기체 확산층의 불균질한 공극율의 분포 및 압축이 고분자 전해질 막 연료전지의 물 분포와 성능에 미치는 영향

Abstract

Polymer electrolyte membrane fuel cells (PEMFC) are a low-pollution, high efficiency energy system which attracts great attention as a next generation automotive power source. The hydrogen supplied by the anode of a fuel cell is separated into hydrogen ions and electrons at the electrode. The electrons move to the cathode of the fuel cell through external circuits and the ionized hydrogen passes through the electrolyte to chemically react with oxygen and electrons at the cathode of the fuel cell thereby generating water as a product. The generated water is accumulated in the electrolyte and electrode to affect the performance of the fuel cell. As such, water in fuel cells is a variable that has a major effect on fuel cell performance. Therefore, many researchers conduct theoretical-experimental studies of water management inside fuel cells. To view water distributions in fuel cells, studies of non-destructive visualization methods using neutrons and X-rays are being conducted and neutron imaging methods are frequently used in visualizing water distributions in fuel cells because these methods can visualize the front side of fuel cells. However, the spatial resolutions of neutron imaging methods range from several ten to several hundred μm which are insufficient to visualize internal structures and water distributions in through-planes of fuel cells. However, X-rays from synchrotron accelerators have spatial resolutions ranging from several hundred nano to several μm and thus are suitable in visualizing the internal structure and water distribution in through-planes of fuel cells. Recently, a study result was reported indicating that the porosity of GDLs was found to be not uniform in through-planes using the X-ray visualization method of synchrotron accelerators. In addition, changes in heterogeneous porosity distribution (HPD) caused by fuel cell fastening are expected to affect mass transfer in GDLs thereby affecting water distributions inside fuel cells and fuel cell performance.In this study, experimental studies and numerical studies of the effects of changes in HPD caused by GDL compression on water distributions inside fuel cells and fuel cell performance were conducted. Using X-ray tomography technology that used the 7B2 X-ray microscope system of a Pohang synchrotron accelerator, changes in the HPD of GDLs according to GDL compression ratios were measured. In order to figure out the effect of the HPD on water distributions inside GDLs, the water distributions inside GDLs were predicted using 1D numerical analysis. Through comparison of the results with experimental results, it could be identified that this analysis predicted experimental results better than analysis where uniform porosity was applied. By analyzing water distributions inside GDLs in relation to GDL compression ratios, it could be seen that when water influx remained constant, as GDL compression ratios increased, water distributions inside GDLs decreased.To analyze the effect of GDL compression on fuel cell performance and water distributions, water distributions in a fuel cell actually in operation were measured. To this end, small cells for X-ray visualization were made and a fuel cell operation evaluation system was installed in Pohang synchrotron accelerator 7B2. Using X-ray radiography techniques, water distributions in the fuel cell in operation and its performance according to GDL compression ratios were visualized. To analyze the effect of the HPD of GDLs on fuel cell performance and water distributions, 2D numerical analysis was conducted. Based on the results of the numerical analysis, it could be seen that the HPD of GDLs affected internal water distributions and oxygen diffusion and in particular, accelerated rapid performance declines in high current density regions. Of accuracy can be improved through works to additionally improve the fuel cell analysis model applied with the HPD of GDLs, it will be very helpful to fuel cell designs and studies for optimizing GDLs.