Overcoming the Conductance versus Crossover Trade-off in State-of-the-Art Proton Exchange Fuel-Cell Membranes by...

Permeance-selectivity trade-offs are inherent to polymeric membranes. In fuel cells, thinner proton exchange membranes (PEMs) could enable higher proton conductance and increased power density with lower area-specific resistance (ASR), smaller ohmic losses, and lower ionomer cost. However, reducing thickness is accompanied by an increase in undesired species crossover harming performance and long-term efficiency. Here, we show that incorporating atomically thin monolayer graphene synthesized via scalable chemical vapor deposition (CVD) and tunable defect density into PEMs (Nafion, ∼5-25 μm thick) can allow for reduced H₂ crossover (∼34-78% of Nafion of a similar thickness) while maintaining adequate areal proton conductance for applications (>4 S cm^-2). In contrast to most prior work using >50 μm symmetric Nafion sandwich structures, we elucidate the interplay of graphene defect density and Nafion proton transport resistance on the performance of Nafion|graphene composite membranes and find high-quality low-defect density CVD graphene (G) supported on Nafion 211 (∼25 μm); i.e., N211|G has a high areal proton conductance (∼6.1 S cm^-2) and the lowest H₂ crossover (∼0.7 mA cm^-2). Fully functional centimeter-scale N211|G fuel-cell membranes demonstrate performance comparable to that of state-of-the-art Nafion N211 at room temperature as well as standard operating conditions (∼80 °C, ∼150-250 kPa-abs) with H₂/air (power density ∼0.57-0.63 W cm^-2) and H₂/O₂ feed (power density ∼1.4-1.62 W cm^-2) and markedly reduced H₂ crossover (∼53-57%).