E

SOO 900 1000'C

Fig. 3. Plots of jo for Pt cathode in 02 against a,on of zirconia electrolytes.

Fig. 3. Plots of jo for Pt cathode in 02 against a,on of zirconia electrolytes.

LSM Cathode As shown in Fig. 4 (B), the adsorbed oxygen can be ionized at the mixed conducting surfaces of LSM besides TPB-path. The j0 at the LSM cathodes increased linearly with the increase of alo„ at each temperature, and also increased with elevating the temperature (Fig. 5). This suggests that the rds was apparently mixed with the transport of oxide ions of step 4 and the reaction step 2 and/or 3 in Fig. 4(B).

A.Pt cathode 1) Difiusion of 02(gas phase) 02 2) Adsorption of 02 to

\ form Old

Pad 3) Surface diffusion of „0«, OadtoTPB

5) Transfer of O2' (from TPB into YSZ)

1) Difiusion of 02(gas phase)

2) Adsorption of 02 to form

4) Transfer of O2* (through LSM into YSZ, or from TPB into YSZ)

Fig. 4. Schematic illustrations of elementary reaction steps at (A) Pt and (B) LSM cathodes.

Pt-LSM Cathode When such surface reactions at step 2 and/or 3 were activated by Pt-microcatalysts dispersed on the surfaces of porous LSM. the j0 was increased dramatically at 800 and 900°C. depending on the a,on. As shown in Fig. 5. all the data obtained at different temperatures and different electrolytes fell into a regression line as shown by the solid line. However, the slope of the line was 1.7. which is larger than that expected for the case controlled simply by the ionic transfer step {jo oc cr,0„). We expect that further improvement of the electrocatalytic activity, up to the upper limit purely o;„„-controlled (dotted line), may be achieved on Pt-/LSM below 900°C, since the performances of catalyzed LSM cathodes were found to increase further with decreasing the particle size of Pt-catalysts (8).

The present study reveals that the high a,0„ in the electrolyte reduces not only the ohmic loss but also the polarization losses at the anode and cathode. Consequently, the combination of the solid electrolyte having high crio„ and the electrodes consisting of good mixed conductors loaded with inicrocatalysts is very important to achieve high performance SOFCs which can operate at low temperatures.

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