Potentiometric CO 2 gas sensor with lithium phosphorous oxynitride electrolyte

Abstract

Potentiometric cell, Au/LiCoO 2 5 m/o Co 3O 4/Li 2.88PO 3.73N 0.14/Li 2CO 3/Au, has been fabricated and investigated for monitoring CO 2 gas. A LiCoO 2–Co 3O 4 mixture was used as the solid-state reference electrode instead of a reference gas. The idea is to keep the lithium activity constant on the reference side using thermodynamic equilibrium at a given temperature. The thermodynamic stability of the reference electrode was studied from the phase stability diagram of Li–Co–C–O system. The Gibb’s free energy of formation of LiCoO 2 was estimated at 500°C from the measured value of the cell emf. The sensors showed good reversibility and fast response toward changing CO 2 concentrations from 200 to 3000 ppm. The emf values were found to follow a logarithmic Nernstian behavior in the 400–500°C temperature range. CH 4 gas did not show any interference effect. Humidity and CO gas decreased the emf values of the sensor slightly. NO and NO 2 gases affect this sensor significantly at low temperatures. However, increased operating temperature seems to reduce the interference.