Electrocatalysis of the charge-transfer process for the Sb(III)/Sb(V) couple at a platinum electrode in concentrated HCl

Electrocatalysis of the electrochemical oxidation and reduction of Sb(V) and Sb(III) by adsorbed I⁻ and I₂ at a rotating Pt disc electrode was investigated in 12 M HCl. The heterogeneous rate constant in the absence of adsorbed halogen was increased by 200x for a maximum surface coverage by halogen. This large rate increase cannot be predicted on the basis of an electrostatic argument and a mechanism based on electron-transfer bridging is proposed.     

Electrochemical oxidation of manganese(II) at a platinum electrode

Electrochemical oxidation of Mn²⁺ in sulphuric acid to form MnO₂ was studied using stationary and rotating platinum/platinum ring-disc electrodes. It appears that nucleation of MnO₂ is governed by an equilibrium involving a Mn(III) intermediate. Growth of MnO₂ involves the reduction of MnO₂ surfaces by Mn²⁺ ions in the solution to form MnOOH intermediates. The subsequent electrochemical oxidation of MnOOH releases a hydrogen ion and results in the formation of MnO₂. The rate constant of MnOOH oxidation to MnO₂ was estimated to be 40 s^–1. With a sufficient supply of Mn²⁺ ions, a layer of MnOOH is built up and the in-solid diffusion of hydrogen ions becomes the ratedetermining-step. With a low Mn²⁺ concentration, diffusion of Mn²⁺ ions from bulk electrolyte to the MnO₂/electrolyte interface is a factor controlling the growth of MnO₂. The activation energy and the pre-exponential term of the diffusion coefficient of Mn²⁺ in 0.5m sulphuric acid were determined to be 44.8 kJ mol^–1 and 100 cm² s^–1, respectively.     

The influence of platinum (phase) oxide on the electrode kinetics of the manganese(III)/manganese(II) and cerium(IV)/cerium(III) redox couples in sulphuric acid

The kinetics of the Ce(III)/Ce(IV) and Mn(II)/Mn(III) redox reactions (both oxidation and reduction) were followed at a rotating Pt disc electrode, on which known amounts of platinum oxide (type II) had been formed. Oxide film inhibits both anodic and cathodic reactions, but to different extents. There is no systematic decrease in i₀ with increase in film thickness suggesting that electron tunnelling is unimportant under these conditions but that electron exchange takes place with the oxide itself. The inhibiting effect of type II oxide is seen even when it is present in sub-monolayer amounts, and this and other effects suggest that the overall behaviour of the system cannot be simply represented, for example in terms of a semi-conducting oxide.     

Electrochemical behaviour of the Cr(II)/Cr(III) couple on silver,silver amalgam and silver-based mercury film electrodes

The electrocatalytic activity of silver, mercury, silver-based mercury films, and silver amalgam electrodes for the reduction of Cr(III) ions in acidic solutions is analysed. An enhancement in the activity of the last two electrodes is observed when they are pretreated with high cathodic current polarization or ultrasonic waves. These effects can be explained by the formation of an unstable highly reactive amalgam. The electrocatalytic enhancement is not observed when the same electrode pretreatments are applied to sitting-drop mercury or silver electrodes.     

Investigation on the electrode process of the Mn(II)/Mn(III) couple in redox flow battery

The Mn(II)/Mn(III) couple has been recognized as a potential anode for redox flow batteries to take the place of the V(IV)/V(V) in all-vanadium redox battery (VRB) and the Br₂/Br⁻ in sodium polysulfide/bromine (PSB) because it has higher standard electrode potential. In this study, the electrochemical behavior of the Mn(II)/Mn(III) couple on carbon felt and spectral pure graphite were investigated by cyclic voltammetry, steady polarization curve, electrochemical impedance spectroscopy, transient potential-step experiment, X-ray diffraction and charge-discharge experiments. Results show that the Mn(III) disproportionation reaction phenomena is obvious on the carbon felt electrode while it is weak on the graphite electrode owing to its fewer active sites. The reaction mechanism on carbon felt was discussed in detail. The reversibility of Mn(II)/Mn(III) is best when the sulfuric acid concentration is 5 M on the graphite electrode. Performance of a RFB employing Mn(II)/Mn(III) couple as anolyte active species and V(III)/V(II) as catholyte ones was evaluated with constant-current charge-discharge tests. The average columbic efficiency is 69.4% and the voltage efficiency is 90.4% at a current density of 20 mA cm⁻². The whole energy efficiency is 62.7% close to that of the all-vanadium battery and the average discharge voltage is about 14% higher than that of an all-vanadium battery. The preliminary exploration shows that the Mn(II)/Mn(III) couple is electrochemically promising for redox flow battery.     

