Noble metal alloy-Zr(Sc)O2 cermet cathode for reduced-temperature SOFCs

Polarization characteristics of noble metal alloy-(Sc0.10Ce0.01)Zr0.89O2 (SSZ) cermet cathodes were studied in order to develop a new cathode for reduced-temperature solid oxide fuel cells (SOFCs). Several noble metal alloy-SSZ cermet cathodes were prepared by mixing Pt, Pd, Rh and/or Ag and their alloy powders with SSZ powder using a high-energy ball mill in vacuum and pasting the cermet onto yttria stabilized zirconia (YSZ) electrolyte. A Pt-Ag/SSZ cermet cathode achieved as high as 12 S/cm(2) of interfacial conductivity, sigma (E), at 973 K and 1.5 S/cm(2) at 873 K in air. The results suggest that this Pt-Ag/SSZ cermet cathode has enough activity not only at 973 K but also at 873 K. This high activity can be obtained by selecting a suitable alloy composition, ball milling a proper ratio of SSZ/noble metal mixture in vacuum and controlling the cathode thickness and the sintering temperature. By replacing the metallic component of cermet from Pt to Pt-Ag alloy (50 wt.% Pt), the quantity of Pt in cermet can be reduced to 19 from 40 Mg/cm(2) in addition to the improvement of activity from 6.7 S/cm(2) at 973 K to 12 S/cm(2) of sigma (E) at 973 K. The activation energies, E-a, of Pt-Ag and Pd-Ag/SSZ cermet were smaller than that of Pt/SSZ cermet. In the case of Pt-Ag/SSZ cermet, the E-a decreased with increasing Ag ratio in the Pt-Ag alloy. The E-a also depends on the SSZ/Pt-Ag ratio. This cathode showed two optima of sigma (E) versus the SSZ/Pt-Ag ratio and a remarkable dependence on cathode thickness. It was considered that the first optimum is based on two-dimensional reaction sites on YSZ electrolyte and the second optimum originates from three-dimensional expansion of reaction sites into the cermet cathode layer. (C) 2001 Elsevier Science B.V. All rights reserved.