Toward Eco-Friendly Proton Exchange Membranes: A Molecular Dynamics Study of the Pendant Group Effects on Phosphoric Acid Doping in Polybenzimidazoles

Polybenzimidazoles (PBIs) are promising fluorine-free candidates for proton exchange membrane (PEM) applications due to their thermal stability and anhydrous proton conduction. While several studies have investigated phosphoric acid (PA) transport within the polymer bulk, the effects of pendant groups remain less understood, particularly at the molecular level. In this study, molecular dynamics simulations were employed to investigate the pendant group effects on PA doping in PBIs. A series of pendant groups was selected for evaluation, including unsubstituted (H-OO-H), bromo-substituted (H-OO-Br), phenyl-substituted (H-OO-Ph), trifluoromethyl-substituted (H-OO-TFM), and 3,5-bis(trifluoromethyl)phenyl-substituted (H-OO-TFMP). The PBI–PA interface models revealed that bulky pendant groups could modulate polymer chain orientations, thereby enhancing the diffusion dynamics. H-OO-Ph and H-OO-TFMP outperformed H-OO-Br and H-OO-TFM in terms of the anomalous diffusion exponent, doping ratio, and interaction energy. Spatial correlation and interaction energy analyses indicated that H-OO-Ph promoted the PA aggregation near imidazole groups, while H-OO-TFMP strengthened the PBI–PA interaction despite exhibiting limited aggregation. These molecular-level insights demonstrate that bulky pendant groups can modulate polymer chain orientations and optimize imidazole group environments, thereby influencing the diffusion dynamics, uptake behavior, spatial correlation, and interaction energy of PA doping in PBIs. This work supports the rational design strategies for high-performance PEMs without reliance on persistent fluorinated substances. It provides a perspective on pendant group design through interface models rather than conventional mixed models. Based on the MD simulation results, this approach holds promise for optimizing pendant groups to other nonfluorinated groups. Furthermore, density functional theory (DFT) could be used to examine how pendant groups affect the PA network and proton transfer in PBIs.