Multiple Confinement Effects in Covalent Organic Framework Membranes for Wide‐Temperature and Wide‐Humidity Proton Transport

ABSTRACT

Phosphoric acid‐doped proton exchange membranes that combine high proton conductivity with strong acid retention are critical for fuel cells. Although elevated temperatures and humidity levels typically enhance conductivity, they also exacerbate undesirable acid leaching. Here we report the design and fabrication of two types of self‐standing covalent organic framework (COF) membranes featuring tailored distributions of quaternary ammonium, which serve as scaffolds for phosphoric acid doping. By integrating either flexible or conformationally constrained alkyl side chains within the porous frameworks, the membranes achieve a synergistic effect between the confined nanochannel structure and robust ion‐pair interactions, enabling efficient proton transport while minimizing acid loss. Experimental analyses reveal that the immobilized quaternary ammonium side chains within the COF nanopores effectively restrict the migration of both acid and water molecules, even under highly humidified conditions. The optimized COF membrane (COF‐CQA/PA) retains 92.4% of the acid content after operation at 100°C and 90% RH, while maintaining structural and chemical integrity during prolonged use. Moreover, it delivers peak power densities ranging from 0.22 to 0.82 W cm ⁻² across a wide operating window (40°C–120°C, 20%–80% RH). This work establishes a design strategy for developing acid‐doped membranes that simultaneously achieve high performance and robust humidity tolerance.