Visualization of Liquid Water Accumulation in PEMFCs Operating at Different Temperatures by Soft X-ray Radiography
Visualization of Liquid Water Accumulation in PEMFCs Operating at Different Temperatures by Soft X-ray Radiography
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通过软 X 射线照相可视化在不同温度下运行的 PEMFC 中的液态水积聚
DOI：
10.1149/05002.0335ecst
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发表时间：
2013
期刊：
影响因子：
--
通讯作者：
S. Hirai
中科院分区：
文献类型：
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作者：
P. Deevanhxay;T. Sasabe;S. Tsushima;S. Hirai
在质子交换膜燃料电池(PEMFC)中，适当的水管理是获得高功率密度和增强其稳健性的关键。膜需要足够的湿化来完成其质子导体的功能。然而，阴极侧的泛滥会阻碍反应物的传输，减少反应面积。了解电解质膜组件(MEA)中的液水传输对于设计有效的燃料电池具有重要意义。已经开发了几种技术来可视化MEA中的液态水。然而，要在微观尺度上观测液态水的输运，还需要提高空间和时间分辨率。最近，我们发展了软X射线照相术，并成功地在室温下运行的质子交换膜燃料电池中显示了液态水[1-3]。在这项研究中，我们的目标是在接近实际操作条件的温度下，研究液态水在膜电极内的传输。使用实验室软X射线显微镜系统(Tohken，TUX-3110FC)对运行中的质子交换膜燃料电池中的液态水进行了可视化。我们用金刚石窗口上的一层钨薄膜作为产生光子能量为8.4-10keV的X射线的靶材料。X线管电压设置为18~20千伏。采用转印法制备了催化剂涂膜。在全氟磺酸膜(Nafion®EC NRE212，50μm厚)上用热压法制备了铂负载量为0.22-0.23mg.cm的催化层(CL)。制备了CL厚度为30~35μm的连铸机。MEAS的活动面积为0.10 cm(0.8 mm×12 mm)。采用SIGRACET24BC(SGL炭)作为气体扩散层(GDL)，一侧有微孔层(MPL)。通道的宽度和深度分别为1.0 mm和0.5 mm，肋骨与通道的比例为1。操作条件如表1所示。观察是在肋骨区域进行的。图像以1fps的速度拍摄，为了减少随机噪声，使用了积分时间为32 S的图像。图1显示了MEA的高倍率软X射线照片。由于化学成分和层密度的不同，MEA的每一层都可以通过衰减差异清楚地识别出来。在OCV和发电过程中进行现场观察。在加载开始时，我们观察了膜的肿胀。为了避免膜膨胀的影响，在30 S时获得的图像被操作后期的图像减去，以显示产生的液态水。图2显示了在210 S负荷后以30进行操作的MEA的减去图像。图像显示了在0.40和0.60A/cm两种操作条件下阴极侧积累的液态水。CL和GDL中的液态水随着电流密度的增加而增加。在MPL/GDL边界处观察到大量的液态水，而在MPL中观察到少量液态水。图3显示了在50℃时在MEA中观察到的液态水。50℃时的液态水含量比30℃时要少。在0.40A/cm下运行的MEA中几乎没有液态水。然而，在电流密度为0.60A/cm时，在GDL中观察到了积累的液态水。在较高的温度下，较少的液态水表明，在较高的温度下反应产生的水大部分以蒸汽的形式传输。
In proton exchange membrane fuel cells (PEMFCs), appropriate water management is critical to achieving a high power density and increased their robustness. The membrane requires sufficient humidification to fulfill its function as proton conductor. However, flooding at cathode side can hamper the transport of reactant and reduce the reactive area. Understanding of liquid water transport in electrolyte membrane assembly (MEA) is important to design an effective fuel cell. Several techniques have been developed for visualizing of liquid water in the MEA. However, it is necessary to improve the spatial and temporal resolution to observe the liquid water transport at micro scale. Recently, we developed soft X-ray radiography and succeeded in visualizing of liquid water in the PEMFCs operating at room temperature [1-3]. In this study, we aim to investigate the liquid water transport in the MEA at the temperature closed to the practical operating condition. The visualization of liquid water in the operating PEMFCs was performed by using laboratory-based soft X-ray microscope system (Tohken, TUX-3110FC). We used a thin-film of tungsten on diamond window as the target material for generating X-ray with the photon energy of 8.4–10 keV. The X-ray tube voltage was set at 18–20 kV. The catalyst-coated membrane (CCM) was fabricated using a transfer printing method. The catalyst layer (CL) with Pt loading of 0.22–0.23 mg.cm was decaled on to the perfluorinated sulfonic acid membrane (Nafion® EC NRE 212, 50 μm-thick) by hot pressing. A CCM with CL thickness of 30–35 μm were made. An active area of the MEAs was 0.10 cm (0.8 mm x 12 mm). SIGRACET24BC (SGL Carbon) was used as gas diffusion layer (GDL) which has micro porous layer (MPL) on one side. The channel width and depth were 1.0 and 0.5 mm, and the rib-to-channel ratio was 1. The operating condition is shown in Table 1. The observation was performed in the rib area. The images were taken at 1 fps and the images with an integration time of 32 s were used in order to reduce random noise. Fig. 1 shows the soft X-ray radiograph of the MEA at high magnification. Each layer of the MEA is clearly identified by attenuation differences due to chemical constituents and layer densities. The in situ observation was performed at the OCV and during power generation. We observed the membrane swelling at the beginning of load. To avoid the effect of membrane swelling, the image obtained at 30 s was subtracted by the images at the later operating time to show the generated liquid water. Fig. 2 displays the subtracted images of the MEA operating at 30 after 210 s of load. The images reveal the accumulated liquid water in the cathode side in both the operating conditions at 0.40 and 0.60 A/cm. The liquid water in the CL and GDL increased with increasing current density. A large amount of liquid water was observed at the boundary of MPL/GDL toward the rib, while small amount of liquid water was observed in the MPL. Fig. 3 shows the observation of liquid water in the MEA operating at 50 . The amount of liquid water at 50 was less than that at 30 . There is almost no liquid water in the MEA operating at 0.40 A/cm. However, the accumulated liquid water in the GDL was observed at the current density of 0.60 A/cm A smaller amount of liquid water at higher temperature suggests that much of water generated from the reaction at higher temperature transport as vapor.