Thin-Film Gd-Doped Ceria for Electrolyte of Micro-SOFC

Abstract

Solid oxide fuel cells (SOFCs) are the highly efficient devices that convert chemicals energy into electricity directly with low pollutant emission. Conventional operating temperature of SOFC is 800~1000 oC, however, micro-SOFCs that operate at reduced temperature (400~600 oC) have been studied recently, and are considered as a promising power source for portable electronics due to many advantages such as higher energy density than conventional rechargeable battery. Low operation temperature of micro-SOFC has many advantages including quick startup, easy heat insulation, and flexibility of component material. However, reducing operation temperature is not easy since the Ohmic resistance of electrolyte increases exponentially as the operating temperature decreases. One of the strategy for solving this problem is fabricating the electrolyte in thin film form by reducing Ohmic resistance of electrolyte. One other way is introducing Gd-doped ceria (GDC) as an electrolyte which has higher electrical conductivity than that of conventional yttria-stabilized zirconia (YSZ). In the series of studies in this dissertation, GDC thin film is considered as a candidate electrolyte material of micro-SOFC. The goal of this dissertation is to use GDC thin film for electrolyte of micro-SOFC. To assess the feasibility of GDC thin film as an electrolyte, the factors affective to the electrical conductivity of GDC film should be studied. In Part 1, GDC (Ce1-xGdxO2-δ, x=0.14~0.16) thin-films were deposited on sapphire substrate by RF-magnetron sputtering and their electrical conductivities of films were measured as a function of temperature (T=300~500oC) and oxygen partial pressure (Po2). All films showed columnar grains with crystallite size of 18~36 nm. The films annealed at high temperature and deposited from dense target exhibited their ionic conductivities similar to or higher than bulk GDC. The electronic conductivities of GDC films shown in low Po2 range, however, were higher than that of bulk GDC. From the results of Part 1, GDC films are deposited on conductive substrates by RF-magnetron sputtering and their across-plane electrical conductivities (perpendicular to film plane) are measured as a function of temperature (300 ≤ T ≤ 400 °C) and oxygen partial pressure (Po2) to assess the feasibility of their use as an electrolyte in miniaturized solid oxide fuel cells (micro-SOFCs). The films deposited at relatively high T = 500 °C, deposited on either dense or porous substrate, show columnar grains with diameter 200 nm. The magnitude of conductivities in air are similar to those of pellet-form GDC. However, the electronic conductivities of GDC films in low Po2 are higher than that of pellet. Finally, in part 3, micro-SOFCs with thin-film components are designed and fabricated. A single cell with GDC electrolyte deposited by RF-magnetron sputter on metal/glass substrate shows high open circuit voltage (OCV) value (1.12 V at 400 oC), close to the theoretical voltage. Porous Ni-alloy film coated on a glass is used as a substrate and the glass is etched for gas flow. The metal/glass support layer enables large cell area, ~5 mm in diameter. Although the cell shows poor polarization resistance of electrodes, the performance of micro-SOFC can be improved by modifying the electrodes. Micro-SOFC using GDC electrolyte was fabricated successfully, and several improvements on electrodes also suggested. In conclusion, this study contributes to the SOFC society by introducing the factors affective to the electrical conductivity in GDC film, fabrication of good quality of GDC film, and new cell design with GDC electrolyte as well as robust structure.