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

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 750EW, 950EW, and 1100EW. The 750EW membrane contained short sulfonic acid side-chains while the 950EW and 1100EW membranes were Nafion®-based with long sulfonic acid side-chains. Membranes were tested in fuel cells for 100h under open circuit voltage, at 90°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 950EW cell had the highest decay metrics including fluoride emission, voltage decay, loss in ECA, and loss in cell performance. In all cases, the 1100EW cell showed the lowest degradation. This has been explained in terms of the lower water content and number of side-chains in the 1100EW polymer and the absence of a tertiary carbon and lower concentration of ether linkages in the side-chains of the 750EW polymer. To reach optimal levels of durability and performance, it is necessary to use the appropriate EW and side-chain length combination.