Date of Award

8-2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Materials Science and Engineering

Committee Chair/Advisor

Kyle S Brinkman

Committee Member

Jianhua Tong

Committee Member

Kai He

Committee Member

Fei Peng

Committee Member

Harry W Abernathy

Abstract

Triple ionic-electronic conductors (TIECs) are a widely studied class of materials for electrodes in ceramic electrochemical devices such as protonic ceramic fuel cells. These ceramics are perovskite-based and exhibit concurrent conductivity of protons, oxide ions, and electrons at elevated temperatures. Despite their numerous references in literature, few studies have systematically probed the fundamental surface- and bulk-level properties of TIECs. In this dissertation, dopant levels are systematically altered in the BaCo0.4Fe0.4Zr0.1Y0.1O3-δ-type perovskite TIEC to reveal the effects on structure, transport properties, and durability of each material. The results of this work unveil the relative tunability of this TIEC system, which can be used to guide further research in these and similar materials.

In Chapter One, background of the unique properties, applications, and challenges of TIECs is summarized along with the scientific objectives of this dissertation. Chapter Two describes the general experimental procedures, including synthesis, characterization, and analytical techniques which apply across the subsequent chapters. In Chapter Three, the robustness of this BCFZY-type system is revealed: changes in zirconium to yttrium and cobalt to iron ratios result in limited changes to the base cubic perovskite structure. Chapter Four describes the effect of the dopants on surface and bulk-level oxide-ionic transport. Generally, increasing Y to Zr ratios result in a tradeoff between increasing surface exchange and decreasing bulk diffusivity, while increasing Co to Fe ratios greatly increase bulk diffusivity while revealing a surface exchange maximum in the equally co-doped composition. In Chapter Five, the dopants’ effect on surface and bulk-level proton transport is revealed. Yttrium substitution for zirconium increases bulk diffusivity and surface exchange up to a maximum, where full yttrium substitution limits the benefits. Increasing cobalt to iron ratios decreases the bulk proton diffusivity. Chapter Six studies the durability of the system through thermodynamic and functional device testing. It is revealed that thermodynamic stability does not preclude functional device stability, nor does it predict functional device performance. However, more thermodynamically stable compositions may predict stability in harsh environments. Additionally, the changes in structural and transport properties are analyzed against functional device performance. Chapter Seven discusses the combination of all these factors on the overall tunability of these materials. Chapter Eight summarizes the most important highlights of this work, while proposing further research which would help complete the dataset and help confirm the results of this work.

Author ORCID Identifier

0000-0003-1102-9020

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