Relating multiscale structure and chemistry with oxygen exchange kinetics in low-thermal-budget Sr(Ti,Fe)O3-d and high-throughput combinatorial thin films

Skiba, Emily J.

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https://hdl.handle.net/2142/122149 Copy

Description

Title

Relating multiscale structure and chemistry with oxygen exchange kinetics in low-thermal-budget Sr(Ti,Fe)O3-d and high-throughput combinatorial thin films

Author(s)

Skiba, Emily J.

Issue Date

2023-12-01

Director of Research (if dissertation) or Advisor (if thesis)

Perry, Nicola H

Doctoral Committee Chair(s)

Perry, Nicola H

Committee Member(s)

• Braun, Paul V
• Ertekin, Elif
• Shoemaker, Daniel P

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• SOFC
• crystallization
• processing-structure-property relationship
• oxygen surface exchange kinetics
• mixed ionic and electronic conductors
• SrTi0.65Fe0.35O3-d
• X-ray absorption spectroscopy
• optical transmission relaxation
• combinatorial thin films

Abstract

With the ever-increasing need for efficient and cost-effective alternative energy generation and storage, reversible solid oxide (fuel/electrolysis) cells (SOFC/SOECs) offer an unprecedented opportunity for alleviating the worst effects of climate change. Various challenges related to the high temperatures necessary for SOFC/SOEC operation have generated increased interest in electrochemically surface-active materials for intermediate-temperature (IT) SOFC/SOECs (500-800 ˚C). Additionally, low-thermal-budget processing of thin films, where the structure may remain amorphous or be subject to a short crystallization step, has been an emerging research focus. Mixed conductors show promise for low-to-intermediate temperature energy conversion/storage applications, where their ability to transport ionic and electronic carriers and catalyze electrochemical reactions at the surface enables rapid and efficient reduction/evolution of molecular oxygen. Towards this goal, the development and accurate measurements of novel compositions with rapid oxygen surface exchange kinetics and high ionic and electronic conductivities are necessary for the development of next generation IT-SOFC/SOEC devices. To shed light on the significant disagreement in oxygen surface exchange kinetics measured by different techniques on nominally the same materials in the literature, we have applied simultaneous in-situ electrical conductivity relaxation, AC-impedance spectroscopy, and optical transmission relaxation to Sr(Ti,Fe)O3-d thin films in order to deconvolute the effects of measurement technique, processing inhomogeneities, and film thermal history on the phenomenological surface exchange rate. Results indicate a 2x improvement in the oxygen incorporation/evolution rate coefficient when films are in full contact with current collectors, with a corresponding increase in the activation energy, indicating an alternate rate limiting step. 2D spatially resolved optical transmission relaxation (2D-OTR) measurements were carried out to better understand the improved surface exchange values derived from techniques that require catalytic current collectors be applied to the surface and showed that orders of magnitude improvement in the oxygen reduction/evolution kinetics is seen at or near the metal current collector. X-ray photoelectron spectroscopy (XPS) analysis revealed that the presence of the metal current collector both reduces the surface contamination in the nearby film region as well as provides catalytically active triple phase boundary sites for oxygen reduction and evolution. An alternative approach toward forming active surfaces with beneficial surface chemistry is low-temperature film growth. To better understand the role of local structure in material functionality and stability in this space, atomic level structural analysis utilizing X-ray absorption spectroscopy (XAS) was performed on amorphous, in-situ crystallized, and crystalline-grown films. XAS results reveal significant structural and defect chemical changes between amorphous and high-temperature, grown-crystalline films. Notably, post-annealed, amorphous-deposited films are more highly disordered and reduced compared with high-temperature grown counterparts, indicating kinetics limitations to the low-temperature crystallization. Annealing of the as-deposited amorphous films in various reducing and oxidizing environments reveals the invariance of the Ti and Sr local structures, whereas the Fe local coordination and oxidation state is primarily affected. Lastly, high-throughput combinatorial screening utilizing the 2D-OTR setup allowed for determination of optimal electrode compositions as well as unexpected thickness dependencies of the oxygen exchange kinetics. Binary solid solutions of the form SrTi1-xFexO3-d and SrTi0.65CoxFe0.35-xO3-d and a ternary film (SrTix (Fe, Co)1-xO3-d) were fabricated via PLD. High surface activity for the oxygen reduction/evolution reactions was found for STF compositions with x>0.35, although no increase was seen by further increasing the Fe content. In light of the decreased electrode stability for higher Fe contents, lower concentrations, e.g., x=0.1 exhibited the fasted kinetics over time with longer term testing. For SrTi0.65CoxFe0.35-xO3-d (with fixed Ti:(Fe+Co) ratio at 0.65:0.35) an optimal composition of SrTi0.65Co0.15Fe0.20O3-d is recommended, while SrTi0.75Co0.12Fe0.13O3-d (without fixed Ti:(Co+Fe) ratio) is superior from analysis of the ternary film. Studies on films with constant lateral composition reveal an unexpected and significant variation in oxygen surface exchange coefficient as a function of thickness, pointing the possible thickness-dependent interplay of strain, microstructure, and Sr surface segregation as a result of the deposition process, chemical expansion and impact of thermal expansion coefficient mismatch. Taken together, the results provide fundamental insight into processing-structure-property relationships of thin-film mixed conductors that can inform efficient, durable electrode development. Further, the in-depth analysis and development of optical relaxation methods facilitates future use of this technique for accurate and high-throughput measurement methods of surface reaction kinetics across a variety of contexts.

Graduation Semester

2023-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Emily J. Skiba

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