The effect of silicate ion on the anodic behaviour of zinc in concentrated alkali

The anodic behaviour of zinc in 8 M NaOH/1 M Na₂Zn(OH)₄, containing 0.1, 1 and 10 v/o sodium silicate solution, has been studied under quiescent conditions at ambient temperature using cyclic linear sweep voltammetry in conjunction with darkfield illumination optical photo-microscopy. Results, together with measurements of the double layer capacitance of the zinc/alkali interphase as a function of potential, suggest the presence of a surface film of silicate which induces early passivation in the anodic potential excursion.     

The electrodeposition and dissolution of zinc and amalgamated zinc in alkaline solutions

The reaction mechanism of zinc and amalgamated zinc was investigated with the galvanostatic transient technique in the concentration range 1.5–10 M KOH. The Tafel slopes of the zinc electrode were 40 mV anodically and 120 mV cathodically. The cathodic reaction order in zincate was found to be +1. From the Tafel slopes and the dependence of the exchange current density on the activity of the KOH, the reaction orders in OH^? were calculated, yielding values of 2.3± 0.8 in the anodic and ?0.8 ± 0.2 in the cathodic direction. These results are consistent with the suggested mechanism of Bockris et al.[16] for the zinc electrode. The Tafel slope in cathodic direction of the amalgamated zinc electrode was a function of the KOH concentration (120 mV at KOH concentrations up to 3 M; about 60 mV in 10 M KOH); the anodic Tafel slope was 30 mV over the whole concentration range. These results and measurements at constant ionic strength suggest a mechanism which involves the participation of water. The difference in behaviour of the zinc electrode and the amalgamated zinc electrode is probably caused by changes in the adsorption characteristics due to amalgamation.     

Cyclic voltammetric study of zinc and zinc oxide electrodes in 5.3 M KOH

The behaviour of zinc and zinc oxide in 5.3 M KOH in the presence of alkaline earth oxides, SnO, Ni(OH)₂ and Co(OH)₂ was examined by cyclic voltammetry. The influence of the alkaline earth oxides was compared with additives of established effects (Bi₂O₃, LiOH, Na₂CO₃ and CdO). The alkaline earth oxide each exhibits a distinct behaviour towards zincate. Whereas, a single process of interaction with zincate was shown by CaO; two modes of reaction were obtained with SrO and BaO. Solid solution formation was noticed with BeO and MgO. The other additives forming solid solution with ZnO were CdO, SnO. The ionic sizes of Ni(OH)₂ and Co(OH)₂ allow solid solution formation with Zn(OH)₂. Both Bi₂O₃ and Na₂CO₃ enter into complexation with zincate. LiOH forms two distinct zincates, of which one is an oxo zincate leaching the `hydroxyl' functionality. Cyclic voltammetry revealed the deposition of the oxide/hydroxide additives as metal prior to the onset of zinc deposition and the potential range for this additive metal deposition is almost the same for different additives (SnO, CdO, Ni(OH)₂). The beneficial action of these additives to zinc alkaline cells is associated with a substrate effect. The implication of this electrocatalytic deposition of metals on a zinc oxide electrode is also discussed.     

Comparative study of the potentiodynamic behaviour of aluminium at low potentials in barrier layer and in pore-forming electrolytes

The potentiodynamic behaviour of a 99.9995% aluminium electrode in several barrier layer and pore-forming electrolytes at a temperature of 25° C, with sweep rates in the range 1–200 mV s^–1 and potentials between –2.00 and 3.00 V (versus SCE), has been studied. In the anodic sweep the potential of zero current,V _j=0, depends on the pH of the electrolyte and corresponds to a corrosion potential (the cathodic and anodic reactions coexist). In the 0.5 M H₃BO₃-borax solution (pH=6.3) and afterV _j=0, the current density increases in a convex form, approaching a steady-state value. For this electrolyte and the potential ranges studied, the anodic charges corresponding to the anodic sweep are independent of the sweep rate and are associated with the formation of a barrier layer oxide. In the neutral 0.50 M propanedioic acid solution (pH=7.0) and the potential ranges studied, the anodic charges are much greater than those corresponding to the H₃BO₃-borax solution, and metal corrosion combined with a solvent effect of the electrolyte is found. The greatest anodic charges are found for the H₂SO₄ solution (pH=1.0); the anodic current increases rapidly in the potential ranges studied due to pore initiation. The cathodic and anodic behaviour of aluminium depend strongly on the electrolyte employed, basically through the effect of the electrolyte on the oxide film and through the pH of the solution.     

