REVERSIBLE ELECTRODES BASED ON COBALT BRONZES FOR SOLID-STATE POTENTIOMETRIC GAS SENSORS

The impedance measuring method has been used to investigate the properties of the interface between a solid sodium-conducting electrolyte of the type NASICON and an ionic-electronic conductivity NaxCoO2 (sodium-cobalt bronze), which is available in four crystallographic modifications with different contents of sodium. It was reported that the less the sodium content in bronze, the higher was the Na+ reversibility of this interface. This is accounted for by greater deviations from the stoichiometry in the sodium sublattice and, therefore, by a greater ability to inject sodium. Bronzes whose compositions range from 0.55 < x < 1.0 show a metallic nature of conductivity, while within the range 0.55 < x < 0.70 (corresponding to the beta-phase) the bronzes exhibit potential independence (Na-reference electrode) from the amount of sodium injected.