As shown in Fig. †, the binding energies of the Mo 3d3/2 (235.1 eV) and Mo 3d5/2 (232.0 eV) peaks indicate that the molybdenum dopant is in the oxidation state of +VI. The high-resolution XP spectrum of Co 2p (Fig. ) shows two core-level spin-orbit coupling peaks located at 797.1 (Co 2p1/2) and 781.2 eV (Co 2p3/2) with typical satellite peaks at 805.4 and 785.7 eV, indicating the presence of Co 2+ . In Fig. , the two spin-orbit splitting peaks at 37.6 eV and 35.4 eV attributed to W 4f5/2 and W 4f7/2 confirm the presence of W 6+ . Fig. shows the J-V curves of various optimized photoanode architectures under back-side illumination (from the FTO-glass side). The BiVO4 and Mo-doped BiVO4 photoanodes reach the photocurrent density of 0.65 mA cm -2 and 2.53 mA cm - 2 at 1.23 V vs. RHE, respectively. After the modification by CoPOM, the highest photocurrent density of Mo-BiVO4/CoPOM increases up to 4.32 mA cm -2 at 1.23 V vs. RHE, which is an enhancement by the factor of 6.6, and 1.7 with respect to BiVO4, and Mo-BiVO4, respectively. Importantly, both Mo-BiVO4 and Mo-BiVO4/CoPOM photoanodes exhibit a remarkable cathodic shift of the photocurrent onset potential by ~0.5 V as compared to the pristine BiVO4 photoanode, whereby the modification with CoPOM significantly increases photocurrents even at very low bias potentials, improving thus the fill-factor of the J-V curve. In other words, the combination of Modoping and CoPOM modifications results in a substantial increase of the photovoltage available for driving the water splitting reaction. In order to further disentangle the effects of Mo-doping and Co-POM modification, we also directly compared the J-V curves of Mo-free (undoped) BiVO4/CoPOM with Mo-BiVO4/CoPOM (Fig. ).