Localized ionomer degradation analysis of sulfonated poly(phenylene sulfones) in fuel cell applications using confocal Raman microscopy

Abstract

The development of highly conductive and stable fluorine-free polymer materials is critical for transitioning from perfluorosulfonic acid-based membranes to non-fluorinated alternatives for proton exchange membrane fuel cells and water electrolyzers. Among these, sulfonated poly(phenylene sulfones) (sPPS) are a promising class of polymers. However, little is known about their stability when used for these applications. To gain deeper insight into the aging mechanisms of sPPS membranes, confocal Raman microscopy was employed as a non-destructive, contact-free technique to determine membrane thickness and local equivalent weight. In this study, two sandwiched sPPS membranes were aged in situ using an accelerated stress test under open-circuit voltage (OCV) conditions in a fuel cell setup. Confocal Raman microscopy revealed that after the OCV-hold test, the combined thickness of sPPS membranes decreased from 27 µm to 15 µm, confirming chemical degradation. 60% of this reduction occurs on the membrane facing the anode side of the cell, indicating localized acceleration of degradation processes near the anode. Notably, despite the observed degradation, the local EW remained unchanged at the end of the test. This hints at a mechanism where chain scission is prevalent over desulfonation. Complementary techniques, nuclear magnetic resonance spectroscopy and gel permeation chromatography, were used on aged polymers to further validate these findings. It was found that despite no clear chemical changes (e.g. the degree of sulfonation or EW), the molecular weight decreased by 50%.