Nanocomposites from Polymer Brushes and Metal Oxide Clusters for Fabrication of High‐Temperature Fuel Cell Proton Exchange Membranes

High-temperature proton exchange membranes (HT-PEMs) are highly desired for fuel cells with high energy density; however, the requirements in balanced anhydrous proton conductivity, mechanical/structural stability, processability and gas-barrier property impose great difficulty for molecular design. Herein, the supramolecular complexation of brush polymers and super acidic metal oxide cluster (H₃PW₁₂O₄₀, abbreviation PW₁₂) affords HT-PEMs with comprehensive performance that contributes to the robust performance of high energy density fuel cells. The polymers brush topology enables the decoupling of mechanical properties and proton conduction: the polyethylene glycol (PEG) side chains possess high affinity to PW₁₂ for proton transport while the rigid backbones help maintain structural stability and mechanical strengths up to 250 °C. The PW₁₂ clusters can be homogeneously dispersed in PEG with high loadings (≈80 wt.%) and it facilitates proton hopping among the crowded PW₁₂ for promising anhydrous proton conduction, e.g., 2 × 10^-3 S cm^-1 at 200 °C. Their dense supramolecular bonds contribute to enhanced mechanical strength, flexibility and gas barrier property with mitigating hydrogen permeation current as 0.73 mA cm^-2, enabling the feasible processability of PEMs and stable operation of fuel cells. The devices show high power density as 218 mW cm^-2 at 180 °C with long-term stable operation.