Anion-conducting electrolyte membrane fuel cells (AEMFCs) have attracted considerable interest due to their significant advantages over proton-conducting electrolyte membrane fuel cells (PEMFC). Because AEMFCs operate under alkaline environments, non-precious metals can be used as active catalysts (e.g. nickel, cobalt and iron) and electrochemical reactions are more facile than that in acidic medium.¹⁾ In recent years, we have developed imidazolium-type AEMs by radiation-induced grafting method and founded that copoly(N-vinylimidazolium/styrene)-grafted AEM shows moderate alkaline stability. However, the AEMs degraded via the β-elimination reaction and the ring opening reaction of C2 position.²⁾ In this work, we synthesized p-(2-imidazoliumyl) styrene (2STZ)-grafted membrane (Pr-PrSTZ-AEM) to exclude β-hydrogen from the chemical structure and protect C2 position.

The 2STZ monomer was synthesized from 4-vinylbenzyl chlorid)e via the 4-vinylbenzaldehyde. And then, the 2STZ was mono N-alkylated by sodium hydrate and propyl iodide in a standard manner to obtain propylated monomer (PrSTZ).³⁾ Poly(ethylene-co-tetrafluoroethylene) (ETFE) films were irradiated with a ⁶⁰Co γ-ray source (QST, Takasaki, Gunma, Japan) at room temperature in argon atmosphere with an absorption dose of 50 kGy. The pre-irradiated ETFE films were immediately immersed into the argon-purged monomer solution consisting of 50 vol% of PrSTZ in 1,4-dioxane for 1~16 h. The imidazole groups in the grafted-ETFE were N-alkylated by propyl iodide in a quantitative manner. The obtained Pr-PrSTZ-based AEMs (an iodide form) were immersed in 1M HCl and 1,4-dioxane (1 : 1 volume ratio) solution at 60 °C for 24 h to convert into a Cl^- form, followed by the ion-exchange reaction in 1M KOH and 1,4-dioxane (1 : 1 volume ratio) solution at room temperature for 16 h to obtain Pr-PrSTZ-AEM (Scheme 1).

The Pr-PrSTZ-based AEMs with IEC_exp. of 0.54~1.72 mmol/g were converted from the grafted membranes with grafting degrees of 18~98% by quantitative N-alkylation and ion-exchange reaction in more than 90% conversion. The prepared AEMs exhibited 7~78% of water uptakes and 50~181 mS/cm of ionic conductivities at 60°C (Figure 1). The alkaline stability of AEMs was evaluated by monitoring the change in conductivity of AEMs in 1M KOH at 80°C. The conductivity of AEM with a grafting degree of 47% decreased from 125 to 8 mS/cm after 600 h, whereas the conductivity of AEM with a grafting degree of 18% increased from 50 to 60 mS/cm after 600 h. This result indicates that the Pr-PrSTZ-based AEMs with the grafting degree of 18% has moderate ionic conductivity with surprised water uptake and quite high alkaline stability, comparing to copoly(N-vinylimidazolium/ styrene)-grafted AEMs. Therefore, the Pr-PrSTZ-based AEMs are expected as a promising membrane for highly alkaline durable 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:

1) J. R. Varcoe, P. Atanassov, D. R. Dekel, A. M. Herring, M. A. Hickner, P. A. Kohl, A. R. Kucernak, W. E. Mustain, K. Nijmeijer, K. Scott, T. Xu and L. Zhuang, Energy Environ. Sci., 7, 3135-3191 (2014).

2) K. Yoshimura, H. Koshikawa, T. Yamaki, H. Shishitani, K. Yamamoto, S. Yamaguchi, H. Tanaka and Y. Maekawa, J. Electrochem. Soc., 161(9), F889- F893 (2014).

3) C. G. Overberger and Y. Kawakami, J. Polym. Sci. Polym. Chem. Ed., 16, 1237-1248 (1978).