Effect of water content and side chain on hydroxide transport in quaternary ammonium functionalized covalent organic frameworks as anion exchange membranes

Abstract

Covalent organic framework (COF) is an excellent candidate of ion transport material for its outstanding properties, yet the correlation between the structures and ion transport in COF is still to be clarified. Here we systematically investigate the effect of water content and side chain on the diffusion of hydroxide in several COFs functionalized with quaternary ammonium (QA) groups via molecular dynamics simulations. The water uptakes of the systems are determined by computing the adsorption isotherm curves at different water fugacity. Different from the previous assumption, we find that the length of the side chain does not change the radius of the channel, which consequently has little impact on the diffusion of hydroxide either. Instead, hydroxide diffusion is mainly determined by the strength of association between the cations and anions. Moving the QA groups from the side chain to the aromatic ring on the framework of the COF dramatically reduces hydroxide diffusion, which is attributed to the significantly enhanced association. Removing a fraction of tethers reduces hydroxide diffusion probably due to the broken of regularity of the hopping sites for the anions. Those results shed light on the interplay between several key parameters and the anion diffusion in the COF-based anion exchange membranes, which provide guidance for the future design.