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

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