Effect of perfluorosulfonic acid membrane equivalent weight on degradation under accelerated stress conditions

    Authors

    M. P. Rodgers; B. P. Pearman; N. Mohajeri; L. J. Bonville;D. K. Slattery

    Comments

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    Abbreviated Journal Title

    Electrochim. Acta

    Keywords

    Equivalent weight; Sulfonated side-chain length; Durability; Proton; exchange membrane fuel cell; Nafion; PFSA; PEM FUEL-CELLS; IONOMER MEMBRANES; PERFLUORINATED MEMBRANES; CHEMICAL; DEGRADATION; PROTON; DURABILITY; ION; Electrochemistry

    Abstract

    The equivalent weight of proton exchange membranes has a large effect on their properties and can impact performance and durability in hydrogen fuel cells. For example, increasing the EW increases the crystallinity of perfluorosulfonic acid membranes, while water content and glass transition temperature decrease. The length of the sulfonic acid side chain also impacts membrane properties. Perfluorosulfonic acid membranes with shorter sulfonic acid side chains, though they exhibit similar gas permeability, have been shown to have higher crystallinity, higher glass transition temperature, slightly lower water content, and lower proton conductivity than membranes with longer sulfonic acid side chains for a given EW. Although many reports have investigated cell performance for membranes as a function of low EW and side chains length, their impact on cell durability is not well understood. Because side chain attack by radicals formed during fuel cell operation is a major source of membrane degradation, it is reasonable to hypothesize that membranes with lower EW and, therefore, more sulfonic acid side chains, would have lower durability. This study evaluates membrane degradation for cells containing PFSA membranes with 750 EW, 950 EW, and 1100 EW. The 750 EW membrane contained short sulfonic acid side-chains while the 950 EW and 1100 EW membranes were Nafion (R)-based with long sulfonic acid side-chains. Membranes were tested in fuel cells for 100 h under open circuit voltage, at 90 degrees C and 30% relative humidity. Diagnostic tests conducted on the cells included hydrogen crossover, fluoride emission, catalyst electrochemical surface area, posttest membrane scanning electron microscopy/transmission electron microscopy evaluation, and defect identification in membranes. The 950 EW cell had the highest decay metrics including fluoride emission, voltage decay, loss in ECA, and loss in cell performance. In all cases, the 1100 EW cell showed the lowest degradation. This has been explained in terms of the lower water content and number of side-chains in the 1100 EW polymer and the absence of a tertiary carbon and lower concentration of ether linkages in the side-chains of the 750 EW polymer. To reach optimal levels of durability and performance, it is necessary to use the appropriate EW and side-chain length combination. (C) 2013 Elsevier Ltd. All rights reserved.

    Journal Title

    Electrochimica Acta

    Volume

    100

    Publication Date

    1-1-2013

    Document Type

    Article

    Language

    English

    First Page

    180

    Last Page

    187

    WOS Identifier

    WOS:000320492400024

    ISSN

    0013-4686

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