Stabilization of counter electrode for nasicon based potentiometric CO ₂ sensor

Two kinds of materials, Bi₂Cu_0.1V_0.9O _5.35 (BICUVOX) and NaCoO₂, were investigated for their applicability as the counter electrode for a potentiometric CO₂ sensor using NASICON (Na⁺ conductor, Na₃Zr ₂Si₂PO₁₂). The BICUVOX counter electrode was more stable than an Au counter electrode. Thermal cycling between the operating temperature (450°C) and room temperature under various conditions indicated the time needed for the BICUVOX electrode to reach the stationary potential after switching to the operating temperature was unacceptably long. NaCoO ₂ was found unsuitable as a counter electrode material due to its reactivity with CO₂. This reactivity was suppressed completely when NaCoO₂ was coated with a layer of glass (SiO₂: Na ₂O: B₂O₃: Al₂O₃ = 44:20:31:5, in molar ratio). The glass-coated NaCoO₂ electrode worked well as a counter electrode at 450°C. Thermal cycling indicated that the glass-coated NaCoO₂ electrode exhibited an initial ageing effect. Once aged, glass-coated NaCoO₂ showed the properties of a reliable counter electrode, i.e., rapid warm-up characteristics and a reproducible stationary potential at 450°C.