A fresh perspective on accelerated degradation studies for proton exchange membranes

Abstract

Driven by the increasing demand for green hydrogen, proton exchange membrane (PEM) water electrolyzers are positioned as a key technology in the sustainable energy market. A primary concern for electrolysis cells remains the aspect of PEM durability. For durability testing, researchers often resort to accelerated degradation procedures utilizing ex situ Fenton reactions, exposing the PEM to high concentrations of Fe²⁺ and H₂O₂ to emulate prolonged exposure that mimics electrolysis conditions. However, these investigations often yield discrepancies compared to degradation occurring under actual long-term operation. The presented work elucidates and contextualizes ex situ Fenton testing by studying the transformations and reactions occurring across different length scales in state-of-the-art long- and short-side-chain PEM materials, revealing that the different weighting of chemical and morphological reaction pathways yields distinct degradation results for the two studied ionomers. PEM degradation mechanisms were classified into two pathways: chemical reactions, consisting of uniform polymer chain unzipping without significant preferential scissoring of polar moieties; and morphological transformations from the formation of voids, driven by pressure build-up inside the gas-impermeable membranes due to the breakdown of peroxide into oxygen. Both pathways operate in conjunction to affect crystallinity and ion conductivity, revealing that accelerated Fenton procedures require modification for transferable durability testing.