Cobalt as a sacrificial metal to increase the photoelectrochemical stability of CuBi2O4 films for water splitting

CuBi2O4 is an excellent photocathode candidate in water-splitting photoelectrochemical cells. However, its poor photoelectrochemical stability caused by the reduction of Cu2+ to Cu metal limits its use. Here, we show a strategy to decrease the reduction of Cu2+ to Cu using cobalt as a sacrificial metal. Co-doped CuBi2O4 films were prepared by spray pyrolysis using Co2+ salt as the precursor. Co2+ ions replace Cu2+ in the CuBi2O4 structure, and subsequent heat treatment at 500 °C leads to partial oxidation of Co2+ to Co3+. As the reduction potential of Co3+/Co2+ is higher than that of Cu2+/Cu, Cu2+ reduction can be minimized. Comparatively, about 72% of the photocurrent produced by the CuBi2O4 film is lost in the first few minutes of illumination. In the Co-doped CuBi2O4 film, the photocurrent drops by less than 7%. Thus, the Co-doping can increase the CuBi2O4 photostability and be helpful for the fabrication of more stable photocathodes.