Influence of anion and pH on the electrochemical behaviour of polypyrrole synthesised in alkaline media

The electrochemical behaviour of conducting polypyrrole films electrosynthesised on vitreous carbon electrodes in alkaline solutions has been investigated by means of cyclic voltammetry and open circuit potentiometric measurements. The shape and evolution of the voltammograms during cycling depend strongly on the kind of anion and solution pH used. From these results, anion and cation participation in the redox process of the polymer has been analysed. The transport of anions like NO₃⁻ or Cl⁻ is dominant, while the transport of OH⁻ is the most important contribution when the polymer is cycled in the presence of anions that cannot be incorporated into the polymer matrix. The incorporation of Na⁺ also takes place and its contribution is more significant as deposition charge increases. The characteristics of the films are compared with those obtained in acid media. The polymerisation efficiency is similar in both cases. Films electrodeposited in alkaline solution can be doped and undoped in acid solution without loosing its redox behaviour.     

Electropolymerization of phenol on a vitreous carbon electrode in alkaline aqueous solution at different temperatures

Electrochemical oxidation of phenol in basic aqueous solution has been studied on a vitreous carbon electrode at different temperatures in the range of 25-85 °C by cyclic voltammetry and chronoamperometry techniques. The electrochemical oxidation of phenol led to a complete deactivation of the electrode, whatever the temperature used, as a result of the deposition of an adhesive and insulating polymeric film. The electrochemical activity of the electrode was progressively restored by repeated potential scans in the range of water stability only when conducted at high temperatures; electrode reactivation was explained by an increase in the polymeric film permeability for both electrons (electron tunneling) and phenol molecules (diffusion). Chronoamperometric measurements carried out in the potential region of water stability have shown that electrode passivation was reduced or prevented at high temperatures. For chronoamperometry performed at the onset of oxygen evolution, the electrode remained active even at low temperatures because the discharge of water involved the production of hydroxyl radicals that destructively oxidized the polymeric film. The effect of temperature on electrode reactivation was determined by the measurement of current at an electrolysis time of 300 s; an increase of the temperature from 25 to 85 °C amplified the current from 0.212 to 5.373 mA.     

Electrodeposition of gallium onto vitreous carbon

Gallium electrodeposition onto vitreous carbon was studied in chloride solution using potentiostatic and potentiodynamic techniques. The morphology and composition of the deposits were analysed by SEM/EDX. The effects of solution pH, applied potential and the hydrodynamic conditions on the electroreduction process were investigated. Results indicate that the reduction to Ga⁺¹ species takes place at the lower overpotentials, while at high overpotentials gallium deposition occurs through progressive nucleation and growth under diffusion control. The influence of the simultaneous hydrogen evolution reaction on the electrodeposition process is discussed. This paper is dedicated to the memory of Professor J. B. Bessone who died on 23rd April 2006     

Iron nucleation mechanisms on vitreous carbon during electrodeposition from sulfate and chloride solutions

Iron nucleation mechanisms from aqueous solutions onto vitreous carbon electrode were comparatively investigated in iron sulfate and iron chloride systems by utilizing the electrochemical techniques of cyclic voltammetry (cv) and chronoamperometry (ca), coupled with atomic force microscopy (AFM) studies. The investigated parameters were pH, scanning rate, iron concentration, deposition potential and temperature. It was found that iron nuclei population density decreased with increase of pH. On the other hand, the population density increased with increase of iron concentration and cathodic deposition potential. Increase of solution temperature resulted in the increase of nuclei population density in the sulfate system, while the dependence of nuclei population density on temperature in the chloride system was more complex. The experimental electrochemical data fitted the theoretical model describing progressive nucleation mechanisms, which was also confirmed by the AFM morphological studies. In addition, the atomic force microscopy was successful in determining the possible crystallographic orientations of electrodeposited iron nuclei.     

