Cyclic voltammetry investigation of diffusion of ferrocene within propylene carbonate organogel formed by gelator

Propylene carbonate organogel containing LiClO₄ was formed in the presence of gelator bis-(4-stearoylaminophenyl) methane (BSAPM). The electrochemical behavior and diffusion of ferrocene (Fc) and ferricenium (Fc⁺) entrapped within the organogel was investigated by cyclic voltammetry. The Fc molecules still show redox activity within the organogels in comparison with corresponding solutions of propylene carbonate containing LiClO₄. The shape of the cyclic voltammograms of the Fc electrooxidation in organogel was similar to that in corresponding solutions. The results indicated that redox reactions of Fc/Fc⁺ were a quasi-reversible process of diffusion-controlled single electron transfer in organogels. The diffusion coefficients of Fc and Fc⁺ in organogels decreased with an increase of the concentration of gelator BSAPM, but increased with an increase of temperature. The temperature dependence of the diffusion coefficient in organogels followed classical Arrhenius equation. The activation energy in organogels was found of no difference from that in corresponding solutions.     

Electrochemical studies of four organometallic redox couples as possible reference redox systems in 1-ethyl-3-methylimidazolium tetrafluoroborate

The electrochemical behavior of four organometallic redox couples has been studied in the room temperature ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate. Diffusion controlled anodic and cathodic peaks were found for the redox couples Fc/Fc⁺ (ferrocene/ferrocenium), Cc/Cc⁺ (cobaltocene/cobaltocenium) on glassy carbon, platinum and gold. Bis(biphenyl)chromium(I) tetraphenylborate (BCr/BCr⁺) yielded a well-behaved diffusion controlled pair of cathodic-anodic peaks only on glassy carbon and gold as working electrodes. The electrode reaction of decamethylferrocene was affected by adsorption of the reduced form on all of the three working electrodes employed by us. The applicability of three of the four redox couples studied as candidates for reference redox systems in this ionic liquid is discussed.     

Electrochemical studies of ferrocene in a lithium ion conducting organic carbonate electrolyte

We carried out a detailed study of the kinetics of oxidation of ferrocene (Fc) to ferrocenium ion (Fc⁺) in the non-aqueous lithium ion conducting electrolyte composed of a solution of 1 M LiPF₆ in 1:1 EC:EMC solvent mixture. This study using cyclic (CV) and rotating disk electrode (RDE) voltammetry showed that the Fc⁰/Fc⁺ redox couple is reversible in this highly concentrated electrolyte. The ferrocene and ferrocenium ion diffusion coefficients (D) were calculated from these results. In addition, the electron transfer rate constant (k⁰) and the exchange current density for the oxidation of ferrocene were determined. A comparison of the kinetic data obtained from the two electrochemical techniques appears to show that the data from the RDE experiments are more reliable because they are collected under strict mass transport control. A Tafel slope of c.a. 79 mV/decade and a transfer coefficient α of 0.3 obtained from analysis of the RDE data for ferrocene oxidation suggest that the structure of the activated complex is closer to that of the oxidized specie due to strong interactions with the carbonate solvents. The experiments reported here are relevant to the study of redox reagents for the chemical overcharge protection of Li-ion batteries.     

Electrochemistry of ferrocene in anionic,cationic and nonionic micellar solutions. Effet of the micelle solubilization of the half-wave potentials

The redox properties of the ferrocene/ferricinium Fc/Fc⁺ couple were studied in aqueous 0.1 M NaCl solution of sodium dodecylsulphate (SDS), N-cetyltrimethylammonium bromide (CTAB) and polyoxyethylene-23 lauryl ether (Brij 35). Solubilities of ferrocene were measured by spectrophotometry and by limiting current measurements in direct current voltammetry. The reversible half-wave potentials were determined by cyclic and differential pulse voltammetry. The results are discussed on the basis of the micelle solubilization equilibrium of both the reductant Fc and the oxidant Fc⁺. Indeed, diffusion coefficient ratios of the oxidant to the reductant, measured by coulometry, show that the micelle solubilization of the electrogenerated ferricinium cation Fc⁺ must also be considered in the nonionic and the anionic micellar solutions. Thus in every solution studied, a standard potential of Fc/Fc⁺ in water may be obtained from the experimental values of the half-wave potentials, the partition coefficients of Fc and Fc⁺ and the ratio of their diffusion coefficients. The behaviour of ferrocene in the SDS solutions and the effect produced by the addition of pentanol seem to confirm that SDS forms micelles with a less ordered structure than the other surfactants studied. The invariance of the half-wave potentials of ferrocene in pure SDS solutions whatever the surfactant and ferrocene concentrations suggests the use of this solute-solvent couple as a reference potential system.     

