| Solid oxide fuel cell(SOFC)is an important method to solve the energy crisis.The unique characteristics of non-pollution and high efficiency have become the research focus of new energy technology.The high temperature operating environment of SOFC leads to high cost and easy to appear the problem of material compatibility in industrial application.Lowering temperature can improve the stability of SOFC but decrease its electrochemical performance.Therefore,it is necessary to study the electrochemical performance of SOFC at medium and low temperature,which can be used to guide the research and development of related materials and preparation technology.Recently,the electrochemical performance of SOFC is mainly focused on improving electrode materials,optimizing operating conditions and identifying reaction mechanism of semi-cell.The theoretical derivation of the electrode reaction mechanism and the application of the results of the theoretical analysis to judge the electrode rate control step(RDS)and explain the characteristics of the performance curve are relatively few in the literature.Therefore,the RDS and electrochemical properties of SOFC under different operating conditions were studied by combining theory with experiment.The main contents are as follows:(1)The relationship between the exchange current density and the partial pressure of the gas is calculated and discussed in detail by using the theory of defect chemistry.RDSs with different reactants as boundary conditions are summarized.This conclusion can be used to determine the RDS of SOFC operation and to explain the performance curves under different gas partial pressure adjustment.(2)A button type SOFC test system was established.The impedance spectra of different hydrogen partial pressure,different oxygen partial pressure and different temperature were studied in the range of 0.01Hz~100KHz by means of AC impedance method.The polarization impedance decreases with the increase of partial pressure and temperature.According to the relationship between the partial pressure of gas and the polarization impedance,the RDS of the experimental SOFC are determined.The RDS in the high frequency region is mainly a charge transport process,the RDS in the intermediate frequency region is mainly a process of adsorption and dissociation,and the RDS in the low frequency region is a gas diffusion process.(3)The electrochemical properties of SOFC at different gas partial pressure and temperature were studied.The performance of the battery increases with the increase of gas partial pressure and temperature.When the partial pressure of the gas is low,the performance of the battery is enhanced greatly,but when the partial pressure of the gas is higher,the performance of the battery is opposite.The kinetic theory gives a reasonable explanation that the reaction rate is determined by the holes and hydrolysates in the low partial pressure region,and the exchange current density increases with the increase of the partial pressure of the gas,and the adsorption gas at the high partial pressure region determines the reaction rate.The exchange current density decreases with the increase of the gas partial pressure,so the performance enhancement amplitude of the cell is different in different gas partial pressure regions.(4)The two-dimensional numerical model of COMSOL 5.3 is used to simulate the electrochemical performance of SOFC.The simulation results are in good agreement with the experimental results,and the maximum error is 3.5.The distribution of gas mole fraction in the cell indicates that the molar fraction gradient of gas in the reaction zone is the largest and decreases gradually along the radius of the cell.The temperature distribution indicates that the temperature increases along the direction of the radius of the cell and increases with the increase of output voltage.The maximum temperature of the battery showed a decreasing trend. |
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