Dimensionally Stable Cross-Linked Hydroxyl-Functionalized Sulfonated Poly(ether ether ketone) with Low H2 Crossover and High Proton Conductivity for High-Performance H2 Sensing.

Reliable H2 leak detection is essential because H2 is highly diffusive and flammable. Although fuel-cell H2 sensors are attractive for room-temperature operation without external bias, their performance is limited by conventional proton exchange membranes (PEMs) such as Nafion, in which high H2 crossover and excessive swelling compromise long-term accuracy and stability. Herein, we report a hydroxylated cross-linked sulfonated poly(ether ether ketone) membrane (CSPEEK-OH) to address this dilemma. Covalent cross-linking endows the CSPEEK-OH membrane with a stable and dense network, markedly reducing H2 permeability to an ultralow value of 2.05 Barrer, 72.6% lower than that of Nafion 212. Solid-state 13C NMR, LF-NMR, and SAXS analyses reveal a constrained hydration architecture, in which residual hydroxyl groups stabilize bound/interfacial water within confined ionic domains, thereby suppressing swelling and gas crossover. This optimized water distribution preserves efficient proton transport while simultaneously enhancing dimensional stability and gas-barrier properties. As a result, the optimized CSPEEK-OH sensor delivers a 6-fold higher sensitivity than commercial Nafion (9.0 nA·ppm-1), a low detection limit of 8.4 ppm, a fast response time of 50 s, and negligible signal loss under drastic humidity fluctuations. This high-performance, PFAS-free PEM provides a molecular design paradigm for overcoming the gas crossover/swelling trade-off in electrochemical sensing.