Anodic Limit Research Articles

Electrochemistry in Room-Temperature Ionic Liquids: Potential Windows at Mercury Electrodes

The cathodic and anodic potential limits of eleven different ionic liquids were determined at a mercury hemisphere electrode. Ionic liquids containing the phosphonium cation (tri(n-hexyl)tetradecylphosphonium, [P14,6,6,6]+) give the largest potential window, especially when coupled to a trifluorotris(pentafluoroethyl)phosphate, [FAP]−, or bis(trifluoromethanesulfonyl)imide, [NTf2]−, anion.

Electrochemical Methods to Enhance the Capacitance in Activated Carbon/Polyaniline Composites

Activated carbon/polyaniline composites have been prepared using different electrochemical methods: single-step potentiostatic polymerization, multiple-step potentiostatic polymerization, and potentiodynamic polymerization with the anodic potential limits being fixed at either 0.75 or (vs ). The prepared composite samples were characterized by cyclic voltammetry, galvanostatic charge-discharge tests, electrochemical impedance spectroscopy, and Fourier transform infrared (FTIR) spectroscopy. The synthesis conditions were found to strongly affect the electrochemical behavior of the samples. High capacitance was achieved by the potentiostatic polymerization methods. As a general trend, higher capacitance and lower resistance were observed in the composites than the sum of these parameters of the individual components. This benefit is attributed to the enhanced electron delocalization along the polymer chains in the composites resulting from the influence of the activated carbon, as evidenced by the FTIR. However, an influence of the polyaniline morphology induced by the porous carbon cannot be discarded.

Poly(3,4-(2-methylene)propylenedioxythiophene) for Electrochemical Grafting of Polystyrene via Methylene Functionality

A monomer 3,4-(2-methylene)propylenedioxythiophene (MProDOT) was synthesized and electrochemically polymerized to yield PMProDOT with methylene groups that have the potential to be further functionalized. Electrochemical redox switching of PMProDOT between −0.6 and +1.5 V (vs ) caused the cross-linking reaction between methylene moieties at the anodic potential limit and resulted in the fast decrease of electroactivity due to destruction of π-conjugation. When PMProODT was subjected to +1.5 V in the presence of styrene (ST) monomers, the loss of electroactivity was not significant and the mass of the PMProDOT was rapidly increased. The mass increase was ascribed to polystyrene (PST) grafting onto PMProDOT. Simultaneous oxidation of methylene groups and ST at +1.5 V led to the coupling of ST to PMProDOT, which reduced cross-linkable free-methylene moieties. PST-grafted PMProDOT showed relatively stable electroactivity over repeated potential cycling. The infrared spectrum of PST-grafted PMProDOT also showed additional C–H stretching peaks possibly due to PST grafting.

Morphological and electrochemical characterisation of graphite electrodes coated with SPANI and Ni-cyclam

The preparation of the composite graphite–sulphonated polyaniline (SPANI)–Ni-cyclam was investigated with the purpose of dispersing a catalyst in a conducting matrix that allows a better electronic transfer from the surface of the electrode towards the reaction site. The graphite is a material with a high porosity, big area and cheaper than other materials. The electrochemical and morphological characterisations show that the best conditions for obtaining SPANI over graphite were reached with an anodic limit of 1,000 mV in H2SO4. The deposit of the Ni-cyclam over graphite–SPANI in basic medium allows the formation of a spongy and open material, which permits better accessibility to the catalytic sites, showing good catalytic properties. These results illustrate that the importance relays on the morphology and dispersion of the catalyst and not on the amount of catalytic material present in the composite. The electrocatalytical properties Ni-cyclam over graphite–SPANI was studied for the methanol oxidation in basic medium.

