PbTe electrodeposition studied by combined electrochemical quartz crystal microbalance and cyclic voltammetry

Electrodeposition of PbTe thin films from alkaline solutions, and the electrochemical behavior of the related precursors TeO₃^2? and PbEDTA^2?, were studied for the first time by means of an electrochemical quartz crystal microbalance (EQCM) combined with cyclic voltammetry. PbTe was found to form by an induced codeposition mechanism via six electron reduction. The reduction mechanism of Te was complicated and sensitive to the substrate surface. On the other hand, the reduction of the PbEDTA^2? complex to Pb⁰ was a simple two electron reaction.     

Electrochemical characterization of polyethylene glycols as solid polymer electrolytes

Polymer electrolytes prepared from polyethylene glycol (PEG)-LiClO₄ complexes have been characterized at a stainless steel electrode using cyclic voltammetry, chronoamperometry, and ac impedance techniques. The charge transfer process appeared to be affected by the oxide film on the stainless steel electrode surface in the early stages of redox processes. With the electrode surface cleaned off by reduction of oxide films with electrogenerated lithium, very well defined, chemically reversible voltammograms were obtained for Li⁺ reduction. Different diffusion properties were observed in three different time zones in potentiostatic experiments. An exchange current density has been determined from the Tafel plot for the Li⁺ reduction using impedance data. The conductivity of the PEG-10000 electrolyte has been determined to be 4.7×10^?5 S cm^?1. This value is higher than that of the corresponding polyethylene oxide electrolyte by about two orders of magnitude, which is attributed to the anionic end groups increasing the polarity of the matrix.     

Scanning electrochemical microscopy (SECM): Study of the formation and reduction of oxides on platinum electrode surfaces in Na2SO4 solution (pH = 7)

The formation and reduction of oxides on polycrystalline platinum were studied in a neutral solution with a scanning electrochemical microscope (SECM). Experiments were carried out with tip—substrate voltammetry where the faradaic current flowing to the tip is recorded while cycling the potential of the substrate, and with tip—substrate chronoamperometry where the faradaic tip current is recorded against time following the application of a potential step to the substrate. The tip current was made pH sensitive by holding the tip potential in a region where a pH dependent reaction occurs. Hydrogen evolution was used to probe pH decreases, oxygen evolution was used for the detection of pH increases and Pt oxide formation was used to detect both pH increases and decreases. The results showed that oxide formation occurs in two stages, each involving the transfer of electrons and the release of protons into the solution. During the first stage the release of H⁺ precedes the transfer of electrons. while in the second stage H⁺ release and electron transfer proceed simultaneously. Results are analysed in terms of the formation of PtOH during stage I and PtO during stage 2. However stage 2 behaves differently under slow potential changes and the release of protons lags behind the transfer of electrons. This is interpreted as the result of a place-exchange mechanism from PtOH to HOPt prior to stage 2, followed by the oxidation of HOPt to OPt during stage 2. Similarly, Oxide reduction was found to occur in two stages, each involving the transfer of electrons and the consumption of protons. During the first stage, the consumption of H⁺ precedes the transfer of electrons. The results suggest that during the transfer of electrons, protons diffuse from the outer layer of the oxide (OPt) into the inner layer to form HOPt. For the second stage results are analysed in terms of a place exchange mechanism from HOPt to PtOH running in parallel with the consumption of H⁺ followed by a surface reduction from PtOH₂⁺ to Pt metal.     

Effect of pH and temperature on carbon-supported manganese oxide oxygen reduction electrocatalysts

