Interplay Between Charge Accumulation and Oxygen Reduction Catalysis in Nanostructured TiO 2 Electrodes Functionalized with a Molecular Catalyst

The catalytic reduction of O 2 by a manganese(III) porphyrin immobilized in a nanostructured semiconductive transparent TiO 2 electrode is here investigated by UV‐Vis spectroelectrochemistry in an aqueous buffered medium. Analysis of the operando spectroelectrochemical data, collected for both the immobilized catalyst and the TiO 2 matrix, demonstrates the coexistence of two faradaic electrochemical processes, namely ( i ) irreversible interfacial electron transfer from TiO 2 to the immobilized porphyrin triggering the catalytic reduction of O 2 , and ( ii ) reversible proton‐coupled electrochemical reduction of TiO 2 leading to the accumulation of electrons in the TiO 2 bulk. The competition between these two processes is modulated by the local concentration of O 2 , which itself varies with the rate of the catalysis. Indeed, when O 2 is locally strongly depleted by catalysis, the process switches from catalysis to charge storage, like a battery. As a result, the electrons stored in TiO 2 were observed to pursue the catalysis even after the electrode polarization was switched‐off ( i. e ., under open circuit). This is an overlooked phenomenon that we believe is important to consider in applications relying on metal oxide‐based photoelectrodes operating in aqueous media.