Potentiostatic voltammetry at spherical electrodes and microelectrodes in the presence of product

An explicit analytical solution for the complete anodic–cathodic I– E– t response of a reversible electrode process in presence/absence of amalgamation under potentiostatic conditions is presented. To obtain this solution we have taken into account the electrode curvature, using in the case of amalgamation, Koutecký’s approximation (i.e. the finite electrode volume has not been considered). Explicit expressions for the concentration profiles and the surface concentrations have been also deduced. All the results are applicable to electrodes of any radius including ultramicroelectrodes when both species are soluble in the electrolytic solution for any value of the ratio D O/ D R. When amalgamation takes place, analytical results were compared with the numerical ones deduced by using the rigorous condition of null net flux in the electrode centre, pointing out that Koutecký’s approximation remains valid even for electrode radius and time values at which the diffusion layer reaches the electrode centre. For high electrode sphericities the I– t curves present a cross-linking for applied potential values higher than the equilibrium potential which is the cause of the appearance of a “peak” near to the anodic limit of the I– E curve.

Cr(VI) Reduction using Polyaniline Coating on RVC Electrodes

Polyaniline coatings electrodeposited by cyclic voltammetry on RVC electrodes are studied as a system to reduce Cr(VI) to Cr (III). Various polymer film electrodeposition conditions are assessed, such as the number of scans, anodic potential limit, monomer concentration and background electrolyte concentration. The results show that to achieve a high Cr(VI) transformation rate, deposits with a large polymer mass must be obtained, and that the structures must be as open as possible in order to afford the Cr(VI) greater permeability, thus increasing the conversion rate.

Electrochemical Oxidation of Nitrite and the Oxidation and Reduction of NO2 in the Room Temperature Ionic Liquid [C2mim][NTf2

The electrochemical oxidation of potassium nitrite has been studied in the room temperature ionic liquid (RTIL) [C2mim][NTf2] by cyclic voltammetry at platinum electrodes. A chemically irreversible oxidation peak was observed, and a solubility of 7.5(+/-0.5) mM and diffusion coefficient of 2.0(+/-0.2)x10(-11) m2 s(-1) were calculated from potential step chronoamperometry on the microdisk electrode. A second, and sometimes third, oxidation peak was also observed when the anodic limit was extended, and these were provisionally assigned to the oxidation of nitrogen dioxide (NO2) and nitrate (NO3-), respectively. The electrochemical oxidation of nitrogen dioxide gas (NO2) was also studied by cyclic voltammetry in [C2mim][NTf2] on Pt electrodes of various size, giving a solubility of ca. 51(+/-0.2) mM and diffusion coefficient of 1.6(+/-0.05)x10(-10) m2 s(-1) (at 25 degrees C). It is likely that NO2 exists predominantly as its dimer, N2O4, at room temperature. The oxidation mechanism follows a CE process, which involves the initial dissociation of the dimer to the monomer, followed by a one-electron oxidation. A second, larger oxidation peak was observed at more positive potentials and is thought to be the direct oxidation of N2O4. In addition to understanding the mechanisms of NO2- and NO2 oxidations, this work has implications in the electrochemical detection of nitrite ions and of NO2 gas in RTIL media, the latter which may be of particular use in gas sensing.

Medium effects on the electro-deposition of MnO 2 on glassy carbon electrode : A comparative study in alkane, perfluoro alkane carboxylic acids and methanesulphonic acid

Voltammetric studies of Mn(II) were carried out in acetic acid (AA), trifluoroacetic acid (TFA), butyric acid (BA), perfluorobutyric acid (PFBA) and methanesulphonic acid (MSA) media. Influence of anodic limit and the nature of acids were investigated. Oxidation of Mn(II) shows two anodic peaks in weak acids namely acetic acid and butyric acid media due to the formation of insoluble Mn(III) and Mn(IV) species. Reduction of these species were also observed as two peaks in the cathodic region. Formation and reduction of solid oxide phases are the dominant electrode processes in such weak acid media. In strong acids like MSA and TFA media sharp anodic and cathodic peaks were obtained due to the formation and reduction of stable soluble Mn(III) species. SEM studies reveal the formation of thicker oxide phases in weak acid media and much thinner oxide layers in strong acid media. γ-MnO 2 phase was formed from AA and TFA while ɛ-MnO 2 phase was formed from BA and PFBA, suggesting a relationship between phase structure and anionic size.

