The integral capacitance,kinetics and mechanisms of the Cu/Cu(II) system in sulphuric acid media

Galvanostatic investigation has been carried out on the Cu/Cu(II) system 2 M H₂SO₄ + 0.7 M CuSO₄, at 298 K. The pseudo-capacitance (integral capacitance) has been extracted as a function of overvoltage from the portion of the charging curve prior to plateaux. It has been found that the pseudo-capacitance, for both anodic and cathodic processes depended upon the current density and time. On the basis of the above findings the reaction mechanism has been suggested to be Cu?Cu(I)_ad^v + ve^?, Cu(I)_ad^v?Cu(I) + (1 ? v)e^?, Cu(I)?Cu(II)e^?.     

The anodic dissolution process on active iron in alkaline solutions

Steady state anodic polarization curves were taken for Armco iron and in some experiments for high purity Puratronic iron in KOH solutions in the concentration range 5 x 10^?2-5M. After the 30 min cathodic pretreatment, well reproducible anodic Tafel plots are obtained. The experimentally obtained diagnostic criteria b_a = 67–70 mV dec^?1, n_OH? = 1.1 and nHFeO₂^?= ?0.45 are interpreted by the anodic reaction mechanism in which FeOH_adsand Fe(OH)_2,ads appear as the intermediates adsorbed under Temkin conditions, the primary stable product of the electrode reaction being HFeO₂^?, and the final product Fe (OH)₂, formed by precipitation.     

Anodic behaviour of α-PbO2 substrates containing different percentages of lower lead oxides

A study has been made of the anodic behavior of lead oxide deposits containing 92, 80 and 60%α-PbO₂. The non-porous deposits were prepared by electrodeposition from a plumbite bath containing different proportions of 30% formaldehyde solution, acting as a reductant in the cases of 80 and 60%α-PbO₂. Measurements of the steady-state oxygen evolution reaction under constant surface conditions of the electrodes, were carried out galvanostatically within the current-density range 30–190μAcm^?2. The oxygen overpotential values were found to increase with increasing the amount of superficially formedβ-PbO₂ during anodization in sulphuric acid solution. In the proposed mechanism for the o.e.r, the rate determining step is an electron-transfer reaction. Tafel slopes having values of 220±10 mV dec^?1 were obtained. Possible interpretation of these higher slopes is given which is based on the dual barrier model. Potential-time decay curves reveal a retarded oxygen diffusion process.     

The anodic oxidation of nickel in alkaline solution

The anodic oxidation of nickel in alkaline solution was studied by cyclovoltammetric and optical techniques. The range of the scanning potential effects the resulting voltammograms. A constant I-E diagram with anodic peaks at 130 and 270 mV (at scan rate 10 mV · s^?1) is obtained after multiple scanning from ?800 to + 1200mV. The layer of Ni(OH)₂ that is formed in the anodic cycle, is only partially reduced by cathodic polarisation. Growth of the Ni(OH)₂ film on Ni occurs by repeated oxidation and reduction. This occurs via oxidation of Ni to α Ni(OH)₂ and conversion of α Ni(OH)₂ to β Ni(OH)₂.     

Electrochemical studies with a Cu-5wt.%Ni alloy in 0.5 M H2SO4

The electrochemical behavior of the annealed Cu-5wt.%Ni alloy in 0.5 M H₂SO₄ was studied by means of open-circuit potential (E_OCP) measurements, cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and quasi-stationary linear potential sweep. The hydrodynamics of the system was also studied. This material is constituted by a single α₁ phase. The anodic behavior of a Cu-Ni alloy in H₂SO₄ consists fundamentally on the electrodissolution of Cu, its main component, and the formation of a sulfur-containing passive layer. The presence of Ni decreases the rate of Cu oxidation, mostly at high positive potentials. The impedance spectra, obtained for the unrotating electrode, can be interpreted in terms of a simple charge-transfer reaction across a surface layer. When the electrode is rotated, the occurrence of an inductive loop evidenced the existence of an adsorbed layer. All the resistance estimated from the proposed equivalent circuits diminished with the electrode rotation rate, emphasizing the influence of ion transport in the overall electrode process. The system presented two anodic Tafel slopes: 40 mV dec⁻¹ for E<255 mV and 67 mV dec⁻¹ for E>275 mV. A Tafel slope of 40 mV dec⁻¹ evidences that copper dissolution can be interpreted in terms of the mechanism proposed by Mattsson and Bockris. The second Tafel suggests that at potentials more positive than 275 mV, copper dissolves according to a mechanism that considers the disproportionation of adsorbed Cu(I) species.     

