Ligand Effects of Penta- and Hexacyanidoferrate-Derived Water Oxidation Catalysts on BiVO4 Photoanodes

The design of an efficient, robust, and cost-competitive photoanode is often considered to be the bottleneck step to an industrial implementation of artificial photosynthesis. To overcome its drawbacks and to improve photoelectrochemical performance, water oxidation co-catalysts can be deposited on the photoanode. Coordination polymers based on Prussian blue, that is, cobalt cyanidoferrates, have emerged as promising water oxidation catalysts because of their low cost and tunable composition, while maintaining high efficiency and stability in a wide range of pH. They present an excellent alternative to typically used metal oxides, which show limited efficiency improvement. Indeed, the hexacyanidoferrate analog K_xCo_y[Fe(CN)₆]·nH₂O appears to be an outstanding co-catalyst for decorating bismuth vanadate (BiVO₄) photoanodes, by enabling fast and irreversible hole transfer, combined with high catalytic efficiency. Because intrinsic catalytic activity can be modulated and enhanced by the introduction of pentacyanidoferrate precursors, we compared the effect of different pentacyanidoferrate derivatives on bismuth vanadate photoanodes for light-driven water oxidation. Our results indicate that the coordination polymers prepared using [Fe(CN)₅(NH₃)]^3– as a precursor lead to higher photocurrents and lower onset potential shifts. It is suggested that the relation between catalytic activity and ligand substitution is a combination of electronic effects imposed by the nature of the ligand and morphological effects, which regulate the number of active sites exposed and charge transport across the catalyst layer.