Yttria-stabilized zirconia (YSZ) based potentiometric gas sensors have been widely utilized for detecting NOx (NO and NO2). Nevertheless, it is still remains challenging issue for YSZ-based sensors to sense total NOx due to the opposite response signals to NO and NO2. Herein, we report an efficient strategy to sense total NOx at high temperature (above 300 °C) by designing a dual functional sensing electrode (SE); namely, the SE will simultaneously convert NO (in NOx mixture) to NO2 and electrocatalyze all of the obtained NO2 to generate the response signal of total NOx. In comparison with those previously reported total NOx sensors, the proposed total NOx sensor will be featured with a simplified sensor configuration and desirable long-term stability. To confirm the practicability of the proposed strategy, the NO conversion rate of several metal oxides and their composites have been measured and it turns out that the Co3O4/NiO shows relatively high NO conversion rate. Further study indicates a YSZ-based sensor consisting of (Co3O4 + 20 wt % NiO)-SE and Mn-based RE demonstrates satisfactory performance in detecting total NOx. For instance, analogous response magnitude to NO and NO2 as well as the mixture of NO/NO2 (within 35 ppm) is witnessed for the sensor; particularly, the sensor gives acceptable stability and response/recovery rate at the operating temperature of 500 °C within the examined period. In summary, the use of dual functional SE (e.g., Co3O4/NiO composite SE) indeed addressed those issues of concern in monitoring the level of total NOx and has provided a promising alternative way for designing future high-performance total NOx sensor.
Keywords: CoO/NiO; electrochemical reaction; gas-phase reaction; total NO; yttria-stabilized zirconia.