Preparation And Properties Of La, Fe Co-doped SrTiO₃ Oxide Anode Material For Solid Oxide Fuel Cell

Ni/YSZ (8.mol% yttria-stabilized zirconia) cermet is the most pratical anode material which has been commonly used in solid oxide fuel cell (SOFC). When H2 was used as fuel, this kind of anode shows excellent performance, but there are still some shortcomings with this material: (1) carbon deposition will happen when hydrocarbon is used as fuel; (2) if the natural gas or coal gas are directly used as fuel, the issue of sulfur poisoning will appears; (3) nickel may sinter during SOFC operating in high temperature for long time, thus affecting the long-term stability of the electrode. Since the storage of H2 is not sufficient enough and the techniques of preparing H2 are limited, coal gas, methane and other fossil fuels should be used directly in the SOFC for power generation, so how to try to use hydrocarbon, such as methane and natural gas, as fuels of SOFCs is the key of study in this field presently. Therefore Donor-substituted SrTiO₃ with mixed ion-electron conductivity and good electro-catalytic properties on hydrocarbon fuel has become a hot spot on research about anodes of SOFCs.In this work, La and Fe co-doped SrTiO₃ materials were synthesized by solid state reaction method in deoxidized atmosphere generated by using active carbon, which can not only simplifies the experimental apparatus, but also eliminate security risks when hydrogen were used as reducing atmosphere. It can be known by X-Ray patterns of La_0.3Sr_0.7TiO₃ that deoxidize atmosphere generating by activated carbon can be used to prepare samples with a similar crystal structure as that of H2. More Ti³⁺ and higher conductivity were obtained in samples synthesized in deoxidize atmosphere.La_0.3Sr_0.7Ti_1-xFe_xO₃ (x =0, 0.05, 0.10, LSTF00,LSTF05,LSTF10) samples were synthesized by solid state reaction and properties of these anode were tested and discussed in this work. A good match of thermal expansion coefficient between LSTF and YSZ electrolyte material was found throughout the temperature range. Different amount of Ti³⁺ is detected in samples synthesized in different atmosphere. The results of high-temperature conductivity show us that Ti³⁺ plays an important role in the electrical properties of LSTF. Samples prepared in deoxidize atmosphere have higher conductivities, in which the conductivity of La_0.3Sr_0.7TiO₃ reached 292 S·cm^-1 at 800oC. After Fe was doped into La_0.3Sr_0.7TiO₃, the conductivities of LSTF05 and LSTF10 decreased, but still reached 131 S·cm^-1 and 110 S·cm^-1 at 800 oC, respectively. YSZ electrolyte supported single cells were fabricated with La and Fe co-doped SrTiO₃ perovskite anode and silver cathode. Output performances and impedance spectras were tested to investigate the influences of doping Fe on the electrochemical performance of LSTF anode materials. The results show that doping Fe can reduce the polarization resistance, and increase the maximum power density of cells. In order to further improve the electrochemical performance of LSTF anode, composite anode materials were studied in this paper. A small amount of Ni was added in LSTF anodes with a method of nitrate solution impregnation, and the performances of anodes were significantly improved. After introducing SDC into LSTF, ionic conductance was improved and anode materials with better performance were obtained. The maximum power density of cells with Ni impregnated anodes were 4⁵ times higher than that that using anodes without Ni, showing that impregnation of Ni can indeed increase the ability of anodes to convert H2 into H+. SDC can accelerated the migration of oxygen ions, reduce the transporting path of fuel, and further improve the cell's power density, a high maximum power density of 366 mW·cm^-2 was obtained at 850 oC when Ni and SDC co-impregnated LSTF10 as anode. At last, because that anodes based on SrTiO₃ can be free of carbon deposition and sulfur poisoning, LSTF05 was employed as anode of a direct carbon fuel cell (DCFC) and a maximum power density of 26 mW·cm^-2 was obtained at 900 oC.In conclusion, La_0.3Sr_0.7Ti_1-xFe_xO₃ is a promising anode candidate for SOFC and DCFC.