Facile deposition of nanostructured cobalt oxide catalysts from molecular cobaloximes for efficient water oxidation

Cobaloximes are usually used as molecular hydrogen evolution reaction (HER) catalysts. Herein we report for the first time the use of molecular cobaloximes as catalyst precursors for water oxidation when anodic potentials were applied. Highly active thin catalyst films were deposited at +1.5 V and +1.1 V (vs. Ag/AgCl) in 0.1 M borate buffer solution at pH = 9.2 containing 1 mM cobaloximes. Four catalyst films (CoOx-1–CoOx-4) were synthesized from four different cobaloximes. The current densities of CoOx-1 were up to ∼5.5 mA cm−2 and ∼2.6 mA cm−2 when the applied potentials were +1.5 V and +1.1 V, respectively, which were higher than the current densities of CoOx-2, CoOx-3 and CoOx-4 under the same conditions. Scanning electron microscopy (SEM) images reveal that the nanometer-sized particles of CoOx-1 possibly contribute to its high activity while the other three catalysts have micrometer-sized amorphous materials on the surface of FTO. X-ray photoelectron spectroscopy (XPS) data displayed the valence state of the cobalt element as Co(II) or Co(III) oxide species. The morphological stability of the CoOx-1 catalyst was further studied using SEM.