Ionic Liquid Electrode Research Articles

Secondary electron emission measurements from imidazolium-based ionic liquids

Abstract The electron-induced secondary electron emission (SEE) yields of imidazolium-based ionic liquids are presented for primary electron beam energies between 30 and 1000 eV. These results are important for understanding plasma synthesis of nanoparticles in plasma discharges with an ionic liquid electrode. Due to their low vapor pressure and high conductivity, ionic liquids can produce metal nanoparticles in low-pressure plasmas through reduction of dissolved metal salts. In this work, the low vapor pressure of ionic liquids is exploited to directly measure SEE yields by bombarding the liquid with electrons and measuring the resulting currents. The ionic liquids studied are [BMIM][Ac], [EMIM][Ac], and [BMIM][BF4]. The SEE yields vary significantly over the energy range, with maximum yields of around 2 at 200 eV for [BMIM][Ac] and [EMIM][Ac], and 1.8 at 250 eV for [BMIM][BF4]. Molecular orbital calculations indicate that the acetate anion is the likely electron donor for [BMIM][Ac] and [EMIM][Ac], while in [BMIM][BF4], the electrons likely originate from the [BMIM]$^+$ cation. The differences in SEE yields are attributed to varying ionization potentials and molecular structures of the ionic liquids. These findings are essential for accurate modeling of plasma discharges and understanding SEE mechanisms in ionic liquids.

Low Reducing Potentials Enabled by CaF2-Supported Graphene Electrodes in High Impedance Solutions.

We report electrochemical measurements using in situ Raman spectroscopy at graphene/D2O interfaces under extremely low applied potentials. Here, the hydrophobic and catalytically inert nature of graphene and the insulating nature of the deionized (DI) water enables potentials as low as Vapplied = -7 V vs Ag/AgCl to be applied without exceeding 200 μA/cm2 of current density. At higher currents, bubble formation (i.e., hydrogen evolution reaction) prohibits reliable spectra from being obtained from the electrode surface. Using CaF2 as the supporting substrate enables significantly lower reducing potentials to be reached compared to glass substrates, likely due to trapped charge and impurities in the glass substrate. G band Raman spectra taken under various applied electrochemical potentials exhibit a linear relationship between the G band shift (ΔωG) and the applied potential, with blueshifts as high as ΔωG = 18 cm-1. These large Raman shifts indicate a large change in the Fermi level of ΔEF = -0.43 eV for graphene electrodes in contact with water, favoring reduction half-reactions. Based on the solution resistance measurement, there is a VIR = 3.1 V voltage drop across the solution for D2O (when the applied potential was Vapplied = -7 V vs Ag/AgCl) and the effective reducing potential on the working electrode is Veffective = -3.9 V vs Ag/AgCl. We have also tested these graphene electrodes in ionic liquids [DEME][TFSI], which are limited to applied potentials above Vapplied = -2.7 V vs Ag/AgCl and a corresponding shift in the Fermi level ΔEF = -0.32 eV, indicating that pure water can provide a more robust electrolyte for reaching low reducing potentials than ionic liquids.

Revealing Interfacial Reactions on Pt Electrodes in Ionic Liquids by In Situ Fourier-Transform Infrared Spectroscopy.

In situ monitoring of the electrolyte/electrode interfacial processes, such as the oxygen reduction reaction (ORR), is crucial for the design of electrolytes for fuel cells. In this study, we investigate the electrochemical behavior of platinum electrodes in protic ionic liquids (PILs) by means of in situ Fourier-transform infrared spectroscopy coupled with cyclic voltammetry. The result provides direct evidence of the change of water at the Pt electrode surface due to Pt oxide formation and reduction. A decrease in the interfacial water was observed in the spectra upon the formation of the Pt oxide. Conversely, the local water concentration at the electrode surface increases if the Pt oxide is reduced and the ORR takes place. At the same time, more cations replace anions on the electrode. The ORR kinetics in the [TFSI]-based PILs is slower than in the [TfO]-based ones, which could result from a blockage of catalytic sites by the adsorbed [TFSI] anions. It suggests that reducing the anion adsorption on the platinum surface could provide an opportunity to enhance the ORR activity.