Enhancement of corrosion resistance of a biomedical grade 316LVM stainless steel by potentiodynamic cyclic polarization

The paper discusses the results on the use of a simple cyclic linear potentiodynamic polarization technique as a method of improving corrosion properties of passive oxide films formed on a biomedical-grade 316LVM stainless steel surface in phosphate buffer. The results demonstrate that the modification of 316LVM surface by cyclic potentiodynamic polarization between the potential of hydrogen and oxygen evolution results in the formation of a passive film that offers significantly increased corrosion resistance (both pitting and general) when compared to the naturally grown passive film. The effect of number of cycles and anodic potential limit on the resulting corrosion properties is discussed. The capacitance analysis demonstrates that the major difference between the electrochemically formed and naturally grown passive film is in the type of semiconductivity in the potential region where pitting on the unmodified surface occurs. The XPS analysis shows that this is due to the presence of Cr(VI)-species in the electrochemically formed passive film, which contribute to the increased density of metal vacancies, and thus to the increased pitting corrosion resistance of the passive film.

Electrochemical Quartz Crystal Microbalance Study on Dissolution of Platinum in Acid Solutions

The influence of chloride ion on the oxidation and dissolution of platinum has been studied by using Electrochemical Quartz Crystal Microbalance (EQCM) in combination with Inductively Coupled Plasma (ICP) analysis to clarify the deterioration mechanism of platinum catalyst in the Polymer Electrolyte Fuel Cell (PEFC). Platinum was electroplated on an Au-QCM electrode in 2% H2PtCl6 solution and used as the working electrode. A mass gain of the platinum electrode was observed during anodic polarization in 0.5 M H2SO4 solution ([Cl−] free solution) due probably to the formation of Pt oxide. The oxide film was not stable in the whole range of potential used in this study. On the other hand, a mass loss due to the dissolution of Pt as chloride complex ions was observed in more positive potentials than 0.85 V vs. SHE in chloride-containing solutions. As the chloride concentration increased, platinum showed a higher dissolution rate with the formation of chloride complexes, PtCl42− and PtCl62−. The amount of dissolved PtCl62− tended to increase with the increase of anodic potential limit. The EQCM results revealed that the dissolution of Pt was accelerated when potential was cycled between cathodic and anodic limits. The ICP analysis of the solution after anodic polarization supported the results obtained by EQCM.

Hybrid Electrochemical Treatment for Persistent Metal Complex at Conductive Diamond Electrodes and Clarification of Its Reaction Route

The hybrid electrochemical treatment for persistent organometallic complexes [Cu–ethylenediaminetetraacetic acid (EDTA)] that was a combination of the reduction recovery of the center metal as nanoparticles and the oxidative decomposition of the organic ligand was attempted using the potential cycling at the oxygen-terminated boron-doped diamond (BDD) electrode. In the step of the recovery of the center metal of the organo-metal complex in the potential cycling from the reduction potential for , by stepping up the anodic potential limit to the potential region for oxygen evolution reaction, the reionization of electrodeposited Cu could be suppressed in the presence of the dissolved oxygen. Moreover, separation of the electrodeposited Cu as nanoparticles from the electrode surface could be achieved by introducing the oxygen-containing functional groups on the diamond surface. The oxidation of the particle surface by the dissolved oxygen might induce the formation of the insulating layer and accelerate the separation by the electrostatic repulsive force between the oxygen-terminated surface and the insulating layer on the Cu particles. The reaction pathway of the treatment was analyzed using the long-term potential cycling at the potential region between the reduction potential of Cu and the oxidation potential of the EDTA. In flow injection analysis with the UV detector, the formation of the Cu particles by the reduction reaction of the ion from was clarified to mainly occur in the initial stage of the electrolysis. Moreover, in high-performance liquid chromatography analysis with the electrogenerated chemiluminescence detector for the treated solution, ethylenediaminetriacetic acid (ED3A) and ethylenediaminediacetic acid (EDDA) were observed. Therefore, it was confirmed that the liberated EDTA was oxidized to ED3A and EDDA through sequential removal of the acetate groups. An unidentified product (supposed to be the small size of a hydrocarbon) appeared after during the electrolysis. Although the concentration of this unidentified product was not decreased in the electrolysis with anodic potential limit of , the concentration decay for this product was observed for the potential cycling up to , suggesting that for the full oxidation of EDTA to , it might be effective for the electrochemical oxidation by the radical available over at BDD.

