Stability and interfacial structure of a NASICON-Based CO₂ Sensor Fitted with a Solid-Reference electrode

We investigated the stability of a NASICON (Na₃Zr ₂Si₂PO₁₂)-based potentiometric CO₂ sensor fitted with a composite of BiCuVOx (Bi₂Cu_0.1V _0.9O_5.35) and perovskite-type-oxide (La _0.6Sr_0.4Co_0.78Ni_0.02Fe _0.2O₃) as a solid-reference electrode. The sensor showed stable electromotive force (EMF) responses to changes in CO₂ concentration (400-2000 ppm) at 400 °C in high humid air (100% RH at 45 °C). Notably, no significant fluctuation in the potential of the solidreference electrode was observed even after the sensor device was kept under high humid conditions at room temperature for a long period (1-2 days). SEM (scanning electron spectroscopy)-EDX (energy dispersive X-ray spectroscopy) analysis revealed the presence of a thick interfacial layer between NASICON and the solid-reference electrode that may prevent NASICON from reacting with water and CO₂.