나노 결정립계 산화물의 표면 교환 반응 및 이온 전도에 관한 연구

Abstract

Significant development in technology and growing population require more energy to consume. Until now, fossil fuels are the most common and useful resources to produce energy such as heat or electrical energy. However, due to their limited amount of stored fuels on earth, and harmful products as a result of combustion, renewable energy has been widely studied. Among those renewable energy conversion systems, solid oxide fuel cells (SOFCs) have been attracted a great deal of attention due to their high fuel conversion efficiency and environmental friendly emissions. However, due to the high operating temperature (800-1000 oC), many thermal problems occurs which facilitate researcher to develop low-temperature SOFCs (LT-SOFCs). To lower the operating temperature, surface kinetics for oxygen reduction reaction and ion conduction in the electrolyte have to be improved to perform as an energy conversion system. In this study, we successfully enhanced ion conduction and surface kinetics by adopting thin film and analyzing grain boundary role in nanocrystalline oxide materials.

First of all, structural investigation was conducted to reduce ohmic and activation losses by adopting thin film electrolyte and three-dimensional (3-D) interlayer. Since ohmic resistance is determined by electrolyte thickness, investigation of minimizing electrolyte thickness has been conducted. By adopting supporting structure, thin film SOFCs could be fabricated. Atomic layer deposition (ALD) was used for ultra-thin film (~180 nm) electrolyte. For designing porous support, Knudsen flow was applied, and by using simulation result, anodized aluminum oxide (AAO) with 50 nm pores was used as a substrate. The high power density of 380 mW/cm2 was achieved with thin film SOFCs. For reducing activation loss, we enhanced surface area of SOFCs since the number of reaction sites can increase with larger surface area at the interface between electrode and electrolyte. Langmuir-Blodgett method combined by reactive ion etching (RIE) was used, and we successfully fabricated 3-D interlayer. As a result, electrochemical impedance spectroscopy (EIS) shows 2-fold decrease in electrode interfacial resistance.

In addition to the structural study, we also investigated nanocrystalline oxide materials in terms of grain boundary. As a result of thin film deposition, most of the electrolytes have high grain boundary density due to extremely small grain sizes. This high grain boundary density yields extraordinary electrochemical properties compared to microcrystalline materials. One of the significant features of grain boundary is defect chemistry. Due to the loss atomic structure, it has been widely known that grain boundary usually have higher defect concentration such as higher oxygen vacancies and dopants. These higher defects affect electrochemical properties in nanocrystalline oxide materials. First of all, space charge layer which is formed near grain boundary hinder ionic conduction across the grain boundary, and thus ionic conductivity. On the other hand, higher defect concentration positively affects surface kinetics by providing preferential oxygen incorporation sites. Through thermal treatment and spectroscopic study, we carefully investigated the grain boundary property, and found that nanocrystalline oxide materials have random distribution of dopants unlike microcrystalline materials which have local concentration near grain boundary. As a result, ionic conductivity of nanocrystalline materials was comparable or even higher than that of microcrystalline one. Also, randomly-distributed dopants further enhanced surface kinetics by providing more reaction sites that the surface with locally-segregated dopants. Therefore, as-deposited nanocrystalline oxide films would more enhance the performance of SOFCs.