Atomic Layer Deposited Ceria and Zirconia-Based Thin Films for Solid Oxide Fuel Cells Applications
thesisposted on 29.10.2016 by Jorge I. Rossero Agudelo
In order to distinguish essays and pre-prints from academic theses, we have a separate category. These are often much longer text based documents than a paper.
This Ph.D. thesis shows the design, fabrication and analysis of solid electrolyte materials that can potentially be used to develop practical and cost-effective set-ups of solid oxide fuel cell stacks operating below 800 °C (IT-SOFC). In this thesis, CeO2-x and yttria doped-CeO2-x (YDC) were deposited via atomic layer deposition (ALD) using novel precursor combination. Under optimized ALD conditions, the growth-rate of CeO2-x and YDC films is 3 times higher than the one reported using β-diketonates precursors and ozone. Characterization show that CeO2-x have a non-stoichiometry gradient, with surface Ce+3 amounts ranging from 10 to 25 %. Evidence suggests tha the analyzed films have bulk-like characteristics and a significant density of point defects. These finding imply that ALD can be an effective way to engineer stable CeO2-x films with varying structures and with large density of point defects not attributed to dopant atoms. YDC films are deposited with precise composition and thickness control which became a very attractive way for scaling down the SOFC electrolyte. It was found that YDC films are stable under reducing atmosphere and that there are doping and thermal induced structural transformations that are relevant to the physical properties that affect the ionic conductivity. YSZ films are studied, at unexplored thickness levels, to better understand thermal and doping induced structural transformations and to relate the physical properties of YSZ at this thickness scale to electrochemical processes. YSZ films were deposited using novel precursor combination. There are doping and thermal induced structural transformations that may be relevant to the electrochemical processes. The lattice parameter obtained for cubic YSZ ultra-thin film is lower than that estimated for cubic YSZ single crystals at the same composition. Activation energy for conduction at low temperatures was similar to that calculated for bulk YSZ films. YSZ/YDC composites were deposited using the same ALD set-up for establishing an atomically defined interfacial region. The composites have an oxygen vacancy gradient as determined by energy dispersive X-ray spectroscopy and electron energy loss spectroscopy. This gradient can progressively provide exchanger of oxygen ions among surface, lattice and most importantly improve the kinetics at the YSZ/YDC interface.