Dynamics of metal electrodeposition inside porous electrodes: Effect of the oxidant reduction reaction

Effect of the more electropositive reaction of the oxidant reduction on the metal deposition process inside a porous electrode (PE) for a direct-flow potentiostatic regime of electrolysis is studied with the aid of a mathematical model that was developed previously. It is shown that the marked worsening of dynamic indicators characterizing the process (decrease in the rate of deposition and final weight of metal, its localization in a narrow layer near the front end of PE) is caused not only by the worsening of potential distribution inside PE at the expense of the oxidant reduction reaction but also by anodic dissolution of the metal deposit in cathodically unprotected areas of PE. The effect various factors exert on the dynamics of the emergence and development of an anodic zone inside a cathodically polarized PE and possible ways to suppress it are considered.     

Dynamics of codeposition of metals inside porous electrode operating in direct-flow mode

Using an extended dynamic model of liquid flow-through porous electrode (PE), the effect of kinetics of deposition of individual components and conditions of potentiostatic electrolysis on the dynamics and final parameters (the cathodic deposit weight, the ratio between the amounts of components, and the spatial distribution of components) of codeposition of two metals M₁ and M₂ is studied. An equipotential PE operating in the direct-flow mode in the absence of anodic dissolution of electronegative component M₂ is considered. The effects of concentration, exchange currents, a difference between the equilibrium potentials M₁ and M₂, a prescribed voltage on PE, and solution flow velocity and direction are analyzed. It is shown that, for this version of codeposition of metals, the rates of M₁ and M₂ deposition averaged over the PE width are constant in time. However, this does not mean that their local deposition rates are constant. The general tendency is that the metal deposition rate on the rear part of PE decreases with the time, whereas the deposition rate on the frontal zone of PE, which is closer to the anode, increases. As a result, the final profiles for M₁ and M₂, which are calculated for equal deposition times taking into account and ignoring the redistribution of current during the deposition, differ essentially.     

Effect of circulating solution volume on codeposition process of two metals within porous electrode at high flow rate

The effect of the solution volume and deposition potential on the dynamics and final parameters of the deposition process was studied using a mathematical model of codeposition of the two metals within a one-dimensional flow porous electrode (PE). The initial PE characteristics, kinetic parameters of the overall polarization curve, and volume solution flow rate were fixed. It was found that a decrease in the solution volume at a high flow rate apart from the expected consequences (a decrease in the metal deposition rate in time and an increase in the time of the porous matrix filling by the deposit) results in a significant improvement of the uniformity of distribution of electropositive component M₁ and a dramatic extension of the deposition region of electronegative component M₂. The positive effect of a decrease in the solution volume is enhanced at an increase in the cathodic potential and the closing in of partial currents of the deposited metals. Eventually, these conditions provide simultaneously a high rate of metal recovery and maximum PE filling by the cathodic deposit.     

The effect of the solution flow rate on the final mass of metals deposited into a porous electrode at individual and joint deposition. Frontal delivery of solution

Calculations in terms of the model of metal codeposition into a porous electrode (PE) and experimental measurements are used for studying how the flow rate m _v and the volume of circulating solution at its frontal delivery affect the final mass of the cathodic deposit and its spatial distribution. For both individual and joint deposition of metals, the correlation between the final mass of the cathodic deposit m _f and the uniformity of its distribution in PE is demonstrated: the more uniform the deposit distribution the higher its final mass. Due to peculiarities of the frontal delivery (the fixed position of the maximums of polarization and metal concentration and the same direction of their shift within the PE), the dependence of the final deposit mass on the volume flow rate demonstrates a very flat maximum shifted to sufficiently high m _v. The increase in cathodic polarization and the transition to circulation with a small solution volume lead to the analogous shift. As a result, for the frontal delivery, the region of high flow rates turns out to be relatively more advantageous for the metal deposition into PE. For codeposition of silver and copper from their alkaline thiosulfate solution at the frontal solution delivery, the qualitative agreement between experimental and calculated m _f = f(m _v) dependences is demonstrated.     

