et al. prepared FeS nanoparticles by a solvothermal decomposition of Fe2S2(CO)6 at 230 °C for HER in neutral conditions.163 Other available reports of Fe based sulphides are accompanying with Co as dopant. Shen and his coworkers reported the formulation of Fe0.5Co0.5S@mesoporous N-doped graphitic carbon as an efficient ORR and OER catalyst in alkaline condition by a solvothermal route where TU was used as the S source.164 Apart from these, the pyrite type FeS2 for HER was reported recently by Faber et al. which was obtained by a two-step procedure that includes the formation of metallic thin film on a desired substrate by electron beam evaporation followed by thermal sulphidization taking elemental S as the source directly.165 Co based sulphides with varying S content such as CoS, CoS2, Co3S4 and Co9S8 are prepared by solvothermal methods and reported with different morphologies. You et al. compared the HER electrocatalytic activity of hollow nanoprisms of CoS prepared by microwave (MW) and solvothermal methods taking TU as the source of S.166 Kornienko et al. have shown the HER electrocatalytic activity of Amorphous CoS with some O content in the CoS2-like small amorphous clusters of CoS.167 Aslan and his co-workers have prepared CoS nanoparticles and anchored them on CNTs by solvothermal method for HER in alkaline conditions.168 An Interesting report by Wang’s group on designing a 3D efficient electrocatalyst CoS@CTs@CP (Figure 6, a-f) was reported for overall water splitting recently.169 25 ACS Paragon Plus Environment

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Figure 6: (a, b) SEM images of CP/CTs/Co-S. (c, d) TEM images, (e) SAED pattern, and (f) element mapping of CT/Co-S. Reproduced with permission from ref. 169. Copyright 2016 American Chemical Society. Besides the CoS, the pyrite type CoS2 is the next well documented Co based sulphide nanocatalysts for water splitting. In the stated report of Faber et al. other two pyrite type MS2 (M = Co and Ni) and the sulphides of their alloys have been obtained by electron beam evaporation of metal target followed by thermal sulphidization and applied to HER.170 These Pyrite CoS2 have begun to draw attention from the mid of 2014 and reported frequently with modifications in morphologies and support materials like graphene oxide (GO), CNTs, NCNTs, etc. Among them following are some significant reports. It was Faber and his co-workers who reported the HER activity of CoS2 for the first time.170 Soon after Zhang’s group reported this pyrite CoS2 prepared by a solvothermal route as a versatile catalyst for HER in a wide pH range of 0 – 14.171 In the same year Peng et al. fabricated the CoS2 nanosheets on graphene@CNT as a flexible electrode for 26 ACS Paragon Plus Environment

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HER.172 Then a similar report by Ganesan et al. with S and N doped graphene as support for CoS2 was published later in 2015 for ORR and OER.173 Other than these two sulphides of Co, the Co3S4 on N doped CNT was prepared by Wang and his Co-workers by a simple anion exchange method taking Co(OH)2/NCNT as the starting material and applied to OER.174 Feng et al. have proposed method for the formulation of Co9S8 armored with C which utilized the thiocycanouric acid as S source and the thermal decomposition at 700 °C under N2 atmosphere.175 Very recently Tang et al. have shown an interesting report on CoS and Zn doped CoS for total water splitting where these catalysts are grown on Ti mesh.176 Like the sulphides of Fe and Co, the Ni based sulphides are reported with good electrocatalytic properties toward water splitting. The NiS and Ni3S2 are the most common Ni based sulphides applied for water splitting. The Exceptional OER performance of Ni3S2 grown on Ni foam with low overpotential was reported first by Zhou et al. in 2013 by a simple solvothermal sulfurization of Ni foam taking S directly.177 Followed in 2015, Tang et al. showed the HER performance of Ni3S2 nanosheets grown on Ni foam by hydrothermal method for a pH range of 0 – 14.178 Then the same Ni3S2 in form of nanorods (NRs) array on the same Ni foam was reported for total water splitting by Ouyang and their group members later in 2015.179 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 of Tang et al. the NiS hierarchical array have been grown on CC for efficient HER electrocatalysis adapting a solvothermal route of synthesis.180 Another interesting report of Yang and cowrokers achieved the highly efficient and stable water splitting catalysis with Fe doped NiS nanaoarrays prepared by a solvothermal route.181 Similarly the electrocatalytic property of NiS was reported for HER and OER individually and also for bifunctional catalytic activity. Unlike the Ni3S2, the bifunctional activity of NiS in 1 M KOH was reported first by Zhu et al. in 2015 following similar hydrothermal route of sulphidizing Ni foam.182 Later in 2015 itself, Li et al. reported the atomic layer deposition of NiS on silica substrate for OER in alkaline conditions.183 Very recently, Mabayoje et al. comparatively reported the role of anions in Ni sulphide, selenide towards OER.184 Apart from the binary individual metal sulphides, it is common to see reports on the sulphides of the alloys of Fe, Co and Ni. The sulphide of Ni-Co alloy is the most often reported alloy sulphide of this kind. The first report for the preparation of NiCo2S4@graphene by a solvothermal approach using TU as S source was published by Liu and his co-workers in 2013 and applied it to ORR and OER in alkaline 27 ACS Paragon Plus Environment

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conditions.185 Recently Liu et al. reported the electrodeposition of Ni-Co-S and applied it for total water splitting.186 Recently, in a couple of report of by Sun et al. the sulphides of Ni and Co have been fabricated into nanowire arrays for total water splitting electrocatalysis application via a solvothermal route.187,188 Ansovini et al. reported the formulation of Co9S8-NixSy/Ni foam 3D electrode for HER recently by a solvothermal method.189 The sulphide of Co-Fe alloy on N doped mesoporous carbon was reported by Shen et al. for ORR and OER recently.164 Wang et al. reported the advantages of Co doped FeS2 nanosheets/CNTs array for HER application.190 Similarly, Long et al. reported the fabrication of Fe-Ni-S ultrathin nanosheets by a topo tactic sulphidization of FeNi layered double hydroxides and examined it’s HER property in acidic conditions.191 Apart from these, the sulphides of these three metals (Fe, Co, Ni) are often alloyed with other metals also to improve their electrocatalytic properties. One such a catalyst was reported by Miao et al. which is Ni-Mo-S on carbon fiber cloth by solvothermal method for HER in neutral conditions.192 For a better comparative overview on latest synthetic methodologies of these metal sulphides, we have tabulated the material, precursors and method as Table 2. Table 2: Methodologies and precursors used in the sulphides of Fe, Co and Ni. S. No.

Material and Morphology

Metal precursor

Source of sulfur

Method and reaction conditions

Ref.

1

Iron Sulfide Nanoparticles Co0.5Fe0.5S@NMC

Fe2S2(CO)6

Fe2S2(CO)6

163

Cobalt(II) acetate and iron(III) nitrate

Thio urea

Silvothermal heating@230°C Self-assmbly by F127 surfactant and annealing @ 900°C in Ar atmosphere Electron beam evaporation and thermal sulfidation Microwave heating for 15 min Electro deposition @ pH 7 Solvothermal method @ 140°C for 24 h, N2 atmosphere Electro deposition for 8 min

167

2

3

4

FeS2, CoS2, NiS2 High purity Fe,Co films and Ni metals

Sulfur powder

cobalt acetate hydroxide

Thio acetamide

5

Hollow cobalt sulfide nanoprisms CoSx film

CoCl2.6H2O

Thio urea

6

CoS@CNT

CoCl2.6H2O

Thio acetamide

7

CoS@CNT@CF P sheets

Co(NO3)2·6H2O

Thio urea 28

ACS Paragon Plus Environment

164

165

166

168

169

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8

CoS2 Micro and nanowires

9

CoS2 pyramids@Ti foil CoS nanosheets@Gra phene@CNT

10

11

12

13

14

15

16

17

18

19

20

21

Co(NO3)2·6H2O and (Co(OH)(CO3)0.5· xH2O CoCl2.6H2O

Sulfur powder

Thermal Sulfidation 500 °C for 1 h

170

Thio urea

Hydrothermal method 180 °C for 15 h Hydrothermal method combined with vacuum filtration solid-state thermolysis @ (400, 500, and 600 °C)

171

174

Co(Ac)2·4H2O

CS2 and Thio urea

CoS nanoparticles@N and S doped Graphene oxide Co3S4@NCNTs

Co(TU)4(NO3)2 complex

Co(TU)4(NO3)2 complex

CoCl2.6H2O

Co9S8 Nanoparticles@ C Zn0.76Co0.24 S/CoS2 nanowires@Ti mesh Ni3S2 nanorods@Ni foam Ni3S2 nanosheets@Ni foam Ni3S2 Nanorod arrays@Nickel foam NiS2 nanosheets@CC

Co(NO3)2·6H2O

Na2S and Thio acetaamide Trithiocyanu ric acid

Iron-doped nickel disulfide nanoarray@Ti Nickel sulfide microsphere film@Ni foam NiSx films

172

173

ZnCo2O4 nanowires

Sulfur powder

Solvothermal method @ 160°C for 15 h Annealing @700°C in N2 atmosphere for 3h annealing@400°C for 2h

Nickel foam

Thio acetamide

Hydrothermal method@ 180°C 4 h

177

Ni(NO3)2 .6H2O

Thio urea

178

Nickel foam

Thio urea

Ni(NO3)2.6H2O

Sulfur powder

NiFe-LDH

Sulfur powder

Nickel foam

Sulfur powder

bis(N,N′-ditert-

Hydrogen sulphide

Hydrothermal method@ 120°C 10 h Hydrothermal method@160°C for 6h Hydrothermal method@100°C for 10 h Annealing@400 °C for 60 min under Ar atmosphere Annealing@300 °C with a rate of 8°C min-1 under Ar atmosphere Vapor-Phase Atomic Layer Deposition


175

176

179

180

181

182

183

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22

NiS film

23

NiCo2S4 nanoparticles@g raphene

24

NiCoS nanosheets films

25

CoS nanosheets@Ti mesh NiCo2S4 nanowires@CC Co9S8– NixSy@Ni foam

26 27

28

Fe1−xCoxS2/CNT

29

Iron−Nickel Sulfide Nanosheets Ni-Mo-S nanosheets@CC

30

butylacetamidinat o)nickel(II) NiCl2

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Thio urea

Electro deposition

184

Ni(CH3COO)2.4H 2O and Co(CH3COO)2.4 H2O Ni(CH3COO)2.4H 2O and Co(CH3COO)2.4 H2O CoCl2.6H2O

Thio urea

Solvothermal method@200 °C for 6 h.

