Performance of La_(1.75)Sr_(0.25)NiO_(4+δ) Cathode on Ce_(0.8)Ln_(0.2)O_(2-δ) (Ln= La, Nd), LSGM and YSZ electrolytes^

摘要

La_(1.75)Sr_(0.25)NiO_(4+δ) layered perovskite has been synthesized to be evaluated as possible cathode material in Solid Oxide Fuel Cells (SOFCs). La2NiO_(4+δ) is a widely studied compound with very appealing transport properties, however it lacks of the adequate electronic conductivity, which can be improved by doping the A position with alkaline-earths. In this regard, the insertion of strontium increases the conductivity from 60 S·cm~(-1) (at 850 oC) for the undoped La2NiO_(4+δ) compound to values around 100 S·cm~(-1) for La_(1.75)Sr_(0.25)NiO_(4+δ). Moreover, this material exhibits a thermal expansion coefficient (TEC) value between 11-12x10~(-6) K~(-1) that matches well with the TEC value of the usual SOFC electrolytes. In this work the performance of La_(1.75)Sr_(0.25)NiO_(4+δ) cathode material on Ce_(0.8)Nd_(0.2)O_(2-δ), Ce_(0.8)La_(0.2)O_(2-δ), LSGM and YSZ electrolytes has been analyzed by interface polarization measurements. The analysis was performed by impedance spectroscopy in air, using symmetrical cells prepared after the deposition of porous La_(1.75)Sr_(0.25)NiO_(4+δ)-electrodes on each electrolyte system. XRD of cathode materials after used in symmetrical cells confirmed the absence of chemical reaction between the La_(1.75)Sr_(0.25)NiO_(4+δ) electrode and ceria based or LSGM electrolytes; however, a slight reaction between the cathode and YSZ forming the insulating La2Zr2O7 phase has been observed. In the studied systems, the use of composites as electrodes (La_(1.75)Sr_(0.25)NiO_(4+δ):electrolyte powder, 2:1 w:w) allowed to reduce significantly the polarization resistance. La_(1.75)Sr_(0.25)NiO_(4+δ)-based composite-cathodes on ceria-based an LSGM electrolytes gave ASR values well below 0.5 ohmcm~2 in the temperature range 850-1000 oC. The use of LSGM in these systems gives the lowest polarization resistance values at high temperatures(1000-850 oC) whereas ceria-based electrolytes are presented as the most promising ones at intermediate and low temperatures (800-600 oC).^