Alkaline stable pyrrolidinium-type main-chain polymer: The synergetic effect between adjacent cations

The structure evolution of individual cation approach to enhance the alkaline stability of anion exchange membranes (AEMs), has attracted considerable attention in recent years. However, the synergetic effect between adjacent cations was ignored even though it actively affects alkaline stability. Herein, a series of small organic compounds and main-chain polymers with pyrrolidinium cations tethered with different methylene groups were successfully synthesized and characterized. The increasing cation charge density of the pyrrolidinium compounds conferred better alkaline stability while the long spacer between adjacent pyrrolidinium cations accelerated the degradation of the cations in alkaline solutions. Then, DFT calculations indicated that the synergetic effect between the adjacent pyrrolidinium cations leads to a lower charge density on the α-C in the pyrrolidinium groups and enhance the energy barrier value for the attack by OH−, thereby improving excellent alkaline stability. The mechanically and chemically robust AEMs were prepared with pyrrolidinium-based main-chain polymers. The uncovered synergetic effect between the cations makes the synthesized AEMs a potential membrane for alkaline fuel cells.