Preparation And Characterization Of Polysulfone Anion Exchange Membranes With Resonant Ionic Groups

Anion exchange membrane fuel cell (AEMFC) has recently attracted tremendous attention because its fast oxygen reduction reaction in basic operating condition allows the use of non-precious catalyst or transition metal oxides in place of platinum, making AEMFC more cost effective; also, fuel crossover and carbonate crystallization can be effectively weakened and avoided. Currently, low ionic conductivity and inferior chemical stability of AEM are two major problems restricting practical application of AEMFC. Therefore, modification of existing AEM or developing new membrane structure is highly important for the development of AEMFC. Imidazolium or guanidinium anion exchange membrane has dual advantages of conductivity and chemical stability thanks to strong basic and conjugated structure of ion exchange groups. In this dissertation, we explored the influence of resonance on membrane’s chemical stability by modulating resonance level of cations. Resonance can possibly promote charge delocalization of cations and reduce its interaction with hydroxide ions so that alkali-resistance of membrane can be improved. Main contents are as follows:Firstly, we introduced guanidine into imidazole molecule to synthesize binary ionic reagent 2-(1,1,3,3-tetramethylguanidine)-imidazole, also named guanidimidazole (GIm), which has double resonance structure. Then. we prepared guanidimidazole-quanternized anion exchange membrane (PSf-MIm-GImOH) using GIm and 1-methylimidazole (MIm) as quaternizing agent simultaneously. PSf-MIm-GImOH membrane had decent anti-swelling effect so that membranes fabricated with and without GIm exhibited similar ion exchange capacity (IEC) and conductivity (ca.14.83 vs.16.05 mS · cm-1) but very different swelling ratio (ca.23 vs.38%). Furthermore, PSf140-MIm-GImOH membrane showed impressive alkali stability; its conductivity remained unchanged after soaking in 3 M NaOH solution for 10 days at room temperature and experienced a 27% conductivity decrease after 15 days treatment while PSf140-MImOH membrane’s conductivity decreased by 73% which demonstrates resonance effect improved membrane’s chemical stability.Secondly, we introduced phenyl into imidazole molecule to synthesize 1-butyl-2-methyl-benzimidazole (Bu-BIm) with stronger resonance relative to imidazole, which was used for membrane preparation. The resulting AEMs, namely benzimidazole quanternized anion exchange membrane (PSf-BImOH) had a 77% conductivity retention after soaking in 1 M NaOH solution at 60℃ for 24 h while the PSf-MImOH membrane’s conductivity decreased by 61%. This demonstrates that alkali-stability of PSf-BImOH membrane is better than PSf-MImOH membrane.Further, we prepared vinyl-imidazole quanternized anion exchange membrane (PSf-VImOH) as a control experiment. PSf-VImOH membrane’s conductivity dropped to 34% of its initial value after soaking in 1 M NaOH solution at 60℃ for 24 h. which is inferior to PSf-MImOH membrane (39% conductivity retention). The weak electron-withdrawing conjugation effect of carbon-carbon double bond decreased the charge density of cations, thus adversely influencing membrane’s chemical stability.Finaly. to further study alkaline stability of inoic groups, Density Functional Theory (DFT) was adopted to caculate lowest unoccupied molecular orbital (LUMO) energy of [GIm]+, [Bu-BIIm]+. [VIm]+ and [MIm]+. and the results well agree with experiments. It provides a new strategy for preparation of high-performance anion exchange membrane and density functional theory (DFT) calculation method could guide for the experiment.