Potentiostatic Oxidation Research Articles

Electrochemical Synthesis of Layered Manganese Oxides Intercalated with Tetraalkylammonium Ions

Thin films of birnessite-type layered manganese oxides with various interlayer spacings have been prepared on a platinum electrode by a one-step electrochemical procedure. The process involves a potentiostatic oxidation of aqueous Mn(2+) ions at around +1.0 V (Ag/AgCl) in the presence of tetraalkylammonium cations with different alkyl chain lengths. X-ray diffraction indicates that the films deposited with tetrabutylammonium (TBA), tetrapropylammonium (TPA), and tetraethylammonium (TEA) ions are composed of a single phase where unhydrated tetraalkylammonium ions are accommodated as a monolayer between manganese oxide layers. The interlayer spacing of the products increases in an order of TEA < TPA < TBA. The film deposited with tetramethylammonium (TMA) is a mixture of two phases relating to hydrated and unhydrated guest cations, the former being predominant probably as a result of less hydrophobic property of TMA compared to that of other tetraalkylammonium ions. The TBA(+)-intercalated Mn oxide film-coated electrode exhibits a good charge/discharge property in a KCl solution between 0 and +0.8 V. In this case, TBA(+) ions between the Mn oxide layers are rapidly replaced with K(+) in solution by ion exchange, accompanying a shrinkage of the interlayer. The incorporated K(+) ions as well as protons play an important role in the electrochemical conversion between Mn(4+) and Mn(3+) in the oxide layer. In the TBACl solution, the interlayer TBA(+) ions can be excluded electrochemically during the positive-going scan, concomitant with the oxidation of Mn(3+) sites. This causes an anodic current and the accompanying shrinkage of the interlayer. On the reverse scan, however, the compressed interlayer does not allow the incorporation of bulky TBA(+) ions from the electrolyte, with virtually no cathodic current observed. Such an obvious difference in electrochemical behavior between the two electrolytes can be recognized by considering that most of the Mn oxide surface is present inside the layered structure, not on the external surface. This indicates that the layered structure is formed over the entire film.

A Direct Electrochemical Route to Construct a Polymer/Manganese Oxide Layered Structure

A layered nanocomposite with poly(diallyldimethylammonium), PDDA, intercalated between manganese oxide layers can be formed on a platinum electrode in a thin film form through a direct electrochemical route. The process involves a potentiostatic oxidation of aqueous Mn(2+) precursors in the presence of PDDA by applying a constant potential (+1.0 V vs Ag/AgCl).

Bismuth Telluride (Bi2Te3) Nanowires Synthesized by Cyclic Electrodeposition/Stripping Coupled with Step Edge Decoration

Polycrystalline bismuth telluride (Bi2Te3) nanowires have been prepared by the step edge selective electrodeposition of Bi2Te3 on highly oriented pyrolytic graphite (HOPG) surfaces. Bi2Te3 nanowires were obtained from an aqueous plating solution containing Bi3+ and HTeO2+ using a three-step procedure: (1) potentiostatic oxidation of the graphite surface at +0.8 V (vs saturated calomel electrode, SCE), (2) potentiostatic nucleation of Bi2Te3 at −0.6 V for 5 ms, (3) growth of Bi2Te3 from these nuclei by cyclic electrodeposition (of both Bi2Te3 and bismuth) and stripping (of bismuth only) between +0.30 V and −0.05 V at 20 mV s-1. Control of the number of electrodeposition/stripping scans allowed the diameter of Bi2Te3 nanowires to be specified in the range from 100 to 300 nm. Bi2Te3 nanowires were narrowly dispersed in diameter (RSDdia = 10−20%), were up to 1.0 mm in length, and were organized into parallel arrays containing hundreds of wires.

Pt–Me (Me = Ir, Ru, Ni) binary alloys as an ammonia oxidation anode

Various Pt–Me (Me = Ir, Ru or Ni) binary alloys were prepared on a glassy carbon (GC) substrate by thermal decomposition and compared as the electrocatalyst for ammonia oxidation in KOH solutions. On the solid solutions of Pt1−xIrx (0≤x≤1.0) and Pt1−xRux (0≤x≤0.6), ammonia oxidation was found to begin at a lower potential (−0.6V versus Ag/AgCl) than on pure Pt by about 0.1V. During potentiostatic oxidation the current density j decreased considerably within about 300s. The arbitrarily selected j60 (j after 60s) tended to saturate at a high set potential region probably due to the deactivation by a poison of Nads. The saturated j60 at a high oxidation potential was apparently higher on Pt1−xIrx (x≤0.8) or Pt1−xRux (x≤0.4) than on Pt, which suggested a positive cooperation of Ir and Ru with Pt in the electrocatalysis. On the other hand, Ni added as a solute to Pt contributed to lower j60 with x in Pt1−xNix (0≤x≤0.7) and did not lower the starting potential of ammonia oxidation at all. The reason why Ir and Ru enhance the activity might be explained by their activity at the dehydrogenatation steps of NH3 at a lower potential, at which ammonia oxidation can never start on the surface of pure Pt.

