Probing of the Noninnocent Role of P in Transition-Metal Phosphide Hydrogen Evolution Reaction Electrocatalysts via Replacement with Electropositive Si

Transition-metal phosphides (TMP) havebeen identifiedas promising electrocatalysts for the hydrogen evolution reaction(HER). Despite recent computational investigations identifying P sitesas being crucial for hydrogen adsorption, the main mode of optimizationfor TMPs has been focused on changing the metal sites.To experimentally verify computational hypotheses and provide a routefor HER electrocatalyst optimization via ternary compounds, we performedsystematic experimental studies of structurally related NiSi1-x P x phases, namely,Ni2SiP, Ni5Si2P3, Ni3SiP2, and Ni7Si2P5, which are ordered derivatives of the NiSi structure (Pnma, oP-8, MnP structure type). We found that P playeda significant role in modulating HER activity in an acidic electrolytebecause the incorporation of P in NiSi reduced the overpotential atcurrent density j = 10 mA/cm(2) from & eta;(10) = 529 mV (NiSi) to & eta;(10) = 97 mV (Ni2SiP). Ni2SiP outperformed the current state-of-the-artNi(5)P(4) electrocatalyst prepared and studied inidentical conditions both in terms of activity and stability, whichis attributed to the presence of covalent Ni-Si bonding inthe structure. Within the family of ternary Ni-Si-Pcompounds, electrocatalytic activity correlates with the number ofNi-3d states at the Fermi energy.