Probing and tuning far-from-equilibrium oxygen exchange kinetics on electrochemical solid-gas interfaces
Abstract/Contents
- Abstract
- The oxygen exchange reaction, which involves the transformation of an oxygen gas molecule to two lattice oxygen ions, is ubiquitous and fundamental in solid-state electrochemistry. During this reaction, the oxide catalysts could change the stoichiometry, surface morphology, electronic structure etc. Without a well-defined surface and in situ probing techniques, the in-depth study of this solid-gas catalysis reaction is complicated. Moreover, to understand the reaction pathway and to identify the rate-determining step, near-equilibrium measurements have been employed to quantify the exchange coefficients. Because the exchange coefficient contains contributions from both forward and reverse reaction rate constants and depends on both oxygen partial pressure and oxygen fugacity in the solid, unique and definitive mechanistic assessment has been challenging. In this thesis, we derive a current density equation as a function of both oxygen partial pressure and solid oxygen fugacity and consider both near and far-from-equilibrium limits. Rather than considering specific reaction pathways, we generalize the multi-step oxygen incorporation reaction into the rate-determining step, preceding and following quasi-equilibrium steps, and consider the number of oxygen ions and electrons involved in each. By evaluating the dependence of current density on oxygen partial pressure and solid oxygen fugacity separately, one obtains the reaction orders for oxygen gas molecules and for solid-state species in the electrode. This model was then applied to (La1-xSrx)FeO3-δ (LSF), a promising candidate for solid-oxide fuel cell (SOFC) cathodes. We fabricated thin film LSF electrochemical cells whose surface morphology, structure, and chemistry were well controlled. We first investigated the surface defect chemistry using in operando synchrotron techniques. Combining the defect concentration results and the away-from-equilibrium rate equation model, we simulated many reaction pathways and compared with experimental data to determine the rate-determining steps. It was revealed from TEM that varying the (La+Sr)/Fe ratio changes the surface structure and shifts the rate-determining step from the molecular pathway to atomic pathway. The individual reaction steps were also simulated by DFT. The surface double Fe-O2 structure causes a substantial incorporation barrier and explains the experimental results. More applications to other LSF thin films showed how the reaction kinetics were influenced by the material composition and the vacancy formation energy. Some deviation on kinetics parameters were also observed and explained by generalizing the rate equation to non-ideal conditions.
Description
| Type of resource | text |
|---|---|
| Form | electronic resource; remote; computer; online resource |
| Extent | 1 online resource. |
| Place | California |
| Place | [Stanford, California] |
| Publisher | [Stanford University] |
| Copyright date | 2018; ©2018 |
| Publication date | 2018; 2018 |
| Issuance | monographic |
| Language | English |
Creators/Contributors
| Author | Guan, Zixuan |
|---|---|
| Degree supervisor | Chueh, William |
| Degree supervisor | Hwang, Harold Yoonsung, 1970- |
| Thesis advisor | Chueh, William |
| Thesis advisor | Hwang, Harold Yoonsung, 1970- |
| Thesis advisor | Clemens, B. M. (Bruce M.) |
| Degree committee member | Clemens, B. M. (Bruce M.) |
| Associated with | Stanford University, Department of Applied Physics. |
Subjects
| Genre | Theses |
|---|---|
| Genre | Text |
Bibliographic information
| Statement of responsibility | Zixuan Guan. |
|---|---|
| Note | Submitted to the Department of Applied Physics. |
| Thesis | Thesis Ph.D. Stanford University 2018. |
| Location | electronic resource |
Access conditions
- Copyright
- © 2018 by Zixuan Guan
- License
- This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).
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