Abstract
Membranes based on 3M's perfluoro imide acid (PFIA) ionomer have been studied using the open circuit voltage (OCV) hold accelerated stress test. For these samples, a decay in the OCV potential within the first 200 hours was observed along with an increase in cell resistance that rapidly grows near the end of life. A multi-membrane electrode assembly (MEA) technique was employed to study the origins of these phenomena. Effluent water collected at the beginning of life and later, during a recovery protocol, was analyzed using liquid chromatography – mass spectroscopy (LC-MS). A variety of small molecule fragments were detected that could be traced to the ionomer side chain. The center layer of a three-membrane MEA was separated at the end of life and analyzed by 19F nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy. Peaks associated with the bis(sulfonyl)imide group of the PFIA ionomer were observed to have reduced in intensity and new peaks, assigned to a perfluorosulfonamide side chain, appeared. The combination of the fragments detected in the effluent water, along with the spectral changes in the membranes after aging, point to one or more degradation processes involving the PFIA ionomer side chain. This decomposition is likely analogous to reactions described for perfluorosulfonic acid (PFSA) ionomers but with new consequences owing to the greater number of potential fragments.
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