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 deg 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 deg 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.