A. Zh. Medvedev

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.

Distribution of geometrical current density inside a flow-by porous electrode: Effect of electrode parameters and electrochemical reactions

The effect of the difference between the equilibrium potentials of the target and side reactions, the solution flow rate, and specific surface area of porous electrode (PE) on the distribution of the geometrical current density along the solution flow ig(y) at various average current densities is studied by the method of mathematical modeling. It is found that the largest range of the variation of ig(y) (the exponential decrease along the solution flow) is typical for the conditions that provide the limiting current mode of the target reaction on the entire PE surface in the absence of simultaneous hydrogen evolution. All changes in the operation conditions, which reduce the uniformity of the distribution of the current inside PE along the X axis and hamper reaching the limiting current mode (for example, a decrease in the equilibrium potential difference, an increase in the flow rate or specific surface area of PE) lead to more uniform distribution of ig(y).

Dynamics of metal deposition onto a porous electrode of poor initial conductivity, with the electrode operating in a direct-flow regime at high solution flow rates

The process of metal electrodeposition onto a porous matrix with poor initial conductivity is studied with the aid of a dynamic model for a porous electrode (PE), which was designed earlier and which was complemented with a block for calculating local conductivity of the solid phase. It is established that, despite a very low initial metal deposition rate, the final weight of the deposit inside the PE in the electrolysis conditions under consideration is greater than that inside a PE with a high conductivity of the solid phase. It is demonstrated that the additional metal amount is localized largely in the rear part of the PE and undergoes deposition chiefly in the initial electrolysis stage, specifically, until the instant of full metallization of the porous matrix and the PE conversion into an equipotential electrode. Specific features characterizing variations in the metal’s deposition rate in the course of its deposition onto a low-conductivity porous matrix and possible reasons for such variations are considered.

Desorption of octanethiol from gold electrode surface during its electrochemical cleaning

The potentialities of electrochemical method of removal of adsorbed octanethiol layer from gold electrode surface are considered. The dependence of a degree of gold macro- and microelectrode cleaning on the electrolyte composition, potential cycling range, and the number of cycles is studied. It is shown that the commonly used method of cleaning by multiple potential cycling in the perchloric acid solution in the potential range from 0 to 1400 mV (SCE) is not sufficiently efficient. Cycling in the water-ethanol solution of perchloric acid promotes the octanethiol desorption and leads to the complete regeneration of surface even after 6–8 cycles. This is evidenced by comparing with the voltammograms, which were obtained on pure Au surface in 4 mM potassium ferri-ferrocyanide and 0.1 M HClO4 solutions. A range of ethanol concentrations, which provides the highest degree of gold surface cleaning, is determined. The electrochemical method of cleaning by using water-ethanol solution of perchloric acid enables one to perform multiple electrode regeneration without destruction of thin gold layer (for example, in the quartz microbalance), which takes place in the case of chemical regeneration in the “piranha” solution based on sulfuric acid and hydrogen peroxide.

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.

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.

Effect of gaseous products of overall electrode process on local solution conductivity and efficiency of operation of flow-through porous electrode

A one-dimensional porous electrode (PE) model and additional consideration of the dependence of the local solution conductivity on its gas saturation was used to study the effect of simultaneous hydrogen evolution on distribution of the potential in PE and the overall rate of the target redox reaction. It was found that this effect depends on the ratio of conductivities of the solid κs and liquid κl phases and direction of solution supply and can be both negative (rear supply at any κs and κl, front supply at κs ≫ κl), and positive (front supply at κs ≤ κl). However, variation of the target reaction rate in all cases for PE with a high specific surface area is low (10–40%). It is shown that in the terms of the model of a homogeneous gas-liquid mixture, a weak effect of gaseous hydrogen is related to the specific form of profiles κl(x) far from the earlier considered ideal (or inverse) liquid-phase conductivity profiles.

Effect of solution flow rate and direction on copper electrodeposition process from sulfuric acid electrolyte on porous electrode with high electric conductivity

The effect of solution flow direction and rate on the dynamics of copper electrodeposition onto a premetallized coal-graphite VINN-250 material from a dilute copper sulfate sulfuric acid solution is experimentally studied in the direct-flow mode. A light effect of solution supply on the copper deposit final mass for high and low solution flow rates is found, while the effect of this parameter on the metal distribution within the porous electrode is significant. The most uniform copper deposit distribution throughout the porous electrode is observed in the case of intermediate solution flow rates at its rear supply. The obtained experimental data agree qualitatively with the earlier published mathematical simulation results. Small systematic deviations from numeric calculations can be due to the process disregarded in the mathematical model: the effect of gas phase formed within the porous cathode due to the simultaneous hydrogen evolution.

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.

Recovery of palladium from spent solutions for manufacture of catalysts

Palladium recovery from [Pd(NH3)4]Cl2 solutions (concentration in terms of the metal ∼1 g l−1) with flow-through porous electrodes was studied. The conditions of effective electrochemical recovery of Pd were found. Various porous cathodes were compared.