Interface reaction and its effect on the performance of a CO2 gas sensor based on Li0.35La0.55TiO3 electrolyte and Li2CO3 sensing electrode

A new potentiometric CO2 gas sensor using lithium–lanthanum–titanate (Li0.35La0.55TiO3, LLTO) electrolyte, Li2CO3 sensing electrode, and Li2TiO3 + TiO2 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 performance of the sensor depended both on the fabrication temperature and the sensor operation temperature. Sensors with the sensing electrode fabricated above 500 °C performed poorly. For sensing electrodes fabricated at 500 °C, as the sensing temperature increased from 300 to 450 °C, the performance of the sensor improved (near Nernstian response), but above 450 °C, the sensor degraded. The proposed hypothesis for the degradation beyond 450 °C is that at low levels of CO2 (ppb in the background), Li2CO3 reacts with LLTO resulting in insertion of Li+ into LLTO that causes changes in the electrical properties of the electrolyte. Poor performance of sensors fabricated at 700 °C was due to formation of a new phase, LaLi1/3Ti2/3O3. Thermodynamic calculations combined with X-ray diffraction of the reaction products are used to support the hypothesis. Introduction of high concentrations of CO2 (~99.99%) during sensor fabrication (650 °C) eliminated the reaction between Li2CO3 and LLTO, and also facilitated the 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 below temperatures of 450 °C, as long as it is operated in conditions where there is a background of CO2, such as in ambient atmosphere or combustion environments. The sensor exhibits minimal interference toward oxygen, but significant interference to humidity.