| A fuel cell is an energy conversion device with a high efficiency, which can convert chemical energy directly into electricity without pollution, which is a new green energy developed in the world for 21 century.Solid Oxide Fuel Cell (SOFC) is attracting substantial interest as it is regarded as the most efficient and versatile power generation system, in particular for distributed power generation. The current operating temperature is around 1000°C, however substantial efforts are under way to reduce the operating temperatures to 500-800°C. Lowering the operation temperature of SOFCs can not only extend the range of material selection, significantly reduce the cost of production and application, but also improve the stability and reliability for the SOFC system. However, the electrochemical activity of the cathode dramatically decreases with decreasing temperature. The cathode becomes the limiting factor in determining the overall cell performance. Therefore, the development of new electrodes with high electrocatalytic activity for the oxygen-reduction reaction is critical for intermediate-temperature (IT)-SOFCs。Studies showed that 112-type structure double-perovskite oxides, LnBaCo2O5+δ(Ln=rare earth),are mixed ionic and electronic conductors with Rapid oxygen ion diffusion and surface exchange kinetics, which have been attracting much attention as potential cathode material for IT-SOFCs. The samples of LnBaCo2O5+δ(LnBCO)(Ln=Pr, Nd, Sm, and Gd) were prepared by the conventional solid state reaction method. The basic physical properties of these materials were investagated. The suitability of LnBCO as cathode materials for IT-SOFCs were evaluated. The results show that LnBCO cathodes are chemically compatible with the intermediate-temperature electrolyte materials such as Sm0.2Ce0.8O1.9 (SDC) and La0.9Sr0.1Ga0.8Mg0.2O3Ωδ(LSGM). No chemical reaction of the binary-mixed LnBCO–SDC and LnBCO–LSGM systems is detected upon sintering at 1000oC for 5h. The thermal expansion coeficients (TECs) of the LnBCO cathodes are relatively high , and is similar to the other Co-based single-perovskite. The average TEC values decrease from Ln =Pr to Gd in the temperature range of 30-1000oC. The conductivity of the LnBCO samples decreases as the measuring temperature is increased, which presentes metallic-like behavior. The faster decrease in conductivity at higher temperatures could be due to the formation of significant amount of oxide ion vacancies. The formation of oxide ion vacancies is accompanied by a reduction of Co4+ to Co3+, resulting in a decrease in the charge carrier concentration and Co–O covalency. In addition, the conductivity of the samples LnBCO decreases with decreasing radius of rare earth ions. The conductivity of samples LnBCO follows the sequence:σPBCO >σNBCO >σSBCO >σGBCO. However, the lowest electrical conductivity for all the samples is still higher than 300 S cm-1 from 300 to 850oC. The polarization resistances for PBCO, NBCO, SBCO and GBCO materials on LSGM electrolyte are 0.025Ωcm2, 0.029Ωcm2, 0.031Ωcm2, 0.053Ωcm2 at 800oC, respectively. The polarization resistances of all the samples are all lower than 0.15Ωcm2 at 700oC for the LnBCO cathodes on LSGM. For LnBCO/LSGM/SDC/Ni-SDC single cell, the maximum power density are 815 mW cm-2, 775 mW cm-2, 723 mW cm-2 and 661mW cm-2 800oC, respectively. The cell performance also presents a trend which reduces in turn from Pr to Gd. This result is consistent with the polarization resistance and the conductivity of LnBCO materials. It can be seen from the SEM micrographs of the cross-section between LnBCO and LSGM electrolyte, the particle distributions in the cathode are not homogeneous and particle sizes are also larger. In addition, the half-cell and single-cell performances with SBCO cathode on SDC electrolyte were studied. The results show that the polarization resistances for SBCO cathode on SDC electrolyte are 056Ωcm2 at 800oC, 0.098Ωcm2 at 750oC and 0.190Ωcm2 at 700oC, respectively. For SBCO/SDC/NiO-SDC single cell, the maximum power density is 641 mW cm-2 at 800oC. LnBCO-SDC composite cathodes were also prepared and characterized. The results