Date of Award

Spring 1-1-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical & Biochemical Engineering

First Advisor

J. Will Medlin

Second Advisor

Ryan O’Hayre

Third Advisor

Daniel Schwartz

Fourth Advisor

Charles Musgrave

Fifth Advisor

Se-Hee Lee

Abstract

Certain acceptor-doped perovskite-type oxides show significant promise for deployment into a number of electrochemical device applications, including fuel cells, batteries, and electrolyzers, owing to their rapid proton conductivities at high temperatures. However, limitations in bulk material hydration and slow grain boundary conductivities have reduced the viability of these materials in intermediate temperatures applications. This thesis work uses density functional theory to gain a fundamental understanding of proton and defect chemistry within various perovskite environments in order to identify strategies to increase proton concentration and improve overall proton conductivity.

First, material hydration was probed within yttrium-doped barium cerate (BCY) to examine how the thermodynamics of material hydration are influenced by dopant concentration. A model was derived from solely first principle techniques to describe hydration within BCY as a function of dopant concentration, temperature, and partial pressure of water. The resulting model can be used to screen for favorable perovskite-dopant combinations with enhanced hydration capabilities.

Next, defect segregation was investigated in the more complex interfacial environment to probe the origin of low proton conductivity across perovskite grain boundaries (GB). The results of this study suggest that screening for perovskite-dopant combinations with strong dopant-oxygen bond strengths may reduce the segregation of dopant ions and oxygen vacancies to the GB interface, mitigating the development of a positive GB core and enhancing proton conduction across the GB.

Finally, proton stability was assessed at various interfacial regions within the perovskite material. An examination of proton adsorption at the BaZrO3-vacuum interface reveals a destabilization of protons in the first subsurface layer of the perovskite, yielding a potential barrier for proton diffusion into and out of the perovskite membrane. An electronic analysis of oxygen ions indicates that the oxygen p-band center influences proton adsorption strength. This p-band center model was shown to accurately describe adsorption trends within bulk BCY as well. An analysis of proton adsorption at BaZrO3-metal heterointerfaces displayed a stabilization of protons in the near heterointerfacial environment compared to the BaZrO3-vacuum surface. This stabilization reduces the barrier for proton diffusion near the perovskite interface and likely leads to increased proton concentrations at the perovskite-metal heterointerface.

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