Regulating phosphoric acid distribution through cationic side-chain length and spatial position in ion-pair proton exchange membranes: Insights from molecular simulations

The phosphoric acid (PA) retention capacity and the hydrogen bond network are pivotal to the stability and proton conductivity of proton exchange membranes (PEMs). This study investigates ion-pair PEMs by examining how the position of the cationic center and the length of the cationic side chain influence PA distribution and proton transport. Molecular simulations reveal the regulatory mechanisms governing local PA distribution at the molecular level. The results indicate that tailoring the position of the cationic center modifies the local electrostatic potential, which promotes the enrichment of H₂PO₄^-/PA species around cationic sites and reinforces the local hydrogen bond network, thereby facilitating the Grotthuss-type proton-hopping pathway that dominates proton conduction in PA-doped systems. Conversely, increasing the length of the alkyl side chain facilitates the formation of larger PA clusters but also expands free volume and enhances PA self-diffusion. This work elucidates the synergistic influence of cationic side-chain configurations on PA distribution, clustering dynamics, and hydrogen bond networks, providing theoretical guidance for the molecular design of PEMs that balance acid retention with efficient proton transport.