Challenges to Hydrocarbon Proton-Exchange Membrane Conductivity: Examining Hydration, Conduction Pathways, and Activation Energy

Hydrocarbon ionomers have emerged as promising per- and polyfluoroalkyl substance (PFAS)-free alternatives to perfluorosulfonic acid (PFSA) ionomers in proton-exchange membrane fuel cells. This study systematically examines hydrocarbon ionomer performance over a broad range of humidity and temperature conditions to identify the key challenges limiting commercial viability. The first challenge is the low water content per acid group at a given relative humidity, as hydrocarbon ionomers exhibit less swelling on an acid-normalized basis. The second challenge is the complexity of proton conduction pathways; analysis using percolation theory suggests that hydrocarbon ionomers exhibit significantly more complex conduction pathways than PFSA ionomers. The third challenge is proton transport activation energy, as determined by Vogel–Fulcher–Tammann analysis, which shows that hydrocarbon ionomers can achieve activation energies comparable to PFSAs, but only with specific ionomer architectures.