Electrocatalytic and thermal activation of anodic oxygen- and cathodic hydrogen-evolution in alkaline water electrolysis

In micro-electrolysis cells (4 cm² electrode area) which possess a sandwich-configuration as used in advanced water electrolysis[1] different anodic and cathodic electrocatalysts, which did not contain noble metals were investigated over the current density range from 10^?4 to 1.0 A cm^?2 and a temperature range from 30 to 130°C (electrolyte: 50 wt% caustic potash). Mechanically activated nickel electrodes, RuO₂ doped anodes and Pt-black covered cathodes served as comparison standards for H₂ and O₂ evolution.IR-drop connections were not applied. Nonetheless the measuring-method used allowed to keep IR-induced mistakes in voltage-reading in single-electrode voltages below 40 mV even at the highest current densities of 1.0 A cm^?2.Plots of voltage vs log current densities for anodic oxygen evolution possess slopes between 40 and 70 mV (100°C) dec^?1 of current density which in some cases—especially at low temperatures—increased up to a value of 2 RT/F at higher current densities. By increasing the temperature these steeper parts of the anodic current—voltage curves very often disappear.The current-voltage curves of anodic oxygen evolution for different temperatures unexpectedly run nearly parallel to each other ie with a slope which is nearly independent of temperature and thus cannot be described according to the Butler—Volmer equation with a constant value of the formal charge-transfer coefficient βⁱ.The effective activation energies obtained from dln i_o/d(1/T) range from 70 to 100 kJ mole^?1.O₂ overpotentials at current densities around 1 A cm^?2 are most efficiently decreased by (i) application of mixed oxides containing cobalt in at least two different valency states (Co^II/Co^III or Co^III/Co^IV) and (ii) by use of higher working temperatures; roughened surfaces, however, are only of limited value in this respect.Voltage vs log current curves for cathodic hydrogen evolution show a pattern which is in agreement with the Butler—Volmer equation. The effective charge-transfer coefficient is close to 0.5 and increases slightly above this value for Raney-metal activated cathodes.The effective activation energies lie between the limits of 40 and 55 kJ mol^?1. Hydrogen evolution overpotentials are most efficiently decreased by (i) preparation of cathode surfaces with high roughness factors, (ii) using Ni, Co or Fe as cathode material and (iii) by increasing the working temperatures.     

The triangular potential sweep voltammetric studies on pure tin in concentrated sodium hydroxide solutions

The triangular potential sweep voltammetric studies were carried out on pure tin (99.9999%) in NaOH solutions (1–10 N). The forward scan revealed two distinguished peaks in the region ?1.12 to ?0.9 V vs sce and at faster sweep rates a single peak appeared. Oscillations were observed in the E-i curve in the potential regions of Sn/Sn(OH)₂ or Sn(OH)₂/Sn(OH)₄ in the forward scan; and increase of stannite concentration increased the oscillations. The non-stoichiometric nature of the passive oxide film is responsible for these oscillations. The first anodic peak potential and current dependencies on sweep rates revealed that a “pore resistance model” holds good. The appearance of a single cathodic peak at far negative potentials, compared to the anodic peak observed at higher sweep rate, was understood to be due to irreversible reduction of oxidised species formed on the forward scan. The fractional dependence of cathodic peak current and potential on stannite ion concentration, at constant OH^? ion concentration and a Tafel slope of 60 mV decade^?1, suggests the reduction of stannite involves activated adsorption of Sn(OH) obeying the Temkin isotherm with its discharge as a rate-determining step.     

Cyclic voltammetry of monel 400 in lithium hydroxide solution at elevated temperatures

The electrochemistry of Monel 400 in 1 mole/kg^?1 LiOH solution at 25, 125 and 250°C has been investigated using the technique of cyclic voltammetry. The general electrochemical behaviour is found to most closely approximate to that of the major component, nickel, although expansion of the current scale reveals anodic and cathodic peaks which probably arise from redox processes involving copper. The general similarity to nickel can be rationalized in terms of either the d electron theory for cupronickel alloys or phase separation, the latter being favoured in the present study. At 25°C the majority of charge consumed on sweeping the potential in the positive direction is involved in the formation of an oxide film at potentials close to oxygen evolution. This process is no longer observed at 250°C, due to a sharp reduction in the oxygen evolution overpotential with temperature. The majority of charge consumed on cyclic sweeping at this temperature is attributed to active dissolution of the nickel component of the alloy to form HNiO^?₂ (or Ni(OH)^?₂ (or Ni(OH)^?₃) at potentials slightly positive to the hydrogen evolution region.     

