Mixed-potential-type zirconia-based NOx sensor using Rh-loaded NiO sensing electrode operating at high temperatures

The improvement in sensing characteristics of mixed-potential-type NO2 sensor based on an yttria-stabilized zirconia (YSZ) plate was examined by an addition of noble metal to NiO sensing-electrode. Among the various noble metals (Pt, Rh, Ir, Pd and Ru) examined, Rh was found to give a significant enhancement in NO2 sensitivity. In addition, this enhancement was maximum when the Rh content in NiO-SE was 3 wt.%. The sensitivity (emf) to 50 ppm NO2 was as high as about 77 mV for the sensor using the 3 wt.% Rh-loaded NiO-SE at 800 °C under the wet condition (5 vol.% H2O). The NO2 sensitivity was hardly affected by the change of H2O and CO2 concentration in the examined range of 5–15 and 5–20 vol.%, respectively, and was almost stable for about 50 days tested at 800 °C. It was speculated that the improved kinetic effect of Rh loading on the cathodic reaction of NO2 at the interface of SE/YSZ was the major contributing factor for the enhanced NO2 sensitivity of the sensor.