Pt-Ru electrodeposited on gold from chloride electrolytes

Voltammetric behavior of submicron-thick electrodeposited Pt-Ru on gold support is studied in sulfuric acid solution as a function of deposition potential and Pt:Ru ratio in chloride bath. In contrast to Pt-Ru, deposition of pure Ru is observed only at potentials of hydrogen evolution. The reason is found to be of kinetic nature, namely an inhibition of Ru deposition in presence of chloride. Chloride ions remain adsorbed on Ru at more negative potentials than on Pt and Au because of more negative ruthenium potential of zero free charge. Cu-UPD is applied to test the surface content of the oxidized Ru on pure Ru and various Pt-Ru surfaces. An enhancement of Ru oxohydroxides reduction in presence of Pt is observed. The electrocatalytic activity of Pt-Ru in respect to methanol oxidation correlates with the content of rechargeable surface Ru oxide. Ageing and ‘training’ of Pt-Ru electrodeposits under various modes is studied in order to determine the conditions of irreversible Ru oxidation. No manifestations of Ru dissolution from Pt-Ru electrodeposits in 0.5 M H 2SO 4 are found for anodic potential limits up to 1.1 V (RHE), in agreement with thermodynamic predictions. Electrodeposited Pt-Ru can be considered as a convenient model system for the study of Ru dissolution and crossover, as well as for determining the nature of the active surface species in the real composite catalysts for methanol oxidation.

Comparative studies of chloride and chloride/citrate based electrolytes for zinc–polyaniline batteries

Electrochemical behavior of zinc and thin polyaniline (PANI) polymerized from 0.1 M HCl to 0.1 M aniline on graphite electrode, in 0.2 M ZnCl2 and 0.50 M NH4Cl (chloride electrolyte) and with addition of 0.33 M Na-citrate (chloride/citrate electrolyte) were investigated. In the chloride/citrate comparing with chloride containing electrolyte zinc electrode shows negative shift of the open circuit potential of ∼130 mV, decreases of exchange current density for more than order of magnitude and increase of cathodic Tafel slope, due to the zinc ions complexation. In citrate/chloride electrolyte zinc dendrite formation were suppressed. In the range of investigated charge/discharge current densities of 0.25–1 mA cm−2, initially obtained specific capacity was in the range of 140–85 mAh g−1, respectively. In cycling regime specific capacity and columbic efficiency were affected with anodic potential limits. For anodic potential limits of 0.32 V (SCE) citrate/chloride electrolyte shows better characteristic than chloride electrolyte, due to the influence of citrate ions on negative shift of doping reaction. Increasing anodic potential limit to 0.5 V (SCE), leads to faster decrease of specific capacity in citrate/chloride than in chloride electrolyte, which was. explained by higher hydrophilic effect of citrate anions.

Changes in Electrochemical and Optical Properties of Oligoalkylthiophene Film Induced by Bipolaron Formation

An unusual electrochemical response is found for oligoalkylthiophene (13T) films in acetonitrile. A pristine 13T film exhibits a stable redox wave in cyclic voltammogram when the anodic potential limit is below 0.6 V, whereas the redox peaks shift clearly to the negative side by ca. 0.1 V once the 13T film experiences a potential beyond 0.7 V. After this electrochemical stimulus is imposed on a pristine 13T film, the absorption and emission spectra change their features. These findings, along with the measurements with varying thicknesses of 13T films, suggest that a pristine 13T film is a mixture of two phases to be oxidized at separate potentials and that one phase can be switched to the other by the potential stimulation. A plausible model proposed for explaining these observations assumes that a key parameter characteristic of the two phases is a conformation of 13T molecules in the solid state and a change in conformation, leading to the shift of the redox potentials and the change of optical properties, is induced by electrochemical generation of bipolarons on the half units of the oligomers in the pristine 13T film.

