NASICON devices attached with Li₂CO₃-BaCO₃ auxiliary phase for CO₂ sensing under ambient conditions

NASICON (Na₃Zr₂Si₂PO₁₂) - based electrochemical devices, constructed in a gas-separation type or planar type, were attached with a Li₂CO₃-BaCO₃ (1:2 in molar ratio) auxiliary phase to investigate their CO₂ sensing properties under ambient conditions. The devices were found to be far more resistant to the disturbance by a change in relative humidity (RH) than those using Na₂CO₃ or NaHCO₃ auxiliary phase. The EMF was correlated linearly with the logarithm of CO₂ concentration in the range of 0.03-50% CO₂ at 30°C, and the linear correlation shifted up or down only slightly with a change in RH between 30 and 90%. For a change in operating temperature, the CO₂ sensing capability was kept almost the same at 45°C, while it was deteriorated significantly at 60 and 75°C, especially in the lower RH range. Applicability of a cobalt oxide bronze (Na_0.6CoO₂) as a solid reference electrode for the planar type device was also investigated. The devices using the bronze reference electrode exhibited the CO₂ sensing capability as good as those using the Au reference electrode did. In addition, the bronze electrode was found to much improve the reproducibility of the EMF to a fixed concentration of CO₂ among devices.