Liquid Chromatography/Mass Spectrometry Analysis of Effluent Water from PFSA Membrane Fuel Cells Operated at OCV

Abstract

Perfluoroalkylsulfonic acid (PFSA) ionomer membranes degrade under accelerated testing conditions such as open circuit voltage (OCV). Fluoride release rate is commonly used for evaluating the membrane degradation rate; however, many proposed degradation mechanisms should result in the release of small molecule polymer fragments. Liquid chromatography/mass spectrometry (LC/MS) methods are well suited to analyze for these fragments and provide insight into the degradation reactions. Accelerated OCV durability tests were conducted on membrane electrode assemblies made with 3M Ionomer ™ or Nafion™ XL membranes. Effluent water was analyzed for fluoride, sulfate, and trifluoroacetic acid by ion chromatography (IC) and other polymer fragments by LC/MS. The detection of partially hydrogenated side chain fragments and long chain dicarboxylic acids suggest hydrogen atoms play a significant role in these reactions. The results of this study show the possibility that more than one reaction may occur at the tertiary fluoride on the polymer backbone. The presence of a tertiary fluoride on the backbone and side chain of the Nafion™ XL membranes allows for these reactions in more than one location on this polymer. Performance loss for the Nafion™ XL samples during these tests is consistent with adsorption of ionomer fragments on the catalyst surface.