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.

Facile Solvent-Free Synthesis of Metal Thiophosphates and Their Examination as Hydrogen Evolution Electrocatalysts.

The facile solvent-free synthesis of several known metal thiophosphates was accomplished by a chemical exchange reaction between anhydrous metal chlorides and elemental phosphorus with sulfur, or combinations of phosphorus with molecular P2S5 at moderate 500 °C temperatures. The crystalline products obtained from this synthetic approach include MPS3 (M = Fe, Co, Ni) and Cu3PS4. The successful reactions benefit from thermochemically favorable PCl3 elimination. This solvent-free route performed at moderate temperatures leads to mixed anion products with complex heteroatomic anions, such as P2S64−. The MPS3 phases are thermally metastable relative to the thermodynamically preferred separate MPx/ MSy and more metal-rich MPxSy phases. The micrometer-sized M-P-S products exhibit room-temperature optical and magnetic properties consistent with isolated metal ion structural arrangements and semiconducting band gaps. The MPS3 materials were examined as electrocatalysts in hydrogen evolution reactions (HER) under acidic conditions. In terms of HER activity at lower applied potentials, the MPS3 materials show the trend of Co > Ni >> Fe. Extended time constant potential HER experiments show reasonable HER stability of ionic and semiconducting MPS3 (M = Co, Ni) structures under acidic reducing conditions.