Flow-through Porous Electrodes Research Articles

Ozone Electrogeneration on Pt-Loaded Reticulated Vitreous Carbon Using Flooded and Flow-Through Assembly

Electrogeneration of ozone was investigated at platinum-loaded reticulated vitreous carbon (RVC). Stationary and flowing solutions, i.e., flooded and flow-through porous electrodes, respectively, were used at room temperature . The study evaluates the flow-through porous electrode for a continuous production of -aqueous solutions. was generated potentiostatically by applying a constant potential for . The flooded electrode showed a negligible current efficiency at the bare RVC compared to a value of 2.2% at the Pt-loaded RVC electrode (RVC/Pt). At the flow-through porous electrode, the effects of acid concentration and electrolyte flow rate on the concentration of generated and on the current efficiency of electrogeneration at RVC/Pt were explored. The concentration increased in the outlet stream with concentration. The current efficiency did not change significantly with the electrolyte flow rate, but the higher concentration of in the outlet stream was obtained at lower flow rates. The current efficiency remained nearly unchanged but the specific electrical energy consumption ( of ) decreased significantly with the acid concentration. The stability of the RVC/Pt electrodes was examined by measuring the time-course of the electrolysis current at a given applied potential and the scanning electron microscopy images of the electrode surfaces before and after the electrolysis.

Modeling of the dynamics of metal deposition inside a flow-through porous electrode with low initial conductivity

Abstract With the help of the previously developed dynamic model of the equipotential porous electrode (PE) supplemented with the unit to calculate the local conductivity of the solid phase, the effect of solution flow direction and rate on the process of metal electrodeposition on the porous matrix with low initial conductivity is investigated. It is established that in comparison with the equipotential PE the noticeable differences in the final mass of the deposit and uniformity of its distribution are observed only in the case of rather low conductivity of the metal deposit (⩽100 Ω −1 cm −1 ), the direction and scale of these differences being dependent on the rate and type of solution feed into the PE. For high flow rates, the effect is positive and weakly dependent on the direction of solution feed. To the contrary, in the region of medium and low solution flow rates, the effect is negative for the back solution feed and essentially positive for the front one. In order to explain the observed regularities, we consider the features of changes in the distribution of polarization, metal ion concentration and local conductivity of the solid phase during the metal deposition process.

Expanded Use of a Battery-Powered Two-Electrode Emitter Cell for Electrospray Mass Spectrometry

A battery-powered, controlled-current, two-electrode electrochemical cell containing a porous flow-through working electrode with high surface area and multiple auxiliary electrodes with small total surface area was incorporated into the electrospray emitter circuit to control the electrochemical reactions of analytes in the electrospray emitter. This cell system provided the ability to control the extent of analyte oxidation in positive ion mode in the electrospray emitter by simply setting the magnitude and polarity of the current at the working electrode. In addition, this cell provided the ability to effectively reduce analytes in positive ion mode and oxidize analytes in negative ion mode. The small size, economics, and ease of use of such a battery-powered controlled-current emitter cell was demonstrated by powering a single resistor and switch circuit with a small-size, 3 V watch battery, all of which might be incorporated on the emitter cell.

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 rate and direction of solution flow on metal deposition in porous electrodes: The final weight and distribution of the deposit

Using an earlier-developed dynamic model for a porous flow-through electrode (PFE) with a high initial conductivity, the effect of the solution’s flow rate (0.05–10 cm/s) and direction on the final metal weight mf and uniformity of the metal distribution in the porous matrix is studied. It is found that mf increases with increasing flow rate. However, the dependence is nonmonotonic: it peaks at intermediate flow rates. The peak is most pronounced in the case of rear supply. At high and very low flow rates, mf is independent of the flow direction. In the first case, the metal distribution profiles almost coincide, while in the second case they are mirror-opposite. The deposit weight correlates well with the index of uniformity of its distribution: all other factors being equal, the more uniform the deposit distribution in PFE, the larger the mf. These effects are explained by taking into account the joint effect of profiles of cathodic polarization and concentration of metal ions in PFE.

