A simple cation substitution strategy to regulate the morphology and properties of anion exchange membrane for fuel cells

Poly(aryl piperidinium)-based anion exchange membranes (AEMs) have manifested remarkable chemical stability, thereby firmly positioning themselves as highly promising candidates for application in anion exchange membrane fuel cells (AEMFCs).. However, the closely-packed polymer stacking configuration, along with the insufficient cation mobility, gives rise to inferior conductivity performance. In the present study, a novel cation substitution strategy was devised specifically for anion exchange membranes. This strategy entailed the utilization of quinuclidinium cations to substitute for piperidinium cations within aryl ether-free backbones, with the replacement process being facilitated by simple superacid catalysis. Molecular dynamics simulation revealed that, in contrast to the piperidinium cations which exhibited relatively stronger intermolecular interactions, the spherical quinuclidinium cations, characterized by their larger steric hindrance, were capable of augmenting the free volume within the AEMs. Consequently, this led to an enhancement in the mobility of cations. Through regulating the quinuclidinium substitution degrees, the cation substitution strategy can integrate the complementary advantages of the two cations to balance the cation interactions and mobility, hence optimizing the microphase separation morphology, and further controlling the properties of AEMs. Benefiting from the enhanced microphase separation architecture and elevated water uptake, the QPPTQP-50% membrane exhibited a remarkably high conductivity of 174.9 mS cm-1 at 80 o C, while simultaneously maintaining a restricted swelling ratio of approximately 13% at the same temperature. This outstanding performance effectively resolved the long-standing conductivity-swelling conundrum. Simultaneously, the QPPTQP-50% manifested remarkable mechanical robustness, registering a tensile strength of 39.7 MPa and an elongation at break reaching up to 15.2%. It also demonstrated outstanding alkaline resistance, as evidenced by a 95.9% retention of OH - conductivity and a 94.7% maintenance of tensile strength upon immersion in a 1 M NaOH solution at 80 °C for a period of 1,680 hours. Moreover, a single cell fabricated with the QPPTQP-50% membrane was capable of attaining a peak power density (PPD) of 1.52 W cm -2 at 80 °C. In addition to this high power output, the cell exhibited excellent in-situ durability, signifying its long-term stability and reliability under operational conditions. • A simple cation substitution strategy was proposed for aryl ether-free AEMs. • Balancing cation interactions and mobility optimized the microphase separation. • Microphase separation contributed to excellent conductivity and stability. • Optimized quinuclidinium substitution degrees guaranteed outstanding cell performance.