Facet-Dependent Hydrogen Evolution Reaction on M ₂ P (M = Ni, Co, Fe) Single Crystals

Transition-metal phosphides (MPs) are promising earth-abundant catalysts for hydrogen evolution reactions (HERs) due to their remarkable activity and stability. To further improve their properties, facet control is a key strategy. The growth of shape-selected nanoparticles may substantially enhance electrocatalytic activity, but this approach requires fundamental studies of facet-specific catalytic properties. There are only a few reports on the facet effects of MPs, which leads to a limited understanding of the activity of each facet and hampers catalyst design. Here, we grew large hexagonal-prism-shaped single crystals of three representative M2P (M = Ni, Co, and Fe) catalysts using metal flux routes. Two facets of M2P single crystals were tested to study facet-dependent HER activities, and it was consistently demonstrated that for all M2P crystals, a tip facet [(0001) for Ni2P/Fe2P and (010) for Co2P] had a higher activity than the side facet [(101̅0) for Ni2P/Fe2P and (100) for Co2P]. HER activity between the same facet elucidated the activity ordered between different transition metals as Fe2P > Co2P > Ni2P under low-potential regions. At high applied potentials, this trend is reversed due to the differences in Tafel slopes, with Ni2P becoming the most active catalyst, such that the activity of the (0001) facet of Ni2P approaches that of Pt. The calculated surface density of states (DOS) of each facet and its local curvature were found to be a useful descriptor for the activity trends among different transition metals of the same facets.