Influence of pH on nickel electrodeposition at low nickel(II) concentrations

The effect of pH on the nucleation stages of nickel electrodeposition on vitreous carbon has been analysed using low nickel concentrations, without additives. The experimental results indicated that there was a change in the electrochemical response of the system at 4 < pH < 4.5. A predischarge adsorption ascribed to nickel(II) species was observed with a different surface coverage depending on the pH. Moreover, different inhibition and 3D nucleation processes were detected in varying experimental conditions.     

Ionization equilibria of nickel(II) bromide in acetonitrile solutions

Uv and visible absorption spectra and the molar conductance curve of NiBr₂ in acetonitrile solution have been determined at 25°C. Equilibrium concentrations and formation constants of the consecutive NiBr⁺, NiBr⁰₂, and NiBr^?₃, complexes have been calculated from the spectrophotometric results making use of the isosbestic points found in an auxiliary study of three-component solutions with variable Br^?:Ni(II) mole ratio. The existence of NiBr⁺ in the form of the [NiBrL₅]⁺ pseudooctahedral complex, and that of NiBr^?₃ in the form of the [NiBr₃L]^? pseudotetrahedral complex is inferred, while the nature of NiBr⁰₂ remains unclear.     

Ionization equilibria in solutions of nickel(II) chloride in dimethyl-sulphoxide

Visible absorption spectra and the molar conductance curve of NiCl₂ dissolved in dimethyl-sulphoxide (DMSO) have been determined at 25°C. The results indicate the formation of the [NiCl(DMSO)₅]⁺ inner-sphere complex followed by the formation of the {NiCl(DMSO)₅]⁺Cl^?} outersphere ion-pair as the concentration of NiCl₂ increases. The [NiCl₂(DMSO)₄] inner-sphere complex in pure dimethyl sulphoxide at 25°C is formed to but a small extent, but its relative content increases upon addition of the non-coordinating diluent, chlorobenzene. Dilutions with chlorobenzene also enhance coordination disproportionation producing the [NiCl₃ DMSO]^? tetrahedral complex. The stability constant of the monochloro complex derived from the conductometric data has the value of 297 (^±10)M^?2 at 25°C, and the estimated value of the association constant of the [NiCl (DMSO)₅]⁺Cl^? complex electrolyte, dominating in moderately concentrated solutions at room temperature, is 7(±3)M^?1.     

Outer-sphere association of ions in DMSO solutions of nickel(II) salts

The visible absorption spectra and the molar conductance curves of solutions of NiI₂, NiBr₂, Ni(BF₄)₂, Ni(ClO₄)₂ and Ni(CF₃COO)₂ DMSO at 25°C have been determined. The results indicate a weak association which is only of the outer-sphere type, in each case involving the Ni(DMSO)^{2 +}₆ complex. The conductance data are analysed using the Lee-Wheaton conductance equation for unsymmetrical electrolytes. The first-step association constants (K₁) and the limiting ionic conductances, λ₀, for the Ni(DMSO)^{2 +}₆ cation as well as for the BF^?₄ and CF₃COO^? anions are obtained. A linear relation between log K₁ and the reciprocal of the anionic radius is observed. On the basis of this relation, the relative contribution of the outer-sphere association for solutions of NiCl₂ in DMSO is estimated.     

Specific adsorption of tetraalkylammonium cations on the 1,2-dicloroethane/water interface

Interfacial tension was used to study the electrochemical properties of the interface between LiCl in water and tetraalkylammonium tetraphenylborate in 1,2-dichloroethane. The interfacial tension decrease with decreasing of the size of the cation of the organic supporting electrolyte on the order TBA⁺ < THA⁺ < THpA⁺ < TOA⁺. The decrease of interfacial tension is also observed at the electrocapillary maximum the potential of which (potential of zero charge) shift towards more positive values in the order TBA⁺ > THA⁺ > THpA⁺ > TOA⁺. This fact maybe an evidence that there is an increase of adsorption when the alkyl chain length of the organic cation is reduced.     

Preparation of Co-Sn alloys by electroreduction of Co(II) and Sn(II) in molten LiCl-KCl

Co-Sn alloys were prepared by an electrochemical route in molten LiCl-KCl between 400 and 550 °C. The Sn(IV)/Sn(II), Sn(II)/Sn(0) and Co(II)/Co(0) redox couples were studied by cyclic voltammetry and/or chronopotentiometry over the temperature range. The diffusion coefficient values of Co(II) ions were measured. For example, it was found that the D_Co(II) values deduced from chronopotentiometry range from D_Co(II) = 1.65 × 10⁻⁵ cm² s⁻¹ at 400 °C to 4.95 × 10⁻⁵ cm² s⁻¹ at 550 °C. The standard potential of the Co(II)/Co(0) redox couple in molten LiCl-KCl was measured at 400 °C: vs Cl₂/Cl⁻. Finally, Co-Sn alloys were prepared in potentiostatic mode. The influence of the temperature of molten LiCl-KCl, the applied potential and the deposition time on the morphology and the composition of the Co-Sn alloys were also investigated. For T > 450 °C, the following tendency has been observed: the more negative the potential, the higher the Sn content in the deposited alloy. Thus, depending on the operating conditions, pure CoSn or CoSn₂ can be prepared.     

