Oxide Ionic Transport and Surface Kinetics in Nanocrystalline Ceramic Electrolyte for Thin-film Solid Oxide Fuel Cells

Abstract

Development of technology and advanced society accelerating depletion of fossil fuels and emission of pollutant are still issued in recent days. Thus increased concerns with replacement of conventional energy are significantly received attention especially renewable energy such as wind, solar energy and fuel cells. Among the renewable energy, solid oxide fuel cells (SOFCs) are greatly considered as a most promising energy conversion device due to their high efficiency of conversion rate, no-emission of pollutant and scalability of conversion system. Generally, the solid oxide fuel cells have been operated in high temperature region (800-1000oC) since ceramic electrolyte which used in solid oxide fuel cells has reasonable oxide-ionic conductivity and electrode/electrolyte reaction kinetics in this temperature regime. However, this high temperature have occurred thermal issues resulting in fast catalyst degradation, selection of materials and difficulty of gas tightness. Therefore many researches are conducted to reduce the operating temperature up to 300-500oC by using a method to decrease the thickness of electrolyte on porous supporting structure, however, this temperature regime adversely arouse sluggish ionic transport in electrolyte and oxygen reduction reaction at electrolyte surface. Thus this dissertation is composed with three main topics regarding the nanoscale thin-film engineering: fabrication of nanoscale-dense ceramic electrolyte on porous supporting structure, increasing electrode/electrolyte interfaces kinetics with a grain controlled layer and scaffolding effects of electrode with oxide capping layer. The first work in this dissertation is a fabrication of thin-film electrolyte as dense enough to using solid oxide fuel cells without defects (i.e. pinholes). This topic describes the fabrication process of thin film via sputtering and investigates restrained columnar grains in YSZ electrolyte to verify the phenomenon of suppressed pinholes generation resulting in dense morphology of electrolyte, which is required in porous supporting structure. Through this work, the restrained columnar grain of YSZ electrolyte demonstrates increased open circuit voltage (OCV) and high performance as employing solid oxide fuel cell due to pinhole-free characteristic in electrolyte. This result can be considered to fabricate thin-film electrolyte on porous supporting structure and design the membrane-based energy conversion device to enhance dense morphology of thin-film, especially performance of solid oxide fuel cells. The second topic of research is new-concept cathode functional layer for electrode/electrolyte interfaces by depositing surface engineered layer. This research describe the concept of grain controlled layer (GCL) which enhance the grain development and boundary density believing in favorable oxygen incorporation sites. The controlled bias voltage during the sputtering process exploits the thin-film YSZ electrolyte having different development of crystalline phase and grain size. It is confirmed via deduced exchange current density that enhanced crystalline phase (better grain boundary density) of YSZ thin film able to enhance oxygen incorporation rate at electrode/electrolyte interfaces by accelerating cathodic surface kinetics. The fuel cell adopted GCL is also revealed that the enhanced grain boundary density of GCL at cathode/electrolyte interfaces enhanced the performance of fuel cell in terms of peak power density up to1.5 folds than pristine fuel cell. This result of grain controlled layer also contribute outstanding performance enhancement for low-temperature solid oxide fuel cells. We investigate the last topic of this dissertation regarded in scaffolding electrode for solid oxide fuel cells. It is regarded that ceria-based materials for electrolyte have better oxide ionic conductivity than YSZ electrolyte. Thus this study employs the gadolinia-doped ceria (GDC) as an oxide-capping layer for Pt cathode, which have been considered as vulnerable material for thermal durability despite of low temperature region (300-500oC). The electrochemical performance of GDC-oxide capped fuel cells indicates the highest peak power (258 mW/cm2) density with 15-nm thickness oxide-capping layer. In addition, the 50-nm thick GDC oxide-capped fuel cell successfully performs with enhanced thermal durability during the 25-hours operation at 450oC with 56.4% of thermal degradation. These results provide an opportunity to improve the thermal durability, as well as enhance the performance of solid oxide fuel cells by surface engineered with nanoscale ultra thin-oxide capping layer. All these achievements and investigations were devoted to enhance the performance and durability of solid oxide fuel cells at low temperature regions (300-500oC) by using dense morphology of electrolyte, catalyzing oxygen reduction reaction (ORR) at electrode/electrolyte interfaces and scaffolding cathode material. We expect that these accomplishments surely contribute to the designing for solid oxide fuel cells with nano-science and engineering.