Photoelectrochemical Oxygen Reduction Reaction Using Phthalocyanines

The oxygen reduction reaction (ORR) has attracted broad interest not only with respect to biological processes, such as cellular respiration, but also in terms of practical energy conversion, i.e. cathodic reactions in fuel cells. Because the use of light energy is one of the promising alternatives to platinum that is utilized as an efficient catalyst for the ORR, we have investigated the photoelectrochemical ORR using various phthalocyanines (Pcs, Figure 1). The cathodic current corresponding to the ORR significantly increases upon visible light irradiation of the thin film consisting of Pc and the polyvinylidene difluoride (PVDF) polymer coated onto an ITO electrode (Figure 2). Although a previous study suggests that the high photocurrent may originate from the initial electron transfer between the photoexcited Pc and O₂ (Pc* + O₂ → Pc⁺ + O₂^-) followed by the hole-transport between Pcs, herein we propose a new mechanism that can explain the efficient photoelectrochemical ORR in the Pc/PVDF thin film. The dependence on the reduction potential of Pcs indicates that the electrochemical reduction of the photoexcited Pc is an initial process. Subsequently, the electron transfer from Pc^- to O₂, i.e. Pc^- + O₂ → Pc + O₂^-, occurs. Based on the comparison between the ZnPc and MgPc derivatives whose excited triplet quantum yields are different, the high photocurrent mainly originates from the electrochemical reduction of Pcs in the lowest excited singlet state, while the reaction via the lowest excited triplet state contributes to an increase in photocurrent only when the electrode potential is below +0.2 V. This study will be useful for developing catalysts for the photoelectrochemical ORR.

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