Mechanism Of Electrode Process Research Articles

Peculiarities of Partial and Joint Electroreduction of Carbon and Tungsten Oxyanions in Chloride Melt

High-temperature electrochemical synthesis (HTES) in molten salts allows one to produce a wide range of interesting in the application terms chemicals: single-phase carbon nanomaterials of various structures and morphologies, as well as carbides, borides, and silicides of refractory metals [1]. A necessary condition for the successful implementation of the synthesis is a detailed study of the electrochemical behavior of the synthesis precursors - compounds of carbon and refractory metal.This work presents the results of studies on the mechanism of carbon and tungsten electrodeposition reactions from their oxide compounds (СО2; Li2СО3; Na2WO4: Na2W2O7; Na3WO3F3) in their partial and joint presence in a chloride melt. The possibility of controlling the mechanisms of electrode processes by changing the acid-base properties of melts is shown.The research algorithm was as follows. At the first stage, a thermodynamic analysis of possible pathways for the electroreduction of precursors and the choice of optimal systems and conditions for the synthesis were carried out. The next step was a voltammetric study of the electrochemical behavior (partial and joint) of synthesis precursors and the establishment of the electrode processes mechanism. After analysis of the obtained results, electrolyzes were performed in potentio- and galvanostatic modes to obtain phases of carbon, tungsten (partial reduction of precursors) and tungsten carbides (joint reduction of precursors) of various compositions. Finally, the optimal synthesis conditions were determined.Based on the analysis of thermodynamic calculations and published data, we have chosen CO2 and Li2CO3 compounds as carbon precursors, and Na2WO4, Na2W2O7, Na3WO3F3 as tungsten precursors. An equimolar mixture of NaCl and KCl was chosen as the supporting electrolyte, since the cathodic decomposition potentials of NaCl and KCl are more negative than the potentials of carbon and tungsten evolution. In addition, these chlorides are cheap and highly water-soluble salts that simplify the washing of the final product. The optimum temperature is 750 °C.For the synthesis of tungsten carbides, the following 7 compositions of electrolytic baths were chosen: (1) Na,K|Cl – Na2WO4 – CO2; (2) Na,K|Cl – Na2WO4 – NaPO3 - CO2; (3) Na,K|Cl – Na2WO4 – MgCl2 - CO2; (4) Na,K|Cl – Na2W2O7 – CO2; (5) Na,K|Cl – Na2W2O7 – Li2CO3; (6) Na,K|Cl – Na2W2O7 – Li2CO3 – CO2; (7) Na,K|Cl – Na3WO3F3 – CO2.The partial electrochemical reduction of the synthesis precursors (СО2; Li2CO3; Na2WO4; Na2WO4 – xMgCl2; Na2WO4 – xNaPO3; Na2W2O7; Na3WO3F3) in a chloride melt NaCl-KCl (1:1) was studied by cyclic voltammetry at different depolarizer concentrations, potential scan rates (0.02-10 Vs-1), reverse potentials and gaseous media in the cell.Based on the cycle voltammogram diagnostics and analysis of the products obtained by the potentiostatic electrolysis mode, the mechanisms of electrode reactions are proposed and the areas of potentials and current densities of the carbon and tungsten deposition are determined. The data obtained made it possible to purposefully select the concentrations of precursors for the electrochemical synthesis of tungsten carbides.A voltammetric study of the joint deposition of tungsten and carbon was carried out. The synthesis of nano-sized powders of carbon, tungsten, tungsten carbides of various compositions and structures (C; W; W|W2C|; W|W2C|WC|; W2C|WC; WC|C) has been implemented. The properties of electrolytic products have been studied by XRD, SEM, BET, and Raman spectroscopy. The influence of the synthesis parameters and conditions on their phase composition, morphological and structural features has been established.The electroreduction of CO2 and Li2CO3 to carbon in molten chlorides can be taken as the basis of high-temperature electrochemical synthesis of various carbonaceous nano-scaled inorganic materials: carbon films and powders of different structures and morphology (carbon nanofibres; nanotubes, graphene oxide, amorphous carbon). Joint electroreduction of CO2 and Li2CO3 with various tungsten oxyanions makes it possible to obtain a whole range of composite electrolytic materials based on tungsten carbides. The morphology and properties of the obtained products can be changed cardinally depending on the nature of the chosen precursors and synthesis conditions. Tungsten carbides have an average particle size of ~ 10 – 30 nm and consist of hollow mesoporous spherical structures with a specific surface area of ~140 m2g-1.The electroreduction of carbon and tungsten anions in molten salts presented in this work providing a promising route for the synthesis of various carbonaceous materials with great application opportunity. It was shown that the use of obtained electrolytic nano-sized powder composites WC|C as cathode material for the hydrogen evolution in sulfuric acid solutions is to be promising.[1] Shapoval VI, Malyshev VV, Novoselova IA, Kushkhov KhB (1995) Modern problems of HTES of transition metal compounds. Russ Chem Rev 64:125–132.