Synthesis, characterization and the electrocatalytic behaviour of nickel (II) tetraamino-phthalocyanine chemically linked to single walled carbon nanotubes

In this work we report on the synthesis, characterization and the electrochemical behavior of amide linked nickel (II) tetraamino-phthalocyanine (NiTAPc)-single walled carbon nanotube (SWCNT) nanomaterials (NiTAPc-SWCNT (linked)). UV-vis, XRD, IR and Raman spectroscopies were used in characterization whilst cyclic voltammetry was used to study the electrochemical behavior of NiTAPc-SWCNT (linked)-GCE. Relative to the bare glassy carbon electrode (bare-GCE), SWCNT-GCE, NiTAPc-GCE, and NiTAPc/SWCNT (mixed)-GCE, the NiTAPc-SWCNT (linked)-GCE gave the best current responses for the oxidation of 2-mercaptoethanol (2-ME). The catalytic rate constant is of the magnitude of 10³ M⁻¹ s⁻¹ while the detection limit (LOD) is 0.15 μM using the 3δ notation, with a sensitivity of 2.53 μA μM⁻¹ cm⁻².     

Electrochemical determination of bromide at a multiwall carbon nanotubes-chitosan modified electrode

A multiwall carbon nanotubes (MWNTs)-chitosan modified glassy carbon electrode (GCE) exhibits attractive ability for highly sensitive cathodic stripping voltammetric measurements of bromide (Br⁻). In pH 1.8 H₂SO₄ solution, a substantial increase in the stripping peak current of Br⁻ (compared to bare GCE and chitosan modified GCE) is observed using MWNTs-chitosan modified electrode. Operational parameters were optimized and the electrochemical behaviors of Br⁻ were studied by different electrochemical methods. The kinetics parameters were measured, the number of electron transfer (n) was 1 and the transfer coefficient (α) is 0.17. A wide linear calibration range (3.6 × 10⁻⁷-1.4 × 10⁻⁵ g mL⁻¹) was achieved, with a detection limit of 9.6 × 10⁻⁸ g mL⁻¹. The mechanism of electrode reaction was fully discussed.     

Electrocatalytic oxidation of hydrazine at a Co(II) complex multi-wall carbon nanotube modified carbon paste electrode

The catalytic oxidation of hydrazine was investigated by a cobalt(II) bis (benzoylacetone) ethylenediimino multi wall carbon nanotube-modified carbon paste electrode (Co(II)BBAEDI-MWCNT-MCPE) as a highly sensitive electrochemical sensor. The effect of variables such as pH and modifier percent on cyclic voltammograms peak current was optimized. The modified electrode showed very efficient electrocatalytic activity for anodic oxidation of hydrazine in 0.1 M phosphate buffer solution (pH 7.0). Anodic peak potential of hydrazine oxidation at the surface of modified electrode shifts by about 500 mV toward negative values compared with that on the bare electrode. The diffusion coefficient and electron transfer coefficient of hydrazine were obtained using electrochemical approaches. The Co(II)BBAEDI-MWCNT-MCPE showed good reproducibility (RSD < 3.3%). The electrocatalytic current increased linearly with the hydrazine concentration in the range of 0.3–70.0 μM and detection limit was 0.1 μM. The effect of various interferences on the hydrazine peak current was studied. This method was applied to determine hydrazine in water samples.     

Powdered activated carbon and carbon paste electrodes: comparison of electrochemical behaviour

Commercial activated carbon (Norit R3ex), de-mineralised with conc. HF and HCl, was oxidised (conc. HNO₃) and heat-treated at various temperatures (180, 300 and 420 °C). The physicochemical properties of the samples obtained were characterised by selective neutralisation and pH-metric titration of surface functional groups (acid–base properties), thermogravimetry (thermal stability—TG), FTIR spectroscopy (chemical structure) and low-temperature nitrogen adsorption (BET surface area). Thermal treatment of the carbon materials caused the surface functional groups to decompose; in consequence, the chemical properties of the carbon surfaces changed. Cyclic voltammetric studies were carried out on all samples using a powdered activated carbon electrode (PACE) and a carbon paste electrode (CPE), as were electrochemical measurements in aqueous electrolyte solutions (0.1 M HNO₃ or NaNO₃) in the presence of Cu²⁺ ions acting as a depolariser. The shapes of the cyclic voltammograms varied according to the form of the electrodes (powder or paste) and to the changes in the surface chemical structure of the carbons. The electrochemical behaviour of the carbons depended on the presence of oxygen-containing surface functional groups. The peak potentials and their charge for the redox reactions of copper ions depended on their interaction with the carbon surface.     

