Study on the fabrication process of the anode and metal-supported solid oxide fuel cells using thin-film electrolyte

Abstract

Lowering the operating temperature of solid oxide fuel cell has been investigated for the last 10-20 years in worldwide institutes. Nano-structuring technologies including thin films have shown remarkable performance at relatively low temperature, however, most of works have not been successfully commercialized yet, because of the lack of scalability and high process cost. In order to overcome this problem and achieve marketability, the fabrication process of the thin-film solid oxide fuel cells supported on anode and metal substrates were developed by utilizing materials and technologies which are all scalable and readily available in market. At first, the fabrication process of the thin-film solid oxide fuel cell supported on the conventional anode substrate was developed. To stably deposit the thin-film electrolyte, the anode functional layer (AFL) was designed. In general, an AFL has been introduced to increase a catalytic activity at the anode-electrolyte interface

however, in this study, the purpose of the AFL design was higher density and flatness. By introducing the AFL, approximately 1.3-μm thin-film electrolyte was uniformly deposited. The button-sized cell (2.5-cm diameter, 0.785-cm$^2$ active area) and the large-sized cell (5 cm, 7.065 cm$^2$) were fabricated via developed fabrication process, and the electrochemical performance and microstructure were measured and analyzed. The fabrication process of the thin-film solid oxide fuel cell supported on a metal substrate was also investigated. An all-ceramic solid oxide fuel cell such as an anode-supported cell is intrinsically brittle and show poor mechanical strength against external shock. Thus, solid oxide fuel cells have been applied to the distributed power generation or large power plant, which are all stationary. Metal-supported solid oxide fuel cells are more resistant to external forces and thermal shocks due to high mechanical strength and high ductility of the metal substrate. If the thin-film electrolyte can be applied to these metal-supported solid oxide fuel cells to operate at a relative low temperature, the application area of the solid oxide fuel cell can be extended to portable and mobile applications. Similar to the developed anode-supported cell, the materials and techniques which are all scalable, cost-effective and readily available in market were used in the fabrication process. Since the heat treatment temperature is strictly limited to a maximum of 1000 °C to prevent oxidation of metal substrate, the surface of anode layer is considerably porous and rough. In order to achieve dense and flat surface at restricted atmosphere, nano-Gd0.1Ce0.9O2-δ (GDC) layer was introduced. Nano-GDC solution was prepared by dispersing GDC nanoparticles in GDC nitrate solution. The viscosity of the solution was controlled to coat the surface with high porosity. Approximately 1.3-μm thin-film electrolyte was uniformly deposited by sputtering on the introduced nano-GDC layer. Through the developed fabrication process, a button-sized cell a large-sized cell was fabricated and the electrochemical performance and the microstructure was evaluated and analyzed. The button-sized cells of both anode and metal-supported solid oxide fuel cells showed high maximum power densities at relatively low temperature

however, large-sized cells showed much lower performance than those of button-sized cells. Since the large-sized cell was completely the same as the button-sized cell except for the dimension in the plane direction, and the performance of the button-sized cell and the large-sized cell was measured in exactly the same way, this change in performance is considered to be caused by the high aspect-ratio of the cell. In the large-sized cell, the area to be electrically connected is 9 times larger than that of the button-sized cell, so that the sheet resistance and the contact resistance may be increased by the high aspect ratio. This problem can be effectively enhanced by introducing a current collecting layer with a highly conductive material on each electrode layer. In addition, by preparing effective flow channel at large area of each electrode surface, increased faradaic impedance of large-sized cell would also be resolved.