Manganese oxides nanoparticles were chemically deposited on a high area (ca. 300 m² g⁻¹) carbon black substrate to act as electrocatalysts for oxygen reduction. The morphology and chemistry of the carbon-supported MnO_x nanoparticles was characterised by Transmission Electron Microscopy), X-ray Diffraction, and chemical analysis. The oxygen reduction reaction (ORR) catalytic activity was studied in the 7–10 pH range using a rotating disk electrode (RDE). High activity towards oxygen reduction and very good stability in neutral and slightly basic solution were obtained. At low current densities, at 25 °C, MnO_x/C displayed a reaction order with respect to OH⁻ ions of −0.5 and Tafel slopes of −0.153 and −0.167 V dec⁻¹ at pH 7 and 10 respectively; showing that the ORR mechanism on MnO_x/C is unchanged in the 7–10 pH range. From the data, we propose that the first electrochemical step of the 4-electron ORR mechanism, in the 7–10 pH range, is the quasi equilibrium proton insertion process in MnO₂ yielding MnOOH (insoluble in neutral or slightly basic solution). The ORR activity of the MnO_x/C materials increased with increasing temperatures from 5 to 40 °C. The 2-electron pathway of oxygen reduction, yielding hydrogen peroxides as intermediates, may however be favoured over the 4-electron O₂ reduction at higher temperatures.     

EIS study of solid-state transformations in the passivation process of bismuth in sulfide solution

Anodic oxidation of polycrystalline bismuth in alkaline medium, containing sulfide ions was investigated in situ. Cyclic voltammetry was used to define the potential regions of formation and stability of anodic Bi₂S₃ and Bi₂O₃ semiconductor films that translate bismuth to the passive state. The mechanism of elementary steps of the passivation process has been investigated using electrochemical impedance spectroscopy (EIS). The electric and dielectric properties of anodic films and structural parameters of the interfaces Bi–growing film–electrolyte in a wide region of potentials and frequencies of six decades, were determined. The EIS data have been analyzed and discussed in the frame of the point defect model (PDM) of anodic film formation and growth. The growth of passive surface films on bismuth takes place via transport of anionic vacancies generated at the metal–film interface. The slow step of the process is the layer-limited diffusion of anionic vacancies (D≈ 10^?16 cm² s^?1).Solid-state transformation of sulfide to the oxide film is a consequence of OH^? ion capture into the anionic vacancies of the sulfide film, the generation and transport of cation vacancies from the film–solution interface, their annihilation and formation of a vacancy condensate of a critical size at the metal–film interface.     

Self-protonation mechanism in the electrochemical reduction of Jatropholone

The electrochemical reduction mechanism of jatropholone (JOH) and derivatives (JOAc) was investigated by cyclic voltammetry, polarography, coulometry and controlled potential electrolysis. It involves at the first wave potential, a self-protonation mechanism, whereby the two-electron reduction product (JOHH^?), an aromatic ketone, is formed together with the conjugate base of the former compound, JO^?. The unusual stoichiometry of the cathodic process at the first wave, 1 mol electron mol^?1, indicates that JOHH^? is not further protonated. The decreased basicity of the anion is due to resonance stabilisation. The second wave is related to the reduction of JO^? and further reduction of JOHH^?, that has a reminiscent electroactive group. In the latter case, the background electrolyte is liable to be the proton donor, through Hoffman elimination. The complete reduction involves 4 e^? and 4 protons. Electrolysis performed near the first wave potential led to the almost exclusive reduction of the exocyclic double bond, without dimerization.     

Investigation of the electrochemical reduction mechanism of 1-[N-(2-pyridyl)aminomethylidene]-2(1H)-naphthalenone (PN)

The electrochemical reduction mechanism of 1-[N-(2-pyridyl)aminomethylidene]-2(1H)-naphthalenone (PN) was investigated by using various electrochemical techniques in 0.1 M tetrabutylammonium tetrafluoroborate (TBATFB) in acetonitrile at glassy carbon (GC) electrode. The number of electrons transferred and diffusion coefficient of the compound were estimated by using an ultramicroelectrode (UME). PN has two reduction peaks in a cyclic voltammogram. Each of them corresponds to a one-electron transfer. In this medium and at the GC electrode surface, the first peak was observed at about ?1.8 V (vs. Ag/Ag⁺) which is more stable and well defined compared to the second peak. It was determined by using cyclic voltammetry that PN is electroreduced by an EC mechanism. It was also explained by multicycle CV experiments and a dopamine test as well as by Raman spectroscopy that the homogenous chemical reaction is dimerization. The EC mechanism was also examined and the kinetics of this process were calculated by digital simulation.     

