The p-type semiconductor copper oxide (Cu 2 O) has a band gap of 2.17 eV. Furthermore, the conduction band edge position is well-suited for the electron transfer reactions associated with the formation of H 2 . 9 Unfortunately, Cu 2 O electrodes undergo photodegradation in aqueous environments due to the position in the band gap of the redox potentials for the reduction and oxidation of Cu 2 O. Thus, Cu 2 O electrodes are unsuited for PEC devices.