Activation and Ripening of Impregnated Manganese Containing Perovskite SOFC Electrodes under Redox Cycling

The impregnation of electrode precursor solutions is a very powerful technique for creating novel electrode microstructures constrained within preformed scaffolds. Here we report on the microstructural evolution of Mn-containing perovskites impregnated into yttria stabilized zirconia scaffolds on heating and redox cycling. Good performances have previously been reported for SOFC anodes with similar structure, and our objective is to better understand the origins of this good performance. For La0.75Sr0.25Cr0.5Mn0.5O3-d a remarkable thin coating with microfissures is formed on the scaffold after firing the electrode precursors at 1200 °C, and such behavior can be considered as wetting of one oxide by another. On further treating this microstructure at 800 °C in H2 the microstructure changes dramatically forming an interconnected array of 10 nm scale particles. This seems to offer a very attractive structure with extensive triple phase boundary regions where electrochemical reactions can occur. On reoxidation at this temperature the particles reagglomerate to form a structure approaching the initial smooth coating. Performing similar procedures on the system La0.33Sr0.67TixMn1-xO3±d, we find that the wetting only occurs if Mn is present in the oxide and that the degree of wetting increases with Mn concentration. This favorable interaction between the Mn containing perovskites and the zirconia scaffold must be associated with a chemical interaction between impregnated oxide and substrate. The strength of this interaction decreases on reduction allowing the perovskite electrode to form nanoscale particles which along with appropriate additional catalysts provide good electrode functionality.