Carbon Dioxide Sensor Using Sodium Ion Conductor and Binary Carbonate Auxiliary Electrode

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Abstract
A NASICON‐basic sensor was improved considerably when it was fitted with a Ba‐Na binary carbonate electrode (Ba content: 46 atom percent). It responded to very quickly with a 90% response time of less than 8 s, at 550°C, without being affected by water vapor. The electromotive force responses followed perfectly a Nernst equation corresponding to a 2‐electron reaction for over a wide concentration range at 450–600°C. The sensor performance was stable for the test period of 200 days. X‐ray diffraction analysis revealed that no free 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 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.

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