Preparation and Characterization of Copper/Yttria Titania Zirconia Cermets for Use as Possible Solid Oxide Fuel Cell Anodes

Abstract

The potential of a new anode cermet based on Cu and titania doped yttria stabilized zirconia (YZT) for high temperature fuel cells was examined. Cermets were prepared by a standard solid-state reaction using Y0.2Ti0.18Zr0.62O1.9 and CuO as starting materials. Green pellets were sintered at 1000 °C for 10 hours resulting in sound pellets. These were successfully reduced in 5 % H2 in Argon again yielding sound pellets with a porosity of 50 %. These were characterized by SEM, XRD and TGA methods and their conductivity measured by ac impedance and the 4 point DC method as a function of both temperature and oxygen partial pressure. On sintering there was evidence of a small amount of reaction between CuO and YZT. This resulted in a slight tetragonal distortion of YZT; however, most of the copper oxide was not incorporated into the zirconia. The cermet was successfully redox cycled and percolation was achieved when the copper composition exceeded 33 % of the volume. Conductivity remains high under a wide range of oxygen partial pressures from the most reducing conditions up to 10–4 atm O2. Electrochemical testing performed using three-electrode geometry showed good performance for hydrogen oxidation for temperatures up to 800 °C. At higher temperatures up to 1000 °C copper was observed to be very mobile with considerable agglomeration of metallic copper particles. Indeed in some instances there was a total segregation of copper from YZT resulting in a copper layer forming at the electrolyte interface with the outer layer of the electrode being essentially YZT. This agglomeration and migration of Cu led to a significant degradation in electrochemical performance with large increases in the series resistance and polarization resistance, especially under anodic bias. Due to these segregation problems copper based cermets produced in this manner are not thought to be good candidates for fuel cell electrodes operating at 1000 °C.