Potentiostatic Oxidation of Polyaniline under Conformational Relaxation Control: Experimental and Theoretical Study

Electrogenerated films of polyaniline were studied by potential steps in acetonitrile solutions to check for the presence of compaction, relaxation, and diffusion processes giving like-nucleation chronoamperograms such as those described by the electrochemically stimulated conformational relaxation (ESCR) model. This solvent was used to avoid both proton discharge during cathodic compaction of the film at high cathodic potentials and proton exchange during redox processes. Chronoamperograms obtained from different cathodic potentials to different anodic potentials under different concentrations of electrolyte or at different temperatures indicate the presence of compaction, relaxation, nucleation, and diffusion processes under electrochemical stimulation. The ESCR model, developed to describe the electrochemical behavior of polypyrrole films by the quantification of these processes, was used to determine if it can be applied to simulate the chronoamperometric responses of the polyaniline and the influence of the different variables on the experimental chronoamperograms. Constants required to simulate the chronoamperograms were obtained from experimental results: the same model indicates the required experimental conditions. Good agreement was obtained between simulated and experimental results.

Loss and Recovery of Faradaic Response on Glassy Carbon Electrodes: Effect of Electrochemical Pretreatments on Oxide Layer Properties

Due to the complexity of the glassy carbon (GC) | electrolyte interface, several physical, chemical, and electrochemical activation methods have been developed, mainly to obtain both high activities for heterogeneous charge transfer and reproducibility. One of the most simple and widely used methods in the pretreatment of GC electrodes is oxidation in aqueous media with different electrolytes at variable pH. This oxidation can be made through dynamic and/or constant programs of potential. In this work we studied the kinetics of heterogeneous charge transfer for several couples after different pretreatment procedures: potential cycling and application of a constant anodic potential. In the dynamic method, the results are correlated with an activation parameter. Total lack of faradaic current is observed when the electrode surface is oxidized under a constant potential regime, indicating that the electrode acts as a dielectric. The loss and recovery of electrochemical activity of the GC electrode occur in a gradual way as a function of the injected oxidation and reduction charges. This fact is very important in systems where carbon materials are used as electrodes under a continuous oxidation regime, such as high-pressure liquid chromatographic (HPLC) electrochemical detectors, batteries, capacitors, and for the generation of polymers, by potentiostatic oxidation. © 2001 The Electrochemical Society. All rights reserved.

Electrooxidation of Phenylhydrazine on Carbon Electrodes

Electrochemical behavior of phenylhydrazine on glassy carbon and carbon fiber in a neutral aqueous solution is studied with differential voltammetry and potentiostatic oxidation. The fiber surface has a considerable catalytic effect, and the oxidation products depend on the fiber potential. The application of carbon fiber as an electrode material in electrochemical purification of waste water from phenylhydrazine is considered.

Mediated Oxidation and Determination of Gaseous Monomethyl Hydrazine in a Solid-State Voltammetric Cell Employing a Sol-Gel Electrolyte

Mesoporous silica that was prepared by a sol-gel process was used as the electrolyte for the amperometric determination of vapor-phase monomethyl hydrazine (MMH). Three detection strategies were tested, namely cyclic voltammetry at a Pt working electrode with the oxidation mediated by FeII,III that was hosted by the silica, pulsed electrochemical detection with a gold working electrode, and potentiostatic oxidation at a mixed-valence ruthenium oxide (mvRuOx) film that was deposited on a Pt working electrode prior to casting the solid electrolyte overlayer. The sensitivities (and linear dynamic ranges) for the respective methods were 1.4 nA/ppm (500–1400 ppm), 0.49 nA/ppm (70–1050 ppm), and 0.30 nA/ppm (1–1050 ppm). With pulsed electrochemical detection and mvRuOx, respective detection limits of 20 ppm and 0.3 ppm were calculated on the basis of a concentration that yielded a response three times the standard deviation of the blank. Because of high uncertainty of the intercept of the FeII,III-mediated method, a detection limit was not calculated for this technique. A cell design that provided a three-phase boundary at a mvRuOx-modified electrode yielded a response of 90 % of the steady-state current at 11 s.

