On the electrodeposition of hard gold

The electrodeposition of hard gold in layers of 2 m was investigated. The electrolyte was an acid citrate bath (pH 3·5) with cobalt as an additive. A flow cell allowed a controlled variation of the hydrodynamic conditions. The following features were examined quantitatively in the experiments: the current efficiency for gold deposition (10–30%), the carbon and cobalt content, as well as the porosity of the deposits, and the morphology [by scanning electron microscope (SEM)]. Above 50 mA cm^–2 the deposition of gold and to a minor extent the incorporation of cobalt become mass transport limited (with certain complications resulting from the complex nature of the diffusion layer). The influence observed below 50 mA cm^–2 seems to be due to the synergic effect of the transport controlled reduction of dissolved oxygen. A simple qualitative model for the incorporation of carbon is proposed. The substantial decrease in current efficiency observed upon the addition of cobalt to the bath is probably causedboth by a decrease of the hydrogen overpotential and by an increase of the overpotential for gold deposition. From the viewpoint of technical application, the most relevant result, is the substantial decrease in porosity at decreasing current density (c.d.) and increasing flow rate.Nomenclature c _e interfacial concentration (mol m^–3) - c ₀ bulk concentration (mol m^–3) - D diffusion coefficient (m² s^–1) - D _h hydraulic diameter (m) - F Faraday's constant (96 500 C equiv.^–1) - j _Au partial c.d. of gold deposition (A m^–2) - j _Co partial c.d. of cobalt deposition (A m^–2) - j _L limiting c.d. for gold deposition (A m^–2) - J _H partial c.d. for hydrogen evolution (A m^–2) - j _t total c.d. j _Au+j _H (A m^–2) - c.d. defined by Equation 7 - k _exp experimental mass transfer coefficient (ms^–1) - k _g mass transfer coefficient for gas stirring alone (m s^–1) - k _t overall mass transfer coefficient (m s^–1) - k _v mass transfer coefficient for stirring by hydrodynamic flow alone (m s^–1) - u flow velocity of solution (m s^–1) - z charge number of electrode reaction (equiv mol^–1) - v kinematic viscosity (m² s^–1) - angular velocity (rad s^–1) - (Re) Reynolds numberuD _h/v - (Sc) Schmidt numberv/D - (Sh) Sherwood numberkD _h/D     

Electrode current distribution in a hypochlorite cell

Electrochemical production of gases, e.g. Cl₂, H₂ and O₂, is generally carried out in vertical electrolysers with a narrow electrode gap. The evolution of gas bubbles, on one hand, speeds up the mass transport; on the other it increases the solution resistance and also the cell potential. The gas void fraction in the cell increases with increasing height and, consequently, the current density is expected to decrease with increasing height. Insight into the effects of various parameters on the current distribution and the ohmic resistance in the cell is of the utmost importance in understanding the electrochemical processes at gas-evolving electrodes. An example of the described phenomena is the on-site production of hypochlorite by means of a vertical cell. Experiments were carried out with a working electrode consisting of 20 equal segments and an undivided counter electrode. It has been found that the current distribution over the anode is affected by various electrolysis parameters. The current density,j, decreased linearly with increasing distance,h, from the leading edge of the electode. The absolute value of the slope of theI/h straight line increased with increasing average current density and temperature, and with decreasing velocity of the solution, NaCl concentration and interelectrode gap.Nomenclature a ₁ constant - b _a anodic Tafel slope (V) - b _c cathodic Tafel slope (V) - B current distribution factor - B ₀ current distribution factor att _e=0 - c _NaCl sodium chloride concentration (kmol m^–3) - d_wt interelectrode gap (mm) - h distance from the leading edge of the segmented electrode (m) - H total height of the segmented electrode (m) - I current (A) - I _s current through a segment (A) - j ₀ exchange current density (kA m^–2) - j _av mean current density (kA m^–2) - j _t current density at the top of the segmented electrode (h=H) (kA m^–2) - j _b current density at the bottom of the segmented electrode (h=0) (kA m^–2) - n _s number of a segment of the segmented electrode from its leading edge - R _s unit surface resistance of solution ( m²) - R _{s, b} unit surface resistance of solution at the bottom of the segmented electrode ( m²) - R _{s, t} unit surface resistance of solution at the top of the segmented electrode ( m²) - t _e time of electrolysis (h) - T temperature (K) - U _c cell voltage (V) - U ₀ reversible cell voltage (V) - v ₀ solution flow rate of the bulk solution in the cell at the level of the leading edge of the electrode (m s^–1) - resistivity of the solution ( m) - _a anodic overpotential (V) - _c cathodic overpotential (V) - gas void fraction - _b gas void fraction ath=0 - _t gas void fraction ath=H Paper presented at the 2nd International Symposium on Electrolytic Bubbles organized jointly by the Electrochemical Technology Group of the Society of Chemical Industry and the Electrochemistry Group of the Royal Society of Chemistry and held at Imperial College, London, 31st May and 1st June 1988.     

