Next, the photocurrent generation was tested at a constant potential of 0.80 V vs. RHE under prolonged (3 hours) AM 1.5G (1 sun) illumination. As shown in Figure , CoPOMdoped BiVO4 showed a relatively fast and continuous decrease of photocurrent. To explore the reasons for the instability of Composite 1 under PEC operation, we analysed in detail the elemental composition of the surface for CoPOM-doped Bi-VO4 before and after the PEC experiments, as shown in Figure and Table , Supporting Information. Interestingly, the XPS analysis revealed that the signal related to the presence of Co is significantly stronger after the PEC operation (Figure , Supporting Information), while the signal of W practically did not change at all after PEC operation (Figure , Supporting Information). Notably, the ratio of W/Co for Composite 1 photoanode significantly decreased after the PEC operation, from 2.29 for before PEC operation to 0.59 for after PEC operation (Table , Supporting Information). In other words, we see a similar trend as discussed above for the case of Composite 1 photoanode (higher ratio) and Composite 3 photoanode (lower ratio). We conclude that the PEC performance is related to the ratio of W/Co on the surface of photoanode, whereby a higher ratio of W/Co will result in better PEC performance. A possible reason for this trend might be that at the lower ratios of W/Co, pronounced leaching of Co from the BiVO4 surface into the borate buffer electrolyte might lead to formation of cobalt borate (CoBi), which is known to exhibit operational instability in Bi-VO4/CoBi systems. Notably, employing CoPOM as a surface co-catalyst to modify the Composite 1 photoanode turned out be highly beneficial as higher and much more stable photocurrents were observed (the green line in Figure ). The small jumps of the photocurrent were most probably due to the bubble release from the testing area, as observed visually during the PEC operation. We used XPS to analyze the elemental composition of the surface for Composite 2 before and after the PEC experiments, as shown in Figure and Table , Supporting Information. In contrast to Composite 1, the signals related to the presence of Co and W were significantly weaker after the PEC operation (Figure and S10d, Supporting Information) with a slightly decreased ratio of W/Co for Composite 2 photoanode than before the PEC operation (Table , Supporting Information). In particular, the changes in signal intensities and ratio of W/Co indicate that a partial dissolution of the CoPOM co-catalyst and/or its conversion to cobalt and tungsten oxides can occur during the PEC operation, which is consistent with the findings discussed in detail in our previous study. Furthermore, the transient photocurrent measurements of pristine BiVO4, Composite 1 and Composite 2 photoanodes were investigated to assess the charge recombination dynamics at +0.80 V vs. RHE under chopped AM 1.5G (1 sun) illumination (Figure , Supporting Information). The strong spike-like behaviour of the photocurrent transients at pristine BiVO4, a typical fingerprint of intense surface recombination, is clearly diminished to a large extent at the Composite 1 and Composite 2 samples, indicating reduced charge carrier recombination for these samples.