Synthesis and electrochemical properties of redox active polyurethanes with ferrocene units in polyether soft segments

Electrochemical active segmented polyurethane with ferrocene units in polyether soft segments (PU‐S‐Fc) has been originally synthesized and identified by ¹H‐NMR spectra. Electrochemical behaviors of PU‐S‐Fc blending with lithium perchlorates were investigated by cyclic voltammetry. In N,N′‐dimethyl formide solution, PU‐S‐Fc exhibited normal cathodic and anodic peaks of the ferrocene/ferricinium (Fc/Fc⁺) couple. Compared with that of ferrocene molecules blended in ordinary polyurethane (PU‐B‐Fc), redox peaks of ferrocene units in PU‐S‐Fc were found to be broader, which may be ascribed to the weak adsorption of the polyurethane on the electrode surface. The influence of lithium perchlorate concentration on peak potentials indicated that supporting electrolytes played an important role in electrochemical redox of PU‐S‐Fc. In the solid state, PU‐S‐Fc/Li⁺ showed discernible redox peaks at temperatures higher than 60°C, and an exponential increase curve of electrochemical response with an increase of temperature was found. Temperature variations of the solid‐state ionic conductivity for PU‐S‐Fc/Li⁺ can be interpreted by the Arrhenius equation. The similarity between the temperature dependence of ionic conductivity and electrochemical response revealed that transport mechanism of ionic and redox species in the polyurethane matrix was controlled by the mobility of polyether chains. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2674–2680, 1999     

Electrochemical reduction of N-(1-butyl)pyridinium cation in 1-methyl-3-ethylimidazolium chloride—aluminium chloride ambient temperature ionic liquids

The reduction of N-(1-butyl)pyridinium cation (BuPy⁺) has been studied in the ambient temperature ionic liquid, 1-methyl-3-ethylimidazolium chloride (ImCl)aluminium chloride. A one-electron wave with convective-diffusion controlled limiting current was observed and the diffusion coefficients of BuPy⁺ were measured in neutral and basic (MC = 0.6) ImClAlCl₃ melts. The Dη/T product was found to be almost independent of melt composition. E_1/2 and peak potentials measured in neutral and basic melts were practically independent of melt composition; however a small but significant shift to less negative potentials was observed in the absence of chloride ions (ie in pure ImAlCl₄). This effect was attributed to ion pair formation (BuPy⁺Cl^?). Coulometry indicated an “n” value of 1 for the reduction of the BuPy⁺ cation. About 15% of the primary product was transformed into the electroactive viologens.     

Non-additive voltammetric currents from multicomponent systems of redox-active substances

Mixtures of two redox-active compounds with dissimilar diffusion coefficients produce non-additive mass-transfer limited currents. Similarly, in the potential range where three redox-active species, decamethylferrocene (dMeFc), ferrocene (Fc) and N-methylphenothiazine (MePTZ), are oxidized simultaneously with rates controlled by linear diffusion, electrogenerated radicals diffusing outwards from the electrode react with original species diffusing towards the electrode from the bulk; thus, Fc⁺ reacts with dMeFc producing Fc and dMeFc⁺, while MePTZ⁺ reacts with both Fc and dMeFc producing MePTZ together with Fc⁺ and dMeFc⁺. These processes replace the flux of dMeFc with Fc at the second current plateau (referring to normal pulse voltammetry), and the fluxes of both dMeFc and Fc with MePTZ at the third plateau. Analogous results have been obtained and analyzed with two other multicomponent systems undergoing multiple sequential electron transfers, namely dMeFc/Fc/TPTA and dMeFc/TTF (TPTA: tri-N-p-tolylamine; TTF: tetrathiafulvalene). Since the diffusion coefficients of the three species are different, the mass-transfer limited currents of the second and third oxidation waves are not equal to the sum of the currents that each component would have produced if it were in the solution alone. Numerical simulations of the experimental voltammograms using diffusion coefficients measured independently support this mechanism. Multicomponent systems are encountered frequently in practice and our results identify one significant (∼10%) source of error in quantitative voltammetric analysis. Ways around the problem are summarized in the conclusions section.     

