An artificial water-oxidizing Co electro-catalyst: Structure and mechanism by in situ x-ray absorption spectroscopy

Abstract

In situ X-ray absorption spectra (XANES and EXAFS) were obtained for a water-oxidizing Co electrocatalyst while operating at a potential that sustains water-oxidation and at open potential. Spectra were also collected at two different thicknesses of Co deposition, one which is extremely thin and almost a monolayer and the other which is thicker. In situ XANES spectra show that the Co valency is greater than 3+ for both samples during water oxidation at 1.25 V vs NHE. In situ EXAFS results indicate that a minimal bis-oxo/hydroxo-bridged Co multinuclear structure cluster, deposited at 1.1 V, grows into a higher nuclearity structure in the thicker films deposited at 1.25 V. On the basis of the growth pattern, and the structural parameters from both the thin and thick electrodes, we have proposed a structural model where the Co forms edge sharing CoO6 oxo/hydroxo octahedra. In the thin layer electrode, the Co cluster has molecular dimensions, that increases in nuclearity to grow into a larger structure in the thicker layer electrode, while maintaining the same basic structural motif.