Structures And Properties Of Cathode And Interconnect Materials For Intermediate Temperature Solid Oxide Fuel Cells

A fuel cell is an energy conversion device with a high efficiency, which can convert chemical energy directly into electricity without pollution. A fuel cell is a new green energy developed in the world for 21 century.A solid oxide fuel cell (SOFC) is a kind of energy generator with high efficiency and low pollution, and it is undoubtedly one of the cleanest energy convertors and a suitable device for environment maintenance. As the key component of SOFC, the cathode plays a very important role in the performance of the whole cell. The properties and microstructures of cathode material determined the cathodic reaction mechanism. To improve the cathode properties, it is necessary to develop the preparation technique.In this study, a cathode material Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃-δ(BSCF) was prepared using glycine nitrate process (GNP). The crystal structures, conductivities and microstructures of samples sintered at different temperatures were investigated in order to optimize the properties of cathode material.X-ray diffraction results showes that the samples sintered at 1000, 1050 and 1100℃(referred to as BSCF1000, BSCF1050 and BSCF1100) presents acubic perovskite structure. The lattice parameter of BSCF1000 and BSCF1050 is a=0.39814nm and a=0.39824nm, respectively. The value is very close to the result reported by Shao and Haile, which is a=0.39830nm and a = 0.39832 nm, respectively.Sintering temperature strongly infulenced electrical properties of BSCF samples. The results show that BSCF1050 sample has the optimal electrical conductivity, which attains 32.2S/cm at 850℃in air. Electrical conductivity of BSCF1100 sample is relatively low, due to the segregation of CoO from substrate due to the evaporation of BaO.The grain sizes of BSCF samples increased with increase of sintering temperature. The pores between grains decreased, and the samples were much dense. However, the segregation of CoO was observed on the surface for BSCF1100 sample.The results of EDAX show that Ba content in BSCF1100 sample decreases and Co content increases. This demostrated the segregation of CoO was caused by the evaporation of BaO.In conclusion, to optimize the properties of cathode material for SOFCs, we prepared BSCF cathode material with GNP. The results show that sample sintered at 1050℃presents a good electrical conductivity.As is well known, the open-circuit voltage of a single cell is very low, this maks it difficult to use such a cell directly. To meet the requirement of application, the interconnect material is used to connect single cells in series and in parallel to achieve high output voltage, current and power. The interconnect material should have good electrical conductivity and phase stability, chemical stability in both reducing and oxidizing conditions. Moreover, the thermal expansion coefficient (TEC) should be close to those of the other cell components.In this study, NdCrO₃-based interconnectors with perovskite-type were synthesized using the solid state reaction method. The crystal structure,electrical conductivity and thermal expansion of NdCr_1-xMg_xO₃ were invesitgated, in order to assess the propertie of NdCr_1-xMg_xO₃ as an interconnect material.The results show that all the Mg-doped NdCrO₃ samples fabricated are single orthorbombic perovskite phase, which was as same as that of NdCrO₃. The cell volumes for the NdCr_1-xMg_xO₃ perovsite phase increased with the Mg contents. The volumes increased rapidly doping with <15mol% Mg on the B site. However, the volumes have no significant change in the Mg content rang from 15 to 25mol%, which may be due to approaching the solubility limit.Direct current electrical conductivity was measured from 300 to 850℃in air by the van der Pauw method. The electrical conductivity increased with increasing Mg content. The electrical conductivity of NdCr_0.75Mg_0.25O₃ sample is superior, which attains 14.2S/cm at 850℃in air. The electrical conductivity was higher by two orders of magnitude than that of pure NdCrO₃.NdCrO₃ possesses poor sinterability because of the presence of a large amount of remaining pores, which is in accordance with the minimum relative density of 89.8%. The samples of NdCrO₃ doped with Mg exhibited more dense microstructures formed by brick-shaped grains. It can be seen that NdCr_0.75Mg_0.25O₃ sample exhibited excellent densification. The results suggest that NdCr1-x MgxO3 samples prepared by the present method were highly sinterable in air. Mg-doped NdCrO₃ samples have very excellent sinterability from microscopic view.The sintering density results show that the relative densities are greatly improved by Mg-doping at B-site for NdCrO₃. The relative density is already more than 95% with the presence of 15mol% Mg on the B site,which NdCr_0.75Mg_0.25O₃ has a highest density, and attains 97.4%. It can be found that Mg-doping of the NdCrO₃ perovskites are effectively for improving theirsintering characteristics. This is in agreement with SEM results.The EDAX results show that the chromium-vapor of undoped NdCrO₃ is heavy. The chromium-vapor decreased with increasing Mg content. This means that Mg doping effectively suppress the chromium-vapor in NdCrO₃. Therefore, Mg doping evidently improved the sintering characteristics of NdCr_1-xMg_xO₃ samples.The higher TEC with increasing Mg content can be explained by the increase of the average ionic radius of B-site metals in the perovskite structure during the thermal expansion measurements. In this study, NdCr0.75 Mg0.25O3 shows an excellent thermal expansion coefficient of 9.53×10^-6/℃. The results indicate that NdCr_0.75Mg_0.25O₃ has well matched TEC with that of YSZ electrolyte.In conclusion, to develop interconnect materials for application in intermediate temperature SOFCs, the NdCrO₃-based solid solutions with perovskite type were prepared and investigated. The results show that NdCr_0.75Mg_0.25O₃ has a high electrical conductivity and high density,indicating that NdCr_0.75Mg_0.25O₃ is a kind of potential interconnect material for application in intermediate temperature SOFCs.