Dynamics of variations in the metal deposit distribution in porous flow-through electrodes: Effect of solution flow velocity and direction

Dynamics of variations in the metal deposit distribution in a porous flow-through electrode (PFE) and a number of integral indicators of the process are studied as a function of the velocity of solution flow and the direction of its supply with the aid of a dynamic model for PFE. It is established that qualitative character of variations in the above parameters with time depends on which of two factors (distribution of polarization or concentration of metal ions) predominantly defines the metal electrodeposition process inside PFE. It is shown that employing reverse of solution flow through PFE with the aim of increasing the weight of metal deposited in it and the uniformness of its distribution gives positive effect only in the case where the metal distribution for opposite supply directions is different.     

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.     

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.     

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 the filling of a porous cathode by the deposited metal: Modeling the process and analyzing the case of high cathode conductance and low solution depletion degree

A dynamic model for a porous electrode is designed on the basis of a one-dimensional representation of the electrode in the form of parallel filaments. The method takes into account the alterations in the local values of the filament diameter (and, correspondingly, in the effective conductances of phases), porosity, the velocity of a linear flow, and the mass transfer coefficient for the deposited metal ions, which occur in the course of the metal electrodeposition. For the simplest version of dynamics, at a high initial conductance of the electrode and a small solution depletion degree, the method predicts the following specific features: (i) the development of the working surface area and an increase in the current efficiency for the metal associated with it, (ii) a decrease in the metal penetration depth into the electrode with time and the metal localization near the most loaded end, and (iii) an irregular change in the current efficiency and concentration of the metal at the exit out of the electrode.Translated from Elektrokhimiya, Vol. 41, No. 3, 2005, pp. 333–342.Original Russian Text Copyright © 2005 by Maslii, Poddubnyi, Medvedev.     

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.     

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.     

Dynamics of the filling of a porous flow-through electrode,which is operating in a circulation mode,with metal

A model, developed previously for describing the filling of a porous flow-through electrode (PFE) with a metallic deposit, is used to demonstrate that, in contradistinction to a straight-through mode, a decrease in the concentration of metal in a circulating solution (approximately by an order of magnitude in the course of the time period required for filling a critical cross-section of PFE with metal) leads to a change both in the direction of the spatial redistribution of metal inside the porous matrix and in the dynamics of the variation of its basic parameters. In a straight-through mode, the metal distribution inside a PFE is defined by the action of two opposite factors (development of a surface inside the working layer L _ef at the expense of the growth of the diameter of fibers and, vice versa, its shrinking at the expense of a decrease in L _ef with time and the expulsion of the process of deposition of metal in the direction of the front end of PFE) and is characterized by nonmonotonous dependences of the current efficiency and the outlet concentration of metal on time. The predominant tendency in the case of a circulation mode is different: L _ef rises with time, which leads to a displacement of the process of the metal deposition in the direction of the rear end of PFE (at rear solution input), increase of maximal amount of deposit by approximately 1.4 times at L > L _ef, as well as to dependences of the current efficiency and the ratio between the concentrations of metal at the outlet and inlet of PFE that are monotonously decreasing with time. At the expense of a continuous variation of the effective working surface area of PFE and the mass transfer coefficient with time, the circulation mode is characterized by a nonlinear dependence of the logarithm of the metal concentration in the circulating solution on the electrolysis duration. A comparison of indicators that are characterizing the dynamics of the filling of a PFE with metal in the course of galvanostatic and potentiostatic modes of electrolysis is performed. It is established that the application of a potentiostatic mode of electrolysis under the conditions that provide for the predominance of the target reaction is accompanied by a slight decrease in the maximum quantity of metal, but a very significant decrease (by 5–6 times) in the specific spendings of electric energy.__________Translated from Elektrokhimiya, Vol. 41, No. 4, 2005, pp. 452–459.Original Russian Text Copyright © 2005 by Maslii, Poddubnyi, Medvedev.     

Nonstationary mathematical models of the electrochemical processes in volumetric porous flow electrodes

Model representations of the nonstationary processes caused by the nonstationary electrolysis regimes on three-dimensional flow electrodes (TFEs) were developed. A change in the electrode and electrolyte parameters with time was taken into account. As an illustration of the efficiency of a model of two-dimensional electrochemical system, co-electrodeposition of two metals with hydrogen evolution on TFE was considered for the case of mutually perpendicular directions of current and electrolyte flow.     

