P2.6.5 Half-cell characterization of a novel NH3 gas sensor

Abstract

The function of a novel electrochemical NH3 gas sensor for application in SCR-systems is investigated. It provides a semi-logarithmic characteristic curve with a high NH3-sensitivity and marginal NOx cross interference at 550 °C. The electrochemical cell of the sensor device can be defined as Au | YSZ | Au, VWT. It is assumed that the sensing mechanism is based on nonequilibrium conditions (mixed potential theory) including electrochemical kinetics. This paper describes the investigation of electrode potentials and polarization curves of the half-cells Au | YSZ and Au, VWT | YSZ in dependence of NH3, which provides essential information about electrochemical reactions at the three-phase boundary. All electrode potentials depend on reactive gas concentration, whereby the electrode potential of the VWT-covered Au-electrode shows a stronger dependency on the NH3 concentration. The formation of mixed potentials at both electrodes is confirmed. Additionally, the influence of the VWT-catalyst coating on the sensing mechanism and the sensitivity is demonstrated. The sensitivity increases with increasing coverage of the Au-electrode with VWT catalyst. Voltage-current curves help to analyze the kinetics of electrochemical reactions at the TPB. A clear shift in cathodic direction (to more negative potentials) can be observed with increasing NH3 concentration and the current increases at a fixed potential due to an enhanced electrochemical NH3 oxidation.