Insight into the origin of the limited activity and stability of p-Cu₂O films in photoelectrochemical proton reduction

The origin of instability of p-Cu₂O films deposited on a platinized Si substrate when used as photocathode in photoelectrochemical water splitting, was studied in the absence or presence of a protective layer of RuO₂. When applied at +0.3 V vs. RHE and at pH 7, p-Cu₂O films were found to show a slightly more stable performance as compared to photoelectrochemical measurements reported in the literature at 0 V vs. RHE and under acidic conditions. In addition, the stability and the photocurrent induced by the Cu₂O films were significantly improved when H₂O₂ was added to the electrolyte, which is explained by efficient scavenging of electrons, yielding oxygen and water as confirmed by gas chromatography (GC). Also, other electron acceptors improved the photocatalytic performance of the p-Cu₂O films, demonstrating that the transfer of photo-excited electrons to protons adsorbed on the surface is the rate determining step in p-Cu₂O based photo-electrochemical water splitting. We confirmed that deposition of RuO₂ improves the stability of the films, but to the expense of a decrease in photocurrent density. The results provided in this study rationalize the attachment of an effective H₂ evolution catalyst as a means to significantly improve the stability of p-Cu₂O electrodes.