INFLUENCE OF HUMIDITY AN A POTENTIOMETRIC CO2 GAS SENSOR USING A COMBINED ELECTROLYTE OF SODIUM-ION CONDUCTING GLASS AND STABILIZED ZIRCONIA

A CO2 gas sensor using a combined solid electrolyte of a Na+ conductin g Na2O-Al2O3-4SiO(2) glass and an yttria-stabilized zirconia was prepa red, and its sensing properties in dry and wet gases were investigated using a variable flow rate of the test gases. The glassy electrolyte in the sensor was suitable as a CO2 sensing material because of its hi gh density and good contact between the electrolytes. In a dry atmosph ere, excellent results were obtained for the CO2 gas concentration dep endence of the electromotive force (EMF), expressed by EMF (mV) = 547. 5 - 74.0 log [C-CO2/ppm] at 470 degrees C in which the sensitivity of 74.0 mV/decade was in fair agreement with the theoretical value based on a two-electron electrochemical reaction. In the case of wet CO2 gas , the sensitivities and response time were depressed even for a high C O2 gas concentration. The decomposition of Na2CO3 into sodium oxide on the Pt electrode was accelerated in the presence of H2O. The reason f or the depression of the sensing properties in a humid condition could be ascribed to its reformation reaction as a side reaction.