Studies concerning the growth of cadmium dendrites. I. Morphology in alkaline media

The morphology of cadmium dendrites formed during potentiostatic electrodeposition onto nickel and cadmium substrates from cadmate solutions in alkaline supporting electrolyte has been investigated. The morphology is potential dependent for deposition under convective diffusion conditions to a nickel substrate. For 1.05×10^?4 mol dm^?3 Cd(OH) ₄ ^2? /30% KOH solutions, 2D- fern dendrites are observed at an overpotential of ?150mV, needle dendrites at ?200 mV, and large ‘filled-in’ fern dendrites at ?300 mV. Similar results were found at the higher concentration, 2.4×10^?4 mol dm^?3 Cd(OH) ₄ ^2? /50% KOH, but the time taken to grow an equivalent morphology and length were reduced in proportion. Crystalline aggregate dendrites were observed on a cadmium substrate in 1.05×10^?4 mol dm^?3 Cd(OH) ₄ ^2? /30% KOH, becoming more crystalline and well defined with increase in overpotential. A significant induction time of the order 8 h was observed for all deposition onto stationary nickel and cadmium wires. Under the well-defined diffusion conditions at a rotating nickel disc electrode only one morphology, namely small ferns, was observed over a wide range of overpotentials. The current-time behaviour is presented, and the current is shown to have a (time)² dependence, indicative of progressive nucleation of dendrites. The induction time, indicated approximately by the current minima, had decreased significantly.     

Modeling of the anode side of a direct methanol fuel cell with analytical solutions

In this work, analytical solutions were derived (for any methanol oxidation reaction order) for the profiles of methanol concentration and proton current density, by assuming diffusion mass transport mechanism, Tafel kinetics, and fast proton transport in the anodic catalyst layer of a direct methanol fuel cell. An expression for the Thiele modulus that allows to express the anodic overpotential as a function of the cell current and kinetic and mass transfer parameters was obtained. For high cell current densities, it was found that the Thiele modulus (?²) varies quadratically with cell current density; yielding a simple correlation between anodic overpotential and cell current density. Analytical solutions were derived for the profiles of both local methanol concentration in the catalyst layer and local anodic current density in the catalyst layer. Under the assumptions of the model presented here, in general, the local methanol concentration in the catalyst layer cannot be expressed as an explicit function of the position in the layer. In spite of this, the equations presented here for the anodic overpotential allow the derivation of new semi-empirical equations.     

MOF nanosheet array-derived NiS with Co,N-doped carbon layer: A highly efficient oxygen evolution electrocatalyst

The development of the hydrogen industry from electrolytic water is confined in great measure to the slow kinetics of oxygen evolution reaction (OER), which means the rational design of a stabilized and efficient OER electrode is the key. The construction of easily accessible hydroxide ion (OH^?) adsorption sites can effectively accelerate the kinetics of the OER. Herein, the NiS/CoNC electrocatalysts, which can effectively adsorb OH^? and exhibited excellent OER performance in an alkaline environment, were obtained by pyrolysis and sulfidation of NiCo-MOF nanosheets on nickel foam (NF). In particular, the optimized NiS/CoNC electrocatalyst achieved an overpotential of 46/106 mV at 10/100 mA cm^?2 and Tafel slope of 57.41 mV dec^?1, which is superior to most reported materials. The outstanding function of OER results from the NiOOH/CoOOH active component generated by surface reconstruction after activation of the NiS/CoNC catalyst and the excellent charge transfer capability of the NiS component. This work offers a scheme to construct the electrocatalysts derived from MOF with excellent OER performance.     

Studies concerning the growth of cadmium dendrites. II. Morphology in acidic media

The morphology of cadmium deposits formed during potentiostatic deposition onto etched cadmium substrates in a range of cadmium sulphate solutions with 0.5 mol dm^?3 sulphuric acid as supporting electrolyte has been investigated. The deposit morphology and induction time was found to be both concentration and overpotential dependent. At 10^?1 mol dm^?3 CdSO₄ for an overpotential range ?20 to ?80 mV, large crystalline aggregates were observed and large dendrites resulted after longer deposition times. The induction time was less than 1 min and the current time curves linear, indicating instantaneous rather than progressive initiation. At 10^?2 mol dm^?3 CdSO₄ the morphology varied from fine, 2D-fern dendrites atη=?75 mV to needle dendrites atη=?150 mV. The morphology at 10^?3 mol dm^?3 CdSO₄ closely resembled that at 10^?2 mol dm^?3 CdSO₄ but, showed finer structural detail with less filling in of the main skeletal structure. The induction time was an order of magnitude greater at 10^?3 mol dm^?3 CdSO₄ than at 10^?2 mol dm^?3 CdSO₄, and the time taken to grow dendrites of the same length was also increased.     

