A novel perspective on accelerated degradation studies of proton exchange membranes

Accelerated degradation studies are widely applied in research on proton exchange membranes (PEMs) for the investigation of the origins and mechanisms of performance loss for electrolysis or fuel cell applications. In a nutshell, it is reported in literature that degradation in PEMs commonly occurs following Fenton-like reactions, where in situ formed H2O2 reacts with transition metal cations to produce radicals. These radicals then alter the ionomer on a chemical level by attacking particularly its polar side chains, causing a loss of functional moieties for proton transport [1]. Fast degradation studies mimic and promote these conditions by subjecting PEMs to high concentrations of H2O2 and Fe2+ cations at elevated temperatures. However, these studies often exhibit discrepancies when compared to degradation occurring during long-term operation [2]. The presented work attempts to elucidate these discrepancies by i) addressing inconsistencies in accelerated degradation and testing procedures, ii) studying the dependence of degradation on PEM chemistry and structure and iii) utilizing both NMR spectroscopy and SEM microscopy among other techniques for a comprehensive picture. Hereby, solid-state magic angle spinning (MAS) NMR spectroscopy provides information on both chemical and local structural transformations of the PEM, while SEM offers concrete insights into structural changes on a microscopic scale. The Fenton-like accelerated degradation experiments were optimized for homogeneity and effectiveness by introducing the catalytic iron centers into the PEMs. Additionally, interferences in the analytical techniques were minimized by careful removal of excess reactants after accelerated degradation. The combined analytical techniques reveal that chemical degradation in PEMs is significantly less pronounced than suggested in literature, although differences were observed depending on the type of PEM material. Moreover, organic radicals that form during Fenton-like reactions could not be detected by EPR spectroscopy. However, all samples experienced significant changes in the local structure, as indicated by NMR relaxometry, and microscopic structure, as illustrated by SEM techniques. Thus, instead of chemical degradation, the PEM may be affected on a structural level by mechanical stress due to microscopic gas pockets and macroscopic bubbles forming inside the gas impermeable material. [1] L. Ghassemzadeh et al., J. Am. Chem. Soc. 135, 8181–8184 (2013). [2] J. Mališ et al., Int. J. Hydrogen Energy 41, 2177–2188 (2016).