Analytical Modeling of an Electrochemical Sensor Used in Electrolysis Mode and Exposed to a Binary Mixture of Gases

Abstract

Today, air pollution has become a major society issue. With the increased development of motorized vehicles, polluting and even toxic gases are being released in increasing quantities into the atmosphere with all known consequences [1] [2]. With the apparition of the concerns about impacts of human activity into the global warming, European legislations have started to impose more and more drastic emission limits through the euro norms (euro 6B currently). The norms concern both particles and polluting gases emitted from motorized vehicles. In a close future, industries, and particularly those having combustion furnaces will also be impacted by the norm. Therefore, controlling emitted gases and particles has become a major issue for car manufacturers and the development of new sensors responding to the requirements of the harsh environments present in exhausts has emerged. In response to those requirements, electrochemical sensors based on solid-state ceramic electrolyte are well-adapted [3] [4]. However, like for gas sensors based on other transducing techniques, a lack of selectivity compels the sensors’ developers to find different methods to extract the analyte’s nature and concentration information. According to the sensing principle, the gas analytes will be discriminated according to different of their properties. For example, in the case of microbalances, the sensor’s response will rely only to the mass deposited. In this case, it seems difficult to discriminate the gas nature since the mass is not specific to one specie in particular. The output sensor’s signal will give concentration information but no information linked to analyte’s nature. Besides, in the case of electrochemical sensors (like for MOX sensors), the sensor’s response will both give information on the analyte’s concentration and on its redox behavior. Different models describing electrochemical sensors already exist in the literature for null current potentiometry based measurement (thermodynamic equilibrium). In this case, models used to describe the operation of the sensor are often based on the mixed potential theory [5]. However, only few of them describe the sensor’s kinetics behavior when used in electrolysis mode (imposed current potentiometry). In this work, two analytical models describing the response of an electrochemical sensor used in his electrolysis mode are proposed and compared. Those model results from the association of the Butler-Volmer equations and an equivalent electrical circuit describing the parallel reactions that can occur at the triple phase interfaces of the electrodes. The first model considers that both oxidizing gases and reducing gases react at the gold cathode whereas the second one considers that only oxidizing gases react at the gold cathode, the reducing gases just adsorbing on the cathode which induce slowing down of the reaction kinetics of other O2 oxidizing gas.Besides, to test those models, different experimental curves, obtained thanks to our gas test bench and our “homemade” screen-printed electrochemical sensors, were used. Apart from O2, which is always present in the dilution air or in the “base gas”, the sensors were exposed to four different polluting analytes: NO2, NO, NH3, CO. To perform curve fitting, MATLAB algorithms were developed. 3 methods were chosen perform curve fitting and parameters extraction: the Least Squares method, the Newton-Gauss method and the Levenberg-Marquardt method. For each one, an iterative algorithm was developed to extract the vector values containing the fitting parameters. First results indicate good fitting performance for both oxidizing and reducing gases in the case of model 2 whereas for the first model, relevant fitting results could only be obtained for oxidizing gases.