higher IPCE values at wavelengths above 350 nm. This is an expected behavior for porous photocathodes since for the backside irradiation the electrons and holes are generated closer to the FTO, and therefore the transport pathway for the holes is relatively short, whereby the electron transport is not a limiting factor, as the electrons are expected to readily react with electron acceptors in the solution that penetrates the whole volume of the porous electrode. In the case of the frontside irradiation, where most of the charges are generated in the vicinity of the electrode/electrolyte interface, the hole transport is typically the limiting factor in porous photocathodes because of the longer pathway for the holes to the FTO across the photoelectrode. Notably, at very low irradiation wavelengths (below 340 nm), the IPCE values are higher under the frontside illumination than under the backside illumination. We assume that this behavior, which is contradictory to the situation at longer wavelengths, is partly As the photocurrents at all photocathodes dropped significantly in the absence of dioxygen, it is clear that dioxygen dissolved in the solution acts as the main scavenger of photogenerated electrons. However, the key question to be addressed was whether H2O2 can be detected as a product of the photoelectrocatalytic conversion. Since the molar concentrations of H2O2 accumulated in our cells were below the detection limit of the conventional colorimetric H2O2 determination using a reaction with titanium peroxo-complex, we measured the H2O2 evolution in-situ using amperometric microsensors . To this end, we used H2O2 microelectrodes (diameter of 25 m) modified with Prussian blue (PB), as an effective electrocatalyst for H2O2 reduction, positioned approx. 50 μm above the photocathode surface as schematically shown in and). The presence of PCN in the photocathode is therefore crucial in order to observe H2O2 as a product. Based on the Faraday's law, the total amount of H2O2 was determined to be 0.32 ± 0.08 pmol (n = 3) and 0.030 ± 0.007 pmol (n = 3) for the NiOx-TiO2-PCN and NiOx-b a PCN photocathodes, respectively (Fig. ). As expected, no H2O2 was detected for PCNcontaining photocathodes in the absence of dioxygen (Supporting Information, Fig. ). These results demonstrate that PCN-containing photocathodes are capable of visible lightdriven reduction of O2 to H2O2, whereby the photocathode architecture comprising the TiO2 interlayer exhibits the best performance both in terms of photocurrent and the amount of H2O2 formed. A simplified energy scheme depicting a possible mechanism of H2O2 formation at NiOx-TiO2-PCN photocathodes under visible light irradiation is shown in Fig. . As discussed above, Ti 3+ -related intra-gap states are formed in TiO2 interfaced with NiOx. As the TiO2 layer is very thin, these intra-gap states are also assumed to facilitate the hole transport across the TiO2 film to the NiOx hole-collecting scaffold. Please note that at least two different mechanisms of H2O2 formation are thermodynamically feasible. The two-electron reduction pathway, typically postulated for PCN materials in presence of additional electron donors , has a larger thermodynamic driving force (1.4 V downhill) than a one-electron reduction to superoxide anion radical (0.9 V downhill), followed by disproportionation to H2O2 upon protonation of the superoxide anion radical: → → .