Photoelectrochemical Oxygen Reduction Reactions Using Phthalocyanine-Based Thin Films on an ITO Electrode

The oxygen reduction reaction (ORR) has attracted much 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 a promising alternative to various kinds of efficient catalysts such as platinum, we have investigated the photoelectrochemical ORR using various phthalocyanines (Pcs). On the irradiation by visible light of an indium tin oxide (ITO) electrode coated by thin films consisting of Pc and poly(vinylidene difluoride) (PVDF) polymer, the cathodic current corresponding to the ORR increased significantly. A previous study suggested that the high photocurrent might originate from the initial electron transfer between the photoexcited Pc and O-2 (Pc* + O-2 + Pc center dot- + O-2(center dot-)) followed by the hole transport between Pcs. However, here we propose a new mechanism that can also explain the efficient photoelectrochemical ORR in Pc/PVDF thin films. The dependence on the reduction potential of the Pcs indicates that the electrochemical reduction of the photoexcited Pc is the initial process. Subsequently, electron transfer from Pc center dot- to O-2 occurs, i.e., Pc center dot- + O-2 -> Pc + O2(center dot-). Based on the comparison between ZnPc and MgPc derivatives, which have different quantum yields for the lowest excited triplet (T-1) state, the high photocurrent mainly originates from the electrochemical reduction of Pcs in the lowest excited singlet (S-1) state. Although the reaction can also occur via the T-1 state, the T-1 state contributes to an increase in photocurrent only when the electrode potential is below +0.2 V. This study will be useful for the development and design of new catalysts for photoelectrochemical ORR.