Effect of sintering temperature on microstructure and performance of LSM–YSZ composite cathodes

The correlation between sintering temperature, microstructure and performance of composite electrodes comprising lanthanum strontium manganate (LSM) and yttria stabilised zirconia (YSZ) with a current collector of LSM has been studied at 1000°C in air. The microstructure was found to be less dense and to contain smaller grains as the sintering temperature was decreased in the range 1300–1150°C. This increased the active triple phase boundary line (TPBL) between electrode, electrolyte and gas phase, leading to a decrease in the polarisation resistance with decreasing sintering temperature. When keeping the sintering temperature of the composite structure constant at 1300°C the performance was found to improve when lowering the sintering temperature of the current collector. This may suggest that a large part of the sites, which are active to oxygen reduction, are situated at the interface between the composite and the current collector. This interface is assumed to become more important with respect to oxygen reduction as the sintering temperature of the composite layer is increased, due to loss of porosity in this layer. An experiment with in-situ sintering of the composite and the current collector was performed in the temperature range 600–1050°C with regular measurements during heat-up and sintering. When measuring at a reference temperature of 850°C the lowest polarisation resistance was found for sintering temperatures of 950–1000°C. When increasing the sintering temperature to 1050°C the increase in the polarisation resistance was counterbalanced by a decrease in the series resistance. The optimum sintering temperature with respect to the initial performance is assumed to be where good physical and electrical contact between LSM and YSZ is obtained in addition to a long TPBL. When the optimum sintering temperature for obtaining high initial performance is found, the next step is to study the long term stability of the electrode structure during operation.