A solid state CO2 sensor operative at room temperature, developed by incorporating a metal oxide sensing electrode and a metal carbonate auxiliary phase into a NASICON (Na3Zr2Si2PO12)-based electrochemical cell, suffers from a humidity-dependent shift of the EMF response to CO2. In an attempt to reduce the humidity dependence, binary carbonate systems of Li2CO3-BaCO3 and Na2CO3-BaCO3 at various compositions were tested for the auxiliary phase of the device. The melting-and-quenching method conventionally used for attaching the carbonate was not applicable well to the present type device due to the corrosion of NASICON surface. The humidity-least dependent device could be obtained when the Li2CO3-BaCO3 composite (1:2 in molar ratio) was adhered to NASICON by calcination at 500°C (non melting method). The device could respond to 300-3000 ppm CO2 quite well almost independent of relative humidity (RH) beyond 30%. In the lower range of RH, however, the EMF response commenced to shift and the CO2 sensing capability was completely lost in the dry atmosphere. A planar type CO2 sensor was fabricated successfully, which could work stably under the conditions of 30% RH and above.
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