The kinetics of the bromine electrode in a silver bromide-sodium bromide melt

Kinetic parameters for the bromine evolution reaction on graphite and carbon electrodes in a silver bromide/sodium bromide melt have been determined at 750°C.Double pulse measurements show that the charge transfer reaction is fast with an exchange cd in the range of 1–3A/cm². Tafel slopes obtained from steady state cd/potential measurements show that the desorption of adsorbed bromine atoms is probably the rate controlling step of the electrode reaction. The adsorption then probably follows a Temkin-type isotherm.Critical parameters for the anode effect have been determined. The critical cd was found to be especially low on polished graphite surfaces.     

The free energy of transfer of Pb(II) ion and electrode kinetics of the Pb(II)/Pb(Hg) system in dimethyl sulfoxide (DMSO) and DMSO-water mixtures

The electrode kinetics of the Pb(II)/Pb(Hg) system has been studied in DMSO and DMSO-water mixtures using the faradaic impedance measurements. With the increase of the mole fraction of the organic component in the mixture rate constant decreases steadily from 2 cm s^?1 in pure water to 5 × 10^?2 cm s^?1 in DMSO in rough proportion to the coverage of the electrode by DMSO. The change of the kinetics with the solvent composition was discussed in the frame of existing models.Basing on the determined potentials of the Pb(II)/Pb(Hg) electrode expressed vs the ferrocinium ion/ferrocene electrode potential, the free energy of transfer of Pb(II) from water to DMSO and its mixtures was calculated and discussed.     

Kinetic behaviour of the Sn(IV)-Sn(II) couple in acidified concentrated bromide solutions at a glassy carbon rotating ring-disk electrode

Kinetic parameters of the Sn(IV)-Sn(II) couple are evaluated at a glassy carbon rotating ring-disk electrode in acidified concentrated bromide media. Both the oxidation and reduction are found to be first order in reactant and to proceed irreversibly. While the oxidation of Sn(II) shows no complications, the reduction of Sn(IV) is probably preceded by a homogeneous chemical reaction involving complexation with bromide ions. Only in highly concentrated bromide media (10 M) is a relatively undisturbed Sn(IV) reduction wave obtained.     

The mechanism of formation of a surface film of silver on a platinum electrode

The mechanism of the growth of thin metal films on inert substrates in potentiostatic metal electrodeposition is given. A possible relation to the porosity of metal deposits is discussed. The effects of pulsating overpotential on the electrodepositon of the first monolayers are described.     

Influence of ionic speciation on electrocatalytic performance of polyaniline coated platinum electrode: Fe(III)/Fe(II) redox reaction

Porous-polyaniline coated Pt electrode (PANI/Pt) was electro-synthesized potentiodynamically in 0.1 M aniline + 0.5 M H₂SO₄ and morphologically characterized by scanning electron microscopy (SEM). Nature of predominant Fe-species in HCl and H₂SO₄ was checked by UV-vis spectrophotometry. Electrocatalysis of Fe(III)/Fe(II) reaction was studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) for three different solution compositions viz. (i) FeCl₃/FeCl₂ in 1 M HCl, (ii) FeCl₃/FeCl₂ in 0.5 M H₂SO₄ and (iii) Fe₂(SO₄)₃/FeSO₄ in 0.5 M H₂SO₄. For different thicknesses of PANI, the peak current increased irrespective of the nature of the Fe-species, but the polarity of the charge on the Fe-species showed great influence on reversibility of electrocatalysis by PANI/Pt. The Donnan interaction of the polyaniline modified electrode for the three compositions was investigated with respect to [Fe(CN)₆]³⁻/H₂[Fe(CN)₆]²⁻ which are believed to be the predominant species present in K₃[Fe(CN)₆]/K₄[Fe(CN)₆] solution in 0.5 M H₂SO₄. The electrocatalytic performance of PANI/Pt for Fe(III)/Fe(II) redox reaction was found superior in HCl compared to that in H₂SO₄.     

Kinetics and mechanism of the silver (I) oxide to silver (II) oxide layer electrooxidation reaction

The electroformation of Ag(II) oxide layer during the anodization of silver in 0.1 M NaOH is investigated under potentiostatic and potentiodynamic conditions. Results are discussed in terms of nucleation and growth models and statistical analysis of induction times related to the nucleation kinetics of Ag(II) oxide crystals. The best fitting of results comes out from the application of a progressive nucleation and 3-D growth model under mass transfer control where diffusion of species from the electrode to growing sites is essential for further expansion.     

