Browsing by Subject "cathode"
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Item Nanoengineered Thin Films for Solid Oxide Fuel Cells(2013-11-21) Su, QingSolid oxide fuel cells (SOFCs) are very attractive as energy generation devices because of their high energy efficiency, flexible fuel selections and clean energy conversion. To avoid cell cracking and formation of non-conducting compounds at electrolyte/electrode interfaces issues caused by high operating temperatures (~1000 ?C for conventional SOFCs), intermediate temperature SOFCs (ITSOFCs) in the range of 500-700 ?C have attracted extensive research interests. However, the polarization loss of cathode and ohmic loss of electrolyte significantly increases under reduced temperatures which lead to decreased cell performance and power output. To address the above issues, the efforts in this work are focused on engineering microstructure of cathode, electrolyte and their interface to achieve high performance. First, a bi-layer method has been developed to prepare La0.5Sr0.5CoO3 (LSCO) cathode by combining a pulsed laser deposition (PLD) technique and a screen printing method. It provides a cost-effective approach to fabricate thick and high quality cathode films and the method could also be applied to many other cathode systems. Second, detailed PLD interlayer thickness effect is investigated. The mechanical and electrochemical properties of those hybrid cathodes are examined and correlated with the microstructure of the cells with different interlayer thicknesses. Third, partial oxygen pressure AC impedance study has been carried on those bi-layer cathodes with different interlayer thicknesses. The guidelines for designing high-performance bi-layer cathodes with optimum performance and low cost are proposed. Fourth, the design of a La0.8Sr0.2MnO3-?/Zr0.92Y0.08O2 thin interlayer with a vertically-aligned nanocomposite (VAN) structure between the electrolyte and oxygen electrode is demonstrated for solid oxide reversible fuel cells. The VAN structure significantly improves the overall cell performance and also acts as a transition layer that improves adhesion and relieves both thermal stress and lattice strain. Fifth, Two-phase (Ce0.9Gd0.1O1.95)0.5/(Zr0.92Y0.08O1.96)0.5 nanocomposite thin films with vertically aligned structure are grown as the electrolyte for thin film solid oxide fuel cells (TFSOFCs). More than 50% increase in overall power density is achieved compared with that of the cells without VAN electrolyte.Item Surface Modification of LiNi0.5Mn0.3Co0.2O2 Cathode for Improved Battery Performance(2012-10-19) Lynch, ThomasThis thesis details electrical and physical measurements of pulsed laser deposition-applied thin film coatings of Alumina, Ceria, and Yttria-stabilized Zirconia (YSZ) on a LiNi0.5Mn0.3Co0.2O2 (NMC) cathode in a Lithium ion battery. Typical NMC cathodes exhibit problems such as decreased rate performance and an opportunity for increased capacity exists by raising operation voltage beyond the electrolyte stability window. Very thin (~10 nm) coatings of stable oxides provide a pathway to solve both problems. As well, the electrochemical impedance spectra of the uncoated and coated cells were measured after different numbers of cycles to reveal the property variation in the cathode. Further understanding of the mechanism of rate performance enhancement and chemical protection by thin oxide coatings will continue to improve battery capability and open up new applications. Ceria-coated Li-NMC cells show the best capacity and rate performance in battery testing. Through electrochemical impedance spectroscopy (EIS), the surface film resistance was found to remain stable or even drop slightly after repeated cycling at high voltage. CeO2 is proposed as a coating for Lithium ion battery cathodes owing to its high chemical stability and the demonstrated but not yet well understood electrical conductivity. Alumina-coated cathode shows comparable performance as that of the uncoated cell in the early stage of the test, but through the course of testing the rate capability and recoverable capacity is improved. This is possibly due to Al2O3?s well-known abilities as HF scavenger and chemically inert nature. YSZ-coated cathode performs worse than the uncoated ones in terms of capacity, rate capability, and EIS-related figures of merit. The reason for the poor performance is not yet known, and repeatability tests are under way to verify performance. High voltage cycling reveals no obvious difference in irreversible loss between the coated or uncoated cells. The reason for the lack of distinction could be the relatively small percentage of surface coating compared to the thick doctor-blade processed cathode layer.