Properties and Morphologies of Anion Exchange Membranes for Fuel Cells with Different Lengths of Fluorinated Hydrophobic Chains

Abstract

1. IntroductionPolymer electrolyte fuel cells are expected as next-generation energy devices because of their high energy conversion efficiency and no carbon dioxide emission. However, expensive noble metals such as platinum are used for the catalyst in the commercialized proton exchange membrane fuel cells. To solve this problem, anion exchange membrane fuel cells (AEMFCs) operable with non-noble metal catalysts have been attracting attentions. The low ionic conductivity and chemical instability of the anion exchange membranes (AEMs), however, are barriers to the practical use of AEMFCs. In our laboratory, we have been developing new AEMs with fluorinated hydrophobic groups, such as BAF-QAF1) and QPAF(C6)-42,3) (Fig. 1) to overcome the problems. QPAF(C6)-4 showed a high conductivity and a high mechanical strength but the thermal stability was insufficient. We have newly synthesized QPAF(C4)-4 with shorter perfluoroalkylene chains than those of QPAF(C6)-4 to tune the interpolymer interaction between the hydrophobic chains. The membrane structures and the properties of two membranes, QPAF(C6)-4 and QPAF(C4)-4, are compared.2. ExperimentalQPAF(C6)-4 ionomers were synthesized, and the membranes were formed as previously reported.2,3) QPAF(C4)-4 ionomers were synthesized in the same manner as QPAF(C6)-42) except PAF(C4) monomers were used instead of PAF(C6) monomers. Transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) were used to understand the arrangements of the hydrophobic/hydrophilic clusters in membranes. To observe the surface morphology and visualize the current images on the film surfaces, current-sensing atomic force microscopy (CS-AFM) was used.4,5) The water uptake and conductivity were measured in water, and the dynamic mechanical analysis (DMA) was conducted. The storage and loss elastic modules were also measured at different temperatures.3. Results and discussionBy TEM, hydrophilic clusters with a diameter of ca. 2 nm were confirmed in both membranes in vacuum (Fig. 2). In situ SAXS measurements under humidified nitrogen gas revealed the existence of periodic structures with the distance more than two times larger than those obtained between the hydrophilic clusters observed by TEM. The rate of the increase in periodicity was larger for QPAF(C4)-4 when the relative humidity was increased. The QPAF(C4)-4 membrane showed a higher water uptake than that of QPAF(C6)-4, but the conductivity was lower. This decrease in conductivity for QPAF(C4)-4, caused by an excessive film swelling (Fig. 3) confirmed by SAXS, may be related with the lower hydrophobicity with the shorter perfluoroalkylene chains.In DMA curves, QPAF(C4)-4 film retained high storage and loss modules up to ca. 80 ˚C, 15 ˚C higher than those of QPAF(C6)-4 (Fig. 4). By the stress-strain measurements, QPAF(C4)-4 showed a lower strain and a higher maximum stress than QPAF(C6)-4 (Fig. 5). The mechanical improvements of QPAF(C4)-4 may be explained by the improved packing between benzene rings due to the shorter perfluoroalkylene chains in the hydrophobic structure.Other properties are discussed based on the chemical structures (Fig. 1) and the three-dimensional conformations of the membranes.References Kimura, A. Matsumoto, J. Inukai, and K. Miyatake, ACS Appl. Energy Mater., 3, 469 (2020).Ono, J. Miyake, S. Shimada, M. Uchida, and K. Miyatake J. Mater. Chem. A, 43, 21779 (2015).Miyake and K. Miyatake, Sustain. Energy Fuels, 3, 1916 (2019). Hara, T. Kimura, T. Nakamura, M. Shimada, H. Ono, S. Shimada, K. Miyatake, M. Uchida, J. Inukai, and M. Watanabe, Langmuir, 32, 9557 (2016).Kimura, R. Akiyama, K. Miyatake, and J. Inukai, J. Power Sources, 375, 397 (2018). Figure 1