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 degrees 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.

Automated summary

Flexible synthetic routes to crystalline metal-rich to phosphorus-rich nickel phosphides were developed for electrocatalytic HER studies. Five different nickel phosphides were synthesized from NiCl2 and phosphorus via solvent-free, direct, and tin-flux-assisted reactions. The activity of these nickel phosphides as HER electrocatalysts was influenced by factors including particle size, phosphorus content, polyphosphide anions, and surface charge.