The small perturbations of the photocurrent are likely related to the release of oxygen bubbles from the electrode surface, which can be observed by naked eye (see Video in the ESI †). The XRD patterns of all samples recorded after the PEC experiment show the typical diffraction peaks of monoclinic scheelite BiVO4, indicating that the BiVO4based light absorber did not undergo any structural changes under the operating conditions (Fig. †, compared with Fig. ). However, a question arises regarding the stability of the CoPOM co-catalyst under PEC operation since it has been reported that this polyoxometalate candepending on experimental conditionsundergo conversion to cobalt oxide (CoOx) which then often plays a chief role in water oxidation catalysis. In order to shed light on the fate of CoPOM in our photoanodes during operation, we analyzed in detail both the composition of the Mo-BiVO4/CoPOM photoanode after the PEC operation and the elemental composition of the electrolyte before and after the PEC experiments. First of all, we note that the small nanospheres attributed to CoPOM that could be seen before the PEC operation (Fig. and), were not observed anymore by HRTEM after the PEC operation (Fig. ), which indicates that the CoPOM cocatalyst has undergone (at least partially) morphological changes. Notably, the XPS analysis revealed that the signals related to the presence of W and Co are significantly weaker after the PEC operation (Fig. ), whereby the XPS signature of Co is still well-detectable in the high-resolution XP spectrum and is very similar to the situation before the PEC operation (Fig. †, compare with Fig. ). Notably, the surface content of both Co and W derived from the XPS analysis is significantly decreased and the W/Co atomic ratio increased after the PEC operation (Tab. S3 †). However, EDX analyses performed both from the top-view (Tab. S4 †) and from the cross-sectional view (Tab. S5 †) clearly show that both W and Co are still present in the photoelectrode in significant amounts, albeit with a significantly higher W/Co ratio than before the PEC operation. In this context, it should be noted that quantitative EDX analysis is rather challenging in this case due to the overlap between the Si-K (from the underlying glass) and W-M lines and since the content of Co and W is very low. Yet, the above differences in the composition before and after the PEC operation, in particular the changing ratio of W/Co, indicate that at least a partial dissolution of CoPOM and/or conversion of CoPOM to cobalt and tungsten oxides can occur during the PEC operation. Interestingly, an analysis of the composition of the electrolyte solution before and after the PEC operation using inductively coupled plasma atomic emission spectrometry (ICP-AES) did not detect any Co, and the concentration of W also did not increase (Tab. S6 †). While we cannot completely rule out the dissolution of CoPOM as the dissolved amount of Co and W might be below the detection limit of ICP-OES, our results indicate that a complete dissolution of CoPOM is not an issue. All the above observations suggest that, though the CoPOM catalyst does not dissolve in the electrolyte solution, it does undergo, at least partially, conversion to cobalt oxide, as previously reported for borate electrolytes by Finke et al. In other words, under our PEC operating conditions, the CoPOM co-catalyst should be rather conceived as a pre-catalyst, whereby a substantial part of water oxidation catalysis might be performed by cobalt oxide-based catalyst derived from CoPOM under operating conditions, as suggested by Finke et al. for CoPOM behavior in electrocatalysis. However, in this context, it should be also noted that the activity and stability of various cobalt oxide-based water oxidation catalysts can strongly depend on their precursor and operating conditions, as minute changes in composition can often exert strong effects on catalysis. For comparison, we have therefore also tested the performance of our Mo-BiVO4 photoanode after modification with a CoPi co-catalyst (i.e., cobalt oxy-hydroxide formed upon photodeposition in Co 2+ containing phosphate buffer) and with a CoO(OH)x co-catalyst (i.e., oxy-hydroxide deposited via a two-step impregnation process). These two photoanodes reached photocurrents slightly lower (by 9% and 33%, respectively) than the Mo-BiVO4/CoPOM photoanode (Fig. ), which confirms that the "pre-history" of the catalyst can make a difference to its activity in photoanodes, and that the CoPOMderived water oxidation catalyst exhibits optimal performance. Furthermore, it is well established that the activity and stability of Co-based electrocatalysts can depend strongly on the electrolyte composition, and our group previously observed strong electrolyte effects on the stability of CoO(OH)x water oxidation co-catalysts in hybrid photoanodes. Therefore, we carried out a comparative analysis of the PEC operational stability of our photoanodes in borate and phosphate electrolytes. Specifically, we measured the linear sweep voltammetry under chopped illumination in the borate and the phosphate electrolyte at pH 9.0, and then repeated the measurement six times. In the borate buffer electrolyte at pH 9.0 (Fig. ), the photocurrents for BiVO4, Mo-BiVO4 and Mo-BiVO4/CoPOM photoanodes remained completely stable over all six cycles. In contrast, in the phosphate electrolyte at pH 9.0 (Fig. ), the Mo-BiVO4/CoPOM exhibits a gradual decrease of photocurrents. The much higher operational stability of Mo-BiVO4/CoPOM photoanodes in a borate electrolyte is also clearly apparent from the long-term (4 hours) chronoamperometric measurement at +0.74 V vs. RHE under 1 sun irradiation (Fig. ). In this context, two points are noteworthy. First, the clearly detrimental effect of phosphate anions on the PEC performance stability of Mo-BiVO4/CoPOM is not simply related to the fact that the phosphate electrolyte is operated at pH 9.0 (i.e., beyond its ideal buffering pH range around pH ~7), whereas the borate electrolyte operates at its optimal buffering capacity (pH ~9), since comparative experiments performed at pH 7.0 also revealed that the photocurrents at Mo-BiVO4/CoPOM were more stable in the borate electrolyte than in the phosphate electrolyte (Fig. ). Second, similar to the case of the borate electrolyte, no cobalt could be detected in the phosphate electrolyte after PEC operation (Tab. S6 †), suggesting that the dissolution of the co-catalyst is not the major reason for lower photoanode stability in phosphate electrolytes. We, therefore, conclude that the PEC operational stability of our photoanodes is related to the presence of specific electrolyte anions, whereby borate electrolytes are more beneficial as compared to phosphate electrolytes. Drawing on our previous discussion, we assume that the Co (III/IV) ions, which are formed in situ during the water oxidation catalytic cycle and which represent, in terms of the HSAB theory, hard acid species, can interact more strongly with phosphate ions which are much harder bases than the relatively large borate anions (present mainly as tetraborates). We speculate that the relatively stronger interaction of cobalt ions with phosphate anions can induce, during the PEC operation, changes in the interaction between the BiVO4 light absorber and the cocatalyst, which eventually results in lower observed stability of PEC performance in phosphate-containing electrolytes.