Development Of Multi-scale And Multi-physics SOFC Electrode Microstructure Model And Reaction Kinetics Analysis

In recent decades,Solid Oxide Fuel Cell（SOFC）has been widely regarded as one of the most promising energy conversion device duo to its excellent advantages such as high efficiency,low emission,and fuel flexibility.There is inevitable trend to reduce the operating temperature from about 800-1000°C to an intermediate temperature（IT）range of 500–750°C for the commercialization of SOFC.Mixed ionic and electronic conductors（MIECs）such as La_0.6Sr_0.4Co_0.2Fe_0.8O₃（LSCF）perovskite are one of the promising cathode materials for IT-SOFC.As is emphasized in lots of literatures,cathode is one of the main limiting factors for SOFC performance improvement.It is of great importance to gain a fundamental understanding on how the electrode microstructure affect the transport of ions/electrons/gas species and the electrochemical reaction processes.The present study focuses on the key scientific issues in the optimization process of SOFC electrode microstructure.Simulation models from meso to macro scale are established to further reveal the complex transmission mechanism inside the SOFC,and provide theoretical support for the electrode microstructure optimization.At the macroscopic scale,the comprehensive steady-state tools including1D and quasi 2D model is developed to investigate the effects of electrode structure on the performance of SOFC,considering detailed heat and mass transfer processes,as well as electronic and ionic charge transport.At the microscopic scale,a complete multi-physics mesoscopic simulation system is established for the evaluation of the microstructure evolution and further assessment of electrochemical activities inside real microstructure.The influence of co-sintering process on microstructure evolution is fully investigated while the valuable sintering control strategy is also proposed.The influence mechanism of LSCF microstructure on electrochemical activities is discussed to gain a fundamental understanding on how the electrode microstructure affect the transport of ions/electrons/gas species and the electrochemical reaction processes.The advantage of LSCF-GDC composite electrode is also observed while the machine learning methods is also carried out for LSCF:GDC composition and sintering time optimization.Infiltration process are generally recognized as an important way to improve effective reaction sites of SOFC electrode,the correlations between electrode type and infiltration catalysts are fully discussed while the best infiltration loads for different types of infiltration electrode are also identified.The main researches are listed as follows:（1）Microstructure analysis and optimization based on multi-dimensional model:an enhanced non-isothermal 1D and quasi-two-dimensional model for SOFC parametric simulation and optimization are proposed considering the complex multi-physics transport process inside the electrode.The percolation theory is introduced to evaluate the effective transport properties.The influence mechanism of electrode microstructure parameters on SOFC performance is systematically studied while the design of non-uniform distribution of microstructure is proposed.Besides,an elementary effect（EE）approach based on Monte Carlo experiments is adopted to comprehensively evaluate the sensitivity degree of totally 24 parameters.Seven microstructural parameters are classified into very sensitive factors.Based on the sensitivity analysis results,a targeted microstructure optimization strategy is proposed.（2）Sintering kinetics analysis and sintering control strategy:an enhanced Kinetic Monte Carlo model is proposed to study the microstructure evolution of LSCF-GDC electrodes.Simulation results show good agreement with experimental data in the literatures.Based on the data accumulated from the KMC simulation,a back propagation（BP）neural network is established for quick prediction of micro-characteristics during sintering process,which solves the problem of low efficiency of sintering control procedure.Besides,a global sensitivity analysis approach is adopted to evaluate the sensitivity of structural and empirical parameters at different sintering stages.The KMC sintering model and sintering control strategy provide strong tools for real porous structure reconstruction.（3）The Lattice Boltzmann model is established to describe the transport properties and coupling effects of ORR activities inside the porous media with the interlink among species and charge transport as well as electrochemical activities all considered.By introducing the concept of virtual diffusion coefficient,the time mismatch problem caused by different diffusion coefficient of charge and reactant transport process is successfully solved.A source term processing solution to deal with the coupling effects the surface and line reaction is proposed.The model shows good conservation properties.The prediction results under different types of electrode and operating conditions are also comprehensively compared with experimental results to confirm the reliability of the model.（4）Reconstruction and optimization of LSCF cathode microstructure.The microstructures of LSCF electrodes are numerically reconstructed by a Kinetic Monte Carlo method.The performance of the reconstructed LSCF electrodes is then evaluated by a pore scale Lattice Boltzmann model.The effect of local O₂ partial pressure on electrode ohmic loss has been firstly evaluated.Another important finding is that the initial states of compact powder have a profound impact on the electrode performance.The study also provides deep insight into the influence of sintering process.The effective conductivity of electrode is mainly controlled by the enhancement of electrode connectivity.The activation loss of the LSCF electrode tends to increase duo to that the effective reaction site is reduced by sintering densification process.（5）Performance assessment of LSCF-GDC composite electrode and optimizations:by simultaneously comparing the differences in electrochemical activities and performance of three types of electrodes,the superiority of LSCF-GDC composite electrode is emphasized.The distribution principle of line and surface reaction rates along the electrode depth direction are also revealed.At different sintering stages,the influence mechanism of LSCF content on electrode performance is quite different.LSCF contents presents a much more significant effects on TPB reaction sites formation than sintering time.Finally,Support Vector Machines and Genetic algorithm method are conducted to provide the optimal range of LSCF contents and sintering time.（6）Microstructure simulation and optimization of infiltration electrodes.A Kinetic Monte Carlo method is conducted for the reconstruction of initial backbones while a three-dimensional infiltration method is introduced to simulate the numerical infiltration process.The correlations between electrode type and infiltration catalysts are firstly discussed while the best infiltration loads for different types of infiltration electrode are also identified.The contribution of surface and line electrochemical reaction on operation current density shows remarkable difference among four types of infiltration electrode.The selection of desirable infiltrated catalysts is closely related with the volume ratio of LSCF:GDC.This paper focuses on the design and optimization of SOFC electrode microstructure.A complete set of multi-scale simulation system from the mesoscopic to the macroscopic scale is established while the real microstructure fabrication and operation process for various types of electrodes are systematically explored.The targeted microstructural optimization strategy is also proposed which is of great significance for guiding the design of electrode microstructure and the optimization of electrode performance.