Aerosol Deposition of BiVO4 Films for Solar Hydrogen Generation

Abstract

Due to its suitable semiconductor band gap energies and associated visible light absorption, bismuth vanadate offers high photon efficiencies in solar photo-anodes, enabling green hydrogen generation in photoelectrochemical water splitting cells. Respective bismuth vanadate films have to ensure high efficiencies in electron / hole pair generation, and sufficiently high rates of charge transfer, for both, electrons to the conducting substrate, as well as holes to the electrolyte. Thus, tuning of coating properties has to aim for high phase purity and good layer integrity. So far, respective films are mainly produced by thin film techniques, but at rather high costs and low deposition rates. Less costly processing routes are opened by thermal spraying or sol-gel techniques, however, these cannot guarantee the required phase purity or absence of remnants from the binder. As solid state and binderless alternative, Aerosol Deposition (AD) offers several advantages: comparative low costs, high deposition rates, no undesired phase transformations, and no impurities or residues that could reduce the photoelectrochemical activity. Under the scope of this research on photo-electrochemically active bismuth vanadate films, powder sizes were tailored by milling, and spray parameter sets like the process gas pressure were varied, in order to elucidate their influence on microstructure and application properties. Covering a wide parameter range in aerosol deposition allowed for the development of a window of deposition. Most promising combinations for layer build-up were derived. The results on stainless steel substrates were transferred to FTO-coated glass substrates, as needed in backlit cell layouts. For fine tuning of maximum photocurrents, layer thickness and conductivity were then systematically adjusted. Homogeneous large-scale prototypes demonstrate that aerosol deposition is suitable for processing layers for solar energy harvesting.