Investigation of WO3-based potentiometric sensor performance (M/YSZ/WO3, M = Au, Pd, and TiO2) with varying counter electrode

Sensing properties of solid-state potentiometric sensors consisting of WO3 and three different counter electrodes (Au, Pd, and TiO2) are compared with those of a WO3/YSZ/Pt sensor studied previously and discussed with regard to catalytic behavior of the electrode elements. The NO2 sensor response is closely related to NO2 decomposition activity of the counter electrode, such that the higher the catalytic activity, the bigger is the NO2 response (Pt > Au > Pd > TiO2). In O2/N2 balance condition, the NO2 selectivity properties of Pt/WO3 (Pt–W), Au/WO3 (Au–W), and Pd/WO3 (Pd–W) combinations are generally similar with one another against CO2, O2, and H2O. A difference is, however, shown against CO, where both Au–W and Pd–W are significantly affected by CO, while Pt–W is NO2 selective against CO. In simulated exhaust, the NO2 sensitivity of an Au/WO3 combination (Au–W) is modified by O2 and H2O, but not by CO and CO2, showing very similar selectivity tendency to Pt–W. Two sensors (Au–W and Pt–W) are also very comparable in their NO selectivity behavior in the exhaust.