Galvanic solid electrolyte cells for the measurement of CO 2 concentrations

Abstract

The first commercially offered CO2 sensors with galvanic solid electrolyte cells have very different properties. A review of the numerous carbonate cells described in the literature shows that it is easy to obtain CO2-sensitive systems with Au, Na2CO3 measuring electrodes. The problems of obtaining reproducible CO2 sensors with long-term stability mainly concern the reference electrode. Only electrodes composed of pure solid substances, stable under the operational conditions, promise the desired properties. Reference electrodes with the oxides of Mo, W, Sn, Ti, Si and Ge have been tested, with different degrees of success. With silica and sodium silicate on β-alumina, a CO2 sensor results that can be used also in reducing gas phases and without calibration because the evaluation of the signals is possible by a thermodynamically precalculated equation. The volatility of Na2CO3 is presumably caused by the vapor pressure of thin layers of the creeping substance and by the formation of gaseous Na2(OH)2. The properties of Na2CO3 are particularly unfavorable for planar sensors. The sensor signals are independent of the partial pressure of O2 and H2O, but the participation of O2 in the electrode reactions causes cross sensitivities not only for carbon-containing gases but also, for example, for NH3 and H2. The cross sensitivities against halogens and SO2 are irreversible. At sudden changes of the CO2 concentration the sensor signal follows within less than a few seconds, but questions remain concerning the observable differences in the response times of differently arranged sensors. The response times are highly important for a sensor arrangement that is aimed at simultaneous measurements of CO2 and O2 in real time for each breath.