Elucidating Surface Restructuring-Induced Catalytic Reactivity of Cobalt Phosphide Nanoparticles under Electrochemical Conditions

Probing and understanding surface restructuring-induced electrocatalytic reactivity is an essential but challenging step toward rational prediction of electrocatalytic properties and design of high-performance catalysts. Cobalt phosphide (CoP) nanoparticles are state-of-the-art electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). However, the structure reactivity correlations are not straightforward because the nanoparticles will restructure under working conditions. Employing a protective sample transfer procedure, we use simple lab XPS to unveil the changes in oxidation state and composition of the nanoparticle surface induced by electrochemical reaction conditions. CoP nanoparticles are naturally oxidized on their surface. In alkaline electrolyte under HER conditions, a Co-rich phosphide surface is generated as a result of polyphosphate dissolution and reduction of the oxidized P and Co species. In alkaline electrolyte under OER conditions, an oxidation and dissolution process occurs, and the surface evolves into hydroxide/oxide with a subtle amount of phosphate residue. In acidic electrolyte under HER conditions, the surface oxidation layer is dissolved by the electrolyte, and a fresh CoP surface is exposed. These surface restructuring results help rationalizing the electrocatalytic reactivity of CoP nanoparticles for water splitting under various electrochemical conditions.