The electrochemical reduction of ruthenium(IV) in perchloric acid solutions on platinum electrodes

Two reversible waves are observed for the reduction of Ru(IV) on platinum electrodes. Each wave is the sum of two one-electron steps according to:

Potentials (vs nhe) and species are given for 1.0 M HClO₄ solutions; more hydrolysed products are formed in less acid solutions. The tetrameric Ru(III) ion slowly decomposes to a more stable Ru(III) species, probably a dimeric ion.     

The Mn(II)/Mn(Hg) electrode reaction in dimethylsulphoxide (DMSO) and its mixtures with water

The electrode reaction of the Mn(II)/Mn(Hg) system in DMSO and its mixtures with water has been studied. Charge transfer rate constants, diffusion coefficients and formal potentials were determined. The transfer energies of Mn(II) from water to water + DMSO mixtures were calculated, based on potentials expressed in the ferrocene electrode scale. No minimum on the dependence of the standard rate constant on DMSO concentration was observed. To describe this dependence the proposed earlier model of the electrode reaction in the mixtures of water with more basic solvents was used. A decrease of cathodic Tafel slope with the increase of DMSO concentration and practically no change of the anodic one indicates that the rate of both steps of Mn(II) reduction in the mixtures rich in DMSO is rather comparable.     

The mechanism of formation of a surface film of silver on a platinum electrode in galvanostatic deposition

A mechanism for the growth of thin metal films on inert substrates in galvanostatic metal deposition is proposed. Qualitative agreement between theoretical and experimental results was obtained.     

Application of a proton-electron mechanism to the cathodic reduction of multilayer oxide on a platinum electrode in perchloric acid solution

The electrochemical reduction of the multilayer oxide on smooth Pt electrode in HClO₄ solution was studied using a galvanostatic technique. This oxide is completely reduced in the potential region where the electrosorption of hydrogen on Pt electrode occurs and the reduction rate depends on the hydrogen coverage. A previously proposed proton-electron mechanism was applied to the reduction of the multilayer oxide. The linear relationship between Q_Hs and log i/Q_Hs, which is expected from this mechanism, was confirmed in the coverage range of H_s from 0.1 to 1, where i is the reduction rate expressed with current density and Q_Hs the quantity of strongly-adsorbed hydrogen on the electrode surface of unit area.     

The reduction of acetophenone to ethylbenzene at a platinised platinum electrode

The efficiency and current density for the reduction of acetophenone to ethylbenzene at a platinised platinum cathode is shown to depend strongly on the electrolysis conditions and in acidic ethanol an acceptable synthetic procedure is possible. The major by-product is generally the alcohol and this may be minimised by periodic current reversal to activate the electrode. Some experiments on the reduction of substituted acetophenones and benzaldehyde in similar conditions are also reported.     

The mechanism of the nitrate (platinum) electrode in molten alkali nitrates

The mechanism for the nitrate electrode, (Pt)NO₂, O₂/NO^?₃, is shown to be: NO^?₂ = NO₂ + e; NO₂ = NO + $\text{1}\text{2}$O₂; NO^?₃ + NO = NO^?₂ + NO₂; with the overall reaction: NO^?₃ = NO₂ + $\text{1}\text{2}$O₂ + e.     

On the kinetics and mechanism of the Cd(II)/Cd(Hg) system in acid media

The kinetics and mechanism have been investigated in the Cd(II)/Cd(Hg) system in 2 M NaClO₄ + 1 M HClO₄, at 25 ± 0.5°C. The galvanostatic single-pulse method has been used. The value of double-layer capacity, exchange current density, as also the α value, k_s, and the partial reaction order of each of the component which takes place in the reaction, have been determined on the grounds of different kind of anions in the solution. The two-step single-electron exchange mechanism has been suggested, as also the one-step exchange reaction in combination with the reaction of disproportionation.     

The role of platinum oxide in the electrode system Pt(O2)/yttria-stabilized zirconia

The existence and role of platinum oxide in the solid state electrode system Pt(O₂)/yttria-stabilized zirconia is discussed. Covering and porous model-type Pt film electrodes on YSZ single crystals are investigated by cyclic voltammetry, electrochemical impedance spectroscopy, and in situ scanning photoelectron microscopy. The formation of Pt oxide and its amount strongly depend on the experimental conditions, such as temperature, oxygen partial pressure, and oxygen flux towards the electrode during anodic polarization. Electrode activation and deactivation processes can be explained by formation and decomposition of Pt oxide, which is reducing or inhibiting the oxygen exchange rate.     

Reductions in aprotic media—I. Cathodic reduction limits at a platinum electrode in acetonitrile

The cathodic behavior of acetonitrile at a platinum electrode is strongly dependent on the cation of the supporting electrolyte, and contamination by moisture. Mixtures of supporting electrolytes can give anomalous effects under certain conditions; the results are consistent with the formation of hydride ion at the platinum surface.