Mechanistic Study of ORR by Cu(II) Based Electrocatalyst Using Simultaneous Electrochemical EPR Spectroscopy

The ORR mechanism as catalyzed by non precious metal catalysts to replace Pt based catalysts at the cathode of PEMFC has been controversial for decades, though this category of catalysts has recently achieved remarkable ORR performance.^1,2 More understanding of the ORR mechanism is essential to the further improvement of the performance of catalysts of this class. Cu(II) based non precious metal catalysts are advantageous for the study of the ORR mechanism due to their biomimetic significance in mimicking copper oxidases and, when not pyrolyzed to their well defined structure.³

    To gain deeper understanding of the ORR mechanism, an in situ electrochemical half cell for EPR has been designed and applied to probe the mechanism of the ORR catalyzed by a Cu(II)-3,5-diamino-1,2,4-triazole based (shown here) and other catalysts. Simultaneous observation of electrochemical behavior and the corresponding Cu(II) EPR signal evolution at different potentials was realized as shown in Figure 1. The intensity of the Cu(II) signal decreases as the potential steps negative because more Cu(II) is reduced to Cu(I) at more negative potentials. In Figure 2, the EPR spectroscopic behaviors of Cu(II) during a cyclic voltammogram are different in the presence and absence of O₂, with a shift in potential indicating a complex formation between O₂and Cu ions during ORR catalysis. A study of the effects of oxygen concentration will be conducted to estimate the amount of active catalytic sites in the catalyst. A quantitative relationship will be investigated to determine the reaction order of Cu species in the ORR catalysis. Finally, this method will be applied to a number of related non-precious metal catalysts.

Acknowledgements:

The authors gratefully acknowledge the support of the US DOE Office of Fuel Cell Technology and the office of Naval Research for financial support of this work.

 

 

References:

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[2]. Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. Science 2011, 332, 443–7

[3]. Thorum, M. S.; Yadav, J.; Gewirth, A. A. Angew. Chem. Int. Ed. Engl. 2009, 48, 165–7