Synthesis, Characterization, and Alkaline Stabilities of 4-Vinylimidazolium-Based Anion Conducting Polymer Electrolyte Membranes Prepared By Radiation-Induced Grafting for Alkaline Fuel Cell Applications
Authors: Yoshimura Kimio , Watanabe Shun , Shishitani Hideyuki , Yamaguchi Susumu , Tanaka Hirohisa , Maekawa Yasunari 
Published: February 2020

There has been a growing interest in noble metal-free alkaline fuel cells, in which anion conducting polymer electrolyte membranes (AEMs) required highly alkaline durability because of high temperature and highly basic fuel cell operating condition in the fuel cells. In recent years, we have developed N-vinylimidazolium-type AEMs by radiation-induced grafting for direct hydrazine hydrate fuel cells (DHFCs).1,2 The prepared poly(N-vinylimidazolium)-grafted AEMs (NVIm-AEM) showed moderate alkaline stability, however, these AEMs degraded via the initial β-elimination at the initial stage and hydrolytic ring opening reaction of the imidazolium unit at the later stage.3 In this work, we synthesized the AEM based on a homo- and co-polymer grafts of 2-methyl-N-methyl-4-vinylimidazolium with styrene (2Me-NMe-4VIm-AEM and 2Me-NMe-4VIm/St-AEM) to prevent the β-elimination and ring opening reaction (Figure 1).
According to the literature, 4-vinylimidazole (4VIm) was transformed to N-methyl-4-vinilimidazole (NMe-4VIm) by the N-methylation using sodium hydrate and methyl iodide in a 64% yield.4 NMe-4VIm was then reacted with n-BuLi and methyl iodide to give a 2-methyl-N-methyl-4-vinylimidazole monomer (2Me-NMe-4VIm) in a 56% yield (Scheme 1).
The poly(ethylene-co-tetrafluoroethylene) (ETFE) films were irradiated with a 60Co γ-ray source (QST Takasaki, Gunma, Japan) at room temperature in argon atmosphere. The pre-irradiated ETFE films were immediately immersed into the argon-purged monomer solution consisting of 5.3 mol/L 2Me-NMe-4VIm or a mixture of 2Me-NMe-4VIm and St (80 : 20 molar ratio) in 1,4-dioxane. The graft reaction proceeds smoothly and gave 2Me-NMe-4VIm and 2Me-NMe-4VIm/St graft ETFE films with the grafting degree of 33% and 54%, respectively. The grafted films were then N-alkylated and ion exchanged to obtain the hydroxide from of 2Me-NMe-4VIm-AEM and 2Me-NMe-4VIm
/St-AEM with IECs of 1.38 and 1.74 mmol/g (Scheme 2). The N-alkylation reaction proceeded quantitatively while the un-methylated analogue, a 4-VIm graft ETFE film, showed less reactivity and saturated at around 66% level. 2Me-NMe-4VIm-AEM and 2Me-NMe-4VIm
/St-AEM showed conductivities of 175 and 157 mS/cm with water uptakes of 53 and 86%. These AEMs show significantly higher conductivities with lower water uptakes, which are desirable properties for fuel cell applications.
The alkaline stability of AEMs was evaluated by the change in conductivity of AEMs in 1M KOH at 80°C. As shown in Figure 2, the conductivity of NVIm-AEM (N-vinyl type AEM) decreased to 0 mS/cm after 100 h, while 2Me-NMe-4VIm-AEM showed higher conductivities than 20 mS/cm after 100 h. The decreasing profile of the 2Me-NMe-4VIm-AEM conductivity indicates that the initial conductivity drop resulting from β-elimination was clearly suppressed by the shift of vinyl substituent from 1- to 4-positon of the imidazolium ring. Also, the decreases of conductivity at the later period, arising from ring opening degradation become much slower for 2Me-NMe-4VIm-AEM owing to the methyl substitution effect at 2-position of the imidazolium ring. The copolymer type 2Me-NMe-4VIm
/St-AEM possesses excellent initial and long term alkaline stabilities. Thus, sharp drops of conductivity at the initial stage probably caused by the graft chain detachment, and the introduction of copolymerized styrene units should suppress the detachment phenomenon by hydrophobicity.5 In conclusion, for vinylimidazolium-containing graft-type AEMs, the control of vinyl position, the methyl-protection at 2-positon, and copolymerization with styrene should be promising strategy to improve the alkaline stability, which is the essential property for alkaline fuel cells.

Acknowledgement
This work was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) from the Japan Science and Technology Agency (JST).

References
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Figure 1