Elimination voltammetry as an innovative tool of electroanalysis

• Overview of principles and applications of elimination voltammetry with linear scan (EVLS). • EVLS as a promising tool for obtaining new information about electrochemical processes. • A critical look on the use of EVLS for LSV or CV experiments. • Perspectives and possibilities of EVLS for description of different electrode process mechanisms (CE, CEE). The review presents the application potential of elimination voltammetry with linear scan (EVLS) in the electroanalysis of organic and inorganic electroactive substances. This innovative method can be considered a software tool that transforms the total current-voltage records measured at different scan rates into elimination functions providing new useful information about a given electrode process. Some EVLS functions are capable of separating overlapped voltammetric signals, identifying the adsorbed state of the analyte, and detecting the preceding chemical reaction before the electron transfer. EVLS finds use in solving the mechanisms of electrode processes and determining significant physicochemical parameters, such as charge transfer coefficients and electrochemical equilibrium protonation constants. This review selects and draws attention to certain problems that may accompany EVLS in voltammetric experiments on solid electrodes.

Unraveling the Mechanism of Water-Mediated Sulfur Tolerance via Operando Surface-Enhanced Raman Spectroscopy.

While several proton-conducting anode materials have shown excellent tolerance to sulfur poisoning, the mechanism is still unclear due largely to the inability to probe miniscule amounts of sulfur-containing species using conventional surface characterization techniques. Here we present our findings in unraveling the mechanism of water-mediated sulfur tolerance of a proton conductor under operating conditions empowered by surface-sensitive, operando surface-enhanced Raman spectroscopy (SERS) coupled with impedance spectroscopy. Contrary to the conventional view that surface-adsorbed sulfur is removed mainly by oxygen anions, it is found that -SO4 groups on the surface of the proton conductor are converted to SO2 by a water-mediated process, as confirmed by operando SERS analysis and density functional theory (DFT)-based calculations. The combination of operando SERS performed on a model electrode and theoretical computation offers an effective approach to investigate into complex mechanisms of electrode processes in various electrochemical systems, providing information vital to achieve the rational design of better electrode materials.

VOLTAMPEROMETRY OF Co–Mo ALLOY DEPOSITION

At present, cobalt–molybdenum alloys are among the most functional galvanic coatings characterized by the following properties: magnetic, chemical resistance, catalytic activity, high wear resistance and corrosion resistance, including in aggressive environments. This paper is devoted to the study of the production of cobalt-molybdenum coatings from simple and complex electrolytes. The effect of ligands, such as trilon B and ammonium sulfate, on the alloy deposition process has been investigated in this paper. The kinetics of the process of electrodeposition of a coating from a polyligand ammonium–trilonate electrolyte on a copper electrode was studied using methods of linear voltammetry in a potentially dynamical regime with variation of the scanning speeds of a potential of 1–100 mV / s. The kinetic regularities of the restoration of cobalt ions in the following systems are studied: – cobalt sulfate; sulphate – cobalt – trilol B; – cobalt – ammonium sulfate and – cobalt – trilol B – ammonium sulfate with different concentrations. The nature of cathode peaks and the mechanism of electrode processes are determined by the analysis of volt-ampere characteristic dependences, and the Semerano criterion is calculated. Based on the Rendls–Shyevchik equation for the irreversible process, the coefficient of diffusion of the electrically active compound was determined and the rate constant of the charge transfer stage was calculated on the basis of J. Hohstein’s equation. Moreover, the product of the transfer coefficient for the number of electrons of the irreversible stage is calculated by the angular coefficient. The mechanism and kinetic equations of alloy deposition from a polyligand electrolyte in general and in separate stages are proposed. The electrochemical behavior of the – sodium molybdate system at various pH values in an acidic environment.

