Properties Of Double-perovskite YBaCo₂O_5+δ Based Cathodes For Intermediate-temperature Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) have become of great interest because of the advantage of directly using hydrocarbon fuels without requiring external fuel and pollution-free energy. However practical application of SOFCs is still limited for economical reasons, particularly as result of the high costs of component materials. The cathode and electrolyte materials are the two key components of SOFCs. Researching some novel materials, simplifying the preparation of materials and using cheaper metals can reduce the costs of the cell and make SOFCs to be commercial.In this study, the oxides YBaCo_2-xFe_xO_5+δ(x=0, 0.2, 0.4 and 0.6) with double-perovskite structure were synthesized with a solid-state method and evaluated as cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Low-cost iron which has similar chemical properties with cobalt was doped in the B-site in order to further reduce costs of the electrode. The double perovskite structure, thermal expansion coefficient (TEC), the chemical compatibility, and power densities of the single cell etc were investigated in order to acquire low cost, thermal expansion coefficient and high performance cathode materials.YBaCo_2-xFe_xO_5+δ(x=0, 0.2, 0.4 and 0.6) samples with double-perovskite structure can been prepared with the solid-state reaction. The cathode materials have good compatibility and connection with the electrolyte La0.9Sr0.1Ga0.8Mg0.115Co0.085O2.85 (LSGMCO). The lattice parameters and cell volumes of YBCF increase with increasing Fe concentration. The samples exhibit a metal-semiconductor transition at about 350 oC. The maximum conductivity of = 315 S cm-1 at 325 oC was observed. The average TEC of YBCF oxides is about between 16.3 and 18.3×10-6 K-1 in air over the temperature rang 30 oC to 900 oC. The ASRs for YBaCo_2-xFe_xO_5+δ(x=0, 0.2, 0.4 and 0.6) samples are 0.11, 0.13, 0.15 and 0.099 cm2 at 700 oC, respectively. The maximum output power density of single cells with YBaCo_2-xFe_xO_5+δ(x=0, 0.2, 0.4 and 0.6) cathodes achieved 873.1, 767.5, 706.0 and 799.0 mW cm?2 severally. From an economic and performance point of view, YBCF2 and YBCF6 are most suitable for the cathode material of SOFCs. The obtained encouraging results in our work suggested that the YBCF oxides were very promising cathode materials for IT-SOFCs.The cathode materials YBa_0.5Sr_0.5Co₂O_5+δand YSrCo2O5+ were prepared with a solid-state method. The performance of the materials were assessed comprehensively by XRD, electrical conductivity, TEC, impedance, single-cell output power density and open circuit voltage test. The results indicated that the content of Sr affect the lattice structure of the materials. The electrical conductivity of YBSC is higher and a metal-semiconductor transition occurred at about 325 oC. When the temperature is higher than 400 oC the electrical conductivity of YSCO tends to a constant. In the temperature range of 30-850 oC the TEC of YSCO and YBSC are 17.7 K-1 and 18.8 K-1. The output power density of single cells with YSCO and YBSC cathodes achieved 768and 650 mW cm?2 at 850 oC respectively. Further study is necessary on the compatibility between the cathode and electrolyte and farther improvement of electrical performance.At last the highly phase-pure perovskite electrolyte, La0.9Sr0.1Ga0.8Mg0.115Co0.085O2.85, was prepared by means of glycine-nitrate process. The perovskite phase evolution, sintering, electrical conductivity and electrochemical performance of LSGMCO were investigated. The results showed that the highly phase-pure perovskite electrolyte LSGMCO can be obtained after calcining at 1150 oC. The TEC of samples LSGMCO and LSGM9182 is 13.1×10-6 K-1 and 11.5×10-6 K-1 between 30 oC and 1000 oC in air, respectively. The partial substitution of Co2+ ion for Mg2+ ion would cause the increase of cell volume, thus resulting in the TEC of LSGMCO is slightly larger than that of LSGM9182. The electrical conductivities of the sample was 0.124 S cm-1 at 850 oC in air. The sample sintered at 1450 oC for 20h in air exhibited a better sinteribility, and the relative density of LSGMCO was higher than 95%. The stoichiometric indexes of the elements in the sintered sample LSGMCO determined experimentally by EDS were in good agreement with the nominal composition but the actual value of Co concentration in the sintered sample was slightly lower than the nominal value. This was mainly due to the volatilization of Co of the sample in high temperature sintering. The maximum power densities of the single cell fabricated with LSGMCO electrolyte with Ce0.8Sm0.2O1.9 (SDC) interlayer, SmBaCo2O5+x cathode and NiO/SDC anode achieved 802 mW cm-2 at 850 oC. These suggested that perovskite oxide LSGMCO was a promising electrolyte for IT-SOFCs.