Ionic Conductivity and Potential Application for Fuel Cell of a Modified Imine-Based Covalent Organic Framework

We present the novel potential application of imine-based covalent organic frameworks (COFs), formed by the direct Schiff reaction between 1,3,5-tris­(4-aminophenyl)­benzene and 1,3,5-benzenetricarbaldehyde building blocks in <i>m</i>-cresol or acetic acid, named <b>RT-COF-1</b> or <b>RT-COF-1Ac</b>/<b>RT-COF-1AcB</b>. The post-synthetic treatment of <b>RT-COF-1</b> with LiCl leads to the formation of <b>LiCl@RT-COF-1</b>. The ionic conductivity of this series of polyimine COFs has been characterized at variable temperature and humidity, using electrochemical impedance spectroscopy. <b>LiCl@RT-COF-1</b> exhibits a conductivity value of 6.45 × 10^–3 S cm^–1 (at 313 K and 100% relative humidity) which is among the highest values so far reported in proton conduction for COFs. The mechanism of conduction has been determined using ¹H and ⁷Li solid-state nuclear magnetic resonance spectroscopy. Interestingly, these materials, in the presence of controlled amounts of acetic acid and under pressure, show a remarkable processability that gives rise to quasi-transparent and flexible films showing in-plane structural order as confirmed by X-ray crystallography. Finally, we prove that these films are useful for the construction of proton exchange membrane fuel cells (PEMFC) reaching values up to 12.95 mW cm^–2 and 53.1 mA cm^–2 for maximum power and current density at 323 K, respectively