Enhanced Water Splitting at Thin Film WO3 Photoanodes Modified with the Electro-Catalysts

Abstract

An effective solar light-driven water splitting in photo-electrochemical devices requires use of materials which combine high photo-conversion efficiency and long term stability. For the latter reason the choice of the suitable systems is restricted to the semiconducting oxides that, in most cases, do not undergo photo-corrosion in aqueous solutions and are able to absorb efficiently the visible light. However, although the nanostructured scaffold of the semiconducting photo-electrodes contributes generally to decrease the bulk charge recombination by reducing the minority charge carriers pathway to reach the solution species, at the same time, it tends to enhance the recombination of charges at the surface due to the increased contact area with the electrolyte. Common approach to reduce recombination losses, involves deposition at the photo-electrode surface of electrocatalysts allowing to overcome slow kinetics of oxygen evolution. There are only few, precious metal (Pt, IrO2), oxygen-evolution electrocatalysts that can be used in pH acidc in which WO3 operates effectively. In this regard, we will present an increase of water photo-oxidation currents at WO3 photo-anodes, caused by presence of polyoxometalates (POMs), such as phosphomolybdate (PMoO40 3-) Keggin type ions. This class of compounds is known to undergo reversible, stepwise multi-electron transfer reactions that are combined with their excellent proton conductivity. What is more, the POMs might act as a buffer layer when form a cover for a metallic nano - particles largely used as a regular catalysts. The POMs may act either as relays for photogenerated electrons or as catalysts directly involved in the hole transfer reactions. In particular, they may affect the pathway of water oxidation by modifying the stability of the intermediates (e.g., peroxides) involved in this processes. Ongoing efforts devoted to minimize the bias voltage required to perform visible light-driven photo-oxidation of water at NS WO3 photoanodes also include the use of dopants or incorporation of plasmonic metal nanostructures allowing to enhance the light capture by the WO3 films1. 1. R. Solarska, K. Bienkowski, S. Zoladek, A. Majcher, T. Stefaniuk, P. J. Kulesza and J. Augustynski, Angew. Chem. Int. Ed. 2014, 53: DOI: 10.1002/anie.201408374