The role of sulfide ions in the electrode processes involving gold in sulfite-thiocarbamide electrolytes

The regularities of electrochemical deposition and dissolution of gold in the mixed sulfite-thiocarbamide electrolytes in the absence and in the presence of sodium sulfide additive are studied by using the voltammetric measurements on a renewable electrode and quarts microgravimetry. It is shown that, in the cathodic metal deposition, an addition of sodium sulfide promotes the depolarization effect, which is caused by the presence of thiocarbamide in the solution. Under the anodic polarization of gold in the mixed sulfite-thiocarbamide solution with pH < 10, the gold dissolution rate is insignificant. An addition of 10^?5 M Na₂S to this solution dramatically accelerates the process. At pH > 10, the gold dissolution in the sulfite-thiocarbamide electrolyte is observed even in the solution free of Na₂S additive. It is evidenced that this is associated with spontaneous accumulation of sulfide-containing species in the solution, probably, as a result of thiocarbamide hydrolysis; the rate of hydrolysis steeply increases with increasing pH value.     

Peculiarities of metal electrodeposition inside a flow-through porous electrode with low initial conductivity in oxidant containing solution

The earlier developed dynamic model of a flow-through electrode is used for studying how the variations in initial conductivity of a porous matrix κ_s,ini and a metal deposit affect the rate of metal deposition from an oxidant-containing solution for the direct-flow operation mode of the porous electrode. It is found that in contrast to an oxidant-free solution in which the decrease of κ_s,ini improves the uniformity of deposit distribution inside the porous cathode and increases the deposit final mass m _f, the opposite situation is observed in the presence of an oxidant, namely, a decrease in κ_s,ini, under otherwise similar conditions reduces the deposit mass and leads to its specific spatial distribution. The final metal deposit is divided into two separate fragments (rear and front) with a region of low conductivity of the initial porous matrix in between. Dynamics of the current and metal redistribution within the porous electrode, the reasons for the formation and stabilization of the rear fragment of coating, the correlation between the metal deposition rate and changes in the anodic zone position and intensity are discussed. It is shown that with the appearance of a specific profile of deposit distribution, the dependence of m _f on the metal conductivity develops a limit that differs considerably from the deposit final mass for an equipotential porous electrode.     

Effect of solution volume on the rate of metal recovery and its distribution in porous cathode

By means of earlier developed dynamic model of porous electrode, numerical analysis is given of the effect of the division into smaller parts of optimal volume V _opt of metal-containing solution circulating through the porous electrode, which ensures both achieving of the metal preset recovery and the prescribed final porosity in the critical cross-section of the porous cathode, most strongly filled with the deposit. It is shown that the result depends significantly on the presence (or absence) of oxidant ions in the solution. In the absence of oxidant, the division of V _opt is desirable; it entails some improving of the distribution uniformity and increase in the deposit eventual mass. On the contrary, in the presence of oxidant such a division brings about marked negative consequences: (1) the metal deposit stronger localization at the porous electrode front edge and (2) decrease in the deposit’s final mass. It was shown that these phenomena result from the formation of anodic zone in the porous electrode rear side in the initial stage of the processing of the second and subsequent portions of solution. This is promoted by low polarization in this part of the porous electrode and the presence of a soluble metal deposited here during the final stage of electrolysis of the solution preceding portion.     

Electro-deposition as a preconcentration step in analysis of multicomponent solutions of metallic ions

An explanation is given for the observation that in the preconcentration of trace metals by electro-deposition, the yield of deposited metal is lower if an oxidant is present. The effect is related to the competition between the deposition and dissolution kinetics of the metal.     

酸性介质中丙烯基硫脲对铜阳极溶出和阴极沉积过程影响的EQCM研究

采用循环伏安(CV)和电化学石英晶体微天平(EQCM)方法研究了酸性介质中铜阳极溶出和阴极沉积过程以及丙烯基硫脲(AT)对该过程的影响. 结果表明, 铜阳极溶出和阴极沉积过程的M/n分别为32.0和34.2 g/mol, 都是两电子过程, 其间未检测到Cu(Ⅰ)中间产物. AT改变了铜阳极溶出和阴极沉积的历程. 在含AT的溶液中, 铜阳极溶出和阴极沉积过程的M/n分别为61.9和65.4 g/mol, 可指认铜阳极溶出产物为CuAT+, 并提出了AT存在下Cu阳极溶出和阴极沉积过程的反应机理; 从电极表面质量定量变化的角度提供了Cu阳极溶出和阴极沉积过程的新数据.     

