High throughput electrochemical screening and dissolution monitoring of Mg–Zn material libraries

A combinatorial study on Mg–Zn material libraries obtained by thermal evaporation is performed in order to investigate the effect of alloying magnesium on the electrochemical behaviour and dissolution rate of zinc in borate buffer of pH 7.4. The surface morphology of the graded samples is complex and subject to a detailed discussion, whereas the crystal composition revealed Mg, MgZn₂ and Zn exclusively.Open circuit potential measurements and potential sweeps along the graded samples are combined with downstream zinc detection and revealed several strongly non linear dependencies between electrochemical features and magnesium content. While the chemical dissolution rate of zinc by the electrolyte was found to reflect the film stoichiometry except in the regions of high surface roughness, the open circuit potential revealed a local minimum around 20 at.% magnesium accompanied by a maximum in the current plateau during the anodic sweep and a high thickness of the native oxides present prior to electrochemical experiments. All compositions showed passive like behaviour during anodic sweeps with high plateau current densities (200–350 μA cm⁻²) originating from slow but constant oxide dissolution as supported by XPS analysis of the surface before and after contact with the electrolyte.     

Importance of water content in formation of porous anodic niobium oxide films in hot phosphate-glycerol electrolyte

Niobium has been anodized at a constant current density to 10 V with a current decay in 0.8 mol dm⁻³ K₂HPO₄-glycerol electrolyte containing 0.08-0.65 mass% water at 433 K to develop porous anodic oxide films. The film growth rate is markedly increased when the water content is reduced to 0.08 mass%; a 28 μm-thick porous film is developed in this electrolyte by anodizing for 3.6 ks, while the thickness is 4.6 and 2.6 μm in the electrolytes containing 0.16 and 0.65 mass% water respectively. For all the electrolytes, the film thickness changes approximately linearly with the charge passed during anodizing, indicating that chemical dissolution of the developing oxide is negligible. SIMS depth profiling analysis was carried for anodic films formed in electrolyte containing ∼0.4 mass% water with and without enrichment of H₂¹⁸O. Findings disclose that water in the electrolyte is a predominant source of oxygen in the anodic oxide films. The anodic films formed in the electrolyte containing 0.65 mass% water are practically free from phosphorus species. Reduction in water content increased the incorporation of phosphorus species.     

Determination of the potential range responsible for the replacement of surface film on LiMn2O4

This study determined the potential range where the dissolution of a surface film on a thin film LiMn₂O₄ electrode, which forms during electrode synthesis, and the formation of a new surface film during cycling at room temperature using an in situ bending beam method (BBM) and an in situ electrochemical quartz crystal microbalance (EQCM) technique with cyclic voltammetry and a galvanostatic charge/discharge cycle. The electrolytes used were LiClO₄/EC-DEC, LiClO₄/PC and LiPF₆/EC-DMC. The deflectogram and massogram showed large peaks during the anodic scan only in the first cycle. These phenomena, were observed regardless of the electrolytes and scan rate used. The tensile strain and the mass reduction in the early stage of the strain peak and the mass peak are related to the dissolution of the initial surface film. In addition, the compressive strain and the mass increase are related to the formation of a new surface film during cycling. The potential ranges where the formation of the new surface film begins ranged from 4.03 to 4.1 V, which appears to terminate at the end of the first anodic scan, and was also observed during the galvanostatic charge/discharge cycle in the same potential range.     

Iron and hydrogen electrodes in liquid ammonia

Equilibrium potentials of the iron(II)—iron electrode vs TlTlCl; 0.1 M NH₄Cl in liquid ammonia are determined from intersections of Tafel lines for iron deposition and dissolution. The hydrogen electrode is shown to behave reversibly at palladium hydride, but not at platinum with respect to the dependence of hydrogen pressure. The standard potential of iron at 293 K is estimated at E° = ?0,356 (±0,010)V in 0.1 M NH₄NO₃. Standard potentials of other electrodes are reevaluated and shown to be consistent with earlier measurements except for the standard potential of lead. Steady state and transient polarization curves indicate in simple mechanism of the iron electrode involving transfer of iron(II) in one step. The temperature-independent anodic transfer coefficient is α₊ = 0.41 (±0.02), the cathodic transfer coefficient α_? = 0.61 (±0.03). The standard exchange current density at E° is j°_° = 7.10^?7 Acm^?2 for 293 K. Exchange current densities of the hydrogen electrode are j_° = 1.6·10^?9 Acm^?2 at platinized platinum in 0.1 M NH₄Cl both for 293 K.     