Electrochemical reduction of silver thiosulphate complexes Part II Mechanism and kinetics

In this paper the thermodynamic data for complex formation between Ag⁺ and S₂O_{3 2–} ions, determined previously, are applied to kinetic investigation of the reduction of silver thiosulphate complexes. Both electrochemical (linear sweep voltammetry on a rotating disc electrode) and surface analytical (Auger electron spectroscopy) techniques are used. The deposits resulting from the electrodeposition of silver thiosulphate complexes are shown to be composed of silver and to be polycrystalline. The reduction follows a mechanism involving mass and charge transfer and chemical reaction steps. The relevant kinetic parameters are calculated and a rate equation describing the kinetics of the reduction is given.List of symbols a activity (M) - c concentration (M) - j current density (A m^–2) - j _c current density of charge transfer (A m^–2) - j _m current density of mass transfer (A m^–2) - k rate constant (m s^–1) - y activity coefficient (molarity scale) - D diffusion coefficient against gradient of concentration (m² s^–1) - D diffusion coefficient against gradient of electrochemical potential (m² s^–1) - E electrode potential vs NHE (V) - I ionic strength (M) - T temperature (K) Greek symbols a transfer coefficient - _1n stability constant of Ag(S₂O₃) _n (^2n–1)- - kinematic viscosity (m² s^–1) - rotation speed of the electrode (rad s^–1) Indices b bulk of the solution - f free (= uncomplexed) - 1,n related to complex Ag(S₂O₃)_n ^(n–1) - t total Constants F Faraday constant (96486 A s mol^–1) - R universal gas constant (8.3145 Jmol^–1 K^–1)     

Oxygen reduction on stainless steel

Oxygen reduction was studied on AISI 304 stainless steel in 0.51 m NaCl solution at pH values ranging from 4 to 10. A rotating disc electrode was employed. It was found that oxygen reduction is under mixed activation-diffusion control. The reaction order with respect to oxygen was found to be one. The values of the Tafel slope depend on the potential scan direction and pH of the solution, and range from – 115 to – 180 mV dec^–1. The apparent number of electrons exchanged was calculated to be four, indicating the absence of H₂O₂ formation.Nomenclature B =0.62 nFcD ^2/3 –^1/6 - c bulk concentration of dissolved oxygen (mol dm^–3) - D molecular diffusion coefficient of oxygen (cm² s^–1) - E electrode potential (V) - EH standard electrode potential (V) - E _H ⁰ Faraday constant (96 500 As mol^–1) - I current (A) - j current density (A cm^–2) - j _k kinetic current density (A cm^–2) - j _L limiting current density (A cm^–2) - m reaction order with respect to dissolved oxygen molecule - M molar mass (g mol^–1) - n number of transferred electrons per molecule oxygen - density (g cm^–3) - kinematic viscosity (cm² s^–1) - angular velocity (s^–1)     

Potentiodynamic electrodeposition of paint (EDP)

