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

Abstract

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_0.03Cu_0.97O) 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_xCu_1−xO 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. Cu₂O grows as distinct crystallites on the surface of Li_xCu_1−xO instead of forming a dense layer. Additionally, there is no obvious Cu₂O gradient inside the films, as Cu₂O seems to form on all Li_xCu_1−xO nanocrystals exposed to water. The application of a thin Ti_0.8Nb_0.2O_x coating by atomic layer deposition and the deposition of a platinum co-catalyst increased the stability of Li_xCu_1−xO against decomposition. These devices showed a stable hydrogen evolution for 15 minutes.