Development of rutin sensor based on graphene quantum dots@nano-carbon ionic liquid electrode

Development of rutin sensor based on graphene quantum dots@nano-carbon ionic liquid electrode

High rate and long-term cycling of silicon anodes with phosphonium-based ionic liquids as electrolytes for lithium-ion batteries

This paper describes the effects of the length of the phosphonium alkyl chain in FSI-based ionic liquids (ILs) used as an electrolyte solvent with Si electrodes in a Li/Si half-cell. The electrochemical performance of Si nanoparticles at a high rate in the synthesized triethyl-n-butylphosphonium bis(fluorosulfonyl)imide) [P2224][FSI] and triethyl-n-octylphosphonium bis(fluorosulfonyl)imide) [P2228][FSI] ILs, which also contain 1 M LiFSI, is compared with that of a common organic electrolyte using either polyacrylic acid (PAA) or polyacrylonitrile (PAN) as a binder. For the three electrolytes, the highest performance was obtained with the PAN binder, while both electrodes in ILs exhibited superior performance compared to the conventional organic electrolyte. For the Si/PAN composite electrode, after 1000 cycles at 1 A.g−1, the delithiation capacities were 1344, 550 and 136 mAh.g−1 for [P2224][FSI] in 1 M LiFSI, [P2228][FSI] in 1 M LiFSI and the commercial organic electrolyte, which corresponded to capacity retentions of 61%, 50% and 5%, respectively. Our results indicate that the chemical structure of the phosphonium cation enabled the tuning of transport properties and significantly influences the electrochemical behavior of the Si anode in the presence of LiFSI/IL as an electrolyte.

Reservoir computing with dielectric relaxation at an electrode\u2013ionic liquid interface

A physical reservoir device with tunable transient dynamics is strongly required to process time-series data with various timescales generated in the edge region. In this study, we proposed using the dielectric relaxation at an electrode–ionic liquid (IL) interface as the physical reservoir by making the most of designable physicochemical properties of ILs. The transient dynamics of a Au/IL/Au reservoir device were characterized as a function of the alkyl chain length of cations in the IL (1-alkyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide). By considering a weighted sum of exponentials expressing a superposition of Debye-type relaxations, the transient dynamics were well reconstructed. Although such complex dynamics governed by multiple relaxation processes were observed, each extracted relaxation time scales with a power law as a function of IL’s viscosity determined by the alkyl chain length of cations. This indicates that the relaxation processes are characterized by bulk properties of the ILs that obey the widely received Vogel-Fulcher-Tammann law. We demonstrated that the 4-bit time-series signals were transformed into the 16 classifiable data, and the data transformation, which enables to achieve higher accuracy in an image classification task, can be easily optimized according to the features of the input signals by controlling the IL’s viscosity.

Investigating the application of caffeine-based ionic liquid modified by zinc bromide as an effective electrode in supercapacitor

In this paper, an efficient ionic liquid (IL) by using caffeine (Caff), trietahnolamine (TEA) and ZnBr2 ([Caff-TEA]+[ZnBr3]-) was used for supercapacitor applications. The supercapacitive properties of the electrode material were studied using cyclic voltammetry (CV), charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS). The specific capacitance was obtained of 165.28 F/g at current density of 1 A/g. Moreover, it retained 80% of the initial capacitance after 20,000 cycles at current density of 3 A/g. These results shows that the ionic liquid electrode is a promising option for use in supercapacitors.

(Invited) Diffusion Control of Metal Ions Close to Electrode in Ionic Liquid -Effect of Local Structure of Ionic Liquid-