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.     

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.     

Progress in the electrochemical reduction of CO2 on hierarchical dendritic metal electrodes

Electrochemical reduction of CO₂ (ERC) is a promising way to deal with CO₂ emissions in the atmosphere, recycle CO₂, and achieve a carbon-neutral economy. Electrodes made of hierarchical dendritic materials are investigated for the ERC owing to their structural advantages, including large surface areas, presence of facets with low-coordinated atoms, and needle-like tips. Selected examples are presented to illustrate the state-of-the-art investigation of these dendritic electrodes. The mechanisms and the function of the dendritic structure are discussed, combined with a comparison of performance results of ERC.     

Studying the process of oxygen ionization on a porous metal hydride electrode

Problems that are connected with utilization of oxygen evolving during overcharge of the nickel oxide electrode in sealed nickel metal hydride batteries are considered. It is established experimentally that the rate of the process of oxygen reduction in conditions of forced gas supply into pores of a metal hydride electrode increases by two orders of magnitude as compared with the intensity of this process during natural convection. Up to 80% of evolved oxygen undergo ionization on a metal hydride electrode in these conditions even in a regime of forced (hour-long) charge of a model sealed nickel-metal hydride battery. The dependence of the current density of oxygen reduction at a metal hydride electrode on the filling of the electrode’s porous space by oxygen is estimated with the aid of manometric and potentiostatic methods. It is shown that practically all the oxygen ionization current is generated at the walls of gas-filled electrode pores, under thin electrolyte films, with a local current intensity of 1–3 mA cm^?2.     

The wax deposition rate of water-in-crude oil emulsions based on the laboratory flow loop experiment

This paper aims to develop a mathematical model to predict the wax deposition rate of waxy crude emulsions, combining heat and mass transfer mechanisms. According to the flow loop experimental results, the wax deposition rate increases with the decreasing average temperature of oil/wall in a manner of linear regularity, and shows a downtrend with the increase of water cut due to diffusion resistance. An applicable model is developed regarding emulsion properties, radial temperature gradient, shear stress, and wax diffusion coefficient. In model validation, the prediction results are in good agreement with experimental data with the relative errors within 28.87%.     

Study on the flow resistance of the dispersion system of deformable preformed particle gel in porous media using LBM-DEM-IMB method

After being injected into the porous media, the dispersion system of preformed particle gel (PPG) tends to enter high permeability regions and block water channeling passages, which forces the subsequent water to turn to the low permeability regions and thus increases sweep efficiency and enhances oil recovery. However, it is still unclear about the influence factors and the mechanisms how PPG increases water flow resistance, which limits the application of PPG in more oilfields. Therefore, the paper combines the lattice Boltzmann method (LBM), the discrete element method (DEM) and the improved immersed moving boundary (IMB) method to simulate the migration of deformable PPG in porous media. On the basis, the paper quantitatively analyzes the variation law of displacement pressure across the porous media and discusses the influence factors such as the PPG diameter, elasticity modulus and the number concentration. Results indicate that, because of the friction and retention of PPG in pore-throat, the displacement pressure across the porous media during PPG flooding is much higher than that during water flooding. In other words, the existence of PPG increases the flow resistance of injected water. Besides, the displacement pressure is always fluctuant resulting from the continuous process of PPG migration, retention, deformation and remigration. Influence factor analysis shows that the incremental value and fluctuation degree of flow resistance increase with the PPG diameter, elasticity modulus and the number concentration. The study not only provides useful reference for future PPG flooding, but also benefits the development of deformable particle flow theory.     

The alkali metal cation effect on the surface-enhanced Raman spectra of phosphate anions adsorbed at silver electrodes