185

Thio urea

Electro deposition

186

Thio urea

Electro deposition

187

NiCl2·6H2O, CoCl2·6H2O Co(NO3)2·6H2O and Nickel foam

Sulfur powder Thio urea

188

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

Thio acetamide Thio acetamide

Na2MoO4 · 2H2O, and NiSO4 · 6H2O

L-Cysteine

Annealed@300 °C for 2 h Annealing of cobalt thiourea coplex on Ni foam @ 300°C for 5 min. 90 °C in an oil bath for 24 h. Hydrothermal method@120°C for 6h Hydrothermal method@200°C for 24 h

189

190 191

192

7.1.2 Synthesis of Metal (Fe, Co and Ni) Selenides. Synthesis of metal selenides resembles the synthesis of sulphides in many ways. The most common method of forming metal selenide nanostructures is the solvothermal route. The Se sources used for metal selenides synthesis are the 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.193 Unlike the sulphides, the reports on the selenides of these three metals is uneven. The most reported one is the CoSe2 for both OER and HER. The lone report on NiSe nanowires (NWs) was first published by Tang et al. in 2015 where the NiSe NWs were solvothermally grown on Ni-foam and applied for water splitting in high alkaline conditions (1 M KOH).194 NaHSe 30 ACS Paragon Plus Environment

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ACS Catalysis

derived from the reaction of crude Se powder and borohydride was used as the Se source during the solvothermal treatment of surface etched clean Ni foam. In case of Fe, there is no lone report to either HER or OER. However, in very recent report of Wang and his Co-workers, it has been reported that the OER activity of NiSe was drastically increased when Fe is introduced in the system with a decrease in the overpotential of >100 mV.195

Figure 7: (a) XRD patterns of the precursor and selenized product scratched down from CC. (b and c) SEM images of Co(OH)F NW/CC. (d) TEM image of a Co(OH)F NW. (e and f) SEM images of CoSe2 NW/CC. (g) TEM image of a CoSe2 NW. (h) HRTEM image and (i) SAED pattern taken from 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 with permission from ref. 206, Copyright 2015 American Chemical Society.


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This indicates that though the catalysts with Fe alone do not possess any significant OER activity so far, their incorporation into the lattices of active catalysts composed of Ni and Co may bring down the overpotential and increases the activity. In this report the Ni to Fe ratio is maintained at 3:1 during the Ni-Fe ultrathin precursor sheet formation and also while selenizing it with NaHSe by hydrothermal route. Apart from these 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 is briefed below. Other than these there are two interesting reports on Ni3Se2 film and NiSe2 nanowire fabricated via electrodeposition and solvothermal routes by Sun et al. for application in total water splitting recently.196,197 Like the other two selenides (NiSe NWs and Ni-Fe-Se nanosheets), the synthesis of CoSe2 is also achieved by solvothermal method. It was Xu and his co-workers who reported the exceptional HER activity of CoSe2 nanobelts to which Ni/NiO was anchored by electrodeposition on GCE in 2013.198 Soon after in 2014, the same was reported as CoSe2 NPs grown on carbon fiber paper (CFP) by Kong et al. who utilized a thermal pyrolysis assisted cobalt oxide NPs formation on CFP followed by selenization under Se vapor atmosphere.199 The HER performance shown by this CoSe2 NPs/CFP was better than the previous report. In 2015, more attention was paid on synthesizing this CoSe2 with various forms using various methods of synthesis with ultimate aim of total water splitting. In such case, for the first time Liu et al. reported the bifunctional catalytic activity of this CoSe2 as electrodeposited amorphous thin films on Ti foils for full water splitting.200 Before this, Carim and co-workers have already reported the HER activity of electrodeposited CoSe amorphous thin films in 2014.201 Then the attention was deflected towards the crystalline CoSe2. Zhang et al. reported the HER activity of polymorphic CoSe2 with mixed orthorhombic and cubic phases.202 Recently, Liao et al. reported the OER activity of coral-like CoSe2 nanostructures prepared by a solvothermal route.203 As expected, the CoSe2 was then composited with various other active catalysts such as MoS2 and Ni-Fe LDH and the synergistically enhancing compounds and substrates such as CeO2, TiO2 and Graphene to improvise the catalytic activity of CoSe2. A clever thing done by Hou and his coworkers was the synthesis of CoSe2/Ni-Fe LDH on graphene composite which had resulted in better bifunctional water splitting catalytic activity than any other report.122 Similarly, Gao et al. synergistically improvised the HER performance of CoSe2/MoS2 by making a composite of them.105 With a different synthetic route of microwave irradiation Ullah 32 ACS Paragon Plus Environment

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et al. reported the same CoSe2 but as a composite with graphene and TiO2 for with improvised HER performance.204 Zheng and his group members were the ones who showed the effect CeO2 on the OER catalysis lead by CoSe2 in nanobelt morphology.205 As a step ahead in the HER catalysis of CoSe2, Liu et al. formed a 3D HER catalyst by a simple hydrothermal growth of CoSe2 on carbon cloth (Figure 7, a-k).206 as given with the metal sulphides, we have summarized the methods and materials used for the synthesis of these metal selenides too here as Table 3.

Table 3: Methods and materials used for the synthesis of selenides of Fe, Co and Ni. S. No.

Material and Morphology

Metal precursor

Source of selenium


Ref.

1

MoS2/ CoSe2

Co(OAc)2.H2O and (NH4)2MoS4

Na2SeO3

105

2

Co0.85Se/NiFe LDH nanosheets

Na2SeO3

3

NiSe Nanowire Film@Ni foam

Co(NO3)2.6H2O, Ni(NO3)2.6H2O and Fe(NO3)3.9H2O Nickel foam

Hydrothermal method@200°C for 10 h Hydrothermal method@150°C for 48 h

194

4

(Ni0.75 Fe0.25) Se2@ CFC

5

Ni3Se2 film@Cu foam

6

NiSe2 nanoparticles Film Ni/NiO/CoSe2 nanocomposite

7

8

9 10

CoSe2 Nanoparticles@ CFP CoSe film@Ti mesh Amorphous cobalt

Se powder

122

Ni(NO3)2.6H2O and Fe(NO3)3.9H2O Ni(CH3COO)2.4 H2O

Se powder

SeO2

Hydrothermal method@140°C for 12 h Hydrothermal method@180°C for 24 h Electro deposition

NiCl2·6H2O

SeO2

Electro deposition

197

Co(AC)2·H2O and Nickel(II) 2,4pentanedionate Co(NO3)2.6H2O

Na2SeO3

Hydrothermal method@180°C for 16 h

198

Se powder

Thermal Selinization in Ar atmosphere

199

Co(AC)2·4H2O

SeO2

Electro deposition

200

Co(AC)2·4H2O

SeO2

Electro deposition

201


195

196

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selenide films@Ti foil 11

Polymorphic CoSe2

CoCl2

SeO2

Electro deposition

202

12

3D coral-like CoSe

Co(NO3)2.6H2O

Se powder

203

13

CoSe2/graphene– TiO2 heterostructure CeO2 /CoSe2 Nanobelts

CoCl2

Se powder

Hydrothermal method@180°C for 15 h Microwave heating for 15 min

Co(AC)2·H2O and Ce(CH3COO)2 Co(NO3)2.6H2O

Na2SeO3

polyol reduction method@ 278 °C for 1 h Hydrothermal method@140°C for 10 h

205

14

15

CoSe2 Nanowires

Se powder

204

206

7.1.2 Synthesis of Metal (Fe, Co and Ni) Phosphides. Though there are two good surveys exist on the synthesis of transition metal phosphides and their applications into HER, it would be better to have a consolidated review on the OER activity of Fe, Co and Ni phosphides too in addition with the sulphide, selenide of the same. Synthesis of metal phosphide nanostructures is much difficult than the simple sulphides and selenides and almost done in high temperature in an inert atmosphere. The metal phosphide synthesis mainly falls into three categories viz., the high temperature liquid phase synthesis where the most common source P source trioctylphosphine (TOP) is used along with the trioctylphosphine oxide (TOPO) as the structure directing agents with varying molar ratio, the high temperature solid state reaction of the desired metal precursor with sodium hypophosphite (Na2HPO2) commercially known as the hypo is used as the source of P under highly oxygen free conditions. Other than hypo, the elemental P (preferably red phosphorous) 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 two fold higher than the one required for the solution phase synthesis using TOP. The inert atmospheres used so far for the synthesis of these metal phosphides nanostructures are Ar, H2, N2 and the mixture of H2 and N2 either in a static condition or in a flowing condition. Similarly, in all liquid phase synthesis the common metal precursor used are the complexes of acetates and acetylacetonate of respective metal ions or the nitrates and 34 ACS Paragon Plus Environment

