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Nanostructure and Chemistry Evolution of Solid Oxide Fuel Cells upon Cell Operation By Song Chen Solid Oxide Fuel Cells (SOFCs) are clean and efficient alternatives to conventional power generation technology. In SOFC, electrochemical reactions take place on the triple-phase boundaries (TPBs) between the electrolyte, the electrode and the gaseous fuel. Small changes in TPB structure or chemistry, and the interface between electrode and electrolyte can drastically affect SOFC performance and lifetime. However, very limited experimental work has been reported on the nanostructure and chemistry evolution of interface and TPBs in the SOFC upon cell operation. The main goal of this dissertation is to investigate the nanostructure and chemistry of the grain boundaries and interfaces including TPB in SOFC, and their evolution with respect to the cell operation, and the variation of the fuel chemistry. The nanostructure and chemistry of the SOFC was investigated using Transmission Electron Microscopy. The analysis of microstructural and chemical evolution of anode was focused on the grain boundaries and TPB junctions of Ni/YSZ anode, in the two different aspects detailed in the following. (1). Comparisons were made between an as-received cell and a cell operated at 800°C for several hundred hours using H2 and syngas as fuel. Significant crystallographic evolution, including the formation of previously un-acknowledged NiO interface ribbon phase and the Y migration along the Ni/YSZ interface, was studied systematically. (2). The interaction of trace (ppm) phosphine with Ni-YSZ anode of commercial SOFC has also been investigated and evaluated for both synthesis gas and hydrogen fuels in an effort to examine the reactions between phosphine and YSZ. A principal benefit of SOFC is that it can utilize diverse fuels, including those derived from fossil and biotic sources. At elevated temperatures, SOFCs demonstrate some tolerance to fuel impurities that poison most other fuel cell systems. However, for coal-derived synthesis gas (syngas) under typical SOFC operating conditions, some trace material species may escape gas cleanup processing to impact the SOFC anode performance and long term stability. Among such trace species, phosphorus is of particular interest because the nickel anode is known to rapidly react with volatile phosphorus compounds to form a series of nickel phosphide products, which results in severe depletion of the nickel component and degradation of the anode. The particular focus of the present study is to analyze the interaction of P with YSZ, so as to elucidate the complete set of cell degradation modes induced by phosphine. In this study, a new YPO4 phase was observed at YSZ/Ni/YSZ triple phase junctions, which indicates the reaction between PH3 and YSZ in SOFCs. In addition to the work on the anode, the microstructure and chemistry evolution of the cathode upon cell operation is studied as well. The focus is on the infiltration of cathode which has been proved to be effective in enhancing the cathode performance. Detailed study on the infiltration include crystallinity of infiltrates, chemistry of infiltrates (such as Fe content in the infiltrates), interaction between infiltrates and backbone materials, and their evolutions upon cell operation. The microstructure and chemistry at electrolyte/cathode interface is studied as well. Cations inter-diffusion between YSZ electrolyte and SDC buffer layer is detected, which leads SrZrO3 formation and structure change of YSZ.