Ionic liquids (ILs) have attracted keen attention due to its applicability to various kinds of electrochemical devices. However, we still lack enough knowledge of the interface structure and spatial distribution of chemical species during the electrochemical processes. In our previous study, we succeeded in the observation of the spatial distribution of Ag+ ions in ionic liquid solution close to the electrode under the applied potential using scanning electrochemical photoelectron spectroscopy (Scanning EC-PES) we originally developed. Interestingly, the diffusion layer where the concentration of metal ions is quite small (We named this region as “depletion layer”) was formed close to the electrode in a wide region. By a numerical simulation, we revealed that the metal ions diffused by the hopping-like mechanism, in which the apparent diffusion coefficient increased with increasing of the concentration of 'holes' (vacancies formed between IL molecules) acting as hopping sites.On the other hand, it can be anticipated that the local structure of ILs can be systematically controlled by changing of IL species (e.g. the alkyl chain length of imidazole cation, shape and size of anion), additive ions and/or temperature of solution. By using these techniques for controlling the local structure of ILs, we can strictly control the diffusion behavior of metal ions close to the electrode. In other words, we will be able to propose a new strategy for controlling the diffusion behavior of solutes in ionic liquid solutions close to the electrode. In this study, we investigated the factors for controlling the diffusion behavior of metal ion in IL electrolyte based on our proposing diffusion model, and revealed that the behavior of metal ions can be controlled by some factors.We investigated the additive effect using Li+ ion. In the case of the ~200 mM Ag+/BMI-TFSA((1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) amide) solution, the large depletion layer was formed close to the electrode, whereas the depletion layer was decreased when ~100 mM Li+ (as an electrochemically inactive additive) was added to the solution. This result indicates that the hopping-like diffusion was suppressed by the addition of the Li+ ion which efficiently occupied the holes (hopping sites) (Note that the depletion layer was formed by the fast hopping-like diffusion mechanism). On the other hand, when we added the excessive amount of additive (~500 mM Li+) to BMI-TFSA solution, a large depletion layer was formed again, indicating that the hopping diffusion was enhanced. This is due to that some Ag+ ions are drive out of the polar domain, and moved to the non-polar domain where the activation energy of hopping diffusion is relatively small. On the other hand, we carried out a similar experiment using HMI-TFSA instead of BMI-TFSA. We found that the hopping diffusion of Ag+ ion is enhanced compared with that in BMI-TFSA. This could also be explained by the local domain effect of ionic liquid. These results indicate that we can control the diffusion behavior of metal ions by adjusting the local structure of ionic liquid.

Electrochemical Characterization of Single Layer Graphene/Electrolyte Interface: Effect of Solvent on the Interfacial Capacitance

AbstractThe development of the basic understanding of the charge storage mechanisms in electrodes for energy storage applications needs deep characterization of the electrode/electrolyte interface. In this work, we studied the charge of the double layer capacitance at single layer graphene (SLG) electrode used as a model material, in neat (EMIm‐TFSI) and solvated (with acetonitrile) ionic liquid electrodes. The combination of electrochemical impedance spectroscopy and gravimetric electrochemical quartz crystal microbalance (EQCM) measurements evidence that the presence of solvent drastically increases the charge carrier density at the SLG/ionic liquid interface. The capacitance is thus governed not only by the electronic properties of the graphene, but also by the specific organization of the electrolyte side at the SLG surface originating from the strong interactions existing between the EMIm+ cations and SLG surface. EQCM measurements also show that the carbon structure, with the presence of sp2 carbons, affects the charge storage mechanism by favoring counter‐ion adsorption on SLG electrode versus ion exchange mechanism in amorphous porous carbons.

Electrochemical Characterization of Single Layer Graphene/Electrolyte Interface: Effect of Solvent on the Interfacial Capacitance.

The development of the basic understanding of the charge storage mechanisms in electrodes for energy storage applications needs deep characterization of the electrode/electrolyte interface. In this work, we studied the charge of the double layer capacitance at single layer graphene (SLG) electrode used as a model material, in neat (EMIm‐TFSI) and solvated (with acetonitrile) ionic liquid electrodes. The combination of electrochemical impedance spectroscopy and gravimetric electrochemical quartz crystal microbalance (EQCM) measurements evidence that the presence of solvent drastically increases the charge carrier density at the SLG/ionic liquid interface. The capacitance is thus governed not only by the electronic properties of the graphene, but also by the specific organization of the electrolyte side at the SLG surface originating from the strong interactions existing between the EMIm+ cations and SLG surface. EQCM measurements also show that the carbon structure, with the presence of sp2 carbons, affects the charge storage mechanism by favoring counter‐ion adsorption on SLG electrode versus ion exchange mechanism in amorphous porous carbons.

In situ measurement of diffusion behavior of metal ion close to electrode in ionic liquid

In situ measurement of diffusion behavior of metal ion close to electrode in ionic liquid

Electrochemical Evaluation of Choline Oxidase on Modified Bamboo-Like CNTs Electrode: from Nano-Chemistry Towards Biotechnological Application

