Cyclic voltammetry at microelectrodes in the absence of added electrolyte using a platinum quasi-reference electrode

The cyclic voltammetric behaviour of the ferrocene/ferricinium (Fc/Fc⁺) couple has been examined in acetonitrile without deliberately added electrolyte using scan rates over the range 5 mV/s to 50 V/s. Platinum (0.5 μm, 2.5 μm and 25 μm radius) and gold (5 μm radius) microelectrodes were used as working electrodes, with platinum wire quasi-reference electrodes to minimize contamination. At slow scan rates (5 to 500 mV/s) sigmoidal shaped steady state voltammograms were generally observed on the forward (oxidative) scan as is the case with electrolyte. Reverse (reductive) scans were not strictly sigmoidal and exhibited small peaks. This phenomenon is not observed in the presence of electrolyte and is attributed to ionic migration of the Fc⁺ cation. With a two electrode configuration, employing a platinum wire quasi-reference electrode, the forward and reverse scans of cyclic voltammograms were not superimposed at low scan rates unless small radii and low ferrocene concentrations are used. This distortion may be attributed to polarization of the reference electrode. Use of a three electrode platinum configuration, in a potentiostatic model while increasing the noise level, decreases this problem. At fast scan rates in excess of 1 V/s, planar diffusion terms are apparent. Additionally, iR (ohmic) distortions are considerable and are enhanced as the electrode radius and ferrocene concentration increase. Despite this problem, which mitigates against obtaining reliable thermodynamic data or assessment of electrochemical reversibility, the important diagnostic criterion of the presence of chemical reversibility or otherwise of an electrode process can still be clearly ascertained at fast scan rates, since the reduction current arising from the presence of the Fc⁺ ion in the reverse scan is observed, as is the case in the presence of electrolyte.     

Redox monomer ionic liquid based on quaternary ammonium: From electrochemistry to polymer brushes

Bifunctional quaternary ammonium ionic liquid bearing redox and polymerisable units was synthesized. The electrochemical investigations of the ferrocene monomer ionic liquid were performed. Following that, surface-initiated atom transfer radical polymerization (SI-ATRP) was used to build polymer brushes onto the electrode surface. The presence of the Poly(ferrocenyl quaternary ammonium) was evidenced by surface and electrochemical analysis. The latter exhibits high electron transfer rate and the presence of ions within the polymer framework leads to record the attached ferrocenyl redox signal in solution without adding supporting electrolyte. Finally, the wettability of the surface was modulated by electrochemical switch and by anion exchange within the polymer.     

Electrochemical Determination of NDPhA via its Electrocatalysis at Porous Au Electrode in Room Temperature Ionic Liquid

N‐Nnitrosodiphenylamine (NDPhA) is a typical kind of nonvolatile nitrosamine. Its electrochemical oxidation occurs usually at relative positive potentials (>1.1 V) even at catalytic noble metal electrodes in aqueous solutions. The formation of oxide and evolution of oxygen at such high potentials makes the analysis of NDPhA on noble metal electrodes difficult. Accordingly, its electrochemical analysis is usually performed in anhydrous organic electrolytes. In this work, room temperature ionic liquid [BMIM⁺] [BF ] acting as electrolyte was introduced in this electrochemical analysis systems. It acts as supporting electrolyte itself, has good solubility of organic compounds, and allows a wide performance potential window of noble electrode, and in turn, highly electrocatalytic noble electrode of platinum or gold can be used as working electrodes. After the investigation of the electrocatalytic behavior of NDPhA at a gold electrode in this room temperature ionic liquid electrolyte, high sensitive determination of NDPhA was designed. It is demonstrated that the electrochemical response of NDPhA is determined by a surface‐controlled process. Therefore, a sensor with high sensitivity was constructed by applying porous Au electrodes with highly electrocatalytic activity and large active surface area. The present approach on one hand broadens the application field of room temperature ionic liquids, and on the other hand provides a sensitive analytical method for environmental detection.     

