Modeling Of Solid Oxide Fuel Cell Electrode Microstructures

The performance of solid oxide fuel cells (SOFC) is sensitive to the electrode microstructures. During fabrication processes, the electrode microstructure is made and the corresponding performance is determined. In long term operation processes, electrode microstructure evolves, leading to degradation in performance. This thesis focuses on the electrode microstructure and its stability during fabrication and operation processes.The conventional composite electrodes fabricated by high temperature co-sintering processes and the nanostructured electrodes prepared by infiltration-calcination processes are state-of-the-art electrode architectures. These two types of electrodes have tremendous differences in both microstructure and performance. In chapter two, according to their fabrication processes, these electrodes are considered as a structure composed of particle layers, and thus a unified model for microstructure-performance relationship is developed. The parameters such as interfacial polarization resistance, three-phase boundary (TPB) length, and effective electrode thickness are formulated as a function of effective TPB resistivity, ionic resistivity, electrode composition, porosity, particle size, and electrode thickness. It is confirmed that infiltrated electrodes exhibits much lower polarization resistance and thinner effective electrode thickness due to the significant enhancement of effective TPB length compared to conventional composite electrodes. The unified model provides basic theory to compare composite and infiltrated electrodes together.In chapter three, refined models are proposed for the composite electrode microstructures. The effects of particle size distribution and electrode thickness on electrode microstructure and polarization resistance are included. The distribution of effective TPB length can be calculated by using particle size distributions of electrode powders, electrode composition, porosity, and electrode thickness. Combining with effective TPB resistivity and ionic resistivity, electrode polarization resistance can be estimated. The models are validated by using the experimental data and literature data for La0.8Sr0.2MnO3(LSM)/yttria stabilized zirconia (YSZ) composite electrodes. It is found that, when particle size has a distribution interms of number/volume, effective TPB length is decreased/increased. The distribution of effective TPB length within electrode is not uniform. Due to the contribution of un-percolated clusters, the effective TPB length is higher near electrolyte and/or current collector layer compared to the electrode bulk. And because of this, the electrode resistance is not so bad when electrode composition is beyond of percolation thresholds. The models provide feasibility to estimate electrode performance form raw powder distributions.Compared with conventional composite electrode, the understanding about infiltrated electrode microstructures is limited. In chapter four, a numerical methodology is developed to simulate the three-dimension microstructures of infiltrated electrodes under various infiltration loadings. Key geometric parameters are calculated as a function of infiltration loading, including percolation probability of infiltrated nanoparticles, effective TPB length, and surface area of infiltrated nanoparticle. The effects of backbone structure, infiltrated nanoparticle size and its aggregation risk are studied. It is found that, backbone structure remarkably affected effective TPB length, but has little effect on nanoparticles surface area; By reducing the nanoparticle size, effective TPB length and nanoparticles surface area are enhanced significantly; The aggregation of nanoparticles slows down the evolution of effective TPB length with infiltration loading. In other words, more infiltration loading is needed to maintain the same TPB length. In addition, nanoparticles surface area is not affected by its aggregation. Analytical models are proposed to calculate TPB length and nanoparticles surface area, and are fully validated by the numerical construction results and literature data. It is found that, the peak TPB length is achieved when63%backbone surface is covered by nanoparticles.High temperature sintering is an indispensable process in fabricating SOFC electrodes. For composite electrodes, the resultant microstructure after sintering process determines electrode performance. In chapter five, a kinetic Monte Carlo (kMC) model is developed to simulate the evolution of three-dimension microstructure of composite electrodes during sintering processes. The model employs LSM/YSZ composites as the example electrodes. The sintering mechanisms include grain boundary migration, surface diffusion, vacancy creation and annihilation. TPB length, porosity, and tortuosity factor of pores are calculated during kMC sintering. And the effects of particle size and its distribution and sintering temperature are studied. The model is validated by literature data. It is found that, the sintering kinetics can be enhanced remarkably by increasing sintering temperature; Small particle size leads to high sinterability and results in high peak TPB length; Particle size distribution (in terms of particle number) exhibites little effect on sintering kinetics, but reduces TPB length. The kMC model is capable of simulating various sintering mechanisms simultaneously. It can serve as a useful tool to design and optimize the sintering processes.In chapter six, the thermal cycle stability of composite and infiltrated electrodes are studied. By combining with the unified model proposed in chapter two and Weibull theory, a durability model is proposed. It is demonstrated that the bonding between electrode particles can break under the stress induced by temperature rang and thermal expansion mismatch between electrode materials. The model employs temperature range of thermal cycle, thermal expansion coefficients and mechanical parameters of electrode materials to calculate electrode resistance as a function of cycle numbers. The calculation results agree nicely with literature data. It is confirmed that the significant thermal cycle stability of infiltrated electrodes derives from the nanosized infiltrated particles, which enhances the survival probability of particle-to-particle bonding under thermal stresses.In chapter seven, the microstructure stability under polarization is studied. The mechanisms of LSM delamination from YSZ electrolyte under anodic polarization are revealed:The anodic polarization creates high oxygen pressure in LSM grains just near the LSM/YSZ interface. In addition, oxygen ions migrate from YSZ to LSM lattice, and create cation vacancies, and therefore induce tensile stress. The combination of these two stresses delaminates LSM electrode from YSZ electrolyte. By using Weibull theory, the survival probability of LSM/YSZ interface is estimated. The relaxation time for oxygen diffusion from LSM/YSZ interface to the microcracks in LSM links the survival probability with time, thus the evolution of survival interface area can be predicted. The model is validated by experimental data. It is confirmed that decreasing polarization current and/or operation temperature can significantly slow down the delamination kinetics.