Zirconia-Based Electrochemical NOx Sensors with Semiconducting Oxide Electrodes

Electrochemical devices based on coupling an oxygen ionic conductor (Y2O3-stabilized ZrO2: YSZ) with semiconducting oxides as sensing electrodes were investigated. LaFeO3, a p-type semiconductor, and WO3, an n-type semiconductor, were chosen because of their known good NO2-sensing properties as resistive-type sensors. The entire sensors were wholly exposed to the same atmosphere. The electromotive force (EMF) was measured for different NO2 concentrations and at fixed temperature. The EMF values of both sensors increased in NO2 atmosphere with a fast response time. The best performance was observed around 450°C for LaFeO3-based and 650°C for WO3-based sensors. Moreover, the response of the LaFeO3-based sensors was found to be strongly affected by the grain size of the powder used for the preparation of the sensing electrode. The polarization curves of all sensors showed a nonlinearity; nevertheless an opposite behavior in the presence of NO2 gas was observed for the p- and ntype- based oxide sensors. The nonlinearity was attributed to the potential drop for LaFeO3 or increase for WO3 at the electrode interface. Amperometric measurements, performed at a fixed potential of 1 V, were found in accordance with polarization curves. The electrochemical impedance spectroscopy (EIS) analysis showed that the resistance and reactance at the electrolyte/electrode interface decreased (for LaFeO3)/ increased (for WO3) on introduction of NO2 gas, because of the lowering/increasing of the electrode overvoltage.