Expanded Electrochemical Capabilities of the Electrospray Ion Source Using Porous Flow-Through Electrodes as the Upstream Ground and Emitter High-Voltage Contact

Use of a porous flow-through electrode at the upstream ground contact or at both the upstream ground contact and the high-voltage emitter contact in an electrospray ion source was shown to provide for new types of electrochemical experiments utilizing only the electrochemistry inherent to electrospray. The normal stainless steel bore-through union serving as the upstream grounding point in a floated electrospray emitter system was replaced with a high surface area porous flow-through electrode assembly to achieve effective electrochemical reduction of analytes at this point in positive ion mode, and effective electrochemical oxidation of analytes in negative ion mode. This was demonstrated by the oxidation of 3,4-dihydroxybenzoic acid and reserpine in negative ion mode and by the reduction of thionine in positive ion mode. In the case of reversible oxidation (3,4-dihydroxybenzoic acid) and reduction (thionine) processes, partial rereduction and reoxidation of the products due to reaction with products generated by cathodic and anodic processes at the emitter were observed, respectively. By implementing two high surface area porous flow-through electrodes in the system, one as the upstream grounding point and the other as the emitter electrode, a multiple-step reaction scheme was achieved that included consecutive electrochemical reduction and oxidation reactions and a following chemical reaction as demonstrated by the hydroquinone tagging of an initially disulfide-linked peptide.

Copper Electrodeposition Dynamics at a Porous Flow-through Electrode

The dynamics of the copper electrodeposition out of a dilute sulfuric acid solution of copper sulfate (straight-through regime, solution supplied from the rear at a velocity of ∼1 cm s−1) onto preliminarily metallized carbonaceous felt of the VINN-250 brand is studied. The profile, which describes the final deposit distribution, qualitatively conforms to predictions made in terms of a dynamic model for a porous electrode described previously, as does the nonmonotonic variation of the gain in the copper weight and the current efficiency for copper with time in the first half of the deposition process. At later stages of the process, along with the enhancement of the tendency towards the forcing of the copper electrodeposition process towards the front end of the electrode, there appears a new factor, which is not taken into account by the method. This factor is the deposition of a loose rough copper layer outside the porous electrode at its front side.

Study and Application of a Controlled-Potential Electrochemistry−Electrospray Emitter for Electrospray Mass Spectrometry

This paper discusses continued studies and new analytical applications of a recently developed three-electrode controlled-potential electrochemical cell incorporated into an electrospray ion source (Van Berkel, G. J.; Asano, K. G.; Granger, M. C. Anal. Chem. 2004, 76, 1493-1499.). This cell contains a porous flow-through working electrode (i.e., the emitter electrode) with high surface area and auxiliary electrodes with small total surface area that are incorporated into the emitter electrode circuit to control the electrochemical reactions of analytes in the electrospray emitter. The current at the working and auxiliary electrodes, and current at the grounding points upstream and downstream of the emitter in the electrospray circuit, were recorded in this study, along with the respective mass spectra of model compound reserpine, under various operating conditions to better understand the electrochemical and electrospray operation of this emitter cell. In addition to the ability to control analyte oxidation in positive ion mode (or reduction in negative ion mode) in the electrospray emitter, this emitter cell system was shown to provide the ability to efficiently reduce analytes in positive ion mode and oxidize analytes in negative ion mode. This was demonstrated by the reduction of methylene blue in positive ion mode and oxidation of 3,4-dihydroxybenzoic acid in negative ion mode. Also, the ability to control electrochemical reactions via potential control was used to selectively ionize (oxidize) analytes with different standard electrochemical potentials within mixtures to different charge states to overcome overlapping molecular ion isotopic clusters. The analytical benefit of this ability was illustrated using a mixture of nickel and cobalt octaethylporphyrin.

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 Lef at the expense of the growth of the diameter of fibers and, vice versa, its shrinking at the expense of a decrease in Lef 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: Lef 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 > Lef, 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.

Efficient analyte oxidation in an electrospray ion source using a porous flow-through electrode emitter

This article describes the components, operation, and use of a porous flow-through electrode emitter in an electrospray ion source. This emitter electrode geometry provided enhanced mass transport to the electrode surface to exploit the inherent electrochemistry of the electrospray process for efficient analyte oxidation at flow rates up to 800 microL/min. An upstream current loop in the electrospray source circuit, formed by a grounded contact to solution upstream of the emitter electrode, was utilized to increase the magnitude of the total current at the emitter electrode to overcome current limits to efficient oxidation. The resistance in this upstream current loop was altered to control the current and "dial-in" the extent of analyte oxidation, and thus, the abundance and nature of the oxidized analyte ions observed in the mass spectrum. The oxidation of reserpine to form a variety of products by multiple electron transfer reactions and oxidation of the ferroceneboronate derivative of pinacol to form the ES active radical cation were used to study and to illustrate the performance of this new emitter electrode design. Flow injection, continuous infusion, and on-line HPLC experiments were performed.

