Sensing Properties and Interfacial Structure of Solid Electrolyte CO₂ Sensor Attached with Na₂CO₃-based Auxiliary Phase

Abstract

Potentiometric devices for which a disc of NASICON (Na⁺ conductor, Na₃Zr₂Si₂PO₁₂) was attached with an auxiliary phase of Na₂CO₃ (I) or Na₂CO₃-BaCO₃ (2 : 3 in molar ratio) (II) were investigated for CO₂ sensing properties and the structure of hetero-junctions involved. SEM observation combined with EPMA analysis indicated that, for device (I), a corrosion layer (20-30 µm) rich in Na content was formed inside NASICON. The corrosion layer was estimated to consist mainly of Na₂ZrSiO₅ and Na₂ZrO₃ from separate experiments using powder samples. For device (II), in contrast, such a corrosion layer was hardly formed between the carbonate and NASICON. This difference in junction structure could account well for the difference in CO₂ sensing capability at lower temperature between the devices.