Efficient Solar Water Splitting Photocathodes Comprising a Copper Oxide Heterostructure Protected by a Thin Carbon Layer

Photoelectrochemical (PEC) solar water splitting has received extensive attention because it promises to provide an alternative and sustainable source of energy. A key challenge is to achieve a stable PEC system in either acidic or basic electrolyte without degradation of the (photo)electrodes. We have used a cubic Cu₂O film and porous granular bilayer Cu₂O/CuO composite with a carbon protection layer as photocathode materials. The films were deposited under different conditions, such as variation of the electrodeposition time, thermal oxidation of the Cu₂O films in air versus nitrogen atmosphere, and deposition of the carbon materials, and were investigated structurally and with regard to their PEC performance. The optimized electrodes showed photocurrents up to 6.5 and 7.5 mA/cm^-2 at potentials of 0 and -0.1 V vs RHE at pH 5.5, respectively. The stabilities of the Cu₂O/C and Cu₂O/CuO/C photocathodes, at a low bias of 0.3 V vs RHE, were retained after 50 h. The strongly improved photostability of the photocathodes in comparison to electrodes in the absence of a carbon overlayer is attributed to a more effective charge transfer and a protective role of carbon against photocorrosion.