(Invited) Partially Fluorinated Anion Exchange Membranes for Electrochemical Applications

There has been a considerable effort to develop high performance and durable anion exchange membranes (AEMs) for the electrochemical applications such as fuel cells, electrolyzers, and redox flow batteries. When these devices are operated under alkaline conditions, possible use of non-precious metal catalysts is advantageous. Although a number of AEMs have been proposed in the literature, chemical and mechanical stability in alkaline media still remains an issue for long-term, reliable operation of the devices. In the pursuit of more stable AEMs, we have recently reported some new polymer structures. Combination of absence of heteroatom linkages such as ether, sulfide, and sulfone in the main chain and employing pendant ammonium groups contributed to improvement of the alkaline and mechanical stability of AEMs. A typical example is QPAF-4 copolymers composed of perfluoroalkylene/phenylene main chain and hexylammonium head groups, which exhibited high hydroxide ion conductivity and alkaline stability. 1 In the present paper, we report effect of other fluorinated components for our AEMs. We have developed a novel series of quaternized aromatic copolymers (BAF-QAFs) containing hexafluoroisopropylidene groups. 2 Similar to QPAF-4 copolymer membranes, BAF-QAF copolymers had good solubility in polar organic solvents, membrane forming capability, and phase-separated morphology. Hexafluoroisopropylidene groups contributed to efficient anion transport as BAF-QAF membranes showed lower water uptake and higher hydroxide ion conductivity than those of QPAF-4 membranes. BAF-QAF membranes were thermally stable up to 95 °C and 90% RH with no obvious glass transition behavior in DMA analyses, while the elongation of BAF-QAF membranes were smaller than that of QPAF-4 membranes. BAF-QAF membranes were alkaline stable in 1 and 4 M KOH at 80 °C for 1000 h. A hydrogen/oxygen fuel cell with the BAF-QAF as a membrane and an electrode binder was successfully operated. The maximum power density reached 319 mW/cm 2 at a current density of 702 mAcm 2 with O 2 . The fuel cell with BAF-QAF copolymers were better in cathode performance than QPAF-4 cell most probably because of the higher hydroxide ion conductivity and better interfacial contact between the membrane and the catalyst layer. Acknowledgements This project was partly supported by NEDO through the Advanced Research Program for Energy and Environmental Technologies, JST through SICORP project, and JSPS/SNSF under the Joint Research Projects (JRPs) program. References Ono, T. Kimura, A. Takano, K. Asazawa, J. Miyake, J. Inukai, K. Miyatake, J. Mater. Chem. A , 5 , 24804 (2017). Kimura, A. Matsumoto, J. Inukai, K. Miyatake, ACS Appl. Energy Mater. , 3 , 469-477 (2020). Figure 1