Research On SrFeO_3-δ-based Electrode Materials For Solid Oxide Cells

In view of the importance of clean energy utilization and CO2 emission reduction,this paper proposes the use of solid oxide cell for efficient and clean power generation of coalbed methane gas and resource utilization of CO2.Solid oxide cells(SOC)are the general term of solid oxide fuel cells(SOFC)and solid oxide electrolysis cells(SOFC),and they are reversible operations of the same cell structure.Considering that the traditional SOFC nickel-based anode will produce serious carbon deposition when using hydrocarbon fuel,which will lead to cell performance degradation,therefore the concept of a split anode catalyst layer is proposed,and SrFeO3-δ perovskite based catalyst materials are further investigated in the case of methane(CH4)or coalbed methane(CBG,eomposition:CH4,82.9975%;O2,2.1853%;N2,10.1839%;C2-C8,3.4731%;CO2,1.1602%)as fuel.In the case of fuel,the application of catalyst layer could improve the cell performance and increase the coking resistance of the Ni-based anode.Based on the above work,A-site-deficiency and Ce-doping(Ce0.08La0.52Sr0.3)Cr0.5Fe0.5O3-δ electrode material[(Ce+A)-LSCrF]is achieved by doping the high catalytic activity Ce and fabricating deficiency as well on the A-sites of(La0.7Sr0.3)Cr0.5Fe0.5O3-δ materials.It is found that A-site defects and Ce-doping in the material could effectively improve the concentration of oxygen vacancies content,thus improves the efficiency of CO2 electrolysis.The 8%-Y2O3 stable ZrO2(YSZ)thin film with high surface roughness was prepared by dry pressing/quenching/calcining process,and combined with(Ce+A)-LSCrF electrode material,the faraday efficiency was 97.2%.They are introduced as follows:Research of La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCFO)independent catalyst layer on SOFC fueled by CH4 and CBG.LSCFO catalyst was prepared by EDTA-CA method,and it was applied as an independent catalyst layer to study its electrochemical performance,coking resistance and discharge stability under CH4 and CBG fuels.Under SOFC operating conditions,LSCFO exhibits high catalytic activity for methane partial oxidation with 88%conversion at 950℃ in a mixture of CH4 and O2(1:1).The application of LSCFO catalyst layer could improve the cell performance.For example,the maximum power density(PPD)of the cell without the catalyst layer in CH4(VH2O:VCH4=3:97)and wet coal bed gas(CBG)(VH2O:VCBG=3:97)is 0.50 W cm-2 and 0.55 W cm-2,the PPD of cells with LSCFO catalyst layer increased to 0.66 W cm-2 and 0.59 W cm-2 under the same conditions,respectively.In addition,the application of the LSCFO catalyst layer can improve the stability of the cell.Traditional blank cells show poor stability and their voltage drops rapidly within 30 minutes in wet CH4 and wet CBG fuels.The larger voltage drop of the modified cell in CBG is attributed to the lower catalytic activity of LSCFO on heavy hydrocarbon compounds in CBG.EDX analysis showed that,for the cell with LSCFO catalyst layer,after 116 h the total carbon content of the anode surface using CH4 is about 4.62 wt.%,more than 3.21 wt.%of that after exposure to CBG for 12 h.Therefore,the application of LSCFO independent catalytic layer could have a good coking resistance and improve the electrochemical performance under wet methane-based fuels.Research of double perovskite Sr2MoFeO6-δ(SMFO)independent catalyst layer on SOFC fueled by CH4 and CBG.It has demonstrated that applying an independent catalyst layer could increase the coking resistance of the Ni-based anode.However,LSCFO is unstable under low oxygen partial pressure and the metal may segregate after long-term operation,which would reduce the cell stability.Thus,a redox-stable double-perovskite SMFO is applied as an independent on-cell reforming catalyst with the aim of improving coking resistance of a Ni-YSZ anode.The results indicate that the SMFO-modified cell shows better performance and stability than those tested of the conventional cell without the catalyst layer when fueled with methane-based fuel,which is attributed to the SMFO catalyst could reform CH4 into the coking insusceptible CO and H2.However,because the SMFO catalyst shows moderate catalytic activity for methane conversion,coking on the Ni-based anode still occurs after a long operation period.For the SMFO-modified cell tested in wet CBG for 55 h,the mean-carbon-content(MCC)on the surface of the anode and catalyst are 49.5 at.%and 13.3 at.%,respectively,which are higher than the MCCs of 19.6 at.%and 13.1 at.