High-density sulfonic acid-grafted covalent organic frameworks with efficient anhydrous proton conduction

Abstract

Covalent organic frameworks (COFs) have attracted much attention in the field of high-temperature anhydrous proton conduction due to their high specific surface area and ordered one-dimensional nanochannels. Herein, a high density sulfonic acid (–SO₃H) functionalized proton conducting COF (TB-COF) was designed and synthesized via anchoring of sulfonic acid groups on the pore walls as a proton source. To the best of our knowledge, the synthesized TB-COF presents the highest intrinsic proton conductivity (1.52 × 10⁻⁴ S cm⁻¹ at 120 °C) reported, due to the presence of high density –SO₃H groups on the ordered 1D nanochannels. Further incorporation of a protic ionic liquid (PIL), 1-methyl-3-(3-sulfopropyl) imidazolium hydrogensulphate ([PSMIm][HSO₄]), into the TB-COF greatly improved its proton conductivity to 2.21 × 10⁻³ S cm⁻¹ at 120 °C. Moreover, density functional theory (DFT) calculations proved the directional migration of protons along the stacked nanochannels of the COFs with the assistance of O atoms on the keto groups. This work provides innovative structural design ideas and performance improvement methods for COFs as proton conducting materials.