SOLID-STATE POTENTIOMETRIC CO2 SENSORS USING ANION CONDUCTOR AND METAL CARBONATE

Solid-state potentiometric CO2 sensors have been fabricated by combini ng a fluoride-ion conductor (LaF3) or an oxide-ion conductor (stabiliz ed zirconia) with an auxiliary phase of a metal carbonate (Li2CO3). Th e e.m.f, response of the LaF3-based sensor fitted with Li2CO3 is found to follow Nernst's equation for the two-electron reaction of CO2 in t he concentration range 40-2000 ppm at 400 and 450 degrees C. The 90% r esponse times are as short as 10-20 s at 400 degrees C. The tubular-ty pe MgO-stabilized zirconia (MSZ) sensor fitted with Li2CO3 also respon ds well to CO2, giving the same Nernstian behaviour as above at 500-65 0 degrees C. The CO2 sensitivity of this sensor is hardly affected by coexistent NO, NO2 or H2O, and is rather stable for the test period of approximate to 2800 h. Furthermore, it is possible to design a planar -type MSZ sensor that is insensitive to O-2 and gives a stable e.m.f. response to CO2 regardless of variations in O-2 concentration. The CO2 sensing mechanisms of these new type sensors are discussed.