Proton–Electron Conductivity in Thin Films of a Cobalt–Oxygen Evolving Catalyst

The conductivities of thin film cobalt oxygen evolving catalysts (Co-OECs) are examined for four variants differing in phosphate and borate anion compositions and pH. The effects of these parameters on the Co-OEC activity is explored by (a) examination of the intermediate range structural order within films through high energy X-ray scattering and pair distribution function analysis, and (b) direct measurement of film conductivity, which has been measured with an interdigitated array microelectrode. We find that the nature of the anion is a critical determinant of the size of cobaltate clusters comprising the Co-OEC thin films which in turn is a critical determinant of film conductivity. Improved conductivity of the Co-OEC films with the size of the metalate clusters is consistent with a model for electron conduction that arises from the hopping of hole equivalents delocalized over metalate clusters. The hole hopping is fast relative to the rate-determining step of OER, which may become limited by proton transport within the films.