One-step electrodeposition of ZnO/eosin Y hybrid films from a hydrogen peroxide oxygen precursor

The deposition of eosin Y/zinc oxide hybrid films is performed by cathodic electrodeposition from a hydrogen peroxide oxygen precursor in chloride medium. Typically the deposition bath contains 5 mM ZnCl₂, 10 mM H₂O₂ and less than 10 μM eosin Y. A rotating disk substrate is used with a rotation speed of 500 rpm. The deposition potentials range between ?1.25 and ?1.50 V versus the mercurous sulfate electrode (MSE). The resulting films are well crystallised and highly textured with the c axis perpendicular to the substrate. They exhibit an electrocatalytic activity for hydrogen peroxide reduction during the deposition. The role of reduced eosin bound to ZnO in the electrocatalysis is discussed. We have measured a maximum of dye loading at 0.21 M and we show that the dye is concentrated in the film compared to the dye/zinc ion molecular ratio in solution. The dye loading decreases slightly with deposition overvoltage. We discuss the loading mechanism in the light of the dye diffusion behaviour in solution and of the relationship between dye loading and film growth rate. The chemical diffusion coefficient of eosin Y under the present conditions is measured an equal to 1.9×10^?5 cm² s^?1. We show that the complexation between reduced eosin Y and free zinc ion in solution and the coprecipitation reaction of this complex are likely to be the key steps of the loading process.     

Competitive adsorption versus surface complexation as models for the simultaneous adsorption of metal complexes and free ligands

A statistical thermodynamical approach to the study of anion-induced adsorption of Cd(II) from halide solutions is presented. The simultaneous adsorption of metal complex and ligand is introduced in the isotherms by considering two possible mechanisms — competitive adsorption and surface complexation. These isotherms have been tested for the system Cd(II) in KBr at several ionic strengths. The experimental surface excesses of Cd(II) calculated from single-step chronocoulometry can be simulated, giving an explanation for the desorption of the metal complex at positive potentials. Also, the change in ligand adsorption promoted by the adsorption of the metal complex has been calculated. Both approaches lead to the conclusion that the anionic tricoordinate metal complex CdBr₃^? and the tetracoordinate CdBr₄^2? are the absorbed species on the electrode surface, with CdBr₄^2? dominating at higher bromide concentrations.     

Evidence of the withdrawing effect of a pyridinium N-cation on the electrochemical reduction of gold (III) tetraphenylporphyrin

1-(Au^III-meso-tetraphenylporphyrin)-4-pyridinium dication Au^IIITPP⁺-β-Py⁺, formed by a pyridinium cation bearing a charged gold porphyrin at the N-position, has been synthetised and studied by stationary voltammetry, cyclic voltammetry, UV–Vis spectroelectrochemistry and exhaustive coulometry. The pyridinium cation and the porphyrin ring are both electroactive species on the investigated range of potentials. The obtained results allowed to discriminate the sites of the different charge transfers and to propose a mechanism for the three first electrochemical reduction processes involved. The gold porphyrin reduces before the reduction of the pyridinium cation whose signal is intercalated between two reduction steps of the porphyrin. A transient dimeric gold porphyrin is detected. The withdrawing effect of the pyridinium cation on the reduction potentials of the porphyrin is discussed and quantified.     

Evidence of self-protonation on the electrodic reduction mechanism of an anti-Helicobacter pylori metronidazole isotere

The electrochemical reduction mechanism of 1-(2-ammoniumethyl)-2-methyl-5-nitroimidazole bromide (2) in DMSO + 0.1 mol l^?1 TBAP has been investigated by cyclic voltammetry and macroscale electrolysis, on a glassy carbon electrode, in comparison with metronidazole (1). The cyclic voltammogram of 2 is represented by three reduction waves, one of them at less negative potential, when compared to the first wave of metronidazole which indicates that it undergoes easier reduction. There is evidence for a self-protonation mechanism in the electroreduction of 2, represented by the absence of the first wave in the successive cyclic voltammogram, by the disappearance of the first reduction wave upon addition of base and increase of the same wave in the presence of exogenous proton donors. The stoichiometry of the reaction, at the first reduction wave, involves 0.8 mol electron mol^?1 and yields 0.2 mol of 4e^?/4H⁺ reduced derivative (probably an unstable hydroxylamine) and 0.8 mol of the amine derivative, the conjugated base of 2. The second and third waves are typical for nitroaromatic reduction and are related to the reduction of the nitro group in this aminoderivative.     