On the Effectiveness Factor of Flow-Through Porous Electrodes

An effectiveness factor, Φ, was introduced to investigate the effects of different parameters on the utilization extent of a flow-through porous electrode operating for simultaneous reactions. A mathematical model helped to study the effects of ohmic resistance, mass transfer, kinetics, and bubble formation on the effectiveness factor for the main reaction (metal deposition taken as an example). This was presented as a set of dimensional and dimensionless groups. These groups are the following: total theoretical limiting current, IL; conductivity group, σ; kinetic ratio, Io; and the bubble group, Ψ. The results showed that Φ increases at higher σ, lower IL and Io, and higher Ψ (high Ψ means lower extents of bubble formation). The ohmic resistance pushed the reaction to a small layer at the front of the electrode, resulting in a situation in which the reaction is limited at the front of the electrode (higher polarizations) but operates at lower currents at the back of the electrode (lower polarizations). ...

Controlling analyte electrochemistry in an electrospray ion source with a three-electrode emitter cell.

The inherent electrochemistry occurring at the emitter electrode of an electrospray ion source was effectively controlled by incorporating a three-electrode controlled-potential electrochemical cell into the controlled-current electrospray emitter circuit. Two different basic cell designs were investigated to accomplish this control, namely, a planar flow-by working electrode and a porous flow-through working electrode design, each operated with a potentiostat floated at the electrospray high voltage. Control of the analyte electrochemistry was tested using the indole alkaloid reserpine, which is often used to test the specifications of electrospray mass spectrometry instrumentation. Reserpine was relatively easy to oxidize (E(p) = 0.73 V vs Ag/AgCl) in the acidic electrospray medium (acetonitrile/water 1:1 v/v, 5.0 mM ammonium acetate, 0.75 vol % acetic acid) and was oxidized when the conventional electrospray emitter was used at low solution flow rate. With the proper cell auxiliary electrode configuration and adjustment of the working electrode potential, it was found that reserpine oxidation could be "turned off" at flow rates as low as 2.5 microL/min as well as at flow rates as high as 30-40 microL/min. Just as important, it was also possible to "turn on" essentially 100% oxidation of reserpine in this flow rate range. The area of the auxiliary electrode along with flow rate, which affect mass transport of analytes to this electrode, were found to be critical in controlling the electrochemical reactions in the emitter cell. Such control over analyte electrochemical reactions in the emitter has been difficult or impossible to achieve with a conventional electrospray emitter. This control is paramount in obtaining experimental results free from electrochemically generated artifacts of the analyte or in exploiting electrochemical reactions involving the analyte to analytical advantage.

Efficiency of three-layered porous electrodes with variable conductance or variable specific areas of layers: A comparative analysis

Efficiency of action of variable conductance and specific area of porous matrix on the current distribution is compared in identical conditions with the aid of numerical calculations based on a one-dimensional model for a porous electrode. Versions of three-layered flow-through porous electrodes with different and identical parameters are considered at different reactant conversion degrees.

Ideal Conductivity Profile of a Porous Matrix for a Flow-through Porous Electrode with a High Conversion Degree of the Electroactive Component

The possibility of designing an equally accessible, in electric respect (η ≈ const), porous electrode (PE) is considered. The design involves creating a special conductivity profile of the solid phase, κs(x), in conditions of considerable variations of current density inside the electrode. A generalized expression for an ideal profile κs(x) at any current distribution is derived, and some concrete solutions for different PE operation modes at the limiting current of the target reaction are obtained. The profiles are exponential over a major fraction of PE and change to hyperbolic at a low conversion degree of the electroactive component. Similarities and differences of the profiles are analyzed. At a high conversion degree, only the exponential profile ensures effective operation of the entire surface of PE and selectivity of the target reaction.

Mathematical modeling of gas evolving flow-through porous electrodes

A system of four equations is developed to simulate gas evolution reaction within flow-through porous electrodes. The equations include the mass transfer resistance and gas bubble formation at definite kinetic and ohmic parameters. Two cases are introduced: the first is when the bubble effects are separated from the mass transfer effects; the second, is the combined bubble and mass transfer effects. These interlinked effects are discussed by examining the overall performance of the electrode and the distributions of the gas void fractions, potential and reaction currents. When the bubble effects are included in the simulation, the features of the polarization curves as well as the current distributions are significantly influenced. The gas bubbles decrease the effective conductivity of the gas-electrolyte dispersion filling the pore space resulting in a more non-uniform distribution of the potential and current.

