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

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 (101 over bar 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 (101 over bar 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 (101 over bar 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 (101 over bar 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 (Delta G(H)*) at different surface sites, we found that the presence of Ni-3-hollow sites on the (0001) surface and bridge Ni-Ni sites on the (101 over bar 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.