Oxygen reduction mechanism on corroded zinc

The mechanism of oxygen reduction on the as-polished and corroded zinc specimens has been studied using a rotating ring disc electrode (RRDE) system. On the as-polished surface, oxygen was reduced into two distinct steps. In the first step, about 44% of O₂ was reduced to H₂O₂ in a 2-electron reaction with the rest being reduced to OH^? in a 4-electron reaction. On the other hand, in the second step, with the increase of overpotential O₂ was almost exclusively reduced to OH^? in a 4-electron reaction. The first step reduction occurred on an air-formed oxide-covered surface at more positive potential than ?1.2 V vs. Ag/AgCl and the second step reduction (E < ?1.2 V) took place on a semi-uniformly active surface. On the corroded surface, the second step was not distinctly observed on the polarization curve, because reduction of the zinc corrosion products simultaneously took place around ?1.2 V. The O₂ reduction in the first step was inhibited by deposition of the corrosion products, though the ratio of amount of O₂ reduced to OH^? in a 4-electron reaction was larger than that on the as-polished surface. The mechanism of oxygen reduction is discussed on the basis of results obtained from the RRDE experiment.     

Microgravimetric monitoring of transport of cations during redox reactions of indium(III) hexacyanoferrate(III,II): Radiotracer evidence for the flux of anions in the film

We demonstrate here that, primarily, electrolyte cations but also, to some extent, anions are capable of penetrating indium hexacyanoferrate films during redox reactions. We find from electrochemical quartz crystal microbalance measurements that the electrolyte cation (K⁺) undergoes sorption and desorption during the system's reduction and oxidation, respectively. The formal potential, which has been determined from the system's well-defined voltammetric peaks recorded with the use of an ultramicroelectrode, decreases ～40 mV per decade of decreasing K⁺ concentration. The latter value is lower than the 60 mV change expected for the involvement of a cation in the reaction mechanism according to the ideal Nernstian dependence. We also demonstrate, using 35-S labelled sulfate, that anion penetrates the reduced film and its concentration markedly increases during oxidation. Careful examination of cyclic voltammetric responses of the system shows that, in addition to the well-defined peaks, capacitance-like currents appear during oxidation. During reduction anion is largely expelled from the film. This complex ionic penetration and transport in indium hexacynoferrate may be explained in terms of formation of two forms: ‘soluble’, KIn^III[Fe^II(CN)₆], and ‘insoluble’ (‘normal’), In^III₄[Fe^II(CN)₆]₃, during the electrochemical growth or potential cycling of the films. These forms would require cations and anions, respectively, to provide charge balance during reactions. Regardless of the actual mechanism, penetration of anions cannot be neglected completely in the discussion of charging of indium hexacyanoferrate.     

Characterization of lapachol in artificial organic-film membrane: Application for the trans-membrane transport of Mg

Species transfer across a liquid|liquid interface is studied by means of a thin film-modified electrode using cyclic voltammetry and square-wave voltammetry. The thin film-modified electrode consists of an edge plane pyrolytic graphite electrode (EPG) covered with a thin film of a water-immiscible electro-inactive organic solvent (nitrobenzene) containing a neutral redox probe and/or a suitable electrolyte. For this study we used, as redox probe in the organic phase, 2-hydroxy-3-isopropenyl-1,4-naphthoquinone also known as lapachol (Q) and an appropriate electrolyte. The redox transformations of Q at the graphite electrode/organic (EG|NB) interface was coupled to an ion-transfer reaction from aqueous to organic phase. The proton transfer at the nitrobenzene/water (NB|W) interface is essential for the electrochemical conversion of Q within the membrane. The voltammograms obtained are influenced by the pH of the aqueous phase. Q has two reduction systems due to the redox transformation of its two tautomeric forms resulting from the migration of a proton between the hydroxyl group in position 2 and the carbonyl group in position 4. The electrochemical mechanism consist of 2e⁻/2H⁺ exchange to form the separate redox compounds H₂Q. The experiments conducted reveal the ability of both tautomers to form 1:1 complexes with Mg²⁺ when this cation is present in the aqueous phase.     

