Surface Modification of Nickel Phosphide During Oxygen Evolution Reaction and Routes to a Dual-Site Mechanism

Nickel phosphide based catalysts precursors were synthesized via electroless deposition and characterized via electrochemical testing. Their surface composition after alkaline oxygen evolution reaction (OER) testing was evaluated via x-ray photoelectron spectroscopy (XPS); it was determined that nickel phosphide catalyst precursors oxidize to nickel phosphate rather than nickel oxyhydroxide under OER conditions in the presence of a phosphate ions within an otherwise standard potassium hydroxide electrolyte (pH 14). Furthermore, the incorporation of tungsten within nickel phosphide catalyst precursors appeared to limit the amount of phosphorus which dissolved into the plain KOH electrolyte during OER testing conditions. Density functional theory (DFT) calculations were performed to evaluate the capability of nickel phosphate to serve as an auxiliary deprotonation site as part of a dual-site mechanism, which has been reported to be a possible route to improve catalyst performance beyond the limits of a widely reported scaling relationship for oxygen evolution reaction electrocatalysts. At $1.99eV$ for $\Delta G3$ for $NiPO4$, the dual-site mechanism does not appear as though it would contribute to catalyst performance with nickel phosphate acting as the proton acceptor in this mechanism.