Interestingly, the Mo-BiVO4/CoPOM shows much higher photocurrents at relatively higher applied potentials (> 0.9 V vs. RHE), but at very low bias potentials (< 0.7 V vs. RHE) the situation is reversed and the Mo-free BiVO4/CoPOM electrode yields even slightly higher photocurrents than the Mo-BiVO4/CoPOM photoanode. These observations imply that (i) the combination of Mo-doping and Co-POM deposition significantly improves the photocurrent response in the whole potential range with respect to pristine BiVO4, (ii) the enhancement due to the deposition of CoPOM is particularly pronounced at low bias potentials, i.e., the beneficial negative shift of the photocurrent onset potential and improved fill-factor is mainly due to the CoPOM deposition, and (iii) at very low bias potentials the Mo-doping might even partially counteract the positive effects induced by the CoPOM deposition. The observation that the Mo-doping might be detrimental at relatively low bias potentials prompted us to measure the open-circuit photopotential (OCP) of all photoanodes (Fig. ). Under illumination, the observed OCP should correspond to the quasi-Fermi level of electrons *EFn (i.e., the electrochemical potential of electrons) in the illuminated semiconductor, which in turn is directly related to the photocurrent onset potential (and the fill-factor at low bias potentials), and thus to the maximum achievable photovoltage of the water-splitting system. Indeed, the OCP results clearly show that for both Mo-BiVO4 and Mo-BiVO4/CoPOM the Mo-doping leads to a positive shift (by ~90 mV) of the maximum achievable quasi-Fermi level of electrons as compared to pristine BiVO4.