Characterization of Cerium Dioxide Thin Films Prepared By Plasma-Enhanced Atomic Layer Deposition for Solid Oxide Fuel Cells

Abstract

Solid oxide fuel cells (SOFCs) are considered to be the next generation of energy conversion devices because of their environmental friendliness and high efficiency. Particularly due to its high operating temperature, it has high fuel flexibility and does not require water management. However, the high operating temperature that causes these advantages accelerates the degradation of fuel cell components, thereby reducing the durability and limiting the usable materials. Therefore, in recent years, a lot of research has been conducted to lower the operating temperature of the SOFCs. One method of lowering the operating temperature of SOFCs is to apply an electrolyte having good ionic conductivity at low temperatures(400-600℃). Potential examples are doped ceria materials. One of the well-known ceria-based electrolytes is Gd-doped ceria (GDC), which exhibits ion conductivity 1-2 times higher than yttria stabilized zirconia, YSZ, which is the most commonly used electrolyte in SOFC. GDC has good ionic conductivity, but when exposed to a reducing environment, Ce4+ is reduced to Ce3+ and begins to have electronic conductivity, which causes a decrease of open circuit voltage (OCV). Therefore, in order to apply GDC as an electrolyte, an additional blocking layer must be added so that it is not exposed to the hydrogen environment of the anode. This additional layer increases not only the thickness of the electrolyte but also the interfacial resistance between the layers, thereby increasing the ohmic resistance. Therefore, the fabrication of GDCs with thinner and better surface properties is critical to improving the performance of low temperature SOFCs. In this study, CeO2 thin film was fabricated using atomic layer deposition (ALD) as a preceding step for the fabrication of GDC thin films. Oxygen plasma was used to oxidize the precursor to widen the ALD window and to produce the highly crystalline CeO2. The growth rate of CeO2 was determined by controlling precursor and oxidizer exposure time, and the thickness of thin film per cycle was confirmed. The surface morphology was observed by scanning electron microscope (SEM), and dense and pinhole-free thin film surface was confirmed. In addition, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses were performed and the characteristic crystalline peaks and chemical compositions were compared with those of the thermal ALD CeO2 thin films.