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

Pulsed polarization of Au|YSZ gas sensors is examined to clarify the mechanism of NO_x 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.