Mediated Oxidation and Determination of Gaseous Monomethyl Hydrazine in a Solid-State Voltammetric Cell Employing a Sol-Gel Electrolyte

Mesoporous silica that was prepared by a sol-gel process was used as the electrolyte for the amperometric determination of vapor-phase monomethyl hydrazine (MMH). Three detection strategies were tested, namely cyclic voltammetry at a Pt working electrode with the oxidation mediated by FeII,III that was hosted by the silica, pulsed electrochemical detection with a gold working electrode, and potentiostatic oxidation at a mixed-valence ruthenium oxide (mvRuOx) film that was deposited on a Pt working electrode prior to casting the solid electrolyte overlayer. The sensitivities (and linear dynamic ranges) for the respective methods were 1.4 nA/ppm (500–1400 ppm), 0.49 nA/ppm (70–1050 ppm), and 0.30 nA/ppm (1–1050 ppm). With pulsed electrochemical detection and mvRuOx, respective detection limits of 20 ppm and 0.3 ppm were calculated on the basis of a concentration that yielded a response three times the standard deviation of the blank. Because of high uncertainty of the intercept of the FeII,III-mediated method, a detection limit was not calculated for this technique. A cell design that provided a three-phase boundary at a mvRuOx-modified electrode yielded a response of 90 % of the steady-state current at 11 s.

Current Oscillations during Iodide Oxidation at a Gold Rotating Disk Electrode

Oscillations in the electrode current were observed during the potentiostatic oxidation of iodide at a gold rotating disk electrode. In a 0.10 M KI solution, the oscillations were periodic, with a peak‐to‐peak amplitude equal to 70% of the peak current and a frequency of approximately 2 Hz. The oscillations continued for longer than 10 min, at which point the experiment was discontinued. Periodic oscillations in current were observed only for a narrow range of applied potentials, from 1.08 to 1.13 V vs. SCE in 0.10 M KI. At lower concentrations of KI, the oscillations were less periodic, had a smaller amplitude, and were sustained for shorter times (about 30–60 s for 20 mM KI). No oscillations were observed for concentrations less than about 5 mM. Maximum and minimum current values were measured from the oscillating currents observed at various rotation rates, and the resulting linear plots of inverse current vs. inverse square root of rotation rate gave n values of 1.28 and 0.24, respectively. Based on these results, a mechanism is discussed that involves the periodic formation and dissolution of a thick film. The presence of this film decreases the rate of oxidation of iodide to iodine, thus yielding an apparent n value of less than one. © 1999 The Electrochemical Society. All rights reserved.

Electrochemistry and spectroelectrochemistry of poly[(tetraethyldisilanylene)quinque(2,5-thienylene)

Electrochemistry of poly[(tetraethyldisilanylene)quinque(2,5-thienylene)] (DS5T) has been investigated in acetonitrile with cyclic voltammetry, potentiostatic oxidation and spectroelectrochemical measurements. In multicycle voltammograms of a DS5T film there are a pair of redox peaks attributable to anion-doping and dedoping. During potential cycling or electrolyzing at certain positive potentials, the polymer is subject to decomposition to some extent. An irreversible oxidation subpeak for the decomposition of the polymer is observed in the first-cycle voltammogram with low scan rates. Spectroelectrochemical measurements demonstrate that the slow decomposition process tends to be inhibited by further oxidation of its decomposed product, followed by redepositing of a solid film onto the DS5T film. The further oxidation of the decomposed product also leads to some unexpected observations in the spectroelectrochemistry of DS5T. Moreover, if the forward potential scanning is returned at more positive potentials, two pairs of oxidation/reduction peaks occur in the cyclic voltammogram of DS5T and the polymer film becomes much more unstable at the potential of the second oxidation peak.

An Electrochemically Induced Oscillatory Instability

We report current oscillations during the potentiostatic oxidation of iodide in aqueous solution. The oscillations become visible as columns of descending liquid with periodic structure colored by iodine (or iodine−starch complex). They can be attributed to a periodically varying flow of solution in front of the electrode due to a density gradient. The instability is explained using a qualitative model with a threshold function for the onset of the flow.

