Understanding and Correcting Unwanted Influences on the Signal from Electrochemical Gas Sensors

Abstract

Humidity- and temperature-dependent errors in concentrations reported by electrochemical sensors for atmospheric nitrogen dioxide significantly limit the reliability of the data. A basic understanding of the source of these errors has been missing. Empirical, software-based corrections are of limited reliability. The sensors feature a 40 wt % (≈4 molal) sulfuric acid electrolyte, and carbon working and quasi-reference (QRE) electrodes. We show that the sensor behaves as a truncated transmission line with resistance and capacitance elements varying with humidity. High-amplitude current fluctuations are due to humidity fluctuations, and are charging currents in response to fluctuations in interfacial capacitance. Baseline currents are due to very small differences in the open-circuit electrode potential between working and reference electrodes. We deduce that acid concentration changes in the meniscus within the porous electrode structure, in response to changes in the ambient temperature and humidity, cause both the capacitance fluctuations and the baseline changes. The open-circuit potential differences driving the baseline current variations are in part due to a difference in the liquid junction potential between the QRE and working electrode, dependent on humidity and temperature and caused by a gradient of acid concentration, and in part due to temperature- and acid-concentration-dependent variations in the rate of the potential-determining reactions. Based on the understanding obtained, we demonstrate a simple hardware change that corrects these unwanted errors.

Calibration of SO2 and NO2 Electrochemical Sensors via a Training and Testing Method in an Industrial Coastal Environment

“…Like humidity in the air, temperature can affect sensor performance. It has been shown that both exert changes in the electrode–electrolyte interface [ 12 ]. In addition, the weaker performance may be due to the effect of temperature on the gas in the electrolyte liquid.…”

“…The evaporation of the electrolyte has been proven to contribute to loss of the area of interfacial contact with the electrode surface [ 12 ]. In the event that the sensors are used for a long period of time, our method faces the limitation of not incorporating the effect of degradation-related drift in the sensor data into the model.…”

“…Though the technology that enabled the development of electrochemical sensors emerged several decades ago [ 9 ], further refinement is required to enable low-cost sensors to achieve the same accuracy as reference air quality monitors [ 10 ]. Studies show that measurements made by low-cost electrochemical sensors are affected by temperature, humidity, and cross-reactions with other pollutants [ 11 , 12 ]. For example, ozone is a known interfering pollutant in NO 2 electrochemical sensors.…”

Low-Cost Formaldehyde Sensor Evaluation and Calibration in a Controlled Environment

“…These sensors have improved over the past few years with respect to their underlying material science [43], [44] and signal processing [45], [46]. However calibration using machine learning when done effectively has also shown encouraging results [47], [48].…”

Considerations of Thermodynamics and Kinetics for the Effects of Relative Humidity on the Electrolyte in Electrochemical Toxic Gas Sensors

“…In any electrochemical sensor, it is clear that the sensing or working electrode (WE) will be exposed not only to the analyte gas but also to the ambient gaseous environment that will include water vapor which can adsorb onto the electrode catalyst and affect the electrochemistry. Such effects have been considered from the point of view of influences on sensor transient signals, 1 but, in addition, depending on the humidity level in the external atmosphere, water can be macroscopically absorbed by or desorbed from the electrolyte, which will also affect the longer-term sensor operating characteristics. We will start by considering electrolyte effects.…”