The proton exchange membrane fuel cell (PEMFC) is one of the most promising renewable energy sources due to benign exhaust emissions and high energy efficiency. Pt/C is currently the primary catalyst for the slow oxygen reduction reaction (ORR) kinetics at the cathode of a PEMFC. The ORR, O₂ + 4H⁺ + 4e^- → 2H₂O, indicates that this process preferentially occurs at a triple phase boundary where reactant, electrolyte, and catalyst are in contact [1, 2]. The depletion of O₂, H⁺ or e^- would significantly affect reaction kinetics. The Nafion^® perfluorosulfonic acid (PFSA) ionomer, composed of a carbon-fluorine backbone and side chains ending with sulfonic acid groups, is conventionally used to prepare catalyst layers by exploiting its binding, proton-conducting and O₂ transport properties. Foreign cations replace ionomer protons and occupy sulfonic acid sites by ion exchange [3-6]. The replacement of H⁺ by foreign cations in the ionomer decreases not only the proton conductivity but also O₂ permeability. Uddin et al. [6] model results showed that the presence of Na⁺ in the ionomer of the cathode catalyst layer, which decreases the O₂ permeability, causes a decrease in dissolved O₂ concentration. Durst et al. [7] found that the diffusion coefficient and the concentration of O₂ in the H₂SO₄ electrolyte, and the kinematic viscosity of the electrolyte decrease in the presence of Co²⁺. Cationic impurities decrease the membrane water content in turn shrinking the ionomer and lowering the active catalyst surface area. Also, foreign cations promote the generation of H₂O₂, a side product of the ORR, which exerts an undetermined influence on the performance and durability of PEMFCs.

Literature and our group studies indicated that foreign cations have severe effects on the performance and durability of the PEMFC. Among all cations, Ca²⁺ deserves special attention because it is one of the most abundant elements within the earth’s crust. Prior efforts have focused on the effects of Ca²⁺ on PEMFC operation [8-10] and membrane transport properties [11]. However, the effects of Ca²⁺ on the ORR kinetics have hardly been studied. Okada et al. [12] studied the effects of Ca²⁺ on the O₂ reduction kinetics at a Nafion^® film covered Pt rotating disk electrode. Their results showed that impurity ions, even in small amounts, significantly hinder the rate of the O₂ reduction charge transfer step. However, the Nafion^® film covered Pt electrode is different from the supported catalyst layer for a PEMFC. The O₂ transport properties in a Ca²⁺ form ionomer are poorly understood. In addition, the H₂O₂ yield has not been measured. In this presentation, detailed analyses of the changes in the Pt/C electrochemical surface area, ORR diffusion-limited and kinetic currents, Tafel slope, H₂O₂ yield, O₂ permeability and ionomer sulfonate site occupation will be presented and discussed.

Acknowledgments

Authors are grateful to the United States Department of Energy (award DE-EE0000467), the Office of Naval Research (award N00014-12-1-0496), and the Hawaiian Electric Company.

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