Operando optical studies of next generation anode materials in high temperature solid oxide fuel cells
Welander, Martha Maria
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Solid oxide fuel cells (SOFCs) are high temperature energy conversion devices capable of efficient and sustainable energy production. Because of the need to electrochemically reduce molecular oxygen and the relatively high activation energy required for oxide ions to diffuse through the dense, solid-state electrolyte, SOFCs typically operate at temperatures > or = 500 °C. High operating temperatures endow SOFCs with many advantages, including fuel flexibility and high conversion efficiencies, distinguishing them from other types of fuel cells. However, high temperatures also present challenges related to the stability of the electrode materials, accelerating cell degradation and limiting the development and integration of SOFCs into large scale power production strategies. These mechanisms are the result of fundamental changes in material properties that remain poorly described and difficult to predict. Studies presented in this work utilized operando Raman spectroscopy and electrochemical measurements to directly correlate material changes with changes in cell performance under various operating conditions. Research focused on developing and characterizing new electro-catalytic materials having improved conversion efficiencies and mechanical resilience to thermal and chemical stress. Because current state of the art SOFC Ni-YSZ cermet anodes are sensitive to oxidation, the first two studies investigated the effects of adding small amounts of Al 2TiO 5 to Ni-YSZ anodes and the impact of resulting secondary (2°) phases that formed on SOFC tolerance to electrochemical and environmental reduction and oxidation (redox) cycling. Results show that Al 2TiO 5 helps improve tolerance to both types of redox cycling by maintaining electrode-electrolyte connectivity and minimizing catalyst coarsening. The third study illustrates how this same dopant improved anode carbon tolerance when operating with hydrocarbon fuels. Because excessive carbon accumulation on SOFC anodes can lead to rapid cell failure, ways to improve carbon tolerance was further explored in the last two studies. These studies investigate the effect of decoupling the electro-catalytic and the electronically conductive phases of the anode under pure methane and biogas-surrogate environments. Collectively, the studies described in this dissertation provide insight into the materials-specific mechanisms responsible for limiting degradation of doped and functionally decoupled anodes to help guide the design of new SOFC electrode materials.