Electrochemical characterisation of tetra- and octa-substituted oxo(phthalocyaninato)titanium(IV) complexes

The synthesis and electrochemical characterisation of the following oxotitanium tetra-substituted phthalocyanines are reported: 1,(4)-(tetrabenzyloxyphthalocyaninato)titanium(IV) oxide (5a); 1,(4)-{tetrakis[4-(benzyloxy)phenoxy]phthalocyaninato}titanium(IV) oxide (5b); 2,(3)-(tetrabenzyloxyphthalocyaninato)titanium(IV) oxide (6a) and 2,(3)-{tetrakis[4-(benzyloxy)phenoxy]phthalocyaninato}titanium(IV) oxide (6b). The electrochemical characterisation of complexes octa-substituted with 4-(benzyloxy)phenoxy (9b), phenoxy (9c) and tert-butylphenoxy (9d) groups is also reported. The cyclic voltammograms of the complexes exhibit reversible couples I-III and couple IV is quasi-reversible for complexes 5a, 5b, 6a and 6b. The first two reductions are metal-based processes, confirmed by spectroelectrochemistry to be due to Ti^IVPc²⁻/Ti^IIIPc²⁻ and Ti^IIIPc²⁻/Ti^IIPc²⁻ redox processes and the last two reductions are ring-based processes due to Ti^IIPc²⁻/Ti^IIPc³⁻ and Ti^IIPc³⁻/Ti^IIPc⁴⁻. Chronocoulometry confirmed a one-electron transfer at each reduction step. The electrochemistry of the above complexes is also compared to the previously reported 5c, 5d, 6c and 6d.     

Application of elimination voltammetry in the study of electroplating processes on the graphite electrode

The electrode reaction mechanism of electrolytical coating by nickel on paraffin impregnated graphite electrode (PIGE) was investigated by cyclic voltammetry (CV) and elimination voltammetry with linear scan (EVLS). The EVLS, a relatively new method of processing electrochemical signals obtained by voltammetry, is able to eliminate some individual chosen currents from total voltammetric currents measured at different scan rates. During the electrodeposition of metals on the graphite electrode, hydrogen evolved from aqueous acidic solutions interferes with the plating process. The elimination of kinetic current arising due to hydrogen evolution enables one to study other processes proceeding at the electrode. Cyclic voltammograms for metal coating deposition/dissolution on the graphite electrode were measured at three scan rates (12.5, 25 and 50 mV/s) and the EVLS functions were calculated for one or two eliminated currents. The results indicate the occurrence of surface reactions with the adsorption of intermediates on graphite. The application of EVLS provides deeper insight into the mechanism of electrode reaction during metal deposition.     

Electrochemical and AFM study of cobalt nucleation mechanisms on glassy carbon from ammonium sulfate solutions

Nucleation mechanisms of cobalt on a glassy carbon electrode (gce) from aqueous ammonium sulfate solutions were investigated through the electrochemical techniques of cyclic voltammetry (cv) and chronoamperometry (ca), coupled with atomic force microscopy (AFM) studies. The studied parameters were pH, cobalt concentration, temperature, scanning rate, and deposition potential. It was found that scanning in the cathodic direction produced two peaks, corresponding to cobalt and hydrogen reduction, respectively. Scanning in the anodic direction was characterized by cobalt dissolution, which was interrupted by formation of cobalt hydroxide, causing a second anodic peak. The amperometric study found progressive nucleation mechanisms, in contrast to the instantaneous nucleation mechanisms determined by the AFM study. An explanation for the contradictory nucleation mechanisms shown in the two studies is provided.     

