| As an energy conversion device for solid oxide fuel cells(SOFCs)and solid oxide electrolysis cells(SOECs),reversible solid oxide cells(RSOCs)is highly favored by researchers because of its high efficiency and no pollution.It can work in the SOFC mode,releasing electric energy.It can also operate in the SOEC mode,thus the exhaust gas from factories can be directly converted into valuable fuel gas through external power supply.RSOCs run alternately in the SOFC and SOEC modes,so that we can achieve the recycling of fuel gas electric energy,which is of great significance in the increasingly critical energy problems.This work explored the physical and electrochemical properties of La and Fe co-doped SrTiO3 perovskite oxide(LSTF)as the fuel electrode of RSOCs,as well as the stability of reversible cells.In order to investigate the electrochemical performance of LSTF fuel electrode for reversible cells in the SOFC mode,in the Chapter 3 of this dissertation,the effects of different La and Fe dopants on the conductivity of SrTiO3 were studied,and the structural stability of LSTF materials in different atmospheres was analysed.It was determined that when the doping amount of La was 30%and Fe was 70%(LSTF0.7),the conductivity of the anode materials was the highest,reaching 4.69 S·cm-1(800?).On this basis,CeO2 and Ni metal catalyst was introduced by the impregnation method,which improved the discharge performance of the cells with LSTF0.7-CeO2 and Ni-LSTF0.7-CeO2 anodes(612 mW·cm-2 and 698 mW·cm-2 in H2 at 800 ?,respectivly).The polarization impedance values of the whole cells were 0.466 ?·cm2 and 0.455?· cm2,respectively.The contribution of the cathode and anode to the polarization impedance of the cell was determined by preparing a symmetric cell with the LSM(La0.2Sr0.8MnO3)-ScSZ(scandium-stabilized ziconia)electrode.The polarization loss for the cell on the LSM-ScSZ cathode was greater than that for the anode material.The activation energies of the electrode reaction could be calculated by the polarization impedance of the anode and cathode.The activation energies of LSTF0.7-ScSZ infiltrated anode in hydrogen and carbon monoxide were 52.2 kJ·mol-1 and 157.5 kJ·mol-1,respectively,indicating that anode had relatively lower activation energy in hydrogen,which made electrochemical reaction easier.In order to investigate the electrochemical performance of LSTF fuel electrode for reversible cells in the SOEC mode,in the Chapter 4 of this dissertation,the electrochemical performance of solid oxide electrolysis cell with LSTF0.7-CeO2 cathode and LSM-ScSZ anode at different temperatures,applied voltages and atmospheres was studied.In the process of CO2 electrolysis,when the applied potential was 2.0 V,the current density of SOEC reached 1.20,2.05,3.14 and 4.44 A·cm-2 at the temperature of 700,750,800 and 850 ?,respectively.Through fitting and analyzing the data of the AC impedance spectra,it was concluded that the contribution of load impedance R1 to the total polarization impedance under a specific voltage was dominant,compared to the mass transfer impedance R2.In addition,the long-term stability of the electrolysis cell had also been systematically studied.It was preliminarily determined that the decay of the electrolytic cell mainly came from the agglomeration of the LSM-ScSZ anode.In order to investigate the electrochemical performance of LSTF fuel electrode used for reversible cells in the two modes of SOFC and SOEC,in the Chapter 5 of this dissertation,the discharge power densities of reversible cell with the LSTF0.7-CeO2|ScSZ|LSM-ScSZ configuration at different temperatures in the SOFC mode were studied.The ohmic impedance and polarization impedance values of all electrodes in the SOEC mode were characterized by electrochemical impedance spectroscopy.Under open circuit conditions,polarization impedance values were 0.68,1.04 and 1.46 cm2 at 800 ? in 30%CO+ 70%CO2,50%CO+ 50%CO2 and 70%CO+ 30%CO2 gas mixtures,respectively.The lower impedance values showed that the LSTF0.7-CeO2 electrode possessed high catalytic activity.In addition,the cycle performance of the reversible solid oxide cell was characterized,and the reason for the cell degradation was the agglomeration of the electrode as characterized by SEM.In the chapter 6 of this dissertation,the electrode process of the symmetric cell with the cell configuration of LSTF0.7-CeO2|ScSZ|LSTF0.7-CeO2 was studied.The electrochemical performance of the symmetric reversible cells in the SOFC and SOEC modes was investigated by controlling the temperatures,applied voltages and gas compositions.The long-term stability of the reversible cells,especially in the SOEC mode,was an important factor restricting their commercial application.To solve the stability problem of the symmetric cell,in this work we applied a voltage to the electrolysis cell with a constant voltage transformation from single direction to reversible voltage operation.The results showed that the reversible voltage operation significantly increased the stability of the CO2 electrolysis cell.EDS results mechanistically demonstrated that this method could eliminate the deposited carbon on the fuel electrode surface caused by the constant voltage.It could effectively enhance the operation time of the cell from 80 h to 550 h,providing an important technical improvement for large-scale application of solid oxide electrolysis cell. |
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