Synergistic interaction and alkali resistance of N-spirocyclic cations with piperidine cations for poly(biphenyl piperidine) membranes

The conformation of N-spirocyclic cations has a high alkaline stability and the construction of microphase-separated structures with poly(biphenyl piperidine) (PBP) is one way to improve the performance of AEM. Here, a variety of N-spirocyclic cations were prepared and formed into polycationic side chains with the PBP main chain via the -(CH2)4- spacer group. The ion transport efficiency was greatly enhanced by the synergistic effect of the main chain piperidine cations, and the hydroxide conductivity and peak power density of the MDSU-MPBP membrane reached 132.7 mS cm-1 and 295.2 mW cm-2 at 80 °C, respectively. The BM-DSU cation was degraded to a single cation mainly by the SN2 reaction after 96 h in a 5 M NaOH/D2O solution at 80 °C. The N-spirocyclic mono-cation has extremely high alkali resistance, especially the MDSU cation without significant degradation at 80 °C for 1008 h. The B-MDSU double cation tends to undergo the SN2 reaction at the C2 and C4 positions. The asymmetrically grafted main-chain piperidine cations exhibit poor chemical stability due to the electron-absorbing effect of the phenyl group. the degradation of the MDSU cations increases the hydrophobicity of the side chains, and the MDSU-MPBP membrane has the best alkali resistance, with a conductivity residual of more than 92.8% after immersion in a 5 M NaOH solution at 80 °C for 480 h.