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ACS Catalysis

chlorides of the metals with a phase transfer catalyst (PTC) such as tetraoctylammonium bromide (TOAB).207,208 In case of high temperature solid-gas phase reaction, the nitrates are the most common metal precursor used. Unlike the sulphides and selenides, the phosphides of Fe, Co and Ni are equally explored in terms of synthesis and their subsequent applications to water splitting. In case of Fe, the phosphides reported so far are the FeP and Fe2P. Between these two, the FeP is the frequently reported one. As we have mentioned in the earlier discussion on the effects of hetero atoms in water splitting, the higher P content in FeP helps it to be more active than Fe2P. Moreover, when the bi-functionality is concerned, among available reports on the phosphides of Fe, there is only one report by Yan et al. who had grown the ZnO NWs on CFP which was then doped with Fe3+ by hydrolysis followed by phosphidization at 300 °C with hypo in Ar atmosphere.209 Similarly, there is a lone report on the OER catalytic activity of FeP NRs supported CFP obtained through a set of hydrothermal and Gas phase phosphidization reactions by Xiong et al. Almost all other reported FePs in various morphologies and crystalline forms are devoted only to the HER catalysis.210 Some of their synthesis sequences is briefed below. FeP is reported as nanowires, nanotubes, nanosheets, thin films, nanocrystals and as composite with N-doped CNTs for HER applications. The first ever report on the HER electrocatalysis on FeP was reported first by the Xu and co-workers who had synthesized the FeP nanoporous nanosheets by an anion exchange method.211 Du and coworkers proposed an interesting method of preparing FeP NRs by a simple hard template method using anodized aluminium oxide (AAO) as the desired hard template to which the Fe3+ precursor was loaded through a sequence of soaking and drying process which was then phosphidized with hypo in an alumina boat at 350 °C.212 Later, Liang et al. obtained an array 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, a-f).213 Using a similar synthetic route Tian and coworkers obtained a 3D FeP NPs thin films on CC which showed better HER performance than other phosphides in both acidic and neutral electrolytes.214 In a similar reported of Yang et al. it is stated that the same CC has been used once again for the formation of rugae-like FeP and FeP2 nanocrystals (NCs) array and the same was used later for HER electrocatalysis in acid.215 Jiang and coworkers chose the Ti Plate as the substrate instead CC to FeP NWs array.216 The synthesis was a two-step process in which the first comes the hydrothermal growth of FeOOH on Ti plate followed by its chemical conversion to FeP NWs array on Ti plate at low temperature. As expected, Liu and coworkers composited the N-doped CNTs to 35 ACS Paragon Plus Environment

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FeP by growing the Fe2O3/NCNT precursor composite first via an hydrothermal route followed by phosphidization under N2 atmopshere with hypo at 350 C and applied to HER.217

Figure 8: 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 EDAX elemental mapping of C, P, and Fe for the FeP NAs/CC. Reproduced with permission from ref. 213, Copyright 2014 American Chemical Society. 36 ACS Paragon Plus Environment

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The phosphides of Co has been studied more than that of Fe in both HER and OER. The known phosphides of Co so far are the CoP and Co2P. As seen in the case of FexPs, the CoP with maximum P content is reported to be the better catalyst for HER and OER than Co2P. Moreover, the number of reports on CoP is three fold higher than that of Co2P. Co2P has been reported as nanoneedles, NWs and as a composite with CNT and NCNT for OER and HER applications by many earlier. The synthesis of Co2P resembles the synthesis of FexPs. One of such recent report with a significantly different synthetic route was made by Dutta and coworkers who had synthesized the Co2P nanoneedles by a liquid phase alkaylamine assisted synthesis but instead of taking the TOP as the P source, the PH3 gas produced ex-situ was purged into the reaction vessel at 230 °C. This method provides a significant advantage of avoiding high possibility for explosion by other methods in case when little O2 is present inside the reaction chamber.218 In case of CoP, there is a cluster of report for the synthesis and their subsequent applications to HER and OER, the synthesis of CoP is done any one of the above seen methods for the synthesis of FexPs viz., the liquid phase metal ion reduction and phosphidization, high temperature solid-vapor phase reaction or the anion exchange reaction. Like the contribution made by Sun and his group members in case of sulphide and selenide catalysts of these three metals, they have done enormous amount of work on cobalt phosphide too by various methodologies and subsequently applied them for both HER and OER applications. Among them fabrication of self-supported nanoporous CoP nanowire arrays,219 CoP nanowire arrays for sensing and photcatalytic HER,220 templated assisted synthesis of CoP nanotubes,221 CoP nanowire array on Ti mesh,222 3D interconnected CoP nanowire arrays,223 and the combine experimental-theoretical work on CoP with varying Fe dopant content are the significant works.145 Popczun et al. reported the HER activity of CoP NPs in acidic conditions which was prepared by the first method mentioned above.224 Followed by which a number of reports had been appeared in literature either with a significant improvement in the catalytic activity or the improvement in relatively easier synthetic routes. CoP has been reported as branched star-like morphologies by Popczun et al.,225 hollow polyhedral by Liu et al.,226 urchin-like NCs by Yang et el.,227 porous NRs bundle by Niu et al.,228 as nanosheets on Ti foil substrate by Pu et al.,229 self-supported mesoporous NRs by Zhu et al.,230 surface oxidized NRs by Chang et al.,231 again as NRs by Huang et al.,232 nanoporous NWs by Gu et al.,233 with a mixed morphology by Jiang et al.234 and as CoOOH covered CoP NPs by Ryu et al.235 Apart from various morphologies, as expected the CoP had been composited with the carbonaceous materials like CNTs by Liu et al.236 37 ACS Paragon Plus Environment

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and with GO by Ma et al.237 via an hydrothermal deposition of CoP on GO. One another important report was made by Hou et al.238 who had decorated the CNTs with ultrafine CoP NPs and applied as a highly active bifunctional water splitting catalysts. Beyond this, an interesting report on CoP@C core-shell nanostructures was first made by wang’s group where they have applied it to HER electrocatalysis.239 Like FeP case, the Cop had also been grown CC by Li et al. utilizing the two-step universal process of growing metal phosphides on CC.240 The phosphides of Ni are quite different from that of Fe and Co. Unlike Fe and Co, the phosphides of Ni has been reported beyond the mono and di metallic centers. So far the reported nickel phosphides are NiP, Ni2P, Ni5P4, Ni12P5 and NiP2. The amount of P content and their direct influence on the catalytic performance on HER is well explained in the very recent review reports on metal phosphides synthesis for HER applications by Shi et al.8 Moreover, the evolution of nickel phosphides with varying P content had lead Kucernak and coworkers to make a linear relationship between the P content and the catalytic activity of the metal phosphide towards HER in acidic medium.241 Like we observed with Fe and Co, the nickel phosphide with high P content (NiP) was found to be the better catalyst in HER than others. However, such a direct correlation between the P content and the OER activity of metal phosphides cannot be made as the mechanism of OER which is much more complicated than that of HER. Synthesis of nickel phosphides were also done in any one of the known three methods of phosphidization as explained for Co and Fe phosphides. The nickel phosphides with varying P content were obtained by just changing the molar ratio between metal ion precursor and the source of P as done by Kucernak et al.241 Among the known nickel phosphides, the Ni2P is the most frequently reported one with various morphologies like metallic nanosheets by Li and coworkers,242 as urchin-like crystals on Ni foam by You and coworkers,243 as nanoaggregates by Li and coworkers,244 as W doped Ni2P microspheres by Jin and coworkers,112 as nanosheets of Ni foam by Shi and coworkers,245 as monodispersed NCs with different phases by Pan and coworkers,148 as nanoflakes (NFs) on graphene/Ni foam hybrid electrode for HER from pH 0–14 by Han and coworkers,246 and as NRs on Ni foam by Wang and coworkers.247 The second most reported nickel phosphide is NiP. The NiP has been reported by Yu et al. as the carbon coated porous nanoplates which was applied OER,248 Wang and coworkers reported the one step formation of NiP nanosheets arrays on Ni foam for efficient HER in acidic electrolyte.249 Very recently, Zhuo et al. reported the effect Se doping onto the NCs of NiP and NiP2 on HER.250 As an interesting advancement in the nickel 38 ACS Paragon Plus Environment

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phosphide research, Jiang and coworkers reported the exceptional HER performance of NiP2 in both acidic and alkaline conditions.251 The higher P content in NiP2 fetches an additional corrosion resistance under harsh alkaline conditions and improved the HER performance of the same. Similarly, the Ni5P4 NCs were found to be highly stable while catalyzing HER in both acidic and alkaline conditions due to the increased P content as reported by the Laursen and coworkers.252 As a step ahead and above the same Ni5P4 was applied to full water splitting by Ledendecker and coworkers afterwards as it holds better stability in alkali than other phosphides of Ni. 253 Huang and coworkers showed the catalytic and photocatalytic HER performance of Ni12P5 NPs.254 The methodologies and methods applied to synthesis the phosphides of Fe, Co and Ni are summarized here as Table 4. Table 4: Methods and materials used in the synthesis of metal (Fe, Co and Ni) phosphides. S. No.

