Oxygen Reduction Reaction at Multi-Copper Complexes Investigated by Modified Electrode and In Situ XAFS

The oxygen reduction reaction (ORR), which occurs at the cathode in polymer electrolyte fuel cells (PEFCs), requires a large overpotential (more than 200 mV) even at the highly active platinum group metal (PGM) catalysts, since this reaction contains 4-electrons and 4-protons transfer to yield two water molecules from a dioxygen molecule. However, the biocathodes which are constructed by covalently attaching multi-copper enzymes, such as laccase (Lac), at electrode surfaces show nonexsistent overpotential (ca. 20 mV) toward ORR[1-2]. Unfortunately, the pH region, where the Lacs works well as an electrocatalyst for ORR, is limited and the spatial density of reactive sites at such the biocathode seems to be insufficient as compared with the state-of-the-art PGM electrocatalysts. Thus, the extraction of reactive sites in Lac and the effective arrangement on catalyst supports have been desired[3].

 Recently, Gewirth and co-workers have achieved a preparation of binuclear copper complex, Cu₂(Hdatrz)₂, as an electrocatalyst for ORR[4-6]. Although the catalyst shows relatively large overpotential for ORR in acidic solution, the activity in alkaline solution is quite high. The active site for ORR in the catalyst has been proved to be bi-copper centers [6-7] and the investigation of ORR mechanism at the catalyst seems to be valuable.

In the present study, we have adopted in situ X-ray absorption fine structure (XAFS) spectroscopy to monitor the electronic structure of Cu centers during ORR and to determine the molecular structure of Cu₂(Hdatrz)₂ complexes on electrodes. In situ XANES measurement has shown that the ORR at the Cu centers requires the formation of Cu^I species and the rate-determining step for ORR is dependent on pH.

For further development of metal complex catalysts by replacing Cu centers to other metal ions, a easy preparation of model complex electrode should be required. Chemically modified model electrodes are also constructed on gold surface to investigate ORR at multi-metal complex using electrochemistry and spectroscopic methods. 

This work was supported by New Energy, Industrial Technology Development Organization (NEDO). XAFS experiments were performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2010G200 and 2013G173)

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