Alkaline Durable 2-Methyl-1-Vinylimidazolium-Based Anion Exchange Membranes Prepared by Radiation-Induced Grafting for Hydrazine Hydrate Fuel Cells

Introduction

An anion exchange membrane (AEM) is one of the key materials for alkaline-type fuel cells (AEMFC), which have potential advantages against proton exchange membrane fuel cells (PEMFC). In the AEMFC, electrochemical reactions are more facile than in acidic medium, and non-precious metals such as nickel, cobalt and iron can replace platinum as the catalyst.^1,2 Because of high temperature and highly basic fuel cell operating condition, it is desirable to develop alkaline durable AEMs. In recent years, we have developed imidazolium-type AEMs by radiation-induced grafting for AEMFCs, especially in direct hydrazine fuel cells and found that poly(1-vinylimidazolium (VIm))-grafted and copoly(VIm/styrene)-grafted AEMs (AEM1 and AEM2) shows moderate alkaline stability; the AEMs degraded via the initial β-elimination and following ring opening reactions of the imidazolium unit.^3,4 Therefore, we synthesized the AEM based on a poly(2-methyl-1-vinylimidazolium (MVIm)) grafts by radiation-induced grafting to improve long-term stability by suppressing β-elimination and ring opening reactions.

Experimental

The ETFE films were irradiated with a ⁶⁰Co γ-ray source (JAEA 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 wt% MVIm or a mixture of MVIm/stylene (8 : 2 weight ratio) in 1,4-dioxane. The imidazole groups in the grafted-ETFE were N-alkylated using methyl iodide. The resultant AEMs (an iodide form) were immersed in 1 M NaHCO₃ solution at 60°C for 24 h to convert into a HCO₃^- form, followed by the ion-exchange reaction with OH^- in 1 M KOH solution at room temperature for 16 h to givethe anion forms of the AEMs (AEM3 and AEM4), as shown in Scheme 1. The ionic conductivity was measured by two-point probe AC impedance spectroscopy at 100% relative humidity in N₂-purged deionized water. The ion exchange capacity (IEC) was measured using standard back titration methods.

Results and discussion

The poly(MVIm)- and copoly(MVIm/styrene)-grafted ETFE films (precursors of AEM3 and AEM4) were prepared with grafting degrees of 36 and 55%, respectively. The copolymer-type AEM4 was synthesized for reduction of steric and electrostatic repulsions between adjacent imidazolium cations. The N-alkylation for AEM3 proceeds quantitatively. Thus, molar ratio of MVIm and stylene in AEM4 determined from the weight gain after N-alkylation given that the N-alkylation of the imidazole groups proceeded in a quantitative maner was estimated as 1.0 : 1.4. AEM3 and AEM4 with IECs of 0.99 and 0.93 mmol g^-1 showed ionic conductivities of 55 and 53 mS cm^-1 at 60°C, respectively.

The alkaline stability of AEMs was evaluated by monitoring the change in conductivity of AEMs in 1 M KOH at 80°C. Figure 1 shows the normalized conductivities of the AEMs on the basis of the initial conductance as a function of immersion time. The homopolymer-type AEM1 and AEM3 showed similar profiles for conductivity decreases,  indicating  the donimant β-elimination at an initial stage for homopolymer-type imidazolium grafts in the AEMs due to the steric and electrostatic repulsions. Copolymer-type AEM3, which had no 2-methyl group on the imidazolium group, showed a slower degradation rate after the fast decrease at the initial stage. Furthermore, copolymer-type AEM4, which had 2-methyl group on the imidazolium group, showed slower decreases at the initial stage and kept excellent long-term stability with keeping maintenance factor of 22% (12 mS cm^-1) after 600 h immersion. The above results strongly indicate that the introducition of methyl group at 2-position of imidazolium unit drastically enhances the long-term durability in alkaline solutions at elevated temperatures due to the resistance of nucleophilic ring opening reactions of the imidazolium cations. Accordingly, the 2-methylimidazolium units in copolymer grafts in AEMs are promising an anion exchange group for highly alkaline durable direct hydrazine 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] A. Serov and C. Kwak, Appl. Catal B-Environ., 98, 1 (2010).

[2] K. Asazawa, K. Yamada, H. Tanaka, A. Oka, M. Taniguchi and T. Kobayashi, Angew. Chem., Int. Ed., 46, 8024 (2007).

[3] K. Yoshimura, H. Koshikawa, T. Yamaki, Y. Maekawa, K. Yamamoto, H. Shishitani, K. Asazawa, S. Yamaguchi and H. Tanaka, ECS Trans., 50, 2075 (2012).

[4] Y. Ye and Y. A. Elabd. Macromolecules, 44, 8494 (2011).

-