Insights into the Origin of High Activity of Ni₅P₄(0001) for Hydrogen Evolution Reaction

Hydrogen evolution reaction (HER) is directly relevant to green hydrogen production from water splitting. Recently, a low-cost Ni₅P₄ material has been demonstrated experimentally and theoretically to exhibit excellent electrocatalytic activity toward HER. However, a fundamental understanding of the origin of Ni₅P₄(0001) activity is still lacking. In this work, density functional theory (DFT) calculations were employed for a comprehensive investigation. The calculation results indicate that the Ni₅P₄(0001) surface exposing Ni₃P₄ termination gains the highest stability, on which a nearly thermoneutral hydrogen adsorption was found at the P₃-hollow sites, providing a high activity for HER. The activity was also observed to be maintained over a wide H-coverage. HER occurs via the Volmer-Heyrovsky mechanism as evidenced from the optimal hydrogen adsorption free energy, but unlikely through the Tafel reaction due to its large energy barrier. Furthermore, the P₃-hollow sites also exhibit a low kinetic barrier for water dissociation, promoting HER in alkaline media. A series of electronic structure analyses were performed in gaining insights into the origin of the HER activity. First, the density of states (DOS) and crystal orbital Hamilton population (COHP) analyses revealed a favorable interaction of electronic states between P and H atoms, leading to stable H adsorption at P₃-hollow sites. In addition, the Bader charge analysis demonstrates that the strength of H adsorption at P₃-hollow sites linearly increases with the electrons carried by the latter. The optimal net charge on the P₃-hollow sites leads to a desired ΔG _H that is close-to-zero. Finally, a highly efficient electron transfer was observed between the P₃-hollow sites and their neighboring atoms, facilitating the HER.