In terms of photocurrent magnitude, in the H-cell configuration, we received a negative photocurrent of ~40 µA/cm 2 upon the excitation of the PB and a positive photocurrent of ~20 µA/cm 2 upon the excitation of the PA. We suggest that the absolute magnitude of the generated photocurrent upon the excitation of PB is larger than the one of PA due to the direct reduction of the former by the WE (in addition to the effect of pH), in a similar manner to the single-cell setup (Figures and). On the other side, the PA present in the CE compartment can only 'interact' with the WE compartment by proton pumping. In summary, as indicated by our results using the H-cell setup, the pH jump in the WE environment upon applying a bias of -1V induces the decrease in cathodic current, while the pH drop induces the opposite.