Electrochemical Behavior of Sulfur in Aqueous Alkaline Solutions

The kinetics and mechanism of the electrode oxidation-reduction of sulfur on an electrically conductive sulfur–graphite electrode in an alkaline solution was studied by the potentiodynamic method. To examine the mechanism of electrode processes occurring during AC polarization on a sulfur–graphite electrode, the cyclic polarization in both directions and anodic polarization curves were recorded. The kinetic parameters: charge transfer coefficients (α), diffusion coefficients (D), heterogeneous rate constants of electrode process (ks), and effective activation energies of the process (Еа) were calculated from the results of polarization measurements. An analysis of the results and calculated kinetic parameters of electrode processes showed that discharge ionization of sulfur in alkaline solutions occurs as a sequence of two stages and is a quasireversible process.

Reductive and oxidative electrochemical study and spectroscopic properties of nickel(II) complexes with N2O2 Schiff bases derived from (±)-trans-N,N′-bis(salicylidene)-1,2-cyclohexanediamine

Cyclic voltammetry, chronopotentiometry, chronocoulometry, controlled potential electrolysis and UV VIS were used in investigation of ligands and Ni(II) complexes with (±)-trans-N,N′-bis(salicylidene)-1,2-cyclohexanediamine substituted in orto- and/or para- positions of phenolate anions with tert-butyl and/or methoxy groups. Irreversible reduction and oxidation of ligands were shown. No influence of solvent on reduction of complexes, which results in Ni(I) complexes, was observed, however, an effect of solvent and substituents of phenolate anions on oxidation processes was proved. Anodic process in (CH3)2SO always results in appropriate Ni(III)-phenolate complex. Oxidation of orto- and para- substituted complexes in CH2Cl2 results in Ni(II)-mono-phenoxyl radicals, which undergo delocalisation and subsequently are oxidised to Ni(II)-bis-phenoxyl radicals. Last oxidation step of radicals produce long-lived Ni(II)-bis-phenoxonium cations. Complexes without substituents in para-and/or orto- position having formed Ni(II)-mono-phenoxyl radicals undergo dimerization. Mechanisms of electrode processes were studied and D and Ef values for processes of reduction and Ist step of oxidation of complexes were calculated.

β-cyclodextrin non-covalently modified ionic liquid-based carbon paste electrode as a novel voltammetric sensor for specific detection of bisphenol A

A highly effective approach was developed for specific detection of bisphenol A (BPA) in real samples, based on a β-cyclodextrin (β-CD) modified ionic liquid-based carbon paste electrode (β-CD/ILCPE). Voltammetry and electrochemical impedance spectroscopy measurements reveal significantly improvements in the conductivity and compatibility of the sensor, when using an ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) as a super conductive binder in the composition of carbon paste. The introduction of β-CD as a modifier as well as a host molecule exhibited obviously enhanced sensitivity and selectivity toward BPA over a wide concentration range of 1.0×10−7 to 1.1×10−5molL−1, with a detection limit (S/N=3) of 8.3×10−8molL−1. Compared to classic carbon paste electrodes, the BPA sensor not only possesses the inherent reusability and cost effectiveness, but also shows better sensing performance and stability. This can be attributed to the combination of the high ion conductivity and non-covalently interaction properties of BMIMBF4, and the supramolecular recognition and enrichment capability of β-CD. The mechanism of electrode processes was also presented. The proposed sensor was successfully used in the determination of BPA in real samples with good recovery.

Electrode processes occurring during the electrodeposition of chromium-carbon coatings from solutions of Cr(III) salts with carbamide and formic acid additions

The regularities and possible mechanisms of electrode processes occurring during the electrodeposition of the Cr-C alloy from an electrolyte based on Cr(III) sulfate with carbamide and formic acid as organic components were analyzed. The chromium electrodeposition was found to impose its kinetics on carbon inclusion in the alloy. The mechanism of carbon incorporation in the deposit was assumed to be the chemical interaction of the adsorbed organic particles with highly active chromium adatoms. During the stage discharge of Cr(III) ions to the metal, the rate of the limiting stage Cr(II) + 2e → Cr(0) increased when OH− or F− ions were included in the structure of the electrochemically active complex.

