Preparation And Properties Of Self Crosslinked Quaternary Ammonium Poly(Aryl Ether Sulfone) Anion Exchange Membranes For Fuel Cell

In order to address the problems of low ion conductivity and poor stability associated with anion exchange membranes for polymer electrolyte membrane fuel cell applications, several types of self-crosslinked anion exchange membranes (AEMs) derived from quaternary ammonium poly(aryl ether sulfone)s were prepared. Their fundamental properties such as water uptaking and swelling behavior, ion conductivity and thermal/chemical stability were investigated, and the chemical structure-crosslinking reagent-property relationship was studied.Firstly, a series of AEMs crosslinked by 1,4-diazabicyclo[2.2.2]octane (DABCO) (named as BHPF-DABCO) with crosslinking degree (DC) of 0～100% and ion exchange capacity (IEC) about 1.20 mmol/g were successfully prepared, the AEMs were obtained from 9,9’-bis(4-hydroxyphenyl) fluorene,4,4’-(hexafluoroisopropylidene) diphenol and 4,4’-difluorodiphenyl sulfone through nucleophilic substitution, Freidel-Crafts chloromethylation, quaternization and crosslinking via DABCO and trimethylamine, and alkalization. The obtained membranes by solution casting all were uniform, transparent and ductile. The results indicated that:water uptakes of the obtained self-crosslinked membranes increased initially and then decreased, whereas the swelling ratio and ion conductivity declined but the hydrolytic and oxidative stability increased with the increase in DC. In the temperature range of 30～90℃, this series of BHPF-DABCO membranes exhibited water uptakes of 14.6～53.2%, swelling ratio of 3.3～13.7% and ion conductivity of 5.2～48.4 mS/cm. They also showed largely improved hydrolytic stability by crosslinking modification, the membranes could keep rather good mechanical strength with weight losses less than 2.6% after treated in water at 140℃ for 24 h. Besides, the membranes with DC≥60% could maintain more than 70% of their original ion conductivity after immersed in 1 mol/LNaOH solution at 60℃ for 168 h.Secondly, a series of AEMs crosslinked by N, N, N’, N’-tetramethylhexanediamine (TMHDA) (named as BHPF-TMHDA) with DC of 0～100% and IEC about 1.36～1.46 mmol/g were successfully prepared by similar procedures as mentioned above, all obtained membranes were ductile and uniform. In the temperature range of 30～90℃, this series of BHPF-TMHDA membranes exhibited water uptakes of 41.7～76.9%, swelling ratio of 4.0%～13.8% and ion conductivity of 5.1～62.0 mS/cm. The membranes with DC≥40% could keep fairly good mechanical strength with weight losses less than 9.4% after treated in water at 140℃ for 24 h. Compared with the BHPF-DABCO membranes, they also exihibited better oxidative stability, keeping original shape about 2.1～15.4 h in Fenton’s reagent at 80℃, while only 2.3-5.5 h for the former membranes.Finally, a series of ductile and uniform AEMs crosslinked by TMHDA (named as BPA-TMHDA) with DC of 0～100% and IEC about 0.80 mmol/g were successfully prepared by replacing 9,9’-bis(4-hydroxyphenyl) fluorene with bisphenol. The results indicated that the BPA-TMHDA showed higher water uptakes (86.3-160.6%), swelling ratio (4.5～33.1%) and ion conductivity (15.4～61.7 mS/cm) at 30～90℃ than the BHPF-TMHDA membranes despite of the 43% lower IEC. However, their hydrolytic and dimensional stability were much worse.In summary, among all the prepared self-crosslinked anion exchange membranes, in the premise of satisfying the basic ion conductivity requirement (> 10 mS/cm), those membranes having fluorenyl moieties in the polymer backbone and crosslinked by TMHDA had good physico-chemical stability. They are promising candidates for practical fuel cell applications.