In Situ FTIR Spectroscopy Study of the Break-In Phenomenon Observed for PPy/PVS Films in Acetonitrile

The in situ Fourier transform infrared (in situ FTIR) technique was used for the first time to investigate the break-in phenomenon observed for polypyrrole/poly(vinyl sulfonate) (PPy/PVS) films in acetonitrile containing 0.1 M LiClO(4). Consecutive potential scans provided a continuous increase of the infrared band intensities, simultaneous to an increase observed in the charge involved in the voltammetric peaks, suggesting a rise in the number of the polymeric chains participating in the infrared signal at the same time as the electroactive participants increase in the redox process. Moreover, in situ FTIR spectra evidence that the new infrared-activated chains in each voltammetric cycle adopt the same polymeric structure achieved by the chains activated in the initial cycles. However, if we achieve a cathodic potential limit of -2.1 V (vs Ag/AgCl), a restructuring of the polymeric morphology is observed. In situ FTIR spectra obtained for PPy/ClO(4) films under the same conditions pointed to a steady-state behavior from the very early voltammetric scans. Moreover, the intensities of FTIR bands obtained for PPy/ClO(4) films in the early voltammetric cycles are much higher than those obtained for PPy/PVS films after several potential scans. Only when high cathodic and high anodic potential limits were used for the consecutive cycles did the FTIR band intensities from PPy/PVS become similar to those obtained from PPy/ClO(4), indicating that in both films a similar number of polymeric chains were infrared active. Polarization at a high anodic potential (+1.3 V vs Ag/AgCl) produced overoxidation of the polymer appearing characteristic 1725 cm(-1) band assigned to the formation of carbonyl groups. Furthermore, the approximately 1540 cm(-1) band shifted to higher wavenumbers, indicating that overoxidation reduced the length of conjugated chains in the polypyrrole.

Electrochemical window of molten LiCl–KCl–CaCl 2 and the Ag +/Ag reference electrode

The electrochemical window of an LiCl–KCl–CaCl 2 eutectic melt (52.3:11.6:36.1 mol%) was determined by cyclic voltammetry and open-circuit potentiometry at 723–873 K. The reaction at the anodic limit was confirmed to be Cl 2 gas evolution. The reaction at the cathodic limit was found to be a liquid Ca–Li alloy formation on the basis of ICP analysis of the deposits. An Ag +/Ag reference electrode separated with a Pyrex membrane showed good stability for more than 1 week. The standard electrode potential of Ag +/Ag was determined in the temperature range of 723–823 K by measuring the potential of a silver electrode in different concentrations of Ag + ions.

Electrochemical removal of nitrate ion from aqueous solution by pulsing potential electrolysis

The main problem in the process of the electrochemical removal of nitrate from aqueous solutions and wastewaters, when the electrolysis is performed at a constant potential, is the production of more toxic side-products than nitrates, such as NO 2 − and NH 3. This work deals with the removal of nitrate by square, sinusoidal and triangular wave pulsing potential electrolysis. In this way, the nitrates are reduced during the cathodic cycle mainly to NO 2 − and NH 3 and consequently, during the anodic cycle, the produced NO 2 − and NH 3 are oxidized to NO 3 − and gaseous products, respectively. The experimental results on a Cu60Zn40 electrode showed that square asymmetrical pulses having a frequency of 50 Hz and duration in the cathodic and anodic limit of 80 and 20% of the period, respectively, are more effective than sinusoidal and triangular as regards to the removal efficiency (%RE) of nitrate. The minimum selectivity (%S) for both nitrite and ammonia was obtained in the frequency range of 30–50 Hz. At frequency values higher than 200 Hz the %RE of nitrate is decreased because the ions are not able to follow the potential changes and this leads to limited Faradaic reactions. When the electrolysis was performed by applying square pulses having a constant cathodic limit of −1.7 V versus Ag/AgCl and anodic limit ranged from 0.50 to 3.00 V, a minimum in the %S of nitrite and ammonia was observed at 1.50 V. This minimum was attributed to the ability of the formed surface oxides of brass to oxidize ammonia and nitrite.