Simultaneous Calibrationless Determination of Zinc, Cadmium, Lead, and Copper by Flow-Through Stripping Chronopotentiometry

Zn, Cd, Pb and Cu are deposited in a porous flow-through electrode plated with mercury and then are stripped by constant current while the stripping time is measured. Since complete electrochemical deposition can be achieved, the analyte concentrations can be directly obtained from Faraday's laws i.e., the method is denoted as calibrationless. The influence of the deposition potential, stripping current, carrier electrolyte composition, Cu content and sample matrix was investigated. The optimum conditions are: deposition potential: –1600 mV, stripping current: 200 μA, carrier electrolyte: 0.1 mol/L sodium sulfate at pH 4–5. The dynamic range of the method is from about 0.1 ng/mL to few mg/mL. The repeatability of the method is 1–2 % in the optimum concentration range. The procedure was applied to the analyses of water samples, geological, and biological materials.

Simultaneous Calibrationless Determination of Zinc, Cadmium, Lead, and Copper by Flow-Through Stripping Chronopotentiometry

Zn, Cd, Pb and Cu are deposited in a porous flow-through electrode plated with mercury and then are stripped by constant current while the stripping time is measured. Since complete electrochemical deposition can be achieved, the analyte concentrations can be directly obtained from Faraday's laws i.e., the method is denoted as calibrationless. The influence of the deposition potential, stripping current, carrier electrolyte composition, Cu content and sample matrix was investigated. The optimum conditions are: deposition potential: –1600 mV, stripping current: 200 μA, carrier electrolyte: 0.1 mol/L sodium sulfate at pH 4–5. The dynamic range of the method is from about 0.1 ng/mL to few mg/mL. The repeatability of the method is 1–2 % in the optimum concentration range. The procedure was applied to the analyses of water samples, geological, and biological materials.

Metal recovery in surface treatment units by using the 3PE reactor

Pollution from heavy metals arises mainly from industrial activities and, in this respect, it may be controlled and consequently reduced. The recovery of metals from industrial wastes is important, not only from an ecological point of view (toxicity), but also for economical reasons (recycling). At the present time, a large number of techniques are available to satisfy the stringent low concentration standards for waste water (Crine 1993; Verma et al. 1993). But for most of these techniques, it is not possible to have an immediate and total recycling of the recovered metal. With the development of the volumic electrodes, also called the flow-through porous electrodes, the electrochemical technique could be applied for the recovery of usable metals from effluents or dilute solutions and depollution. These electrodes are made up of a good conductor which has a porous structure through which the solution flows to be treated. In order to avoid disadvantages, such as the potential drop in the liquid phase and the problem of sealing, a new type of reactor called the Pulsating Porous Percolated Electrode (3PE) has been developed. This reactor eliminates the problem of plugging the particle bed by moving the matrix conductor during a fraction of time of the functioning of the reactor. This is done by using pulsed columns, which are largely used in different processes in chemical engineering. In this manner, one could expect to obtain the advantages of both the fixed and fluidized bed.

Influence of solid phase conductivity on spatial localization of electrochemical processes in flow-through porous electrodes: Part I: Electrodes with uniform conducting matrix

Mathematical simulation of the flow-through porous electrode (PE) operation on the basis of a one-dimensional model with a uniform conducting matrix and a cathodic process involving the main and side reactions (i.e., hydrogen evolution) has been made. The influence of current density and rate and direction of the solution flow on the depth of the main process penetration into the PE has been analysed for different relationships between phase conductivities. It has been shown that when the polarization curve of the side reaction is Tafelian, and the rate of solution circulation is high, there is a limit for the main process penetration into the PE. This limiting value is close to the thickness of the layer, Ld, capable of working at the limiting diffusion current obtained by Sioda's method. The dependence of the Ld layer thickness on phase conductivity has been analysed. In the limiting cases (low fractional conversion, high or identical phase conductivities) analytical expressions for Ld have been obtained. At low flow rates, the depth of the main process penetration increases up to the value of the entire thickness of the PE. It can be concluded that the possibility of increasing the PE efficiency for the uniform matrix by changing phase conductivities is limited.

Influence of solid phase conductivity on spatial localization of electrochemical processes in flow-through porous electrodes Part II: Nonuniform porous matrix with a variable conductivity profile

The possibilities of improving the current distribution uniformity of the main reaction and of increasing porous electrode (PE) performance efficiency by using a porous matrix with a definite profile of conductivity κs(x) have been theoretically analysed. A more general deduction of an equation for the κs(x) profile providing the electrode with constant polarization is given. Peculiarities of this profile and ways of its approximate realization are discussed. The possibility of significant increase in the performance efficiency of electrodes of this type has been experimentally confirmed.