In order to shed more light on the factors governing the photoresponse of CoPOM-containing photoanodes, we performed an analysis of charge separation (ηsep) and hole transfer (ηtr) efficiencies according to established protocols. This analysis is based on the assumption that the measured photocurrent density in water oxidation can be calculated by multiplying the maximum possible photocurrent (obtained from the absorbed photon flux) by ηsep and ηtr, whereby the hole transfer efficiency ηtr in the presence of a readily oxidizable reducing agent (here Na2SO3) is taken as 100%; for details see the experimental section. The calculated ηtr and ηsep values for the both CoPOM-containing electrode are depicted in Figure , calculated from data in Figures and. As expected, both efficiencies show clear dependence on the applied potential as stronger positive applied bias is beneficial for both charge separation and hole transfer. Notably, the charge separation efficiency ηsep is very similar for both PEI linker-free CoPOM-CNx-TiO2 and PEI-containing CoPOM-PEI-CNx-TiO2, which is also in line with only minor differences in photoluminescence spectra of the corresponding photoanodes that are possibly related to slightly more efficient quenching of emissive states in CNx (Supporting Information, Figure ). In stark contrast, the CoPOM-PEI-CNx-TiO2 exhibits significantly higher charge transfer efficiencies than CoPOM-CNx-TiO2 in the whole potential range. In other words, this data suggests that the superior photoelectrocatalytic behavior of the PEI linker-containing CoPOM-PEI-CNx-TiO2 photoanode arises from a more efficient transfer of photogenerated holes from CNx to water, which can be attributed to the higher CoPOM loading due to the beneficial effect of the cationic PEI polymer linker. However, in addition to its beneficial effect on the CoPOM immobilization, a question arises whether the cationic PEI polymer could potentially also act as a sacrificial electron donor that can be simply more easily oxidized than water by holes from CNx. In order to address directly this issue, we measured the photocurrents from the CNx-TiO2 electrode modified with PEI polymer only (dipped in PEI solution 5 times), and the measurements at PEI-CNx-TiO2 photoanodes were repeated subsequently in four cycles. The deposition of PEI enhanced photocurrents, but a gradual decrease of photocurrents was observed, ending up at the same values as those for the CNx-TiO2 photoanode (Figure ). Hence, in the absence of the CoPOM catalyst, the PEI does extract effectively the holes from CNx, but is thereby oxidatively degraded. As a next step, the same protocol was also applied to the CoPOM-PEI-CNx-TiO2 photoanode. Contrary to the gradual decline of photocurrents observed for PEI-CNx-TiO2, the photocurrents at CoPOM-PEI-CNx-TiO2 remain completely stable over all four cycles (Figure ). This is also in line with the chronoamperometric photocurrent measurements of the three electrodes (Figure ), which show that PEI-CNx-TiO2 exhibits a fast decline of photocurrent due to degradation of PEI in the absence of CoPOM, whereby the photocurrent at CoPOM-PEI-CNx-TiO2 is practically stable. Therefore, we conclude that in the presence of CoPOM, the holes are efficiently transferred from PEI to the CoPOM catalyst where they drive water oxidation, and the cationic PEI linker is thereby effectively stabilized. In order to unambiguously prove the dioxygen evolution at CoPOM modified photoanodes, we performed photoelectrocatalytic OER measurements (Figure ) in a borate solution (pH 8) under prolonged (1 hour) visible light irradiation (λ > 420 nm, 150 mW/cm 2 ). Both CoPOM-containing hybrid photoanodes, CoPOM-CNx-TiO2 and CoPOM-PEI-CNx-TiO2, clearly exhibit OER activity under visible light illumination. This confirms our assumption that charge transfer from CNx to CoPOM is feasible and that the presence of the CoPOM cocatalyst is necessary to trigger the OER. Importantly, at the PEI-containing electrode the oxygen evolution rate was doubled compared to the counterpart photoanode without PEI. Importantly, no oxygen evolution was observed at the CoPOM-free CNx-TiO2 photoanode despite substantial photocurrents that can be ascribed to photocorrosion. This result is also in line with our previous studies that confirmed that the presence of an effective OER catalyst is absolutely necessary to observe oxygen as a product of water oxidation at CNx-TiO2 hybrid photoanodes. On the other hand, CNx-free pristine TiO2 photoanodes modified with CoPOM exhibited neither photocurrents nor oxygen evolution since pristine TiO2 does not absorb in the visible range. The apparent (based on dissolved O2 and uncorrected for losses in the headspace) Faradaic efficiencies (FE) of oxygen evolution for CoPOM-PEI-CNx-TiO2 (15% ± 4%) and for CoPOM-CNx-TiO2 (12% ± 4%) are rather low, which suggest that even in the best photoanodes the overall utilization of holes generated in CNx for water oxidation is still far from optimum, and a substantial portion of holes does not induce the OER but instead contributes to the photocorrosion of CNx. The improved photocurrent onset potential, higher oxygen production rate and FE at the PEI-containing CoPOM-PEI-CNx-TiO2 photoanode clearly confirm the beneficial effect of the cationic PEI polymer that serves as an effective linker by establishing the electrostatic attraction between the CNx sensitizer and CoPOM catalyst that are both negatively charged. This results in a more efficient hole extraction from CNx and more effective utilization of photogenerated holes for water oxidation due to the more effective immobilization (i.e., higher loading) of the CoPOM water oxidation catalyst.