Development Of High Temperature Mixed-potential CO/C₃H₈/NO₂Sensors

With a rapid increase in the number of the automobiles in the modern human society, the harmful exhaustive gas emissions such as hydrocarbons or CO, NOx (x=1,2) from the tailpipe have become the main source of urban air pollution. Those toxic gases have a detrimental effect on the environment and human health. In order to decrease fuel consumption and protect environment, many countries or regions have enacted more and more strict laws to abate such gases. Consequently, more and more gas sensors are needed for inline monitoring the exhaust gases. The planar-type mixed potential sensor based on the YSZ electrolyte become one of the most potential online sensors of monitoring such gases at high temperatures. So far significant efforts have been made to develop metal oxide sensing electrodes to further improve sensitivity, selectivity, and long-term stability of the sensor.This work systematically studied the sensor with a mixed-potential working principle based on YSZ electrolyte. The thickness of the electrolyte, the configuration of sensing electrode and the reference electrode of the sensor have been investigated to optimize sensor the configuration of the sensing elements include the sensing electrodes and electrolyte. In this work, the sensing properties of the four new sensing electrodes of Zn-Sn-O (ZniSnO4+SnO2) composite, ZnCo2O4, Zn2TiO4and Ni-Zn-O (Nio.8Zno.2O+ZnO) composite to CO, C3H8and NO2at high temperatures above600℃were investigated. The composite oxides were prepared by the coprecipitation method and the microstructure was determined by XRD and SEM. The planar-type mixed-potential sensor coupled with Zn-Sn-O as sensing electrode indicated a linear relationship between the responses of the sensor and the logarithm of CO concentrations within a range from60ppm to1200ppm. The sensor showed good signal reproducibility at600℃while some interference from NO2and C3H8were also observed. The sensor using the ZnCo2O4sensing electrode indicated a higher response to propane and excellent selectivity to NO2and CO at600℃. These former two new sensing electrodes demonstrated a significant potential to be employed for the selective mixed-potential CO/C3H8sensors for online monitoring automobile gas emissions. The sensor utilizing Zn2TiO4or Ni-Zn-O composite as sensing electrode indicated a poor selectivity and need further improving. The sensing mechanism involved with the mixed-potential theory and more general differential potential theory as well was discussed and analyzed based on the above results.