Relationship between aging time and EMF change of potentiometric CO2 device using Na1+xZr2SixP3−xO12 (0 < x < 3)

Four kinds of NASICON (Na1+xZr2SixP3−xO12, x = 1, 1.5, 2 or 2.75)-based CO2 devices were combined with a Li2CO3–BaCO3 (1:2 in molar ratio) auxiliary phase to investigate their CO2 sensing properties. The EMF values of NASICON devices were correlated linearly with the logarithm of CO2 concentrations in the range of 250–2500 ppm at 450 °C. When exposed to 250 ppm CO2 for 200 h under dry condition, the EMF of each NASICON device decreased with aging time. To research the cause of the EMF drift, a mixture that consists of NASICON and Li2CO3 was heated at 750 °C in air. From X-ray diffraction measurement, it was confirmed that alkali-rich crystal phases such as Li2ZrO3 and Na5Zr(PO4)3 were formed instead of Na3Zr2Si2PO12 phase. This suggests that the EMF drift relates with the formation of alkali-rich crystal phase at the interface between NASICON and auxiliary phase.