| A new potentiometric CO2 gas sensor using lithium-lanthanum-titanate (Li0.35La0.55TiO3) electrolyte, Li2 CO3 sensing electrode, and Li2TiO3+TiO 2 reference electrode was investigated. The microstructure and electrical properties of the optimized solid electrolyte were examined and the measured conductivity values were found consistent with those reported in literature. The sensor was tested under dry condition in 21% O2/N2 at temperatures ranging from 250 to 550°C. As the temperature increased, the percentage of Nernstian behavior improved from 50% at 250°C to 95% at 450°C, but the performance degraded above 450°C. The proposed hypothesis for the degradation is as follows. Depending on CO2 partial pressure, Li2CO3 can decompose and react with Li0.35La 0.55TiO3 around 475-500°C resulting in insertion of Li + into Li0.35La0.55TiO3 that causes structural distortion. When the reaction between Li2CO3 and Li0.35La0.55TiO3 occurs at elevated temperatures such as at 700°C, the distorted structure transforms to disordered LaLi 1/3Ti2/3O3 and the sensor performance degrades irreversibly. Thermodynamic calculations combined with solid-state reaction under controlled atmosphere followed by X-ray diffraction (XRD) are used to confirm the hypothesis. Finally, for device fabrication, it is demonstrated that introduction of high concentration of CO2 (∼99.99%) can avoid the reaction between Li2CO3 and Li0.35La 0.55TiO3 at high temperatures, which also facilitates good bonding between the electrode and the electrolyte. As for long-term device performance, it is shown that the sensor can measure changes in CO2 concentrations reproducibly as long as it is operated in conditions where there is a background of CO2, such as in ambient atmosphere or combustion systems. |