Potentiodynamic electropainting at a rotating iron disc electrode has been investigated with three different EDP resins, two anodic from the acrylate type and one cathodic from the epoxide type, and a wide variation of conditions. Voltage scan rate ( _s=1 to 200 Vs^–1), voltage range (40 to 200V) and electrode rotation speed (n=60 and 1000rpm) were the most important parameters. The (cyclic) voltammetric curves obtained generally exhibit three characteristic features: (1) The current rises steeply at the start of the experiment. Bath resistance transforms the potentiodynamic curve simultaneously into a galvanodynamic curve. After a transition time, , a critical pH is attained at the phase boundary and electrocoagulation occurs. This leads to a rapidly decreasing current density. The sharp c.d. maximum thus established has a peak voltage,U _p, which increases with _s according to the relation logU _P 1/3 log _s in accordance with theory. (2) At high voltages, a limiting current density is observed, increasing with the square root of _s. This could be quantitatively interpreted in terms of dynamic growth of film thickness governed by Ohmic ion transport in the film. The preceding part of theU/j curve declines withj t _–1/2, which indicates the prevalence of space charge effects. (3) Ohmic lines are measured in the course of the first reverse scan and in all quasi steady state follow up cycles. They are flatter by a factor of 1000 in regard to the initial Ohmic line and reflect low voltage Ohmic behaviour of the EDP-film. At high voltages positive current deviations occur due to Child's law. The curves can be measured easily and reproducibly. Due to their salient features it is proposed to use them for characterization of EDP-paints.Nomenclature a current density scan rate (mAcm^–2s^–1) - A electrode area (cm²) - c ^* critical hydrogen ion- (or hydroxyl ion-) concentration at the electrode for electrocoagulation (mol dm^–3) - C _A capacitance of EDP-film per unit area (Fcm^–2) - E electric field strength (Vcm^–1) - I cell current (mA) - j current density, c.d. (mA cm^–2) - j _c capacitance current density (mA cm^–2) - j ^lim limiting current density (mA cm^–2) - j _p peak current density (Section3) (mA cm–2) - J _r residual current density (mA cm^–2) - j ^* critical current density (for EDP) (mA cm^–2) - K constant in Equations 9 and 10 (Vs^1/2) - L _F thickness of polymer film (cm) - L _sc thickness of space charge layer (cm) - m _e electrochemical equivalent (gC^–1 - n _c exponent in Child's law - n rotating disc electrode rotation speed (rpm) - N particle number concentration (cm^–3) - R _B bath resistance () - R _F film resistance () - s density (g cm^–3) - transition time (s) - U (cell) voltage (V) - U _max maximum voltage, point of reversion of voltage scan direction (V) - U _p peak voltage, section3 (V) - _s voltage scan (or sweep) rate (Vs^–1)     

Surface treatment for mitigation of hydrogen absorption and penetration into AISI 4340 steel and Inconel 718 alloy

It is shown that the underpotential deposition of zinc on AISI 4340 steel and Inconel 718 alloys inhibits the hydrogen evolution reaction and the degree of hydrogen ingress. In the presence of monolayer coverage of zinc on the substrate surfaces, the hydrogen evolution current densities are reduced 46% and 68% compared with the values obtained on bare AISI 4340 steel and Inconel 718 alloy, respectively. As a consequence, the underpotential deposition of zinc on AISI 4340 steel and Inconel 718 alloy membrane reduces the steady state hydrogen permeation current density by 51% and 40%, respectively.List of symbols C _S surface hydrogen concentration (mol cm^–3) - D hydrogen diffusion coefficient (cm² S^–1) - E potential (V) - E _pdep predeposition potential (V) - F Faraday constant (96 500 C mol^–1) - i current density (A cm^–2) - i _a HER current density in the absence of predeposition of zinc (A cm^–2) - i ₀ exchange current density (A cm^–2) - i _p HER current density in the presence of predeposition of zinc (A cm^–2) - j _t transition hydrogen permeation current density (A cm^–2) - j _o initial hydrogen permeation current density (A cm^–2) - j steady state hydrogen permeation current density (A cm^–2) - k thickness dependent absorption-adsorption constant (mol cm^–3) - L membrane thickness (cm) - Q _max maximum charge required for one complete layer of atoms on a surface (C cm^–2) - t time (s) Greek symbols _c cathodic transfer coefficient, dimensionless - _H hydrogen surface coverage, dimensionless - _Zn zinc surface coverage, dimensionless - work function (eV) - = t D/L ² (dimensionless time)     

