9:45 AM - EN04.01.05
Structural Designs of Alkaline Durable Imidazolium-Containing Anion Conducting Membranes Prepared by Radiation-Induced Grafting for Pt-Free Fuel Cells
Kimio Yoshimura1,Yue Zhao1,Ahmed Mahmoud1,Akihiro Hiroki1,Hideyuki Shishitani2,Susumu Yamaguchi2,Hirohisa Tanaka3,Yasunari Maekawa1
National Institutes for Quantum and Radiological Science and Technology1,Daihatsu Motor Co., Ltd.2,Kwansei Gakuin University3
We have been developing alkaline-durable anion conducting electrolyte membranes (AEMs) for liquid fuel (hydrated hydrazine) type fuel cell (FC) vehicles. Even though the AEM-FC system have been significantly attractive because of non-precious metals used as active catalysts, there is no commercially available AEMs due to the severe damage of the membranes in alkaline operating conditions . Thus, we applied the radiation-induced grafting technique to introduce various anion conducting graft-polymers into a thermally and mechanically tough poly(ethylene-co-tetrafluoroethylene) (ETFE) film to develop new AEMs.
Recently, we investigated a series of imidazolium-type AEMs because of the low basicity of imidazolium hydroxide as an Arrhenius base . Most of the imidazolium-type AEMs exhibited lower water uptake and higher thermal stability than those of the corresponding AEMs containing trimethylammonium hydroxide. Furthermore, AEMs containing weak base imidazolium groups suffered less damage of polymer backbones via self-base catalyzed degradation. Even though N-vinylimidazolium graft-type AEMs are subjected to β-elimination and hydrolytic ring opening degradation to reduce the anionic conductivity in an alkaline solution at elevated temperature (1M KOH, 80°C), we introduced the graft-copolymers of vinylimidazolium with styrene because the adjacent vinylimiazolium cations led very fast elimination due to the double β-positioned hydrogen. Furthermore, for suppressing hydrolysis, the protecting group is introduced at 2-posion of imidazolium rings, which are subject to hydroxide attack. The alkaline durability of the AEMs having the abovementioned molecular designs showed drastically improved alkaline durability. Namely, a poly(2-methyl-N-vinylimidazolium hydroxide-co-styrene)-grafted ETFE (MIm/St-AEM, MIm/St ratio of 60/40) retained 26% of the initial conductivity (>14 mS cm−1) after 1300 h in the actual fuel of 5% hydrated hydrazine in 1 M KOH at 80 °C. The membrane-electrode-assembly (MEA) consisting of MIm/St-AEM with the anion conducting ionomer having a similar polymer structure to the grafts, showed a maximum power density of 230 mW cm−2 in a direct hydrazine hydrate FC .
Since the electrolyte properties, durabilities, and fuel cell performance of the graft-type AEMs could not be elucidated only by these chemical structures of the graft-type AEMs, we performed the hierarchical structure analysis of the synthesized AEMs. Contrary to the above mentioned MIm/St-AEM, the AEM with higher styrene ratios (MIm/St = 40/60 and 20/80) showed less durability in the alkaline solution. By a SANS contrast variation method, only the AEMs with MIm/St = 40/60 and 20/80 showed water puddle sphere with a diameter of 3-4 nm. Thus, we concluded that the imidazolium groups around the water puddle are subjected to the hydrolytic attack to lead the degradation of the AEMs . We find out that the elucidation of hierarchical structures of the AEMs takes important role to develop alkaline durable AEMs.
This work was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) from the Japan Science and Technology Agency (JST).
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 (2014).
2) B-S. Ko, K. Yoshimura, S. Warapon, H. Shishitani, S. Yamaguchi, H. Tanaka and Y. Maekawa, J. Polym. Sci., A, Polym. Chem., 57, 503 (2019).
3) K. Yoshimura, A. Hiroki, H-C. Yu, Y. Zhao, H. Shishitani, S. Yamaguchi, H. Tanaka and Y. Maekawa, J. Membr. Sci., 573, 403 (2019).
4) K. Yoshimura, Y. Zhao, A. Hiroki, Y. Kishiyama, H. Shishitani, S. Yamaguchi, H. Tanaka, S. Koizumi, J. E. Houston, A. Radulescu, M-S. Appavou, D. Richterf and Y. Maekawa, Soft Matter, 1, 9118 (2018).