Properties Of SrCoO_3-δ Based Cathode Materials For Intermediate-temperature Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) represent one of the cleanest, most efficient and versatile power generating technologies that convert chemical energy directly into electrical energy. The traditional SOFCs are operated at high temperature (~1000°C). The high operating temperature can cause complex materials problems, such as electrode sintering and interfacial reaction between electrolyte and electrode materials. It is thus desirable to operate SOFCs at an intermediate temperature range of around 600-800°C. A major requirement for the intermediate temperature-operating SOFCs (IT-SOFCs) is to develop new mixed ionic-electronic conductors (MIECs) as cathode materials with better performance that also fulfill with the cell requirements. The developments of many cathode materials for IT-SOFCs have been reviewed recently by Tsipis and Kharton.Perovskite oxide SrCoO3-δis a very important parent compound that can be further developed into many functional materials. The A-site and B-site doped SrCoO3-δoxides have been paid much more attention due to their potential and technical applications in oxygen separation membranes, methane conversion reactors and SOFC cathodes. The structural evolution research showed that the SrCoO3-δoxide exists in three different polymorphs: (i) the orthorhombic "O" brownmillerite phase between room temperature and 653 oC, (ii) the hexagonal "H" phase between 653 oC and 920 oC, and (iii) the cubic perovskite "C" phase above 920 oC, which is transformed again into the "H" phase at 774 oC when cooled. In the different polymorphs of SrCoO3-δ, high-temperature SrCoO3-δphases with cubic 3C-like crystal structures are MIECs, which exhibits the highest electrical conductivity and oxygen permeability values. However, the SrCoO3-δoxide with 2H BaNiO3-type structure (2H-like hexagonal structures) undergoes phase transitions when it is heated in air. This phase transition causes abrupt changes in the thermal expansion coefficient that would result in cracking problems during the preparation and operation of SOFCs. Moreover, the SrCoO3-δoxide with 2H-like hexagonal structure at room temperature was demonstrated to be almost non-oxygen permeable. The structural instabilities thus limit this material for further application in oxygen separation devices and SOFCs. Therefore, it is important to inhibit the structural phase transition in SrCoO3-δ, and to stabilize the high temperature cubic phase to low temperature. In order to stabilize the cubic phase in SrCoO3-δoxide, one commonly used method is to dope proper cations for either A-site or B-site to improve the phase stability. Many efforts have been made to stabilize the SrCoO3-δcubic perovskite by doping with various elements. Among these dopant efforts, Nagai et al.demonstrated that Nb was the most effective dopant in SrCo0.9M0.1O3-δ(M = Cr, Fe, Al, Ga, Ti, Zr, Sn, V and Nb) oxides for improving the phase stability and oxygen permeability. More recently, Zhang et al. We systematically investigated new SrCo1?yNbyO3?δ(SCNy) ceramic membranes with high oxygen semi-permeability. Their results indicated that the Nb doping content had a significant effect on the phase structure stability, electrical conductivity and oxygen permeability of the SrCoO3-δoxides. Moreover, the substitution of Nb for Co in SCNy oxides stabilizes the cubic perovskite phase to room temperature at y≤0.2, thus significantly extends the application scopes of the SCNy materials. However, there is little information on the performance of SCNy as cathode materials for IT-SOFCs up to date. In this paper, the perovskite oxides SrCo1-yNbyO3-δ(SCNy, y = 0.00-0.20) were synthesized by a solid-state reaction. The properties of SCNy oxides as a potential cathode material were systematically investigated and assessed. The performance of single-cell with SCNy cathodes based on La0.9Sr0.1Ga0.8Mg0.2O3-δ(LSGM) electrolyte was also tested. As the B-site doped successful attempt, the paper also have did some valuable tests of the A-site SrCoO3-δoxides with doping Ce.Perovskite oxides SrCo1-yNbyO3-δ(SCNy, y = 0.00-0.20) are investigated on LSGM electrolyte. Compared to the undoped SrCoO3-δ, the Nb doping significantly improves the thermal stability and enhances the electrical conductivity of the SCNy oxides. The cubic phase of the SCNy oxides with high thermal stability can be totally obtained when the Nb doping content y≥0.10. Among the investigated compositions, the SrCo0.9Nb0.1O3?δoxide exhibits the highest electrical conductivity of 461-145 S cm-1 over the temperature range of 300-800 oC in air. The SCNy cathode has a good chemical compatibility with the LSGM electrolyte for temperatures up to 1050°C for 5 h. The area specific resistances of SCNy with y = 0.10, 0.15 and 0.20 cathodes on LSGM electrolyte are 0.083, 0.099 and 0.110 ? cm2 at 700 oC, respectively. At y = 0.10, 0.15 and 0.20, the maximum power densities of a single-cell with SCNy cathodes on 300-μm thick LSGM electrolyte achieve 675, 642 and 625 mW cm-2 at 800 oC, respectively. These results indicate that SCNy perovskite oxides with cubic phase are potential cathode materials for application in IT-SOFCs.Perovskite oxides Sr1-xCexCoO3-δ(SCCx, x=0.00-0.20) are investigated on SDC electrolyte. Compared to the undoped SrCoO3-δ, the Ce doping significantly improves the thermal stability and enhances the electrical conductivity of the SCCx oxides. The cubic phase of the SCCx oxides with high thermal stability can be totally obtained when the Ce doping content x≦ 0.15. Among the investigated compositions, the Sr0.95Ceo.o5CoO3-δoxide exhibits the highest electrical conductivity of 362-521 S cm-1 over the temperature range of 600-800 oC in air. The SCCx cathode has a good chemical compatibility with the SDC electrolyte for temperatures up to 1100°C for 2 h. The area specific resistances of SCCx with x = 0.05, 0.10 and 0.15 cathodes on SDC electrolyte are 0.0384, 0.0424 and 0.0472 ? cm2 at 800 oC, respectively. At y = 0.05, 0.10 and 0.15, the maximum power densities of a single-cell with SCNy cathodes on 300μm thick SDC electrolyte achieve 470, 369and168 mW cm-2 at 800 oC, respectively. These results indicate that SCCx perovskite oxides are potential cathode materials for application in IT-SOFCs.