Recovery of pickling effluents by electrochemical oxidation of ferrous to ferric chloride

The characteristics of the effluents from the preparatory pickling step of zinc plating are presented and the various methods of oxidizing ferrous to ferric chloride are briefly considered. An electrochemical oxidation method is proposed to recover these effluents by using an electrochemical cell with three-dimensional electrodes and an anion selective membrane. A near exhausted hydrochloric acid solution was used as catholyte. The experimental data obtained from the proposed cell show a faradic yield of 100% and easy control of the parasitic reactions. The three-dimensional anode was modelled and it is shown that at high values of current only the felt entrance region works efficiently.Nomenclature A membrane surface (cm²) - a specific felt surface (cm^–1) - C concentration difference (mol dm-^–3) - D average diffusion coefficient through the membrane (cm² s^–1) - i _n felt wall flux of species (mol cm^–2 s^–1) - j total current density (A cm^–2) - j ₀ exchange current density (A cm^–2) - j ₁ current density in matrix (A cm^–2) - j ₂ current density in felt solution (A cm^–2) - j _n transfer current density (A cm^–2) - L thickness of felt electrode (cm) - L _m thickness of membrane (cm) - M transport of ferrous and ferric ions through the membrane (mol) - N superficial flux of ion reactant (mol cm^–2 s^–1) - u superficial fluid velocity (cm s^–1) - x distance through felt electrode (cm) - R universal gas constant (8.3143 J mol^–1 K^–1) - T absolute temperature (K) - t time (s) Greek letters _a, _c anodic and cathodic transfer coefficient - local overpotential ( = ₁ – ₂) (V) - conductivity of solution (mS cm^–1) - µ solution viscosity (Pa s) - solution density (g cm^–3) - conductivity of solid matrix (mS cm^–1) - ₁ electrostatic potential in matrix phase (V) - ₂ electrostatic potential in solution (V)     

Mass transfer at rough gas-sparged electrodes

Rates of mass transfer were measured by the limiting current technique at a smooth and rough inner surface of an annular gas sparged cell in the bubbly regime. Roughness was created by cutting 55°V-threads in the electrode normal to the flow. Mass transfer data at the smooth surface were correlated according to the expression j = 0.126(Fr Re)^–0.226 Surface roughness of peak to valley height ranging from 0.25 to 1.5 mm was found to have a negligible effect on the mass transfer coefficient calculated using the true electrode area. The presence of surface active agent (triton) in the solution was found to decrease the mass transfer coefficient by an amount ranging from 5% to 30% depending on triton concentration and superficial air velocity. The reduction in the mass transfer coefficient increased with surfactant concentration and decreased with increasing superficial gas velocity.Nomenclature a constant - A electrode area (cm²) - C _p specific heat capacity Jg^–1 (K^–1) - C ferricyanide concentration (m) - d _c annulus equivalent diameter, (d _o – d _i) (cm) - d _o outer annulus diameter (cm) - d _i inner annulus diameter (cm) - D diffusivity of ferricyanide (cm²s^–1) - e peak-to-valley height of the roughness elements (cm) - e ⁺ dimensionless roughness height (eu ^*/) - f friction coefficient - F Faraday constant (96 500 Cmol^–1) - g acceleration due to gravity (cm s^–2) - h heat transfer coefficient (J cm^–2 s K) - I _L limiting current (A) - K mass transfer coefficient (cm s^–1) - K thermal conductivity (W cm^–1 K^–1) - V _g superficial air velocity (cm s^–1) - Z number of electrons involved in the reaction - Re Reynolds number (_L V _g d _e/) - J mass or heat transfer J factor (St Sc ^0.66) or (St Pr ^0.66), respectively - St Stanton number (K/V _g for mass transfer and h/C _p V _g for heat transfer) - Fr Froude number (V _g ² /d _e g) - Sc Schmidt number (/D) - Pr Prandtl number (C _p/K) - PL solution density (g cm^–3) - kinematic viscosity (cm²s^–1) - gas holdup - u ^* friction velocity = V _L(f/2) - diffusion layer thickness (cm) - solution viscosity (gcm^–1 s^–1)     

