CARBON-DIOXIDE SENSOR USING SODIUM-ION CONDUCTOR AND BINARY CARBONATE AUXILIARY ELECTRODE

A NASICON-basic CO2 sensor was improved considerably when it was fitted with a Ba-Na binary carbonate electrode (Ba content: 46 atom percent). It responded to CO2 very quickly with a 90% response time of less than 8 s, at 550-degrees-C, without being affected by water vapor. The electromotive force responses followed perfectly a Nernst equation corresponding to a 2-electron reaction for CO2 over a wide CO2 concentration range at 450-600-degrees-C. The sensor performance was stable for the test period of 200 days. X-ray diffraction analysis revealed that no free Na2CO3 remained in the binary carbonate electrode, while scanning electron microscopy observation of the electrode revealed deposits of micro-needles containing both Ba and Na. It was concluded that the disappearance of free Na2CO3 to form needle-like deposits gave rise to a resistance to water vapor and the quick response rates of the sensor element. Similar improvements were also observed for other binary carbonate systems, such as Sr-Na and Ca-Na.