Studies on Pr1-xLnxBa0.5Sr0.5Co0.5Fe1.5O5+δ (Ln = Nd, Gd, La; x = 0, 0.25, 0.5, 0.75 and 1) Double Perovskite Oxides for SOFC Application

Abstract

Good cathode material for solid oxide fuel cell (SOFC) should possess sufficient electronic conductivity (EC), compatible thermal expansion coefficient (TEC), phase stability, large triple phase boundary, able to transmit both ions and electrons, and low polarization resistance. Among perovskite materials possessing mixed ion and electronic conductivity (MIEC) properties, layered perovskites (AAʹB2O5+δ) appeared to be more advantageous due to their layered stacking, easier oxygen ion diffusion, and faster surface oxygen exchange. By introducing A-site cation doping, chemical and physical properties such as TEC, EC, oxygen non-stoichiometry, and ionic conductivity of materials can be altered. Thus, the aim of the present study is to observe the influence of A-site cation doping with Ln = Nd, Gd, La on the properties of Pr1 xLnxBa0.5Sr0.5Co0.5Fe1.5O5+δ (where x = 0, 0.25, 0.5, 0.75 and 1.0). The cathode powders and electrolyte gadolinium doped ceria (GDC) were synthesized by the Glycine-Nitrate method. Optimization of process parameters such as fuel to oxidant ratio and calcination temperature was performed to achieve phase purity of reference cathode material Pr1 xLnxBa0.5Sr0.5Co0.5Fe1.5O5+δ (x = 0). The optimized parameters were used to synthesis other doped compositions. Further, structural refinement was performed to obtain crystallographic information of the phase. Bulk samples were prepared from powders to attain dense ceramic properties. Variation in sintering time and the temperature was also conducted to study their effect on reduction in percentage porosity, grain growth, and on the EC behavior exhibited by the A-site doped Fe rich cathode systems. Thermal and chemical compatibility of cathode systems with GDC were examined from TEC measurements and by carrying out XRD analysis of sintered cathode-GDC mixtures respectively. Cathode inks were prepared and coated on GDC circular pellets for impedance analysis to analyze oxygen reduction kinetics. Finally, the electrochemical performance of the cathode systems was evaluated by fabricating symmetric cells with the configuration of Pr1 xLnxBa0.5Sr0.5Co0.5Fe1.5O5+δ/GDC/ Pr1 xLnxBa0.5Sr0.5Co0.5Fe1.5O5+δ. Electrochemical study exhibited that A-site doping had shown a significant effect on the cathode polarization resistance (Rp) and activation energy (Ea). The Rp and Ea values obtained for Ln = Nd, Gd, La-substituted cathode systems fall in the suitability criterion for cathode applications. Therefore, the above findings and observations led to recommend the cathode systems prepared in the present study as working electrode materials in SOFC stack for a power grid system.