Solid-State Sulfonation of Dense Poly(2,6-Dimethyl-1,4-Phenylene Oxide) Films: Membrane Properties and Electrochemical Assessment at MEA Level

Fluorine-free hydrocarbon proton-exchange membranes offer an alternative to perfluorosulfonic acid (PFSA) electrolytes, but good membrane-level properties do not always translate into membrane-electrode assembly (MEA) performance. Here, dense poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) films were sulfonated in the solid state, yielding a sulfur distribution that appeared homogeneous through the membrane thickness and ion-exchange capacities (IEC) of 1.17–2.20 meq g−1, corresponding to sulfonation degrees of 16–32 mol% and water uptake from 55 to 92%. The sulfonated PPO (sPPO) membrane with the highest IEC that remained self-standing was tested in a single-cell MEA and compared with a Nafion® 212-based MEA prepared and operated in the same way. The sPPO-based MEA showed lower hydrogen crossover and higher apparent short-circuit resistance than the Nafion® 212 reference. However, its high-frequency resistance (HFR) was also higher, corresponding to an apparent MEA-level conductivity of 11 mS cm−1 at 80 °C and 70% RH, compared with 40 mS cm−1 for Nafion® 212. This gap persists even though sPPO has a higher hydration number. Since the catalyst layers contain a PFSA ionomer, this HFR-based conductivity includes membrane, electrode, and interfacial contributions. The sPPO membrane reached a peak power density of 0.45 W cm−2 at 80 °C and 90% RH. In a short-term potentiostatic test, the sPPO-based MEA failed after approximately 6 h. Together with swelling, tensile, and Fenton data, these results identify dimensional control, oxidative stabilization, and better membrane–ionomer compatibility as the next steps and provide a quantitative MEA-level baseline for solid-state sulfonated PPO membranes.