Electrochemistry of ionophore-coordinated Cs and Sr ions in the tri-1-butylmethylammonium bis((trifluoromethyl)sulfonyl)imide ionic liquid

The selective extraction of Cs⁺ and Sr²⁺ from aqueous solutions by using the ionophores calix[4]arene-bis(tert-octylbenzo-crown-6) (BOBCalixC6) and dicyclohexano-18-crown-6 (DCH18C6), respectively, was demonstrated in the hydrophobic, room-temperature ionic liquid (RTIL), tri-1-butylmethylammonium bis((trifluoromethyl)sulfonyl)imide (Tf₂N⁻). The electrochemistry of Cs⁺ coordinated by BOBCalixC6 and Sr²⁺ coordinated by DCH18C6 was examined at a mercury film electrode (MFE) in this ionic liquid by using cyclic staircase voltammetry, sampled current voltammetry at a rotating electrode, and chronoamperometry. Both BOBCalixC6·2Cs⁺ and DCH18C6·Sr²⁺ exhibit well-defined reduction waves at approximately −2.4 and −2.9 V versus the ferrocene/ferrocenium (Fc/Fc⁺) couple, respectively, in which the coordinated ions are reduced to their respective amalgams, permitting the recycling of the ionophores. The diffusion coefficients of BOBCalixC6·2Cs⁺ and DCH18C6·Sr²⁺ are (2.7 ± 0.1) × 10⁻⁹ and (2.1 ± 0.1) × 10⁻⁹ cm² s⁻¹, respectively, at 30 °C. The coulometric efficiency for the reduction and stripping of Cs at mercury pool electrodes was about 90% and was independent of the deposition time, whereas the efficiency for Sr was slightly less than 90% at short times and decreased with the deposition time, probably due to the formation of a passive layer of Sr(Tf₂N)₂.     

Electrochemical studies and self diffusion coefficients in cyclic ammonium based ionic liquids with allyl substituents

Several cyclic ammonium-based ionic liquids (ILs) with allyl substituent are synthesized, these allyl substituent ILs have high ionic conductivity (up to 4.72 mS cm⁻¹ at 30 °C) and wide electrochemical window of 5 V. The electrochemical behaviors of two organometallic redox couples Fc/Fc⁺ (ferrocene/ferrocenium) and Cc/Cc⁺ (cobaltocene/cobaltocenium) have been studied in these ILs, the calculated Stokes–Einstein product (Dη T⁻¹) of ferrocene in ILs is larger than that of cobaltocenium in ILs. The self-diffusion coefficients of cation and anion in these ILs are studied using pulsed gradient spin-echo NMR technique. There are very few reports where electrochemically derived diffusion coefficients and self diffusion coefficients are available for comparison, so a new key concept in electrochemistry could be developed in this paper.     

