The CO2 concentration is a good indicator of the indoor air quality, and the ground-level CO2 concentration needs to be measured precisely in order to monitor global warming. Several designs for a cost-effective CO2 sensor in order to replace the commercialized infrared spectroscopic CO2 sensor have been suggested. Among them, the potentiometric-type sensor that consists of a Na+ (or Li+) conductor, a metal carbonate auxiliary phase and a reference electrode is promising because it shows a satisfactory EMF (electromotive force) over a wide CO2 concentration range. In order to commercialize the electrochemical CO2 sensor, minimizing the signal drift is crucial. The EMF of a potentiometric sensor is the result of the Na+ activity difference between a CO2-sensitive sensing electrode (SE) and a CO2-insensitive counter electrode (CE). As the sensor ages in air, Na+-ions migrate spontaneously from the SE toward the CE, which forms various Na-containing deposits by a reaction with the humidity, O2 and CO2. This makes it difficult to define the Na+ activity at the CE, which is a major reason for the signal drift. There have been two different approaches to fix the Na+ activity at the CE: One is the placement of the CE in the reference gas such as CO2-free air, which requires tight gas sealing between the SE and CE. The other is the attachment of a two-phase mixture to the CE, which fixes the Na+-ion activity thermodynamically.
Compared to potentiometric sensor, amperometric sensor is more advantageous for the linear and reliable sensing within a narrow concentration regime. An amperometric CO2 sensor was fabricated using a Pt|NASICON (Na+ conductor, Na3Zr2Si2PO12)|Pt cell and a porous Na2CO3-BaCO3 (1:1.7 in molar ratio) auxiliary layer. The sensor with the Na2CO3-BaCO3 layer showed a faster response than that with the Na2CO3 layer, and the sensor signal was barely influenced by the humidity. When operated in an amperometric mode with an applied external electric field, the sensor could detect the CO2 concentration stably and precisely without the need for a reference gas or a reference electrode material, and the sensor signal was constant for extended test periods (60 days.). The sensing mechanism and the long-term stability is discussed in terms of the amperometric operation of the sensor.
|1||Ji-Sun Lee, Jong-Heun Lee,* and Seong-Hyeon Hong, "Solid-state amperometeric CO2 sensor using a Na-ion conductor," J. Euro. Ceram. Soc., 24, 1431-1434 (2004)|
|2||Ji-Sun Lee, Jong-Heun Lee,* and Seong-Hyeon Hong, "NASICON-based amperometric CO2 sensor using a Na2CO3-BaCO3 auxiliary phase," Sensors and Actuators B, 96, 663-338 (2003)|
|3||Ji-Sun Lee, Jong-Heun Lee,* and Seong-Hyeon Hong, "Solid-state amperometeric CO2 sensor using a lithium ion conductor," Sensors and Actuators B, 89(3), 311-314 (2003)|