Electrode materials that exhibit mixed conductivity (that is, both electronic and ionic conductivity) have been investigated in a continuing effort to improve the performance of the alkali metal thermal-to-electric converter (AMTEC). These electrode materials are intended primarily for use on the cathode side of the sodium-ion-conducting solid electrolyte of a sodium-based AMTEC cell. They may also prove useful in sodium-sulfur batteries, which are under study for use in electric vehicles.
An understanding of the roles played by the two types of conduction in the cathode of a sodium-based AMTEC cell is prerequisite to understanding the advantages afforded by these materials. In a sodium-based AMTEC cell, the anode face of an anode/solid-electrolyte/cathode sandwich is exposed to Na vapor at a suitable pressure. Upon making contact with the solid electrolyte on the anode side, Na atoms oxidize to form Na+ ions and electrons. Na+ ions then travel through the electrolyte to the cathode. Na+ ions leave the electrolyte at the cathode/electrolyte interface and are reduced by electrons that have been conducted through an external electrical load from the anode to the cathode. Once the Na+ ions have been reduced to Na atoms, they travel through the cathode to vaporize into a volume where the Na vapor pressure is much lower than it is on the anode side. Thus, the cathode design is subject to competing requirements to be thin enough to allow transport of sodium to the low-pressure side, yet thick enough to afford adequate electronic conductivity.
The concept underlying the development of the present mixed conducting electrode materials is the following: The constraint on the thickness of the cathode can be eased by incorporating Na+-ion-conducting material to facilitate transport of sodium through the cathode in ionic form. At the same time, by virtue of the electronically conducting material mixed with the ionically conducting material, reduction of Na+ ions to Na atoms can take place throughout the thickness of the cathode. The net effect is to reduce the diffusion and flow resistance to sodium through the electrode while reducing the electronic resistance by providing shorter conduction paths for electrons. Reduced resistance to both sodium transport and electronic conductivity results in an increase in electric power output.
Previous research had shown that mixed-conducting electrodes to improve the performance of an AMTEC cell could be made from mixtures of Mo (an electronic conductor) and Na2MoO4 (an ionic conductor) or of W (an electronic conductor) and Na2WO4 (an ionic conductor). Unfortunately, electrodes made of these mixtures do not last long: the vapor pressures of Na2MoO4 and Na2WO4 are so high at the typical operating temperature of a sodium AMTEC cell (between 1,020 and 1,120 K) that these ionically conducting materials evaporate within a few hundred hours of operation.
The present mixed-conducting electrode materials are mixtures of Mo (as before, an electronic conductor) and NaxTiO2 (a conductor of both electrons and Na+ ions). NaxTiO2 can be formed by exposing TiO2 to Na vapor at a temperature >900 °C. NaxTiO2 is a mixture of Na-Ti-O compounds, all of which are electronically conducting by virtue of the conductivity of TiO2, and ionically conducting toward Na+ ions. In an experiment, an electrode made from a mixture of equal weight proportions of Mo and TiO2 was found to perform well at a temperature of 830 °C (1,103 K) for >1,000 hours, with no significant change in either electrode power or transport properties. Moreover, the performance of this electrode was found to equal or exceed the performance of the best previously known AMTEC electrode, which was made of RhW.
This work was done by Margaret Ryan, Roger Williams, Margie Homer, and Liana Lara of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Materials category. NPO-20920.