Nanostructured Thin Film Electrolyte for Thin Film Solid Oxide Fuel Cells
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Solid oxide fuel cells (SOFCs) are very attractive as energy generation devices because they are clean, reliable, and almost entirely pollution-free. SOFCs have flexible fuel selections compared with other fuel cell technologies. The main disadvantage of SOFCs is their high operating temperature (~1000?C for conventional SOFCs) which leads to cell cracking and formation of non-conducting compounds at electrolyte/electrode interfaces. Therefore, intermediate temperature SOFCs (ITSOFCs) in the range of 500-700 ?C has attracted extensive research interests. To achieve high cell performance at reduced temperatures, it requires high-catalytic activity, high ionic conductivity, and comparable thermal expansion coefficient (TEC) of the cell components. To address the above issues, the research focuses on two main approaches (i.e., the interlayer approach and the electrolyte approach) in order to improve the overall cell performance. First, the design of a thin layer of a vertically-aligned nanocomposite (VAN) structure as an interlayer between the electrolyte and cathode is demonstrated. The development of the VAN structures consisted of the cathode material as a perovskite or ordered double perovskite structure, La0.5Sr0.5CoO3 (LSCO) or PrBaCo2O5 delta (PBCO), and the electrolyte material as a fluorite structure, Ce0.9Gd0.1O1.95 (CGO or GDC), were achieved for thin film solid oxide fuel cell (TFSOFCs). The VAN structure significantly improves the overall performance of the TFSOFC by increasing the interfacial area between the electrolyte and cathode and also acts as a transition layer that improves adhesion and relieves both thermal stress and lattice strain. Second, microstructural and electrical properties of Gd-doped CeO2 (GDC, Ce0.9Gd0.1O1.95) thin films electrolyte are studied for intermediate temperature solid oxide fuel cells (SOFCs). The GDC thin film electrolytes with different grain sizes and grain morphologies were prepared by varying the deposition parameters such as substrate temperature, oxygen partial pressure, target repetition rate, and laser ablation energy. The electrical property of the GDC thin film is strongly affected by the grain size. Third, bilayer electrolytes composed of a gadolinium-doped CeO2 (GDC) layer (~6 micrometer thickness) and an yttria-stabilized ZrO2 (YSZ) layer with various thicknesses (~330 nm, ~440 nm, and ~1 micrometer) are achieved by a pulsed laser deposition (PLD) technique for thin film solid oxide fuel cells (TFSOFCs). One effective approach is to incorporate YSZ thin film as a blocking layer in between the GDC and anode for preventing chemical reduction of GDC and electrical current leakage. This bilayer approach effectively improves the GDC's chemical/ mechanical stability and reduces the OCV loss under reducing conditions. The results suggest that the YSZ thin film serves as a blocking layer for preventing electrical current leakage in the GDC layer and also provides chemical, mechanical, and structural integrity in the cell, which leads to the overall enhanced performance.