Humidity influence on the CO₂ response of potentiometric sensors based on NASICON pellets with new compositions, Na₃Zr_2-(x/4)Si_2-xP_1+xO₁₂ (x = 1.333)

CO₂-gas sensors were fabricated using NASICON-type dense ceramics with new compositions in the Na₃Zr_2-(x/4)Si_2-xP_1+xO₁₂ system. The bulk ceramics were prepared from sol-gel-processed powders. Dense electrolytes were obtained for compositions with x = 0.667 and x = 1.333. An improved sinterability was observed with respect to NASICON with conventional composition, which was attributed to liquidphase sintering. The CO₂-gas sensors using these dense samples showed a stable electromotive force (emf) response in dry atmosphere that was similar to the theoretical Nernstian value for a two-electron electrochemical reaction. This was observed also for the NASICON sample with composition x = 1.333 that showed a conductivity far lower than that of the NASICON with conventional composition. The emf changed quickly with changes of the CO₂-gas concentration, and steady-state values were observed. The response time, in adsorption and desorption of CO₂, was very fast, especially at high CO₂ concentrations. The influence of humidity on the CO₂-sensing performance was investigated. A lower sensitivity and slower response were obtained in humid CO₂ gas, especially at low CO₂ concentrations. CO₂-sensing measurements at various gas-flow rates were performed to evaluate the reactions occurring at the measuring electrode. The occurrence of a side reaction on the measuring electrode was observed, i.e., the formation of sodium oxides. In dry gas, the reaction took place at low CO₂ concentrations and small flow rates, whereas the reaction was strongly enhanced in humid environments and occurred over the entire CO₂ concentration range. However, the sensor performance recovered after switching from humid gas to dry gas. This demonstrated that the humidity affects the emf because of the Na₂O_x formation at the electrode, and, thus, the solid electrolyte itself was not degraded by humidity.