An investigation of the adsorption of pyrazine and pyridine on nickel electrodes by in situ surface-enhanced Raman spectroscopy

This paper describes a surface-enhanced Raman spectroscopic study on nickel electrodes. The enhancement factor of the roughened Ni substrate was found to be from two to three orders, depending on the surface pre-treatment. High-quality SERS spectra of pyrazine and pyridine were obtained at different potentials. The bands assigned to Ni–N_pyrazine and Ni–N_pyridine were observed around 260 cm^?1 in the low-frequency region of two sets of SERS spectra, respectively. The adsorption behaviour of pyrazine and pyridine on Ni electrode was studied and compared based on the surface selection rule. The result suggests that both pyrazine and pyridine were strongly adsorbed onto the substrates. It also implies that pyridine was adsorbed perpendicularly onto the substrate, while pyrazine adsorbed onto the substrate in a slightly tilted vertical configuration. The observation of forbidden bands of the pyrazine molecule is due to the lowering of the symmetry from D_2h to C_2V when adsorption onto the surface takes place. The electrochemical reduction of pyrazine on the Ni electrode was observed at extremely negative potentials in the presence of Cl^?.     

Oxygen reduction reaction kinetics and mechanism on platinum nanoparticles inside Nafion®

The aim of this work is to study kinetic parameters and the mechanism of the oxygen reduction reaction (orr) on platinum nanoparticles supported on carbon, inside Nafion^® (i.e. in PEMFC cathode conditions). Stationary and electrochemical impedance spectroscopy techniques were used to measure exchange current densities, Tafel slopes, and reaction orders with respect to O₂ pressure and H⁺ activity. The platinum nanoparticle size effect was confirmed. A specific low frequency inductive behaviour of the cathode impedance was observed. The latter demonstrates the presence of (at least) two electrochemical steps in the orr mechanism. Secondly, dc and ac modelling of the reaction in a gas diffusion electrode is proposed in order to simulate current–potential curves and impedance spectra. This paper reveals that an ECE mechanism for oxygen reduction proposed by Damjanovic and coworkers on bulk platinum in acidic medium is also valid for that on Pt nanoparticles.     

The influence of nitrate concentration and acidity on the electrocatalytic reduction of nitrate on platinum

A study was performed to determine the influence of nitrate concentration and acidity on the reaction rate and selectivity of the electrocatalytic nitrate reduction on platinum. There are two different nitrate reduction mechanisms on platinum: a direct mechanism (0.4–0.1 V vs. SHE) and an indirect mechanism (0.9–0.5 V vs. SHE). In the direct mechanism the dependence of the reaction rate on the nitrate concentration changes with increasing nitrate concentration. Whereas at low concentrations (<0.1 M) the reaction order in nitrate is positive, at high concentrations (>0.1 M) the reaction order is negative. This suggests that at high concentrations the amount of free surface sites determines the reaction rate. These free surface sites are needed either for the adsorption of a second species necessary for the reaction (water or hydrogen) or for the dissociation of nitrate to nitrite. Both at low and high nitrate concentrations the direct reduction is mainly selective towards ammonia, although small amounts of N₂O and N₂ were observed using differential electrochemical mass spectrometry (DEMS) at potentials between 0.4 and 0.2 V at high concentrations of nitrate. This N₂ and N₂O formation seems to be related to the NO_ads coverage on the electrode. The indirect reduction mechanism is autocatalytic as is illustrated by its unusual stirring behavior. Large amounts of NO were observed using DEMS. This suggests that not nitrate but NO⁺ (? HNO₂) is involved in the actual electron transfer. NO⁺ is reduced to NO, which then reacts with HNO₃ to reproduce NO⁺, resulting in an overall reaction of nitrate to nitrite. Both nitrite and a high acidity are needed for this mechanism to develop, but addition of nitrite is not necessary since nitrite is present in small amounts in HNO₃ solutions of concentrations over 4 M. The autocatalytic reduction mechanism slows down and eventually terminates when NO starts to react to N₂O, as was observed using DEMS.     

