It has to be considered especially at low NH3 concentrations. Lambda variations, i.e., simultaneous variations of the concentrations of O2, CO2, and water in dependence on the combustion process in lean, oxygen rich exhaust almost do not affect the sensor response once the O2 concentration exceeds 3 vol.%.This planar sensor was manufactured in thick-film technique, with a screen-printed YSZ film on top of an alumina substrate [17]. The device could be self-operated due to a heater film on its reverse side. In this study, the focus is on experiments to elucidate the sensor mechanism. For that purpose, effects contributing to the sensor response are investigated in a half-cell setup. The electrode processes at the pure Au electrode and at the VWT-covered Au electrode are analyzed separately with reference to a Pt electrode in a defined reference atmosphere. Initial investigations of the electrode effects confirmed that electrochemical reactions occurring at the three phase boundary and the catalytic activity of the VWT-catalyst layer play a crucial role in the sensor mechanism [24]. A detailed investigation of the electrode potential and electrochemical kinetics with respect to mixed potential formation is the focus of this work. Half-cell measurements of the electrode potentials are conducted in dependence on the gas composition. Polarization curves are investigated in different gas atmospheres for a Au electrode and a VWT-covered Au electrode.The objective of this study is to validate if the sensor behavior and NH3 detection mechanism can be described solely by classic mixed potential theory or if other potential changing processes need to be considered which reflect limitations of the mixed potential theory.2.?Experimental Section2.1. Sample PreparationTwo types of samples were prepared, as illustrated in Figure 1: a sensor-like setup and a half-cell type setup. In each setup, YSZ ceramic discs (8 mol.% Y2O3-doped ZrO2 (Kerafol)) served as a substrate. Electrodes and cover layer were applied by screen printing. For the sensor-like setup (Figure 1(a)) both porous Au electrodes (Au thick-film paste (DuPont), fired for 20 min at 850 ��C) were applied on top of the YSZ disc. One electrode, denoted as sensing electrode and inhibitor SB203580 abbreviated by SE, was covered afterwards by a screen-printed porous catalyst film. For that purpose, a commercial extruded VWT-based SCR catalyst (Argillon, now Johnson-Matthey) was ground in a planetary ball-mill and organic binders were added to obtain a screen-printable paste. In the case of the half-cell probe, the electrodes were arranged on top and on bottom of the electrolyte disc (Figure 1(b)). For the reference electrode (RE), which was later exposed to a reference atmosphere, either Au (processed as described) or Pt (thick-film paste (Heraeus) fired at 1,200 ��C for 20 min) was used. The SE consisted of a Au layer covered by the porous SCR-catalyst VWT thick film.