Preparation and Properties of Interconnect Materials for Solid Oxide Fuel Cells
|Course||Condensed Matter Physics|
|Keywords||Solid oxide fuel cell Perovskite Interconnect material Doped lanthanide chromite Sintering Electrical conductivity Thermal expansion coefficient|
Solid oxide fuel cells （SOFCs） are high efficiency and clean power system, which convert the chemical energy of the fuels directly to electrical energy. As an open circuit voltage of single cell at operation is approximately 1 V, the cells must be connected electrically in series with the help of an interconnect material to provide higher voltage and power. In SOFCs, the interconnect not only serves as an electrical contact between individual cells but also separates the fuel gas from the oxidant simultaneously. Therefore, a number of stringent requirements are needed to be satisfied for a good interconnect material at operating temperature of the fuel cells, such as adequate electronic conductivity in both reducing and oxidizing atmospheres, good chemical stability in both atmospheres, and matched thermal expansion coefficient with other cell components under operating conditions. There are few materials that could be used as interconnect materials. The results showed that alkaline earth （Mg, Ca, or Sr） doped LaCrO3 perovskites are considered the most promising interconnect materials because they meet most of these criteria.In this study, NdCrO3-based interconnect materials (Nd1-xSrxCrO3) with an orthorhombic perovskite structure, were synthesized with solid state reaction method. The crystal structure, electrical conductivity, thermal expansion behavior and sinterability of the samples Nd1-xSrxCrO3 were assessed and analyzed. The effects of Sr doping at A-site and Cr-deficiency at B-site on the crystal structure and properties of NdCrO3-based interconnect materials were studied systemically. In order to develop novel interconnect material with high sintered density and electrical conductivity for SOFCs.X-ray diffraction （XRD） characterization of Nd1-xSrxCrO3 solid solutions shows that they all possessed a single phase orthorhombic perovskite crystal structure. No any phase changes are observed from room temperature to 1000oC. It turns out either doping of Sr ions or introduction of Cr defects into the lattice didn’t change the crystal structure but only lattice parameters. And the solubility of doping in A-site is up to 0.25.Sr doping at A-site has greatly improved the sinterability and thermal expansion coefficient （TEC） of Nd1-xSrxCrO3 materials. The relative density of Nd0.75Sr0.25CrO3 is 97.8%, which also possesses the highest electrical conductivity of 17.3S cm-1 at 850oC in air and TEC of 9.45×10-6K-1 between 30 and 1000℃. The thermal expansion behavior of sample Nd0.75Sr0.25CrO3 is in good compatibility with that of YSZ (10.3×10-6K-1). And observation through scanning electron microscope （SEM） revealed that sample Nd0.75Sr0.25CrO3 also has tightly coalesced grains with the smallest average size and lowest porosity.The sinterability, electrical conductivity and thermal expansion behavior of Cr-deficient Nd0.75Sr0.25Cr1-δO3 samples all get worse with increase of the fraction of Cr-vacancies. Sample Nd0.75Sr0.25Cr0.98O3 has the highest relative density of 95.5% as well as the highest electrical conductivity of 18.8S cm-1 at 850oC in air and TEC of 9.62×10-6 K-1 between 30 and 1000 oC. All the results besides sinterability are better than those of Nd0.75Sr0.25CrO3 and Nd0.75Sr0.25Cr0.98O3 is in better compatibility with YSZ. Nd0.75Sr0.25CrO3 and Nd0.75Sr0.25Cr0.98O3 are a promising interconnect material for SOFCs in all respects of densification, electrical conductivity and TEC.Nano-structured La0.70Ca0.30CrO3 solid solution was synthesized with glycine-nitrate process （GNP）. The powders of La0.70Ca0.30CrO3 with an average crystalline size of 22 nm were obtained through calcining the precursors at 700 oC. XRD results of samples sintered at 1400 oC, 1450 oC and 1500oC suggests that all the samples possess a crystal structure of single phase perovskite at room temperature and no phase change has ever been observed up to 1000oC. The sample sintered at 1450oC has the best sinterability. The relative density is nearly 100%. Its electrical conductivity is higher than the other two as well, which reaches 55.5S cm-1 in air and 4.90S cm-1 in H2 at 850℃. The TEC of the sample is 9.93×10-6 K-1 between 30 and 1000℃, nearly the same as that of YSZ. All these results show that 1450oC is the best sintering temperature for La0.70Ca0.30CrO3 solid solution. The sintering temperature of the samples can be efficiently reduced through using the powders of the nanocrystalline La0.70Ca0.30CrO3 synthesized by GNP, and improve obviously the sintering and electrical properties.