Performance Of Two Types Of Double Perovskite Cathode Materials For Intermediate-temperature Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are a type of electrochemical energy conversion devicesand provide electrical power with high efficiency and low pollution. So SOFCs are animportant clean power generation technology in future sustainable energy systems.Compared to traditional SOFC, low operation temperature of intermediate-temperaturesolid oxide fuel cells (IT-SOFCs) significantly reduce the cost of production and application.However, reducing the operation temperature inevitably leads to a large overpotential andcathodic polarization loss. Accordingly, it is necessary to develop a novel cathode materialwith high performance in the intermediate temperature range (600800°C).In this paper, two types of double perovskite cathode materials were studied as cathodematerials: A-site order double perovskite LnBaCo2O5+(Ln=rare rearth element); B-siteorder double perovskite A2B′B″O5+(A=alkali metal element, B=transition metalelement). The effect of the replacement of one transition-metal ion by another on the crystalstructure, valence sate, conductivity, thermal expansion and electrochemical performance,were investigated in detail. Moreover, the optimum component ratio of composite cathodeshas also been prepared to further optimize properties.112-type structure double-perovskite oxides LnBaCo2O5+have been extensively used asIT-SOFC cathode materials because of their excellent oxygen diffusivity and conductivityproperties. However, they have certain disadvantages hamper practical application, such as,high thermal expansion coefficient (TEC) and cost of cobalt, and a poor long-term chemicalstability. We synthesized the double perovskite LnBaCoFeO5+(Ln=Pr, Nd; PBCF, NBCF)cathodes using sol-gel method. PBCF and NBCF materials are chemically compatible withLa0.9Sr0.1Ga0.8Mg0.2O3–(LSGM) and Sm0.2Ce0.8O1.9(SDC) electrolyte at temperatures below1000°C and1150°C, respectively. The XPS analysis shows that the PBCF andNBCF double-perovskites exist different valence states (i.e.,[Pr3+/4+][Ba2+][Co3+/4+][Fe3+/4+]O5+,[Nd3+][Ba2+][Co3+/4+][Fe3+/4+]O5+). Thermal expansioncoefficients (TECs) of PBCF and NBCF are21.0×106and19.5×106K1, respectively,between30and1000°C; these are lower than the TECs of undoped LnBaCo2O5+. The bestelectrical conductivity is321and172S cm1for PBCF and NBCF, respectively.Polarization resistances of PBCF and NBCF cathodes on LSGM electrolyte are0.049and0.062cm2at800°C, respectively. Maximum power densities of the single-cell withNi SDC as anode on a0.3mm-thick LSGM electrolyte reach749and669mW cm2forPBCF and NBCF cathodes at800C, respectively. Besides, P(N)BCF–xCe0.8Sm0.2O1.9(x=20–50wt.%; P(N)BCF–xSDC) composite materials efficiently reduce the TECs andimprove electrochemical perforemance. The polarization resistances are0.037Ω cm2and0.046Ω cm2at800oC for the optimum composition of PBCF–40SDC and NBCF–30SDCcomposite cathodes, respectively. The maximum power densities are960and891mW cm–2for PBCF–40SDC and NBCF–30SDC composite cathodes at800oC, respectively.Although the TECs of PBCF and NBCF are lower than that of LnBaCo2O5+, the TECsare still larger. Therefore, Fe and Cu co-doped PrBaCo2/3Fe2/3Cu2/3O5+(PBCFC) andNdBaCo2/3Fe2/3Cu2/3O5+(NBCFC) are synthesized by the sol-gel method. PBCFC andNBCFC are found to crystallize in a tetragonal structure, have good chemical compatibilitywith the LSGM, SDC, and Ce0.9Gd0.1O1.95(GDC) electrolytes. The XPS analysis shows thatthe PBCFC and NBCFC exist different valence states (i.e.,[Pr3+/4+][Ba2+][Co3+/4+]2/3[Fe3+/4+]2/3[Cu+/2+]2/3O5+,[Nd3+][Ba2+][Co3+/4+]2/3[Fe3+/4+]2/3[Cu+/2+]2/3O5+). The maximum electrical conductivity of PBCFC and NBCFC samples are144S cm1and92S cm1, respectively. TEC values ofPBCFC and NBCFC samples are16.6×106K1and15.7×106K1between30and850C, respectively. The polarization resistances of PBCFC and NBCFC cathodes on GDCelectrolyte are0.038and0.056Ω cm2at800C, respectively. The maximum powerdensities of single cells with PBCFC and NBCFC cathodes