Engineering transition metal phosphide nanomaterials as highly active electrocatalysts for water splitting

Yanmei Shi and Bin Zhang *
Department of Chemistry, School of Science, Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China. E-mail:

Received 28th September 2017 , Accepted 26th October 2017

First published on 26th October 2017

Transition metal phosphide (TMP) nanomaterials are recently considered to be versatile electrocatalysts with excellent activity and stability towards water splitting. This Frontier article will highlight recent advances in engineering the composition and structure of TMPs for higher electrochemical performances.


The rapid depletion of limited fossil fuels and the ever faster growing energy demand have forced researchers to explore other sustainable energies.1 One of the most promising energy candidates is considered to be molecular hydrogen. Water electrolysis is regarded as a promising approach for hydrogen production.2,3 But the current application of water electrolysis is severely restricted by its high cost, which arises mainly from either noble metal electrocatalysts or anti-corrosion equipment.4 To substantially lower the cost of water electrolysis is an effective way to completely solve the energy crisis, especially when coupled with non-thermal power generation.

Many efforts have been made to search for appropriate electrocatalysts with high performance, low cost and wide pH tolerability.5–7 Although Pt has the well-accepted best hydrogen evolution reaction (HER) activity, its scarcity and accompanying high cost prompt scientists to find other highly active electrocatalysts based on earth abundant elements.8 Nickel-based transition metal alloys are widely used in alkaline water electrolysis. Despite the acceptable cost of these alloys, they also expose the shortcomings including low HER efficiency and instability in acid media, which are widely used in proton-exchange membrane systems.9 Researchers have found that alloying some non-metal elements (like sulphur and phosphorus) with nickel-based alloys can dramatically enhance the HER activity and the stability of the new amorphous alloys in acid conditions.10 In 2005, Liu and Rodriguez predicted the high HER activity of the Ni2P(001) facet by density functional theory (DFT) calculations.11 Till 2013, our group,12 and Lewis and Schaak's joint group13 took the lead in practically using nanostructured transition metal phosphides (TMPs) as HER electrocatalysts in acid media. After this, Sun and coworkers improved both the activity and the stability of TMPs by growing different TMP nanostructure arrays directly on three-dimensional (3D) substrates.14–16 These pioneering reports brought the prospect of using TMPs as efficient water splitting electrocatalysts in the following years.

TMPs are gradually found to be versatile electrocatalysts for water splitting (Fig. 1). At first, relevant research focuses only on the acidic conditions.17,18 It is a surprise to find that most of the TMP electrocatalysts function well in a wide pH range from 0 to 14.19–22 Further study shows that TMP electrocatalysts can also perform very well towards the oxygen evolution reaction (OER)-the other half reaction of water splitting.23,24 This discovery makes it possible to use the same two TMP electrodes in one water electrolysis configuration simultaneously.25,26 It should be pointed out that although the mechanism of TMPs towards the OER is not completely clear yet, the well-accepted fact is that it is the oxidation product derived from the TMP under anodic conditions rather than the TMP itself that serves as the true OER-active species.27,28 Since the OER is an energy-intensive process with sluggish kinetics, its low efficiency and high energy consumption drive researchers to find other substitute reactions to lower the applied potential for more efficient hydrogen generation. For TMPs, some successful anodic alternative reactions include the oxidation of biomass,29,30 hydrazine oxidation reaction,31 urea oxidation,32etc. Although TMP electrocatalysts show inherently high activity towards diverse reactions, the composition and structure of TMPs can still be further optimized to achieve much higher electrochemical activity. So in this Frontier article, we will highlight the different strategies from the aspects of the composition and structure to promote the efficiency of TMPs as electrocatalysts for water splitting.

image file: c7dt03648e-f1.tif
Fig. 1 Different reactions catalyzed by TMPs as electrocatalysts for water splitting.

