Methane Recognition and Quantification by Differential Capacitance at the Hydrophobic Ionic Liquid-Electrified Metal Electrode Interface

Abstract

We present a method to identify and quantify methane using a hydrophobic ionic liquid (IL)-electrified metal electrode interface by electrochemical impedance spectroscopy. We investigated the mechanisms of the responses of the IL-electrified electrode interface to the exposure of methane and other interfering gases (H 2 , C 6 H 12 , SO 2 , NO, NO 2 , CO 2 , O 2 , H 2 O). Our results show that at low frequency the IL-electrified electrode interface shows a predominantly capacitive response. The IL-electrode double layer (EDL) was found to be the primary response layer while the transition zone and bulk region of the IL-electrode interface contribute little to the overall signal change. For recognition and quantification of methane using the Langmuir adsorption model and measurement of differential capacitance change, an optimum EDL interface structure was found to form at a specific DC bias potential. The cumulative results shown in this work suggest that an ideal IL-electrode interface can be formed by varying IL structure and applied DC bias electrode potential for a specific analyte and that the semi-ordered structure of IL-electrified interface can act as a recognition element for the sensitive and selective adsorption and detection of gaseous molecules.Due to their unique properties and increasing availability, room temperature ionic liquids (ILs) have received great interests in electrochemistry, catalysis, electronics, and energy conversion as well as interdisciplinary investigations on both fundamental and practical applications. 1-5 For example, ILs as solvent free and ion-coupled material exhibit strong benefits as non-aqueous electrolytes for enhancing the safety and robustness for sensor and transistor devices. 6-12 However, the low intrinsic conductivity of the ILs correlates with their high viscosity, which limits the response time and sensitivity of detection methods such as those based on amperometry and potentiometry. [13][14][15][16][17] Methane, which has been considered as a clean energy source and one of the most important greenhouse gases, has attracted significant interest to the characterization of its adsorption on surfaces and its quantification in atmosphere as well. Since methane is relative chemically and electrochemically inert, current methods for methane detection are either relatively high-cost (e.g. optical), which prevents widespread deployment, or lack the selectivity (e.g. catalytic bead) 18 demanded by various applications. Many methane sensors also need improvement regarding their size, power consumption and the ease of use. [19][20][21] Current results show that the potential-dependent interface of an IL and metal electrode is very sensitive to surface conditions on the electrode, such as the proton adsorption on an oxide electric interface 22 and the adsorption of CO on a metal electrode 23 . As shown in Figure 1, the molecular selectivity of an IL-electrode interface comes from the ordering of the electric double layer (EDL) as well as molecular interactions between the IL...

In Situ Generated Platinum Catalyst for Methanol Oxidation via Electrochemical Oxidation of Bis(trifluoromethylsulfonyl)imide Anion in Ionic Liquids at Anaerobic Condition

“…The unique double layer structure of [Bmpy][NTf 2 ] provides the selectivity for small molecule such as methanol. 16 Both of the cation and anion of [Bmpy][NTf 2 ] are large with weak electrostatic cation–anion interaction densities which allow a large number of unoccupied space in the IL for methanol adsorption but not for larger molecules. 50 …”

Electrode–Electrolyte Interfacial Processes in Ionic Liquids and Sensor Applications

“…Our studies and others have shown that the capacitance of IL/electrode interface is potential dependent, and the adsorption of gas on the IL/electrode interface will decrease the double-layer capacitance under a specific DC bias potential [80]. In contrast to the EDL typically observed in dilute aqueous or nonaqueous electrolytes, the ions of ILs are strongly oriented near the electrode into an ordered layer structure with local ion density at maximum possible value [81,82].…”

“…The current can be mainly attributed to the Faradaic current of oxygen reduction, and the double-layer charging current is negligible after 300 s, which is much longer than that in aqueous electrolytes (lower than few seconds). And the double-layer capacitance is also potential-dependent [80], because under different potentials the orientations of IL are different and the values of relative capacitances vary as well. Thus, cell time constant τ is not a constant and varies in different systems.…”

“…Thus, EIS-based sensors not only provide orthogonal detection modes to amperometric sensors but they also permit self-monitoring of the sensor's stability and automated calibration for drift mechanisms. We have demonstrated an innovative capacitance sensor based on the change of the surface charge at IL-electrified metal interfaces either due to the change of ion or electron transport in ILs resulted from the redox process of analyte or the polarization reaction due to adsorption/dissolution of analyte in ILs [80]. At room temperature, ILs have high viscosity, and the diffusion and conductivity of ions are normally lower in ILs than in aqueous electrolyte solutions.…”

“…For example, the IL [C 4 mPy][NTf 2 ], which has a double-layer structure favoring small gas molecule adsorption, can sensitively and selectively measure concentrations of CH 4 ( Fig. 2.17) using EIS with a −0.3 V DC bias [80]. Figure 2.17c also clearly shows that the sensor returns to its baseline value when the analyte is removed, demonstrating the reaction is fully reversible with less than 0.1 % drift.…”