Composite Proton-Exchange Membrane with Highly Improved Proton Conductivity Prepared by in Situ Crystallization of Porous Organic Cage

Porous organic cage, a kind of newly emerging soluble crystalline porous material, is introduced to proton-exchange membrane by in situ crystallization. The crystallized Cage 3 with intrinsic water-meditated three-dimensional interconnected proton pathways working together with Nafion matrix generates a composite membrane with highly improved proton conductivity. Different from inorganic crystalline porous materials, like metal–organic frameworks, the organic porous material shows better compatibility with Nafion matrix due to the absence of inorganic elements. In addition, Cage 3 can absorb water up to 20.1 wt %, which effectively facilitates proton conduction under both high- and low-humidity conditions. Meanwhile, the selectivity of Nafion–Cage 3 composite membrane is also elevated upon the loading of Cage 3. The proton conductivity is evidently enhanced without obvious increased methanol permeability. At 90 °C and 95% RH, the proton conductivity of NC3-5 reaches 0.27 S·cm–1, highly improved compared to 0.08 S·cm–1 of recast Nafion under the same condition. This study offers a new strategy for modifying proton-exchange membrane with crystalline porous materials.