Anodic dissolution of electronegative component during the process of metal codeposition in a flow-through porous cathode

Possible magnitude of the electronegative component (M₂) dissolution during a two metal (M₁ and M₂) codeposition inside a porous electrode is studied theoretically (by modeling) and experimentally. Model calculations based on the substituting of the pure metal (M₂) dissolution rate for its selective dissolution rate from the alloy gave an overestimated evaluation of the effect. The dissolution effect is shown to be small when the porous electrode is filled up with metal deposited from a solution large single portion; however, it increased significantly when the solution is divided into smaller portions. Experimental studies of Ag and Cu deposition dynamics in thiosulfate solution showed that the turning from a direct-flow to a circulation mode results in significant increase in the Cu mass and widening its deposition zone up to the porous electrode entire thickness. When the 2nd and 3rd portions of the solution are subjected to electrolysis, the solution is temporarily enriched with Cu ions, which evidences the copper partial dissolution whose scale is close to calculated estimates. The explanation of specific features of Cu dissolution in repeated cycles of the metal recovery was suggested and experimentally proved.     

A comparison of the effects of chemisorbed sulfide ions on the electrochemical behavior of gold and silver in thiourea electrolytes

It is shown that, in the case of gold, the catalytic activity of sulfur adatoms is significantly higher in the anodic dissolution of metal than in its deposition. In both processes, the catalytic activity increases with increasing overvoltage. The catalytic activity of sulfur adatoms is considerably lower for silver than for gold both in the metal deposition and dissolution, and it is virtually independent of overvoltage. In the case of silver, the catalytic activity of sulfur adatoms in the electrodeposition is slightly higher than in the metal dissolution. There results are compared with the action of heavy metal adatoms on the dissolution and deposition of gold and silver in the cyanide solutions. It is shown that the above regularities are qualitatively true for both systems. The main distinction is that the catalytic activity of sulfide ions in the thiourea solutions reaches a plateau with increasing surface coverage with sulfur adatoms, whereas the activity of heavy metals passes through a maximum with increasing surface coverage. The results are explained in view of earlier determined regularities of kinetics of electrode processes in these systems and the effect of electrocatalysis on these regularities.     

The effect of the solution flow rate on the final mass of metals deposited into a porous electrode at individual and joint deposition. Rear delivery of solution

Pursuing the studies of the effect of solution flow rate and direction on filling of a porous electrode with deposited metals, the peculiarities of individual and joint deposition of metals are studied for the rear solution delivery. In the rear delivery mode, the nonmonotonic dependence of the final mass of the cathodic deposit m _f on the solution flow rate m _v with the maximum at middle flow rates is observed to hold. The main differences from the frontal delivery mode are the much higher mass values in the maximum m _{f, max} (1.5–3.5-fold higher) and the lower concomitant flow rates m _{v, max} (0.05–1 cm³/s). Moreover, the highest mass gains and rate shifts correspond to the lower cathodic polarization values. These differences are associated with the opposite orientation of the polarization and metal-concentration profiles inside the PE, which is typical of the rear delivery, and also with the gradual shift of the PE critical section from the frontal to rear plane as the solution flow rate decreases. For the rear delivery, the m _{f, max} value observed in the direct-flow mode turns out to be even a little higher as compared with the circulation mode. This makes the direct-flow mode of electrolysis in the middle range of flow rates the most efficient condition for the metal codeposition into a PE at the rear delivery. Experimental data on the individual and joint deposition of silver and copper on carbon felt VINN-250 from alkaline thiosulfate solutions at the rear delivery and different solution rates qualitatively agrees with the model calculations for a PE with the same specific surface.     

