Materials engineering strategies on nanostructured WO3 photoanodes for solar water splitting

Photoelectrochemical water splitting cells could offer a simple, yet efficient approach to solar hydrogen production. Known materials lack the required efficiencies and stabilities to make this process commercially viable, however. This thesis aims to contribute to our understanding of existing materials engineering strategies and to develop new approaches to the design of efficient WO3 based photoanodes. The results of a full factorial optimisation strategy for the synthesis of hydrothermal WO3 nanorod photoanodes are presented first. The external quantum efficiencies of the optimised photoanodes are amongst the highest photon conversion efficiencies reported to date for nanostructured WO3, demonstrating that the full factorial approach can be effective in designing high efficiency WO3 photoanodes. The optimised photoanodes exhibit comparatively fast electron extraction kinetics, which, in contrast to previous studies, shift to earlier timescales with increasing anodic bias. Measurements of charge extraction kinetics as a function of oxygen vacancy concentration demonstrate that this difference in bias dependence can be attributed to a comparatively small degree of oxygen deficiency in these photoanodes. Photovoltammetry measurements in the presence of hole scavengers suggest that bulk recombination limits photoanode performance at all biases, while surface recombination is only limiting at low anodic biases. Motivated by these findings, the impacts of electrocatalyst deposition on photoanode performance are finally investigated. Two different Earth abundant electrocatalysts were chosen for this purpose: FeOx and CoFePbOx. Interestingly, both catalysts have a similar impact on performance. Deposition of either catalyst improves photoanode activity at low biases but reduces performance at high anodic biases. Catalyst stability is an issue for both systems. The work presented here therefore expands our understanding of existing engineering strategies and informs future strategies for the design of efficient WO3 photoanodes.Open Acces