Electrochemical Behavior of the Li+/Li Couple and Stability of Lithium Deposits in Tri‐1‐Butylmethylammonium Bis((Trifluoromethyl)Sulfonyl)Imide Room Temperature Ionic Liquid

The electrochemical behavior of the Li⁺/Li couple was studied at polycrystalline tungsten, platinum, copper and aluminum electrodes in tri‐1‐butylmethylammonium bis((trifluoromethyl)sulfonyl)imide ionic liquid mixed with a little propylene carbonate at 30 °C. Lithium cations were introduced into the ionic liquid by dissolution of lithium bis((trifluoromethyl)sulfonyl)imide which is highly soluble in ionic liquid. Propylene carbonate was used to reduce the viscosity of this ionic liquid in order to enhance the mass transfer and to additionally improve the stability of lithium deposits. At the tungsten and copper electrodes, the cyclic voltammetric behavior of a Li⁺/Li couple is a quasi‐reversible reaction. At the platinum electrode, the behavior becomes very complicated because of the alloy formation. Coulombic efficiency was used to evaluate the stability of lithium deposits at each electrode. The aluminum electrode showed the best efficiency due to the formation of Li‐Al alloy. However, lowest efficiency was obtained at the platinum electrode because of the low redox reversibility of the lithium in the Li‐Pt alloy. The diffusion coefficient of lithium cation in this solution was 1.0 ± 0.1 × 10^?;7 cm² s^?;1 as determined by chronopotentiometry. The best coulombic efficiency obtained at the Al electrode is 97% but dropped to about 90% after 12 hours. The self‐discharge current of the lithium deposits at the Al electrode was 0.4 μA/cm² during the experimental period.     

Redox-active electrolyte supercapacitors using electroactive ionic liquids

Here we present redox ionic liquid supercapacitors (RILSCs) which use electrolytes made from ionic liquids modified with an electroactive function to increase the energy density of activated carbon electrodes via faradaic reactions. More specifically, two different ionic liquids were made by modifying either the imidazolium cation or the bis(trifluoromethanesulfonyl)imide anion with ferrocene in order to determine the importance of the electroactive ion's polarity. The functionalization of an ionic liquid with ferrocene led to high concentrations of redox moieties in the electrolyte (2.4 M) and a large maximum operating voltage (2.5 V). An energy density of up to 13.2 Wh per kg (both electrodes) was obtained which represents an 83% increase vs. the unmodified ionic liquid. When the ionic liquid's anion is modified with ferrocene, the self-discharge at the positive electrode is fully suppressed due to the deposition of a film on the electrode. The results presented herein demonstrate that electroactive ionic liquids constitute a promising alternative to conventional solute in solvent electrolytes found in energy storage devices, and are particularly well-suited for redox-active electrolyte supercapacitors.     

The Effect of Ionic Liquid Covalent Bonding to Sol‐Gel Processed Film on Ion Accumulation and Transfer

Accumulation of electroactive anions into a silicate film with covalently bonded room temperature ionic liquid film deposited on an indium tin oxide electrode was studied and compared with an electrode modified with an unconfined room temperature ionic liquid. A thin film containing imidazolium cationic groups was obtained by sol‐gel processing of the ionic liquid precursor 1‐methyl‐3‐(3‐trimethoxysilylpropyl)imidazolium bis(trifluoromethylsulfonyl)imide together with tetramethylorthosilicate on the electrode surface. Profilometry shows that the obtained film is not smooth and its approximate thickness is above 1 μm. It is to some extent permeable for a neutral redox probe – 1,1′‐ferrocene dimethanol. However, it acts as a sponge for electroactive ions like Fe(CN)₆^3?, Fe(CN)₆^4? and IrCl₆^3?. This effect can be traced by cyclic voltammetry down to a concentration equal to 10^?7 mol dm^?3. Some accumulation of the redox active ions also occurs at the electrode modified with the ionic liquid precursor, but the voltammetric signal is significantly smaller compare with the bare electrode. The electrochemical oxidation of the redox liquid t‐butyloferrocene deposited on silicate confined ionic liquid film is followed by the expulsion of the electrogenerated cation into an aqueous solution. On the other hand, the voltammetry obtained with the electrode modified with t‐butyloferrocene solution in the ionic liquid precursor exhibits anion sensitive voltammetry. This is explained by anion insertion into the unconfined ionic liquid deposit following t‐butylferricinium cation formation.     

Modification of carbon electrode in ionic liquid through the reduction of phenyl diazonium salt. Electrochemical evidence in ionic liquid

Electrochemical reduction of the 4-nitrophenyl diazonium salt in ionic liquid media has been investigated at carbon electrode. The ionic liquid chosen for this study was 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMIM][TFSI]. The cyclic voltammetry study demonstrated the possibility of the electrochemical grafting of the nitrophenyl groups onto carbon electrode after the reduction of its corresponding diazonium in ionic liquid. The electrochemical characterization of the modified electrode achieved on ionic liquid displays the presence of the nitrophenyl group at the carbon surface. Moreover, the surface concentration of the attached group obtained in this media was found to be around 1.7 × 10⁻¹⁰ mol cm⁻², this value may suggest the possibility of the formation of monolayer. Furthermore, the characterization of the modified electrode in [EMIM][TFSI] showed the conversion of some NO₂-phenyl groups to NHOH-phenyl. This observation could indicate the presence of surface interaction between the reduced NO₂-phenyl and the ionic liquid cation, thanks to the presence of acidic proton in the ionic liquid cation.     