show that the TECs of the LnBCO cathodes can be reduced through adding SDC electrolyte into the LnBCO to form the composite cathodes. However, it is found that the addition of SDC into LnBCO results in a slight inscrease of the polarization resistance compared to the LnBCO cathodes.. The polarization resistance is still lower than 0.15Ωcm2 at 700oC. The power density of cell with composite cathode is slightly lower than that of LnBCO cathodes. The maximum and the minimum power density for LnBCO-SDC/LSGM/SDC/Ni-SDC single cell are 758 mW cm-2 and 608 mW cm-2 at 800oC, respectively.Double-perovskite oxides LnBaCuMO5+δhave been used as the catalyst materials and the semiconductor gas sensor materials. However, the information as the high temperature cell materials has not been reported to date. Double-perovskites cuprate materials LnBaCuMO5+δ(LnBCM)(Ln=La, Gd; M=Fe, Co) were synthesized by solid-state reaction method. The high temperature conductivity, chemical compatibility, TEC and electrochemical performance of the materials were investigated. In the IT-SOFC operating temperature ranges 500-800oC, the conductivity of the sample LBCF is 71-157 S cm-1, the conductivity of the sample LBCC is 291-408 S cm-1, which is adequate for the material to be used as a cathode in SOFCs. However, the conductivity for sample GBCC is 20-58 S cm-1, which is relatively low compared to the both materials. LBCF, LBCC and GBCC cathodes are chemically compatible with SDC electrolyte. The average TECs of the LBCF, LBCC and GBCC materials are 17.0×10-6 K-1, 18.3×10-6 K-1 and 15.1×10-6 K-1in the temperature range of 30-850oC, respectively. LBCF, LBCC and GBCC materials presents the high-electrocatalytic activity. The polarization resistances on SDC electrolyte are 0.11Ωcm2, 0.06Ωcm2 and 0.13Ωcm2 at 750oC, respectively. The power density of the cell with LBCF, LBCC and GBCC as cathodes attains 557 mW cm-2, 603 mW cm-2 and 528 mW cm-2 at 800oC, respectively. It is found from the SEM micrographs of the cross-section between LBCF(LBCC and GBCC) and SDC electrolyte, the particle distributions in the cathode are not homogeneous and particle sizes are also larger.In order to improve the properties of the LnBaCuCoO5+δ(Ln=La, Gd) cathode materials, and reduce the TECs. The composite cathodes of the LBCO-SDC and GBCC-SDC were prepared. The results show that the TECs of LBCC-SDC and GBCC-SDC composite cathodes reduce through adding 10wt%, 20wt%, 30wt%, 40wt% SDC to LBCO and GBCC, respectively. The TECs of the LBCC-SDC composite cathodes are 17.7×10-6 K-1, 16.0×10-6 K-1, 15.4×10-6 K-1 and 14.7×10-6 K-1 in the temperature range of 30-850oC, respectively. The TECs for GBCC-SDC composite cathodes are 14.7×10-6 K-1, 14.5×10-6 K-1, 14.1×10-6 K-1, and 13.5×10-6 K-1 over the same range, respectively. The results of the conductivity for the composite cathodes show that the conductivity decreases regularly with the increase of SDC addition. However, the conducting mechanism is the same for the both pure cathodes and the composite cathode. For LBCC cathode, the conductivity of the LBCC-SDC40 composite cathode reduces to 64 S cm–1 as the SDC addition is 40wt%. However, an exciting result is that the conductivity of the composite cathodes is still higher than 100 S cm-1 as the SDC addition is less 30wt%. For GBCC cathode,.The conductivity of compesite cathodes is 40 S cm-1 for GBCC-SDC10,30 S cm-1 for GBCC-SDC20,23 S cm-1 for GBCC-SDC30, and 17 S cm-1 for GBCC-SDC40. In addition, the polarization resistance of the composit cathodes is also improved. The optimum electrochemical performance of composite cathodes can achieve through adding 20wt% SDC into both the LBCC and GBCC cathodes. The polarization resistance is 0.028Ωcm2 for LBCC-SDC20 and 0.066Ωcm2 for GBCC-SDC20 at 750oC, respectively.In conclusion, to develop new cathode materials for application in IT-SOFCs, double-perovskite materials, LnBaCo2O5+δand LnBaCuMO5+δ(Ln = rare earth elements), were prepared and investigated. The results show that double-perovskite cathode is a very promising potential cathode material for application in IT-SOFCs.
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