Investigating mechanisms of anodic film formation by electrochemical probe beam deflection (EPBD)

Electrochemical probe beam deflection (EPBD) was used to investigate the behaviour of a silver electrode immersed in 0.1 M KClO₄ solution containing various concentrations of KBr. The EPBD response was followed under potentiodynamic conditions and after potential steps. The EPBD transients were interpreted using standard models for the transport of ions in the diffusion layer. The AgBr_(s) films formed after potential steps to mild anodic limits follow a nucleation-growth mechanism with progressive nucleation. The good agreement between the EPBD data obtained at intermediate anodic limits and linear diffusion models indicate that the film thickening was controlled by the diffusion of bromide ions from the bulk solution. EPBD experiments performed at high positive overpotentials and low bromide concentrations showed a net flux of ions away from the surface, suggesting that soluble Ag_(aq)⁺ ions were produced through the porous AgBr_(s) structure in these conditions. The EPBD curves obtained during the reduction of the film agreed well with a discontinuous diffusion model. However, EPBD experiments performed after film formation under high anodic limits and/or high bromide concentrations showed a complex pattern for the beam deflection during the reduction. In these cases, the EPBD signal was complicated by additional chemical process that occurred simultaneously with the reduction of the film.     

The anodic behaviour of zinc in aqueous KOH solution—II. passivation experiments using linear sweep voltammetry

Potentiodynamic experiments in the range 60–0· m V/s, designed to elucidate the processes by which the zinc electrode in alkali becomes passive, are described. At low overpotentials the dissolution process is a complex solution mechanism in which soluble zincate species leave the electrode by the processes of mass transport in solution. At overpotentials in the region 0·4–1·5 V the dissolution process is slow and involves the formation of a layer of ZnO on the electrode. The lattice dissolves via this layer at a low rate. The electrode is not covered by a mechanically sound film of ZnO until the overpotential has risen to ca 1·5 V. It is not possible to decide whether the initially formed film of ZnO comes from the electrolyte or is formed in situ on the lattice by direct oxidation.     

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.     

Passivity and passivity breakdown of a zinc electrode in aerated neutral sodium nitrate solutions

The passivation and pitting corrosion behaviour of a zinc electrode in aerated neutral sodium nitrate solutions was investigated by cyclic voltammetry and chronopotentiometry techniques, complemented by ex situ scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray (EDX) examinations of the electrode surface. Measurements were conducted under different experimental conditions. The potentiodynamic anodic polarization curves do not exhibit active dissolution region due to spontaneous passivation. The passivity is due to the presence of thin film of ZnO on the anode surface. The passive region is followed by pitting corrosion as a result of breakdown of the passive film. SEM images confirmed the existence of pits on the electrode surface. The breakdown potential decreases with an increase in NO₃⁻ concentration and temperature, but increases with increasing potential scan rate. Addition of SO₄²⁻ ions to the nitrate solution accelerates pitting corrosion, while addition of WO₄²⁻ and MoO₄²⁻ ions inhibits pitting corrosion. The chronopotentiometry measurements show that the incubation time for pitting initiation decreases with increasing NO₃⁻ concentration, temperature and applied anodic current density. Addition of SO₄²⁻ ions decreases the rate of passive film growth and the incubation time, while the reverse changes produced by addition of either WO₄²⁻ or MoO₄²⁻ ions.     

Untersuchung der teilströme der legierungs-auflösung und der deckschichtbildung an Cu70Zn30 in ammoniakalischer lösung mit der rotierenden scheiben-ring-elektrode

A quantitative electrochemical investigation of the formation of copper oxide films on Cu70Zn30 in [NH₃ + (NH₄)₂SO₄] solutions was carried out on the basis of measurements obtained using a rotating ring-disc electrode. It was found that the non-steady partial current densities of the alloy dissolution are prominent during the formation phase of the Cu₂O film and Cu₂/CuO transformation and that the film formation partial current densities in these ranges are considerably lower than during the quasi-stationary growth of both oxide films. Investigations into the influence of the rotation speed of the disc electrode on the disc current density have shown that a diffusion-controlled electrochemical process takes place during Cu₂O formation and that the Cu₂O/CuO transformation characterises the independence of the rotation speed of the electrode that is typical for film formation processes.     

Effects of introduced electrolytes on galvanic deposition of ZnO films

The effects of supporting electrolytes on galvanic deposition of ZnO films have been investigated in detail by respectively introducing K₂SO₄, KNO₃ and KCl into Zn(Ac)₂ electrolytes. It reveals that the chemical nature of introduced electrolytes plays important roles in acting on the growth of ZnO films. ZnO nanorods tend to grow in KNO₃ and KCl electrolytes, while sheet-like zinc hydroxysulfate tends to be formed in K₂SO₄ electrolyte. Besides, KNO₃ electrolyte is inclined to accelerate the passivation of Zn anode, resulting in the sharp decrease of driving force for galvanic deposition. In contrast, KCl and K₂SO₄ electrolytes facilitate zinc dissolution by anionic adsorption on the metal surface and subsequent participation in the active dissolution process, thus leading to the incorporation of anions in nanocrystals.     

