Structure and Electrocatalytic Reactivity of Cobalt Phosphosulfide Nanomaterials

Structure and electrocatalytic reactivity of cobalt mono-phosphosulfide nanoparticles supported on carbon nanotubes are investigated. Employing two different synthetic methods, we successfully synthesize cobalt mono-phosphosulfide nanoparticles adopting either the CoP crystal structure (CoPlS with the S/P ratio < 93%) or the CoS crystal structure (CoPlS with the P/S ratio < 73%). S substitution in the CoP structure makes the surface P atoms more oxidized whereas the S atoms stay completely reduced. P substitution in the CoS structure makes the surface S atoms more reduced but the Co atoms more oxidized whereas the P atoms stay completely oxidized. All the cobalt phosphosulfide nanoparticles exhibit relatively P-rich surface. The electrocatalytic performance of the nanoparticles for the hydrogen evolution reaction is studied. In acidic electrolyte, CoSlP nanoparticles show negligible dependence of activity on composition, whereas CoPlS nanoparticles are not stable enough to give any steady activity. In alkaline electrolyte, the activity of CoSlP nanoparticles decreases as the S substitution level increases, whereas that of CoSlP nanoparticles is weakly composition dependent.