Numerical simulation and experiment in the systems of labile complexes of metals. Silver reduction from the thiourea electrolyte

Voltammograms of silver reduction from complex thiourea electrolytes with the ligand to metal concentration ratio varied from 1 to 50 were measured on the freshly prepared surface of silver electrode. The results were analyzed using the numerical model of this electrode process which takes into account homogeneous chemical reactions proceeding in solution, diffusion–migration transfer of the species present in solution, and electrode reactions participated by electroactive components of the electrolyte. We estimated the values of standard exchange current density for partial electrode reactions in which silver complexes with different number of ligands can take part. The highest electrochemical activity is exhibited in solutions with the low concentration of complex-forming agent – by silver aqua complexes, while in solutions with high thiourea concentration – by silver complexes with one ligand. It was found that kinetic hindrances to discharge of the thiourea silver complexes unlikely to be due to the adsorption of thiourea molecules blocking the growth sites or to the adsorption of complex silver ions. The results admit interpretation of the mechanism of electrode process either from the viewpoint of simultaneous discharge of all the complex silver ions present or from the viewpoint of discharge of silver complex with one ligand in combination with rapid chemical reactions of dissociation of other, non-discharging silver complexes.

Mechanism of electrochemical reactions of polyaniline films formed under the conditions of cathodic oxygen reduction

Polyaniline films (further, CPANI) were obtained under the conditions of oxygen cathodic reduction in the aniline-containing solution on the electrodes of mixed indium tin oxide (ITO), graphite, and gold. CPANI films are characterized by redox processes in the potential ranges of 0.1–0.2 V and ∼0.4 V (SCE). These processes are caused by the polymer chain fragments of different structure and the ratio between the peaks corresponding to these processes varies significantly as dependent on the synthesis conditions (electrode material, stirring, etc). The mechanism of electrode processes on CPANI is studied using the methods of cyclic voltammetry and quartz microgravimetry. It is found that only hydrogen cations and supporting electrolyte anions participate in the electrode process at the potentials of 0.1–0.2 V. The mechanism of redox processes on the obtained polymer films is discussed.

Synthesis of Glycidyl Ether of Poly(bisphenol-A 1,1′-ferrocene dicarboxylate) and Its Electrochemical Behavior

A novel ferrocene-based epoxy compound of the glycidyl ether of poly(bisphenol-A 1,1′-ferrocene dicarboxylate) (GEPBFC) was synthesized and the electrochemical behavior of its films and solutions was investigated by cyclic voltammetry (CV). It was found that the GEPBFC films and solutions present different CV behavior in different solvents and the swelling ability of the solvent is the main factor that affects the CV behavior of GEPBFC films. The kinetic parameters of the film-electrode process, including the surface transfer coefficient, the apparent diffusion coefficient and the standard rate constant in LiClO4 ethanol solution, were calculated, and the corresponding electrode-process mechanism was discussed.

Electrochemical behavior of 5-substituted 2-alkylidene-4-oxothiazolidine derivatives studied by cyclic voltammetry

This study demonstrates the electrochemical activity of selected 2-alkylidene-4-oxothiazolidine compounds containing different substituents at the C5-position on the Pt-electrode in 0.1 M TBAHFP + MeCN in the potential range from −2 V to 2 V. The mechanism of electrode processes is discussed. It is shown that a chemical step is involved in irreversible electrochemical processes. The effect of the substituent at the C5-position on reduction and oxidation potentials is also noted. The regions of a linear dependence of the current peak on the concentration of the compounds examined are determined.

A comprehensive study on the polarographic behaviour of α-arylhydrazono-3-ketimino-nitriles

The polarographic behaviour of some α-arylhydrazono-3-ketimino-nitriles (Ia—h) in 50% ethanolic aqueous buffer is reported and discussed. The mechanism of electrode processes corresponding to the observed cathodic waves is proposed and confirmed by the aid of model compounds as well as through identification of controlled potential electrolysis products. The E1/2 and pKa values of the title compounds were evaluated and correlated to Hammett constants. The physical characteristics of the studied compounds in DMF regarding their redox potential (Δ1/2); electron affinity (EA); ionization potential (IP); electronic transition energy (ECT) and coulomb repulsion integral (J12) were computed. A linear correlation was shown to exist between Δ1/2 and ECT.