Carbon nanotube (CNT) can be considered as one of the best materials for enzyme nanobiosensor. In this study, we investigated comprehensively electron transfer properties in three kinds of carbon nanotubes for finding the best supporting material for comprising of the electrochemistry and electrocatalysis of enzymes in nano-dimension. Choline oxidase was selected as a sample enzyme for evaluating the performance of the electrodes. The enzyme was absorbed on modified SWCNT, MWCNT and Bamboo-like CNTs electrode in ionic liquid. Electron transfer and electroanalytical responses were investigated by Metrohm Autolab. Functionalization was studied by Raman spectroscopy and FTIR and also, the morphological evaluation was conducted by SEM. Raman spectroscopy showed that Bamboo-like CNTs had better properties in loading enzyme and other electrochemical properties for electron transfer. FTIR spectroscopy showed that Bamboo-like CNTs had high covalent binding on the surface wall of nanotubes. High loading of the enzyme was detected in Bamboo-like CNTs in comparison to two other nano-tubes by SEM. The electrochemical and amperometric response showed that Bamboo-like CNTs had a better response in different scan rates, wide linear range and detection limit. ChOx/RTIL/Bamboo-like CNTs/GC electrodes showed the high amounts of enzyme loading, high enzyme-substrate affinity, low detection limit, best sensitivity and wider linear range.

Electrodeposition of Pr-Mg-Co ternary alloy films from the choline chloride-Urea ionic liquids and their corrosion properties

This paper aims to obtain rare earth magnesium alloy with good adhesion and corrosion resistance. The work was concerned with the electrochemical behaviors of Co(II), Mg(II), Pr(III) on Pt and Cu electrodes in choline chloride-urea ionic liquids by electrochemical techniques of cyclic voltammetry. The electrodeposition conditions of Pr-Mg-Co ternary alloy films on copper substrate from the choline chloride-urea ionic liquids with different metal salts were investigated by potentiostatic electrolysis. The author has used scanning electron microscopy coupled with energy dispersive X-ray spectroscopy to study the surface morphology and composition of Pr-Mg-Co ternary alloy film, Polarization curves are tested. Results shows under −1.15 V voltage, the alloy films with evening is obtained when the concentration ratio of Pr(III)/Mg(II) is 4:3, and deposition time is 30 min .it showed that the content of Pr in the alloy film has reached up to 13.22 wt.%, and the morphology of black alloy coating is amorphous and has micro-cracks, which reflects the alloy films has good corrosion performance in alkaline solution. This can be attributed to its dense film structure and good adherence to the substrate. It provides some technology for the production of corrosion- resistant materials.

Silver thin film electrodes for supercapacitor application

Abstract The potential of a radio frequency (RF) magnetron sputtered silver (Ag) thin film electrodes, for electrochemical energy storage application is presented in this paper. Scanning electron microscope (SEM) was used to reveal the information on the morphology and growth mode of the deposited silver thin film samples. The stoichiometry of the RF sputtered silver thin films was evaluated using X-ray diffraction (XRD), while the elemental constituents of the deposited thin films were confirmed using energy dispersive X-ray spectroscopy (EDX). The surface area, wettability and surface energy of silver thin film electrodes were determined, using Brunauer-Emmett-Teller (BET) and contact angle measurements. Furthermore, the ion diffusion, Faradaic redox reactions and the specific capacitance of the produced Ag thin film electrodes in ionic liquid (1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) were probed using electrochemical impedance spectroscopy (EIS), charge-discharge and cyclic voltammetry (CV). XRD result reveals that the sputtered silver thin films are crystalline, with metallic silver been the predominant element found on the EDX spectra. The specific capacitance of 431 F/g at 2 mV/s scan rate was achieved for Ag thin film electrode produced at 350 W forward power, demonstrating its promising potential as an active electrode for supercapacitor application.

Analysis of Oxide Film on Aluminum Electrode in Ionic Liquid Based Aluminum-Air Battery

In this research, aluminum air battery system was revisited to investigate key issues which have challenged its full application as a rechargeable battery. The Al-air battery had received intense attention once due to the excellent benefits such as low cost and high energy density, but due to the challenging issues such as hydrogen evolution, and inactive oxide film formed on the Al surface, it could not be fully applied. Recently, ionic liquid which is a salt in liquid form at room temperature started to be applied in the battery system to overcome the issues in aqueous electrolyte [1,2], but the inactive oxide film on Al surface is still dominant in decreasing cell performance and detailed research to understand how to control the oxide film was not conducted. In this study, 1-Ethyl 3-Methyl Imidazolium Chloride ([EmIm]Cl) and Aluminum Chloride (AlCl3) was applied in the Al-air battery, and its effect on oxide film on the surface aluminum electrode was analyzed through cell-base test, and SEM image analysis. It was found that the inert oxide film on aluminum, which is one of the most challenging issues, could be controlled and removed by using of proper ratio of [EmIm]Cl and AlCl3, and the growth/breakdown mechanism of the oxide film was theoretically considered. This research results are expected to suggest a new direction to control the surface of aluminum electrode to make it the most promising system.

