The solid electrolytes employed were zirconia doped with yttria, ytterbia or scandia. Zirconias with the composition of (Zr02)i.x(Y203)x (X= 0.03-0.08) and (ZrO:)0 .«(YIhOJoos were prepared by the same manner as in ref. 5. Zirconia with the highest a,„„, (ZrOiJogsCScaO^o.ioCAKO^o.oi, was also prepared (9). The zirconia specimens will be denoted as 3Y, 4Y, 8Y, 8Yb, and lOSc, respectively. The relative density of each specimen to the theoretical one was more than 96 %. The construction of the experimental fuel cell is as described in ref. 6. The anodes tested were porous SDC. (Ce02)n8(Sm0i 5)o2, with and without loading Ru catalysts. The cathodes tested were porous Pt and LSM. LaossSro uMnOj, with and without loading Pt catalysts. Porous Pt counter electrode was used for each cell. These electrodes were prepared on the zirconia disk by screen-printing method, followed by firing at 1050°C (SDC, Pt) or 1030°C (LSM) for 4 h. Microcrystalline Ru and Pt catalysts were highly dispersed with 0.5 mg/cm2 on SDC and LSM, respectively, in the same manner as in ref. 6. The projected surface area for each electrode was 0.26 cm3. Two gold wires for current supply and potential probe were contacted to a gold-mesh current collector attached to each electrode. A platinum wire was wound around the lateral of the electrolyte disk with Pt paste as a reference electrode which exhibited a reversible oxygen potential in air. The anode and cathode compartments were separated by the electrolyte disk and each compartment was sealed by a glass ring gasket.

Hydrogen gas saturated with water vapor at 30°C (P[H2OJ = 0.042 atm) was introduced to the anode compartment, and oxygen gas at 1 atm was supplied to the cathode compartment. The IR-free polarization characteristics of the electrodes were measured by a current-interruption method at 800~1000°C. The exchange current density,/a, was determined from the polarization resistance (Rp) in a low overpotential region less than 0.1 V; Rp = (RT/2F) jo'1.

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