Electroless preparation and electrochemical behavior of a platinum-doped nickel hexacyanoferrate film–zinc modified electrode: catalytic ability of the electrode for electrooxidation of methanol

The use of a zinc substrate as an electrode and the modification of its surface by means of a thin film of platinum-doped nickel hexacyanoferrate (Pt-NiHCF) were developed. The modification conditions of the zinc surface including the electroless deposition of metallic nickel on the electrode surface from NiCl₂ solution, chemical derivatization of the deposited nickel to the NiHCF film in 0.5 M K₃[Fe(CN)₆] solution, and electrochemical penetration of metallic platinum into the modified film are described. The modified zinc electrodes prepared under optimum conditions show a well-defined redox couple due to the [Ni^IIFe^III/II(CN)₆]^1–/2– system. The effects of pH, the alkali metal cation, and the anion of the supporting electrolyte on the electrochemical characteristics of the modified electrode were studied in detail. The diffusion coefficients of hydrated alkali metal cations in the film (D), the transfer coefficient (), and the transfer rate constant for the electron (k_s) were calculated in the presence of some alkali metal cations. The electrocatalytic activity of the modified electrode for methanol oxidation was demonstrated. The stability of the modified electrode under various experimental conditions was investigated.     

Ion insertion into ionic liquid supported toluene generated by electrochemical redox reaction

Ion transfer across the toluene|water, toluene–ionic liquid mixture|water and ionic liquid|water boundary generated by electrochemical redox reaction of tert-butylferrocene (tBuFc) was studied with the glassy carbon (GC) electrode partially covered by the organic liquid deposit and immersed in the aqueous electrolyte solution. The electrooxidation of the redox probe in toluene deposit is followed by ejection of newly formed cation into the aqueous solution. The same reaction in the toluene–ionic liquid deposit promotes anion insertion. This pathway is also found at the electrode modified with ionic liquid.     

Preparation,Characterization, and Electrocatalytic Properties of Cobalt Oxide and Cobalt Hexacyanoferrate Hybrid Films

Polynuclear mixed‐valent films of cobalt oxide and cobalt hexacyanoferrate (CoOCoHCF) have been deposited on electrode surfaces from a solution of Co²⁺ and Fe(CN)₆^3? ions by repetitive potential cycling method. Simultaneous cyclic voltammetry and electrochemical quartz crystal microbalance measurements demonstrate the steady growth of modified film. The effect of type of monovalent cations as well as acidity of the supporting electrolyte on film growth and redox behavior of resulting film was investigated. In pure supporting electrolyte, electrochemical responses of modified electrode resemble with that of a surface immobilized redox couple. The hybrid film electrodes showed electrocatalytic activity toward oxidation of NADH, hydrazine and hydroxylamine. The feasibility of using our modified electrodes for analytical application was also explored.     

Electrochemistry of nanopore electrodes in low ionic strength solutions

The steady-state voltammetric behavior of truncated conical nanopore electrodes (20-200 nm orifice radii) has been investigated in low ionic strength solutions. Voltammetric currents at the nanopore electrode reflect both diffusive and migrational fluxes of the redox molecule and, thus, are strongly dependent on the charge of the redox molecule and the relative concentrations of the supporting electrolyte and redox molecule. In acetonitrile solutions, the limiting current for the oxidation of the positively charged ferrocenylmethyltrimethylammonium ion is suppressed at low supporting electrolyte concentrations, while the limiting current for the oxidation of the neutral species ferrocene is unaffected by the ionic strength. The dependence of the limiting current on the relative concentrations of the supporting electrolyte and redox molecule is accurately predicted by theory previously developed for microdisk electrodes. Anomalous values of the voltammetric half-wave potential observed at very small nanopore electrodes (<50 nm radius orifice radii) are ascribed to a boundary potential between the pore interior and bulk solution (i.e., a Donnan-type potential).     

