Tunable Synthesis of Metal-Rich and Phosphorus-Rich Nickel Phosphides and Their Comparative Evaluation as Hydrogen Evolution Electrocatalysts

Flexible synthetic routes to crystalline metal-rich to phosphorus-rich nickel phosphides are highly desired for comparable electrocatalytic HER studies. This report details solvent-free, direct, and tin-flux-assisted synthesis of five different nickel phosphides from NiCl₂ and phosphorus at moderate temperatures (500 °C). Direct reactions are thermodynamically driven via PCl₃ formation and tuned through reaction stoichiometry to produce crystalline Ni-P materials from metal-rich (Ni₂P, Ni₅P₄) to phosphorus-rich (cubic NiP₂) compositions. A tin flux in NiCl₂/P reactions allows access to monoclinic NiP₂ and NiP₃. Intermediates in tin flux reactions were isolated to help identify phosphorus-rich Ni-P formation mechanisms. These crystalline micrometer-sized nickel phosphide powders were affixed to carbon-wax electrodes and investigated as HER electrocatalysts in acidic electrolyte. All nickel phosphides show moderate HER activity in a potential range of -160 to -260 mV to achieve current densities of 10 mA/cm² ordered as c-NiP₂ ≥ Ni₅P₄ > NiP₃ > m-NiP₂ > Ni₂P, with NiP₃ activity showing some particle size influence. Phosphorus-rich c/m-NiP₂ appears most stable under acidic conditions during extended reactions. The HER activity of these different nickel phosphides appears influenced by a combination of factors such as particle size, phosphorus content, polyphosphide anions, and surface charge.