The electrochemical redox processes in PMMA gel electrolytes—behaviour of transition metal complexes

Electrochemical properties of polymer gel electrolytes based on polymethylmethacrylate (PMMA) were studied by cyclic voltammetry and impedance spectroscopy using new solid-state PMMA-Cd-Cd²⁺ reference electrode. The suitable potential window of the PC-PMMA system was estimated from -0.2 to + 1.5 V versus Cd-Cd²⁺. New polymer gels containing ferrocene-ferricinium (Fc-Fc⁺) couple and other transition metal complexes were prepared by the direct polymerisation of methylmethacrylate (MMA) monomer and the solution of metal complex and supporting electrolyte in anhydrous aprotic solvent—propylene carbonate (PC). The half-wave potentials and apparent diffusion coefficients of used complexes and their dependence on the composition of the system (liquid or gel) were estimated. Time dependent electrochemical measurements showed almost three order decrease of the diffusion coefficients of ferrocene (Fc) and ferricinium (Fc⁺) cation from 6 × 10⁻⁵ to 2 × 10⁻⁹ cm² s⁻¹ during the polymerisation from the liquid to the polymer state. The results show that the PC-PMMA gel electrolyte can be described as a system of embedded solvent in the polymer network of PMMA without present monomer.     

Electrochemical behavior of europium (III) in N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide

N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPyNTf₂) was synthesized and characterized by CHNS elemental analysis, ¹H and ¹³C NMR and IR spectroscopy. Europium tris[bis(trifluoromethylsulfonyl)imide] (Eu(NTf₂)₃) was prepared and studied for the electrochemical behavior of Eu(III) in BMPyNTf₂ at glassy carbon and stainless steel working electrodes at 298-373 K by cyclic voltammetry, chronopotentiometry and chronoamperometry. Cyclic voltammogram of Eu(III) in BMPyNTf₂ consisted of a quasi-reversible cathodic wave at −0.45 V (vs. Fc/Fc⁺, 373 K), which could be attributed to the reduction of Eu(III) to Eu(II) and an irreversible wave at −2.79 V (vs. Fc/Fc⁺) due to reduction of Eu(II) to Eu(0). The diffusion coefficient of Eu(III) in BMPyNTf₂ was determined to be in the range of ∼10⁻⁷ cm² s⁻¹ by various electrochemical methods and the charge transfer rate constant was determined to be ∼10⁻⁵ cm s⁻¹ by cyclic voltammetry.     

Mass transport and charge transfer rates for Co/Co redox couple in a thin-layer cell

Recently, the complex Co(dtb)₃ⁿ⁺ (dtb = 4,4 di tert-butyl-2,2′ bipyridine) in methoxypropionitrile (MPN) solvent has been proposed as an alternative redox mediator in the thin-layer dye sensitized solar cells. The electrochemical properties of this new mediator as a function of temperature were investigated by mean of symmetric golden electrodes thin-layer cell, using three electro-analytical techniques: electrochemical impedance spectroscopy (EIS), slow scan cyclic voltammetry (SCCV) and chronoamperometry (CA). Our study pointed out that, at room temperature, both the electron transfer rate k° = 1.24 10⁻⁴ cm s⁻¹ as well as the diffusion coefficient D = 5.85 × 10⁻⁷ cm s⁻¹ are rather low. Raising the temperature has a beneficial effect, increasing more than 6 times the standard rate constant of electron transfer and more than 3 times the ionic diffusion coefficient at 80 °C. However, for all the studied temperatures, the slow mass transport of Co^(III)/Co^(II) species still remains the rate determining step. Viscosity measurements have demonstrated that the ionic mass transport in MPN follows the Stokes’ law and the Walden product is constant, in the temperature range investigated.     

Ion Exchange Kinetics of Heavy Metal Ions on Organic–Inorganic Composite Cation Exchanger Poly-o-toluidine Zr(IV) Tungstate

To study the ion exchange kinetics of heavy metal ions on the organic–inorganic composite cation exchanger poly-o-toluidine Zr(IV) tungstate, Nernst–Planck was computer simulated. Simulated numerical results for counter ions (Cu, Zn, Cd and Pb) of equal valence and four different ionic mobilities are presented to understand the ionic diffusion process. These results are based on the fractional attainment of equilibrium U(τ), of the counter ions under study. The forward (M²⁺–H⁺) and reverse (H⁺–M²⁺) ion exchange processes are justified as the particle diffusion phenomenon. The self-diffusion coefficient (D _o ), energy of activation (E _a ), and entropy of activation (?S*) have also been estimated to understand the ion exchange process occurring over the surface of this cation exchanger and indicated that the ion exchange process is feasible and spontaneous. It is concluded that the difference in activation energies and entropy of activation may facilitate the separation of metal ions. The regeneration capability of this cation exchanger was also explained.     

