| Alkaline anion exchange membrane (AAEM) is one of the key components in alkaline anion exchange membrane fuel cell (AAEMFC). Due to the fast fuel cell reaction kinetics under alkaline conditions, the need of noble metal catalysts Pt can be replaced by conventional metals (such as Co, Ni and Fe) which can reduce the manufacturing cost of the fuel cells. In order to solve the problem of low conductivity, poor stability and mechanical properties of AAEM, poly(arylene ether sulfone) with different backbone structures were synthesized in our study. At first we anlyzed the influence of electronic effects and steric effects of backbone structure on imidazole ionic groups, then, we found out the structure which can improve the stability of imidazole ion. At the same time, a graft polymer was synthesized to weaken the effects of main-chain on ionic group. With this method the stability of the AAEM can be improved.The following are the details of our study:(1)Synthesis of poly(arylene ether sulfone), membrane preparation and characterization. In order to find the relationship between main-chain structure and ionic groups, three kinds of polysulfone with different main-chain structure and quite molecular weight were prepared by the reaction of 2,2-bis(4-hydroxyphenyl)propane,4,4-dihydroxybiphenyl, and 2,2-bis(4-hydroxyphenyl) hexafluoropropane with 4,4-difluorodiphenyl sulfone, respectively. Followed by the reaction with chloromethyl methyl ether, with 1-methylimidazole as an ionization reagent, a series of imidazolium-functionalized polysulfone were prepared by the steps of ionization, cast membrane and alkalization to chloromethylation of polysulfone. The result shows that the polysulfone have high molecular weight (Mw>104), and the membranes have good solubility in polar aprotic solvent. The water uptake and swelling ratio of ImPSf-A-OH, ImPSf-B-OH and ImPSf-C-OH membranes increases with IEC and temperature. While, the conductivity of membranes increases with temperature, all membranes showed conductivities above 10 mS/cm at room temperature, and the conductivity of ImPSf-A-OH, ImPSf-B-OH and ImPSf-C-OH is up to 53.98 mS/cm,47.19 mS/cm, and 42.46 mS/cm at 80℃, respectively. After soaked in 3 M NaOH at room temperature for 168 h, conductivity has fallen by 17.6%, 11.3%, and 7.14%, respectively, which confirmed the speculation that main-chain structures have effects on the ionic groups. (2) Preparation of grafted poly(arylene ether sulfone) membrane and characterization. In order to weaken the effects on ionic groups from main-chain structure, novel imidazolium-functionalized alkaline anion exchange membrane was prepared by atom transfer radical polymerization (ATRP) reaction of 3-chloro-2-methyl-l-propene with chloromethylated polysulfone under the condition of water and oxygen are completely removed. We also investigated the property of AAEM, including water uptake, swelling ratio, ionic capacity, conductivity, mechanical properties, and alkaline stability. The result shows that the water uptake and swelling ratio membrane increase with IEC and temperature, ImPSf-A-g-PMAC-OH membrane with the IEC of 1.75 mmol/g, displayed the highest conductivity, varied from 20.7 mS/cm to 64 mS/cm over the range 20℃-80℃. The water uptake and swelling ratio was 85.3% and 59.5% at 80℃. Comparaed with ImPSf-A-OH, ImPSf-A-g-PMAC-OH membrane has higher conductivity, alkaline stability, and thermal stability.In this paper, we studied the influence of electronic effects and steric effects of backbone structure on imidazole ionic groups, and improved the properities of membranes, which will provide a new method in the design and preparation of high property alkaline anion exchange membranes. The result will be significant in the developing of AAEMFCs. |
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