Study On Preparation Of La_n+1Ni_nO_3n+1 Cathode Material And Its Application In SOFC

Reducing the operation temperature of the solid oxide fuel cell(SOFC) to the intermediate temperature could overcome the problems caused by the high temperature operation. Lan+1Nin O3n+1, which are classified as the Ruddlesden-Popper series, are composed of perovskite alternating with La O rock salt layers. In this paper, we systematically studied the physical and electrochemical properties of Lan+1Nin O3n+1 in order to get the new high performance cathode materials.Firstly, Lan+1Nin O3n+1(n= 1, 2, 3, 4, 5) pure phase was synthesized in 1100 o C by sol-gel methode and studied the chemical compatibility with common electrolyte Sm0.2Ce0.8O1.9(SDC). No obvious chemical reaction occurs under the condition of1100 o C, but diffraction peaks of SDC shifted to the lower angle direction, which may be a solid solution between Sm3+ and La3+ causing the lattice expansion. Conductivity testing by DC four terminal methode found that its conductivity increasing as the perovskite layers n increasing at the same temperature.In the low temperature region,Arrhenius curve is almost linear, indicating that the main conductive manner consistent with the small polaron hopping mechanism. The electrical conductivity of La5Ni4O13 and La6Ni5O16 are all above 100Scm-1from room temperature to 800 oC, which meeting the requirement of SOFC cathode material for electrical conductivity. Thermal expansion performance analysis shows that TEC generally decreased with the perovskite layers n increasing, and the reduction in 1.9%-4.2%.In addition, in order to study the electrochemical properties of materials, five kinds of cathode materials were used to produce cathode slurry, making half cell using SDC as electrolyte and the AC impedance spectroscopy test using conventional three terminal methode. The impedance spectra at 700 oC showed that the La3Ni2O7 cathode material having the smallest polarization impedance, which about 0.28Ωcm2, and the polarization impedance gradually increased with n increasing from 2 to 5. In order to study the electrochemical reaction control step of Lan+1Nin O3n+1(n= 1, 2, 3), the EIS of the half-cell were carried out in different oxygen partial pressure. The low frequence arc represents the progress of oxygen adsorption and dissociation, and the high frequence arc represents the charge transfer progress in TPB. Charge transfer progress is the rate control step of cathode reaction. The section SEM of half-cell shows that cathode firmly attached to the SDC electrolyte.Finally, electrolyte supported Ni-SDC/SDC/Lan+1Nin O3n+1 single cell was prepared, and tested in double chamber. Comparing the output of five single cell at700 o C, the open circuit voltage is between 0.8 to 0.85 V. The maximum power densityof cells can reach to 49.49, 37.63, 46.03, 38.77 and 44.73 m Wcm-2, respectively.In general, Lan+1Nin O3n+1 cathode material has good performance in the role of conductivity and electrochemical properties. The material properties could be further improved by optimizing the synthesis conditions and microstructure, but it needs to be further studied.