Cyclic voltammetric studies of electroless cobalt in NaOH

The electrochemical behaviour of electroless cobalt in 1N NaOH was studied at 35, 60 and 85°C by cyclic voltammetry. Three anodic peaks corresponding to the formation of Co(OH)₂/CoO, Co₃O₄, Co OOH were observed. The observed cathodic peaks were due to the reduction of the higher cobalt oxides to Co(OH)₂ and also due to the reduction of Co(OH)₂ to cobalt. The variation of peak potential with sweep rate was found to be appreciable only in the case of anodic peak (I). The total charge Q_T, for the passivation to occur was obtained by estimating the area under the first anodic peak at each sweep rate. It was found that the Q_T value decreases with increase of sweep rate. The increase of peak current with increase of sweep rate has been explained as due to the insufficient time available for the film growth at the reversible potential, and that the dissolution of the metal continues until a potential at which the rate of film growth exceeds that of active dissolution is reached. It has been suggested that the passivating film is CoO while the prepassive film is Co(OH)₂. The shift of the anodic peak (I) potentials to more anodic values compared to that for pure cobalt is probably due to the presence of phosphorus in the cobalt deposit.     

Kinetics and mechanism of deposition of zinc from zincate in concentrated alkali hydroxide solutions

Galvanostatic investigation has been carried out of the kinetics of the reaction on zinc amalgam hanging mercury drop immersed in alkaline zincate solutions at KOH concentrations ranging from 1 to 14 normal. Three kinds of data have been extracted: (i) the pseudo-capacitance as a function of overpotential from the portion of the charging curves prior to plateaux, (ii) the Tafel plots from quasi-steady state values at the plateaux and (iii) the Sand's products as function of current density from transition times. In (i), the pseudo-capacitance was found to increase with overpotential, in (ii), two slopes (60 mV dec^?1 and 120 mV dec^?1) have been found in the cathodic direction and a limiting current in the anodic one; in (iii), the Sand's product was found to decrease with increasing (i). On the basis of the above findings the reaction mechanism has been suggested to be of the cece type, with the chemical dissociation of the intermediate univalent zinc complex in between the two elctrochemical steps as rate-determining. A modification of the method of evaluating the reaction orders was introduced to account for a simultaneous change of more than one activity of reactants in concentrated solutions. Using this, reaction order was found to be 2 with respect to both OH^? ions and water, suggesting that the electroactive species is not only reduced in the number of ligands but also dehydrated with respect to the prevailing species. The anodic limiting current appears to be due to the accumulation of Zn(OH) species to cover all the surface free of adsorbed Zn(OH)₂. Exchange cd and rate constants of all the four steps of the reaction have been estimated.     

Voltage decay at passivated zinc anodes

A study has been made of the passivating process at a zinc electrode in KOH solutions. Zinc electrodes were passivated at a constant overpotential and the current response during passivation was measured. The potential response after the passivating potential was removed was also measured.The current during passivation soon reached a semi-steady-state value which increased with increasing overpotential but varied only slightly with changing KOH concentrations.When electrodes were passivated at overpotentials >325 mV, the open circuit voltage decay showed an arrest, the duration of which decreased with increasing KOH concentration. This duration increased when ZnO was dissolved in the electrolyte, when the temperature was decreased, and when the passivating overpotential was increased.The results are interpreted by assuming that passivation is due to the formation of a film, possibly Zn(OH)₂, which can dissolve in the electrolyte. The potential of the electrode is a mixed potential.     