1

2

Material Metal precursor and Morpholog y Iron (Fe(NO3)3.9H2O Phosphide Nanotubes FeP FeCl3.6H2O nanorods@ CP

Source of phosphorou s


Ref.

NaH2PO2

Heated@300 C for 30 min under Ar flow phosphorization treatment in P vapor at 500 oC for 30 min Anion-exchange reaction by diffusion@>300 oC Heated@350 o C for 2 h under Ar flow Heated@300 o C for 2 h under Ar flow Heated@300 o C for 2 h under Ar flow phosphidation@500 o C

209

o

phosphorus red

FeP nanosheets

Fe18S25–TETAH

Trioctyl phosphine

4

FeP nanorods

FeCl2

NaH2PO2

5

FeP nanorod arrays FeP nanoparticl e film@CC Rugae-like FeP nanocrystal @CC

FeCl3.6H2O

NaH2PO2

(Fe(NO3)3.9H2O

NaH2PO2

FeSO4

NaH2PO2

3

6

7


210

211

212

213

214

215

ACS Catalysis

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

8

3D FeP Nanowires

FeCl3

NaH2PO2

9

FeP@N doped CNT

FeCl3.6H2O

NaH2PO2

10

Surfaceoxidized Co2P nanoneedle s 3D CoP nanowires @CC CoP nanowires

CoCl2.6H2O

Ca3P2

Co(NO3)2.6H2O

NaH2PO2

Co(AC)2

NaH2PO2

11

12

13

CoP nanotubes

CoCl2.6H2O

NaH2PO2

14

CoP @Ti mesh

Co(NO3)2.6H2O

NaH2PO2

15

CoP nanoparticl es@CC CoP Nanoparticl es Branched CoP nanostructu res

CoCl2.6H2O

NaH2PO2

Co2(CO)8

Trioctyl phosphine

Cobalt(II) acetylacetonate

Trioctyl phosphine and Trioctyl phosphine oxide NaH2PO2.H2 O

17

18

19

CoP hollow polyhedron

Co(NO3)2.6H2O

20

Urchin-like CoP Nanocrystal s CoP nanorod arrays@Ti

Co(NO3)2.6H2O

NaH2PO2

Co(NO3)2.6H2O

NaH2PO2

21


Page 40 of 87

Heated@300 °C for 2 h in a static Ar atmosphere Calcined@350oC under N2 flow for 2 h Solvothermal method@230 °C for 30 min

216

Heated@300 °C for 1 h in a static Ar atmosphere Heated@300 °C for 2h

219

Heated@300 °C for 2 h in a static Ar atmosphere Heated@300 °C for 2 h in a static Ar atmosphere Heated@300 °C for 2 h in Ar flow

221

Solvothermal method@320 °C for 1h Heated@120 °C for 1 h under vacuum

224

Heated@250 °C for 2 h in static N2 atmosphere Heated@300 °C for 150 min in static N2 atmosphere

226

Heated@300 °C for 2 h in a static Ar atmosphere

228

217

218

220

222

223

225

227

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22

CoP nanosheet arrays@Ti plate CoP nanorod arrays@Ni foam Surface oxidized CoP Nanorods Co2P nanorods

Co(NO3)2.6H2O

NaH2PO2

Heated@300 °C for 1 h in a static Ar atmosphere

229

CoCl2

NaH2PO2

Electro deposition

230

CoCl2.6H2O

NaH2PO2.H2 O

Heated@300 °C for 2 h in static N2 atmosphere

231

Co(AC)2·4H2O

Trioctyl phosphine

232

CoP Nanoparticl es@C CNT decorated with CoP Nanocrystal s CoP nanoparticl es@RGO CoP Nanoparticl es@CNT CoP@C coreeshell nanocables Nickel phosphide


Trioctyl phosphine

Solvothermal method@120 °C for 30 min in a N2 flow Solvothermal method@300 °C

Co(AC)2

NaH2PO2

Heated@300 °C for 2h

236

Co(AC)2·4H2O

NaH2PO2

Heated@300 °C for 2h

237

Co(AC)2

NaH2PO2

Heated@300 °C for 2 h in Ar flow

238


Triphenyl phosphine

239

NiSO4

NaH2PO2

Heated in sealed tube@400 °C for 100 min Electro deposition

33

Ni2P nanosheets

NiCl2·6H2O

NaH2PO2

Heated@ 380 °C for 15 h

242

34

UrchinLike Ni2P@Ni foam NixPy

Ni foam

NaH2PO2. H2O

Heated@ 400 °C for 2 h in static Ar atmosphere

243

Ni(NO3)2.6H2O

NaH2PO2. H2O

Heated@(275-475 °C) for 2 h in static Ar atmosphere

244

23

24

25

27

28

29

30

31

32

35


235

241

ACS Catalysis

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36

37

38

39

40

41

Ni2P nanosheets @Ni Foam Ni2P nanoflakes @graphene/ Ni foam Ni5P4-Ni2P nanosheet array Carbon coated nickel phosphide nanoplates Ni2Pnanorods@ Ni foam Se-doped NiP2

Ni foam

Trioctyl phosphine

Ni foam

KH2PO4

Ni foam

Red phosphorous

potassium tetracyanidonickelat e(II) and NiCl2.xH2O

NaH2PO2

Ni foam

Red phosphorous

Ni(NO3)2.6H2O

Red phosphorous

42

Ni2P NPs films@Ti

Ni(NO3)2.6H2O

NaH2PO2

43

Nano crystalline Ni5P4 Ni5P4 Films

Ni(acetylacetonate)2 ∙xH2O

Tri‐n‐ octylphosphi ne oxide Red phosphorous

Ni12P5 nanoparticl es W-doped NixP microspher es FexCo1−xP Nanowire Array

Ni(Ac)2.4H2O

Triphenyl phosphine

NiSO4

Na2PO2

FeCl3.6H2O, (Fe(NO3)3.9H2O, CoCl2.6H2O and Co(NO3)2.6H2O

NaH2PO2

44

45

46

47

Ni foil

Page 42 of 87

Solvothermal method@320°C for 2h Chemical vapor deposition process

245

Heated@500 °C for 6 h in N2 flow atmosphere Heated@300 °C for 2 h in static Ar atmosphere

247

Hydrothermal method@200 °C for 48 h Heated@500 °C for 30 min in Ar flow atmosphere Heated@300 °C for 1 h in static Ar atmosphere Solvothermal method@390°C for 1.5 h Heated@550 °C for 1 h in inert atmosphere Solvothermal method@390°C for for 30 min under N2 Solvothermal method@80-90 °C for 2 h

249

Heated@300 C for 2 h under Ar flow

145

o


246

248

250

251

252

253

254

112

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Beyond the monometallic phosphides, the phosphides of mixed metals and their electrocatalytic performance towards water splitting has also started appearing recently in literature. The first such mixed metallic phosphides of Ni and Co was reported by Feng and coworkers who prepared the quasi-hollow nanocubes of Ni-Co-P and applied it to HER in alkaline solution.255 Little later, a ternary array of Ni-Co-P nanosheets were synthesized by Li and coworkers and applied it for total water splitting under alkaline conditions.256 The advantages of making such bimetallic phosphides helps to achieve better durability performance during both HER and OER as reported by Li et al.256 Having discussed, the briefed synthetic strategies of sulphides, selenides and the phosphides of Fe, Co and Ni, we can come to a conclusion that there are still rooms to improvise the synthetic routes of these sulphide, selenide 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 out of solution based synthesis. We have also seen that the phosphides of Fe, Co and Ni have been found to be the better catalysts than that of sulphide, selenide of the same. Now it is time to find out simplified, easier, one step quick synthetic strategies to make the efficient metal phosphides to be affordable into industrial level large-scale H2 production more conveniently. Beyond the extensive contribution made by Sun and his group members to this particular field of electrocatalysis of water splitting using sulphide, selenide and phosphide of Fe, Co and Ni, they have newly reported two other catalysts such as Ni-Mo hollow nanorod array for total water splitting257 and amorphous NiB alloy NPs grown on Ni foam for total water splitting again.258 These reports have basically alarm us that there are still rooms to improvise the catalytic efficiencies of these catalysts. From the above discussion, it is quite obvious to expert a review summary on Co catalysts based materials for the electrocatalysis of water splitting from Sun and his group members and the same has appeared very recently in literature.259 This particular review have nicely corroborated the activity trends of various Co catalysts such as its oxides, sulphides, selenides and phosphides for both HER and OER in acidic, basic and neutral environments. However, it could be noted here that this review had been centered only around the Co based catalysts and not covering Ni and Fe completely which has been rectified by this review article now with updated literature reports. 8. Applications of Group VIII 3d Metal (Fe, Co and Ni) Sulphide, Selenide and Phosphide Nanostructures in Water Splitting. 43 ACS Paragon Plus Environment

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Before running into the applications of sulphide, 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 the Nanostructured Catalysts in Water Splitting. One of the major problem encountered with the nanostructured catalyst is achieving reasonable stability without compromising its catalytic activity. The sources for stability failure can be any of the following such as the binder used to tether the nanostructured catalysts to the substrates/the current collectors, the pH at which electrolysis is carried out, high surface energy of nanomaterials, decrease in conductivity due to over oxidation of catalyst (in case of OER) and the loading effect on the conductivity.

