고분자 연료전지의 다공성물질 내부 물관리의 정량적 가시화연구

Abstract

Water management of polymer electrolyte fuel cell (PEFC) has been extensively studied because of its effect on the performance of a PEFC system. The transport and congelation of water significantly affect the efficiency and durability of a PEFC. Several empirical methods have been employed to visualize the spatial distribution of water in a PEFC. Experimental studies using high-resolution imaging techniques have been conducted to reveal the unknown morphological aspects that reduce the performance of a PEFC system. The X-ray imaging technique is the preferred method over other imaging techniques because of its high spatial and temporal resolution. Recently, X-ray micro computed tomography (X-ray μCT) is introduced to better characterize the anisotropic structure of a gas diffusion layer (GDL) by reconstructing a three-dimensional structure. With the development of advanced software and hardware, the X-ray imaging technique is essential in the visualization of water management in PEFCs. In this study, three different X-ray imaging techniques are used to visualize water accumulation, temporal evolution, spatial distribution and morphological structure of a gas diffusion layer (GDL). Each imaging technique has its own merits and limitations. Therefore, the imaging technique is selected, depending on the research objectives and experimental conditions. Synchrotron X-ray radiography is employed to visualize the in-plane temporal evolution of water inside the gas diffusion layer (GDL) of an operating (in situ) polymer electrolyte fuel cell (PEFC). A single-cell PEFC test kit is specially designed for the convenient capture of X-ray images. X-ray images of water in the PEFC components, such as the polymer membrane, GDL, and endplate, are captured consecutively. The synchrotron X-ray radiography of high-spatial and high-temporal resolution is suitable for observing the transport of a liquid layer and for visualizing water distribution in in-plane direction of the PEFC. As a result, the spatial distribution of water in the PEFC components is clearly and quantitatively visualized. The temporal evolution of water in the anode GDL because of the back diffusion effect is clearly observed by adopting the image normalization method. The water-saturation characteristics at the cathode GDL, including saturation time and speed, are quite different from those at the anode GDL.In order to understand morphological structure of porous media in a PEFC, a gas diffusion layer (GDL) in a polymer electrolyte fuel cell (PEFC) is quantitatively visualized using synchrotron X-ray micro computed tomography. For three-dimensional reconstruction, adaptive threshold method is used. This method is compared with conventional method, the Otsu’s method. Additionally, the spatial and temporal variations of porosity distribution of GDL under freeze and thaw cycles are investigated experimentally. The freeze and thaw cycles are established simply using CRYO system and light source illumination, respectively. Structural defects are found to largely affect the porosity of GDL. In addition, cyclic porosity variation is observed in GDL under freeze and thaw cycles. The heterogeneous porosity is irreversibly decreased with the progress of repetitive cycles.The water content in the through-plane direction of an operating polymer electrolyte fuel cell (PEFC) is visualized using a tube-based medical X-ray im-aging device. Since the X-ray source emits non-monochromatic beam composed of wide energy, a calibration experiment is performed for the quantitative evaluation of water content. The water content is measured through a normali-zation process, based on the Beer-Lambert’s Law. For X-ray imaging experiment, a specially designed single cell PEFC kit and a chamber are employed. The water thickness of an operating PEFC is quantitatively visualized with varying ohmic resistances. The result shows that the accumulated water volume and the cell temperature exhibit a close relationship at various operational conditions. In addition, the evaluated water volume is verified by comparing with theoretical prediction.