Anion exchange membrane fuel cells (AEMFCs) have attracted a considerable attention because of the potential use of non-precious metal catalysts and the lower overpotentials for the oxygen reduction reaction. However, a low hydroxide ion conductivity of the anion exchange membranes (AEMs) is a drawback, which eventually lowers the power density of AEMFCs. To achieve higher performances of AEMFCs, understanding the nano-scale phase separations inside the membranes is essential. The ion conduction at the catalyst layer/membrane interfaces also needs to be understood. Small angle X-ray scattering (SAXS) is known as a technique to reveal the structures inside the bulk membranes on the nanometer scale, whereas current-sensing atomic force microscopy (CS-AFM) is capable of observing the morphology and the ion conductive region simultaneously on the membrane surfaces on the nanometer scale. At present, most of the structural studies have been carried out on proton exchange membranes, and few results were reported on AEMs.1,2) In this study, the phase separations and the distributions of anion conductive areas of AEMs with different but similar chemical structures were systematically investigated by SAXS for bulks and by CS-AFM for surfaces. Fig. 1 shows the chemical structures of two membranes, QPE-bl-11a(C1) and QPE-bl-11b(C3), synthesized in our laboratory.3) The membranes have the same hydrophobic structure but slightly different hydrophilic structures. SAXS analyses were carried out in an environmental chamber. The X-ray wavelength was 0.154 nm. The range of the scatter vector, q, was from 0.10 to 2.0 nm-1 (ca. 3.1 – 63 nm). The temperature was set at 40 ºC, while the humidity was controlled from 30% to 90% RH under the nitrogen atmosphere. The membrane samples were equilibrated at least for 2 h at each humidity. For the CS-AFM measurements, the membrane was pressed on a gas-diffusion electrode (GDE) with a catalyst layer composed of Pt/C and a commercial binder (AS-5, Tokuyama Corp.). The membrane on the GDE was installed in a homemade environment-control chamber.2,4) The CS-AFM measurements were carried out in an ultrapure air at 40 ºC and 70% RH using a Pt-coated cantilever at the contact mode with the contact force of 10 nN. The sample voltage between the AFM tip and the GDE was set at -2.0 V. In the SAXS profile of QPE-bl-11a(C1), a well-defined but broad peak was observed at d = ca. 20 nm. The peak intensity increased with increasing humidity, which might indicate the development of a periodic structure with increasing humidity. For QPE-bl-11b(C3), a well-defined peak was observed at d = ca. 14 nm, smaller than that for QPE-bl-11a(C1). Interestingly, a small difference in the length of the pendant aliphatic groups made a considerable difference in the phase separation. Fig. 2 shows the topographies and the current images obtained on the two AEMs. The surface of QPE-bl-11a(C1) was very flat with the maximum height difference of only 10 nm in the scanned area of 1 μm x 1 μm, but small, nanometer corrugations were also observed over the surface. The surface of QPE-bl-11b(C3) was flatter than that on QPE-bl-11a(C1). Fig. 2c shows the current image on QPE-bl-11a(C1). Anion conductive spots were observed over the surface, and the non-conductive regions (current < 0.5 pA of the background) were only 1% of the entire surface area. On the other hand, it should be also noted that the current differed from 0 to 30 pA at different locations on the surface. In the current image on QPE-bl-11b(C3) (Fig. 2d), the anion conductive regions were also observed over the surface. Interestingly, each conductive spot was not evident, showing a larger homogeneousness. The current difference was smaller, 2 – 20 pA, than that on QPE-bl-11a(C1). For QPE-bl-11a(C1) and QPE-bl-11b(C3), the pseudo current density,4) or the averaged current measured in a unit area, was 2.29 and 2.65 pA nm-2, respectively, whereas the OH- conductivities at 40 ºC in pure water were 23.6 and 34.7 mS cm-1, respectively. The difference in the pseudo current density was 14%, whereas that in the OH- conductivity was 32%. The homogeneousness of the surface conductivity, the pseudo current density, and the OH- conductivity of the membrane bulk must be together considered for the cell performance, 2) which will be further discussed using the membranes. This work was supported by a CREST program of Japanese Science and Technology Agency (JST). 1) Q. He and X. Ren, J. Power Sources, 220, 373, (2012). 2) M. Hara et al., ACS Appl. Mater. Interfaces, submitted. 3) R. Akiyama et al., Macromolecules, submitted 4) M. Hara et al., J. Phys. Chem. B, 117, 3892 (2013). Figure 1
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Open Access
Jul 27, 2017
Wei Zhang + 1
