Development of FET-type CO2 sensor operative at room temperature

An FET-type CO2 sensor operative at room temperature was developed by stacking a powder mixture of metal carbonate (auxiliary phase)—indium tin oxide (ITO, sensing electrode) as well as Na+-exchanged cation exchange membrane (ionic conductor gate) on an FET chip. Both ITO and the cation exchange membrane were indispensable to obtain stable response to CO2 at room temperature, while the auxiliary phase was optimized to be Li2CO3–BaCO3 (1:3 in molar ratio) from the stability of response against the disturbance by humidity. The device, Li2CO3–BaCO3–ITO (1:3:1)/Na+-exchanged cation exchange membrane/FET, exhibited excellent CO2-sensing characteristics at 30 °C. The response in gate-source voltage (VGS) was linear to the logarithm of CO2 concentration, with its slopes indicating n=2.0 where n is the number of reaction electrons involved in the electrochemical reduction per CO2. The Nernst’s correlation was fairly stable against a change in RH between 30 and 70%. The times for 90% response and 90% recovery to switching-on and -off 3000 ppm CO2 were 1 and 2 min, respectively.