Investigation on Electrochemical Properties of Low-Temperature Ionic Liquid-Based Electrolytes for MET Sensor

Abstract

The molecular electronic transducer (MET) sensor that operates based on an electrochemical principle provides an alternative approach for motion sensing applications and seismic investigations. As the liquid state proof mass responds to detected seismic waves, the electroactive species, iodide and triiodide, are brought to the electrodes by the induced fluid motions. At the cathode, the triiodide reduction currents are generated correspondingly and serve as a record of the detected ground activities. Compared to the conventional motion sensor and seismometer with solid state mass-spring system, by utilizing a liquid state electrolyte as the proof mass, the MET sensor presents advantages such as installation angle independence, superior shock tolerance, low mass, and low power consumption. The abovementioned characteristics have made MET sensor an appealing candidate for seismic investigations in space explorations, especially on planetary objects with deployment challenges. For potential seismic studies on icy ocean worlds, such as Europa, Titanic, and Enceladus where the extremely low temperatures can be challenging for liquid state electrolytes, our group has developed several ionic liquid-based electrolyte systems with broad liquidus range down to -120 ˚C. While enhancements in thermal and transport properties of the electrolyte systems via optimization of intermolecular interactions were validated, we have yet to fully explore their electrochemical properties at low temperatures that are critical for the MET sensor functionality. In this work, we present investigations on electrochemical properties of the developed electrolyte systems over a broad temperature range. Using electrochemical impedance spectroscopy, we observed effects of tailored intermolecular interactions between ionic liquid and incorporated molecular solvents on conductivity evolution from room temperature to -75 ˚C. Specifically, the resulting bulk conductivity of the overall system was governed by an interplay between ion mobility and ion disassociation, and its temperature dependence can be depicted by the Vogel-Fulcher-Tammann model. Furthermore, we also investigated kinetics of the targeted iodide/triiodide redox reactions via cyclic voltammetry for the MET sensing technology. The electrochemical reversibility of triiodide reduction for signal generation was correlated to the employed molecular solvents and their interactions with the electroactive species. Lastly, the electrochemical windows were also identified for respective electrolyte systems at various temperatures as references for future MET sensor operations. Discoveries of the presented work disclose effects of intermolecular interactions on electrochemical properties and contribute to applications of the MET sensing technology at room temperature and low temperatures.