One-electron redox reaction of di-tert-butyl nitroxide at platinum electrode in acetonitrile

The electrochemical oxidation of di-tert-butyl nitroxide (DTBN) at a platinum electrode in acetonitrile was examined. The cyclic voltammogram indicated an irreversible response during a normal time scale measurement, whereas chemically reversible voltammograms were obtained during a shorter time using a micro disk electrode with relatively fast sweep rates. The apparent formal redox potential and heterogeneous electron transfer rate constant were estimated to be 0.218 V (versus Fc⁺|Fc) and 0.035 ± 0.015 cm s⁻¹ from the digital simulation analysis.     

Diffusion regimes at nanoelectrode ensembles in different ionic liquids

The electrochemical and diffusion behaviour of different redox probes in different ionic liquids is studied at gold nanoelectrode ensembles (NEEs) in comparison with millimetre sized gold (Au-macro) and glassy carbon (GC) disk electrodes. The redox probes are neutral ferrocene (Fc), the ferrocenylmethyltrimetylammonium cation (FA⁺) and the ferrocenylmonocarboxylate anion (FcCOO⁻). The ILs are the dicyanamide, [N(CN)₂] or bis(trifluoromethylsulfonyl)amide), [N(Tf)₂] salts of the following cations: 1-butyl-3-methylimidazolium, [BMIm], 1-butyl-3-methylpyrrolidonium, [BMPy], or tris(n-hexyl)tetradecylphosphonium [P_14,666]. These ILs are characterized by different viscosities, ranging from 32 to 277 cP. The cyclic voltammetric behaviour of the redox probes is reversible and diffusion controlled at GC electrodes. Diffusion coefficients (D) calculated by the Randles-Sevcik equation scales inversely with the IL viscosity, ranging from 2 × 10⁻⁸ to 3 × 10⁻⁷ cm² s⁻¹. Ionic solutes, namely FA⁺ and FcCOO⁻, present slightly lower D values than neutral Fc. At the Au-macro the electrochemical behaviour of the redox probes is diffusion controlled in the ILs containing the [N(Tf)₂] anion, while it involves relevant adsorption processes in the [N(CN)₂] containing electrolyte. For this reason the diffusion at gold NEEs is studied only in the former ILs.The CVs of the redox probes at the NEEs are peak shaped at low scan rate (v), while they are sigmoidally shaped at high v, but with some shift between forward and backward patterns. This is indicative of the occurrence of a total overlap (TO) diffusion condition when v is low which becomes a mixed diffusion layers (MDL) regime, with only a partial overlapping of individual diffusion layers, at high v values. In the most viscous IL, namely [P_14,666] [N(Tf)₂], at v higher than 0.8 V s⁻¹, a plateau current independent on the scan rate is achieved, indicating the tendency to reach the pure radial regime in this IL. The v values at which the transition between TO and MDL is observed scales directly with D and inversely with the IL viscosity. This behaviour is interpreted on the basis of the dependence of individual diffusion layers at each nanoelectrode on redox probe/IL interaction which fits with existing theoretical models very recently developed for nanoelectrode arrays.     

Cation transfer across a hydrogel/organic phase: Effect of cation size, hydrophobicity and acid-base properties

The transfers of tetraethylammonium (TEA⁺) and protonated triflupromazine (HTFP⁺) through a hydrogel/liquid interface (g/o) and a liquid/liquid interface (w/o) were compared using cyclic voltammetry. After the two phases were put in contact, the behavior of each molecule was analyzed at different pH values and at different time points. The gel induces hydrophobic and electrostatic interactions with TEA⁺ and HTFP⁺, shifting the peak potentials to more positive values. The diffusion coefficients, D, in both phases (g and w) at different pH values were calculated. In the case of TEA⁺, the D value remains constant in both systems. However, the D value of HTFP⁺ is lower in the gel phase than in the liquid phase.HTFP⁺ is transferred from the aqueous phase to the organic phase via a direct mechanism that involves coupled acid-base and partition processes. At the g/o interface, the coupled chemical reactions of HTFP⁺ were inhibited by the drug/gel interaction. The results demonstrate that the g/o system could be used as a model to study the controlled release of charged drugs.     

