Understanding how the presence of cations causes polymer electrolyte membranes to degrade is important to advancing PEM research. Fraunhofer ISE has been focused on analyzing various types of cations for their impact on perfluorosulfonic acid (PFSA) membranes. This analysis is significant in understanding the catalytic effect individual cations have on forming radicals that attack PFSA polymers. How chemical stable these polymers are was investigated using Fenton’s reaction.

This ex-situ method for accelerated chemical degradation employs an ion-selective electrode (ISE) to measure fluoride ion (F^–) release and determine the fluoride emission ratio (FER). It can be used together with other methods such as NMR, ATR-IR and Raman spectroscopy to analyze PFSA polymer degradation.

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One of the main challenges to ensuring that PEM fuel cells remain stable is how to retain membrane durability. Due to their excellent stability, perfluorosulfonic acid (PFSA) membranes are now widely used in PEMFCs [1]. And yet, some conditions may cause membranes to become thinner or pinholes to form [1, 2], which increases gas crossover and can lead to lower performance or create safety issues [3].

A membrane can degrade chemically when hydroxyl (•OH) or hydroperoxyl (•OOH) radical species attack its polymer material [2]. These radicals form during oxygen reduction reactions or the decomposition of hydrogen peroxide (H₂O₂), which is created under certain conditions and in secondary reactions inside a PEMFC [2, 3]. For example, in open circuit (where electrodes are supplied with reactant gases but no current is drawn) and dry conditions, H₂O₂ forms from gases crossing the membrane, especially at the anode, because of existing potential [3, 4]. The process by which oxygen or hydrogen permeates the very thin membrane is called gas crossover. Although it is undesirable, it is an inevitable part of operating PEMFCs [3]. Since it creates H₂O₂ and, ultimately, radicals that attack the membrane and increase the crossover of gases, the negative effects become more pronounced over time.

When catalysts are absent, the rate of H₂O₂ decomposition is comparatively low. By contrast, transition metal ions can accelerate decomposition and create •OH and •OOH radicals that will degrade membranes relatively quickly. Iron cations (Fe²⁺) are known to be a particularly effective catalyst when breaking down H₂O₂ [5, 6]. The catalytic effect of any cation other than iron is rarely discussed in the literature, despite the fact that the water in PEMFC stacks contains various kinds of cations. They originate from, for example, assembly materials or salts contained in air humidifiers and enter the system during operation [7, 8]. Although most of them are present in low concentrations, they can still accelerate polymer degradation within the membrane.

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read more in H2-international August 2020

Authors:
Sebastian Prass
Dr. Anneke Georg
Dr. Nada Zamel, all for Fraunhofer ISE, Freiburg, Germany