Potentiometric CO2-Sensors with Ion-Conducting Glasses as Solid Electrolytes

Alkali-ion conducting glasses/glass ceramics of the system Me2O-Al2O3-SiO2 (Me=Li, Na) were applied as solid electrolytes in potentiometric gas sensors to detect CO2 in the presence of O-2 at increased temperatures. The corresponding Me-Carbonates were utilized as auxiliary electrodes. Sensors using the direct Au-glass contact as a kind of reference electrode (type I), as well as symmetrical sensors with carbonate phase at the reference and measuring electrode (type II - for comparative measurements) were manufactured. By applying Au as electrode metal, the theoretically expected EMF difference and the observed EMF difference of both sensor types agree quite well with the expected values according to the Nernst equation between 500 and 600 degrees C (over four orders of magnitude of CO2 partial pressure (10(-5) - 10(-1) bar) at constant O-2 partial pressure (2.1x10(-1) bar)). A long time stability of 120 days for sensors of type I with Li glasses has been observed, although evaporation of carbonate phase (Li2CO3) was detected under the conditions of sensor application. Sensors of type I (with Li2CO3) show thermodynamically unexpected cross-sensitivities to H2O.