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 NiCl 2 and phosphorus at moderate temperatures (500 °C). Direct reactions are thermodynamically driven via PCl 3 formation and tuned through reaction stoichiometry to produce crystalline Ni-P materials from metal-rich (Ni 2 P, Ni 5 P 4 ) to phosphorus-rich (cubic NiP 2 ) compositions. A tin flux in NiCl 2 /P reactions allows access to monoclinic NiP 2 and NiP 3 . 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/cm 2 ordered as c -NiP 2 ≥ Ni 5 P 4 > NiP 3 > m -NiP 2 > Ni 2 P, with NiP 3 activity showing some particle size influence. Phosphorus-rich c/m -NiP 2 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.