Molecular Photoelectrode for Water Oxidation Inspired by Photosystem II

In artificial photosynthesis, the sun drives water splitting into H2 and O2 or converts CO2 into a useful form of carbon. In most schemes, water oxidation is typically the limiting half-reaction. Here, we introduce a molecular approach to the design of a photoanode that incorporates an electron acceptor, a sensitizer, an electron donor, and a water oxidation catalyst in a single molecular assembly. The strategy mimics the key elements in Photosystem II by initiating light-driven water oxidation with integration of a light absorber, an electron acceptor, an electron donor, and a catalyst in a controlled molecular environment on the surface of a conducting oxide electrode. Visible excitation of the assembly results in the appearance of reductive equivalents at the electrode and oxidative equivalents at a catalyst that persist for seconds in aqueous solutions. Steady-state illumination of the assembly with 440 nm light with an applied bias results in photoelectrochemical water oxidation with a per-photon absorbed efficiency of 2.3%. The results are notable in demonstrating that light-driven water oxidation can be carried out at a conductive electrode in a structure with the functional elements of Photosystem II including charge separation and water oxidation.