Morphology and proton transport in sulfonated poly(ether ether ketone) membranes: a molecular dynamics and DFT study

The performance of sulfonated poly(ether ether ketone) (SPEEK), a promising proton exchange membrane material, is constrained by the synergistic mechanism between water content (λ) and degree of sulfonation (DS). The effects of water content (λ = 0, 5, 10, 15, 20, and 25) and degree of sulfonation (DS = 50%, 60%, 70%, and 80%) are investigated via molecular dynamics simulations (MD) and density functional theory (DFT). The results demonstrate that the membrane density peaks with optimal swelling at λ = 10, where the hydrophilic-hydrophobic interface completes its phase transition and stabilizes. Through hydrogen bond counting analysis, λ = 10 is also identified as the optimal hydration level, while increasing DS generally enhances proton transport until excessive DS disrupts the hydrogen bond network and consequently impairs proton conduction. Transport kinetic analysis reveals a maximized hydronium ion diffusion coefficient at DS = 60-70%, consistent with experimental results. Transport kinetic analysis identifies λ = 10 and DS = 60-70% as the optimal parameter range for SPEEK membranes, and radial distribution function analysis confirms the formation of well-developed proton transport channels in this regime. This study deciphers the intrinsic correlation between hydration and sulfonation in SPEEK membranes, providing quantitative guidelines for optimizing proton exchange membrane architectures.