Electrochemical properties of double wall carbon nanotube electrodes

Electrochemical properties of double wall carbon nanotubes (DWNT) were assessed and compared to their single wall (SWNT) counterparts. The double and single wall carbon nanotube materials were characterized by Raman spectroscopy, scanning and transmission electron microscopy and electrochemistry. The electrochemical behavior of DWNT film electrodes was characterized by using cyclic voltammetry of ferricyanide and NADH. It is shown that while both DWNT and SWNT were significantly functionalized with oxygen containing groups, double wall carbon nanotube film electrodes show a fast electron transfer and substantial decrease of overpotential of NADH when compared to the same way treated single wall carbon nanotubes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Reduction process of uranium(IV) and uranium(III) in molten fluorides

This study focused on the electroreduction process of uranium cations in molten fluorides. It involved cyclic voltammetry, chronopotentiometry with and without current reversal, and square wave voltammetry.The results indicate a two-step reduction process for uranium(IV). The first step U(IV)/U(III) exchanging one electron corresponds to a soluble/soluble system and is limited by U(IV) diffusion with D_U(IV) = 1.25 ± 0.35 × 10⁻⁵ cm² s⁻¹ in LiF-NaF at 720 °C.In order to perform a thorough study of the second step U(III)/U(0) in the reduction process, the melt was chemically reduced in U(III) with U metal as reducing agent. Alternatively to the use of LiF-NaF where U metal is unstable at 720 °C, the chemical reduction of U(IV) in U(III) was performed in a LiF-CaF₂-UF₄ solution containing U metal at 810 °C. It has been confirmed that the reduction of U(III) proceeds in one step exchanging three electrons and by a diffusion controlled process with D_U(III) = 2.2 ± 0.7 × 10⁻⁵ cm² s⁻¹ in LiF-CaF₂ at 810 °C.     

核黄素(VB2)在液体石蜡碳糊电极上的伏安特性及脉冲伏安法测定

以液体石腊碳糊电极为工作电极,在0.1 mol/L NaCl(pH=2.5)底液中研究VB2的伏安特性.VB2有一对氧化还原峰,峰电位为氧化峰(I)-110 mV/还原峰(Ⅱ)-160 mV,结果表明溶液中VB2的电极过程主要受VB2表面吸附控制.VB2溶液受光照后,增加一对光色素的氧化还原峰,其峰电位为氧化峰(Ⅳ)-220mV/还原峰(Ⅲ)-280 mV.采用微分脉冲伏安法进行VB2的定量分析,线性范围为1×10-5～6.0×10-8moL/L,检测下限为2.00×10-8moL/L.测定复合VB药片中的VB2,RSD为2.1%,回收率为96.8～104%,其结果与药典中标准方法测定结果一致.     

Electrochemical reduction of 2-ethyl-9,10-anthraquinone (EAQ) and mediated formation of hydrogen peroxide in a two-phase medium Part I: Electrochemical behaviour of EAQ on a vitreous carbon rotating disc electrode (RDE) in the two-phase medium

The electrochemical reduction of 2-ethyl-9,10-anthraquinone (EAQ) has been examined by voltammetry on a vitreous carbon rotating disk electrode (RDE) in a two-phase medium. The medium consisted of an organic phase consisting of a mixture of tributylphosphate (TBP) and diethylbenzene (DEB) (15/85,vol/vol) and an aqueous phase, 2m NaOH. The ratio between the two phases was 40% for the organic phase and 60% for the aqueous phase. The electrochemical behaviour of EAQ in this medium was examined on a vitreous carbon RDE in the presence and absence of oxygen. The formation of hydrogen peroxide mediated by the electrochemical reduction of EAQ was studied analytically. The voltammetric results indicate that hydrogen peroxide formation mediated by the electrochemical reduction of EAQ is feasible.     

Electrochemical synthesis of Cr(II) at carbon electrodes in acidic aqueous solutions

The electrochemical synthesis of Cr(II) has been investigated on a vitreous carbon rotating disc electrode and a graphite felt electrode using cyclic voltammetry, impedance spectroscopy and chronoamperometry. The results show that in 0.1 M Cr(III) + 0.5 M sulphuric acid and in 0.1 M Cr(III) + 1 M hydrochloric acid over an electrode potential range of –0.8 to 0.8 V vs SCE, the electrochemical reaction at carbon electrodes is essentially a surface process of proton adsorption and desorption, without significant hydrogen evolution and chromium(II) formation. At electrode potentials more negative than –0.8 V vs SCE, both hydrogen evolution and chromium(II) formation occurred simultaneously. At electrode potentials –0.8 to –1.2 V vs SCE, the electrochemical reduction of Cr(III) on carbon electrodes is controlled mainly by charge transfer rather than mass transport. Measurements on vitreous carbon and graphite felt electrodes in 1 M HCl, with and without 0.1 M CrCl₃, allowed the exchange current density and Tafel slope for hydrogen evolution, and for the reduction of Cr(III) to Cr(II), to be determined. The chromium(III) reduction on vitreous carbon and graphite electrodes can be predicted by the extended high field approximation of the Butler–Volmer equation, with a term reflecting the conversion rate of Cr(III) to Cr(II).     