8.1.1 The Binders Used to Tether the Nanostructured Catalysts and Associated Demerits in Water Splitting. In general, the binder will be chosen depending on the medium in which electrolysis is carried out. Since, the most reactive HER and OER catalysts are found to be catalyzing these electrochemical reactions either at extremely low pH (highly acidic ) or at extremely high pH (highly alkaline), the most obvious choice is to go with a binder which can offer excellent proton transfer in acidic solutions and good hydroxide conductivity in alkaline conditions. The common commercial binder used in this purpose is the Nafion® which is actually a perfluorinated alkyl sulphonate ionomer and available in various wt. % ranging from 5% to 40% as a solution of water and methanol/propanol mixture. Because of very high fluorine content in the carbon back bone and high hydrophobicity associated with sufficiently high polarity and the sulphonate functional group on the other head with hydrophilicity 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 being widely used as a binder for HER and OER in acidic conditions. But when it is electrolysis in alkaline conditions, this proton transfer ionomer fails to deliver the same efficiency which it did in acidic medium. Due to this, the resistance of the catalyst-electrolyte interface is considerably increased which leads to drastic degradation in the catalytic activity and affects the stability of nanostructured catalysts modified electrodes. However, the same Nafion® 44 ACS Paragon Plus Environment

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ACS Catalysis

5 wt. % solution is being used with additives such as propyl alcohol or iso-propyl alcohol in water in the volume ratio of 0.5:2.0:7.5 as 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 substrate electrode surface. Though, this modification made the Nafion® to be 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 high acidic and alkaline conditions. Due to this, the thickness of the catalyst layer on the substrate electrode is gradually increased and leading to the increased catalyst resistance which in turn decreases the catalytic activity and affecting stability of the catalyst directly. To overcome this problem, people now has diverted their attention towards some anion exchange ionomer made up of some indole based polymers and ionic liquids. But, these are quite expensive and efforts have been made to reduce the overall expenses associated with water electrolysis by making these nanostructured catalysts would become meaningless, if we 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 polytetrafluro ethane (PTFE), polyvinlylpyrrolidine (PVP), polyvinylalcohol (PVA), N-methyl pyrrolidone (NMP) and dimethyl formamide (DMF). However, except the DMF other listed binders above are nonconductive and requires 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 catalysts or additionally contributing to the overall activity. Though DMF does not require any such additive conductive additives, the stability of the catalyst modified surface is poor when compared to other binders particularly while subjecting it to 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 the ultra-small Pt NPs for HER in acidic conditions and IrO2 NPs for OER in alkaline conditions.17, 260 These Pt NPs anchored DNA molecular self-assemblies have shown extreme stability and increased catalytic performance over the commercial Pt/C catalyst under identical experimental conditions.17 The mechanism of binding with DNA molecular self-assemblies is mainly dependent on the electrostatic interaction between the DNA molecules with various charged and polarized entities while polarizing the electrode to 45 ACS Paragon Plus Environment

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both anodic and cathodic directions, apart from these, the catalyst resistance is not increased due to the conductivity associated with the DNA as a consequence of free electron movements by resonance among the purine and pyrimidine bases in their back bone. This study now has opened up pathways of utilizing other biomolecules with charged and polarized entities such as amino acids, peptides, proteins and polysaccharides as binders for other materials in future.

8.1.2 pH of the Medium of Water Electrolysis. The medium of water electrolysis has crucial role in water electrolysis which is elaborated earlier under various categories. Here, the role solution pH of water electrolysis on the stability of nanostructure catalyst is discussed. We have seen that at high acidic and alkaline pH the non-noble metal catalyst such as the ones we considered in this review are highly prone to corrosion and affects the stability directly. To overcome this issue many catalysts have been now designed to be active at neutral pH. In this concern, the metal phosphides, sulphides and selenides have more advantages than the metals, metal oxides and metal hydroxides. As the oxides of S, Se and P are highly resistive towards corrosion resistance. Hence, these materials find very limited stability problems due to the solution pH of water electrolysis.

8.1.2 High Surface Energy and Over Oxidation of Catalyst Surface. These are two other serious problems that reduces the stability of that catalytic water electrolysis. It is known that the nanomaterials are having higher surface energy than their counterpart and when they are exposed to such a high applied electric potential field, there are fair chances of agglomeration of nearby particles. This will ultimately reduce the overall available active sites when compared to the initial stage of the catalytic process. Besides, agglomeration, there are chances for over oxidation of the nanomaterial surface due to these higher surface energies in case where the agglomeration of nearby particles are restricted by high dilution of catalyst concentration. As a consequence of over oxidation of catalyst surface, the resistance associated with the catalysts will also increase and will ultimately result in reduced catalytic activity. Efforts should be taken to avoid all the above explained problems associated with nanostructured catalysts by optimizing the experimental conditions with care.


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ACS Catalysis

8.2 Recent Trends in Electrode Fabrication with Nanostructured Catalysts for Prolonged Water Electrolysis. All the stability problems elaborated in the previous section can be overcome by simple and brilliant electrode fabrication methods. The most stable nanostructured catalyst modified interfaces are recently being reported by making use of either of the following two methods viz., electrochemical deposition and the hydrothermal/solvothermal growth of nanostructured catalysts on 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 desired current collectors by electrochemical deposition over preparing the same by other methods and running behind the suitable binders. However, this method is not always suitable for all kinds of materials. It is most reported for metal, metal oxides and metal hydroxide based nanostructures. Additionally, there are reports for sulphides and selenides too. The common substrates employed are the metal foils such as Ti and Cu, metals such as Ni in the form of Ni-mesh and Ni-foam, tin doped indium oxide (ITO), fluorine doped tin oxide (FTO), carbon fiber paper and cloth. Depending on the material any of the above substrate can be used for electrodeposition. For example, in photoelectrocatalytic water splitting, the ITO or 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 thickens. Similarly, disadvantages are the inefficiency in producing nanomaterials with various other morphologies and requirements of sophisticated instruments such as electrochemical work stations.

8.2.2 Hydrothermal or Solvothermal Growth Assisted Improvement in the Stability of Nanostructured catalysts. This is the recently explored method by materialists and it is suitable for almost all sort of materials. In this method the substrate to 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 fiber cloth. Materials prepared by these methods offer the advantages of preparing catalysts in various morphologies with hierarchy nature. As a 47 ACS Paragon Plus Environment

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Page 48 of 87

consequence of this advantage, it enables some special studies such as morphology and shape selective catalytic studies. The main disadvantage of this method is the lack of control on the quantity of the catalyst grown on desired substrates. Both methods are efficient in fabricating highly stable 3D catalyst electrode and should be chosen according to the material of interest.

8.3 The Sulphides of Fe, Co and Ni in Water Splitting. As we have discussed the methods of evaluating an electrocatalyst for water splitting and the synthesis of the catalyst materials, we would proceed the discussion of this section by benchmarking all the reported sulphides of Fe, Co and Ni metals on overpotential (η) at 10 mAcm2

and the Tafel slope values under two categories viz., metal sulphides for HER and metal sulphides

for OER. The activity parameters for total water splitting catalysts are split up and included along with other catalysts used for HER and OER. The same trend will be followed for the selenides and phosphides too. When the total water splitting is concerned, there is no Fe sulphide reported so far. There are only few reports on the total water splitting by these metal (Fe, Co, and Ni) sulphides. The report by Wang et al. on CoS supported on CT which in turn grown on CC is the lone report on the total water splitting activity of a Co sulphide.169 However, the literature says that the Ni sulphides are better bifunctional catalysts than Fe and Co sulphides. NiS microsphere grown on Ni foam by Zhu et al. Ni3S2 nanosheets grown on Ni foam are the good in catalyzing both HER and OER in alkaline conditions.172 However, significant advancement in the bifunctional activity of these sulphides are achieved when they are present as bimetallic sulphides. The electrodeposited Ni-Co-S as reported by Liu et al.187 and Ni promoted formation of CoS2 NWs array on CC as reported by Fang et al. are the example of this kind.188

8.3.1 Activity Trends of Metal (Fe, Co, Ni) Sulphides in Electrocatalytic HER. The available reports on the sulphides of Fe, Co and Ni, reveals one thing clearly that they have been employed more for HER than for OER and total water splitting. It has been highlighted in synthesis part that till date there is no report except the FeS for HER by Giovanni et al.163 and pyrite type FeS2 for HER by Faber et al.165 On the other hand, reports on Co based sulphides are rich in literature than both Fe and Ni. Interestingly, it can be found that almost 80% of the available reports on the Co based sulphides were studied for HER and the reports for OER and total water splitting are limited. Among the various Co based sulphides such as CoS, CoS2, Co3S4 and Co9S8, 48 ACS Paragon Plus Environment

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ACS Catalysis

the CoS and the pyrite type CoS2 are the ones frequently reported for HER. In case of Ni based sulphides, there are only two reports for HER which are the report of Faber et al. on NiS 2165 and report of Tang et al. on Ni3S2.188 Apart from these, there are reports on HER activity of bimetallic sulphides such as Co-Fe-S, Co-Ni-S, Fe-Ni-S and Ni-Co-S. To have a comparative view on the catalytic activity of these sulphides towards HER, they have been benchmarked against their overpotential (η) at 10 mAcm-2 and Tafel slope. The same is provided as Figure 9. More info on the electrochemical conditions at which these data were acquired and others are provided in Table 1. From Figure 6, it is clear that the FeNiS catalyst reported by Long and coworkers is the best catalyst with the lowest HER overpotential of 105 mV at 10 mAcm-2. In Table 5, other catalysts are listed in the order of increasing overpotential (η) at 10 mAcm-2.

