Author

Yeting Wen

Date of Award

Spring 2019

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Kevin Huang

Abstract

The bulk-to-surface Sr-segregation can seriously compromise the stability of oxygen electrocatalysis in Sr-doped perovskite oxides such as La1-xSrxCoO3-δ and La1-xSrxCo1- yFeyO3-δ and limit their practical applications as cathode materials in solid oxide fuel cells. Although such chemical instability has been actively studied in recent years, fundamental understanding of Sr-segregation process vs temperature and time, particularly under real-world conditions, as well as the suppression method, are still needed. This PhD dissertation aims to acquire fundamental knowledge of Sr-segregation process under practical conditions in solid oxide fuel cells and develop suppression method to ensure the long-term stability through surface modifications.

To gain scientific understanding, the pristine and ZrO2 atomic layer deposition (ALD) modified La0.6Sr0.4CoO3-δ (LSCo) epitaxial films were used as the model system to investigate how the temperature, time and surface coating affect the surface concentration of Sr via in situ synchrotron-based ambient pressure XPS. This information was also correlated with their electrochemical performances characterized by electrochemical impedance spectroscopy. From an engineering perspective, ALD-ZrO2 overcoat was also applied on nano-structured La0.6Sr0.4Co0.8Fe0.2O3-δ (LSCF) to study the effect of Sr-segregation suppression on its electrochemical performance. EIS results showed that the ALD-ZrO2 indeed reduced the performance degradation rate of LSCF and the modification was also optimized by adjusting the thickness of the overcoat. In addition, the Sr-segregation-free SrCo0.9Ta0.1O3-δ (SCT) which is reported with high oxygen permeability and good chemical stability was studied as a multifunctional coating on a benchmark cathode LSCF-GDC (SCT@LSCF-GDC) to enhance ORR activity and mitigate coarsening and Sr-segregation. We also observed significant performance improvement against Cr-poison of SCT@LSCF-GDC composite due to its Sr- segregation-free feature.

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