Study on electroreduction of titanium tetrachloride in acetonitrile solutions

The reaction of electroreduction of TiCl₄ in acetonitrilic solution in the presence of 0.1 M LiBF₄ has been investigated. It has been suggested that processes of reduction-oxidation Ti(IV)Ti(III) proceed relatively easy. The experimental values of and k_Ep/2 have been calculated. The mechanism of overall reduction reaction: Ti(IV)Ti(III)Ti(II)Ti(O) has been postulated. It has been found that in the presence of Li⁺ ions in solution the process of chemical Ti(III)Ti(O) reduction can occur     

Selective transport of cobalt (II) from ammoniacal solutions containing cobalt (II) and nickel (II) by emulsion liquid membranes using 8-hydroxyquinoline

The selective transport of cobalt (II) from ammoniacal solutions containing nickel (II) and cobalt (II) by emulsion liquid membranes (ELMs) using 8-hydroxyquinoline (8-HQ) as extractant has been presented. Membrane solution consists of a diluent (kerosene), a surfactant (ECA 4360J), and an extractant (8-HQ). Very dilute sulphuric solution buffered at pH 5.0 has been used as a stripping solution. The ammoniacal feed solution pH was adjusted to 9.0 with hydrochloric acid. The important variables governing the permeation of cobalt (II) have been studied. These variables are membrane composition, pH of the feed solution, cobalt (II) and nickel (II) concentrations of the feed solution, stirring speed, surfactant concentration, extractant concentration, complexing agent concentration and pH of the stripping solution, and phase ratio. After the optimum conditions had been determined, it was possible to selectively transport 95.0% of cobalt (II) from ammoniacal feed solution containing Co²⁺ and Ni²⁺ ions. The separation factors of cobalt (II) with respect to nickel (II), based on initial feed concentration, have experimentally found to be of as high as 31 for equimolar Co(II)–Ni(II) feed solution.     

Influence of manganese(II), cobalt(II), and nickel(II) additives in electrolyte on performance of graphite anode for lithium-ion batteries

Manganese dissolution into an electrolyte from the spinel LiMn₂O₄ in the lithium-ion cell has been recently investigated. In order to study the influence of the dissolved manganese species on the lithium intercalation/deintercalation into a natural graphite electrode, the electrochemical behavior of graphite was investigated in 1 mol dm⁻³ LiClO₄ electrolyte solution containing a small amount of Mn(II) by the addition of manganese(II) perchlorate. During the charging process, Mn(II) ions were firstly electroreduced on the electrode around 1.0 V versus Li/Li⁺ followed by irreversible decomposition of the electrolyte and lithium intercalation into the graphite. By microscopic observation of the graphite surface, manganese deposition was confirmed after the charge/discharge test. Due to the manganese deposition, the reversible capacity of the graphite electrode was drastically decreased. Furthermore, the cyclability of the anode was degraded with the amount of the manganese additive increasing. We compared these results with those of the cobalt(II) and nickel(II) additives by dissolving the corresponding perchlorates. Furthermore, we discussed the influence in practical cells based on the consideration of electrochemistry of the deposited metals.     

Nature of the polarographic prewaves of Ni(II) in the presence of thiocyanate

The nature of the polarographic prewaves of Ni(II) in the presence of thiocyanate has been analysed using different criteria. The catalytic character of both prewaves has been shown although the second one has a mixed adsorption-catalytic character. The bulk nature of the first prewave and the surface nature of the second prewave are also demonstrated.     

Comparative investigation on electroreduction of Cu(II) using polypyrrole electrode and stainless steel electrode

The electroreduction of Cu(II) by polypyrrole (ppy)-modified electrode and stainless steel (SS) electrode under potentiostatic and potentiodynamic conditions was studied. Compared with SS electrode, ppy-modified electrode presented higher Cu(II) removal ability since it acted as electrocatalyst during reduction process. Extremely high current efficiency was achieved on ppy-modified electrode due to the inhibition effect on the side-reaction of hydrogen evolution, exhibiting important advantage over other electrode materials for heavy metal treatment. Conductivities of ppy films after different treatments were detected to evaluate the stability of the polymer.     

Dynamic aspects of the electroreduction of chromic acid solutions

Potentiodynamic and impedance spectroscopy measurements were performed on ARMCO iron, steels (C30, C40, C50), cast iron and graphite electrodes in a bath of industrial composition: CrO₃ 250 g dm^–3, H₂SO₄ 2.5 g dm^–3. Attention focused on the electrochemical reactions occurring before the start of the chromium deposition. The electrochemical evidence is markedly affected by the chemical nature of the cathode. This is rationalized by proposing a reaction mechanism involving the formation, depending on the chemical nature of the substrate, of a passive adsorbed film consisting of mixed chromium and iron oxides. Moreover, the proposed overall scheme accounts for the presence of periodic current oscillations observed both in potentiodynamic and potentiostatic curves. In close agreement, impedance spectra show negative loops when measured at the corresponding potentials.     

