Numerical Simulation Of Gas Transfer Mechanism And Polarization In Solid Oxide Fuel Cell

The solid oxide fuel cell?SOFC?is a direct electrical generation device widely adaptable to multiple fuel gases,with all solid,clean,efficient,noiseless and continuous working features.SOFC has attracted much attention and is considered to be the third-generation fuel cell that can be widely used in the future to solve energy crisis and environmental pollution.The SOFC power generation system operates under high temperature,closed and complicated working conditions,which brings difficulties to the measurement of internal working conditions,performance and parameters.Evaluating SOFC performance by high-cost and long-cycle experimental analysis subjects to the multi-variable measurement errors caused by preparation conditions and processes,and some are also limited by measurement technology and safety.The method of numerical simulation and optimization is an accurate and effective method to replace the experiment.Most of the existing numerical simulation studies focus on flow field,temperature distribution,current distribution,heat conduction,power and voltage output.In the report on polarization loss,there are few studies on SOFC electrolyte structure,electrode micropore structure,and electrode thermal expansion.At the same time,there are few reports that consider the effects of gas transmission and battery structure on the polarization loss after electrode poisoning.The aim of the work is to establish a quantitative evaluation mechanism for gas transportation and microstructure of fuel cells,as well as provide a way to rationally design electrode structures,and achieve high power and long-life output of fuel cells.This thesis work combines the working principle of macroscopic single cell and microscopic electrochemical reaction to establish the relevant mathematical model of SOFC.The detail work is in the following aspects:?1?Thermal shock and interfacial stress during SOFC operation will cause defects in the electrolyte microstructure and pinholes will cause gas penetration.A model for studing the polarization of batteries caused by electrolyte gas leakage is proposed,and the leakage is compared between the SOFC??SOFC?and the traditional SOFC?c SOFC?in the electrolyte of nanometers and micrometers thick.The effect of chemical loss is analyzed,and the degree of chemical reaction between the leaked gases is analyzed.When there is no chemical reaction between the leaking gases,it mainly affects the increase of the cathode CP.When the leakage gas reacts directly,it has a great influence on the anode and cathode CP.?SOFCs have higher requirements for electrolyte tightness,and the CP caused by leakage is several orders of magnitude larger than the polarization value of c SOFCs under the same conditions.?2?The theoretical evaluation of Knudsen diffusion introduced in the gas transmission of the fuel cell is only applicable to round pores,not to the actual irregular porous structure,which affects the accuracy of the evaluation.A numerical model to study the advantages and disadvantages of electrode pore cross sections is proposed to study the effect of pore morphology on Knudsen diffusion coefficient and effective diffusion coefficient of porous electrodes.Compared with the isotropic pore cross-section,the gas diffusion coefficient in the anisotropic pore cross-section is relatively small.This phenomenon becomes more significant as the pore size becomes smaller and the operating temperature becomes higher,which in turn affects the limiting current density?LCD?and CP.As the aspect ratio of the pore cross-section increases,the gas diffusion coefficient on the electrode decreases,which in turn causes significant changes in LCD and CP.Adjusting the electrode parameters,such as increasing the pore size,reducing the porosity/curvature??/??ratio,adjusting the electrode thickness,etc.,can reduce the influence of the pore cross section on the CP.By lowering the operating temperature and output current density,the influence of pore morphology on CP can be reduced.?3?The existence of thermal expansion at high temperature makes the evaluation of SOFC using parameters at room temperature no longer accurate.A method to study the thermal expansion and battery loss of battery materials is proposed.By introducing the coefficient of thermal expansion?CTE?of the material and the change in the diffusion coefficient caused by temperature changes,the changes in fuel cell mass transfer caused by thermal expansion and polarization during the startup process are discussed.The results show that the electrode thickness change caused by thermal expansion and the temperature-dependent effective binary diffusion coefficient have a greater impact on energy loss.The polarization caused by thermal expansion depends on the output current density and operating temperature.This work helps to accurately evaluate battery polarization and effective operation to reduce the energy loss of SOFCs during the startup phase.While discussing the change of CP,the law of ohmic polarization change caused by thermal expansion is also given.?4?Low-concentration impurity gas is an important factor that causes catalyst poisoning and sharp deterioration of battery performance.The synergistic effect of poisoning gas diffusion and anode reaction is usually ignored in the previous analysis due to the complexity of the process.?a?In the anode,a model to study the sufer poisoning effect under different H₂S concentrations?<100 ppm?,and microscopic parameters porous anodes was established.The results show that the electrode pore size and?/?are small.At low temperature,sulfur poisoning is closely related to gas diffusion.The voltage drops caused by CP,activated polarization?AP?and sulfur poisoning are all sensitive to the microstructure of the anode pores.Compared with CP,thin-layer anodes and large?/?H₂S-diffusion-induced battery APs are particularly serious.We should avoid operating at low temperatures,low current densities,and high concentrations of H₂S.?b?At the LSM-YSZ cathode,a mathematical model was established to quantitatively evaluate the effect of different microstructures and Cr poisoning on performance using gas diffusion and Cr electrochemical deposition mechanisms.Cr₂O₃ deposition will reduce the cathode pore size and TPB area,hinder gas diffusion,and increase CP and AP.In addition,the effects of Cr poisoning on the lifetime and local current density of three types of microstructure cathodes were compared.The results show that the effect of Cr poisoning on the performance of the SOFC cathode depends on the cathode microstructure.The cathode is uniform in size,has uniform Cr poisoning throughout the cathode,and has the best tolerance to Cr poisoning.