Reduction of silver complexes: towards an ever more elaborated insight in the influence of the type of ligand on the reaction rate

The present work deals with the investigation of the electrochemical reduction of silver thiosulphate (1,2-Ag(S₂O₃)₂^3?), thiocyanate (1,3-Ag(SCN)₃^2?) and 1,8-dihydroxy-3,6-dithiaoctane (1,2-Ag(DTO)₂⁺) complexes. The influence of the ligand type on the charge transfer rate is explained by the changing positions of the density distributions of electronic energy levels of the three complexes. The basics for this approach are the theories of energy band models (EBMs). An experimental methodology is developed to determine the energy density distributions. A Ti/TiO₂ substrate, obtained by galvanostatically anodising Ti, is put forward as an appropriate substrate for this investigation, and its semiconducting properties are determined. On this substrate, charge transfer (CT) controlled currents can be measured in a sufficiently large potential domain for the three systems. A method of pre-plating is optimised such that the overall semiconducting character of the substrate is kept during the monitoring of the (quasi-)stationary current/voltage diagrams. The active surface areas, necessary for the calculation of the current density/potential curves, are calculated. The positions of the energy density distributions, obtained by the derivation of the current density/voltage diagrams, of the three complexes, show that thiosulphate exhibits the smallest density of accepting energy levels in the given potential domain. For potentials above 0.5 V vs. SCE, the DTO complex has the largest density of vacant energy levels, but for lower potentials the situation is reversed.     

Electrochemical behaviour of 2-(4′-hydroxybenzeneazo)benzoic-acid at pyrolytic graphite electrodes

The electrochemical reduction of 2-(4′-hydroxybenzeneazo)benzoic acid (I) has been studied at pyrolytic graphite electrodes in the pH range 2.0–10.4. The cyclic voltammetric behaviour clearly indicated an ECE mechanism in acidic medium in which the two-electron two-proton reduction of I gives the hydrazo derivative. The acid catalysed disproportionation of the hydrazo intermediate was also studied in the pH range 2.0–6.0 and the value of k′/[H⁺] was found to be 1.4 × 10^?2 1 mol^?1 s^?1 The products of the reduction have been isolated and characterized using IR, melting point, mass and related techniques.     

Nitrite reduction in acidic solution at a GC/Eastman-AQ-Os(bpy)32+-PVP composite modified electrode

The negatively charged polymer polyester sulfonic acid (Eastman-AQ, abbreviated to AQ), positively charged polymer polyvinyl pyridine (PVP) and mediator Os(bpy)₃²⁺ were used to construct composite modified glassy carbon (GC) electrodes (abbreviated to GC/AQ-Os(bpy)₃²⁺-PVP). The reduction of NO₂^? in acidic solution was taken as a model reaction to explore the properties of the modified electrode. On the steady state polarization curve of NO₂^? reduction there were two well-developed waves with much enhanced plateau current densities and positively shifted half-wave potentials compared with bare GC electrodes. In 0.05 mol/l H₂SO₄ + 5 mmol/l NO₂^? the modified electrode exhibited plateau current densities of 723 and 1153 μA cm^?2 and half-wave potential shifts of 0.29 and 0.56 V for the first and second current wave, respectively, showing promising potential for nitrite detection. The catalytic activity for NO₂^? reduction did not decrease appreciably over 4 months. A number of relevant kinetic and thermodynamic parameters are estimated experimentally. NO₂^? reduction at the composite modified electrodes is suggested to follow the SR mechanism (pure kinetic conditions involving mutual compensation between a catalytic reaction and substrate diffusion in the film in addition to diffusion in the solution phase) according to the Savéant–Andrieux theory.