Semester

Spring

Date of Graduation

2021

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Xingbo Liu

Committee Co-Chair

Daniel J. Haynes

Committee Member

Harry O. Finklea

Committee Member

Edward M. Sabolsky

Committee Member

David Mebane

Committee Member

Konstantinos A Sierros

Abstract

Mixed ion and electron conductive materials especially those with perovskite-related structures have been widely used as cathode and anode of solid oxide fuel cells (SOFC). Double perovskite LnBaCo2O6-δ (Ln=Pr, Nd, Sm, Nd, Y) cathodes have been shown better performance than single perovskite Ln0.5Ba0.5CoO3-δ in SOFC cathode studies. However, the effect of different double perovskite phases on oxygen transport properties has not been investigated. In this study, the electrochemical performance of different double perovskite materials PrBaCo2O6-δ (PBCO), NdBaCo2O6-δ (NBCO), YBaCo2O6-δ (YBCO), and single perovskite La0.5Ba0.5CoO3-δ (LBCO) were investigated and compared. The PBCO material was determined to be a potential SOFC cathode with excellent electrocatalytic activity to oxygen reduction. Tetragonal and orthorhombic PBCO perovskite powders are synthesized by controlling the sintering atmosphere and temperatures to study and compare the oxygen transport mechanism. In-situ X-ray Diffraction (XRD) was adopted to characterize the structure of PBCO in the SOFC cathode operation range, and it proves that there is a phase transition that occurred for pristine orthorhombic PBCO when above 550 ºC. The oxygen vacancy in tetragonal PBCO was proved to be disordered by the Neutron Powder Diffraction (NPD) characterization, while that in orthorhombic was ordered distributed. The Selected Area Electron Diffraction (SAED) patterns and the High-Resolution Transmission Electron Microscopy (HRTEM) morphology also tells that PBCO treated in the air has a higher disorder degree than that of treated in nitrogen. The oxygen non-stoichiometry δ, a critical factor to electro-catalytic activity on oxygen reduction, was determined by Thermal Gravimetric Analysis (TGA). Further, thermodynamic quantities were obtained through the solving of the thermodynamic equation. The abnormal partial molar entropy and enthalpy values were related to the complex activation of oxygen vacancy caused by the phase transition, the appearance of a new crystal lattice that has been detected by NPD. The oxygen transport on dense PBCO electrodes has been investigated with the Electrical Conductivity Relaxation (ECR) technique. The study of oxygen bulk diffusion coefficients together with the structure factor from NPD and the thermodynamic property from TGA have promoted the possibility of calculating the migration entropy and enthalpy. The performance of PBCO oxides as SOFC cathodes was evaluated by electrochemical impedance spectroscopy. Both the as-synthesized tetragonal and orthorhombic show good electrocatalytic activity to oxygen reduction, with polarization resistance 0.078 Ω·cm2 and 0.10 Ω·cm2 respectively at 650 ºC. Oxygen bulk diffusion and surface chemical exchange co-limited the oxygen reduction reaction (ORR) on the PBCO cathodes. Besides, oxygen reduction reactions on electrodes treated in nitrogen were retard mainly by oxygen incorporation into the lattice step.

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