Fundamental and practical aspects of CO₂ sensors based of nasicon electrolytes

Abstract

Potentiometric CO₂ sensors based on the Na⁺ conducting solid electrolyte Nasicon have been investigated. The sensor arrangement may be described as chemical sensor of type III,

$$( - )Pt(or Au), Na_{0.9} CoO_2 |Nasicon|Na_2 CO_3 , Pt(or Au) ( + ),$$

, with Na_0.9CoO₂ as reference electrode and Na₂CO₃ as auxiliary or sensing electrode.

It is shown that major problems are related to the magnitude of the voltage and long-term stability of this type of sensor. The measured EMFs are generally reduced compared to the values calculated from literature data of standard Gibbs energies of formation. This observation is ascribed to reactions at the interfaces and competing surface reactions. The interfacial processes may occur at both electrolyte / sensing and electrolyte / reference electrode, and represent charged capacitors. It has also been observed that the slope of the measured EMF as a function of the CO₂ partial pressure is reversed in some cases. This phenomenon may be related to the possible reduction of Na₂O which is dissolved in Nasicon during the formation of Na₂CO₃ upon an increase of the CO₂ partial pressure.