The Structure of a Water-oxidizing Cobalt Oxide Film and Comparison to the Photosynthetic Manganese Complex

In photosynthesis, water is oxidized at a protein-bound Mn4Ca complex. Artificial water-oxidation catalysts that are similarly efficient and based on inexpensive and abundant materials are of great interest. A recently reported inorganic cobalt catalyst (CoCat) forms by electrodeposition as an amorphous layer on inert anodes, starting from an aqueous solution of cobalt ions and buffered salts such as potassium phosphate (KPi). X-ray absorption spectroscopy (XAS) indicates that the central unit of the CoCat is a cluster of edge-sharing CoIII(μ-O)6 octahedra. We find that the apparent cluster nuclearity is higher for film formation at anode voltages below the water-oxidation threshold. These films exhibit only minimally lower cobalt oxidation states than the films of the same thickness but deposited at voltages supporting water-oxidation. The similarities in structure, function, and oxidative self-assembly of the all-inorganic CoCat and the photosynthetic Mn4Ca complex are striking, despite stark differences in the chemical environment.