Ytterbium and erbium derivatives of 2-methoxyethanol and their use in the thin film deposition of Er-doped Yb3Al5O12

We have proved different methods for yttrium and erbium 2-methoxyethoxides preparation. Our aim was to prepare a solution applicable at the subsequent sol–gel preparation of Yb₃Al₅O₁₂ garnet thin films. We tested the direct reaction of a metal with 2-methoxyethanol in the presence of HgCl₂, the electrolytic dissolution of metals, an alcohol interchange using isopropoxides and, finally, the exchange reaction of acetates with 2-methoxyethanol. Because our demand was to prepare a solution without the necessity of purification, we omitted chlorides as a source of cations. The formation of the metal alkoxides was examined by IR spectroscopy in each case. Both exchange reactions were fully successful; only in the case of acetate use, the arising acetic acid forms ester immediately. The direct reaction of metals with the alcohol did not achieve a full yield; on the other hand, after electrolysis application, the metals dissolved readily. However, in both cases, the filtration of either unreacted metal (direct reaction) or eroded metal pieces (electrolysis) was necessary. Because the filtration has to be processed under inert atmosphere these methods are less convenient for the subsequent use in the sol–gel deposition. Er:Yb₃Al₅O₁₂ thin films deposited on silicon substrates were also prepared using the two appropriate intermediates—the solutions prepared either from acetates or isopropoxides. Both layers were monophase; however, the microstructure and luminescent properties were influenced by the solution used. The Er:YbAG layers exhibited the sharp and discreet luminescence peaks of the Er³⁺ transition ⁴I_13/2 → ⁴I_15/2.     

Silver based mercury film electrode: Part II. The influence of aging of electrode on the electrode processes of various metal ions

The cyclic voltammetric behaviour of 8 metal ions at solid silver amalgam electrodes prepared by aging of a thin silver based mercury film electrode (SBMFE) and by deposition of silver and mercury on platinum were investigated. It was established that such electrodes behave in relation to some metals (Pb, Bi, Sn) similarly as silver electrodes i.e. the cyclic curves obtained with these electrodes at concentration 10^?3M range show a prepeak-postpeak system corresponding to deposition and dissolution of the monolayer of deposit. On the other hand under the same conditions no prepeaks were observed for cadmium, zinc and thallium. In all cases investigated the heights of anodic stripping peaks were lower on curves obtained with aged SBMFE than on those obtained with fresh SBMFE having a mercury layer 1 μm thick.     

Electrochemical preparation and characterization of a cobalt oxide–platinum composite with promising capacitive and electrocatalytic features

In the present work, boron-doped diamond polycrystalline films were used as support for direct anodic deposition of the cobalt oxide, and continuous Co₃O₄ coatings with reasonably good conductivity were obtained by appropriately adjusting the deposition charge. Further electrochemical deposition of platinum particles on the oxide substrate enabled the formation of a stable composite with a specific capacitance of ca. 431 F?cm^?3 that compares well with that available with similar materials obtained by non-electrochemical methods. Additional advantages of electrochemically obtained composites are the lower content of noble metal, the uniform distribution of the charge over an extended potential range, and, importantly, the simplicity of the preparation method. It was also found that when deposited on a Co₃O₄ substrate, Pt particles show, besides an enhanced active surface area, an improved catalytic activity for methanol anodic oxidation. This behavior was tentatively ascribed to the presence of a high amount of platinum-oxidized species.     

Magnesium alloys spontaneous dissolution features under external anodic polarization in presence of inhibitors

The effect of benzotriazole (BTA) and sodium ethylenediamine tetraacetate (EDTA) to the magnesium dissolution intensity under external anodic polarization was studied. The inhibiting properties of BTA and EDTA in electrochemical corrosion of magnesium were found in the concentration range from 0.005 to 0.0125 M in sodium sulfate solution and from 0.0025 to 0.03 M in sodium chloride solution. In the presence of inhibitors, magnesium dissolution becomes uniform with decreasing surface hydrogenation and crumbling intensity of small particles into solution.     

