Atomistic Insight into Molecular Structure and Hydronium Ion Transport in Proton Exchange Membranes of Hydrogen Fuel Cells: Effect of Solvent Hydrophobicity

The limited efficiency of proton exchange membranes under low-humidity conditions remains a major challenge for next-generation fuel cells. Recent studies have explored deep eutectic solvents (DESs) as alternative proton-conducting media to overcome hydration-dependent limitations. In this work, we investigate the effect of solvent hydrophobicity on molecular structure and hydronium ion transport in polyacrylate-based membranes, using three distinct DESs: a hydrophilic system of choline chloride and ethylene glycol (1:2), an amphiphilic system of choline chloride and decanoic acid (1:1), and a hydrophobic system of menthol and lauric acid (1:1), employing density functional theory (DFT) calculations and classical all-atom molecular dynamics (MD) simulations. DFT calculations were used to optimize molecular structures and evaluate electrostatic potential distributions, frontier orbital energies, and hydrogen-bonding capabilities, revealing that the hydrophilic system exhibits the strongest interactions with hydronium ions via stabilized charge delocalization and orbital overlap. MD simulations further elucidated the structural organization of solvent-polymer systems under hydrated conditions, with analyses confirming preferential coordination of hydronium ions with specific functional groups depending on solvent polarity. Interaction energy and diffusion coefficient calculations demonstrated that both water and hydronium mobility are highest in the hydrophilic system, while transport is progressively restricted in amphiphilic and hydrophobic systems. These findings provide fundamental insights into how solvent polarity modulates proton transport, guiding the design of high-performance fuel cell membranes.