Review
A critical review on transition metal phosphide based catalyst for electrochemical hydrogen evolution reaction: Gibbs free energy, composition, stability, and true identity of active site

https://doi.org/10.1016/j.ccr.2022.214956Get rights and content

Highlights

  • Summarized effect of composition, microstructure, hydrophilic, and aerophobic natures of the transition metal phosphide (TMP) towards electrochemical hydrogen evolution reaction.

  • The current manuscript elaborated detailed reaction mechanism of the HER at the TMP-based catalyst and the role of the catalyst composition on the ΔG for HER.

  • The evaluation of the electrochemical stability of the TMP-based catalyst.

  • Identification of the true active site of TMP-based catalyst for HER under potential.

Abstract

The current state of global green H2 demand from water required an efficient and robust catalyst to mitigate greenhouse-gas emissions to the atmosphere. The transition metal phosphides (TMPs) based catalysts are found to be stand out an alternative to the state-of-art Pt/C catalysts for hydrogen evolution reaction (HER). However, systematic desertion is required to assess the recent development and progress towards TMPs based catalyst for HER for thoughtful knowledge to encourage the develop an efficient, robust, and durable catalyst. The current progress made to improve the efficiency, and durability of as-synthesized TMPs based catalyst for HER, include nanostructure synthesis, integration with supports materials, and hybrid structure synthesis. The realization of the individual role of each constituting element of the TMPs-based catalyst is the fundamental pillar to develop an efficient electrocatalyst. Herein, an overview is presented of recent research progress on TMPs for electrochemical HER. The review elaborated on the fundamental aspect of HER kinetics at the TMPs-based catalyst surface, the effect of composition, and pH for 3d (Fe, Co, Ni, and Cu) to higher transition metal (Mo, W, Rh, Ru, Pd, Ir, and Pt) phosphide. The current review also systematically compiled an experimental and theoretical study on HER. Furthermore, we also highlight the urgent need for pre and post-analysis to evaluate the true identity active site and the durability of the TMPs. Finally, the conclusions are drawn with future opportunities and challenges for the development of transition TMPs based.