Electrochemical and corrosion studies of poly(nickel-tetraaminophthalocyanine) on carbon steel

Nickel-tetraaminophthalocyanine [TAPcNi] was electropolymerized from the complex monomeric solution, onto carbon steel substrates, yielding thin adherent films of poly[TAPcNi]. The investigation of such polymer-modified electrodes was carried out by means of cyclic voltammetry, UV-vis spectroscopy, FT-IR spectroscopy and spectroelectrochemistry. The preparation of TAPcNi modified electrode was also carried out by electropolymerization of a preformed molecular film of TAPcNi, after applying a drop of TAPcNi dimethylsulphoxide solution onto carbon steel, and allowing it to dry. The comparison of the corrosion behavior of the two types of polymer-coated electrodes was carried out by electrochemical impedance spectroscopy (EIS) in acid medium. It was found that the structure and morphology of each polymer greatly influence their redox behavior and corrosion inhibition performance for steel in hydrochloric acid. The film prepared by the drop-dry method offered a better corrosion protective efficiency while the electropolymerized film presented a more conductive behavior.     

Electrochemical behaviour of haemoglobin at the liquid/liquid interface

The electrochemical behaviour of haemoglobin (Hb) at the liquid/liquid interface was investigated. Based on ion transfer cyclic voltammetry and bulk ionolysis experiments, it is proposed that Hb adsorbs at the liquid/liquid interface and forms a multi-layer deposit thereafter repetitive cycling. However Hb does not cross the interface. The transfer peak current measured increased non-linearly with the bulk concentration of Hb in the aqueous phase but it did vary linearly with the square root of the scan rate. This diffusion-controlled process is ascribed to the Hb-facilitated transfer of the anion of the organic phase electrolyte. The electrochemical signal also increases with time suggesting a conformational re-arrangement of Hb at the liquid/liquid interface. The transfer peak potential was seen to vary with the nature of the anion, in agreement with the trend in hydrophobicity of the anions examined. Furthermore, at pH values ≥pI of Hb, no transfer peak was observed, suggesting that neutral or negatively charged Hb does not facilitate the transfer process.     

Electrochemical oxidation of benzene at a glassy carbon electrode

Benzene oxidation in sulfuric acid at a glassy carbon electrode was investigated using voltammetric, chronoamperometric, and spectroscopic methods. The results are compared with those at a Pt electrode. Benzene was observed to be oxidized to benzoquinone presumably by active oxygen that was adsorbed on the GC electrode in the oxygen evolution region. It is concluded that oxidation at glassy carbon can produce benzoquinone or quinone-like compounds from an aqueous benzene solution. The applied potential for benzene oxidation should be less than 2.1 V vs RHE in order to prevent glassy carbon electrode damage by oxidation during long operation.     

Synthesis and characterization of electrocatalytic conjugates of tetraamino cobalt (II) phthalocyanine and single wall carbon nanotubes

In this paper we report on the synthesis and characterization of electrocatalytic conjugates of tetraamino cobalt (II) phthalocyanine and single walled carbon nanotubes (CoTAPc–SWCNT-linked) for use as electrode surface modifiers. FTIR, UV–vis and Raman spectroscopies were used to ascertain the chemical linkage between CoTAPc and SWCNT while cyclic voltammetry and rotating disk electrode voltammetry were used to assess the electrocatalytic efficiency of the linked product towards the oxidation of 2-mercaptoethanol. The CoTAPc–SWCNT-linked-GCE demonstrated very good catalytic efficiency relative to CoTAPc–SWCNT-mixed-GCE, CoTAPc-GCE and f-SWCNTs-GCE (functionalised SWCNT). CoTAPc–SWCNT-linked-GCE gave a sensitivity of 0.2 μA/μM and a limit of detection (LOD) of 1.2 × 10⁻⁷ M for 2-mercaptoethanol (2-ME) at pH 4.