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Rational Design of Higher Conductivity Solid Oxide Electrolytes <Mr. Dilpuneet (DP) Aidhy>
Electrolyte and Electrode in Solid Oxide Fuel Cell (SOFC) <Mr. Stephen Xu>
Rational Design of Higher Conductivity Solid Oxide Electrolytes
1. Introuduction
Cubic ¥ä-Bi2O3 is used as a model material to understand the structural and electronic changes during oxygen diffusion in fluorite materials. It has very high oxygen conductivity compared to other fluorite materials.
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Structure of ¥ä-Bi2O3:
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Fluorite based structure
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25% vacant oxygen sites
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Cubic phase is stable between 725 ? 830 ¡ÆC
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Motivation:
Understand the ordering mechanism of the vacancies
Understand the effect of radius and polarizability of the cation
Structural changes occurring due to vacancies
Computational Methodology
2. Computational Methodology
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Molecular dynamics (MD)
Code:
- HELL
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Buckingham potential
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NPT ensemble
- Shell model
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Density functional theory (DFT)
Code:
- VASP
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GGA type exchange-correlation
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2x2x2 system
- Plane wave cutoff of 500eV
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MD simulations are performed to understand the vacancy behavior at elevated temperatures. Oxygen diffusion occurs which leads to an equilibrated vacancy ordered structure.
DFT simulations are performed to obtain the minimum energy structure under different vacancy configurations.
Ordering of vacancies
3. Ordering of vacancies
Possible vacancy ordering mechanisms in d-Bi2O3. (a) <100> (b) <110> (c) <111> |
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| Independent simulations by MD and DFT predict the same final structure. Out of the three possible vacancy ordering mechanisms in a unit-cell, the final structure is a combined ordering in <110> and <111> directions. |
A 2x2x2 fluorite super-structure with a combined vacancy ordering in <110> and <111> directions. Only the anion-vacancy sub-lattice is shown here.
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Stability of the different ordered structures from DFT
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4. Cation Polarizability
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Mean square displacement (MSD) of oxygen is plotted with time to understand the oxygen diffusion.
A continuous increase in the oxygen diffusion is observed in polarizable system.
In the non-polarizable system, the conductivity is limited.
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A continuous increase in the oxygen MSD as the cation polarizability is increased.
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Electrolyte and Electrode in Solid Oxide Fuel Cell (SOFC)
1. Introduction
| Ceria (Fig.1) is an important material used in many state-of-the-art technologies. It is used as an excellent oxygen buffer due to the favorable oxygen transport ability. Variation in oxidation states of the cation (Ce4+?and Ce3+) helps in achieving the buffer property. It is also used as automobile exhaust catalyst, gate dielectric material for metal-oxide-semiconductor devices and gas sensors. In addition, doped ceria is extensively studied as a potential material for electrolyte and electrode in solid oxide fuel cell (SOFC). |
<Fig1> |
2. Simulation Results
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 <Fig3> |
Atomic-level simulations are used to characterize the structure and mechanical properties of stoichiometric and sub-stoichiometric ceria and of aliovalently doped ceria. In agreement with previous experimental results, we find that sub-stoichiometry leads to a significant softening of the elastic constant (Fig.2). By simulating a series of artificial ceria-based materials, we identity the decrease in the strength of the electrostatic interactions as the dominant cause of this elastic softening. Similar results are observed for doped ceria systems
3. The Diffusion Mechanism of Ceria
| The diffusion mechanism of ceria based electrolytes have been also studied using molecular dynamics simulation. The activation energies of various doped ceria systems are calculated and found to be in good? agreement with the experimental results. (Fig.3) The diffusion barrier calculated by? nudged elastic band (NEB) method shows <100> has the lowest migration energy. (Fig.4) The association effects between dopant cations and oxygen vacancy have been also investigated.
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<Fig4> |
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