The interaction of adsorbed phosphate anions with alkali metal cations at the Ag|aqueous solution interface has been investigated by surface-enhanced Raman spectroscopy (SERS). Formation of ion pairs at the interface was evident from the cation-induced perturbations in the SER spectra of anions. The frequency of the external vibration, silver–oxygen (Ag---O′), was not sensitive to the nature of cation, while the relative intensity of this mode was cation-dependent and was explored as a sensitive probe for the monitoring of coadsorption of ions at the interface. From the internal phosphate vibrations, both asymmetric modes, δ_as(PO) and ν_as(PO), were found to be the most sensitive to the nature of the cation. At a relatively positive potential (0.00 V vs. Ag | AgCl) the spectral parameters for the Cs⁺ and K⁺ cations were very similar indicating the same bonding type with anions. A more inhomogeneous chemical environment for the phosphate oxygen atoms was detected in the case of Na⁺ and Li⁺ cations. An increase in ν_as(PO) frequency by ca. 10 cm⁻¹ was the characteristic spectral signature for the interaction of phosphates with Li⁺. The formation of water-shared ion pairs at the interface was suggested based on the absence of splitting in the ν_as(PO) mode and the previously observed frequency sensitivity of this band to solvent H₂O substitution by D₂O. At negative potential (−0.80 V), a stabilization effect of Cs⁺ on the phosphate adlayer was detected based on the twofold increase in intensity of the ν(Ag---O′) mode compared with Li⁺. Splitting of the ν_as(PO) mode suggested the contact interaction of anions with specifically adsorbed Cs⁺ cations.     

Temperature dependence of the electronic factor in the nonadiabatic electron transfer at metal and semiconductor electrodes

The temperature dependence of the electronic contribution to the nonadiabatic electron transfer rate constant (k_ET) at metal electrodes is discussed. It is found in these calculations that this contribution is proportional to the absolute temperature T. A simple interpretation is given. We also consider the nonadiabatic rate constant for electron transfer at a semiconductor electrode. Under conditions for the maximum rate constant, the electronic contribution is also estimated to be proportional to T, but for different reasons than in the case of metals (Boltzmann statistics and transfer at the conduction band edge for the semiconductor versus Fermi–Dirac statistics and transfer at the Fermi level, which is far from the band edge, of the metal).     

Effect of method of solution on observed molecular weight distribution of barley starch and starch derivatives in size exclusion chromatography

Summary Samples of native barley starch and six starch derivatives were suspended (0.1% sample concentration) in four different solvents: the eluent (pH 11 buffer), dimethylsulfoxide, 0.1 M NaOH or 0.5 M NaOH and kept in a boilling water bath for 5 to 60 minutes or shaken for 60 minutes. The average molecular weight values , and the polydispersity value were determined with a TSK PW-type column using narrow standard calibration. Only a small part of the samples dissolved in the eluent. The dissolution of sample in dimethylsulfoxide was dependent on sample type. Of the NaOH solutions, 0.5 M NaOH was the only one that dissolved all the samples. Therefore, 0.5 M NaOH appears to be the solvent of choice for starch molecules.     

The effect of the electrolyte concentration in the solution on the voltammetric response of insertion electrodes

A solid-state redox reaction involving an insertion of ions is analyzed with respect to the influence of the concentration of inserting ions in the solution phase. The voltammetric response is independent of the mass transfer in the solution provided that z = (D _ss/D _aq)^1/2 ρ/[C⁺]* is smaller than 0.1 (D _ss: diffusion coefficient of the cation C⁺ in the crystal; D _aq: diffusion coefficient of the cation C⁺ in the solution; ρ: density of the solid compound; [C⁺]*: concentration of cations in the bulk of the solution). In real cases this condition will be satisfied at solution concentrations above 1 mol/l. Received: 15 December 1997 / Accepted: 5 March 1998     

Influence of gas flow rate and sodium carboxymethylcellulose on foam properties of fatty alcohol sodium polyoxyethylene ether sulfate solution

Foamability, foam initial liquid volume, and bubble size of fatty alcohol sodium polyoxyethylene ether sulfate (AES) surfactant solution were studied with and without the addition of sodium carboxymethylcellulose (CMC) at different gas flow rates, using a sparging method. The generation time decreased with increasing gas flow rate. At low gas flow rates, the added CMC greatly enhanced the foamability by preventing bubble collapse. The initial liquid volume of the foam first increased rapidly, and then gradually decreased. Increasing the CMC concentration increased the initial liquid volume of the foam. The mean bubble diameter first clearly decreased, then increased slowly with increasing gas flow rate. CMC showed different effects on bubble size at high and low gas flow rates. Adsorption of CMC on AES molecules forms a network structure and improves bubble film stability, which can explain the above results. These findings provide guidelines for generating foam with excellent properties suitable for coal mine dust control by adjusting the gas flow rate and the concentration of the added water-soluble polymer.