Influences of water vapor on NASICON-based CO2 sensor operative at room temperature

Electrochemical CO2 sensors were fabricated with NASICON (Na3Zr2Si2PO12) disks containing different levels of Na3PO4. The marked differences of the effect of aging in humid condition revealed that the auxiliary phase (NaHCO3 and/or Na2CO3) was formed spontaneously from the Na3PO4 impurity of NASICON in the presence of water vapor and CO2. Even when the auxiliary phase was formed or intentionally added, CO2 sensing properties were easily lost completely under exposure to dry atmosphere, indicating active involvement of water vapor in the CO2 sensing electrode reactions. EMF versus CO2 concentration correlations usually tended to shift up with increasing relative humidity. Remarkably, this shift was reduced effectively by introducing Li2CO3 as an auxiliary phase, suggesting that the shift originates from the instability of NaHCO3 and/or Na2CO3 in humid atmosphere.