Development of hydrophilicity on the proton exchange using sulfonic acid on PEEK in the presence of water: a density functional theory study

The introduction of a protogenic group such as sulfonic acid enables the operation of polymer electrolyte membrane for fuel cells at intermediate temperatures (> 100 °C) and very low humidity. It has been reported that the addition of a strongly acidic sulfonic acid group to hydrophobic polyether ether ketone (PEEK) creates the water permeability and proton transfer. In order to understand how sulfonic acid develops hydrophilicity, we conducted density functional theory calculations to determine the adsorption affinity of water for sulfonated PEEK (SPEEK), which represents the binding energy and band gap between HOMO (highest occupied molecular orbital) of SPEEK and LUMO (lowest unoccupied molecular orbital) of water molecules. Moreover, we designed disulfonated PEEKs (DSPEEK) with cis- and trans-conformations and found that cis-DSPEEK exhibits higher adsorption affinity for water with strong hydrogen bonds. This is attributed to the narrow energy gap of water molecules on cis-DSPEEK. Furthermore, we investigated proton adsorption in the presence of water to determine the effect of hydrophilic environment on the proton exchange in SPEEK. We found that cis-DSPEEK shows high repulsion for hydrogen transfer and moderate adsorption affinity for protons. Theoretical findings confirm that sulfonation ultimately yields hydrophilicity and developed proton transfer ability for PEEK, leading to a suitable structure for preferable proton exchange membrane.