Non-equilibrium effects in the nickel hydroxide electrode

The electrochemical reaction Ni(OH)₂=NiOOH+H⁺+e occurring under potentiodynamic conditions in 1 N KOH using complex triangular potential sweep revealed the existence of non-equilibrium effects of both reactant and products which cause the splitting of the anodic current peak already known. By a suitable choice of the perturbation variables, the rate of the chemical change the Ni(OH)₂ species undergoes can be estimated. The reaction presenting the nickel hydroxide electrode is interpreted as two parallel electron transfer steps coupled to two chemical reactions through a square-type reaction model.     

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.     

Structural and electrochemical characterization of mechanochemically synthesized calcium zincate as rechargeable anodic materials

Hydrated calcium zincate was synthesized by mechanical ball milling of ZnO and Ca(OH)₂ in water at room temperature. The structural and electrochemical properties of this material used as rechargeable anodic material were examined by microelectrode voltammetry, charge–discharge measurements and structural analysis. The results showed that during mechanical milling, ZnO, Ca(OH)₂ and H₂O reacted rapidly to form Ca[Zn(OH)₃]₂ · 2H₂O which was subsequently transformed to a stable structure CaZn₂(OH)₆ · 2H₂O. Since this composite oxide has lower solubility in KOH solution (<35 wt %) and better electrochemical reversibility than ZnO-based negative materials, the zinc anodes using this material can overcome the problems of shape changes and dendritc formation, and therefore exhibit improved cycling life.     

Electrochemical study on the inhibitory effect of the underpotential deposition of zinc on Zn-Co alloy electrodeposition

Zn-Co alloy electrodeposition from chloride baths containing different Zn²⁺/Co²⁺ ratios was investigated by cyclic voltammetry and anodic linear sweep voltammetry using a Pt electrode. The peaks were attributed by means of EDX analysis, SEM and TEM observations performed on some alloys potentiostatically deposited. In the range of potential where zinc deposits underpotential, cyclic voltammetry showed a complex cathodic peak with one maximum and two shoulders, correlated with the deposition of different cobalt rich alloys. Up to four anodic peaks, two correlated with zinc oxidation from η and γ phases and two correlated with oxidation of solid solutions of zinc in cobalt, were observed. ALSV and TEM indicated that the remarkable increase in Zn content of the alloy, which occurs with a strong inhibition of the process at potentials more negative than that of the cathodic peak and more positive than the bulk deposition potential of zinc, is due to the deposition of γ phase. No inhibition of the alloy deposition process was observed with very low concentrations of zinc (<0.015 M) in the bath containing 0.19 M Co²⁺.     

Structural and catalytic promotion of skeletal copper catalysts by zinc and chromium oxides

ZnO and Cr₂O₃ promoted skeletal copper catalysts have been prepared by leaching CuAl₂ alloy particles in aqueous sodium hydroxide containing various concentrations of sodium zincate (Na₂Zn(OH)₄) (0–0.62 M), of sodium chromite (NaCrO₂) (0–0.1 M) or of a mixture of sodium zincate (0.005 M) and sodium chromite (0.02 M). Both sodium zincate and sodium chromite in the caustic solution were found to affect the leaching process. The presence of sodium zincate retarded the leaching rate and enhanced the specific surface area of the catalyst but did not protect copper rearrangement on extended leaching. On the other hand, sodium chromite in the leachant did not influence the leaching rate but the leached catalyst was more stable with the structure and surface area of the skeletal copper being maintained. The activities of ZnO and Cr₂O₃ promoted skeletal copper catalysts for the reactions of methanol synthesis, water gas shift and methanol steam reforming were determined separately. The results indicated that deposition of both ZnO and Cr₂O₃ on skeletal copper catalysts significantly improved the activities for these reactions. Chromia is found to act mainly as a structural promoter. This revised version was published online in June 2006 with corrections to the Cover Date.     

