Multiplex Analysis on Phase Separation and Ion Conductivity at Bulks and Surfaces of Designed Anion Exchange Membranes

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Anion exchange membrane fuel cells (AEMFCs) are attracting much attention because of the potential use of non-platinum catalysts. The hydroxide ion conductivity of the anion exchange membranes (AEMs) is still not sufficient, which is a drawback for the commercialization. To achieve higher performances of AEMFCs, understanding the nano-scale phase separations inside the membranes is essential. So far, only few results have been reported on the anion conducting behavior either inside the bulk or on the surface of AEMs.1,2) In this study, AEMs strategically designed with different chemical structures were systematically investigated by transmission electron microscopy (TEM) in vacuum and small angle X-ray scattering (SAXS) for the structures inside the bulk membranes on the nanometer scale, and by in-situcurrent-sensing atomic force microscopy (CS-AFM) with an environmental chamber for the morphology and the ion conductive region on the membrane surfaces. Fig. 1 shows the chemical structure of the QPE-bl-11 membrane synthesized in our laboratory.3) The membranes with the same chemical component but with different ion exchange capacities (IECs) of 1.23 (X = 4, m: n = 1:2.9) and 1.95 mequiv g−1 (X = 4, m: n = 1:5.4) were prepared in a chloride-ion form. Fig. 2 shows the SAXS profiles of the QPE-bl-11 membrane. For both membranes, the peak intensity increased with increasing humidity, which indicates the development of periodic structures. With 1.23 mequiv g-1 (Fig. 2(a)), weak and broad peaks were observed at the real-space distance (d) of ca. 17 nm. With 1.95 mequiv g-1 (Fig. 2(b), a peak at d = ca. 17 nm first observed at 30% RH became larger and located at d = ca. 18 nm at 90% RH. The peak intensity was larger with IEC =1.95 mequiv g-1 especially at higher humidities. For the CS-AFM measurements at 40 °C, the membrane was pressed on a gas-diffusion electrode (GDE) with a catalyst layer composed of Pt/C and a binder (AS-5, Tokuyama Corp.).2,4) Fig. 3 shows the current images on the two membranes. At 30% RH with IEC =1.23 mequiv g-1 (Fig. 3(a)), the tip current was always less than the detection limit of 0.5 pA in the scanned area of 1 μm x 1 μm. On the membrane with IEC =1.95 mequiv g-1 (Fig. 3(b)), the OH− conductive area covered nearly 100% of the surface. The average current was approximately 10 pA. Fig. 3(c) shows the current image with IEC =1.23 mequiv g-1 obtained at 70% RH. Anion conductive spots were observed over the surface unlike that at 30% RH. Still, the current ranged from 0 to 30 pA. With IEC =1.95 mequiv g-1at 70% RH (Fig. 3(d)), the anion conductive regions were observed over the surface and more homogeneous. The tip current was larger than 5 pA at any location with an average current of 15 pA. The different IECs gave not only quantitative but also qualitative influences on the membrane morphology and properties, and the differences became larger at higher relative humidity. In the presentation, the membranes with other structures will also be discussed. 1) Q. He and X. Ren, J. Power Sources, 220 (2012) 373. 2) M. Hara et al., Langmuir, 32 (2016) 9557. 3) R. Akiyama et al., Macromolecules, 49 (2016) 4480. 4) T. Kimura et al., J. Power Sources, submitted. Figure 1