Remarkably enhancing proton conductivity by intrinsic surface sulfonation of a pyrazine-linked covalent organic framework

Enhancing the proton conductivity and long-life reusability of the electrolytes by surface modification is a feasible way to fabricate effective proton-conductive electrolytes for the practical application of proton exchange membrane. Herein, on account of its framework robustness and the proton-accepting nitrogen atoms uniformly located in its skeleton, a pyrazine-based covalent organic framework (COF) was selected as a masterplate, and its tert-butyl-absent COF structure, PyHATP-1, was constructed. Experimental results reveal that after its intrinsic pore surface is treated by post-sulfonation, the proton conductivity of the sulfonated sample, PyHATP-1-SO3H, exponentially increases to 1.3 × 10−3 from 7.2 × 10−6 S cm−1 of PyHATP-1 at 353 K and 98% relative humidity. Moreover, while loading H3PO4 molecules into the skeletons, the sulfonated H3PO4@PyHATP-1-SO3H not only displays a remarkable increase in the proton conductivity (0.88 × 10−1 S cm−1) compared with the unsulfonated H3PO4@PyHATP-1 (2.0 × 10−3 S cm−1), but also exhibits a longer reusability. Contact angle tests and density functional theory calculations reveal that its remarkable enhancement in the proton conductivity and long-life reusability are attributed to the post-sulfonation of the pore surface, which significantly improves the affinity towards proton carriers (H2O and H3PO4 molecules). This work demonstrates that by modifying the intrinsic surface of porous materials, effective proton-conductive electrolytes with high proton conductivity and long-life reusability can be achieved.