(Invited) Digital Voltage-Current Time Differential Method for Operating Zirconia-Based Impedancemetric Gas Sensors for Combustion Applications

Abstract

Gas sensors are needed for monitoring and controlling combustion technologies. In particular, NOx compounds (NO and NO2) that are present in diesel exhaust are known to cause poor air quality and to act as both pollutants and greenhouse gases. To mitigate NOx compounds, selective catalytic reduction (SCR) technologies employ ammonia as a reducing agent, where ammonia is also a pollutant and greenhouse gas. Commercially available sensors for vehicle applications use high-temperature solid-state ceramic electrochemical technology similar to the ubiquitous automotive oxygen sensor; however, commercial sensors are much more expensive than their oxygen sensor counterparts due to their complicated design with multiple cells (as opposed to the single-cell oxygen sensor) and their expensive electronics for measuring low-current signals (as opposed to the voltage signal from oxygen sensors). Therefore, a gas sensor alternative that meets sensor performance requirements at a reduced cost is desired for automotive and other combustion applications. Conventional solid-state electrochemical sensors operate with direct current (dc) methods that are either current-based/amperometric or voltage-based/potentiometric. In this work, we use alternating current (ac) impedance-based/impedancemetric operation of zirconia-electrolyte-based sensors for gas detection. The solid-state electrochemical cells are comprised of two electrodes separated by the zirconia electrolyte, where both electrodes are exposed to the gas. The impedance-based response of the simple single-cell sensor relies primarily on multiple concurrent non-equilibrium steady-state interfacial redox reactions. Impedance spectroscopy was used to investigate sensing mechanisms, and sensor operation was performed at pre-determined frequencies to demonstrate fast, stable, reproducible responses to varying concentrations of NOx down to single part-per-million (ppm) concentrations. Since laboratory impedance spectroscopy evaluation usually employs expensive analytical equipment, a novel, low-cost, portable signal processing method was developed using a digital voltage-current time differential method. The applied signal was an alternating current electrical waveform, and the response of the sensor was digitally measured directly in the time domain; laboratory impedance evaluation usually measures in the frequency domain. Results of the low-cost digital signal processing indicated increased gas sensitivity and the potential for improved sensor performance compared to the frequency-domain impedancemetric method, including differentiating between NO, NO2, and ammonia. System design considerations for deployment and performance tradeoffs will be discussed as well as work to better understand the working principle in terms of the sensor behavior with different gases in the frequency domain and in the time domain.