Deciphering the Role of Sulfonated Side-Chain Length in Modulating Proton Transport in TpBd-COFs

Abstract

Covalent organic frameworks (COFs) functionalized with sulfonated side chains have emerged as promising materials for proton exchange membranes due to their ordered nanoporous structures and tunable ionic functionalities. Here, we investigate the effect of side-chain length on water distribution, hydrogen-bond networks, and proton transport in TpBd-COFs using a combination of ab initio molecular dynamics (AIMD) and reactive force field molecular dynamics (ReaxFF MD) simulations. Our results show that increasing side-chain length shifts sulfonic acid groups toward the channel center, enhances hydration shell overlap, and reorganizes water molecules to form continuous three-dimensional short hydrogen-bond networks. Interestingly, while water diffusivity decreases monotonically with longer side chains due to steric confinement, proton conductivity exhibits a non-monotonic trend, reaching a maximum for intermediate sidechain lengths. This decoupling indicates that proton transport is dominated by Grotthuss and surface mechanisms rather than by vehicle-mediated water diffusion. These insights highlight the critical role of side-chain engineering in optimizing hydrogen-bond connectivity and proton hopping pathways, providing a theoretical basis for designing high-performance proton-conducting COFs for fuel cell applications.