Author

Chunyang Yang

Date of Award

Fall 2020

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Xingjian Xue

Abstract

Solid oxide fuel cells (SOFCs) have attracted considerable attentions due to its high conversion efficiency, fuel flexibility, and clean nature. However, the high cost and poor long-term stability induced by the high operating temperature (> 800 °C) of the conventional SOFCs hinder the further application. Therefore, lowering operating temperature to intermediate temperature range (IT, 650-800 °C) or even low temperature range (LT, 400-650 °C) is of practical importance for realizing SOFCs’ full commercial potential. Nevertheless, due to the large activation energy associated with oxygen reduction reaction (ORR), cathode polarization resistance becomes the dominate loss limiting SOFCs performance as operating temperature is reduced. This dissertation aims to develop cathode materials with excellent catalytic activity and stability for IT-SOFCs and LT-SOFCs.

The research focuses on altering the physical and chemical properties of cathode materials by various methods. Four cathode materials developed with different strategies are systematically investigated with respect to activity and stability. In Chapter 2, Yb doping in the B-site of BaCo0.7Fe0.3O3-δ perovskite successfully stabilize the ORRfavored cubic structure. The lower electronegativity of Yb could induce a slightly lower valance of Co and/or Fe, facilitating oxygen vacancy generation and electrochemical kinetic process. BaCo0.7Fe0.2Yb0.1O3-δ is among the best as a cathode material for ITSOFCs. In Chapter 3, systematic studies of partial substitution A-site Sr with Yb in SrCoO3-d are conducted as cathode for IT-SOFCs. The Yb doping in the A-site brings a

structural evolution and leads to less ordered oxygen vacancies, subsequently highly promotes the catalytic activity of Sr0.90Yb0.10CoO3-d. Furthermore, Sr0.90Yb0.10CoO3-d also exhibits excellent thermal stability as well as CO2 tolerance. Cathode materials for LTSOFCs are investigated in Chapter 4 and 5. SmxBa1-xCo0.8Fe0.2O3-δ nanocomposites is prepared through a thermally induced self-assembled process. The intimate contacts between different phases and nanoparticles decorated on the cathode surface bring beneficial effect on the electrochemical performance, which are systematically studied and presented in Chapter 4. In Chapter 5, the introduction of A-site cation deficiency into Ba1-xCo0.6Fe0.2Zr0.1Y0.1O3-d has effectively improved its kinetic property and stability as a cathode material for LT-SOFCs. This dissertation contributes to the research and development of high-performance cathode materials and provides valuable insight into methodology of altering electrochemical properties.

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