Composition engineering

Different kinds of binary TMPs with only one metal have been researched comprehensively. Among all kinds of simple TMPs, CoP has been practically and theoretically proven to show a higher HER efficiency in acid media (Fig. 2a).33 However, the HER efficiency of CoP can still be promoted by doping other metal elements like Fe and Ni,34,35 or some other non-metal elements, such as S and Se.36,37 After doping, the electrical conductivity of the electrocatalyst is improved,38 and the corresponding hydrogen adsorption free energy (ΔGH) can be optimized.33
image file: c7dt03648e-f2.tif
Fig. 2 (a) Activity volcano for the HER showing the electrochemical surface area (ECSA) normalized current density at η = 100 mV as a function of ΔGH. Reproduced with permission from ref. 33, Copyright 2015 Royal Society of Chemistry. (b) Polarization curves for nanoparticles of CoMnP, CoMnO2, and Co2P in 1.0 M KOH. Reproduced with permission from ref. 41, Copyright 2016 American Chemical Society.

Of course this doping strategy can be applied to other metal phosphides and other doping elements. It is worth mentioning that some metal elements, even those which cannot independently form stable phosphides as electrocatalysts, can also be doped into other TMPs to form a much more efficient catalyst.39,40 A typical example is manganese (Mn). Manganese phosphides are found unstable under oxidation potential because of the high oxophilicity of manganese.41 Nevertheless, doping manganese into Co2P can greatly enhance its OER performance (Fig. 2b). The reason is supposed that the high oxophilic nature of manganese is moderated by cobalt.41 In this way, both the overpotential and the turnover frequency of CoMnP can be facilitated. Doping manganese into FeP is also achieved for efficient overall water splitting.42 So it is attractive to get out of the elemental limit of the existing TMP electrocatalysts and explore other doping elements for a higher electrochemical activity.

Structure engineering

The traditional powder electrocatalysts inevitably possess common shortcomings. They always suffer from aggregation and exfoliation during electrolysis, thus causing a dramatic loss of the exposed active sites. And the polymer binder used to combine the electrocatalysts onto the current collectors will block active sites, increase the series resistance and inhibit diffusion, which also leads to a weakened electrocatalytic activity.43 The TMP electrocatalysts are no exception. So fabricating three-dimensional (3D) self-supported electrodes by direct growth of catalysts on 3D substrates is of great significance in promoting both the activity and stability of TMP materials.44–46 The 3D self-supported electrocatalysts are endowed not merely with high apparent activity and stability, but also with tremendous catalyst loading per unit area. Such an activity improvement greatly depends on the increased catalyst loading, which is often ignored when compared with other electrocatalysts. In this sense, the intrinsic activity of the self-supported electrodes is actually not improved.

A strategy to improve the intrinsically catalytic activity is the fabrication of porous TMP electrocatalysts. The unique pore structure in these electrocatalysts provides large surface area without easy aggregation. Guiding by this awareness, our group designed a facile anion-exchange routine to prepare nanoporous FeP nanosheets as HER electrocatalysts.12 The abundant pores in the particle-consisted nanosheets facilitated free infiltration of the electrolyte and the mass transport of the involving species, therefore improving the electrocatalytic performance. However, the real size of these nanoparticles is still too large. Fabrication of TMPs with an ultrasmall size (like ultrafine nanoparticles and ultrathin 2D nanosheets) is a meaningful attempt to further improve the intrinsic activity of TMPs.

Here we take an ultrathin 2D electrocatalyst as an example. Ultrathin 2D nanomaterials have recently attracted great attention because of their compelling chemical, physical, electronic and optical properties.47 As for electrocatalysis, ultrathin 2D nanomaterials display the merits of high atomic utilization and large surface area, especially the large area of a specific exposed facet.48–50 It is attractive to fabricate electrocatalysts by integrating ultrathin 2D nanomaterials with nanoporosity. The abundant dangling bonds in these kinds of materials will provide a large amount of active sites, thus facilitating the reaction. However, since TMP materials are not inherently layered materials, traditional routines such as liquid exfoliation are not appropriate to synthesize ultrathin 2D TMP materials. The preparation of porous ultrathin 2D TMP nanomaterials is still a big challenge until now.

