Planar NASICON-based CO₂ sensor using BiCuVO_x /Perovskite-type oxide as a solid-reference electrode

To improve the stability of a NASICON (Na3 Zr2 Si2 PO12; Na+ conductor)-based potentiometric CO2 sensor under humid conditions, a composite of BiCuVOx (Bi2 Cu0.1 V0.9 O5.35) and perovskite-type oxide (La0.6 Sr0.4 Co0.78 Ni0.02 Fe0.2 O3) was used as a solid-reference electrode. The CO2 sensing properties and stability of a NASICON-based planar device fitted with Li2 CO3 - BaCO3 (auxiliary phase) and the composite reference electrode were examined under humid conditions. The planar device mounted on an alumina substrate with a Pt heater showed stable electromotive force (emf) responses to changes in the CO2 concentration (100-400 ppm) at 400 and 450°C in humid air without degradation. The sensor device also exhibited a good warming-up characteristic, i.e., the emf of the device quickly reached a steady and constant value when the sensor operation was restarted even after the sensor was exposed to humid air (86% relative humidity at 25°C) at room temperature for a long time.