Introduction

Environmental degradation over the years and extensive amount of fossil fuel consumption with civilization has forced us to think of new alternative energy resources. Hydrogen (H2) is a clean energy resource and has the highest gravimetric energy. It is compatible with CO2-free electrochemical processes and can be produced from water electrolysis using renewable power resources. However, most of the H2 consumed for industrial and fuel purposes is produced from steam reforming of natural gas or coal still to date. The H2 production from steam reforming is dominated over the years, due to the low cost of crude oil/gas in comparison with renewable energy. Consequently, the CO2 emission associated with steam reforming is increased exponentially.[1] Therefore, alternative energy technology needs to be put forward to produce H2 in order to compensate for over-dependency on fossil fuels. Electrochemical and photoelectrochemical water splitting methods have progressed in recent decades with huge prospects and are considered one of the prominent methods to convert water to H2 using renewable energy resources.[2] However, the water-to-H2 conversion is a difficult task without using a noble metal catalyst. Currently, Pt-group or Pt-based materials are considered a benchmark catalysts for HER. However, it is an expensive and scarce element, is one of the major obstacle for industrial water electrolyzers. To make the electrochemical water splitting method feasible for large-scale H2 production, it is important to design and develop earth abundant noble metal-free electrocatalysts with high efficiency and durability. The objective has motivated the scientific community to find out earth-abundant Pt-free inexpensive catalyst for HER. Over the year, mostly transition metal-based sulfides[3], [4], phosphides,[5], [6] nitrides,[7], [8] and carbides[9], [10] are explored as a potential alternative to Pt/C. Thanks to all the research efforts made in the recent past to evolve this area of research and identification of efficient noble metal-free catalyst for HER. It is obvious that a large applied potential across the electrode can produce enough H2 from water electrolysis at the catalyst surface. However, in this process, Faradic efficiency (FE) is eventually decreased, which contradicts the fundamental objective of HER. catalyst has the ability to evolve H2 from water at lower potentials with the high exchange current density is desired. In general, the Tafel slope value is exclusively used to evaluate the intrinsic catalytic activity of the catalyst experimentally. A lower Tafel slope directly indicates higher catalyst activity. Theoretical investigation of the HER kinetics at different catalyst surfaces also provides a thoughtful inside to design catalysts. For example, the Density Functional Theory (DFT) calculation of the Gibbs free energy for H adsorption energy (ΔGH*) of the particular catalyst surface is an important parameter to assess any catalyst and is thoughtfully used to correlate the performance of the catalyst. For an ideal catalyst, ΔGH* is supposed to be nearly zero for HER.[11] This implies that the atomic hydrogen formed during the progression of the reaction should bind the catalyst surface with optimum strength so that both the catalyst to proton-electron transfer and the removal of the molecular H2 process should not be inhibited. Not only did the hydrogen binding energy of the catalyst controlled the efficiency of HER, but various other factors also significantly influence and controlled the catalyst efficiency, such as roughness factor, conductivity, durability, charge transfer kinetics across the electrode-electrolyte interface, and so on. These are all necessary parameters that need to be assessed carefully to enhance the efficiency of a catalyst, but not the ultimate criterion. Taking into consideration all such important notes, over the year, a great varity of catalysts developed for HER with pros and cons. For example, Ni-based alloys are considered a potential substitute for Pt due to its low cost, and high stability in alkaline medium. However, Ni-based alloy catalyst is found to be less corrosion resistance in acid medium, where a high production rate is achievable in the acidic medium at lower potentials. This was found to be a significant obstacle for Ni-based alloyed electrocatalysts.[12] The synthesis of the earth-abundant low-cost catalyst with high durability is a bound criterion apart from efficiency. In recent years, many non-Pt catalysts are developed for HER. For example, Zheng et al. grown three-dimensional cobalt diselenide on the carbon fiber felt for HER.[13] The catalyst required only 141 mV overpotential to attain 10 mA/cm2 current density and a Tafel slope of 64 mV/dec. The catalyst retained its activity after 30,000 potential cycles in 0.5 M H2SO4. The exclusive charge transfers between carbon felt and cobalt selenide, open nanostructure (NS) cobalt selenide offers a more active site, which renders hydrogen adsorption during the HER. Two dimensional (2D) MoS2 is another important noble metal-free electrocatalyst is extensively studied for HER, and the edge site S atoms are responsible for HER kinetics.[14] However, most of the S atoms stayed in the basal plane rather than the edge site. The 2D MoS2 nanosheets are aggregated due to the van der Waals force of attraction, which conceals the edge site of MoS2. As a result, the mass transport toward the active site of the catalyst decreases. So, the activity of the catalyst is decreased.[15], [16] To enhance the electrocatalytic activity of the MoS2, Meng et al. synthesized 3D hierarchical Co-doped MoS2/graphene as electrocatalyst for HER.[17] The graphene is introduced to increase the electron conductivity of MoS2, and it also facilitates uniform dispersion of MoS2 NPs. Hence, catalysts exposed with more edge sites are necessary for HER. Lu et al. synthesized dual-phase carbide nanocrystal integrated with porous nitrogen-doped carbon (MoC-Mo2C/PNCDs) for HER.[18] The catalyst MoC-Mo2C/PNCDs needed an overpotential of 121 mV to reach a current density of 10 mA/cm2. Moreover, the overpotential required to reach a current density of 100 mA/cm2 is 182 mV, 87 mV and 49 mV lower than those of MoC/PNCDs and Mo2C/PNCDs, respectively. The highly dispersed carbide nanocrystal is obtained at high temperatures from the carbonization of ordered MO4 units present in the porous ZIF framework. Carbide nanocrystals embedded in porous carbon framework favored the exposure of more active sites and facilitates the charge transfer kinetics across the electrode/electrolyte interface.[19] The Nitrogen–site is acts as electron acceptors and downshifted d-band center of metal to boost HER kinetics.[19], [20], [21] Heteroatom-doped carbon-based nanostructure materials are also studied as metal-free electrocatalysts for HER. Pure carbon-based nanostructures are poor electrocatalysts. To further improve the pure carbon-based materials electrocatalytic activity mono, dual, and or multiple heteroatom-doped carbon-based nanostructures are synthesized and explored. For example, Qu et al. rationalized the effect of B, P, and S doping in N-doped carbons nanosheets for HER.[22] The author reveals that S has the highest promotional effect followed by P. However, B-doping decreases the HER activity of N-doped carbons. The theoretical calculation of Gibbs free energy for hydrogen adsorption (ΔG*H) reveals that N, S-dual doped carbon shows a lower ΔG*H value of 0.23 eV which is found to be significantly lower than the N-doped carbon (0.81 eV). However, the ΔG*H has increased for B, N-dual doped carbon. The lower hydrogen adsorption value is directly correlated with the performance HER catalyst. The theoretical and experimental studies are quite evident about role of the specific active site and their requirement to attain our objective.

Incredible progress is made over the years in the search for highly efficient durable catalysts. It took tremendous work efforts from the scientific community. As a result, the details of the reaction kinetics by means of experimental and theoretical studies are explored. The successful catalyst synthesis is intricate, and it is not a simple one-way process. It strongly depends on multiple factors. The progress of the TMP-based electrocatalyst over the past decades for HER is summarized in Fig. 1.[23], [24], [25], [26], [27], [28], [29], [30] The current research approaches are mainly focused on how to improve or mold a catalyst to an efficient one and rationalize the true active site. In this regard, we thoroughly discussed the recent progress made on TMPs-based catalysts and elaborate exclusive findings for its inherent electrocatalytic activity towards HER. The primary reason for the efficient catalytic activity of the TMP based catalyst required elaborate discussion. In this context, we discussed all such key aspects of different TMPs based catalysts and highlights the key feature of the respective TMP-based electrocatalysts towards improved HER in terms of Tafel slope, overpotential, stability and correlated with theoretical calculation with the experimental results.