Deposition of Ru Adatoms on Pt Using Organometallic Chemistry: Electro‐oxidation of Methanol, Ethanol, 1,2‐Ethanediol, and D‐Glucose over a Surface Optimized for Oxidation of Methanol

A surface with 10% Ru adatoms was prepared by hydrogenating (1, COD is 1,5‐cyclooctadiene) over Pt. The onsets of stabilized potentiodynamic oxidation for several fuels ([fuel] = = 0.5 M, sweep range 0.025 to 0.60 V (vs. SHE), 22°C) were ethanol (∼0.2 V), methanol (∼0.3 V), and 1,2‐ethanediol (∼0.45 V). The activity of D‐glucose was low under these conditions. The ratio of specific current densities for potentiostatic oxidation (30 min, [fuel] = = 0.5 M, E = 0.4 V, 22°C) over and over Pt were methanol (42) 1,2‐ethanediol (4.0), ethanol (2.3), and D‐glucose (1.8).

Preconcentration and Electroanalysis of Copper Species at Electrochemically Activated Glassy Carbon Electrodes

Electrochemically activated glassy carbon electrodes were applied to the preconcentration and electroanalysis of metal species. Activated glassy carbon electrodes were obtained by potentiostatic oxidation at +2.0 V in 0.5 M sulfuric acid. Copper species were readily extracted onto the modified electrode and were determined electrochemically. Voltammetric peak currents showed linear response for copper concentrations in the range between 1 to 20 μM with a large slope of 2.11 A M−1 and a correlation coefficient of 0.997. The reproducibility of the voltammetric measurements was found to be within 7 % RSD for five replicate measurements on copper concentration of 10 μM. The modified electrodes could be applied for repeated copper determinations without appreciable deterioration in electrode response. Results suggest that the preconcentration of copper species onto activated electrodes involved primarily cation-exchange interactions with the surface graphite oxide film.

Potentiostatic Oxide Formation on Lead Selenide Single Crystals in Alkaline Solutions

Lead selenide is a narrow‐gap semiconductor material. It finds applications in infrared emitting and detecting devices, with a performance closely related to surface properties. Charge‐carrier recombination at the surface can be reduced by passivation, for instance by formation of a native oxide by anodic oxidation. In this work, the anodic behavior of single‐crystal lead selenide in aqueous solutions at pH 10 is studied under potentiostatic conditions. Surface analysis using a scanning electron microscope and energy dispersive X‐ray spectroscopy was performed. A ternary oxide was characterized as the oxidation product. Two different growth mechanisms were found: an “island growth” (two‐dimensional nucleus growth) and the growth of a homogeneous film following the high‐field model. The process of oxidation depends significantly on the pretreatment of the lead selenide surface. The electronic properties of the oxidation layer were investigated with photocurrent measurements, resulting in a bandgap of the anodic oxide of 1.6 eV. From I‐V curves a specific conductivity was determined in the order of for the oxide film.

Deposition of Ru Adatoms on Pt Using Organometallic Chemistry: Catalysts for Electrooxidation of MeOH and Adsorbed Carbon Monoxide

Ru adatoms were deposited on black Pt gauze by hydrogenating Ru(COD)(η3-C3H5)2 (1, COD is 1,5-cyclooctadiene) over the gauze at low temperatures in hexanes solution under 1 atm of dihydrogen gas. A series of Pt−Ruad surfaces were prepared by interrupting the hydrogenation after deposition of 0.05, 0.10, 0.30, 0.32, 0.44, 0.70, and 3.5 equiv of Ru adatoms. The ratio of currents in the “double layer” to those in the “hydride” region in the potentiodynamic responses of these surfaces (0.5 M H2SO4, 25 °C, sweep range 0.025−0.70 V, 5 mV/s) increased as the equivalents of Ru adatoms increased. Stripping voltammetry of adsorbed monolayers of carbon monoxide from these surfaces (0.5 M H2SO4, 25 °C, sweep range 0.025−0.70 V, 5 mV/s) showed a drop by 120 mV in the CO stripping peak potential upon deposition of 0.05 equiv of Ru adatoms on Pt. Deposition of more Ru adatoms did not cause significant further drops in the CO stripping peak potential. The surfaces were evaluated as catalysts for the electrooxidation of MeOH. The measured currents were normalized to the specific surface areas of the gauzes measured before deposition of Ru adatoms by hydrogenation of 1. The order of activity of MeOH-poisoned Pt−Ruad surfaces toward the potentiodynamic oxidation of MeOH (0.5 M H2SO4, 25 °C, sweep range 0.025−0.60 V, 5 mV/s) was 0.05 > 0.10 > 0.30 ∼ 0.44 > 0.7 > 0 (Pt) equiv of Ru adatoms. The activity of Pt increased relative to the other surfaces as the potential increased, becoming more active than 0.30, 0.70, and 0.44 equiv Pt−Ruad at the upper limit of the sweeps. Surfaces with low equivalents of Ru adatoms (from 0.05 to 0.10 equiv) were the most active toward the potentiostatic oxidation of MeOH (E = 0.4 V, 0.5 M H2SO4, 0.5 M MeOH, 25 °C) with between 50 and 28 times higher turnover numbers than black Pt. The activation energies for oxidation of MeOH over 0.05 and 0.10 equiv of Pt−Ruad were 37 and 45 kJ/mol, respectively, at 0.4 V, 0.5 M H2SO4, and 0.5 M MeOH.

