In Situ Observation of the Formation and Structure of Hydrogen-Evolving Amorphous Cobalt Electrocatalysts

We have used in situ and operando X-ray absorption spectroscopy at the cobalt K-edge to study the formation of cobalt nanoparticles from a molecular precursor, as well as their structural evolution under hydrogen-evolving electrocatalytic conditions. We show that these particles, which are about 100–150 nm in diameter overall, are made of an uncommon form of amorphous metallic cobalt, the smallest ordered unit being 1 nm clusters of ∼50 cobalt atoms. In aqueous solution, these porous particles are partly oxidized into cobalt(II), a fraction of which remains present as an outer shell during hydrogen evolution electrocatalysis, even at very high cathodic potentials. Our operando measurements show that the activity of the particles is correlated to the oxidized layer thickness, a thinner layer exposing a larger fraction of the active metallic cobalt and leading to a higher activity. These findings expand our current understanding of the solid–liquid interface in hydrogen evolution catalytic species in neutral pH and suggest new directions for the improvement of hydrogen-evolving catalytic systems.