Bio-Inspired Electrocatalysis of Oxygen Reduction

Polymer electrolyte fuel cells (PEFCs) are clean power sources, where a fuel is “burned” by means of two electrocatalytic reactions occurring simultaneously on the anode (fuel oxidation) and the cathode (oxygen reduction reaction, ORR) of the cell. Platinum is the most frequently used cathode catalyst, but its world resources are limited and the platinum market is volatile. These factors virtually prevent commercialization of PEFCs for transportation purposes.

                In biological environments, oxygen reduction is catalyzed by enzymes and some enzymatic oxygen reduction catalysts, e.g., copper laccase exhibit the catalytic activity superior to that of platinum.¹While practical application of copper laccase is not possible due to temperature, pH, and other limitations, its excellent catalytic activity demonstrates that it is possible to develop non‑precious metal based ORR electrocatalysts with higher activities than Pt.

It is well known that the biological function and activity of heme proteins, which contain Fe coordinated by a porphyrin ring, can be modified through axial coordination of the metal center. These phenomena have become a foundation of our approach to create an effective bio‑inspired oxygen reduction catalyst. As opposed to copper laccase, porphyrin based ORR catalysts exhibit quite reasonable stability in strongly acidic environments, such as those in PEFCs. In our talk we will present new results in our recent study on the effects of axial coordination of the iron center on oxygen reduction catalysis by selected iron porphyrins.²Special emphasis will be placed on the correlations between redox and electronic properties of the porphyrins determined using spectroelectrochemical techniques (Fig. 1) and their catalytic activity in ORR.

Acknowledgements

The financial support from the University of California is gratefully acknowledged.

References

1)       H. Schweiger, E. Vayner, and A. B. Anderson, Electrochemical and Solid-State Letters, 8(11) A585-A587, 2005

2)       J. Chlistunoff, J.-M. Sansiñena, Effects of Axial Ligation and Electron Donors on Oxygen Reduction Catalysis by Metalloporphyrins, 224^th ECS Meeting, San Francisco, CA, 10/27/2013-11/01/2013