To evaluate the performance of our photoanodes in water photooxidation, PEC measurements were conducted in a threeelectrode setup under AM 1.5G (1 sun) illumination with a 0.5 The photoanodes were optimized for the best PEC performance with respect to the amount of CoPOM used either as a bulk dopant and/or surface co-catalyst, as shown in Figure and S9b, Supporting Information. As shown in Figure , the pristine BiVO4 photoanodes exhibit a relatively low photocurrent density (0.62 mA cm -2 at 1.23 V vs. RHE), which is mainly attributed to the poor electronic conductivity and sluggish oxygen evolution kinetics at the photoanode/electrolyte interfaces. After CoPOM-doping, for Composite 1 a significantly improved photocurrent density (5-fold increase compared with pristine BiVO4) is observed, reaching 2.86 mA cm - 2 at 1.23 V vs. RHE. When the surface of the sample is further loaded with CoPOM as co-catalyst, the photocurrent density is further increased by a factor of ~1.3 to 3.67 mA cm -2 at 1.23 V vs. RHE. Importantly, we observe a cathodic shift of the photocurrent onset potential by ~0.4 V for Composite 2 and ~ 0.3 V for Composite 1 compared with the non-modified BiVO4 photoanode. In other words, both CoPOM-doping and surface modification by CoPOM result in a significant increase of the photovoltage available for driving the water splitting reaction. The reasons for this performance enhancement are discussed below.