Characteristics of NASICON-Based Thick-Film ${\rm CO}_{2}$ Sensor Attached With Integrated Auxiliary Electrode

A planar-type NASICON thick-film CO₂ sensor attached with integrated auxiliary electrode composed of NASICON, Li₂CO₃, and a little Pt paste is investigated. The electromotive force (EMF) values of the device in the CO₂ concentration (300-750 ppm) are measured at 300-500°C under dry as well as humid conditions. The relationship between EMF values and logarithm of CO₂ concentration is linear with the number of reaction electrons n=1.9, 2.0 at 400 and 500°C, respectively, which are excellent agreement with the theoretical values (n=2.0). Compared with the conventional sintered pellet sensor fitted with binary carbonate (Li₂CO₃-BaCO₃) as auxiliary electrode, the sensor showed smaller cross-sensitivity against humidity and faster CO₂ response time as short as 50 s for 90% saturation. And most important, the EMF of the sensor is more stable than that of the conventional sensor during the heating-cooling process and without any degradation. The long-term stability of the sensor is also perfect.