Improving NO2 sensitivity by adding WO3 during processing of NiO sensing-electrode of mixed-potential-type zirconia-based sensor

A planar NO 2 sensor was fabricated by using an yttria-stabilized zirconia plate and NiO-based sensing electrode (SE). The improvement in sensing characteristic of mixed-potential-type planar NO 2 sensor was examined by the addition of various metal oxides to NiO-SE. Among the metal-oxide additives tested, WO 3 was found to give a significant enhancement in NO 2 * sensitivity. In addition, this enhancement was maximum when the initial WO 3 content in the NiO-based paste before sintering was 10 wt %. The sensitivity (A electromotive force) to 50 ppm NO 2 was as high as about 70 mV for the sensor using the 10 wt % WO 3 -added NiO-SE at 800°C under the wet condition (containing 5 vol % H 2 O). The sensitivity of this sensor was still 8 mV even down to 1 ppm NO 2 at 800°C. The NO 2 sensitivity was found to be invariant to different concentrations of H 2 O and CO 2 in the examined range of 5-15 and 5-20 vol %, respectively. It is speculated that the modification of morphology of the NiO-SE layer by the addition of WO 3 is the main contributing factor for the improved NO 2 sensitivity of the sensor.