on a300-μm thick GDCelectrolyte reach659and535mW cm2at800C, respectively. Addition of appropriateamounts of GDC into PBCFC and NBCFC to form PBCFC–GDC and NBCFC–GDCcomposite cathodes further reduce the TEC and improve cathode performance.B-site order double perovskite oxides La(Ca)SrCoTiO5+(LCCT, LSCT) have beensuccessfully synthesized by sol-gel method. Rietveld refinement has revealed that LCCTand LSCT samples exhibit a cubic structure space group Fm3m (225), with the adoptionof an ideal Co/Ti order double perovskites. LCCT and LSCT have good chemicalcompatibility with LSGM and SDC electrolytes at firing temperature of1000°C for10h.XPS results indicated that mixed valence states of Co3+/Co2+(a ratio of55:45and54:46)coexist in LCCT and LSCT samples. Electrical conductivity with increasing temperatureexhibits a typical p-type semiconducting behavior. Substitution of Ti for Co efficientlyreduced the average TEC of LSCT compared to undoped LaSrCo2O5+(La0.5Sr0.5CoO3).The area specific resistance and the maximum power density are0.246cm2and354mWcm2at800C for LCCT cathode, are0.056cm2and776mW cm2at800oC for LSCTcathode respectively. The addition of SDC into LCCT and LSCT cathodes formLCCT–SDC and LSCT–SDC composite cathodes further reduce the TEC and improveelectrochemical performance. The TEC values of LCCT–50SDC and LCCT–50SDCcomposite cathodes are13.9×10–6K–1and15.1×10–6K–1. The polarization resistances are0.133Ω cm2and0.036Ω cm2at800oC for the optimum composition of LCCT–20SDCand LSCT–30SDC composite cathodes, respectively. The maximum power densities of0.3mm thick LSGM electrolyte-supported single cells are454mW cm–2and828mW cm–2at800oC for LCCT–20SDC and LCCT–30SDC composite cathodes, respectively. Theseresults demonstrate that LSCT and its composite materials are promising candidates ascathodes.Double perovskite Sr2CoFeO5+(SCF) has been successfully synthesized with cubicstructure. The SCF is chemically compatible with LSGM, SDC, and GDC electrolytes attemperature below950oC. The electrical conductivity of SCF sample reach maximum is321S cm1. The XPS analysis shows that Fe and Co ions exist in two different valencestates, i.e. Co4+/Co3+and Fe4+/Fe3+. Oxygen temperature programmed desorption (O2TPD)reveals SCF–SDC composite is more chemical stability than that of pure SCF sample.Compared to pure SCF (24.4×106K1), the SCF–xSDC composite materials efficientlyreduce TECs, e.g., the TEC of SCF–40SDC is16.3×106K1. At800C, the polarizationresistance and maximum power density are0.036Ω cm2and757mW cm–2for theoptimum composition of SCF–40SDC composite cathode, respectively. These resultsdemonstrate that SCF SDC composite materials are promising candidates as cathode forapplication in IT-SOFC.In order to improve properties of SCF, Al and Mo doped Sr2Co0.6X0.4FeO5+(X=Al, Mo;SCAF, SCMF) as a novel cathode has been successfully synthesized in air by sol-gelmethod. X-ray diffraction refinement reveals cubicdouble perovskite structure with thespace group Fm3m (225), and have good chemical compatibility with LSGM and SDCelectrolytes at temperature below950C. O2TPD demonstrates that SCAF and SCMF are more chemical stability than that of SCF sample. The XPS analysis shows that the SCAFand SCMF double-perovskites exist different valence states (i.e.,[Sr2+][Co3+/4+]0.6[Al3+]0.4[Fe3+/4+]O5.57,[Sr2+]2[Co3+/4+]0.6[Mo5+/6+]0.4[Fe3+/4+]O5.68). SCAFand SCMF displays a typical p-type semiconducting behavior, the maximum conductivityare45S cm1and129S cm1, respectively. TECs of SCAF and SCMF are20.6×106K1and19.4×106K1between30and1000°C, which is significantly lower than that ofundoped SCF. At750°C, the polarization resistances values with LSGM and SDCelectrolytes for SCAF cathode are0.070and0.068Ω cm2, for SCMF cathode are0.068and0.056Ω cm2, respectively. The maximum power densities of0.3mm thick SDCelectrolyte-supported single cells at800°C are706mW cm2and718mW cm2for SCAFand SCMF cathodes, respectively. These results demonstrate that SCAF and SCMF cathodematerials are promising candidates as cathode for application in IT-SOFC.