How photocorrosion can trick you: a detailed study on low-bandgap Li doped CuO photocathodes for solar hydrogen production

The efficiency of photoelectrochemical tandem cells is still limited by the availability of stable low band gap electrodes. In this work, we report a photocathode based on lithium doped copper( ii ) oxide, a black p-type semiconductor. Density functional theory calculations with a Hubbard U term show that low concentrations of Li (Li 0.03 Cu 0.97 O) lead to an upward shift of the valence band maximum that crosses the Fermi level and results in a p-type semiconductor. Therefore, Li doping emerged as a suitable approach to manipulate the electronic structure of copper oxide based photocathodes. As this material class suffers from instability in water under operating conditions, the recorded photocurrents are repeatedly misinterpreted as hydrogen evolution evidence. We investigated the photocorrosion behavior of Li x Cu 1− x O cathodes in detail and give the first mechanistic study of the fundamental physical process. The reduced copper oxide species were localized by electron energy loss spectroscopy mapping. Cu 2 O grows as distinct crystallites on the surface of Li x Cu 1− x O instead of forming a dense layer. Additionally, there is no obvious Cu 2 O gradient inside the films, as Cu 2 O seems to form on all Li x Cu 1− x O nanocrystals exposed to water. The application of a thin Ti 0.8 Nb 0.2 O x coating by atomic layer deposition and the deposition of a platinum co-catalyst increased the stability of Li x Cu 1− x O against decomposition. These devices showed a stable hydrogen evolution for 15 minutes. This work reveals deep insights into the photocorrosion mechanism of nanostructured p-type Li-doped CuO cathodes used for photoelectrochemical hydrogen production.