Phosphide-Mediated Hydrogen Evolution by a Well-Defined Molecular Model for Amorphous Cobalt Phosphide Surfaces

Cobalt phosphide is a promising non-noble metal electrocatalyst for the electrocatalytic production of hydrogen gas, although the mechanism by which it operates at its amorphized surfaces is not well understood. In this work, we report a highly stable cobalt phosphide cluster, Co3(μ3-P)(CO)6(CNArDipp2)3 (ArDipp2 = 2,6-(2,6-(iPr)2C6H3)2C6H3), which is a competent catalyst for electrocatalytic hydrogen evolution (HER) under acidic conditions and serves as a functional model system for amorphous cobalt phosphide surfaces. The active catalyst was identified as being the protonated cluster [Co3(μ3-PH)(CO)6(CNArDipp2)3]+, which was isolated and fully characterized. Additionally, P-functionalized derivatives were found to have impeded or absent catalytic activity, implicating the phosphide atom as playing a crucial role in the catalytic mechanism. This work suggests that the phosphide anions on heterogeneous cobalt phosphide surfaces serve as important mediators of proton delivery, analogous to the well-established importance of Lewis basic moieties in molecular and biological hydrogen evolution catalysts.