| Résumé | The catalytic reduction of O₂ by a manganese(III) porphyrin immobilized in a nanostructured semiconductive transparent TiO₂ 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 TiO₂ matrix, demonstrates the coexistence of two faradaic electrochemical processes, namely (i) irreversible interfacial electron transfer from TiO₂ to the immobilized porphyrin triggering the catalytic reduction of O₂, and (ii) reversible proton-coupled electrochemical reduction of TiO₂ leading to the accumulation of electrons in the TiO₂ bulk. The competition between these two processes is modulated by the local concentration of O₂, which itself varies with the rate of the catalysis. Indeed, when O₂ is locally strongly depleted by catalysis, the process switches from catalysis to charge storage, like a battery. As a result, the electrons stored in TiO₂ 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. |
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