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/cm2 of interfacial conductivity, σE, at 973 K and 1.5 S/cm2 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/cm2 in addition to the improvement of activity from 6.7 S/cm2 at 973 K to 12 S/cm2 of σE at 973 K. The activation energies, Ea, 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 Ea decreased with increasing Ag ratio in the Pt–Ag alloy. The Ea also depends on the SSZ/Pt–Ag ratio. This cathode showed two optima of σE 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.