Impedimetric-type NO2 sensor based on the p-NiO/n-NiNb2O6 heterojunction sensing electrode

Abstract

Development of high-performance NO2 gas sensors with good sensitivity and long-term stability at high temperatures is expected for exhaust gas monitoring. The sensor performance mainly depends on sensing material. In this work, p-NiO/n-NiNb2O6 heterojunction material is successfully synthesized and an impedimetric NO2 sensor was assembled using the above material as sensing electrode (SE) and gadolinia-doped ceria (CGO) with a dense/porous bilayer structure as a solid electrolyte. Introduction of SE to the CGO porous layer by screen printing followed by cold isostatic pressing treatment before calcination allows SE to successfully enter inside the porous layer and greatly increases the area of the three-phase boundary (TPB) and improves the long-term stability of the sensor. Using NiO/NiNb2O6 heterojunction SE can effectively improve the modulus (|Z|) response value (S|Z| = (|Z|air - |Z|gas)/|Z|air) of the sensor. Among the sensors, 90NiO/10NiNb2O6 (the mass ratio of NiO to Nb2O5 is 9:1) based NO2 sensor exhibits the best response performance and the highest sensitivity (0.34/ppm at 500 °C). The sensor also shows an excellent anti-interference property. Introduction of 300 ppm CO2, CH4, NO, CO, or H2 into 300 ppm NO2 makes response signal change below 3 % and 300 ppm NH3 only leads to signal reduction by 8.9 %, which is relatively low in a similar sensing system. The sensor response toward NO2 is slightly affected by O2 concentration change in 5–21 %. The superior performance of the sensor can be principally attributed to the heterojunction effect, which promotes the charge transfer and the adsorption capacity for NO2.