Direct evidence of initial pitting corrosion

The chloride ion (Cl⁻) is known as an aggressive species in the aqueous solutions that adsorbs to the imperfect sites on the metal surfaces, such as defects, impurities and second-phase particles. This adsorption process could change the chemical composition and properties, such as ion conductivity, of the passive film. As a result, the passive film becomes less protective and breaks down at some places where the underlying metals are exposed to the electrolyte and dissolved through anodic reaction forming a pit. In the meantime, the hydrolysis (cathodic reaction) occurs inside the pit giving rise to a lower pH and leading to an increased dissolution rate of the metals. Subsequently, more chloride ions migrate into the pit in order to maintain electrical neutrality and adsorb onto the pit surface. The repeating of such process causes the pitting propagation. However, anodic polarization allows the chloride ions to migrate into the passive film more easily. With growth of the pit, more corrosion products make both in plane diffusion across the sample surface and out-of-plane diffusion to bulk solution from the pit mouth [M.G. Fontana, N.D. Greene, Corrosion Engineering, second ed., McGraw-Hill, New York, 1978].     

Recovery of Metals from Anodic Dissolution Slime of Waste from Electric and Electronic Equipment(WEEE) by Extraction in Ionic Liquids

The recovery of metals from a multi-component alloy obtained by crushing, melting and anodic dissolution of waste from electric and electronic equipment(WEEE) was investigated. The anodic dissolution of the alloy was carried out in an electrolysis cell with one copper cathode and a central cast anode, immersed in the electrolyte formed by choline chloride-ethylene glycol-iodine. The temperature of the electrolyte during the process was 343 K. Depending on the electrolysis parameters(current density and cell voltage), cathodic deposits of Sn, Pb and Zn of >99% purity were obtained. Cyclic voltammetry was used in order to determine the deposition potentials of the studied metals. The obtained metallic deposits were subject of determination of XRD, SEM/EDX and AFM in order to evidence the deposits structure and morphology. The experiments performed demonstrated the possibility of separating/selective recovery of metals from the multi-component alloy resulted from the waste from electrical and electronic equipment by anodic dissolution in ionic liquids.     

Corrosion of Cu-xZn alloys in slightly alkaline chloride solutions studied by stripping voltammetry and microanalysis

The mechanism of corrosion of Cu-xZn alloys (x = 10-40 wt %) in slightly alkaline chloride solutions was investigated by analysing solid reaction products by energy dispersive X-ray analysis (EDS) and dissolved reaction products by differential anodic pulse stripping (DAPS) voltammetry. The corrosion process was studied under open circuit and under potentiostatic conditions at selected potentials. Pure metals were studied comparatively so that an interacting effect of particular metal components in the alloy could be determined. All four Cu-xZn alloys show an improved behaviour compared to pure metals. Under open-circuit condition both components dissolve simultaneously in the solution. With increasing immersion time the preferential, dissolution of zinc in the solution becomes pronounced. It is the highest for Cu-10Zn and the lowest for Cu-30Zn alloy. Under potentiostatic control the dissolution mechanism depends on the electrode potential and changes from exclusive dissolution of zinc to simultaneous dissolution of both components with preferential dissolution of zinc. The latter decreases, as the electrode potential becomes more positive.     

Copper in Acid Chloride Solutions: Electrochemical Behavior by Quartz Microgravimetry and Voltammetry

The cathodic deposition of copper on a gold electrode and its subsequent anodic dissolution in an acid chloride solution, where two successive stages of a one-electron transfer are distinguishable because of the stability of chloride complexes of Cu(I), is studied by voltammetry and quartz microgravimetry. The formation of a film of an intermediate compound of Cu(I) during the deposition and dissolution of copper is revealed experimentally. Techniques for identifying the intermediate solid species are suggested. During a cathodic polarization, a film of intermediate compound CuCl forms at a reduced concentration of chloride ions in the Cu(II)/Cu(I) process, while during the anodic dissolution of the copper deposit formed during the cathodic polarization the intermediates appear in the Cu(0)/Cu(I) process, the concentration of chloride ions notwithstanding. The change in the electrochemical behavior of the system caused by a decrease in the concentration of chloride ions is explained.