Cathodic reduction of coenzyme Q 10 on glassy carbon electrode in acetic acid–acetonitrile solutions

The electrochemical reduction of coenzyme CoQ 10 and CoQ 0 on glassy carbon (GC) has been investigated in mixed solvent containing 80 vol.% acetic acid and 20 vol.% acetonitrile. A combination of cyclic voltammetry (CV) and rotating disk electrode technique (RDE) was employed to elucidate the mechanism of electrode processes. The results obtained were interpreted in terms of an E r E q mechanism involving the inverted ordering of formal potentials, i.e. E 2 0′ > E 1 0′. The cathodic processes of both compounds consist of two successive one-electron one-proton steps, whereas the second electron transfer is thermodynamically more facile than the first. The processes occur with the generation of unstable semiquinone radicals as primary products. The results presented can help in explanation of the biochemical properties of CoQ 10 in the living cell.

Electrochemical behavior on poly(ferrocenyldimethylsilane)-b-poly(benzyl ether) linear-dendritic organometallic polymer films

The linear-dendritic polymer poly(ferrocenyldimethylsilane)-b-poly(benzyl ether) (PFS-PBE) films were prepared and their electrochemical behavior in LiClO 4 aqueous electrolyte solutions was investigated with cyclic voltammetry (CV). Reversible CV processes were obtained for PFS 27-PBE 1 film and irreversible CV processes were obtained for PFS 20-PBE 2 film in aqueous electrolyte solutions. The kinetic parameters such as the surface transfer coefficient, the apparent diffusion coefficient, and the standard rate constant for electron transfer, for the two films in aqueous LiClO 4 solutions were measured, and the electrode process mechanism of the films was also discussed.

Electrochemical behavior of high‐molecular‐weight poly(ferrocenylsilane) films in aqueous electrolyte solutions

AbstractThe electrochemical behavior of high‐molecular‐weight poly(ferrocenyldimethylsilane) films and poly(ferrocenylmethylphenylsilane) films, which contained about 2.8 × 10−6 mol cm−2 ferrocene sites in eight kinds of aqueous electrolyte solutions, was investigated with cyclic voltammetry (CV). In some aqueous electrolyte solutions, the CV peak currents diminished gradually with an increase in the scanning times, whereas in other aqueous electrolyte solutions, stable and repeated cyclic voltammograms were obtained. The polymer films were poor‐solvent‐swollen in aqueous electrolyte solutions, and this resulted in a high resistivity of mass transfer and a slow rate of electrode reaction; therefore; quasireversible or irreversible CV processes were obtained. The kinetic parameters of the film‐electrode processes, such as the surface transfer coefficient, the apparent diffusion coefficient, and the standard rate constant for electron transfer, for the two films in aqueous LiClO4 solutions were measured, and the electrode process mechanism of the films was examined. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2245–2253, 2004

The mechanism of electrode process in the system silver/silver cyanide complexes

The electrode's surface inhomogeneity was taken into account studying the mechanism of process. The effect of CN − adsorption and partial surface coverage was investigated by electrochemical fast Fourier transform (FFT) impedance spectroscopy (0.03–3900 Hz). The isopotential solutions ( E 0=−0.350 V) as well as solutions containing constant silver cyanide complex concentration (0.05 M) and different free cyanide ion concentrations (from 0.01 to 1.0 M) were studied. The exchange current density, re-calculated for only active area, gives electrochemical reaction order ( R CN=1.8 for α=0.5, and R CN=2 for α=0.6) which is independent from CN − concentration and from the composition of complexes prevailing in the bulk of electrolyte. The value of R CN close to 2, obtained in a series of isopotential solutions confirms that in all cases the complex ions Ag(CN) 2 − take direct part in the charge transfer step.

Synthesis of heteroleptic anthryl-substituted beta-ketoenolates of rhodium(III) and iridium(III): photophysical, electrochemical, and EPR study of the fluorophore-metal interaction.