Reference Electrode for Ionic Liquids

Reference electrodes for room temperature ionic liquid (RTIL) applications were constructed that have a known and reproducible potential versus the ferrocene/ferrocenium couple. They are based on reference electrodes of the first kind, Ag/Ag⁺ couple type, or of the second kind, based on Ag/AgCl in M⁺Cl^?. The former uses AgNO₃ salt and the latter tetrabutylammonium chloride, Bu₄NCl, dissolved in acetonitrile which are then introduced to the ionic liquid of choice for a final concentration of 0.1 M. The reference electrodes can be easily and reproducibly constructed. An ionic contact of these reference systems with the test electrolyte was made using an asbestos fiber liquid junction. The internal compartment of the reference system was filled with the same ionic liquid as used for the electrochemical experiment. The performance of these reference electrodes was tested in selected ionic liquids using the ferrocene/ferrocenium redox couple. The stability, reproducibility, and temperature behavior of the two reference systems have been characterized in the following ionic liquids: 1‐butyl‐3‐methylimidazolium tetrafluoroborate (BMIBF₄), 1‐butyl‐3‐methylimidazolium bis(trifluoromethanesulfonyl)imide (BMI(CF₃SO₂)₂N), and 1‐butyl‐3‐methylimidazolium hexafluorophosphate (BMIPF₆). It has been found that the formal potentials of the examined reference systems are stable over several days. There is a linear relationship for the temperature studied in the range from 25 to 60 °C.     

Remarkable Electrochemical Responses of Ferrocene/NaY Zeolite Composite modified Electrode Based on Hydrophobic Ionic Liquid

The ferrocene/NaY zeolite composites (Fc/NaY) are introduced on the surface of a glassy carbon electrode together with the hydrophobic ionic liquid 1‐butyl‐3‐methylimidazolium hexafluorophosphate (BMIMPF₆). The modified electrode thus constructed exhibits a pair of reversible redox peaks corresponding to ferrocene. Additionally the peak separation remains almost constant (58–75 mV) and the value of the ratio i_pa/i_pc is close to 1 for scan rates in the range from 10 to 1000 mV s^?1. The effects of the scan rate, aqueous supporting electrolytes, hydrophobic ionic liquid and the contents of ferrocene encapsulated by electrochemistry are investigated. The extrazeolite electron transfer process is discussed. Furthermore, the Fc/NaY/IL‐modified electrode shows good mediation towards oxidation of ascorbic acid, dopamine, hydroquinone, and catechol.     

On the interaction of carbon electrodes and non conventional electrolytes in high-voltage electrochemical capacitors

This study is essentially based on innovative electrolytes such as the organic salt N-methyl-N-butylpyrrolidinium tetrafluoroborate (Pyr₁₄BF₄) dissolved in propylene carbonate (PC) and the pure ionic liquid (N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr₁₄TFSI) and its solution in PC. Activated carbon cloths were used as self-standing binder-free electrodes. It is found that the presence of impurities in carbon electrodes may lead to electrolyte decomposition and electrode degradation which notably affect the electrochemical double-layer capacitor (EDLC) performance. Such processes greatly depend on the composition of both the electrode and the electrolyte, being much less significant with solvent-containing electrolytes. By raising the operation temperature to 60 °C, the EDLC performance in the ionic liquid Pyr₁₄TFSI is notably improved due to a relevant decrease in the viscosity and increase in ionic conductivity. By contrast, the presence of impurities, e.g., Zn and Al, in the electrodes remarkably reduces the electrolyte stability and a thick layer of decomposition products completely covers the carbon fibers after cycling at high temperature. The ionic liquid in solution maintains the high maximum operative voltage of the net ionic liquid whereas its viscosity and ionic conductivity are close to those of the conventional Et₄NBF₄/PC. Furthermore, the presence of propylene carbonate as solvent prevents to some extent the ionic liquid degradation.     

The influence of alkali metal cations on the rate of the Fe(CN)64−/Fe(CN)63− electrode process

The rate of the hexacyanoferrate redox system shows a first order dependence on the concentration of the cationic component of the supporting electrolyte. The catalytic influence of the alkali metal cations on the electrode process increases in the order Li⁺<Na⁺<K⁺～Cs⁺. The temperature dependence of the rate constant of the electrode process in KF and LiNO₃ has been measured and the results show that the activated complex is formed by the collision or association of a cation of the supporting electrolyte with the reactant anion, which may already be paired with one cation. It is suggested that this mechanism may be applicable to other electrode reactions involving highly charged species.     

