Further valuable insights into the effects of Mo-doping and the role of CoPOM co-catalyst on the PEC performance can be gained by determining the charge separation efficiency (𝜂 𝑠𝑒𝑝 ) and the hole transfer efficiency (𝜂 𝑡𝑟 ) as a function of applied potential (Fig. and), employing the methodology developed by Dotan et al. and Hamann et al. (for details see ESI †). Firstly, by integrating the electronic absorption spectra (Fig. ) over the reference AM1.5G photon flux spectra, the maximum theoretically achievable photocurrents 𝐽 𝑚𝑎𝑥 were calculated to be 7.37 mA cm -2 , 7.36 mA cm -2 and 7.34 mA cm -2 for BiVO4, Mo-BiVO4 and Mo-BiVO4/CoPOM, respectively. The photocurrent density of both Mo-BiVO4 and Mo-BiVO4/CoPOM photoanodes in the presence of Na2SO3 acting as a readily oxidizable reducing agent (i.e., sacrificial hole scavenger) is significantly higher than that of pristine BiVO4 and reaches 6.5 mA cm -2 as compared to 3.2 mA cm -2 at 1.23 V vs. RHE, respectively (Fig. ). Based on the above results, the charge separation efficiency 𝜂 𝑠𝑒𝑝 was calculated and plotted in Fig. . Notably, 𝜂 𝑠𝑒𝑝 for Mo-BiVO4 and Mo-BiVO4/CoPOM is significantly higher than for pristine BiVO4 in a wide potential range, whereby the enhancement is increasingly pronounced at more positive bias potentials. This behavior is in line with our finding that the beneficial effects of the improvement of electronic conductivity upon Mo-doping are more pronounced at positive bias potentials, whereby at very low bias potentials the positive effects are traded off against the detrimental effects of Mo-doping associated with increased concentration of intragap electron polaronic states. Interestingly, at higher bias potentials (> 0.7 V vs. RHE), the Mo-doped Mo-BiVO4 exhibits also an improved interfacial hole transfer efficiency 𝜂 𝑡𝑟 with respect to pristine BiVO4 (Fig. ). This positive effect of Mo-doping on 𝜂 𝑡𝑟 can be interpreted as partial mitigation of excessive hole accumulation in the surface states, as reported recently by Selli et al. After the deposition of Co-POM, the hole transfer efficiency 𝜂 𝑡𝑟 (Fig. ) is greatly enhanced over the entire potential range and shifted to more cathodic potentials. For example, at 1.23 V vs. RHE, the 𝜂 𝑡𝑟 increases from 13.1% for BiVO4 to 37.8% after Mo-doping (Mo-BiVO4) up to 63.0% for Mo-BiVO4/CoPOM photoanode. Importantly, the charge separation efficiencies (𝜂 𝑠𝑒𝑝 ) for Mo-BiVO4/CoPOM and Mo-BiVO4 are nearly identical in the whole potential range (Fig. ), which clearly indicates that the beneficial effect of CoPOM deposition consists chiefly in enhancing the catalysis of water oxidation. This is a significant result since the effect of co-catalysts on photocurrent enhancement at BiVO4 photoanodes is a much-debated issue and, apart from enhancing the catalysis of water oxidation, other effects of various co-catalyst (e.g., improved charge extraction or passivation of surface states) are often reported to be responsible for photocurrent enhancement in the literatures. Our results unambiguously demonstrate that in our photoanodes the beneficial effect of CoPOM deposition consists mainly in enhancing the rate of water oxidation by photogenerated holes. Finally, in order to provide a direct evidence for dioxygen evolution at our photoanodes, we performed photoelectrocatalytic OER measurements (Fig. ) in a borate solution (pH 9.0) at 0.74 V vs. RHE under AM 1.5G (1 sun) illumination. As expected, the Mo-BiVO4/CoPOM photoanode exhibits an excellent oxygen evolution rate with an average faradaic efficiency of oxygen evolution of 102.1% ± 6.2% (calculated from three measurements; the error is taken as ±1σ, σ = standard deviation), which demonstrates that the conversion of H2O to O2 by photogenerated holes is practically quantitative and no side reactions (e.g., photocorrosion or H2O2 evolution) occur. Similar results were obtained for Mo-BiVO4 (100.0% ± 12.0%), whereby at pristine BiVO4 the faradaic efficiency was slightly lower (90.3% ± 8.3%), suggesting a minor role of side reactions, possibly related to the photocorrosion. These results suggest that Mo-doping has also a beneficial effect on the stability of the photoanodes under the PEC operation, which again might be related to the decreased charge accumulation in the surface states, since excessive hole accumulation in the surface states can be expected to render the dissolution of BiVO4 more facile.