Passivation of zinc in concentrated alkaline solution—I. Characteristics of active dissolution prior to passivation

As a preliminary stage in the study of the active—passive transition on zinc in concentrated (6.8 mol dm⁻³ solutions of KOH the reported mechanisms have been reviewed and the characteristics of active dissolution prior to passivation have been investigated. The theory of Armstrong and Bulman has been extended in order to relate all the experimentally important parameters. These include electrode potential, current, angular electrode rotation speed and surface concentrations of Zn(OH)²⁻₄ and OH⁻ ions. In common with other observations, it is demonstrated that zinc dissolves quasi-reversibly over a considerable range of potentials. The relationship of this behaviour to other metal/metal-ion systems is discussed. At high current densities (> 100 mA cm⁻²) depletion of hydroxide ions occurs at the zinc surface causing a severe deviation in the exponential dependence of the active dissolution current on potential.     

Influence of methylcellulose-coated paper separators on the corrosion,polarization and impedance characteristics of zinc in concentrated ammonium chloride solution. I. Behaviour in flooded electrolyte

The corrosion characteristics of both pure and amalgamated zinc have been studied in 6.0 M NH₄ Cl using steady-state polarization and a.c. impedance methods, and the influence of methylcellulose-coated and uncoated paper separators have been determined. Irrespective of the type of separator present, the results can be interpreted almost exclusively in terms of charge-transfer effects. In the presence of the base paper the anodic zinc dissolution process is inhibited by 79% whilst the rate of cathodic hydrogen evolution is inhibited by only 43%. Similarly, in the presence of the complete methylcellulose-coated separator formulation the anodic reaction is inhibited by 98–99% whilst the cathodic reaction is inhibited by only 60–74%. In either case it is shown that the anodic inhibition is considerably greater than that expected on the basis of a simple blocking type model, whereas the cathodic inhibition is considerably less. Evidence is presented which suggests that the excessive inhibition of the anodic current is a consequence of the specific influence of the separator on the stability and adsorption behaviour of Zn(I) intermediates in the zinc dissolution reaction. It is also postulated that the deficient inhibition of the cathodic current may result from the small size and high mobility of the proton which allows it to penetrate those regions of the metal-separator interface normally inaccessible to bulkier ions. Overall, the corrosion-inhibiting efficiency of the base paper is 41% whilst that of the complete methylcellulose-coated paper separator formulation is 70–75%. These results reflect a substantial influence of the separator material on the corrosion behaviour of zinc.     

Anodic film studies on nickel under high rate transpassive dissolution conditions

Coulometry and Auger Electron Spectroscopy (AES) were employed to study thickness and composition of anodic films formed on nickel under high rate transpassive dissolution conditions. Nickel anodes were polarized at constant current densities up to 30 A/cm² in alkaline nitrate electrolytes of different nitrate and hydroxyl ion concentration using a flow channel cell with a constant electrolyte flow velocity of 10m/sec. Results show that with increasing current density film thickness goes through a maximum. Nitrogen is detected at the apparent film metal interface in the current region where metal dissolution occurs. No correlation between anodic film thickness and dissolution efficiency is found. The data, together with previous observations, suggest that high rate transpassive dissolution takes place from film free sites.     

Corrosion and polarization characteristics of zinc in neutral—acid media — II. Effect of NH+4 ions and the role of battery zinc alloying constituents

The corrosion and polarization characteristics of zinc have been studied in molar, quiescent solutions of NH₄ClO₄, (NH₄)₂SO₄, NH₄Cl and NH₄Br in the pH range 3.8–5.8. Corrosion rates have been determined by the Tafel line extrapolation procedure and the polarization resistance method where possible. Numerical values of the corrosion currents are compared. The role of NH⁺₄ ions, anions, pH and battery zinc alloying constituents on the nature of the rate controlling anodic and cathodic processes are discussed.     

Impedance study of hydrogen evolution on Ni/Zn and Ni–Co/Zn stainless steel based electrodeposits

A study on the electrocatalytic performance of Ni/Zn and Ni–Co/Zn alloys for hydrogen evolution reaction (HER) in alkaline media (30 wt.% KOH solution) has been carried out. After preparing by electrodeposition on stainless steel supports, the alloys were leached of to remove part of the zinc and generate a porous layer. For the developed electrodes, the surface roughness factor, R_f, was evaluated by electrochemical impedance spectroscopy (EIS). The HER on these electrodes was evaluated by means of steady-state polarization curves and EIS. The obtained electrodes were characterized by large R_f for HER, and very low overpotentials at the current density of 250 mA cm⁻², η₂₅₀ ∼ 0.138 V at 30 °C. The high electrode activity was mainly attributed to the high surface area of the developed electrodes.     