Hydrogen production during zinc deposition from alkaline zincate solutions

The determination of the amount of hydrogen produced during the electrodeposition of zinc from alkaline zincate solutions was carried out using the rotating ring-disc electrode (RRDE) technique. The experimental conditions for which the RRDE technique offers reliable results are discussed. Hydrogen production during zinc deposition was studied for a range of cathodic (disc) current densities (20–500 A m^–2) and electrolyte compositions (1–7 M KOH, 0.01–0.2 M zincate). It was found that an increasing amount of hydrogen was formed with increasing (disc) current density and decreasing KOH and zincate concentration. The impact of hydrogen formation during the charging process on nickel oxide/zinc secondary battery performance is expected to be small. It is concluded that in battery electrolytes (8 M KOH, 1 M zincate) hydrogen is formed chiefly by corrosion of the zinc electrode rather than by electrochemical formation during the electrochemical reduction of zinc.     

Perchlorate and oxygen reduction during Zn corrosion in a neutral medium

A comparison between the potentiodynamic behaviour of the stationary and the rotating Zn disc electrodes in naturally aerated and de-aerated 0.1 M KClO₄ solution was performed. The voltammograms of the stationary electrode in both solutions exhibited one anodic peak and two cathodic peaks. The anodic peak is replaced by two mass transport controlled O₂ reduction cathodic current plateaus in the forward scan by rotating the electrode in naturally aerated solutions. However, the reverse scan is characterized by only one cathodic peak observed at a potential depends on the experimental conditions. The more cathodic reduction peak was referred to the reduction of the passive layer and split into two peaks at low scan rates. Interpretation of these data was made adopting a multi-path mechanism and a two layer passive film model. A correlation between the ClO₄⁻ and dissolved O₂ reduction and the thickness of the two passive layers was performed. The protective nature of the passive layers formed in different experimental conditions was found to decrease with rotating the electrode and de-aerating the solution. Chrono-potentiometry and electrochemical impedance measurements were also used in this study. Impedance technique showed a change in the ZnO thickness with the experimental conditions as a result of changing the reactions occurring in the electrode vicinity.     

Triangular potential sweep voltammetric study of porous iron electrodes in alkali solutions

The cyclic voltammograms of pure iron and sintered iron electrodes in 6.0m KOH solutions revealed a plateau and two anodic peaks in the forward direction and two cathodic peaks in the backward direction when polarized from –1.3 to –0.3 V vs Hg/HgO. In the forward scan the formation of Fe(OH)₂ and FeOOH occurs and these are subsequently reduced to Fe(OH)₂ and iron in the backward scan. The peak potential separation of the Fe/Fe(II) and Fe(II)/Fe(III) couples at zero sweep rate and the ratio of cathodic to anodic charges at zero sweep rates for the above two redox couples have been used to evaluate the reversibility of porous iron electrodes. Additions of LiOH, Na₂S, FeS, sulphur, Sb₂O₃ and As₂O₃ on the reversibility of these redox couples have been discussed. A suitable electrode fabrication condition has been suggested.     

Effects of a small addition of Mn on the corrosion behaviour of Zn in a mixed solution

The effects of a small addition of Mn (0.4 wt%) on the corrosion behaviour of pure Zn (99.995 wt%) in a mixed solution (0.1 M NaCl + 0.1 M Na₂SO₄ + 0.01 M NaHCO₃, pH 8.4) were investigated by electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and X-ray photoelectron spectroscopy (XPS). The electrochemical impedances of both Zn and Zn–0.4Mn have been successfully fitted with a suitable EIS equivalent circuit model. Fitted impedance results revealed that 0.4 wt% Mn improved both the pore resistance and charge transfer resistance of Zn in the mixed solution. As a result, both anodic and cathodic reaction rates were reduced. X-ray photoelectron spectroscopy (XPS) analysis showed that the corrosion films formed in the mixed solution consisted of zinc oxide (ZnO), zinc hydroxide (Zn(OH)₂) and zinc hydrozincite (Zn₅(CO₃)₂(OH)₆). The role of small addition of Mn is that it promotes the precipitation of hydrozincite in the pores of corrosion film. An “alleviation of local acidification” mechanism is proposed to explain the investigated results.     

