Results And Discussion

As reported previously (5), the ionic transport number for all the electrolytes was confirmed to be unit)' at 800~1000°C, i.e.. no contribution of the electronic conduction. The ionic conductivities, cw, decreased in the order. 10Sc>8Yb>8Y>4Y> 3Y in the whole temperature region examined.

SDC Anode The values of log jo for H2 oxidation at the SDC anodes are plotted as a function of log (Tioti in Fig. 1. The jo at the SDC anodes were much larger than that at Pt anodes (5). and were independent of the al0„ at 900 and 1000°C. At 800°C. however, the jo increased linearly with the a,0„. Since these behaviors are quite different from those for the Pt anode (5). the reaction mechanism for the anodic oxidation of H2 at the SDC/zirconia is completely different from that at the Pt/zirconia. At high temperature in a H2 atmosphere. SDC is a good mixed conductor (10). The a,0„ of the SDC used in the present work is nearly comparable to that of lOSc-zirconia. and the electronic conductivity ae is about one-order of magnitude higher than the cw Therefore, oxide ions may be transported into SDC particles through the entire SDC/zirconia interface, and take part in the anodic oxidation of H2 at the SDC surface sites;

02"(zirconia) -> 0""(SDC): ion transfer through the interface [1]

0""(SDC) + H2(g) H20(g) + 2 e"(SDC): electron transfer at SDC surface [2]

As described previously (5). the supply rate of O"' to the SDC/zirconia interface is proportional to the (t,on of zirconia electrolyte. When step [1] is fast enough compared to the following step, the jo at the SDC must be independent of the supply rate of 02\ or ai0„, and the rate determining step (rds) at the SDC anode is considered to be step [2], which is the case experimentally observed at 900 and 1000°C. This is supported by our previous experimental evidences that the anodic polarization resistance Rp and its activation energy were greatly decreased by loading only a small amount of metal microcalatysts such as Ru, Rh, Ir and Pt onto the SDC particle surfaces at the cell using the 8Y-electrolyte (6). On the other hand, when step [2] is fast enough compared to step [1], the supply rate of O2' into the SDC must control the j0. The first order-dependence of j0 on <r,0„ {j0 ec alo„) observed at 800°C can be explained by this mechanism if one oxide ion is included in the overall reaction (n = 2) at the SDC as written by step [2].

Ru-SDC Anode In order to enhance step [2], Ru-microciystals were highly dispersed on the porous SDC surfaces where a large part of O2' transported via the SDC/zirconia interface is expected to contribute to the electron transfer reaction.

Ionic conductivity, crion IS cm'1

Fig. 1. Plots of the exchange current density jo for SDC anodes against <Jl0n of zirconia solid electrolytes in wet hydrogen (P[H2Q] = 0.042 atm).

Ionic conductivity, crion IS cm'1

Fig. 1. Plots of the exchange current density jo for SDC anodes against <Jl0n of zirconia solid electrolytes in wet hydrogen (P[H2Q] = 0.042 atm).

Figure 2 shows the values of logy0 for the Ru-SDC anodes as a function of log <j,c„ of the electrolytes. It must be emphasized that all of the data obtained at different temperatures and different electrolytes fall into a regression line as shown by the solid line, which is parallel to that for the SDC without Ru-catalysts at 800°C (dotted line). The slope of the least-squares fitting line was 1.1 and the correlation factor of the line was 0.97. Thus, the j0 increases linearly on the Ru-SDC anode with the increase of <j,0„ in the entire temperature region of 800 ~ 1000°C examined, resulting in the dramatic enhancement of step [2] at the SDC surfaces, while the jo on the anodes of SDC alone was leveled-off at 900 and 1000°C due to the insufficient catalytic activity. Therefore, the rds at 900 and 1000°C is shifted from step [2] to step [1] by loading Ru-microcatalysts. The parallel increase in the j0 at 800°C indicates that the effective surface area increases by the dispersion of Ru-micro-catalysts without changing the rds, i.e., step |1| for both SDC and Ru-SDC. It is very striking that the j0 on the lOSc-electrolyte at 800°C is higher than that on 4Y at 900"C. indicating the importance of the high a,on in electrolytes for low temperature operating SOFCs.

Pt Cathode As shown in Fig. 3, the values of jo for oxygen reduction at the Pt cathodes were independent of the a,„„ at 900 and 1000°C, but they increased linearly with the increase of aio„ at 800°C. The effective reaction zone for the Pt/zirconia interface is restricted to the portion around the physical triple-phase boundary (TPB) as shown in Fig. 4 (A). The constant jo independent of the am„ at 900 and 1000°C can be well explained when the rds at the Pt cathode is either dissociative adsorption of 02 or surface diffusion of Oad (ID. On the other hand, when the all steps other than step 5 are fast enough, the transport rate of O2' at the interface must control the jo- The first order-dependence of jo on a,0„ observed at 800°C can be explained by this mcclianism.

800 900 1000'C

O O 8Yb

0 0

Post a comment