Molten state synthesis of nickel phosphides: mechanism and composition-activity correlation for electrochemical applications

Nickel phosphides are highly attractive low-cost (electro)catalysts, thanks to their unique electronic structure, versatile phase diagram, and chemical stability. Herein, we describe a single-step, simple, and scalable synthesis of nickel phosphides, with good control over phase composition, size, and catalytic activity, by a direct thermal reaction of nickel nitrate hexahydrate and triphenylphosphine (PPh₃). Advanced analytic tools combined with theoretical calculations reveal that upon heating, nickel ions are dissolved and coordinated by PPh₃, enabling the synthesis of fine-tuned particles, with nickel phosphide phases ranging from Ni₃P to Ni₂P. The new synthetic method enables comprehending the correlation between phase composition and the phosphides' catalytic activity. This work shows a clear composition-activity trend of the optimized nickel phosphides both as electrocatalysts for the hydrogen evolution reaction in acidic media and as anode materials in Li-ion batteries.