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• 800'C o 800°C - 24 h B90œc □ 900°C-24h ■ HXWC O1000'C-24h

• 800'C o 800°C - 24 h B90œc □ 900°C-24h ■ HXWC O1000'C-24h

r| cathode

IR Electrolyte ' il

0 200 400 600 800 1000 1200 1400 Current Density (mA/cm2)

Figure 2. Initial T|-j relations of the Lao.79Srn2MnOj composition sintered on the electrolyte at 1200°C and calcined at _various temperatures._

Figure 3. ri-j relations initially and after 24 h of operation for the Lao.79Sim.jMn03 compositions sintered on the electrolyte at 1200°C and calcined at 800,900, and 1000°C.

powder further coarsening during operation which would give rise to a larger contact area and a reduced number of TPBs. The smallest particles are expected to have the highest driving force for densification which may explain the largest change in the electrochemical results for the 800°C calcination. Higher calcination temperatures (>1000°C) may also show similar behavior for operation times greater than 24 h.

Effect of Composition and Sintering Temperature on Cathode Performance

The compositions studied were Lao.79Sro.2Mn03 (1% excess Mn), Lao.75Sro.2Mn03 (5% excess Mn), and Lao.7oSro.2Mn03 (10% excess Mn) calcined at 900°C for 4 h then sintered on the YSZ electrolyte at temperatures between 1100-1400°C. Figures 4 and 5 are ri-j plots of the three compositions sintered onto the YSZ electrolyte 1200 and 1400°C. Clearly the 1200°C firing condition yields a large variation for the compositions, the 10 % excess Mn shows an extremely small overpotential (- 40 mV at 1000 mA/crn2). The 5% excess Mn composition also shows a lower overpotential (~ 80 mV at 1000 mA/cm2) than the near stoichiometric sample (~ 105 mV at 1000 mA/cm2). Because the grain sizes for the cathodes were observed to be very similar, this cannot be used as the' basis for the large variation in the electrochemical behavior. X-ray diffraction on bilayers was performed to see if a second phase (i.e. La2Zr207) could be detected at the interface. The XRD patterns for the three compositions annealed at 1200°C showed the same diffraction peaks corresponding to the YSZ and LSM phases except for the Lao.79Sro.2Mn03 sample which had an extra peak at -28.5°, corresponding to the 100 % intensity line of the La2Zr207 phase. Therefore, the addition of excess Mn is effective in decreasing the pyrochtore phase formation at the interface at higher temperatures (< 1200°C).

Current Density (mA/cm2) Current Density (mA/cm2)

Figure 4. T|-j relations of the LSM Figure 5. T(-j relations of the three LSM compositions calcined at 900° compositions calcined at 900°C _and sintered at 1200°C._and sintered at 1400"C._

Current Density (mA/cm2) Current Density (mA/cm2)

Figure 4. T|-j relations of the LSM Figure 5. T(-j relations of the three LSM compositions calcined at 900° compositions calcined at 900°C _and sintered at 1200°C._and sintered at 1400"C._

The overpotential results for the three compositions fired at 1400°C are shown in Figure 5. The 1 % and 10 % excess Mn compositions show a consistent trend with the lower sintering temperatures, namely, that the overpotentials are larger due to both a larger grain size and an increased reaction at the interface. The overpotential values for the 1 and 10 % excess samples at 1000 mA/cm2 are - 210 and 100 mV. The 5 % excess sample has a very high overpotential 320 mV at lOOOmA/cm2), and is not consistent with the previously observed trends. Some possible explanations could be that the sample was cracked or improperly sealed which allowed the mixing of the fuel and oxidant which could reduce the oxygen pressure in the cathode chamber. Another possible explanation is that the sample has not equilibrated within the 24 h period.

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