Effect of Local Water Uptake on Proton Transport 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 intermediate-temperature fuel cells [1]. COFs are expected to demonstrate excellent proton conductivity due to the formation of proton transport path with low tortuosity [2]. However, reported conductivities vary significantly, and it is difficult to experimentally evaluate the factor that dominates proton conductivity. For elucidations, we conduct molecular dynamics simulations using machine-learned force fields (MLFFs) [3] with varying the water content, the number of sulfonic acid groups, and their side chain lengths. MLFFs that precisely reproduce first principles interatomic potentials are generated by an efficient active learning scheme implemented in Vienna Ab initio Simulation Package on the fly during first-principles calculations. The proton diffusivity 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.

[1] Minami, S., et al., Chem. Matter., 2024, DOI: 10.1021/acs.chemmater.4c01351; [2] Shi, B., et al., Nat Commun, 2022, 13(1), 6666; [3] Minami, S., et al., J. Mat. Chem. A, 2023, 16104