Development Of LaFeO₃-Based Symmetric Electrode For Reversible Solid Oxide Cell

Solid oxide cell(SOC)is an all-solid-state reversible electrochemical device,which can convert H2 and CO to electricity in fuel cell(SOFC)mode and electrolyzes CO2 and H2O into fuel gas in electrolytic cell(SOEC)mode,exhibiting good application prospect in the development of carbon neutrality.Perovskite electrode materials are considered as potential substitutes for traditional Ni-based materials due to good redox stability.Among them,La1-xSrxFeO3-based materials are more widely used as oxygen electrodes due to high ionic conductivity,while the problem of poor stability and insufficient catalytic activity needs to be solved urgently when it is used for fuel electrodes.In this study,the La1-xSrxFeO3-based perovskite material is used as the matrix,and the stability and catalytic activity of the La1-xSrxFeO3-based perovskite are improved as the symmetrical electrode of SOC.The influence mechanism of A-site and B-site ions on the structural stability and oxygen vacancy of the material is analyzed,and the contributions of electrode stability,electrical conductivity and gas adsorption in chemical reactions are clearly determined.Finally,excellent symmetric electrode of reversible solid oxide cell(RSOC)is obtained.Firstly,the Ti4+,Nb5+ and Mo6+ by introduced into the B site to obtain La0.5Sr0.5Fe0.9Ti0.1O3-δ(LSFTi5591),La0.5Sr0.5Fe0.9Nb0.1O3-δ(LSFNb5591)and La0.5Sr0.5Fe0.9Mo0.1O3-δ(LSFMo5591).The three materials show good reduction stability and CO2 tolerance in wet H2 and CO2:CO=1:1.Under the condition of ensuring stability,the lower-valence doping is more conducive to the formation of oxygen vacancies.After Ti4+,Nb5+ and Mo6+ doping,the content of oxygen adsorption on the material surface is 52.5%,50.4%and 49.8%,respectively.The restrictive step in the polarization resistances is the CO2 adsorption and dissociation in medium-frequency,which is improved as the oxygen vacancy content increases.The current density of LSFTi5591 cell reaches 1.35 A cm-2,which is 14%and 30%higher than that of LSFNb5591 and LSFMo5591,respectively.Therefore,LSFTi5591 is excellent fuel electrode material of SOEC,and LSFMo5591 material has better stability.Considering the Mo ion doping is more advantageous to the structural stability of the material,it is introduced into the highly-active La0.6Sr0.4Fe0.9Ni0.1O3-δ(LSFN)to improve the stability of the material as fuel electrode.La0.6Sr0.4Fe0.8NiO0.1Mo0.1O3δ(LSFNM)can maintain the main phase of perovskite,and the conductivity of LSFNM is improved,which makes the polarization resistance(Rp)is decreased from 0.25 Ωcm2 to 0.18 Ω cm2 at 850℃.When LSFNM is used as the SOFC fuel electrode,the higher conductivity makes the ohmic resistance(Rs)decrease,and the stronger catalytic activity leads to the diminution of Rp,so the peak power density is improved from 740 mW cm-2 to 975 mW cm-2 at 850℃.A high current density of 1.3 A cm-2 is obtained at 1.5 V and 800℃ when LSFNM is used as SOEC fuel electrode.However,its performance is significantly decreased in symmetric cell.Therefore,LSFNM is only suitable for fuel electrode.Since Ti4+doping can improve the catalytic performance of material,the feasibility of materials with different Ti contents is studied as symmetric electrode.Excessive Ti doping restrains the conductivity and oxygen vacancy formation,and the 10%Ti-doped La0.3Sr0.7Fe0.9Ti0.1O3-δ(LSFTi3791)exhibits higher oxygen vacancy content and electrical conductivity.When it is used as SOEC fuel electrode,LSFTi3791 reduces the ohmic resistance and promotes the charge transfer and CO2 adsorption,so the electrolytic performance of LSFTi3791 is 47%higher than that of La0.3Sr0.7Fe0.7Ti0.3O3-δ(LSFTi3773)at 2 V.The LSFTi3791 shows the highest performance among single-phase perovskite electrodes(1.57 A cm-2 at 1.5 V)when it is used as symmetric electrode of SOEC at 800℃.The excellent performance of 847 mW cm-2 is also obtained as SOFC symmetric electrode.The symmetric cell exhibits good cycling reversibility in cycle tests between SOEC and SOFC modes.Therefore,LSFTi3791 is an excellent symmetric electrode for RSOC.Based on the research of LSFTi3791,the effect of Sr2+ content at A site is further studied.The material is more likely to form oxygen vacancies with the increase of Sr2+ content,while the structural stability and CO2 tolerance is gradually decreased in reducing atmosphere.SrFe0.9Ti0.1O3-δ(SFTi91)is seriously decomposed to produce a large number of SrCO3 and FeO phases in CO2:CO=1:1 atmosphere at 800％,while the SrLaFeO4 and Fe phases are produced in wet H2 atmosphere.Different decomposition products lead to obvious differences in electrochemical performance of SOFC and SOEC modes.As SOFC symmetric electrode,the increase of oxygen vacancies and the precipitation of Fe nanoparticles accelerate electrode activity,so the electrochemical performance enhances with the increase of Sr2+ content,and the performance of LSFTi5591,LSFTi3791 and SFTi cells reach 602 mW cm-2,847 mW cm-2 and 870 mW cm-2 at 800℃ respectively.The oxygen vacancy promotes the electrolytic performance in SOEC mode,but SrCO3 and FeO inhibit the electrolytic performance.The electrolytic performance reaches the highest value when Sr2 +content is 70%,while the performance is decreased to 0.39 A cm-2 when Sr2+ content reaches 100%.Therefore,SFTi91 is an excellent symmetric electrode for SOFC,but not suitable for SOEC.Aiming at the restrictive step of LSFTi3791 as symmetric electrode,the Ni ions is introduced to enhance the catalytic activity.At 800 ℃,La0.3Sr0.7Fe0.8Ti0.1Ni0.1O3-δ(LSFTNi811)can maintain the main phase of perovskite in CO2:CO=1:1 atmosphere,while it is completely decomposed into SrLaFeO4 and FeNi alloys in wet H2.Ni doping increases the Fe4+/3+ and oxygen vacancy content,which lead to the promotion of the electrical conductivity and gas adsorption,so the Rp of LSFTNi811 is decreased significantly in air,H2 and CO2:CO=1:1 atmosphere.When LSFTNi811 is used as SOEC symmetrical electrode to electrolyze CO2,the current density is increased from 1.57 A cm-2 to 1.78 A cm-2 at 800℃ and 1.5 V.In SOFC mode,the performance of LSFTNi811 cell is improved by 33%at 700℃,and the precipitation of NiFe nanoparticles makes the LSFTNi811 electrode maintain excellent catalytic activity at 800℃.Compared with LSFTi3791,LSFTNi811 has better performance as symmetric electrode in both SOFC and SOEC modes,while the decomposition in SOFC mode limits its application in reversible cycling.Therefore,LSFTi811 is an excellent symmetric electrode in single SOFC and SOEC mode,while LSFTi3791 has more advantages in reversible cycling.