A Hydrogen Evolution Reaction Catalyst Using Nickel Phosphides with Mixed Crystalline Structure
Authors: Liu Gaoyang , Xu Juyuan , Wang Xindong , Li Hui , Wang Haijiang 
Published: February 2020

Introduction
Proton Exchange Membrane (PEM) water electrolysis provides a sustainable solution for the production of hydrogen, and is well suited to couple with the intermittent nature of energy sources such as wind and solar. To spur large-scale commercial application of PEM water electrolysis, more efficient and less expensive non-noble metal electrocatalysts towards the hydrogen evolution reaction (HER) are required. Recently, transition metal phosphides (TMPs) have become the typical representatives of the burgeoning non-noble metal HER electrocatalysts due to that their intrinsic structures meet the criteria of outstanding electrocatalysts. It should be noted that the currently studied TMPs are mainly focusing on improving their HER performance by a series of structural engineering methods [1-3]. To the best of our knowledge, there are very few reports on promoting the intrinsic activity of the TMPs in water electrolysis.
In this communication, we reported the scalable synthesis of nickel phosphides with a mixed crystalline structure (referred to as Ni2P&Ni3P) toward HER by a solution-phase reaction. To elucidate the superior catalytic activity of Ni2P&Ni3P, we also explored the preparation of the nickel phosphides in a single crystalline state (referred to as Ni2P) and that in a single amorphous state (referred to as Ni-P) for comparison.
Result and Discussion
X-ray diffraction patterns of Ni2P&Ni3P、Ni2P and Ni-P are shown in Fig 1a. The analysis of the diffraction data reveals that both tiny Ni2P nanocrystallines and Ni3P nanocrystallines are observed for Ni2P&Ni3P. As for Ni2P, the Ni3P tiny nanocrystallines are vanished and only the intensive diffraction peaks corresponding to Ni2P are observed. In comparison with the crystalline state nickel phosphides, the Ni-P shows poor crystalline order with a very broad peak. The crystalline structures could take effects on the intermediate reaction of HER. In order to compare the hydrogen desorption ability of the three electrocatalysts, the hydrogen oxidation reaction (HOR) was investigated by LSV (Linear scan voltammetry) at 20 mA cm-2 for 5 min immediately after the hydrogen evolution reaction (HER). As it is shown in Fig 1b, the characteristic mixed crystalline structure is expected to promote the hydrogen desorption ability. Thus, the largest peak current density is obtained with Ni2P&Ni3P. Furthermore, steady state polarization curves of different electrocatalysts were shown in Fig 1c. As expected, the Ni2P&Ni3P shows the best HER activity, and the overpotentials required for Ni2P&Ni3P、Ni2P and Ni-P to produce cathodic current densities of 20 mA cm-2 are 154 mV, 221 mV, 262 mV, respectively.

Fig 1 (a) XRD patterns of Ni2P&Ni3P, Ni2P and Ni-P; (b) LSV and (c) Steady-state polarization curves of the three types of prepared catalysts at a scan rate of 1 mV s-1. All the electrochemical characterizations are tested in the mixture of 0.1 M H2SO4 and 0.5 M Na2SO4 at 25 ◦C and atmosphere pressure.
Reference
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[2] Popczun E. J., Read C. G., Roske C. W., Lewis S., Schaak R. E. Highly Active Electrocatalysis of the Hydrogen Evolution Reaction by Cobalt Phosphide Nanoparticles [J]. Angewandte Chemie International Edition, 2014,53(21):5427-5430.
[3] Liu Q., Pu Z. H., Asiri A. M., Sun X. P. Nitrogen-doped carbon nanotube supported iron phosphide nanocomposites for highly active electrocatalysis of the hydrogen evolution reaction [J]. Electrochim Acta, 2014,149(10):324-329.

Figure 1