NO x Detection By Pulse Polarization: Influence of Gold Electrodes

Introduction Nitrogen oxides (NO x ; NO and NO 2 ) are limited emissions from combustion processes. They are not only harmful to human health, but also to the environment. This makes it necessary to measure and reduce nitrogen oxide emissions. Pulse polarization is a novel method for NO x concentration measurements. In contrast to existing static sensors, this method utilizes the dynamic response of the sensor, similar to cyclic voltammetry or impedance spectroscopy. For this purpose, the sensor is polarized with a constant voltage U pol for t pol (Fig. 1). After applying the voltage, the self-discharge of the sensor is recorded over a defined time t discharge . Charging and discharging phases are repeated continuously, with alternating change of the charging voltage polarity. The Pt|YSZ system has shown that NO x selectively accelerates the discharge of the sensor [1]. This accelerated discharge can be used as a sensor signal by evaluating the voltages at a fixed time during the discharge phase. Due to the faster discharge, these voltages are below the values without nitrogen oxides and thus indicate the concentration of the analyte gas. A semi-log dependency between U and c NOx was found. However, the effects that lead to faster and selective discharge have not yet been fully understood. In order to investigate the effect of the catalytically active Pt electrodes in particular, they were replaced by gold electrodes, which should yield a significantly lower catalytic impact. Experimental To prepare the sensors, two rectangular gold electrodes were screen printed on both sides on an 8YSZ substrate and then fired at 850 °C. The sensors were contacted with Au wires by gap welding. The sensors were operated at 400 °C in a tube furnace. For pulse polarization, a sourcemeter was periodically connected to the sensors via relays and the voltages were recorded. A polarization voltage of U pol = 1 V, a polarization duration t pol = 1 s and a discharge time t discharge = 10 s were chosen. This lead to a total cycle time t cycle = 22 s. To determine the gas concentration dependence in the discharge phase, a mixture of 10 % O 2 with 2 % H 2 O in nitrogen was defined as base gas. In addition, NO, NO 2 , and a mixture of 50 % NO and NO 2 (NO x ) in concentrations between 50 and 200 ppm were added to the gas flow. Results and discussion The sensor signals are shown in Figure 2. The voltages U 4s_neg shown were measured during self-discharging 4 s after each negative pola