Pseudo-capacitance of silver oxide thin film electrodes in ionic liquid for electrochemical energy applications

Abstract The energy storage potential of silver oxide (Ag2O) thin film electrodes, deposited via radio frequency reactive magnetron sputtering, was investigated in an ionic electrolyte (1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide for supercapacitor applications. X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Fourier Transform infrared spectroscopy (FTIR) tools were used to evaluate the structural and oxide phases present in the sputtered silver oxide thin film electrodes. The growth mode, morphology, surface area, wettability and surface energy of the deposited nano-structure silver oxide thin films were confirmed by scanning electron microscope (SEM) data, the Brunauer-Emmett-Teller (BET) analysis and by goniometer and tensiometer studies. Furthermore, the ion diffusion, the Faradaic redox reactions and the capacitance of the sputtered thin films exposed to 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic electrolyte, were monitored with electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The SEM micrographs depict that silver oxide thin films exhibit a columnar growth mode. The wettability analysis reveals that Ag2O thin films are hydrophilic, an indication for excellent electrochemical behaviour. Cyclic voltammetry measurements show that Ag2O thin films exhibit a specific capacitance of 650 F/g at higher sputtering power, demonstrating its promising potential as an active electrode for supercapacitor applications.

Direct electroiodimetric sensing of reducing biomolecules using a modified multiwall carbon nanotube/ionic liquid paste electrode by tetra-n-octylammonium triiodide

An effective, sensitive and simple method as a direct electroiodimetric determination of reducing biological substances using a modified multiwall carbon nanotube/ionic liquid paste electrode has been introduced. Tetra-n-octylammonium triiodide (TOAI3) was used as a new modifier which was simply synthesized and characterized. By combining the advantages of both cyclic voltammetry and iodimetric technique, the problems associated with the classical iodimetry such as time consuming stages and indicator usage were removed. Three types of carbon paste electrodes (CPEs) with different structures were studied and their compositions were optimized. The appropriate one which was MWCNT/TOAI3/CILE (multiwall carbon nanotube/tetra-n-octylammonium triiodide/carbon ionic liquid electrode), displayed unique electrocatalytic behavior in acidic media and was validated for determination of ascorbic acid (AA), cysteine (Cys) and dopamine (DA) as typical reducing biomolecules in pharmaceutical and human blood serum samples. By applying cyclic voltammetry, the linear response ranges for the determination of AA, Cys and DA were recorded as 1.0–2500.0 μM, 1.0–1500.0 μM, and 0.8–800.0 μM, in order. The theoretical detection limits were found to be 1.0, 0.8, and 0.6 μM for the determination of AA, Cys, and DA, respectively. The effect of some coexisting biomolecules in determination of these analytes was investigated and glucose effectively interfered.

Surface Structure Sensitivity of CO2 Electroreduction on Low‐Index Gold Single Crystal Electrodes in Ionic Liquids

Surface Structure Sensitivity of CO₂ Electroreduction on Low‐Index Gold Single Crystal Electrodes in Ionic Liquids

Investigation into Effects of Membrane Thickness on Electromechanical Properties of Biopolymer Chitosan-Based Electroactive Paper

ABSTRACTChitosan-based electroactive paper has been reported as a smart material, which has merits in terms of lightweight, dry condition, biodegradability, sustainability, large displacement output, and low actuation voltage. However, our recent investigations found its actuation performance is highly sensitive to the membrane thicknesses, both on the electrolyte layer and the electrode layer. Focused on this issue, in this paper, we introduce a biopolymer ionic actuator made by multiwalled carbon nanotube, ionic liquid electrode, and polymer-supported chitosan. As a result, we find that chitosan polymer actuator with the thick electrode layer (0.7 mm) behaves with a larger blocking force (9.66 mN) and a smaller displacement (9.53 mm), and the lifetime under applied voltage of 3 V at 0.25 Hz is 1.75 times surpassed the thin one (0.3 mm). In addition to that, we investigate effects of membrane thicknesses on the electrical properties of chitosan polymer actuator, and figure out the relationship between the tensile strength of the membrane and the volume of the ionic solution in the electrolyte layer.

Microfluidic capacitive sensors with ionic liquid electrodes and CNT/PDMS nanocomposites for simultaneous sensing of pressure and temperature

A multimodal microfluidic capacitive sensor for detecting lateral pressure movements and temperature variations is presented.