Generator‐collector Voltammetry at Dual‐plate Gold‐gold Microtrench Electrodes as Diagnostic Tool in Ionic Liquids

Ionic liquids provide high viscosity solvent environments with interesting voltammetric characteristics and new electrochemical mechanisms. Here, a gold‐gold dual‐plate microtrench electrode is employed in generator‐collector mode to enhance viscosity‐limited currents in ionic liquids due to fast feedback within small inter‐electrode gaps (5 μm inter‐electrode gap, 27 μm microtrench depth) and to provide a mechanistic diagnosis tool. Three redox systems in the ionic liquid BMIm⁺BF₄^? are investigated: (i) ferrocene oxidation, (ii) oxygen reduction, and (iii) 2‐phenyl‐naphthyl‐1,4‐dione reduction. Both transient and steady state voltammetric responses are compared. Asymmetric diffusion processes, reaction intermediates, and solubility changes in the ionic liquid are revealed.     

Electrochemical redox reaction in a silica sol-gel glass with embedded organic electrolyte

The electrochemical redox reaction of ferrocene was studied in silica sol-gel glass with embedded organic electrolyte. This material has been prepared by mixing hydrolysed tetramethylorthosilicate sol with propylene carbonate LiClO₄ solution containing ferrocene and further gelation in a form of block or film. The electrochemical behaviour of encapsulated ferrocene was studied by cyclic voltammetry, chronoamperometry, differential pulse voltammetry and impedance spectroscopy on ultramicroelectrodes. The shape of the cyclic voltammograms corresponding to the electrooxidation of ferrocene in sol-gel block is similar to that obtained in liquid electrolyte and it does not depend on gel aging. The current substantially decreases during the first few days after gelation. Later it becomes weakly dependent on aging and the apparent diffusion coefficient of ferrocene in the gel block is about half of that in liquid electrolyte. The electrooxidation of ferrocene also occurs in film of the analogous sol-gel material cast on the surface of the electrode assembly.     

Quasi in situ XPS study of anion intercalation into HOPG from the ionic liquid [EMIM][BF4]

Highly oriented pyrolytic graphite (HOPG) electrodes were electrochemically oxidized in the ionic liquid [EMIM][BF₄]. Both, the electrolyte and the electrode surface were investigated using X-ray photoelectron spectroscopy (XPS) after electrochemical treatment. For that purpose an electrochemical preparation chamber was attached to the ultra high vacuum system allowing for preparation of electrodes in non-aqueous electrolyte and subsequent sample transfer under inert nitrogen atmosphere. The XP-spectra of all species detected on the oxidized HOPG surface show core level shifts towards lower binding energies referring to a Fermi level shift and proving that a graphite intercalation compound was formed. Anion intercalation occurs together with co-intercalation of cations at 2 V vs. carbon quasi-reference electrode and is found to be irreversible. XPS analysis of the ionic liquid prior to and after electrochemical treatment indicates a change in electrolyte composition.     

Electrochemical synthesis and the functionalization of few layer graphene in ionic liquid and redox ionic liquid

Electrochemical reductive exfoliation of graphite to few layered graphene(FLG) in presence of 1-ethyl-2,3-dimethyl imidazolium bis(trifluoromethylsulfonyl) imide ionic liquid and redox ionic liquid based ferrocene has been investigated. Thus, by applying a mild negative potential(-2.7 V vs. Fc/Fc~+) to carbon electrode in ionic liquid graphene flakes could be generated. The generated materials have been characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, high resolution transmission electron microscopy and atomic force microscopy. XPS and Raman analysis show that the electrochemical reductive exfoliation provides the formation of FLG. The thickness of the resulting FLG was found to be ranged between 4 and1 nm. HR-TEM images reveal the formation of few graphene layers and in some cases single graphene layer was observed.Moreover, this electrochemical route conduces to the formation of ionic liquid functionalized FLG. Finally, the reductive exfoliation was further investigated in the presence of redox ionic liquid. XPS and electrochemical measurements confirm the presence of ferrocene.     

Electrolyte effects on electrochemical properties of osmium complex polymer modified electrodes.

The electrolyte effects on the electrochemical behaviors of osmium complex polymer modified electrodes were investigated by a comparison between two osmium complexes, [Os(bpy)2(PVI)10Cl]Cl (Os-PVI10) and [Os(bpy)2(PVP)10Cl]Cl (Os-PVP10). The electrode process at Os-PVI10 modified electrodes is reaction-controlled, while a diffusion-controlled electrode process exists at Os-PVP10 modified electrodes. Both the cation and anion in supporting electrolytes strongly affect their electrochemical behaviors, such as the redox potential, wave shape and peak current. These phenomena are attributed to a change in the film structure and polymer swelling in various supporting electrolytes. The influence of electrolyte anions on the electrochemical behaviors is related to their hydrophobicity. The electrode process of Os-PVP10 depends on the pH value of solutions, exhibiting different electron transfer mechanisms.