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 P 2 S 5 at moderate 500 °C temperatures. The crystalline products obtained from this synthetic approach include MPS 3 (M = Fe, Co, Ni) and Cu 3 PS 4 . The successful reactions benefit from thermochemically favorable PCl 3 elimination. This solvent-free route performed at moderate temperatures leads to mixed anion products with complex heteroatomic anions, such as P 2 S 6 4- . The MPS 3 phases are thermally metastable relative to the thermodynamically preferred separate MP x / MS y and more metal-rich MP x S y 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 MPS 3 materials were examined as electrocatalysts in hydrogen evolution reactions (HER) under acidic conditions. In terms of HER activity at lower applied potentials, the MPS 3 materials show the trend of Co > Ni >> Fe. Extended time constant potential HER experiments show reasonable HER stability of ionic and semiconducting MPS 3 (M = Co, Ni) structures under acidic reducing conditions.