Figure 9: Benchmarking the metal (Fe, Co, Ni) sulphides against their HER overpotential (10 mAcm-2) and the corresponding trends in the Tafel slopes.

Since there had been a serious discussion on the selection of activity parameters to evaluate an electrocatalyst’s efficiency on water splitting, it is fruitful to arrive at a conclusion whether the use of overpotential (η) at 10 mAcm-2 with the Tafel slope as primary activity parameter is good for evaluating catalyst or not. The Figure 1 reveals clearly one thing that the overpotential has nothing to do with the kinetics of the Electrocatalytic water splitting process. As we can see that the catalysts with one among the lowest overpotential (η) at 10 mAcm-2 have larger Tafel slopes than 49 ACS Paragon Plus Environment

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some catalysts with relatively larger overpotential (η) at 10 mAcm-2 such as CoPS/CNT reported by Liu and coworkers which had shown overpotential of 480 mV at 10 mAcm-2 with a Tafel slope of 55 mV/dec.261

Table 5: Benchmarking the metal (Fe, Co, Ni) sulphides against their HER overpotential at 10 mAcm-2. Tafel slope (mV/dec) 40

S. No

Catalyst

1.

A-FeNiS

2.

B-FeNiS

0.5 M H2SO4

117

48

191

3.

CoS2

0.5 M H2SO4

128

52

193

4.

Ni2.3%CoS2/CC

1 M KOH

136

106

236

5.

Co3S4/NCNTs

0.5 M H2SO4

140

70

190

6.

NiCoS/CC NSs

1 M KOH

140

96

178

7.

CoS2 NS/RGO/ CNT

0.5 M H2SO4

142

51

(Fe0.48Co0.52)S2

0.5 M H2SO4

143

1 M KOH

150

83

182

1 M KOH 0.5 M H2SO4

150 160

106 46

187 191

0.5 M H2SO4

160

79

1 M KOH

163

8.

9.

NiS/Ni Foam

10. Ni2.3%CoS2/CC 11. Fe0.9Co0.1S2/CNT

12.

Co3S4

13. Co9S8-NixSy/NiF

Electroly te 0.5 M H2SO4

Overpotential @10 mAcm-2 (mV) 105

Ref. 191

172 47.5 165

174 88


189

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ACS Catalysis

14. (Co0.59Ni0.41)S 2

0.5 M H2SO4

170

50.4

171

15.

0.5 M H2SO4

190

72

171

1 M KOH 1 M KOH

~190 195

141 107

188 179

CoS2/Ti (pH = 0.3)

16. NiCo2S4 NA/CC 17. Ni3S2-AT Ni 18.

Fe0.07Ni0.91S2

0.5 M H2SO4

196

58.7

165

19.

NiMoS/C 1:1

pH=7

200

85.3

192

20. Zn0.76Co0.24S/CoS 1 M KOH on Ti mesh 21. B-NiS 0.5 M H2SO4

~200

164

176

202

-

191

22.

0.5 M H2SO4

217

56.4

165

FeS2 23.

NiS2

0.5 M H2SO4

230

48.8

165

24.

NixSy/NiF

1 M KOH

230

87

189

25. MW-CoS (Nano Prism)

0.5 M H2SO4

230

76

176

26.

ST-CoS (Nano Prism)

0.5 M H2SO4

240

90

176

27. 28.

NiS on CC CoS2/Ti (pH = 13.37)

pH = 7 0.5 M H2SO4

243 244

69 133

180

29. 30.

Fe0.1NiS2 NA/Ti Ni3S2-Ni

1 M KOH 1 M KOH

~250 270

108 141

181 179

31.

C09S8/C

Phosphate buffer

280

-

175

32.

CoS2 NS/RGO

0.5 M H2SO4

280

82

172

184


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33.

3D G/CoSx

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1 M PBS

330

93

177

34. Co9S8 (for all pH) Phosphate buffer

340

-

175

35.

FeS (pH = 7)

0.1 M Phosphate buffer

350

150

163

36.

Co9S8-700

Phosphate buffer

370

-

175

37.

COS|P/CNT

0.5 M H2SO4

480

55 261

Note: “hyphenated” cells in this table indicates that the corresponding data are not available with the respective report cited here.

This primarily alarm us that the thermodynamic activity parameter (i.e., the Overpotential) is not affecting or least affecting the kinetic activity parameter (i.e., the Tafel slope). Similar trends are also observed with selenides and phosphides based catalysts for both HER and OER and discussed in subsequent sections.

8.3.2 Activity Trends of Metal (Fe, Co, Ni) Sulphides in Electrocatalytic OER. Though sulphides of Fe, Co and Ni had been devoted more towards HER, there are reports also available on the OER catalytic performance of these sulphides particularly the sulphides of Co and Ni. There is no data yelling that the OER and total water splitting activity of Fe based sulphides alone. However, it has been reported by as a bimetallic sulphide with Co as Co-Fe-S on N-doped mesoporous carbon for OER along with ORR by Shen and coworkers.164 The story with Co based sulphide is different from that Fe that of based sulphides. Though Co based sulphides are also much devoted to HER, there are reports for their OER and total water splitting activity. Another thing to be noticed here is that wherever the Co based sulphides are applied to OER or total water splitting, it is always applied with a substrate material like carbon nanostructures and Ti foil. Among them, the report by Wang et al. on Co3S4 on N-doped CNT,174 CoS nanosheets grown Ti by Liu et al.263 and CoS2 grown on N and S co-doped GO by Ganesan et al.173 In case 52 ACS Paragon Plus Environment

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Ni, there is a lone report on Ni3S2 nanosheets grown on Ni foam which had shown low overpotential for OER in alkaline conditions. Other than these, the bimetallic NiCo2S4@graphene has been reported by Liu et al.185 for OER and ORR in alkaline conditions. Like HER, as a comparative measure, the plots of the overpotential (η) at 10 mAcm-2 and Tafel slope have been given as Figure 10 as listed in Table 6.

Figure 10: Benchmarking the metal (Fe, Co, Ni) sulphides against their OER overpotential (10 mAcm-2) and the corresponding trends in the Tafel slopes.

Table 6: Benchmarking the metal (Fe, Co, Ni) sulphides against their OER overpotential at 10 mAcm-2. S. No.

Catalyst

Electrolyte

1 2

Fe0.1NiS2 NA/Ti Ni2.3%CoS2/CC

1 M KOH 1 M KOH

Overpotentia l @ 10 mAcm-2 (mV) ~205 297

3 4

NiS/Ni Foam Zn0.76Co0.24S/Co S on Ti mesh Ni2.3%CoS2/CC Ni3S2-AT Ni

1 M KOH 1 M KOH

300 ~300

89 79

180 176

1 M KOH 0.1 KOH

~300 330

119 163

187 179

5 6

Tafel Slope (mV/dec) 43 117


Ref. 181 263

ACS Catalysis

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

7 8 9

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1 M KOH 1 M KOH 0.1 KOH

330 340 355

109 89 -

178 188 185

10

NiCoS/CC NSs NiCo2S4 NA/CC NiCo2S4@N/SRGO CoS2/N-S GO

1 M KOH

370

-

173

11 12

ALD NiSx Co3S4

1 M KOH 0.1 KOH

372 375

41 -

183 185

13

Ni3S2-Ni

0.1 KOH

410

330

179

14

CP/CNT/CoS

1 M KOH

450

-

169

15

Ni3S4

0.1 KOH

555

-

185


Like we have seen in HER, the activity trends of these metal sulphide catalysts benchmarked against their overpotential at 10 mAcm-2 and 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 with the overpotential (thermodynamics) of a water splitting catalyst.

8.4 The Selenides of Fe, Co and Ni in Water Splitting. The irregularity seen in the trends of selenides of Fe, Co and Ni are more vigorous in both OER and HER than the one observed with the sulphides. This indicates the strong influence of a ligand belongs to 3d series of periodic table with 4s shells and comparable ionic radii to that of the central metal atom. However, the best correlation among these metal selenides are given for a comparative evaluation via the bench marking plots (Figure 11 and Figure 12) of overpotential (η) at 10 mAcm-2 and Table 7 and Table 8 for HER and OER activity trends respectively.

8.4.1 Activity Trends of Metal (Fe, Co, Ni) Selenides in Electrocatalytic HER. As we highlighted in the synthetic methodologies part, there is no lone report on Ni and Fe selenides for HER. However, there are some bifunctional water splitting selenides as bi- and trimetallic selenides. On the other hand, the most reported CoSe2 with various morphology and substrate materials has itself showed different trends in the HER catalytic performance. The same


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ACS Catalysis

can be seen when they are benchmarked against the overpotential (η) at 10 mAcm-2 and Tafel slope as shown in Figure 11.

