Contaminant Cation Effect on Oxygen Transport through the Ionomers of Polymer Electrolyte Membrane Fuel Cells

Abstract

We characterized the effects of cobalt (Co2+) and other cation contaminants on the oxygen (O2) transport properties of the PFSA ionomer used in polymer electrolyte membrane fuel cells (PEMFCs) and gained insight into the mechanisms by which contaminant cations inhibit O2 transport. Such cations can be released by alloy catalysts and environmental conditions and pose a significant challenge to maintaining high current density performance with low platinum (Pt) loadings. We used a test cell capable of isolating the ionomer from the membrane electrode assembly (MEA), allowing for O2 transport resistance (RO2) measurements using a limiting current technique. We contaminated ionomer membranes with Li+, Na+, Ni2+, Co2+, and Ce3+ and found a general increase in RO2 for increased contamination levels and decreased water activity. In addition, our Co2+ results indicated distinct concentration-dependent regimes. The other cation-form ionomers allowed us to separate the impacts of ion pair strength, multivalency, and reduced water uptake. We believe that these factors cause a more compressed hydrophilic domain and tortuous O2 diffusion path, and a commensurate increase in RO2. Finally, we studied the impact of Co2+ on an operating PEMFC and found an increase in RO2consistent with the results of our isolated membrane tests.