Study on localized ion transportation phenomena in YSZ at high temperature

• Main Author: Ng, Chee Seng
• Other Authors: Su Pei-Chen
• Format: Theses and Dissertations
• Language: English
• Published: 2017
• Subjects: DRNTU::Engineering::Materials::Ceramic materials; DRNTU::Engineering::Mathematics and analysis::Simulations; DRNTU::Engineering::Materials::Material testing and characterization
• Online Access: http://hdl.handle.net/10356/69961
• Institution: Nanyang Technological University

Yttria stabilized zirconia (YSZ) is a popular solid oxide electrolyte material for solid oxide fuel cell (SOFC) due to its high ionic conductivity and high stability properties at high operating temperature. Much of the research effort has been focused on lowering the operating temperature of SOFC for wider material selection and reduction on the thermal related damage when operating at high temperature. The drawbacks of reduction of operating temperature in SOFC include increase in the ionic resistivity of the electrolyte and decrease in reaction kinetic of the fuel cell compared to higher operating temperature. One of the strategy to reduce the ionic resistance of the electrolyte is through reduction of the electrolyte layer thickness. Thin film YSZ with thickness in the range of nm to µm can be used to improve the ionic conductance of the electrolyte layer. There has been reports on variation of ionic conductivity in nanocrystalline structure of YSZ thin film where grain boundary in YSZ has played an important role in ionic transport of oxygen ions in YSZ electrolyte. Reported studies confirmed that diffusion of oxygen ion across the grain boundary is slower than in the bulk region. However, there is not much work on verifying the contribution of ionic transport parallel to the grain boundary in nano thin film YSZ. The hypothesis of this work is that the diffusion of atom in grain boundary region could be faster due to higher defect concentration. The aim of this work is to investigate on the existence of fast ionic conduction for oxygen ion diffusion along the grain boundary in YSZ electrolyte. In this work, atomic force microscopy (AFM) conductive probe was used as working electrode for high temperature electrochemical studies of the thin film YSZ electrolyte while resolving the grain boundary feature of thin film YSZ. For electrochemical characterization of the grain boundary of YSZ, electrochemical impedance spectroscopy (EIS) were applied to analyze the ionic conductivity of the thin film. High temperature beyond 500 °C is needed for EIS measurement with ionic resistance below 100 MΩ at contact radius of 100 nm, due to instrument limitation of 1 TΩ impedance measurement. Due to unavailable commercial AFM high temperature measurement beyond 250 °C, a MTS was designed and fabricated as a heating solution for thin film YSZ electrolyte to be compatible in conventional AFM setup. Pt-metal AFM probes were fabricated for better resistance towards thermal degradation when in contact with high temperature thin film surface. Characterization were performed for the MTS and Pt-metal tip. Molecular dynamic (MD) simulations were used to understand and predict the behavior of oxygen ion transport YSZ with the presence of grain boundary. Two approaches were used which target at the local variation and regional variation of properties in YSZ simulation cell. The first approach calculates the number of oxygen vacancies and the oxygen ion jump rate at every possible oxygen lattice location. In the second approach, electric field was applied parallel to the grain boundary and the ionic transport of the oxygen ions at different regions were calculated. As conclusion, the MD simulations results so far pointed toward blocking behavior of grain boundary in ionic conductivity in all direction. For local characterization of electrochemical properties using AFM, preliminary results have demonstrated the viability of such approach. In future, experimental works on local measurement of ionic conductivity of YSZ can be performed to support the prediction from the MD simulation.