Figure 11: Benchmarking the metal (Fe, Co, Ni) selenides against their HER overpotential (10 mAcm-2) and the corresponding trends in the Tafel slopes.

As all of them are the same material, it is hard to make a conclusion on the trends of 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 mAcm-2 for HER among all other selenide based HER catalysts as reported by Liu and coworkers.263 The Selenides which are reported so far within the group VIII 3d metals have been listed in the increasing order of their overpotential at 10 mAcm2

along with the experimental conditions under which the evaluations were made are listed in Table

7. Like we observed with the sulphides, the selenides too show similar trends in the correlation between the thermodynamic activity parameter (overpotential) and the kinetic activity parameter (Tafel slope). This is emphasizing the independency of overpotential from the nature of kinetics of HER and vice-versa once again.

Table 7: Benchmarking the metal (Fe, Co, Ni) selenides against their HER overpotential at 10 mAcm-2. S. No

Catalyst

Electrolyte

Overpotentia l @ 10

Tafel Slope (mV/dec)


Ref.

ACS Catalysis

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1

Co0.13Ni0.87Se2/Ti

1 M KOH

mAcm-2 (mV) 64

2

MoS2/CoSe2

0.5 M

68

36

105

1 M KOH

70

82

196

63

263

H2SO4 3

Ni3Se2 electrodeposited

4

NiSe/NF

1 M KOH

96

120

194

5

Ni3Se2/CF

1 M KOH

100

98

197

6

Ni/NiO/CoSe2

0.5 M

100

39

198

100

43

250

130

32

206

137

40

199

170

44

250

175

33

250

193

50

206

H2SO4 7

NiPSe(1.93:0.07)

0.5 M H2SO4

8

CoSe2-NW/CC

0.5 M H2SO4

9

CoSe2/CP

0.5 M H2SO4

10

NiPSe(0.09:1.91)

0.5 M H2SO4

11

NiSe2

0.5 M H2SO4

12

CoSe2-MP/CC

0.5 M H2SO4

13

EG/CoSe-NiFe-LDH

1 M KOH

260

-

122

14

Co0.13Ni0.87Se2/Ti

1 M KOH

270

94

263

15

CoPSe/MWCNT/GC

0.5 M

370

46

118

E

H2SO4

Note: “hyphenated” cells in this table indicates that the corresponding data are not available with the respective report cited here. 8.4.2 Activity Trends of Metal (Fe, Co, Ni) Selenides in Electrocatalytic OER. 56 ACS Paragon Plus Environment

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Like HER, the OER catalysis by these metal selenides is mainly occupied by the CoSe 2 and other sulphides of Co. Some of the significant works are highlighted subsequently. The report of CoSe oxohalides by Rabbani et al.,264 Coral-like 3D CoSe2 crystals reported by Liao et al.203 and CeO2 doped CoSe2 reported by Zheng et al.205 Similarly for NiSe2, there is a report by Tang et al.194 Likewise we did in HER, a benchmarking plot on the activity parameters i.e., the overpotential (η) at 10 mAcm-2 and the Tafel slope of these selenides are provided as Figure 12. Other relevant data on the same is included with Table 8. As expected, the similar irregular trend in the Tafel slopes when these selenide based catalysts are queued according to their increasing overpotential order. This too emphasize the independency of overpotential from the nature of kinetics of HER and vice-versa in a row again.

Figure 12: Benchmarking the metal (Fe, Co, Ni) selenides against their OER overpotential (10 mAcm-2) and the corresponding trends in the Tafel slopes.

Table 8: Benchmarking the metal (Fe, Co, Ni) selenides against their OER overpotential at 10 mAcm-2. S. No .

Overpotential Catalyst

Electrolyte

@ 10 mAcm-2

Tafel Slope

(mV)

(mV/dec)


Ref.

ACS Catalysis

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

EG/CoSe-NiFe-

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1 M KOH

250

57

122

1 M KOH

290

40

173

1 M KOH

290

82

196

LDH 2

Coral like CoSe2

3

Ni3Se2 electrodeposite d

4

CoSe2/Ti mesh

1 M KOH

292

69

200

5

Ni3Se2/CF

1 M KOH

~300

80

197

6

CeO2/CoSe2

1 M KOH

310

44

205

7

CoSe2 NPs

1 M KOH

350

49

173

8

NiSe/NF

1 M KOH

400

-

194

9

Mn3O4-CoSe2

0.1 KOH

450

49

186


8.5 The Phosphides of Fe, Co and Ni in Water Splitting. Similar to the sulphides 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 Electrocatalytic HER. Unlike the selenides, the phosphides of Fe, Co and Ni are equally reported for HER. When the HER is concerned, the maximum number of report (~40) has been published using the phosphides. The sulphides comes the second and the quantity of selenide based HER catalysis is under minor level when compared to the sulphides and phosphides. The plot of overpotential (η) at 10 mAcm-2 and the Tafel slope of metal phosphides for HER is given as Figure 13 with the Table 9 carrying the experimental details under which the measurements on the HER activity on 58 ACS Paragon Plus Environment

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ACS Catalysis

these phosphide catalysts were made. From Figure 13 and Table 9, an interesting thing is noticed which is the lower HER overpotentials (η) at 10 mAcm-2 were shown predominantly by the Fe phosphides than the Co and Ni phosphides.

Figure 13: Benchmarking the metal (Fe, Co, Ni) phosphides against their HER overpotential (10 mAcm-2) and the corresponding trends in the Tafel slopes.

This is in sharp contrast when the HER activity trends of the sulphides and selenides of Fe, Co and Ni are concerned. In case of sulphides and selenides, either Co or Ni or the mixture of both Co and Ni occupies the top position in the Table of HER and OER activity trends in accordance with the predicted activity trends of 3d metal oxides and hydroxides by Subbaraman and coworkers earlier.151 Table 9: Benchmarking the metal (Fe, Co, Ni) phosphides against their HER overpotential at 10 mAcm-2.

S.

Catalyst

Electrolyte

No 1

Ni5P4

0.5 M

Overpotential

Tafel

@ 10 mAcm-2

Slope

(mV)

(mV/dec)

24

27

H2SO4 59 ACS Paragon Plus Environment

Ref.

252

ACS Catalysis

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2

FeP/CC

0.5 M

Nanocrystal -

H2SO4

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34

29.2

215

39

30

223

40

46.1

214

48

70

222

300OC 3

4

Fe0.5Co0.5P/C

0.5 M

C

H2SO4

NixPy

0.5 M H2SO4

5

CoP NPs/CC

0.5 M H2SO4

6

Ni5P4

1 M NaOH

50

14

252

7

CoP MNA

0.5 M

54

51

230

55

30

246

58

45

213

pH 0-14

67

51

219

H2SO4 8

Ni2P-G@NiF

0.5 M H2SO4

9

FeP NAS/CC

0.5 M H2SO4

10

CoP Nanoporous

11

CoP/Ti mesh

1 M KOH

72

65

222

12

NiP2-NS/CC

0.5 M

75

51

251

~80

65

223

90

40

228

92

49

237

98

-

225

H2SO4 13

14

3D CoP Nws

0.5 M

on TI

H2SO4

CoP-NBAs/Ti

0.5 M H2SO4

15

CoP/NCNT

0.5 M H2SO4

16

CoP branched

0.5 M

NS on Ti Foil

H2SO4


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17

Ni2P/Ni/NF

H2 sat 1 M

98

72

243

KOH 18

Ni2P-G@NiF

1 M KOH

100

30

246

19

CoP NWs

0.5 M

110

54

234

113

59

163

H2SO4 20

FeP/NCNT

0.5 M H2SO4

21

NiP2-NS/CC

1 M KOH

110

64

251

22

CoP@NPC

0.5 M

120

69

116

120

79.1

247

123

54

141

H2SO4 23

24

Ni5P4-Ni2P-

0.5 M

NS

H2SO4

CoP/ CNT

0.5 M H2SO4

25

CoP/C

0.1 M KOH

130

-

235

26

CoP NTs

0.5 M

130

60

221

130

106.1

249

138

-

212

1 M KOH

145

60.6

255

0.5 M

148

62

237

150

93

243

159

59

226

160

40

246

H2SO4 27

Ni2P-NRs-Ni

N2 sat 0.5 M H2SO4

28

Fe2P/NGR

0.5 M H2SO4

29

NiCoP hollow NCs

30

Co2P/NCNT

H2SO4 31

Ni2P/NF

H2 sat 1 M KOH

32

33

CoP hollow

0.5 M

polyhedron

H2SO4

Ni2P-G@NiF

1 M Kpi


ACS Catalysis

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

34

CoP NSs

0.5 M

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164

61

234

165

68

237

168

46

227

170

61

239

195

74

237

221

87

234

240

104.8

237

383

90

237

406

101

237

H2SO4 35

CoP/CNT

0.5 M H2SO4

36

CoP

0.5 M H2SO4

37

CoP@C

Ar sat 0.5 M H2SO4

38

Co2P/CNT

0.5 M H2SO4

39

CoP NPs

0.5 M H2SO4

40

41

CoP/RGO

0.5 M

(0.30)

H2SO4

CoP

0.5 M H2SO4

42

Co2P

0.5 M H2SO4


However, there are some specific phosphides of Ni and Co with specific P content such as Ni5P4, NiP2 and CoP which had shown competently good HER performances to that of Fe phosphides. As observed with the sulphides and selenides, the phosphide catalysts ranked by the overpotential (η) at 10 mAcm-2 have shown irregular trends in the Tafel slope values. Catalysts with low overpotentials found to be showing higher Tafel slope and vice-versa. This indicates us that the phosphides are not exceptions and they too have independent thermodynamics 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 sulphides and selenides. 62 ACS Paragon Plus Environment

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8.5.2 Activity Trends of Metal (Fe, Co, Ni) Phosphides in Electrocatalytic OER.

