Solid oxide fuel cells (SOFCs) are the highly efficient devices that convert chemicals into electricity with non-toxic byproducts such as water and carbon dioxide and have been operated conventionally at 800~1000oC. Recently, micro-solid oxide fuel cells (micro-SOFCs) have been developed and are considered a promising power source for portable electronics due to their high energy density. Miniaturization of the cell components is able to allow the operation at low temperature, under 500oC, by reducing the diffusion length of oxygen ions. However, as the cell components are getting thinner into the sub-micrometer scale, the stability is also inevitably reduced.
The objective of this dissertation is to investigate the ionic transport of acceptor-doped ceria thin film known as a highly-conductive oxygen-ion conductor and understand electrochemical properties of acceptor-doped ceria thin film compared to the bulk. Finally, micro-SOFCs supported by metal with enhanced cell stability are fabricated and evaluated at 450oC.
Part I (Chapter 3) deals with the enhanced ionic conductivity of acceptor-doped ceria thin films by additives. Anode-supported SOFCs using an acceptor-doped ceria electrolyte were studied as model studies to investigate the effect of electrolyte thickness and novel cathode materials for low-temperature operation in Part II (Chapter 4, 5). In Part III (Chapter 6, 7), micro-SOFCs using metal support were fabricated and evaluated its electrochemical properties.
For acceptor-doped ceria, thin films have usually shown lower ionic conductivity compared to the bulk. In Chapter 3, the effect of additives (1 mol% of Ga2O3 or Al2O3) has been studied in terms of the film quality and the magnitude of the ionic conductivity of Gd-doped ceria (GDC).
As model studies, It was found that the thickness of a GDC electrolyte have a significant effect on the OCV (Open Circuit Voltage), impedance spectra, I-V (current-voltage) and I-P (current-power density) curves of anode-supported SOFCs (Chapter 4) and the single-phase GdBaCo2O5+x (GBCO) and GBCO-Gd0.2Ce0.8O1.9 (GDC) composites (10, 30 wt. %) were tested as a cathode material on the anode-supported SOFCs for the low temperature operation (Chapter 5). Micro-SOFCs were successfully fabricated and evaluated on a metal substrate. In Chapter 6, ~1 μm-thick Gd-doped ceria on a nano-porous nickel film was fabricated to assess whether a thin-film, metal-supported GDC can be deposited as a pore-free layer and would thus be suitable as an electrolyte of micro-SOFCs. Finally, a micro-SOFC using on a porous metallic Ni/stainless-steel bi-layer, which has the similar thermal-expansion coefficient compared to electrolyte materials, is successfully fabricated and evaluated its electrochemical properties at 450oC and it shows good long-term stability up to ~100 h (Chapter 7).