Photoelectrochemical water oxidation by cobalt catalyst ("Co-Pi")/alpha-Fe(2)O(3) composite photoanodes: oxygen evolution and resolution of a kinetic bottleneck.

A cobalt-phosphate water oxidation catalyst ("Co-Pi") has been electrodeposited onto mesostructured alpha-Fe(2)O(3) photoanodes. The photoelectrochemical properties of the resulting composite photoanodes were optimized for solar water oxidation under frontside illumination in pH 8 electrolytes. A kinetic bottleneck limiting the performance of such photoanodes was identified and shown to be largely overcome by more sparse deposition of Co-Pi onto the alpha-Fe(2)O(3). Relative to alpha-Fe(2)O(3) photoanodes, a sustained 5-fold enhancement in the photocurrent density and O(2) evolution rate was observed at +1.0 V vs RHE with the Co-Pi/alpha-Fe(2)O(3) composite photoanodes. These results demonstrate that integration of this promising water oxidation catalyst with a photon-absorbing substrate can provide a substantial reduction in the external power needed to drive the catalyst's electrolysis chemistry.