Electrochemical investigation of the dimeric oxo-bridged ruthenium complex in aqueous solution and its incorporation within a cation-exchange polymeric film on the electrode surface for electrocatalytic activity of hydrogen peroxide oxidation

Electrochemical behavior of oxo-bridged dinuclear ruthenium(III) complex ([(bpy)2(H₂O)Ru^III-O-Ru^III(H₂O)(bpy)2]⁴⁺) has been studied in aqueous solution (KCl 0.5 mol L⁻¹) by both cyclic and rotating disk electrode (RDE) voltammetry in order to identify and elucidate the reaction mechanism. Modified electrode containing the oxo-bridged ruthenium complex incorporated into a cation-exchange polymeric film deposited onto platinum electrode surface was studied. Cyclic voltammetry at the modified electrode in KCl solution showed a single-electron reduction/oxidation of the couple Ru^III-O-Ru^III/Ru^III-O-Ru^IV. The modified electrode exhibited electrocatalytic property toward hydrogen peroxide oxidation in KCl solution with a decrease of the overpotential of 340 mV compared with the platinum electrode. The Tafel plot analyses have been used to elucidate the kinetics and mechanism of the hydrogen peroxide oxidation. The first at low overpotential region there is no significant change in the Tafel slope (∼0.130 V dec⁻¹) with varying peroxide concentration. The second region at higher overpotential the slope values (0.91–0.47 V dec⁻¹) were depended on the peroxide concentration. The apparent reaction order for H₂O₂ varies from 0.16 to 0.50 in function of the applied potential. The apparent reaction order (at constant potential) with respect to H⁺ concentration of 10⁻⁵ to 10⁻¹ mol L⁻¹ was 0.25. A plot of the anodic current vs. the H₂O₂ concentration for chronoamperometry (potential fixed = +0.61 V) at the modified electrode was linear in the 1.0 × 10⁻⁵ to 2.5 × 10⁻⁴ mol L⁻¹ concentration range.     

Studies concerning charged nickel hydroxide electrodes. VII. Influence of alkali concentration on anodic peak positions

After repetitive potential cycling employing a high positive potential limit (>700 mV wrt Hg/HgO/ KOH) three anodic and one cathodic peak can be observed using aβ-Ni(OH)₂ starting material. Anodic peaks found at 425, 470 and 555 mV in 5 mol dm^?3 KOH shift to less positive potentials as the alkali concentration is increased appearing at 365, 410 and 455 mV respectively in 12.5 mol dm^?3 KOH. Four anodic processes involving various pairs of coexisting phases within both theβ andα-/γ-phase system can be identified as summarized below in order of increasing positive potential: Peak A $$\begin{gathered} Peak A{\text{ }}U_\alpha ^A \to {\text{ }}V_\gamma ^A \hfill \\ Peak B{\text{ }}U_\beta ^B \to {\text{ }}V_\beta ^B \hfill \\ {\text{ }}\mathop C\limits^ + {\text{ }}U_\alpha ^C \to {\text{ }}V_\gamma ^C \hfill \\ Peak E{\text{ }}V_\beta ^B \to {\text{ }}V_\gamma ^E \hfill \\ \end{gathered} $$ Observed shifts in anodic and cathodic peak potentials are consistent with the known influence of alkali and water activity on the reversible potentials for the above processes.     

Kinetics of armco iron dissolution in an anhydrous methanol and hydrochloric acid solution

The polarization behaviour of Armco iron in anhydrous solutions of CH₃OH+HCl and CH₃OH+HCl+LiCl was studied by the potentiostatic method. The following reaction orders were found for the anodic process: Z_a(Cl^?) = 1.7 –1.82 and Z_a(H⁺) = 0. The following mechanism of dissolution is proposed: Fe+Cl^? ? FeCl^?_ads, FeCl^?_ads+Cl^? → FeCl₂+2e (rate-determining step), FeCl₂?FeP²⁺ + 2Cl^?.     

