Inorganic chemistry in service of renewable energy science: solar driven water oxidation by bioinspired manganese catalysts
Catalysts made from earth-abundant materials are needed for the splitting of water into its elements for renewable hydrogen fuel production. We have coupled a dye-sensitized titania film with a manganese-oxo "cubium" water-oxidation catalyst, [Mn4O4L6]+ (1+, L = bis(methoxyphenyl)phosphinate) to achieve the direct oxidation of water using visible light. The photo-catalyst assembly was prepared by sensitizing a titania-coated FTO conductive glass electrode with a ruthenium(II) dye, followed by over-coating with a proton-conducting membrane (Nafion) doped with 1+. Illumination of this photo-anode, when immersed in an aqueous electrolyte, generates a photocurrent that is sustained for multiple hours without the need for an applied potential. The complete photo-anode releases protons to solution and oxygen gas, simultaneously with photocurrent generation. The cooperation of all three components: catalyst, proton conductor and dye sensitized titania film were necessary for water oxidation to be achieved using only light as the driving force. This photoanode represents a functional molecular analog of photosystem II, the natural apparatus found in photosynthetic organisms.