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 (CoP|S with the S/P ratio < 93%) or the CoS crystal structure (CoS|P 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, CoP|S nanoparticles show negligible dependence of activity on composition, whereas CoS|P nanoparticles are not stable enough to give any steady activity. In alkaline electrolyte, the activity of CoP|S nanoparticles decreases as the S substitution level increases, whereas that of CoS|P nanoparticles is weakly composition dependent.