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The gas sensing properties of NASICON-based CO 2 sensor attached with a composite of Bi 8 Nb 2 O 17 plus small amount of Pt as a solid-reference electrode has been investigated for application of the sensor at high CO 2 concentration. Compared with conventional sensor coated with Pt reference electrode, the sensor attached with solid-reference electrode was found to efficiently improve the detection limit due to the presence of solid-reference electrode prevented NASICON reacting with CO 2 and water to form carbonate or bicarbonate. A good bonding interface between the solid-reference electrode and NASICON was formed as revealed by the scanning electron spectroscopy (SEM), which provided a powerful guarantee for the thermal stability of the sensor. Furthermore, the as-fabricated sensor also exhibited fast response time, small cross-sensitivity to humidity and little interference from the coexistent oxygen partial pressure at 500 °C. All the present results suggest that using the composite of Bi 8 Nb 2 O 17 plus Pt as a solid-reference electrode is a promising candidate for NASICON-based CO 2 sensor.

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.

In general, solid electrolyte based potentiometric CO 2 sensors respond well to changes in CO 2 concentration, following Nernst equation when dry CO 2 is used. Unfortunately, the sensing capability will be fatally disturbed when these sensors are exposed to humid conditions. To overcome this problem, a sensor using a porous BaCO 3 film as counter electrode (p-Sensor) was fabricated. For the purpose of comparison, sensor without this porous structure (o-Sensor, i.e. an opened counter electrode) and sensor with a dense BaCO 3 film (d-Sensor) also have been fabricated. The electromotive force (EMF) of all sensors exhibited excellence Nernstian behavior with the logarithm of CO 2 concentration in the range 300–755 ppm at 400 °C under dry condition. However, the EMF values of each sensor tended to shift upward with increase of relative humidity. It was found that the relative humidity dependence of EMF originates from both of auxiliary and counter electrodes. Under the same humid conditions, the order of the EMF deviation of three types of sensors was shown as the following: d-sensor > p-sensor > o-sensor. Nevertheless, only p-sensor still remained the Nernstain behavior even under humid conditions. The electron transfer numbers are in good agreement with theoretical value of n = 2. Moreover, the transients were sufficiently sharp, taking less than 1 min for 90% response or recovery. The most important thing is EMF can rapidly recover the original value without any deterioration. The reason for the satisfactory performance of p-sensor under humid condition was suggested to be due to the amount of H 2 O molecular adsorbed on the porous counter electrode is very close to that of auxiliary electrode.