Effect of Electrode Material and Design on Sensitivity and Selectivity for High Temperature Impedancemetric NO[sub x] Sensors

Abstract

Solid-state electrochemical sensors using two different sensing electrode com- positions, gold and strontium-doped lanthanum manganite (LSM), were evaluated for gas phase sensing of NOx (NO and NO2) using an impedance- metric technique. An asymmetric cell design utilizing porous YSZ electrolyte exposed both electrodes to the test gas (i.e., no reference gas). Sensitivity to less than 5 ppm NO and response/recovery times (10-90%) less than 10 s were demonstrated. Using an LSM sensing electrode, virtual identical sen- sitivity towards NO and NO2 was obtained, indicating that the equilibrium gas concentration was measured by the sensing electrode. In contrast, for cells employing a gold sensing electrode the NOx sensitivity varied depend- ing on the cell design: increasing the amount of porous YSZ electrolyte on the sensor surface produced higher NO2 sensitivity compared to NO. In or- der to achieve comparable sensitivity for both NO and NO2, the cell with the LSM sensing electrode required operation at a lower temperature (575 C) than the cell with the gold sensing electrode (650 C). The role of surface reactions are proposed to explain the di erences in NO and NO2 selectivity using the two di erent electrode materials.