Effects of composition and structure of sensing electrode on NO2 sensing properties of mixed potential-type YSZ-based gas sensors

NO2 sensing properties of yttria-stabilized zirconia (YSZ)-based gas sensors attached with various kinds of single-layered or NiO-based hetero-layered sensing electrodes (SE) have been examined in the operating temperature range of 500∼800°C. Although a sensor with a small-area Pt SE showed negligible NO2 response at 600°C, the lamination with a large-area NiO layer on the small-area Pt SE improved the NO2 response. In addition, the lamination with the large-area NiO layer on small-area Pt-NiO, Au or ZnO SE showed rather large NO2 response. Among all sensors tested, the sensor with the small-area Au SE laminated with the large-area NiO layer showed the largest NO2 response (ca. 49.1mV to 80ppm NO2 at 600°C). The electrochemical impedance properties of the sensor with the small-area Au SE showed larger charge transfer resistance (namely, lower electrocatalytic activity) for oxygen redox reaction than that of the sensor with the small-area Pt SE. In addition, the total of charge transfer resistance of the sensor with the small-area Au SE laminated with the large-area NiO layer decreased upon exposure to NO2 due to the simultaneous occurrence of NO2 reduction reaction. These results indicate that the large NO2 response of the sensor with the small-area Au SE laminated with the large-area NiO layer is caused by the high electrochemical activity of NO2 reduction reaction at the interface of NiO/YSZ and the low electrochemical activity for the oxygen redox reaction at Au/YSZ/gas triple phase boundaries.