Fig. : A simplified energy scheme for H2O2 production at NiOx-TiO2-PCN photocathode. The approximate position of the valence band edge of NiOx is taken from Ref. ; the positions of the band edges of TiO2 and PCN are taken from Ref. . All excitation processes apart from PCN and all possible recombination pathways are omitted for the sake of clarity. The presence of Ti 3+ intra-gap states (see the text for details) is postulated to facilitate the hole transport through the TiO2 layer. The formation of H2O2 is thermodynamically feasible both via a two-electron reduction pathway, as well as via a oneelectron reduction to superoxide anion radical, followed by disproportionation to H2O2 and O2 upon protonation of the superoxide anion radical .