Citation
Thomas, Vaughan Lamar (2012) Particlebased modeling of NiYSZ anodes. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:03302012133547448
Abstract
In this work we examine the performance of particlebased models with respect to the NiYSZ composite anode system. The conductivity and triplephase boundary (tpb) of particlebased systems is estimated. The systems considered have monodispersed particle size distributions, bimodal particle size distributions with a YSZ:Ni particle size ratio of 1:0.781, and particle size distributions based on experimental measurements. All three types of systems show qualitative behavioral agreement in terms of conductivity, with clear transition from nonconducting behavior to high conducting behavior over a small transition regime which varied from a nickel phase fraction of .22.28 for the mono dispersed cases, 0.19.0.25 for the bimodal cases, and 0.190.30 for the experimentally based cases. Monodispersed and simplepolydispersed particle size distribution show very low variation from case to case, with σ/μ ≤ 0.04. Cases based on empirical particle size distribution data demonstrated significantly higher variances which varied over a very large range, 0.3 ≤ σ/μ ≤ 1.1. With respect to the calculations of the TPB length, we find that the same pattern of variance in the measure of the triplephase boundary length. The TPB length for the monodispersed and simple polydispersed systems was in the range of 3 × 1012 –4 × 1013 m/m3 . For empirical particle size distribution data the TPB length density was in the range of 8×109–2×1011 m/m3. The variance of the TPB length density follows the same pattern as the conductivity measurements with very low variance for the monodispersed and simple polydispersed systems and much larger variance for the empiricallybased systems. We also examine the association between the TPB length and the availability of conducting pathways for the participating particles xv of individual TPBs. The probability of a TPB having a conducting pathway in the gas phase is essentially 100% in all cases. The probability of an individual tpb section having conducting pathways in either of the solid phases is directly related to percolation condition of that phase.
We also considered a particlebased composite electrode realization based on a three dimensional reconstruction of an actual NiYSZ composite electrode. For this model we used particles which vary in nominal size from 85–465 nm, with size increments of 42.5 nm. We paid particular attention to the coordination numbers between particles and the distribution of particle size interconnections. We found that homogeneous interparticle connections were far more common than would occur using a random distribution of particles. In particular we found that for a random collection of particles of similar composition the likelihood NiNi particle connections was between 0.18–0.30. For the reconstruction we found the likelihood of NiNi particle connections to be greater than 0.56 in all cases. Similarly, the distribution of connections between particles, with respect to particle size of the participating particles, deviated from what would be expected using a random distribution of particles. Particles in the range of 85–169 nm showed the highest coordination with particles of the same size. Particles in the range of 211–338 nm have the highest coordination with particles of radius 169 nm with very similar distributions. Particles with radius greater than 338 nm represented only 7.2 × 10−3 % of the particles within the reconstruction, and showed the highest coordination with particles of radius of 211 nm, but the distributions vary widely.
In the final chapter, we build a model which can account for mass transfer, hetero geneous chemistry, surface chemistry, and electrochemistry within a porous electrode. The electric potential is calculated on a particle basis using a network model; gas phase concentrations and surface coverages are calculated with a onedimensional porous me dia model. Properties of the porous media are calculated via a TPMC method. TPB electrochemistry is calculated at individual triple phase boundaries within the particle xvi model, based on local gas phase concentrations, surface coverages and particle poten tials, and then added to the porous media model. Using this tool we are able to calculate the spatial distribution of the Faradaic current within the electrode, and variation in gas phase concentrations within the porous media.
Item Type:  Thesis (Dissertation (Ph.D.))  

Subject Keywords:  Solid Oxide Fuel Cell; Fuel Cell Anode Model; Fuel Cell Anode Simulation; Fuel Cell Particle Model; Fuel Cell Electrode Model; Particle Electrode Model  
Degree Grantor:  California Institute of Technology  
Division:  Engineering and Applied Science  
Major Option:  Mechanical Engineering  
Thesis Availability:  Public (worldwide access)  
Research Advisor(s): 
 
Thesis Committee: 
 
Defense Date:  10 January 2012  
Author Email:  vaughantel1 (AT) gmail.com  
Funders: 
 
Record Number:  CaltechTHESIS:03302012133547448  
Persistent URL:  http://resolver.caltech.edu/CaltechTHESIS:03302012133547448  
Default Usage Policy:  No commercial reproduction, distribution, display or performance rights in this work are provided.  
ID Code:  6881  
Collection:  CaltechTHESIS  
Deposited By:  Vaughan Thomas  
Deposited On:  08 Jun 2012 21:23  
Last Modified:  26 Dec 2012 04:41 
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