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Coupled Polyoxometalate – Hydrogenase Catalysts for Photocatalytic Water Splitting

Final Report Summary - POMHYDCAT (Coupled Polyoxometalate – Hydrogenase Catalysts for Photocatalytic Water Splitting)

The EU aims to obtain 20% of all energy from renewable sources by 2020.1 Beyond 2020, expansion of renewable energy is likely to depend upon efficient conversion of solar energy into chemical fuels, such as H2. This fellowship aims to address light-driven H2 production by using polyoxometalate (POM) water oxidation catalysts (WOCs).
POMs such as [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2]10- (1) offer significant advantages as WOCs – they are oxidatively stable, and highly tuneable. The Hill group found that 1 is an efficient WOC at neutral pH, and can be driven by light with [Ru(bpy)3]2+ as sensitizer.2 Simultaneously, photocatalytic H2 production was achieved at neutral pH by the Armstrong group using an [NiFeSe] hydrogenase on dye-sensitized TiO2.3 The compatible pH and photosensitizing regime makes this combination of catalysts an excellent starting point for a light-driven water splitting system.
In the first phase, the fellow (John Fielden, JF) worked with Prof. Craig Hill at Emory University (USA), to create a water splitting photoanode where the WOC 1 is interfaced with dye-sensitized TiO2 (Fig. 1). The return phase, with Prof. Chris Pickett at the UEA (collaborator: Prof. Fraser Armstrong, Oxford) originally aimed to combine the water splitting photoanodes with hydrogenases to assemble a complete water splitting system. However, during the outgoing phase Prof. Armstrong’s group discovered that the target enzyme was more difficult to express than expected. For this reason, we modified the return phase to develop redox-active photocathodes that could be used to support a wide range of light-driven fuel production chemistries.

The first four objectives of the project, relating to the photoanodes, were achieved at Emory with follow-up studies at the UEA. At the UEA we have obtained the first ever covalently connected POM-polypyrrole materials and deposited them on p-type Si. The project has demonstrated that:
• TiO2/Dye/1 photoelectrodes show rapid WOC-to-dye electron transfer, and can oxidize water. Performance and stability will be improved by faster WOCs and more strongly binding dyes.
• Practical methods for light-driven water splitting are likely to require either segregation of light absorption and catalysis, or efficient, stable, inorganic photoelectrodes.
• The cobalt(II) centred Keggin anion is a prototype inorganic photosensitizer with an excited state lifetime of up to 1.7 ns, which is potentially chemically useful and far longer than for any other POM.
• Polyoxometalates can be covalently linked to pyrrole groups, allowing electro-synthesis of stable, redox-active polymer materials on carbon, platinum and p-type Si surfaces.