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Document Type

Campus-Only Access for One (1) Year

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Mechanical Engineering

Year Degree Awarded

2019

Month Degree Awarded

February

First Advisor

Stephen S. Nonnenmann

Subject Categories

Catalysis and Reaction Engineering | Ceramic Materials | Condensed Matter Physics | Materials Science and Engineering | Semiconductor and Optical Materials

Abstract

Oxygen vacancy and ion dynamics in functional oxides are critical factors influencing electrical conductivity and electrochemical activity of oxides assemblies. The recent advancements in deposition and fabrication of oxide heterostructured films with atomic-level precision has led to discovery of intriguing physical properties and new artificial materials. While still under debate, researchers most often attribute these observed behaviors to unique oxygen vacancy distributions in the substrate near heterointerfaces. In electroactive oxides devices such as solid oxide cells (SOCs), oxygen vacancy and ion transport at the triple-phase boundary determines the performance of the device. This complex process motivates numerous remaining questions regarding the redox reaction and performance limitation mechanism, both of which are waiting to be addressed.

Improving the fundamental understanding of the aforementioned local physical properties at size scales approaching the mesoscale and nanoscale requires local characterization of oxygen vacancy dynamics and electrochemical activity under operating conditions. A miniature environmental chamber capable of operating at temperature above 500 ℃ and various gas environment was developed and introduced into this in situ research.

I first demonstrated this novel in situ SSPM method to explore the local ionic conductivity / activation energy of electrolyte material in SOC assemblies. These results are subsequently compared to macroscopic electrochemical impedance results and bulk literature values, thus supporting the validity of the approach.

I subsequently extended the high temperature SSPM technique to probe the surface potential variation across a multilayer complex oxide heterostructure films deposited on Nb:doped SrTiO3 substrates. Through classic semiconductor energy band diagram model analysis, the surface potential profiles measured were converted to spatially defined (< 100 nm) [VO.. ] profiles within STO. The results presented herein i) introduce the means to spatially resolve quantitative vacancy distributions across oxide films, and ii) pose the mechanism by which oxide thin film getters both enhance then deplete vacancies within the underlying substrate.

Finally, I made use of the ~100 nm resolution feature of the SSPM technique to investigate charged surface adsorbate-oxygen vacancy interactions at the triple-phase boundary region in a CO2 environment, manifested as small perturbations of opposite sign in reference to the applied biases. Through fabricating 2D planar structured SOEC assemblies and varying the GdO1.5 dopant concentration in ceria electrode material, the correlation between the potential loss at the electrode | electrolyte interface and oxygen vacancy concentration in ceria electrode was established based on the measured potential loss at the interfacial region. And potential works involving in situ APXPS to provide chemical information and theoretical modeling to further enhance our understanding of the CO2 reduction reaction mechanisms on ceria based SOEC electrode were proposed.

In summary, the works in this thesis demonstrate functional imaging of electroactive oxide heterointerfaces in dynamic environments combined with dopant analysis to yield quantitative defect distributions within oxide films. I also show to probe the local ionic conductivity of SOC electrolyte and charge distribution / potential loss near electrode | electrolyte interface. These results are theoretically interpreted as local activation energy for anionic oxygen transportation in electrolyte and oxygen vacancy / surface adsorbates interactions at triple-phase boundary. The developed in situ SSPM technique provides a new approach to access the fundamental information hiding behind the surface electrical response and questions it is potentially able to address will broadly impact the materials science, electrochemical, and solid-state physics communities.

Available for download on Thursday, August 01, 2019

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