First-Principles Mechanistic Insights into the Hydrogen Evolution Reaction on Ni2P Electrocatalyst in Alkaline Medium

Russell W Cross,Nelson Y Dzade

doi:10.3390/catal10030307

Abstract

Nickel phosphide (Ni2P) is a promising material for the electrocatalytic generation of hydrogen from water. Here, we present a chemical picture of the fundamental mechanism of Volmer–Tafel steps in hydrogen evolution reaction (HER) activity under alkaline conditions at the (0001) and (10 1 ¯ 0) surfaces of Ni2P using dispersion-corrected density functional theory calculations. Two terminations of each surface (Ni3P2- and Ni3P-terminated (0001); and Ni2P- and NiP-terminated (10 1 ¯ 0)), which have been shown to coexist in Ni2P samples depending on the experimental conditions, were studied. Water adsorption on the different terminations of the Ni2P (0001) and (10 1 ¯ 0) surfaces is shown to be exothermic (binding energy in the range of 0.33−0.68 eV) and characterized by negligible charge transfer to/from the catalyst surface (0.01−0.04 e−). High activation energy barriers (0.86−1.53 eV) were predicted for the dissociation of water on each termination of the Ni2P (0001) and (10 1 ¯ 0) surfaces, indicating sluggish kinetics for the initial Volmer step in the hydrogen evolution reaction over a Ni2P catalyst. Based on the predicted Gibbs free energy of hydrogen adsorption (ΔGH*) at different surface sites, we found that the presence of Ni3-hollow sites on the (0001) surface and bridge Ni-Ni sites on the (10 1 ¯ 0) surface bind the H atom too strongly. To achieve facile kinetics for both the Volmer and Heyrovsky–Tafel steps, modification of the surface structure and tuning of the electronic properties through transition metal doping is recommended as an important strategy.

Highlights

Rising global energy demands and the serious concerns of environmental contamination necessitate the development of renewable energy sources to alleviate our reliance on fossil fuels and simultaneously satisfy increasingly stringent environmental regulations
The calculated partial density of states (PDOS) shown in Figure 1b reveals the metallic character of Ni2 P, whereby the Ni d-states dominate the regions around the Fermi level, which is consistent with earlier density functional theory (DFT) results [23,42]
Surfaces have been investigated by means of first-principles DFT-D3 calculations

Summary

Introduction

Rising global energy demands and the serious concerns of environmental contamination necessitate the development of renewable energy sources to alleviate our reliance on fossil fuels and simultaneously satisfy increasingly stringent environmental regulations. Ni2 P is considered an attractive alternative to noble metal catalysts for HER [20] because both of its constituent elements nickel and phosphorous are cheap, abundant, and non-toxic, which makes Ni2 P a promising cost-effective material for scalable renewable energy conversion systems. Earlier investigations have considered HER activity in acidic medium over Ni2 P catalysts, whereby the Volmer–Tafel mechanism (H+ (aq) → H* , 2H* → ↑H2 ) involves only characterizing the Gibb’s free energy hydrogen adsorption to the catalyst surface [22,23]. Volmer–Tafel steps in the hydrogen evolution reaction over Ni2 P catalyst has not been investigated comprehensively This information and insights are, vital in the quest to rationally design more active and stable Ni2 P electrocatalysts.

Ni2 P Bulk and Surface Characterization

Molecular

Activation

Summary and Conclusions

Computational Details