A dinuclear copper(II) complex of 3,5-diamino-1,2,4-triazole is one of the highly active copper-based catalysts for the oxygen reduction reaction (ORR) in basic solutions. Our in situ X-ray absorption near edge structure measurements revealed that deprotonation of the triazole ligand might cause coordination geometrical changes, resulting in the enhancement of the ORR activity.

We report two-dimensional model systems to study electrocatalytic activities of dinuclear copper complexes for various electrocatalytic reactions including the oxygen reduction reaction (ORR), where we can use electrochemical techniques as well as surface-sensitive techniques such as Xray photoelectron spectroscopy and vibrational sum frequency generation spectroscopy. Heteroaromatic thiols including four triazoles and a thiadiazole are used as metal ligands as well as anchors to a polycrystalline gold electrode. The thiols are self-assembled on the polycrystalline gold elecctrode and then react with copper(II) ions to give monolayers of copperbased ORR catalysts on the surface. The dinuclear copper complexes of 1,2,4-triazole-3-thiol and 3-amino-1,2,4-triazole-5-thiol show ORR activity and similar pH-dependent catalytic behavior to that of counterparts supported on carbon black, suggesting that our two-dimensional systems can serve as model catalysts for carbon-supported molecular catalysts. We have also self-assembled dinuclear copper complexes with long alkyl or perfluoroalkyl chains on the surface and studied their orientation on the surface and oxygen transport.

The oxygen reduction reaction (ORR) to water is catalyzed at cathodes in polymer electrolyte fuel cells (PEFCs). For widespread applications of PEFCs, low-cost cathode materials with high catalytic activity and durability are highly desirable. Since natural ORR catalysts such as multi-copper oxidases catalyze the ORR with almost no overpotential (ca. 20 mV)[1]and the enzymatic ORR occurs at a multinuclear copper complex, mimicking enzymatic active sites would allow us to develop non-precious metal-based ORR electrocatalysts for practical applications. Herein we report our research progress in synthetic and mechanistic studies on copper-based electrocatalysts for the ORR.[3,4]Our findings suggest that the design and synthesis of multinuclear copper cores might be a key to develop highly efficient and durable copper-based electrocatalysts for the ORR. References. [1] N. Mano, V. Soukharev, A. Heller, J. Phys. Chem. B, 110, 11180 (2006). [2] M.S. Thorum, J. Yadav, A.A. Gewirth, Ang. Chem. Int. Ed., 48, 165 (2008). [3] M. Kato et al., Phys. Chem. Chem. Phys., 17, 8638-8641 (2015). [4] M. Kato, N. Oyaizu, K. Shimazu, I. Yagi, J. Phys. Chem. C, in press. DOI: 10.1021/acs.jpcc.5b11663

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 CuI 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) [1] N. Mano, V. Soukharev, A. Heller, J. Phys. Chem. B, 110, 11180 (2006) [2] M.S. Thorum, C.A. Anderson, J.J. Hatch, A.S. Campbell, N.M. Marshall, S.C. Zimmerman, Y. Liu, A.A. Gewirth, J. Phys. Chem. Lett, 1, 2251 (2010) [3] C.H. Kjargaard, J. Rossmeisi, J.K. Norskov, Inorg. Chem., 49, 3567 (2010) [4] M.S. Thorum, J. Yadav, A.A. Gewirth, Ang. Chem. Int. Ed., 48, 165 (2008) [5] F.R. Brushett, M.S. Thorum, N.S. Lioutas, M.S. Naughton, C. Tornow, H-R. M. Jhong, A.A. Gewirth, P.A. Kenis, J. Am. Chem. Soc., 132, 12185 (2010) [6] M.S. Thorum, J.M. Hankett, A.A. Gewirth, J. Phys. Chem. Lett., 2, 295 (2011) [7] C.C.L. McCrory, A. Devadoss, X. Otterwaelder, R.D. Lowe, T.D.P. Stack, C.E.D. Chidsey, J. Am. Chem. Soc., 133, 3696 (2011)