Sputtered cobalt-carbon-nitrogen thin films as oxygen reduction electrocatalysts - II. Electrochemical stability and proposed mechanism

Thin films containing Co, C, and N were co-sputtered in vacuum for kinetic studies of oxygen reduction in alkaline electrolytes. Stability tests show that the oxygen reduction activity of the Co-C-N catalysts deteriorates with time. Films were characterized with X-ray photon spectroscopy (XPS) and scanning electron microscopy (SEM) surface techniques before and after electrochemical studies. The analyses reveal a loss of cobalt and nitrogen from the surface region and the formation of cobalt oxide species through its reaction with oxygen. However, the activity of the optimal film, which decreased by less than 50% after two months of continuous operation, is considerably stable compared to metal chelates. The oxygen adsorption on the film surface appears to be consistent with the Griffith model, while the rate-determining step is the primary charge transfer. The mechanism for a four-electron oxygen reduction has been proposed.