Microstructure Theory And Multiscale Modeling Of Solid Oxide Fuel Cell Performances And Degradations

Solid oxide fuel cell (SOFC) has been recognized as a promising candidate for the future electricity power generation technology due to its high efficiency, fuel flexibility as well as other benefits. The experimental method is the decisive factor to promote the progress of SOFC technology, but there are many disadvantages in the experimental approach, e.g., expensive, time-consuming, et al. These shortcomings severely hinder the development of SOFC technology. In contrast, the theory and numerical simulation analysis are low-cost and efficient. Multi-physics model, based on the principle and the material properties of SOFC, can provide more detailed physical and chemical processes in the SOFC and optimize various parameters, such as working conditions, electrode composition and particle sizes et al. In a word, the theory and numerical simulation analysis play an important role in the process of the development of SOFC technology.This dissertation focuses on developing multi-scale and multi-physics model and theory to study the electrochemical performance, mechanical properties and performance degradation of SOFC. This dissertation includes the following contents:(1) In the first section, the fuel cell background and the history of fuel cell development are briefly introduced. And then, the working principle and basic thermodynamic theory of SOFC are described in detail. In addition, we also give a brief description about finite element method. Finally, a brief literature overview at home and abroad about the SOFC simulation and theory is presented.(2) In the second section, a finite element method based multi-physics numerical model is built for micro tubular solid oxide fuel cell (mtSOFC) that is two-dimensional in nature due to its axial symmetry. The effective properties of the composite electrodes in the model are deduced from the microstructure and property relationship theory. The model is capable of describing the coupled gas diffusion, electronic and ionic conduction, electrochemical reaction, and thermal transport. The effects of the working conditions, the methods for collecting current and composition ratio and particle size of electrode and electrolyte materials in the composite electrode on the electrochemical are systematically examined. It is found that the mtSOFC performance can improve largely by using current collected from two side of anode. Higher fuel flow rates can improve the performance of mtSOFC, however its effects is limit at mid-range current densities. When fuel composition is considered, higher hydrogen contents are favorable for power output. The electrode microstructure is optimized based on both the electrochemical performance and mechanical property of mtSOFC. It is found that increasing the Ni content or reducing the Ni particle size can be generally helpful for improving the electrochemical performance. Low LSM contents are detrimental to the electrochemical performance.(3) In the third section, based on the temperature profile and sintering processes of fuel cell, a model, which can calculate the residual stress and thermal stress distribution of mtSOFC under working condition, is development. Based on the thermal stress distribution, the probability of mechanical failure of the ceramic material may be predicted. The effects of the composition ratio of electrode and electrolyte materials in the composite electrode on mechanical properties are systematically examined. The mechanical stability decreases dramatically with the increased Ni content. The LSM content is less consequential on the mechanical stability.(4) In the fourth section, considering the surface diffusion and the microstructure of the anode of SOFC, a new model based on two particle system is developed to describe the nickel particle growth in SOFC anode. The growth of mean Ni particle size is due to the integrating of pairs of big particle and small particle which are contacting with each other. And the difference in metal particle diameter leading to diffusion is the driving force for such process. Surface diffusion of metal atoms on the particle surface is the dominant diffusion mechanism. In this model, the number of connect surfaces is calculated by coordination number theory, and a probabilistic assumption is used for describing the block of Yttria-stabilized zirconia (YSZ) on the growth of Ni particle size in the cermet material. The found analytical function for the growth kinetics was compared to experimental results for the growth of nickel particles in anode. The theoretical model was in good agreement with the experiment and described the time dependence of the observed particle radii in an adequate way. At last, the maximal mean particle size is given by the function and compared to the results in other literature.(5) In the fifth section, based on the model of nickel particle growth in the anode of SOFC, the theory of coordination number and percolation, a multi-scale and multi-physics model, which can describe the performance of SOFC stack with the time evolution, is development. The theoretical and experimental I-V relations are in excellent agreement, demonstrating the usefulness of the theoretical model. Then, the effects of the coarsening of Ni on the length of TPB and conductivity are systematically examined by the coordination number and percolation theory. The change of TPB length and conductivity are mainly due to the change of average coordination number of anode in SOFC. Based on the multi-scale and multi-physics model, the effects of the coarsening of Ni in the anode on the performance of the SOFC stack are also systematically examined. It is found that high Ni content and big YSZ particles are beneficial to reduce the effect of Ni particle coarsening on the performance the stack.(6) In the last section, a brief summary of this dissertation is presented.