Half-Cell Potential Analysis ot an Ammonia Sensor with the Electrochemical Cell Au degree ' YSZ degree ' Au, V sub(2)O sub(5)-WO sub(3)-TiO sub(2)

Half-cell potentials of the electrochemical cell Au, VWT degree ' YSZ degree ' Au are analyzed in dependence on oxygen and ammonia concentration at 550 degree 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 sub(3) sensitivity and selectivity. The analyses of the Au degree ' YSZ and Au, VWT degree ' 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 sub(3) concentration, whereby VWT, Au degree ' 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 sub(2) and NH sub(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 sub(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.