가스 확산 층 열화에 의한 고분자 전해질막 연료전지의 성능 저하 연구

Abstract

Greenhouse gases such as carbon dioxide emitted from the reckless use of fossil fuels have accelerated global warming. Countries around the world are conducting various researches on new and renewable energy to replace fossil fuels to solve the various environmental problems caused by global warming. Among the different renewable energy technologies, polymer electrolyte membrane fuel cells (PEMFC) have high energy density, high output efficiency, and no emission of pollutants, attracting significant attention as future energy in various industrial fields. However, PEMFC has not yet become popular due to its high price and short lifetime. PEMFC converts chemical energy to electrical energy, which reduces the durability of internal components due to electrochemical degradation during operation, resulting in reduced operating performance. PEMFC's component degradation and reduced performance are closely related to water management controlled by the hydrophobicity of the gas diffusion layer (GDL). GDL's substrate and micro porous layer (MPL) gradually deteriorate as the operation time of the fuel cell increases, reducing hydrophobicity and cause flooding and drying, which lowers the overall efficiency of PEMFC. Currently, PEMFC research is focused on improving the performance of the electrolyte membrane where the electrochemical reaction takes place and the catalyst layer containing platinum. However, as the drying and flooding phenomenon caused by the failure of water management accelerates the degradation of other parts and reduces the performance of the PEMFC, various studies need to be conducted on the cause and improvement of the performance of GDL, which plays the role of water management. In this study, an accelerated decomposition method using hydrogen peroxide was used to investigate the effect of GDL degradation. To confirm the characteristic change of GDL due to the degradation of GDL, the contact angle of the surface and the pressure change during water injection were measured and compared in real time. GDL samples with different degradation times were combined into a single cell to observe the performance degradation of PEMFC. In addition, to analyze the cause of the performance decrease, the change in the two-phase flow in the channel due to the deterioration of the GDL was observed through a specially manufactured visualization cell. The flow occurring during the operation was captured and analyzed with a high-speed camera. The performance measurement cell and visualization cell were fabricated with fresh parts except for the degraded GDL of the cathode to observe whether the degradation of GDL affects the performance of the PEMFC. The experimental results showed that the performance of a single cell decreased even though only the GDL was degraded, and the maximum power density and current density decreased according to the degree of degradation of the GDL. This means that the maximum power and operating time of the cell have been reduced. Due to the decreased hydrophobicity and the degradation of GDL, the contact angle of the surface gradually decreased with the degradation time. The water pressure required to penetrate the surface of the GDL also gradually decreased, and it was confirmed that the time taken to reach saturation inside of the GDL is getting shorter. This means that the polytetrafluoroethylene (PTFE) added for hydrophobicity inside the GDL is gradually lost over time, making it easier for water to pass through. Due to these reasons, as the GDL was degraded, the two-phase flow shape in the channel changed from the droplet shape to the annular flow. After the flow, the amount of water and residence time remaining on the surface of the GDL and the ribs of the channel gradually increased. The increase in the residence time of water in the channel and clogging of the internal pores due to the decrease in the hydrophobicity of the GDL prevents oxygen supplied as a reactant from diffusing toward the catalyst layer of the polymer electrolyte membrane, thereby increasing the mass transport loss. Due to these losses, the number of reactants required for a chemical reaction decreased as the current density increases, resulting in reduced performance.

Keywords: Proton exchange membrane fuel cell (PEMFC), Gas diffusion layer(GDL) degradation, Flow visualization, Reduced hydrophobicity, Mass transport loss