Preparation and characterization of copper based cermet anodes for use in solid oxide fuel cells at intermediate temperatures

Two Cu-based anode cermets suitable for direct hydrocarbon oxidation in Solid Oxide Fuel Cells (SOFC) based on yttria stabilized zirconia (YSZ) electrolyte were tested in the temperature range (500-800A degrees C). The ceramic components were CeO2 and the perovskite La0.75Sr0.25Cr0.5Mn0.5O3-d (LSCM). The cermets were made in both the form of pellets and films applied onto the YSZ electrolytes. Pellets exhibited good mechanical strength and resistance to fracture in both oxidized and reduced state. Cu-LSCM cermets exhibited good redox cycling behavior between 700-800A degrees C. Reduction temperature plays a significant role on final morphology with Cu segregation occurring at 800A degrees C. Cu-LSCM films were found to exhibit lower polarization resistances than Cu-CeO2 under 5% H-2. Examination of the data revealed a poorer contact of the Cu-CeO2 electrode with the YSZ surface than the Cu-LSCM electrode. Reduction temperature should be less than 750A degrees C to ensure suitable microstructure and adhesion of both film electrodes with the electrolyte.</p

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

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 Y(0.2)Ti(0.18)Zr(0.62)O(1.9) and CuO as starting materials. Green pellets were sintered at 1000 degrees C for 10 hours resulting in sound pellets. These were successfully reduced in 5 % H(2) 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 sucessfully 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 O(2). Electrochemical testing performed using three-electrode geometry showed good performance for hydrogen oxidation for temperatures up to 800 degrees C. At higher temperatures up to 1000 degrees 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 degrees C.</p

Advanced anodes for high-temperature fuel cells

Fuel cells will undoubtedly find widespread use in this new millennium in the conversion of chemical to electrical energy, as they offer very high efficiencies and have unique scalability in electricity generation applications. The solid oxide fuel cell (SOFC) is one of the most exciting of these energy technologies; it is an all-ceramic device that operates at temperatures in the range 500-1000ºC. The SOFC offers certain advantages over lower temperature fuel cells, notably its ability to utilise CO as a fuel rather than being poisoned and the availability of high-grade exhaust heat for combined heat and power or combined cycle gas turbine applications. Although cost is clearly the most important barrier to widespread SOFC implementation, perhaps the most important technical barriers currently being addressed relate to the electrodes, particularly the fuel electrode or anode. In terms of mitigating global warming, the ability of the SOFC to utilise commonly available fuels at high efficiency, promises an effective and early reduction in carbon dioxide emissions and hence is one of the lead new technologies to improve the environment. Herein, we discuss recent developments of SOFC fuel electrodes that will enable the better utilisation of readily available fuels