Study On The Electrochemical Performance Of Lanthanum Ferrite-based Electrode For Solid Oxide Cells

Solid oxide cells(SOCs)are favored for their high efficiency.It can convert the chemical energy of fuel(H₂,CO,CH₄,etc.)into clean electric energy in solid oxide fuel cell(SOFC)mode,or use intermittent clean energy(wind energy,solar energy,tidal energy,etc.)to electrolyze H₂O or CO₂in order to store energy in a solid oxide electrolysis cell mode(SOEC).Electrodes with rapid oxygen reduction reaction(ORR),carbon dioxide reduction reaction(CO₂RR),oxygen evolution reaction(OER)and sufficient durability play a key role in SOCs.The traditional cermet still has some challenges,as impurity gas poisoning,Ni redox instability and carbon deposition.lanthanum manganese and lanthanum cobaltate-based oxide electrodes have low electrocatalytic activity at reduced temperature and high thermal expansion coefficient problems,respectively.In order to overcome these challenges,this thesis selects iron-based perovskite oxide as the electrode of SOCs,aiming to improve the electrochemical performance of SOCs by using its excellent electronic and ionic conductivity,its good ORR,OER and CO₂RR electrocatalytic activity.In this thesis,the basic physical and chemical properties of Srand Ni co-doped lanthanum ferrite perovskite materials(La_0.6Sr_0.4Fe_0.8Ni_0.2O_3-?,LSFN)are studied and then is used as the oxygen electrode of reversible solid oxide cells(RSOCs);LSFN is used as the electrode for symmetrical solid oxide electrolysis cell(SSOEC)to electrolysis CO₂.Further,the strategy of in-situ exsolved nano Fe-Ni alloy is used to improve the performance of electrolysis CO₂.Finally,in order to solve the problem that Sris easy to segregate at high temperature,using Ca instead of Srto prepare LCaFN materials.The main content and conclusions of this thesis are as follows:(1)LSFN based perovskite oxide as oxygen electrode for RSOCs.LSFN perovskite has been developed and investigated as an oxygen electrode for RSOCs.The electrochemical performance of LSFN oxygen electrode in the SOFC mode and SOEC mode is investigated in detail.The maximum power density of 961 m W/cm²and polarization resistance of 0.142?·cm²at 800? can be achieved in SOFC mode.While the cell is operated in SOEC mode,the current density reaches 0.53 A/cm²at750? with 50 vol%AH at 1.3 V,and the hydrogen generation rate can reach up to1348.5 m L/(cm²·h)with 90 vol%AH at 800?.The RSOCs show excellent reversibility and stability during 144 h medium-term reversible operation.(2)Direct electrolysis of CO₂in SSOEC based on LSFN electrode.In this work,the SSOEC with the configuration of LSFN-GDC/GDC/YSZ/GDC/LSFN-GDC has been evaluated with electrochemical performance for direct electrolysis of pure CO₂.The current density reaches 1.03 A/cm²at800? under an electrolysis voltage of 2.0 V and the polarization resistance(R_p)was as low as 0.12?·cm²at 800?.The formation rates of CO reached 5.16 m L/(min·cm²)and corresponding Faraday efficiency of 92%under the voltages of 1.8 V at 800?,respectively.Furthermore,the cell also displayed good stability in the short-term CO₂electrolysis test.(3)CO₂ electrolysis performance and mechanism based LSFN electrocatalyst with in-situ exsolved Fe-Ni nanoparticles.Herein,we demonstrated a novel quasi-symmetrical solid oxide electrolysis cell with reduced LSFN as cathode and oxide LSFN as anode for pure CO₂electrolysis.After reduction,Fe-Ni alloy nanoparticles were in-situ exsolved and oxygen vacancies formed in LSFN lattices.The cell with reduced LSFN cathode showed higher current density of1.42 A/cm²for CO₂electrolysis,lower polarization resistance(0.06?·cm²),higher Faraday efficiency(98%)and better long-term stability compared to the cell with LSFN cathode.The enhancement mechanism has also been discussed which due to the synergistic effect of the in-situ exsolved Ni-Fe nanoparticles and oxygen vacancies.(4)Novel LCaFN catalyst and its electrocatalytic performance and mechanism for CO₂electrolysis.In the long-term test electrolysis CO₂,Sr-containing perovskite materials can easily form SrCO₃deposited on the electrode surface,resulting in the degradation of electrode performance.Herein,a novel perovskite La_0.6Ca_0.4Fe_0.8Ni_0.2O_3-?(LCaFN)has been investigated as an electrode of SSOEC for CO₂electrolysis.The SSOEC has a low polarization resistance of only 0.04?·cm²and achieves a maximum electrolysis current density of 1.41 A/cm²at 800? with pure CO₂at 2.0 V.LCaFN electrode shows good stability and can be recovered by simple air treatment after a period of operation to avoid the formation of carbonate with alkaline earth elements in perovskite as LSFN.In addition,the mechanism of self-recovery of the LCaFN electrode by air treatment has been discussed and analyzed.