The mechanism of copper powder formation in potentiostatic deposition

A mechanism for copper powder formation in potentiostatic deposition is proposed, and the critical overpotential of copper powder formation is determined. A good agreement between theoretical and experimental results has been obtained.List of symbols C ₀ bulk concentration (mol cm^–3) - D diffusion coefficient (cm² s^–1) - F Faraday's constant (C mol^–1) - h height of protrusion (cm) - h _c height at which dendrites crack (cm) - h _i height (cm) - h ₀ initial height of protrusion (cm) - h _j,t elevation at pointj and timet (cm) - h _j,0 initial elevation at pointj (cm) - I limiting diffusion current (A) - I ₀ initial limiting diffusion current (A) - i limiting current density (A cm^–2) - i _d current density on the tip of dendrite of height h (A cm^–2) - i _t total current (A cm^–2) - j number - k proportionality factor [cm (mol cm^–3)^m] - k constant - M number of dendrites - m number - N number of elevated points - n number of electrons - p concentration exponent - Q _c quantity of electricity (C) - R gas constant (J mol^–1 K^–1) - S electrode surface area (cm²) - T temperature (K) - t time (s) - t _a longest time in which approximation h is valid (s) - t _i induction time (s) - V molar volume (cm³ mol^–1) - surface tension (J cm^–2) - thickness of diffusion layer (cm) - overpotential (V) - _c,p critical overpotential of powder formation (V) - fraction of flat surface - apparent induction time (s)     

Mass transfer at the gas evolving inner electrode of a concentric cylindrical reactor

Mass transfer coefficients for an oxygen evolving vertical PbO₂ coated cylinder electrode were measured for the anodic oxidation of acidified ferrous sulphate above the limiting current. Variables studied included the ferrous sulphate concentration, the anode height, the oxygen discharge rate and the anode surface roughness. The mass transfer coefficient was found to increase with increasing O₂ discharge rate,V, and electrode height,h, according to the proportionality expressionK V ^0.34 h ^0.2. Surface roughness with a peak to valley height up to 2.6 mm was found to increase the rate of mass transfer by a modest amount which ranged from 33.3 to 50.8% depending on the degree of roughness and oxygen discharge rate. The present data, as well as previous data at vertical oxygen evolving electrodes where bubble coalescence is negligible, were correlated by the equationJ=7.63 (Re. Fr)^–0.12, whereJ is the mass transferJ factor (St. Sc ^0.66).Notation a ₁,a ₂ constants - A electrode area (cm²) - C concentration of Fe²⁺ (M) - d bubble diameter (cm) - D diffusivity (cm² s^–1) - e electrochemical equivalent (g C^–1) - F Faraday's constant - g acceleration due to gravity (cm s^–2) - h electrode height (cm) - I _Fe ²⁺ current consumed in Fe²⁺ oxidation A - I _{o 2} current consumed in O₂ evolution, A - K mass transfer coefficient (cm s^–1) - m amount of Fe²⁺ oxidized (g) - P gas pressure (atm) - p pitch of the threaded surface (cm) - Q volume of oxygen gas passing any point at the electrode surface (cm³ s^–1) - R gas constant (atm cm³ mol^–1 K^–1) - r peak-to-valley height of the threaded surface (cm) - t time of electrolysis (s) - T temperature (K) - solution viscosity (g cm^–1 s^–1) - V oxygen discharge velocity as defined by Equation 3 (cm s^–1) - Z number of electrons involved in the reaction - Sh Sherwood number (Kd/D) - Re Reynolds number (Vd/) - Sc Schmidt number (v/D) - J mass transferJ factor (St. Sc ^0.66) - St Stanton number (K/V) - Fr Froude number (V ²/dg) - Solution density, g cm^–3 - v Kinematic viscosity (cm² s^–1) - bubble geometrical parameter defined in [31] - fractional surface coverage - diffusion layer thickness (cm)     

