NOx Detection By Pulse Polarization: Influence of Gold Electrodes
Authors: Donker Nils , Ruchets Anastasiya , Schönauer-Kamin Daniela , Zosel Jens , Guth Ulrich , Moos Ralf 
Published: May 2020

Introduction
Nitrogen oxides (NOx; NO and NO2) 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 NOx 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 NOx 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 % O2 with 2 % H2O in nitrogen was defined as base gas. In addition, NO, NO2, and a mixture of 50 % NO and NO2 (NOx) 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 polarization. The long measuring time of 18 h and the cycle duration of 22 s result in over 2900 cycles during the measurement. The baseline of the voltage curve shows that the cycles are very stable during the entire measuring period. It is also noticeable that NO and NO2 have an opposite effect on the self-discharge of the sensor. While NO2 gas as well as a 50/50 mixture of NO and NO2 accelerate the discharge, NO gas decelerates it. This means, the resulting absolute value of U
4s_neg is lower than that in NOx-free gas in case of accelerated discharge and higher at decelerated discharge. The mixture of 50% NO and 50% NO2 has an accelerating effect.
The accelerated self-discharge in the presence of NO2 was already observed for platinum electrodes. In contrast to this, the discharge decelerated by NO as found here for gold electrodes has not yet been observed on Pt electrodes. We attribute the behavior on platinum electrodes to the gas phase reactions that occur during diffusion through the electrode and to the assumption that the gases are almost in thermodynamic equilibrium at the three-phase contact between electrode, electrolyte and gas phase [2]. At a temperature of 400 °C and an oxygen content of 10 %, this equilibrium is about 50 % NO and 50 % NO2. This would result in conditions similar to those for NOx to be dosed, which also accelerates the discharge. The equilibrium thus explains the same signal for NO and NO2.
The catalytically less active gold electrodes make it possible to separate NO and NO2 effects. The strong oxidizing effect of NO2 is expected to play a key role in the sensor effect [3]. By applying the voltages, oxygen is pumped from the cathode to the anode. This leads to a lack of oxygen at the cathode and an excess at the anode. After polarization, NO2 probably supplies the electrode, which is depleted of oxygen due to polarization, with additional oxide. This helps to reduce the oxygen gradient and thus leads to an accelerated discharge. In contrast, NO seems to slow down the oxygen supply at the oxygen depleted electrode and thus decelerate the discharge. The removal of oxygen at the oxygen-rich electrode seems to play a minor role. If both the oxygen supply on one side and the oxygen removal on the other were equally important, NOx would provide the strongest acceleration of the discharge.

References

[1] J. Exner, et al., Pulsed Polarization-Based NOx Sensors of YSZ Films Produced by the Aerosol Deposition Method and by Screen-Printing, Sensors, 1715 (2017); doi: 10.3390/s17081715.
[2] T. Ritter, et al., On the influence of the NO equilibrium reaction on mixed potential sensor signals: A comparison between FE modelling and experimental data, Sens. Actuators B, 126627 (2019); doi: 10.1016/j.snb.2019.126627.
[3] D. Bhatia, et al., Experimental and kinetic study of NO oxidation on model Pt catalysts, J. Catal., 106–119 (2009); doi: 10.1016/j.jcat.2009.05.020.

Figure 1