Lawrence Berkeley National Laboratory

Polymer-electrolyte membranes (PEMs) are a key component in electrochemical energy conversion devices where their main function is to selectively transport ionic species. Reducing PEM thickness is an effective strategy for improving performance by minimizing transport losses. However, how thickness affects the intrinsic properties of a membrane remains unexplored. This work aims to understand the effect of membrane thickness on structure-property relationships of 3 M perfluorosulfonic acid (PFSA) ionomer. We carried out a systematic investigation of membranes in a thickness range of 5-70 μm to examine their hydration behavior, morphology, crystallinity, mechanical properties, and gas and proton transport, with a discussion on the effect of thermal treatments. The collected dataset demonstrates PFSA membranes exhibit transitions in certain structural features below 10 μm, accompanied by an increased anisotropy in swelling and conductivity. Many properties deviate within 10%-20% without monotonic changes with thickness, however, linear correlations are observed between thickness and thermal-mechanical properties and gas permeability, although the latter is less significant. Identifying the thickness-dependence of PFSA properties could help expand the parameter window of PEMs, thereby enabling their optimization for automotive fuel cells, heavy-duty applications, and electrolyzers, especially if the membrane thickness is considered as part of dispersion-casting and reinforcement strategies.