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

Transition-metal phosphides ( TM P) have been identified as promising electrocatalysts for the hydrogen evolution reaction (HER). Despite recent computational investigations identifying P sites as being crucial for hydrogen adsorption, the main mode of optimization for TM Ps has been focused on changing the metal sites. To experimentally verify computational hypotheses and provide a route for HER electrocatalyst optimization via ternary compounds, we performed systematic experimental studies of structurally related NiSi 1 –x P x phases, namely, Ni 2 SiP, Ni 5 Si 2 P 3, Ni 3 SiP 2, and Ni 7 Si 2 P 5, which are ordered derivatives of the NiSi structure ( Pnma, oP -8, MnP structure type). We found that P played a significant role in modulating HER activity in an acidic electrolyte because the incorporation of P in NiSi reduced the overpotential at current density j = 10 mA/cm 2 from η 10 = 529 mV (NiSi) to η 10 = 97 mV (Ni 2 SiP). Ni 2 SiP outperformed the current state-of-the-art Ni 5 P 4 electrocatalyst prepared and studied in identical conditions both in terms of activity and stability, which is attributed to the presence of covalent Ni–Si bonding in the structure. Within the family of ternary Ni–Si–P compounds, electrocatalytic activity correlates with the number of Ni-3 d states at the Fermi energy.