Interplay Between Charge Accumulation and Oxygen Reduction Catalysis in Nanostructured TiO₂ Electrodes Functionalized with a Molecular Catalyst

Abstract

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.

Impact of Reversible Proton Insertion on the Electrochemistry of Electrode Materials Operating in Mild Aqueous Electrolytes: A Case Study with TiO2

“…[26] In our group, we also revealed the faradaic interplay between the reversible insertion of protons in TiO 2 and the catalytic reduction of O 2 via an iron porphyrin immobilized within the mesoporous structure of a TiO 2 electrode. [27] More recently, the Augustyn and Mpourmpakis/McKone groups have both suggested that protons inserted into WO 3 govern the reactivity toward the hydrogen evolution reaction. [28,29] All these elements reinforce the idea that there is a crucial need to elucidate the key factors governing PICET reactions, and more specifically in aqueous unbuffered media.…”

Photoelectrochemical Proton-Coupled Electron Transfer of TiO2 Thin Films on Silicon

“…Interestingly, mesoporous TiO 2 electrodes or other high-surface-area assemblies containing TiO 2 often appear to be stable photocathodes and supports for tethered molecular catalysts, despite the fact that the TiO 2 is likely being reduced during catalysis. Indeed, Balland et al recently demonstrated that a high-surface-area assembly consisting of a TiO 2 -tethered molecular O 2 reduction catalyst undergoes both catalysis and proton-coupled reduction of the TiO 2 . In this case, the reduced TiOOH is not a source of instability but rather a charge storage reservoir that facilitates catalysis.…”

“…This need for e − /H + accumulation for TiO 2 to act as an electrode has long been known. 34,102,103 Modeling the CV response, however, is challenging, and the TiO 2 /Si electrode is even more complex. The concentrations of electrons and holes at the Si surface and the potential drop within the Si vary with applied potential and other factors.…”

“…Indeed, Balland et al recently demonstrated that a high-surface-area assembly consisting of a TiO 2 -tethered molecular O 2 reduction catalyst undergoes both catalysis and proton-coupled reduction of the TiO 2 . 34 In this case, the reduced TiOOH is not a source of instability but rather a charge storage reservoir that facilitates catalysis. Overall, it is clear that TiO 2 films are noninnocent supports when used for reductive catalysis.…”

“…While many studies of TiO 2 reactivity have emphasized the behavior of electrons and/or holes, it has been well demonstrated that the reduction of TiO 2 in protic media proceeds by the addition of both electrons and protons. This proton-coupled electron transfer (PCET) reactivity has been explored electrochemically, with thin films, nanowires, and nanoparticles on electrodes, and chemically with colloidal nanoparticles. − Redox reactions of many other metal oxides have also been shown to involve PCET, including NiO x , , CeO x , , WO x , − IrO 2 , etc. , Reductive addition of protons or metal cations to a metal oxide has implications for its stability, conductivity, and reactivity. ,− …”