Water Oxidation by Molecular Catalyst Bilayers Self-Assembled on Bismuth Vanadium Oxide (BVO) Photoelectrodes

Sunlight-driven water splitting stands out as one of the viable strategies for the generation of a sustainable fuel. For this purpose, the coupling of proton reduction and water oxidation catalysis, with light absorbing materials is crucial. Here we develop a strategy that integrates a powerful molecular water-oxidation catalyst, based on oligomeric Ru-tda complexes (tda2- is [2,2':6',2″-terpyridine]-6,6″-dicarboxylate), with a light absorbing BiVO4 material through self-assembly and formation of a bilayer. The self-assembly process also allows for the integration of a redox mediator, Ru-bpy (bpy is 2,2'-bipyridine), that was found to accelerate hole-transfer from the BiVO4 to the molecular catalyst leading to more efficient catalysis, as measured by transient absorption and intensity modulated photocurrent (IMPS) spectroscopies. With an applied potential of 0.82 V vs NHE (1.23 vs RHE) at pH 7 and 1 sun irradiation, photocatalytic water oxidation was observed with current densities of ca. 2 mA cm-2 and Faradaic efficiencies of 98%. This corresponds to turnover of over 23,500 per Ru unit (280,000 per Ru oligomer), ranking among the best reported at neutral pH. This new strategy yields photoelectrodes with enhanced stability and with very low catalyst loadings, that are expected to be versatile and easily implemented with other catalysts and semiconducting materials.