The long-term stability of the photoanode and its PEC performance is an important issue not only with respect to possible applications, but in our case also with respect to fundamental questions regarding the stability and/or compositional changes of the used CoPOM polyoxometalate catalyst under PEC operational conditions. As illustrated in Fig. , the photocurrent generated by the investigated photoanodes was measured at a constant potential of +0.74 V vs. RHE for four hours under simulated AM 1.5G (1 sun) illumination, and only a very slight (by ~15%), yet continuous, decline of photocurrent density was observed for the optimized Mo-BiVO4/CoPOM electrode.