Results And Discussion

A typical cell with a single Pt voltage probe buried into the center of the electrolyte (100 pm from the cathode and anode) is shown in Figure 1. The Pt probe is - 5 pm thick and 200 pm wide. Since all electrolytes had thicknesses of 200 |im and cathodes and anodes had electrode areas of 0.4 cm2, the resistive loss associated with the electrolyte ( 82 mV at 1000. mA/cm2) was used as a baseline for all cell experiments.

Effect of LSM Calcination Temperature on the Electrode Reaction

Lao.79Sro.2Mn03 (LSM) powders were calcined between 800 and 1200°C in increments of 100° for 4 h, prepared as pastes, screen printed onto YSZ electrolytes, and densified at 1200°C. The initial overpotential (T|) - current density (j) results for Lao.79Sro.2Mn03 (LSM) powders which were calcined between 800 and I200°C are shown in Figure 2. All powders calcined below 1100°C show a similar electrochemical response with the 800 and 900°C calcinations showing the best performance (~ 60 and 80 mV at 1000 mA/cm2). The lower calcination temperatures have a smaller particle size and would therefore be expected to contain more reaction sites (TPBs) for the oxygen reduction reaction to occur. The opposite would be true for the 1200°C calcination temperature. Although the samples are sintered onto the YSZ electrolyte at the same temperature, their still is the tendency for the lower calcination temperatures to initially have a smaller particle size (more TPBs). The sintering of the LSM onto the electrolyte is constrained even at 1200° due to the densified YSZ support, this is also enables porosity to be introduced into the cathode microstructure. All sintering occurs in the z-direction rather than the x-y plane, therefore there is no change in the area of the electrode before and after sintering.

Measurements were again performed on all samples after 24 h of operation. Powders calcined at 800 and 900°C were not stable with time and resulted in higher overpotentials. Powders calcined at temperatures > 1000°C were stable within the 24 h time frame. Figure 3 illustrates the T| - j behavior of samples calcined at 800, 900 and 1000°C as a function of time. The sample calcined at 800°C had the largest change in the overpotential and was comparable to the sample calcined at 1200°C (- 170 mV at 1000 mA/cm2). The 900°C sample also increased its overpotential but to a smaller extent (~ 100 mV at 1000 mA/cm2). This can be explained by the

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