Figure 14: Benchmarking the metal (Fe, Co, Ni) phosphides against their OER overpotential (10 mAcm-2) and the corresponding trends in the Tafel slopes.

Application of phosphides of Fe, Co and Ni to OER is new and there are relatively limited number of reports when compared to the reports available on the same for HER application. However, here we have made similar comparative evaluation on the available reports on the metal phosphides for OER applications via the benchmarking plots of overpotential at (η) at 10 mAcm-2 and the Tafel slope which is given as Figure 14 along with a Table 10 carrying information on the experimental conditions under which the results on the OER activity of these metal phosphides were acquired. As expected, the irregular trends in the Tafel slope when these catalysts are queued according to their overpotential (η) at 10 mAcm-2 in increasing order. Hence, the same conclusion of independency of kinetics of an electrocatalyst from the thermodynamics of the catalysts can be drawn. Table 10: Benchmarking the metal (Fe, Co, Ni) selenides against their OER overpotential at 10 mAcm-2. Overpotential S. No.

Catalyst

Electrolyte

@ 10 mAcm-2 Tafel Slope Ref. (mV)


(mV/dec)

ACS Catalysis

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 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Ni2P/Ni/NF

O2 sat 1 M

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205

-

243

220

-

243

KOH 2

Ni2P/NF

O2 sat 1 M KOH

3

Ni2P

1 M KOH

300

64

249

4

Co2P NPs

1 M KOH

310

50

223

5

CoP/Ti mesh

1 M KOH

310

87

222

6

CoP MNA

1 M KOH

310

65

230

7

Ni5P4

1 M KOH

330

-

253

8

CoP/C

0.1 M KOH

358

66

235

9

CP@FeP

1 M KOH

365

63.6

210

10

CoP hollow

1 M KOH

400

57

226

polyhedron Note: “hyphenated” cells in this table indicates that the corresponding data are not available with the respective report cited here.

8.6 Activity Trends of Metal (Fe, Co, Ni) Phosphosulphides and Phosphoselenides in Electrocatalytic HER and OER. Beyond the sulphides, selenides and phosphides alone as electrocatalysts, people have now moved forward in combining the effect two different ligands such as P and S and P and Se together with a single metal out of Fe, Co and Ni. Following are the reports where such kind of materials have been employed for HER, OER and TWS so far. A report on pyrite structure CoPS for HER was very recently reported by Liu and coworkers.262 Similarly, another report by Xiao and coworkers stating the exceptional HER performance of Co-P-Se NPs.118 Another interesting report was given by Zhuo and coworkers recently on the HER activity of Se doped pyrite type nickel diphosphide (NiP2).251 These catalysts are included with the sulphides and selenides benchmarking plots (Figure 6 and Figure 8) for comparative evaluation and the respective experimental details are included with the Table 1 and Table 3 respectively. This kind of materials are quite promising and can offer new pathways in increasing the catalytic activities of these sulphides and selenides consequently. 64 ACS Paragon Plus Environment

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a

b

Figure 15: (a) The benchmarking plot of the best ever metal (Fe, Co, Ni) sulphide, selenide and phosphide HER catalysts according to their overpotential at 10 mAcm-2. (b) The benchmarking plot of the best ever metal (Fe, Co, Ni) sulphide, selenide and phosphide OER catalysts according to their overpotential at 10 mAcm-2. Having seen a detailed comparative evaluation approaches by means benchmarking plots and the trends observed with the Tafel slopes, it is now essential to compare the best HER catalysts among the sulphides, selenides and phosphides as well as the best OER catalysts among the sulphides, selenides and phosphides. Such a comparative plots are provided as Figure 15, a-b. From both 65 ACS Paragon Plus Environment

ACS Catalysis

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Figure 12a and Figure 12b, we can conclude that the phosphides are the better catalysts than the sulphides and selenides of Fe, Co and Ni for both HER and OER. This offers us a new pathways of modifying the catalytic properties of these materials by tuning heteroatoms (S, Se, P) content and/or mixing them with a single metal atom among Fe, Co and Ni or with a bi- or tri-metallic alloy of the same. With a given comparative view, the knowledge on the activity trends of Fe, Co and Ni base sulphide, selenide and phosphide catalysts is now clearly exposed. Hence, herby 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 from Table 5, Table 7 and Table 9 have revealed that the anomaly observed in the Tafel slope values against their overpotential is mainly due to the pH of the medium of electrolysis. It is clear that all lower values of Tafel slopes (approximately up to 55 mV/dec) were almost seen when the studies were carried in acidic solution. This again indicates that though there are potential catalysts being developed for HER in, there is no catalyst with better kinetics as observed in acidic HER electrocatalysis. One more thing needed to be emphasized here in the benchmarking of catalysts is the lowest overpotential showing fabricated electrodes such as nano arrays on various supports like Ti mesh, CC, CF, Ni foam and carbon paper. With these cases the loading of catalysts is normally 5-10 folds higher than the normal nanocatalysts used in other studies. It leads us to misunderstand that they are better than the other studies but actually not. This again emphasizes the support that the mass normalized current density would be better than a geometrical area or the ECSA normalized current density for a fair comparison of different catalysts. Having reviewed intensively the current perspectives in electrochemical water splitting catalysis, we recommend the following activity parameters be compulsorily incorporated in future reports of any such electrocatalysts for OER and HER viz. Overpotential at defined current density normalized by geometrical area and mass of loaded catalyst, Tafel slope, exchange current density (for HER), Mass activity and 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 out alternative and viable sources of energies to enlighten the decedents of mankind in near future. As per the recent studies carried on energy consumption and the availability of sources, by 2015 the 66 ACS Paragon Plus Environment

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world will require 30 TW of power from new and non-conventional 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 are that they are seasonal and hence requires means for large scale energy storage systems beyond the incapable batteries and supercapacitors. One such indirect way of large-scale energy storage is the water splitting. As we are aware that the conventional combustion engines running on conventional carbon fuels are badly contaminating the environment which is being considered as the most serious threat for the mankind in near future, people have now diverted their attention to new zero carbon emitting and/or least carbon emitting engines such as the fuel cells. The fuel cell technology is the greener available way of generating electrical energy from various sources such as hydrogen, oxygen, methanol, glycerol, borohydride, formic acids, and more as fuels. Among them, the one which uses the H2 and O2 as fuel is the greenest way of producing 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 not certainly the one that gives the purest H2. The H2 obtained by coal reformation will have 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 and thereby reducing the number of active sites available for the desired electrochemical reaction. A great attention has now been given to the 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 huge energy loss while doing it which increases the overpotential of HER and OER at cathode and anode further. Hence, both 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 so still). Similarly for OER, either Ir or Ru and their compounds are used as the state-ofthe-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 OER is the process that can occur at very high anodic overpotential than HER. To avoid such pitfalls, people have then started looking at the 3d transition metal oxides and hydroxides for OER Electrocatalysis and the chalcogenides 67 ACS Paragon Plus Environment

ACS Catalysis

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of W, Mo, Ta and Ti for HER Electrocatalysis. Very recently, people have found that the sulphides, selenides and phosphides of 3d transition metals particularly the group VIII metals (Fe, Co, Ni) are catalyzing both HER and OER in all pH with almost the same kinetics. Advantages of these 3d transition metals based catalysts are that they highly abundant, cheaper, efficient and easy to design them in a desired shape and structure depending on the needs for electrolysis. Hence, to make use of these advantages, we should be aware with the activity trends, mechanisms of OER and HER on the surface of these group VIII 3d metals (Fe, Co, Ni) based sulphide, selenide and phosphide catalysts. Here in this review, we have made such a comparative measure on group VIII 3d metals (Fe, Co, Ni) based sulphide, selenide and phosphide catalysts to enable the people 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 HER and OER. With this comparative review, we enable the people to acquire knowledge on the trends of activities of these catalysts towards HER and OER and help them to formulate new highly efficient water splitting catalysts for the sake of our global future energy requirements.

10. Acknowledgements. We wish to acknowledge Dr. Vijayamohanan K Pillai, Director, CSIRCECRI, Karaikudi, India. for his continuous support and encouragement. S. Anantharaj, S. R. Ede and K. Sakthikumar wishes to acknowledge CSIR, New Delhi, and K. Karthick acknowledges UGC, New Delhi, for the afforded funding through 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.

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Table of Content Recent Trends and Perspectives in Electrochemical Water Splitting with an Emphasis to Sulphide, Selenide and Phosphide Catalysts of Fe, Co and Ni: A Review

The group VIII 3d metal (Fe, Co, Ni) sulphide, selenide and phosphide nanostructure catalysts have recently evolved as highly efficient, non-precious, stable electrocatalysts materials for electrochemical water splitting. In this review, the recent trends and perspectives of electrochemical water splitting catalysis with an emphasis to these mentioned catalysts have been presented in a coherent manner to enable the reader to be updated from the basics to the frontiers in this field.


Recent Trends and Perspectives in Electrochemical Water Splitting

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