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.     

Electrochemical studies with tin electrodes in citric acid solutions

The electrochemical behaviour of tin in 0.5 M citric acid solution (pH=1.8) was studied by means of the potentiodynamic method. TheE-I profiles show an anodic current peak associated with a dissolution of the metal and the formation of a passivating film, and two cathodic current peaks which are related to the reduction of soluble Sn(II) species and reduction of the film. During the potential sweep in the cathodic direction, an anodic current peak can be observed and is interpreted in terms of a reactivation process. The data suggest that the passivation of tin in this medium occurs by a dissolution-precipitation mechanism. Depending on the potential sweep rate, the process is controlled either by mass transport or by the solution resistance in the pores of the film.     

Methanol Synthesis from CO2 Using Skeletal Copper Catalysts Containing Co-precipitated Cr2O3 and ZnO

Skeletal Cu-Cr₂O₃-ZnO catalysts have been prepared by leaching CuAl₂ alloy particles at 273 K using 6.1 M aqueous NaOH solutions containing sodium chromate (Na₂CrO₄) and sodium zincate (Na₂Zn(OH)₄). The presence of sodium chromate and sodium zincate in the caustic solution was found to affect the pore structure and surface areas of the resulting catalysts. Both BET and Cu surface areas were increased by increasing the concentration of Na₂CrO₄ and of Na₂Zn(OH)₄.Increasing the Na₂CrO₄ level from 0 to 0.06 M in a 6.1M NaOH solution containing 0.2M Na₂Zn(OH)₄ caused the content of ZnO in the catalyst to decrease from 8.8 to 3.0 wt% whilst increasing the Cr₂ O₃ content from 0 to 1.7 wt%, indicating that the presence of Na₂CrO₄ in the leach liquor not only resulted in deposition of a Cr compound but also inhibited precipitation of zinc hydroxide onto skeletal Cu catalysts. On the other hand, increasing the concentration of Na₂Zn(OH)₄ from 0 to 0.6 M in a 6.1 M NaOH solution containing 0.008 M Na₂ CrO₄ resulted in increasing the ZnO loading from 0 to 8.9wt% with an almost constant content of Cr₂ O₃ (1.3 ± 0.2%) in the catalysts, revealing that sodium zincate only led to precipitation of zinc hydroxide and did not suppress Cr₂O₃ formation.Hydrogenation of CO₂ was studied using a gas mixture of 24% CO₂ in H₂ at a total pressure of 4MPa, space velocities up to 210000L kg^-1h^-1 and temperatures in the range 493-533K. The catalysts were found to be both highly active and selective for methanol synthesis. This study confirms the role of ZnO in promoting the activity of copper for methanol synthesis from CO₂ and improving the selectivity by inhibiting the reverse water-gas shift reaction. The role of Cr₂O₃ is to improve the structural development of high surface area skeletal copper.     

Aspects of electrochemical behaviour of carbon steel in different Bayer plant solutions

Cyclic voltammetric and potentiodynamic studies were carried out on 300W carbon steel in Bayer plant solution, at 100 °C, with different alumina concentrations. Alumina behaves as an anodic inhibitor, shifting the critical passivation potentials positively and decreasing the critical passivation current with increasing concentration. Increase in alumina concentration promotes the formation of a uniform and less porous film. The pore resistance model describes the properties of the oxide films. Aluminium was found in all oxides formed, supporting the formation of a mixed oxide Fe_3–x Al _x O₄. Thermodynamic calculation of some equilibrium potentials was carried out using the Fe(OH)₃ ^– ion rather than HFeO₂ ^– ion. Moreover, the Al(OH)₄ ^– ion was considered instead of AlO₂ ^– ion in the oxidation process.