Zirconia‐Based Electrochemical NO _<i>x</i> Sensors with Semiconducting Oxide Electrodes

Abstract

Electrochemical devices based on coupling an oxygen ionic conductor (Y 2 O 3 ‐stabilized ZrO 2 : YSZ) with semiconducting oxides as sensing electrodes were investigated. LaFeO 3 , a p‐type semiconductor, and WO 3 , an n‐type semiconductor, were chosen because of their known good NO 2 ‐sensing properties as resistive‐type sensors. The entire sensors were wholly exposed to the same atmosphere. The electromotive force (EMF) was measured for different NO 2 concentrations and at fixed temperature. The EMF values of both sensors increased in NO 2 atmosphere with a fast response time. The best performance was observed around 450°C for LaFeO 3 ‐based and 650°C for WO 3 ‐based sensors. Moreover, the response of the LaFeO 3 ‐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 NO 2 gas was observed for the p‐ and n‐ type‐based oxide sensors. The nonlinearity was attributed to the potential drop for LaFeO 3 or increase for WO 3 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 LaFeO 3 )/increased (for WO 3 ) on introduction of NO 2 gas, because of the lowering/increasing of the electrode overvoltage.