Effects of Cobalt Cation on Low Pt-loaded PEM Fuel Cell Performance

PtCo and PtNi alloys are attractive as cathode catalysts in PEM fuel cell systems due to their high catalytic activities. However, dissolution of cobalt or nickel in a fuel cell causes not only oxygen reduction kinetic losses, but also ohmic losses, especially at high current densities, due to the migration of the cations into the ionomer. Several groups [1-3] have developed models to understand these effects on PEM fuel cell performance, mainly focusing on their macroscopic transport properties. To meet stringent cost requirements, the Pt loading in the electrode must be reduced significantly from the levels used in these previous studies. We have found that low Pt loaded cathodes showed larger high-current-density performance losses than higher loaded cathodes for a given cation doping concentration. This result is not readily explained using the current models. In this study, we performed controlled experiments where known amounts of Co or Ni cations were exchanged into the membranes in order to investigate the effect of cation concentration on the performance of cathodes with different Pt loadings, as shown in Figure 1.  We used impedance spectroscopy and limiting current method to study the effect of cation contamination on membrane and electrode proton resistances, local oxygen transport resistance, properties not been quantified in previous studies. These newly acquired results will be used to improve our current model and provide direction for future fuel cell development.

This work was partially supported by the U.S. DOE-EERE under grant DE-EE0000458.

References

[1] B. Kienitz, H. Baskaran, T. Zawodzinski and B. Pivovar, ECS Trans., 11, 777 (2007)

[2] T. A. Greszler, T. E. Moylan and H. A. Gasteiger, Chapter 49, Handbook of Fuel Cells – Fundamentals, Technology and Applications. Edited by Wolf Vielstich, Harumi Yokokawa, Hubert A. Gasteriger. Volume 6: Advances in Electrocatalysis, Materials, diagnostics and Durability. 2009 John Wiley & Sons, Ltd.

[3] T. Okada, H. Satou, M. Okuno and M. Yuasa, J. Phys. Chem. B, 106 1267 (2002)