Periodically Pulsed Polarization Gas Sensors Based on Au|YSZ: Mechanism of NOx Detection

Pulsed polarization of Au|YSZ gas sensors is examined to clarify the mechanism of NOx detection under dynamic operation and to disentangle catalytic surface effects from electrochemical relaxation. Using gold electrodes with substantially lower catalytic activity than platinum explicitly enables this mechanistic separation. During pulsed polarization, periodic voltage pulses are followed by self-discharge under open-circuit conditions, and the response is measured based on the self-discharge rate. NO2 consistently accelerates the self-discharge from the beginning, whereas NO slows the relaxation predominantly at later times. CO and H2 produce similar delaying effects, and C3H6 shows no measurable influence under the tested conditions. Decreasing ambient O2 slows the discharge and amplifies the NO2 effect, which indicates that oxygen supply and surface exchange at the triple-phase boundary are rate determining. A Pt-containing catalytic overlayer drives local NO/NO2 interconversion toward equilibrium so that both gases yield to an accelerated self-discharge. These findings support a mechanistic picture in which NO2 provides effective oxygen equivalents that accelerate discharge, whereas NO, CO, and H2 consume oxygen and slow down discharge. Overall, this establishes a materials-based approach for distinguishing between NO and NO2 and evaluating the underlying mechanism during pulsed polarization.