Synthesis and Properties of Partially Fluorinated Anion Exchange Membranes Containing Ammonium-Functionalized Fluorenyl Groups

Abstract

Anion exchange membrane fuel cells (AEMFCs) are one of the promising energy conversion devices because of their high efficiency and possible use of non-precious metal catalysts. However, the existing anion exchange membranes, main component of AEMFCs, do not show sufficient ion conductivity and stability. In this study, we report a novel series of AEMs composed of perfluoroalyl chains and ammonium-functionalized fluorenyl groups (Fig. 1). The perfluoroalkyl groups are expected to provide chemical stability as well as flexible membrane forming capability, while the ammonium-functionalized fluorenyl groups contain high ionic density for high ion conductivity. We have investigated in details synthetic procedure, structure, and properties of the title AEMs. The hydrophobic monomer containing perfluroaklyl groups was synthesized by Cu-catalyzed Ullmann coupling reaction, and fluorenyl-containing monomers were synthesized by Sandmeyer reaction. Precursor polymers were synthesized by Ni-mediated Ullman coupling reaction. Then, chloromethyl groups were introduced onto the precursor polymers by Friedel-Crafts reaction. The chloromethyl groups were quarternized with trimethyl amine by Menshutkin reaction. It is known that the chloromethylation reaction often accompanies with unfavorable side reactions such as cross-linking. By carefully optimizing the Friedel-Crafts reaction conditions, we have successfully controlled the degree of chloromethylation. In fact, the number and the position of chloromethyl groups substituted on fluorenyl groups were controllable. The chloromethylated precursors were soluble in organic solvent and the solutions were cast to thin films. The ion exchange capacities obtained by titration were lower than calculate from DC. The difference of IEC value was increased with increase of DC. It is suggest that when ionic group density becomes higher, there is an ammonium group which is not functioning as ionic groups by their steric hindrance. The quaternized polymer membranes exhibited high hydroxide ion conductivity and the maximum conductivity was 29 mS/cm in water at 80 °C for the membrane with IEC = 1.6 meq/g. It was found that meta-linked polymers in the hydrophilic component showed slightly higher ion conductivity than that of para-linked polymers. Other properties such as morphologies and stabilities will also be reported. Acknowledgement This work was partly supported by Japanese Science and Technology Agency (JST), CREST. References 1) Manai Shimada, Shigefumi Shimada, Junpei Miyake, Makoto Uchida, Kenji Miyatake, J. Polym. Sci., A: Polym. Chem., 2015, 7,935-944. 2) Hideaki Ono, Junpei Miyake, Shigehumi Shimada, Makoto Uchida and Kenji Miyatake, J. Mater. Chem. A, 2015, 3, 21779–21788. Figure 1