Electrochemical doping of poly(vinyl chloride) obtained by photodehydrochlorination from laminated poly(vinyl chloride) + polypyrrole films

Abstract The poly(vinyl chloride) (PVC) in laminated PVC + polypyrrol (PPy) films has been converted to an electrically conducting structure by photodehydrochlorination and subsequent electrochemical doping. The maximum conductivity (6 × 10 −1 S cm −1 ) of the PVC film was obtained by potentiostatic oxidation at 0.3 V vs. SCE in HCl solution. The reflection spectra of the PVC film showed a new band around 600 to 800 nm after the electrochemical doping, indicating the formation of a charge-transfer complex. The maximum conductivity of the PVC film was reached with the highest concentration of the charge transfer complex, polyene + … X − , formed between the photogenerated polyenes and dopant anions.

An X-ray absorption fine structure study of the initial stages of the anodic oxidation of silver

The grazing incidence X-ray absorption fine structure technique was used for the in situ investigation of the initial stages of the anodic oxidation of silver electrodes in 1M NaOH. The reflectivity fine structure of the oxide covered electrode was measured for several potentials corresponding to different oxide thicknesses. The comparison of the experimental data with those of model calculations enabled the extraction of short-range order structural information about the anodic overlayer. Considerable changes of the reflectivity fine structure data are observed for silver(I) oxides formed at different potentials. In addition, the potentiostatic oxide layer formation as well as the electrochemical reduction causes significant changes of the X-ray reflectivity of the silver working electrode. Even the reduction of thin oxide layers of about 2–5 nm thickness causes a microroughening of the electrode surface.

Quick-Scanning EXAFS in the Reflection Mode as a Probe for Structural Information of Electrode Surfaces with Time Resolution: An in Situ Study of Anodic Silver Oxide Formation

Time-dependent extended X-ray absorption fine structure (QEXAFS) measurements using the total external reflection geometry were used for the in situ investigation of the potentiostatic silver oxide formation in 1 M NaOH. It is demonstrated that this EXAFS tool yielding near range order structural information is well suited for time-resolved studies of electrode surfaces under potential control. As examples, the initial growth stages of thin silver(I) oxide layers are presented as well as changes of the near range order of thick Ag2O films induced by potentiostatic transients. A comparison of the experimental results with those of model calculations permits the determination of the reaction kinetics of electrochemical phase formation.

Anodic oxidation of pyrrhotite in simulated CIP liquors

Natural pyrrhotite (Fe 7S 8) can be oxidized in alkali (pH 10) at 25°C at potentials above −0.2 V (SCE). The voltammetry of a ground pyrrhotite disc shows current peaks consistent with the formation of a ferric surface phase such as Fe(OH) 3 by air oxidation or by potentiostatic oxidation. In unbuffered solution, proton production can be demonstrated implicating the formation of sulfate, rather than sulfur as another product of reaction. In unbuffered or buffered solution, electrode rotation increases the initial oxidation rate due to the limiting diffusion of base in the hydrodynamic boundary layer between the bulk solution and the mineral surface. At longer reaction times parabolic kinetics apply suggestive of a limiting transport of base through the pores of the thickening product layer on the mineral surface. This layer persists in cyanide solution for conditions in which dissolved iron cyanide species are thermodynamically stable. Dissolved lead (1.5–3 ppm) decreases the oxidation rate probably due to lead sulphate precipitation within the porous layer. The oxidation rate of pyrrhotite is much lower than the cyanidation rate of gold for similar conditions. The most likely mechanism of pyrrhotite interference in carbon-in-pulp (CIP) gold processing plants involves the precipitation of gold on pyrrhotite driven by the oxidation of surface ferrous hydroxide to ferric hydroxide; and this mechanism is discussed using the data of Koch et al.