전극의 미세구조 제어에 의한 프로톤전도성 고체산화물 연료전지 성능향상 연구

Abstract

Recently, interest in fuel cells has increased, efforts are being made to apply fuel cells in various industrial fields. However, despite the high performance and efficiency of solid oxide fuel cells (SOFCs), SOFCs are limited in their use. The main reason is cost problems due to high operating temperatures and long-term stability. Although many researchers have been studied to lower the operating temperature, the reactivity of the cathode also decreases with such low temperature, resulting in a disadvantage that the overall performance is degraded. In this paper, proton conducting solid oxide fuel cell (PCFC) was studied for intermediate temperature fuel cell and various cathode structures were proposed to overcome the long term stability problem. In the PCFC, water as a reaction product is generated on the cathode side. When the generated water, remained in the cathode, is not discharged to the outside, the electrolyte or cathode material reacts with this water vapor and generate an undesired secondary phase. This secondary phase increases the resistance component of the fuel cell and, when deepened, causes delamination of the electrolyte and the anode. Therefore, many researchers have been carried out to suppress the reactivity of the electrolyte and the cathode with water. However, studies in this paper have developed methods for improving the long - term stability by facilitating the discharge of water generated in the cathode through the change of the cathode structure, and introduced an easy and simple process using the deposition technique called the electrostatic slurry spray deposition (ESSD). In chapter 3, the cathode of dual porosity structure with both macro pores and micro pores was deposited using by the ESSD. It is a structure for allowing the moving of water vapor through the large pore paths and the oxygen gas to reach the inside of the cathode well through the micro pore paths. The single cells fabricated with dual porosity structure; Honeycomb structure and snowball structure. These single cells with honeycomb and snowball structure has excellent durability through the long term stability evaluation of 100 hours showed only 5% and 2% performance degradation, respectively. In Chapter 4, the cathode of the gradient porosity structure was deposited using the ESSD with multi-nozzle. This gradient porosity structure is designed so that the water vapor generated from the inside of the cathode get out to the outside. The ESSD of the multi-nozzle is a device designed to deposit gradient structure by spraying different slurry flow rates. The unit cell with porosity gradient cathode has high performance and also excellent durability with performance degradation within 2% in the long-term stability evaluation of 100 hours. In the last chapter 5, the possibility of making a compositionally graded structure cathode by ESSD with multi-nozzle was evaluated. Recent studies of the composition gradient structure were about improving the performance by deposition several layers with different cathode composition. These studies have the disadvantage of having a lot of time in the process (deposition and heat treatment, etc) and the thickness limitation. In this chapter, ESSD process has advantages of shortening the process time and thickness by depositing the cathode with the only one process. The cathode composition consists of a gradually increasing cathode materials and decreasing electrolyte contents from the underlying dense electrolyte to the current collector on the top. In this structure, the ohmic resistance and the polarization resistance are greatly reduced compared with the general cathode, and the performance is also excellent. In conclusion, this study has a great advantage that the performance of the PCFC can be improved only by the structural modification, not the development of the electrolyte and the electrode component or the doping. The structure of the cathode has been successfully changed by an easy and simple method using an ESSD, and it is a promising structure that can be used in future fuel cell applications.