Cathodic reduction of the anodically formed zinc species as a contribution to the study of the potentiodynamic passivation of zinc in alkaline media

The cathodic part of the potentiodynamic curves obtained for upward-facing horizontal 99.9% zinc electrodes in KOH solutions 0.4, 1.0, 2.0 and 3.0 M and sweep rates in the range 1–100 mV s⁻¹ have been systematically analyzed in order to assign the possible species formed and contribute to the study of the potentiodynamic passivation of Zn in alkaline media. The anodic limit of the potentiodynamic cycles was changed and set for significant points of the total curve (between hydrogen and oxygen evolution). Also, the anodic sweep was interrupted at the potentials corresponding to the anodic limit and the cathodic sweep applied immediately from a potential near that of zero current of the cathodic half-cycle. Only two cathodic peaks have been found for the non-interrupted cycles. The assignation of peaks according to the equilibrium potentials of the reduction of the possible species implied, ie Zn(OH)²⁻₄, Zn(OH)₂ and ZnO, is not possible because local pH changes are expected and the zincate concentration near the electrode is unknown. The peak placed at more positive potentials for KOH concentrations 0.4 and 1.0 M is assigned to zincate and that at more negative potentials, to the reduction of the film. Just the opposite assignation has been found for 2.0 and 3.0 M KOH solutions. The experimental results can be interpreted assuming that the product formed at the passivation potential consists of the same chemical species as those corresponding to the first anodic peak, probably Zn(OH)₂ or hydrated ZnO. From calculating the maximum film thickness according to the charge passed and taking into account the recent theoretical analysis made by Chang and Prentice, it is concluded that the direct formation of ZnO on the electrode at the passivation potential as a consequence of local pH changes is not probable.     

The nucleation and growth of a lead phosphate film on homogeneous lead amalgam

The electrochemical oxidation of Pb(Hg) in acidic phosphate medium results in formation of a monomolecular PbHPO₄ passivating film. Analysis of potentiostatic current-time transients demonstrated that the crystallization occurred through a 2-dimensional progressive nucleation and growth mechanism. The potential dependence of the combined nucleation and crystallization rate constants was [7.8 ± 0.7 mV/decade]^?1. The formation of a soluble lead phosphate species prior to film growth was observed under certain conditions. Examination of it curves in this region by the low overpotential form of the Cottrell equation indicated that the soluble species was PbHPO₄.     

Platinum-iridium catalyzed titanium anode. II. Oxygen evolution

The oxygen evolution reaction on titanium based Pt-Ir catalyzed DSA has been investigated. The steady state polarization curves of the electrodes with the various contents and different pH values of electrolyte have been determined. A good Tafel linearity with slopes of 40 and 120 mV dec^?1 were obtained. The conclusion is drawn that the second electrochemical step is rate-determining. The overvoltage of the oxygen evolution reaction at 1 A cm^?2 compares most favourably with published data for other materials.     

The faradaic impedance of the carbon anode in cryolite-alumina melt

'The anode reaction on pyrolytic graphite, regular graphite and baked carbon in cryolite-alumina melts was studied by impedance measurements within the cd range 0.05–0.7 A cm⁻². The anode process was found to be reaction controlled on pyrolytic graphite at above i_a = 0.25 A cm⁻² and on regular graphite at above i_a = 0.7 A cm⁻². At lower cds a combination of reaction and diffusion control was observed, and for baked carbon this was the case over the entire cd range. The diminishing influence of diffusion control with increasing cd may be caused by suppression of the anode reaction within pres due to blocking by anode gas. The double layer capacity (C_dl) increased considerably with increasing cd on pyrolytic and regular graphites, while on baked carbon it remained uneffected above i_a = 0.1 A cm⁻². The increase in C_dl was attributed to enlargement of the anode surface area during electrolysis. The anodic overvoltage, porosity and reactivity were also investigated. For all tested anodes Tafel slopes of about 0.3 V dec⁻¹ were found. The overpotential increased with decreasing porosity and reactivity.     

Relative reactivity of phenylboronic acid to phenylboronate ion toward L-lactate and mandelate at physiological pH

Kinetic studies were performed on the reactions of phenylboronic acid with L-lactic acid and mandelic acid in acidic aqueous and alkaline solutions in order to specify reactive species in these reactions. It was confirmed that the diprotonated ligand (H₂L: L-lactic acid or mandelic acid) is less reactive than the monoprotonated ligand (HL^?: L-lactate ion or mandelate ion), which made possible direct determination of the rate constants of phenylboronic acid (PhB(OH)₂) and its conjugate base, phenylboronate ion (PhB(OH)₃^?). It was found that PhB(OH)₂ is more reactive than PhB(OH)₃^?. On the basis of kinetic results, it was concluded that the most reactive species are PhB(OH)₂ and HL^? at physiological pH 7.4, so the reaction in the boronic acid-based sensor for L-lactate mainly would occur between these species.