Half-cell potentials of the electrochemical cell Au,？VWT？|？YSZ？|？Au are analyzed in dependence on oxygen and ammonia concentration at 550 °C. One of the gold electrodes is covered with a porous SCR catalyst, vanadia-tungstenia-titania (VWT). The cell is utilized as a potentiometric ammonia gas sensor and provides a semi-logarithmic characteristic curve with a high NH 3 sensitivity and selectivity. The analyses of the Au？|？YSZ and Au,？VWT？|？YSZ half-cells are conducted to describe the non-equilibrium behavior of the sensor device in light of mixed potential theory. Both electrode potentials provide a dependency on the NH 3 concentration, whereby VWT,？Au？|？YSZ shows a stronger effect which increases with increasing VWT coverage. The potential shifts in the anodic direction confirm the formation of mixed potentials at both electrodes resulting from electrochemical reactions of O 2 and NH 3 at the three-phase boundary. Polarization curves indicate Butler-Volmer-type kinetics. Modified polarization curves of the VWT covered electrode show an enhanced anodic reaction and an almost unaltered cathodic reaction. The NH 3 dependency is dominated by the VWT coverage and it turns out that the catalytic properties of the VWT thick film are responsible for the electrode potential shift
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