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

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of the American Chemical Society, © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://pubs.acs.org/doi/abs/10.1021/jacs.7b05182We 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 m-cresol or acetic acid, named RT-COF-1 or RT-COF-1Ac/RT-COF-1AcB. The post-synthetic treatment of RT-COF-1 with LiCl leads to the formation of LiCl@RT-COF-1. The ionic conductivity of this series of polyimine COFs has been characterized at variable temperature and humidity, using electrochemical impedance spectroscopy. LiCl@RT-COF-1 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, respectivelyWe thank the MINECO (Spain) for financial support through Juan de la Cierva postdoctoral fellowship. We thank the Spanish Ministry of Economy (MAT2016-77608-C3-1-P, MAT2013-46753-C2-1-P, CTQ2014-53486-R, ENE2016-77055-C3-1-R), and Comunidad de Madrid (S2013/MAE2882) for generous fundin