%in wet CH4 under the same condition,indicating that the heavy carbon compounds in CBG may accelerate carbon deposition.In fact,the loading mode of the catalyst layer facilitates the detailed analysis of the anode surface because the catalyst layer and the anode are mutually independent.In addition,the catalyst loading mode could be applied in the tubular SOFC stacks,in which the solid oxide tube functions as the anode or cathode,and other cell components are coated around the tube in layers.Research of LaO.7Sr0.3Cr0.8Fe0.2O3-δ(LSCrFO)independent catalyst layer on SOFC fueled by CH4 and CBG.Although SMFO protective layer could greatly improve the cell durability,there are still some problems.For example,SMFO can be decomposed into poorly conductive SrMoO4 and SrFeO3-δ at high temperatures in air,which may affect its catalytic activity.Besides,it was reported that SMFO could react with H2O to form Sr(OH)2 at low temperatures,restricting its potential applications in SOCs.In this work,a pure perovskite LSCrFO was prepared and it shows desirable stability under both oxidizing and reducing conditions.Compared with the traditional blank cell,the LSCrFO-modified cell exhibits good electrochemical performance and high durability under methane-based fuel Compared with the blank cell,under wet CH4 and wet CBG fuel the PPD of the LSCrFO-modified cell improved by 20%and 7%,respectively.The EDX analysis shows that,after operation on wet CH4 over 100 h,the related MCC of the catalyst and anode surface of the LSCrFO-modified cell were about 7.34 at.%and 18.61at.%,respectively.While MCC on the fresh the catalyst and anode surface were 6.38 at.%and 22.4 at.%,respectively.After operation on wet CBG for 35 h,MCC on the catalyst and anode surface were 7.88 at.%and 42.34 at.%,which was higher than that on wet CH4.This may ascribe to the influence of the heavy carbon compounds in CBG.The above results indicate that the application of the LSCrFO catalyst layer could improve coking resistance of a Ni-YSZ anode.Indeed,the independent catalyst layer could perform not only as a catalyst for fuel pre-reforming,but also as a protection layer to avoid direct contact between methane and the Ni-based anode Therefore,the probability of CH4 cracking on Ni particles greatly decreased.In fact,it is also necessary to adjust the porous properties and thickness of the catalyst layer to improve coking resistance.For the catalysts LSCFO,SMFO and LSCrFO,the redox-stabilized perovskite LSCrFO catalyst has become the most preferred among these three catalysts in terms of the stability of the catalyst material and the catalytic activity;On the basis of the above work,non-stoichiometric A-site defects and Ce-doping were further introduced for the La0.7Sr0.3Cr0.5Fe0.5O3-δ catalyst as cathode material of YSZ supported SOEC for high-temperature CO2 electrolysis.CO2 electrolysis using SOEC has been drawing a remarkable amount of interest due to their high efficiencies in converting the greenhouse gas CO2 to high-value-added chemicals from renewable electrical energy The YSZ electrolyte film prepared by the traditional method of dry pressing/calcination/grinding is thick and the surface is not enough rough for electrolyte-supported SOCs.A thin and rough YSZ film can be prepared by the dry pressing/quenching/calcining method.The thicknesses of the as-prepared YSZ films were as low as 78 μm and the root-mean-square(RMS)roughness of the films was 493 nm.For a YSZ-supported SOCs,a thin YSZ film could reduce the ohmic resistance and the rough surface of the electrolyte facilitates close contact between the electrodes and electrolyte,reducing the interlayer mass transfer resistance,thereby improving the performance of the cell However,the further application of SOEC for CO2 electrolysis is still limited by the low electrocatalytic activity and the weak chemical adsorption/activation of CO2 for fuel electrode materials.In this article,A-site-deficiency and Ce-doping(Ce0.08La0.52Sr0.3)Cr0.5Fe0.5O3-δ electrode material[(Ce+A)-LSCrF]is achieved by doping the high catalytic activity Ce and fabricating deficiency as well on the A-sites of(La0.7Sr0.3)Cr0.5Fe0.5O3-δ materials.This novel material shows favorable redox reversibility under reducing and oxidizing atmospheres.TGA,XPS and CO2-TPD analysis showed that A-site-deficiency and Ce in(Ce+A)-LSCrF synergistically promote the formation of oxygen vacancies and greatly improve the ability to chemisorb/activate CO2.DFT calculations indicate that the higher oxygen vacancy concentration in the LaSrCrFe-based perovskite material will cause it to release more energy when adsorbing CO2,thereby facilitating the adsorption of CO2.Combining the high-surface roughness YSZ electrolyte film that prepared by the dry pressing/quenching/calcining method,a significant improved current density of 2.21 A cm-2 is achieved for(Ce+A)-LSCrF at 850℃.Moreover,the cell demonstrated desirable electrolysis durability for 106 h in CO2/CO(70:30)fuel.Therefore,the redox-stable(Ce+A)-LSCrF material would provide a promising alternative for high-performance CO2 electrolysis.