Recently, our group succeeded in fabricating porous ultrathin 2D nanosheets of CoP and other intrinsically non-layered materials as highly efficient HER electrocatalysts via a simple chemical transformation strategy (Fig. 3).51 The previously reported porous ultrathin Co3O4 nanosheets52 were chosen as the precursors. After the well-designed gas-phase phosphidation, the porous ultrathin CoP nanosheets were obtained successfully. The as-obtained CoP nanosheets showed a thickness of less than 1.1 nm and dominant exposed {200} facets. DFT calculations revealed the metallic nature of CoP, the high electroactivity of the {200} facets and the high utilization efficiency of the active sites on the {200} facets. These were the reasons why the porous ultrathin CoP nanosheets exhibited excellent electrochemical HER activity. The porous ultrathin CoP nanosheets exhibited a low overpotential of only 56 mV to reach a current density of 10 mA cm−2 and a small Tafel slope of 44 mV dec−1. In addition, the porous ultrathin CoP nanosheets demonstrated a high mass activity of 151 A g−1, which is 80 times higher than that of CoP nanoparticles. All these results indicate that the intrinsic HER activity of porous ultrathin CoP nanosheets is promoted effectively.

image file: c7dt03648e-f3.tif
Fig. 3 (a) High resolution transmission electron microscopy (HRTEM) image and the associated fast Fourier transform (FFT) pattern (inset a), and (b) Atomic force microscopy (AFM) image and the side-illuminating lighting photograph (inset b) of porous ultrathin CoP nanosheets. (c) IR corrected polarization curves of porous ultrathin CoP nanosheets, CoP nanoparticles, bare GC and 20% Pt/C. (d) Simulated ΔGH* dependent on hydrogen coverage θH* on the P terminated CoP (100) facet. Reproduced with permission from ref. 51, Copyright 2017 Royal Society of Chemistry.

Conclusions and outlooks

In summary, recent advances in engineering TMPs as electrocatalysts for more effective water splitting are highlighted from two aspects, the composition and the structure. In the section of composition engineering, we mainly focus on elemental doping, especially for elements which cannot form stable TMP electrocatalysts by themselves. Regarding structure engineering, we point out that the outstanding performance of the self-supported electrodes primarily comes from the large catalyst loading rather than the improvement of the intrinsic activity. To improve the intrinsic activity of the TMPs, it is necessary to fabricate TMP electrocatalysts with ultrasmall size, porosity and exposed crystal planes.

Despite the fact that the booming research on TMPs as water splitting electrocatalysts has deepened our understanding about TMPs, we should never ignore the many unsolved problems. The mechanism of the promoted performance caused by elemental doping has not been fully understood. And the type of doping element deserves more positive exploration. On the other hand, although the porous ultrathin 2D CoP nanosheets perform very well, the high-performance electrocatalyst also faces the same issues as powder electrocatalysts. Combining the ultrathin nanosheets with the self-supported electrodes is still a challenge. And the controllable synthesis of specific facets and their effects on the electrochemical activity need more exploration.

Additionally, TMPs have been proven to be effective OER electrocatalysts. But their real mechanism is not completely understood. Not just the OER mechanism, the real mechanism of HER also deserves much investigation. The current research on TMPs as HER electrocatalysts mainly focuses on the macroscopic properties of TMPs such as the composition and morphology. We are blind to the local structure reconstruction after the adsorption of various electrochemical intermediates. We also don't know how the real microscopic interface structure influences the electrode reaction. We are even not sure whether TMPs themselves are the true HER-active species. All these answers need to be found out by modern advanced characterization techniques and computational tools. Overall, TMPs have attracted increasing attention due to their high intrinsic activity, not only towards water electrolysis, but also towards batteries53 and many other fields of energy conversion and storage. We hope that TMPs can someday be actually used in industrial production to solve the energy crisis.

Conflicts of interest

There are no conflicts to declare.


This work was financially supported by the National Natural Science Foundation of China (no. 21422104) and the Key Project of Natural Science Foundation of Tianjin City (no. 16JCZDJC30600).

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