Impedance study of protecting film formation on lithium and lithiated graphite induced by bis(fluorosulfonyl) imide anion

The process of Li⁺ reduction from room temperature ionic liquids consisting of N-methyl-N-propylpyrrolidinium cation (MPPyr⁺) and bis(fluorosulfonyl) imide (FSI⁻) or bis(trifluoromethanesulfonyl) imide (TFSI⁻) anions was studied with the use of impedance spectroscopy. Reduction was carried out on both metallic lithium (Li) and graphite (G) electrodes. It has been found that the FSI anion in high amounts is able to form a protective film on both graphite and metallic lithium. The Li⁺/Li couple should rather be represented by a Li⁺/SEI/Li system. The SEI structure depends on the manner of its formation (chemical or electrochemical) and is not stable with time. The rate constant for the Li⁺ + e⁻ → Li process at the Li/SEI/Li⁺ (in MPPyrFSI) interface is k_o = 4.2 × 10⁻⁵ cm/s. In the case of carbon electrodes (G/SEI/Li⁺ interface), lithium diffusion in solid graphite is the rate determining step, reducing current by ca. two orders of magnitude, from ca. 10⁻⁴ A/cm², characteristic of the Li/SEI/Li⁺ electrode, to ca. 10⁻⁶ A/cm².     

Physical and electrochemical properties of 1-(2-hydroxyethyl)-3-methyl imidazolium and N-(2-hydroxyethyl)-N-methyl morpholinium ionic liquids

Various ionic liquids (ILs) were prepared via metathesis reaction from two kinds of 1-(2-hydroxyethyl)-3-methyl imidazolium ([HEMIm]⁺) and N-(2-hydroxyethyl)-N-methyl morphorinium ([HEMMor]⁺) cations and three kinds of tetrafluoroborate ([BF₄]⁻), bis(trifluoromethanesulfonyl)imide ([TFSI]⁻) and hexafluorophosphate ([PF₆]⁻) anions. All the [HEMIm]⁺ derivatives were in a liquid state at room temperature. In particular, [HEMIm][BF₄] and [HEMIm][TFSI] showed no possible melting point from −150 °C to 200 °C by DSC analysis, and their high thermal stability until 380-400 °C was verified by TGA analysis. Also, their stable electrochemical property (electrochemical window of more than 6.0 V) and high ionic conductivity (0.002-0.004 S cm⁻¹) further confirm that the suggested ILs are potential electrolytes for use in electrochemical devices. Simultaneously, the [HEMMor]⁺ derivatives have practical value in electrolyte applications because of their easy synthesis procedures, cheap morpholinium cation sources and possibilities of high Li⁺ mobility by oxygen group in the morpholinium cation. However, [HEMMor]⁺ derivatives showing high viscosity usually had lower ionic conductivities than [HEMIm]⁺ derivatives.     

Experimental study of swelling and shrinking kinetics of spherical poly(N,N-diethylacrylamide) gel with continuous phase transition

The kinetics of swelling and shrinking of spherical particles of poly(N,N-diethylacrylamide) (DEAA) gel, prepared with static mixing technology, was investigated experimentally. The shrinking process was separated into late shrinking and initial shrinking by the skin layer effect. When the final temperature of temperature swing experiments was above the lower critical solution temperature (LCST) of polyDEAA, the apparent increase in the response rate constant (k_v) and the polymer diffusion constant (D) were due to reduction of the friction coefficient between polymer chains and water, arising from decrease in the water viscosity. When the final temperature was below the LCST, the main factor determining k_v and D was the change in the network structure of DEAA particles caused by the final temperature. The initial temperature affected the values of k_v and apparent diffusion constant D_app only in initial shrinking, in which the decrease in D with increase in the initial temperature came mainly from the spatial limitation.