Before we go through the details about the TMPs catalyst for HER, it is important to know why is there so much fantasy about hydrogen. The general perception of hydrogen is that it can solve the world’s fuel and environmental problems. So, the initial thought about hydrogen is that it can reduce dependency on crude oil and natural gas. However, we must discuss the positive and negative impacts of the hydrogen economy in the current context. At present, relatively low-cost hydrogen is available in the market which is believed to be the hope for the prospect. We all know that hydrogen combustion produced only environmentally friendly water molecules. However, this believe is true to a certain extent. For example, hydrogen-powered vehicles can reduce greenhouse gas emissions. But the exact question we must raise is where does this hydrogen come from? Hydrogen is produced exclusively from natural gas or crude oil reforming. For example, steam–methane reforming methods are exclusively used for commercial H2 production is carried out at high temperatures (above 700 °C) in presence of Ni-based catalysts and yield a mixture of H2 and CO. Further reaction of steam finally produces H2 and CO2. Therefore, to perceive such high-temperature chemical conversion, steam reforming consumes enormous amounts of natural gas and emitted an equivalent amount of CO2 to the earth's atmosphere. Furthermore, hydrogen obtained often carries sulfur-containing impurity are poisonous to the fuel cell’s catalyst. The problem needs to be addressed precisely in order to make our beliefs correct, and we are able to produce environmentally sustainable and friendly hydrogen. To make this possible, various educational institutions have come up with new ideas and taken leading roles in this regard. However, it is not all about hydrogen production. There are many key areas associated with the hydrogen economy, which starts from hydrogen production followed by hydrogen storage, transportation, distribution, safety, etc. Still to date, total hydrogen comes from steam reforming, and the byproduct CO2 emission from steam reforming is alarming. The CO2 emission over the decade is presented in Fig. 2.[31].

Therefore, if we are unable to find out alternative technology for hydrogen production in this century, the perception of the hydrogen economy towards environmental protection will not work anymore. In other word, we would be failed to the live up the perception. In recent years, many alternative methods are evolved with huge prospects, categorically can be divided into two ways; (i) electrochemical water splitting, (ii) solar-driven photovoltaic cell and photocatalytic hydrogen generation. We hope solar and or electrochemical hydrogen production would live up to global expectations in the nearer future.

Section snippets

Outline of the review

Over the past decade, enormous progress is made by the scientific community toward electrochemical H2 evolution reactions especially the development of hybrid electrode materials, identification of the active side, and improve the durability of the catalyst. Hence, significant advancement is made in achieving a higher hydrogen production rate and FE. TMPs based electrocatalysts found to be a very efficient electrocatalyst for HER among the many noble metal-free electrocatalysts. A comprehensive

Iron phosphide

In search for an efficient electrocatalyst for HER, a wide range of iron phosphide with desired Fe/P ratio and phase is synthesized. Among the 3d transition metal compound, iron phosphide is gaining more interest as iron is an earth-rich element among the other transition metals. The electrochemical properties of the as-synthesized iron phosphide based catalyst are strongly depend on the composition, crystalline phase, and synthesis parameter. Over the year different compositions FexPy are

Importance of TMP surface chemical state and its characterization

Oxide based materials are most stable thermodynamically. However, phosphides based materials are less stable compared to metal oxide, whereas nitride and phosphide are list stable compared to their native oxide. Therefore, we could expect that the phosphide based materials are going to get easily oxidized at extreme electrochemical water splitting environments. As electrocatalysis is taking place at the surface of the catalyst, it is obvious to investigate the surface and surface properties of

Conclusions and outlook

The transition metal-based nanomaterials are found to be promising HER catalysts to facilitate electrochemical water-splitting reactions and develop effective renewable energy conversion devices. The review summarizes the recent advancement taken to boost the HER performance of TMP based electrocatalysts. We first reviewed step by step the fundamentals of the HER process and the basic properties of the catalyst required to enhance catalytic performance and durability. Optimization of ΔGH* of

Challenges

Enormous progress has been made to synthesize efficient TMP-based electrocatalysts for electrochemical HER which include experimental and theoretical findings. But, we still believe there are major challenges out there which need to address carefully to fully understand the underline reaction mechanism, stability, and true active site of TMP-based catalyst. The major challenges are summarized below.

  • (1)

    Theoretically, ΔGH* and ΔGH2O are calculated to establish and explain the TMP-based catalyst

Future directions

As we have discussed the series of TMP-based catalysts is designed to improve the HER efficiency of the synthesized catalyst. Although several electrocatalysts are tested, are suitably modified. For example, HER study strongly dependent on catalyst composition, size, and shape. In particular, in-situ measurement to identify the true active site of the catalyst could be a possible future of the study to design the most advanced catalyst The in-situ measurements could also unveil the reaction

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Basic Science Research Program through the National Research.

Foundation of Korea (NRF), South Korea grant funded by the Korea government (MSIT)

(2021R1A4A2000934/ 2022R11A1A01053999).

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