Impact of Local Water Uptake on Proton Conduction in Covalent Organic Framework Revealed by Machine-Learning Potentials

We present a factor dominating the proton conduction in covalent organic frameworks (COFs), which are attracting attention as a proton-conducting material for fuel cells. For elucidations, we conduct molecular dynamics simulations using machine-learned force fields with varying the water content, the number of sulfonic acid groups, and their side chain lengths. Under all conditions, the simulated proton diffusivity in the through-pore direction is roughly twice as high as the ones in other directions. The diffusivity in this direction spanned over an order of magnitude depending on the three variables; the proton diffusivity increases with higher water content, lower sulfonic acid concentration and shorter side chain length. The detailed analysis on distributions of water and sulfonic acid groups in the pore reveals that the diffusivity is controlled by the local water uptake around sulfonic acid groups. The greater the local water uptake, the more continuous the connectivity of water within the pores, and the lesser the constraining imposed by the sulfonic acid groups, leading to increased proton diffusivity.