Nanostructured catalysts for oxygen electroreduction based on bimetallic monoethanolamine complexes of Co(III) and Ni(II)

Oxygen reduction electrocatalysts based on the monoethanolmine complexes {[CoEtm]₂(μ-Etm)₄Ni(NO₃)₂} and {[CoEtm]₂(μ-Etm)₄Ni(NO₃)₂} + activated carbon AG-3 have been obtained by high-temperature synthesis. The nature of active centers on the synthesized electrocatalysts was described. Using potentiostatic and cyclic potentiodynamic voltammetry, the kinetic characteristics of catalysts in the oxygen electroreduction reaction were determined. Thermal decomposition of the thermally unstable complexes was described and character of the active centers formed was discussed. The optimal synthesis temperature of electrocatalysts is 600 °C in an inert atmosphere. The calculated exchange current densities for the oxygen electroreduction reaction at the catalysts in 1 M KOH at 20 °C was j ₀  = 1.01 × 10^?3 A g^?1–3.3 × 10^?3 A g^?1. The Tafel slopes of stationary polarization curves are 0.054–0.063 V for b ₁ and 0.106–0.125 V for b ₂ . The prepared electrocatalysts can be recommended only for electrochemical systems with alkaline electrolyte.     

Electrode reactions of nickel(II) at mercury electrodes in aqueous solutions of pyridine at high ionic strengths

At high ionic strength the ion pair (NiPy²⁺₄, nX^?) or complex (NiPy₄X₂), n = 0, 1, 2; X^? = Cl^?, Br^?, SCN^?, N^?₃, F^?, NO^?₃, ClO^?₄; is adsorbed at the surface of mercury electrode. Under specified conditions in chloride, bromide, and thiocyanates solutions the electroreduction is preceded by a crystallization of a complex on the electrode surface. The inductive role of specifically coadsorbed Cl^? ions is discussed.     

Investigation of the mechanism and kinetics of dissolution of nickel(II) and nickel(III) oxides in sulfuric acid

Based on the experimental data on the effect of pH and the H₂SO₄ concentration on the dissolution rate of nickel oxides, the nature of the limiting stage is revealed, which consists in the formation of surface compounds and their subsequent transfer into the electrolyte solution. The intermediate products of dissolution of nickel(II) oxide in sulfuric acid are identified.     

Acetylacetonates of nickel(II) and copper(II) as accelerators for the epoxy resin system

The acetylacetonates of nickel(II) and copper(II) serve as accelerators for the anhydridecurable epoxy resin system. An appreciable lowering in the cure gel time and a substantial increase in the rate of curing is observed with enhanced concentrations of the metal chelates. Cure kinetic studies and cure schedules have been followed using a differential scanning calorimeter. The thermal and electrical insulation characteristics of the cured epoxy system are little affected by the variation of temperature, voltage, and aging under UV radiation. © 1994 John Wiley & Sons, Inc.     

Dynamics of hydrated iron(II) cations in nafion polymer membranes

Mössbauer studies of Fe²⁺ in water-soaked nafion polymer membranes in the temperature range between 90 K and 250 K have been performed. Above a critical temperature (～ 180 K) the spectra exhibit both elastic narrow absorption lines and quasielastic broad lines. These spectra are typical of bounded diffusion phenomena observed by Mössbauer spectroscopy in macromolecular systems like haemoglobin, myoglobin and ferritin. Similar spectral shapes have been observed by quasielastic neutron scattering from water in nafion membranes. Within 50 K above the critical temperature the total Mössbauer absorption area decreases by an order of magnitude whereas the narrow absorption line decreases by two orders of magnitude. The results are interpreted in terms of bounded diffusive motion of the iron. Using a model based on overdamped harmonically bound Brownian motion, the essential parameters of the iron motion can be derived as a function of temperature. The iron motion most probably reflects the motion of a large Fe²⁺ complex, e.g. Fe(H₂O)²⁺₆, which is attached to the polymer side chains via the sulphonic group.     

The potentiodynamic electroformation and electroreduction of the O-containing layer on gold in alkaline solutions

The O-containing layer electroformed on Au in alkaline solutions under potentiodynamic conditions is interpreted by a complex reaction pattern which initiates through the electroformation of an OH_ad monolayer. The latter undergoes further electrooxidations and simultaneous chemical transformations yielding the Au(OH)₃-type layer, under different aging conditions accordingly with the perturbation programme used. The open circuit rest potential is interpreted through an equilibrium involving the partial coverage of Au by adsorbed OH.     

A pigment based on coprecipitated iron(III) and nickel(II) hydroxides

The formation of precipitates of iron(III) and nickel(II) hydroxides by coprecipitation is considered. It is established that by changing the pH a hydroxide precipitate that contains Fe and Ni with a ferrite proportion of 2:1 can be prepared. The methods of IR spectroscopy, DTA, and x-ray phase analysis are used to show the possibility of formation of nickel ferrites that are suitable as overglaze paints for decorating porcelain and faience.