Electrosynthesis of Cu−Se Films on Copper Electrodes in Alkaline Media: A Voltammetric, Electrochemical Quartz Crystal Microbalance and I/t Transient Study

The electroformation of Cu-Se phases, obtained by selenizing a thin film of copper deposited on the quartz/gold electrode system, was studied with an electrochemical quartz crystal microbalance (EQCM) and by cyclic voltammetry (CV) in an alkaline solution (0.05 M Na(2)B(4)O(7)) containing selenide ion. Potentiodynamic parameters showed that the formation of the initial Cu-Se phases (Cu(2-x)Se/Cu(3)Se(2)) is ruled by an irreversible diffusion controlled mechanism, where a first electron transfer is the rate-determining step. A CV study was also performed with a bulk copper electrode in 1 M NaOH solution containing selenide ion. The deconvolution of the anodic and cathodic I/E profiles corresponding to the electroformation and electroreduction of the Cu-Se film formed allowed us to establish that, depending on the anodic potential limit of the potentiodynamic scan, the Cu-Se phases formed were either a mixture of Cu(2)(-x)Se/Cu(3)Se(2) or Cu(2-x)Se/Cu(3)Se(2)/CuSe. An EQCM study showed that, during the initial stage of Cu-Se phase electroformation, water molecules were released from the electrode. In advanced stages of the process, when the electrode was completely covered by Cu-Se compounds, selenide anions were adsorbed on the formed phase. When the anodic potential limit was extended to -0.2 V, copper oxide compounds were formed. The analysis of the cathodic charge related to Cu-Se phase electroreduction and Energy Dispersive X-ray Spectroscopy (EDXS) analysis confirmed that when the anodic limit was -0.8 V, a mixture of different Cu-Se phases was formed. A I/t transient study performed with a bulk copper electrode in alkaline solution containing selenide established that the nucleation and growth mechanism (NGM) of the Cu-Se phases takes place through an initial bidimensional-instantaneous nucleation (IN2D), followed by four bidimensional-progressive nucleations (PN2D). These results and atomic force microscopy (AFM) experiences supported that the growth of the Cu-Se films occurs through a layer-by-layer mechanism.

Electrochemical Window of a LiCl-KCl-CsCl Melt

The electrochemical window of the LiCl-KCl-CsCl melt (57.5:13.3:29.2 mol %, m.p.; 538 K) was investigated at 573-723 K. The cathodic limit of this melt for a nickel electrode was confirmed as lithium deposition from the results of differential scanning calorimetry and inductively coupled plasma atomic emission spectroscopy. Conceming the anodic limit of this melt, chlorine was confirmed to be evolved on a glassy carbon electrode from the result of the gas analysis. The electrochemical windows were determined as 3.75 and 3.62 V at 573 and 723 K, respectively. These values agreed well with theoretical decomposition voltages of LiCl.

Electrochemical Window and the Characteristics of ( α + β ) Al-Li Alloy Reference Electrode for a LiBr-KBr-CsBr Eutectic Melt

The electrochemical window and an availability of the Al-Li alloy electrode as a reference electrode were investigated at 523-673 K in a LiBr-KBr-CsBr eutectic melt. From differential scanning calorimetry and inductively coupled plasma atomic emission spectroscopy, it was confirmed that the cathodic limit of this melt is lithium metal deposition. The anodic limits of bromine gas evolution were 3.38 V (vs. at 523 K and 3.22 V at 673 K, which determined the electrochemical window of this melt. The electrochemical formation of AlLi(β) alloy phase was confirmed by X-ray diffraction analysis. The Al-Li alloy electrode was proved to have excellent characteristics as a reference electrode in terms of stability, reproducibility, and reversibility. The thermodynamic properties were estimated for the Al-Li alloy in the coexisting phase state at 523-673 K from the measured equilibrium potential. © 2004 The Electrochemical Society. All rights reserved.