(Invited) Deprotonation of a Multi-Nuclear Copper Complex for High Oxygen Reduction Reaction Activity Investigated By in Situ X-Ray Absorption Fine Structure Spectroscopy

The oxygen reduction reaction (ORR) occurs at the cathode in polymer electrolyte fuel cells (PEFCs) and its sluggish kinetics is one of the limiting factors for widespread applications of PEFCs. Although ORR catalysts have been extensively studied, platinum-based ORR catalysts with a high overpotential (>200 mV) are used even in the present state-of-the-art PEFCs.

In nature enzymatic oxygen reduction is known to occur at low overpotentials: multi-copper oxidases such as laccase catalyze ORR with an overpotential of ca. 20 mV on electrode surfaces and have been employed in enzymatic fuel cells [1-2]. These enzymes, however, are fragile and their footnotes are large, making them unsuitable for use in practical PEFCs. It is highly desirable to design and develop artificial ORR catalysts with electrocatalytic properties similar to those of the enzymes [3].

Here, we present our resent studies on in situ X-ray absorption fine structure (XAFS) spectroscopy of a dinuclear copper complex of 3,5-diamiono-1,2,4-triazole, Cu-Hdatrz [4,5], supported on a carbon black under catalytic and non-catalytic conditions to gain insights into designing highly active copper-based catalysts. It is known that the Cu-Hdatrz catalyst shows high ORR activity in basic solution [5]. Our in situ XAFS measurements in neutral and basic solutions revealed that the electron transfer from a Cu^I species to O₂ was faster under basic conditions than that under the neutral condition and deprotonation from the triazole ligand might cause the coordination geometrical changes under a basic condition, suggesting that the deprotonated structure of the Cu^Iactive species may be a key to understand the high ORR activity of Cu-Hdatrz or to design multinuclear copper-triazole catalysts with low overpotentials. 

Acknowledgements

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)

 References

[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] E. Aznar, S. Ferrer, J. Borras, F. Lloret, M. Liu-Gonzalez, H. Rodriguez-Prieto, S. Garcia-Granda, Eur. J. Inorg. Chem., 2006, 5115 (2006)

[5] M.S. Thorum, J. Yadav, A.A. Gewirth, Ang. Chem. Int. Ed., 48, 165 (2008)