The anthryl-substituted rhodium(III) and iridium(III) heteroleptic beta-ketoenolato derivatives of general formula [M(acac)(2)(anCOacac)] [acac = pentane-2,4-dionate; anCOacac = 3-(9-anthroyl)pentane-2,4-dionate], 3 (M = Rh) and 4 (M = Ir), and [M(acac)(2)(anCH(2)acac)] [anCH(2)acac = 3-(9-anthrylmethyl)pentane-2,4-dionate], 5 (M = Rh) and 6 (M = Ir), were prepared by reacting the corresponding tris(pentane-2,4-dionate)metal complexes, [M(acac)(3)], with 9-anthroyl chloride and 9-chloromethylanthracene, respectively, under Friedel-Crafts conditions. 3-6 were characterized by elemental analysis, ion spray mass spectrometry (IS-MS), (1)H NMR, and UV-vis spectroscopy. The structure of 3 was also elucidated by single-crystal X-ray analysis. When excited at 365 nm, 3-6 result to be poorly luminescent compounds; while the free diketone, i.e., 3-(9-anthrylmethyl)pentane-2,4-dione 1, whose structure was established also by single-crystal X-ray analysis, results to be a strongly light emitting molecule. The study of the electrochemical behavior of 3-6 as well as of the corresponding tris-acetylacetonates of rhodium(III) and iridium(III) allows a satisfactory interpretation of their electrode process mechanism, and gives information about the location of the redox sites along with the thermodynamic and kinetic characterization of the corresponding redox processes. All data are in agreement with the hypothesis that the quenching of the anthracene fluorescence, observed for compounds 3-6, can be due to an intramolecular electron transfer process between the anthryl moiety and the metal-beta-ketoenolato component. Moreover, a study was carried out of the redox behavior of the dyads 3-6 under chemical activation. The one-electron oxidation of compounds 3-6 by thallium(III) trifluoroacetate leads to the formation of the corresponding cation radicals, 3(+)-6(+), whose highly resolved X-band EPR spectra were fully interpreted by computer simulation as well as by semiempirical and DFT calculations of spin density distribution.

Mechanisms of electrode processes on the iron metals

Mechanisms of the electrode processes occurring on the iron metals which are complicated by the generation of a chemisorbed film on the surface are discussed. The film can be either partially reduced or desorbed at elevated temperatures. It can also be removed during the generation of fresh surface. In its absence, the overall reversible electrode reaction includes consecutive elementary steps of the charge transfer from active centers of adsorption and the step where unstable intermediate univalent metal ions undergo disproportionation.

Half a Century of Research Using Polarography

Polarography has been used in investigations of equilibria between carbonyl compounds and Schiff bases. Formation of mercury compounds of thiols, some urea, and thiourea derivatives is manifested by anodic polarographic waves. Electrochemical data for numerous organic compounds can be correlated with structural parameters using LFERs. Such correlations can be used in mechanistic studies and in choice of representative model compounds. For organic compounds bearing two electroactive centers the degree of their mutual interaction is reflected by their electrochemical behavior. Such interactions range from weak to medium strength (manifested by substituent effects) to strong interactions (resulting in changes in mechanisms of electrode processes). Strong interactions may result in reversal of the sequence of the reducibility of the two electroactive groups. Changes of polarographic current–voltage curves with solution composition enable determination of equilibrium constants for rapidly and slowly established equilibria preceding electron transfer. From currents governed by homogeneous antecedent reactions rate constants of fast reactions (between 104and 1010l mol−1s−1) can be obtained. For slower reactions occurring in the bulk of the solution variations of currents with time are used in kinetic studies. Polarographic measurements can be carried out directly in the reaction mixture and are complementary to spectrophotometry. Kinetics can be followed polarographically even in heterogeneous systems. This was used in studies of formation of slightly soluble complexes, e.g., of heavy metal ions with BAL and bile salts. Adsorption of organic (e.g., pesticides) and inorganic species on lignin can be followed, as well as formation of aromatic aldehydes and unsaturated ketones in alkaline cleavage of lignin. Examples of practical applications of polarographic analysis in food chemistry and pharmaceutical and medical science are given.