Reversibility of electrochemical magnesium deposition from tetrahydrofuran solutions containing pyrrolidinyl magnesium halide

In this paper we reported for the first time the electrochemical behavior of pyrrolidinyl magnesium halide (C₄H₈NMgX, X = Br and Cl) dissolved in tetrahydrofuran (THF) with regard to potential application as electrolytes for rechargeable magnesium batteries. C₄H₈NMgX/THF solutions were characterized in term of conductivity, anodic stability, and reversibility of magnesium deposition and dissolution. Furthermore, the effect of metal substrates on magnesium deposition process was studied. Simple preparation, high cycling reversibility and moderate anodic stability of C₄H₈NMgBr/THF solution indicate the feasible application as the electrolyte for rechargeable magnesium batteries.     

Method for Recovering Anhydrous ZnCl2 from Aqueous Solutions

Abstract

To develop technology to assure an ample supply of zinc and to reduce environmental pollution, the U.S. Bureau of Mines investigated alternatives to the roast-leach process for treating complex sulfide concentrates. Previous studies proved that low-grade zinc sulfide (ZnS) concentrates could be leached using chlorine-oxygen to produce zinc chloride (ZnCl₂). The process involves high energy requirements for evaporating the pregnant solution to produce anhydrous ZnCl₂ needed for electrolytic cell feed. An efficient hydrometallurgical process would facilitate treatment of lower grade ores that can be used in conventional processing and would render roasting unnecessary.

It is difficult to keep ZnCl₂ anhydrous as it is hygroscopic and deliquescent. Therefore, an alternate method of producing a feed material from solution, which could be stored without absorbing H₂O, was sought. Zinc diamine chloride [Zn (NH₃)₂Cl₂], was precipitated from solution by adding ammonium chloride (NH₄Cl) and sparging with ammonia (NH₃) to a pH of 6 to 7.5. The spent solution was treated with calcium hydroxide [Ca (OH)₂] at 60° to 80°C for 1 to 4 h to remove remaining zinc and NH₃. The Zn (NH₃)₂Cl₂ was heated to 300° to 400°C to remove NH₃ and produce anhydrous ZnCl₂. A possible flowsheet was devised and will be presented.     

Photocatalytic Activity of Highly Porous Zinc Ferrite Prepared from a Zinc-Iron(III)-Sulfate Layered Double Hydroxide Precursor

A zinc-iron(III)-sulfate layered double hydroxide (LDH) has been prepared from zinc and iron(II) precursors. Calcination at 500°C or above affords a mixture of ZnO (zincite) and ZnFe₂O₄ (spinel) phases. Treatment of the calcined products with aqueous NaOH leads to dissolution of the ZnO and the formation of a pure zinc ferrite phase with high surface area and pore volume. When used as a photocatalyst for the degradation of phenol, the active lifetime of the catalyst was extended compared with a sample of zinc ferrite prepared by a conventional co-precipitation route followed by calcination under the same conditions. The highly porous material also exhibited a higher selectivity to low molecular weight oxidation products than the co-precipitated catalyst.     

The anodic dissolution of copper single crystals

The electrochemical dissolution of copper single crystal spheres in 0.5 M H₂SO₄ with CuSO₄ (0.02 or 0.1 M) results in the Dissolution-forms [111] (Octahedron) at low (< 130 mV) and spheres at high (> 130 mV) anodic overpotentials. The formation of low indexed steps can be observed. The charge-transfer reaction given by the Butler—Volmer equation is the rate determining step of the dissolution at high overpotentials. In the case of low overpotential surface diffusion depending on the crystal-orientation affects the dissolution process also. The dissolution rate at low overpotential depends strongly on the dislocation density of the copper single crystals. Two dimensional models for the construction of dissolution forms are presented.     

The influence of sorbitol on zinc film deposition, zinc dissolution process and morphology of deposits obtained from alkaline bath

A novel cyanide-free zinc deposition bath was developed in which sorbitol was added at various concentrations. Voltammetric studies indicated that the reduction process is influenced thermodynamically and kinetically by the sorbitol concentration. Also, two cathodic processes were observed, one (wave) associated with the hydrogen evolution reaction (HER) on 1010 steel, the other (peak) with zinc bulk reduction, simultaneous to the HER. Furthermore, the plating-process kinetics was controlled by mass transport. The presence of sorbitol in the bath led to formation of light-grey zinc films, even during the HER, without cracks and dendrites. Plating current efficiency decreased from ∼ ∼62% to 43% on increasing the sorbitol concentration in the plating bath. In the presence of 0.1 M and/or sorbitol concentrations higher than 0.2 M, Zn electrode dissolution was inhibited. However, a small dissolution of zinc electrode was observed with 0.05 M sorbitol in alkaline solution without zincate. SEM micrographs showed that the 1010 steel substrate was fully covered by Zn film and that the sorbitol affected the morphology of zinc films, acting as a grain refiner.