Ionic Liquid Impedance Sensor for Continuous Low Temperature CO2 monitoring
Authors: Sridhar Arun SIddarth , Chen Xiaoyu , Yang Ziming , Xu Yong , Zeng Xiangqun 
Published: July 2021

There is a significant need to develop sensor technology for applications in extreme environmental conditions. Robust gas sensors that can continuously monitor specific gaseous analytes in harsh environments are essential for a variety of applications such as environmental monitoring and greenhouse gas emissions in Arctic where temperatures are constantly in the sub-zero range. For efficient operation and long-term stability, these devices require sensing materials that can function at subzero temperatures as well as non-invasive readout mechanisms. Electrochemical techniques are advantageous for continuous gas sensing due to their cost effectiveness, simplicity, portability and easy miniaturization of the instrumentation. However, the application of constant or varying DC potential in commonly used voltametric and amperometric detection methods may result in interfacial redox reactions and formation of new products that could poison the sensor materials and take part in unfavorable side reactions. Electrochemical impedance spectroscopy (EIS) is a non-invasive alternate technique which measures impedance changes originating from change in polarization of the electrode-electrolyte interface upon addition of analyte gases. Compared to traditional aqueous electrolytes, the unique solvent and electrolyte properties of ionic liquids (ILs) enable them to operate in extreme environments without undergoing physical or chemical changes. In this presentation, we will discuss our results for the development of an ionic liquid (IL) based miniaturized impedance sensor for continuous CO2 monitoring in the -15 to 40oC temperature range using a microfabricated electrochemical sensor designed and fabricated with Pt black electrodes and hydrophobic [Bmpy][NTf2] ionic liquid electrolyte. The large hydrophobic NTf2 anion is shown to weaken cation−anion interactions, enabling a higher absorptivity of CO2. It also weakens water−anion interactions and thus impedes the absorption of H2O, minimizing the interference from humidity at the ambient conditions. Pt black provides a high surface area that allows simple and robust fabrication of planar electrodes on flexible polymer membranes. A non-invasive single frequency impedance spectroscopy (SF-EIS) method was demonstrated for the continuous and quantitative detection of CO2. In this method, impedance changes arising from increasing amounts of added CO2 analyte and [Bmpy][NTf2] IL at open circuit potential were measured at a set frequency of 1 Hz at different temperatures. It was seen that the impedance increased with decreasing temperature, and decreased with increasing amounts of added CO2. A linear relationship between the CO2 gas concentration and impedance was obtained, and increased sensitivity for CO2 detection was observed at low temperatures with a maximum slope of -5.092 ohm/%CO2 seen at -15oC. The same miniaturized and flexible planar electrochemical sensor with [Bmpy][NTf2] IL electrolyte was tested repeatedly for continuous sensing of CO2 at subzero temperatures over a two and a half month period, during which good stability and repeatability were obtained. To explain our impedance trend, we hypothesize that the high viscosity of IL at low temperature will result in the formation of a more packed, concentrated layer of ionic charges on the electrode surface. In the presence of CO2, the relatively rigid IL-electrode double layer at low temperature can be beneficial for chemical sensing since the molecular interactions between the CO2 analyte and [Bmpy][NTf2] could result in decreasing resistance and increasing conductivity by decreasing ionic attractions in the IL. Consequently, the high viscosity of ILs at low temperature, which is usually considered a limitation to practical in amperometric sensing, converts to an advantage in impedance sensing since ordered and concentrated electric double layer could provide high sensitivity and selectivity. In addition, the increased initial impedance at low temperature due to high IL viscosity and density is beneficial for our signal-off sensing mechanism. This reported CO2 sensor is among the first impedance based gas sensors shown to function continuously at sub-zero temperatures, providing CO2 measurements with good sensitivity at low cost and low power. These results obtained from our sensor show promise for potential use in real-time monitoring of carbon dioxide emissions in harsh low temperature environments, uniting the benefits of minimally invasive impedance transduction methods and robust IL sensing materials.

Figure 1