Stability of NASICON-based CO₂ sensor under humid conditions at low temperature

The NASICON-based CO₂ sensor using Li₂CO₃-BaCO₃ auxiliary phase was tested for stability under exposure to the humid air containing 3.6 kPa H₂O and 1000 ppm CO₂ at 50°C. For this purpose, the sensor devices were attached with a reference electrode (RE) which was always kept from contacting the humid air, in addition to the sensing electrode (SE) and counter electrode (CE). With this structure, the SE and CE potentials to CO₂ containing atmospheres at 450°C were measured relative to RE before and after the humid air treatment for 3 days. It was found that the humid air treatment caused the CE potential to shift largely from the original value, while the SE potential remained intact. The shift of CE potential decreased gradually with increasing time of operation at 450°C, disappearing almost completely in several days. The SEM observation revealed that a number of tiny deposits showed up on the surface of NASICON after the 3-day treatment and the deposits grew into discrete crystalline particles after the 14-day treatment. It was found in separate experiments that, when the NASICON disk was soaked in hot water, a significant amount of Na₃PO₄ was eluted from the disk, and that the resulting disk could have the CE potential more stabilized to the humid air treatment. Based on these results, it is concluded that the elution of Na₃PO₄ from the bulk of the NASICON disks to the surface is responsible for the degradation of CO₂ sensing properties in this type sensor after being kept in a humid atmosphere at low temperature.