Application of metallic foams in electrochemical reactors of the filter-press type: Part II Mass transfer performance

This paper focuses on mass transfer characteristics of classical filter-press electrochemical reactors without membranes. In the tested configuration, the working electrode consists of a lane plate with a sheet of foam and the counter-electrode consists of a plane plate with a turbulence promoter. The global mass transfer coefficients of the two electrodes have the same order of magnitude. Moreover, a comparison with literature data shows that their values remain in the range of those previously presented. Due to the high specific surface area of the foam used (A _ve, = 6400 m^–1), the ratio of the surface area of the working electrode to that of the counter electrode is 15. The electroreduction of ferricyanide has been carried out to test the performance of this configuration. The value of the final conversion has been compared to that calculated from mass transfer coefficients and surface areas of the electrodes.List of symbols A _ve dynamic specific surface area of the foam: surface area per volume of material (m^–1) - A_ve dynamic specific surface area of the electrode consisting of a plate and a sheet of foam: surface area per volume of electrode (m^–1) - A _vs static specific surface area (m^–1) - C _in ferricyanide concentration at the inlet of the cell (mol m^–3) - C _out ferricyanide concentration at the outlet of the cell (molm^–3) - D diffusion coefficient (m² s^–1) - d _h equivalent hydraulic diameter, dh = 2lh (l + h)^–1 (m) - F Faraday number (C mol^–1) - h channel thickness (m) - I limiting diffusion current (A) - I _{c a} final limiting diffusion current intensity at the anode (A) - I _cf final limiting diffusion current intensity at the cathode (A) - k _a mass transfer coefficient at the anode (m s^–1) - k _c mass transfer coefficient at the cathode (ms^–1) - k _d mass transfer coefficient (m s^–1) - l channel width (m) - n number of electrons in the electrochemical reaction - Q _v volumetric flow rate in the channel (m³ s^–1) - Re Reynolds number, Re = U ₀ d _h v ^–1 - S active surface area of the electrode (m²) - S _a surface area of the anode (m²) - S _c surface area of the cathode (m ²) - S _c Schmidt number, Sc = v D ^–1 - Sh Sherwood number, Sh = k _d D _h/D - U ₀ superficial velocity (m s^–1) - V volume offered to fluid flow in the volumic electrode (m³) - V volume of one tank reactor in the cascade (m³) - X conversion - X _f final conversion Greek letters porosity - v kinematic viscosity (m² s^–1) - density (kg s^–1) - residence time in a continuous stirred tank reactor = /Q _v (s)     

Mass transfer studies at iron felts

Mass transfer has been studied at flow-through iron felts using the reduction of ferricyanide or copper cementation on iron as test reactions. Empirical correlations between a modified Sherwood number and the Reynolds number are proposed. Comparisons of the mass-transfer performance of iron felts with other three-dimensional structures are made.List of symbols a ₃ specific surface area per unit felt volume (m^–1) - A empty cross-section of the reactor (m²) - C concentration (mol m^–3) - C ₀ inlet concentration (mol m^–3) - d _h hydraulic diameter (m) - e fibre thickness (m) - E electrode potential (V) - D diffusion coefficient (m²s^–1) - F Faraday constant (A s mol^–1) - i current density (A m^–2) - I total current (A) - I _L limiting current (A) - J _m mass transfer j-factor=(k/v)Sc ^2/3 - K mass transfer coefficient (m s^–1) - l fibre width (m) - L electrode thickness (m) - Re Reynolds number - vd _h/ - Re modified Reynolds number - vl/ - Sc Schmidt number = /D - Sh modified sherwood number = ka _e l ²/D - t time (s) - T Temperature (K) - superficial liquid flow velocity (m s^–1) Greek characters void fraction - dynamic viscosity (kg m^–1 s^–1) - kinematic viscosity (m² s^–1) - ₃ charge number of the electrode reaction - iron density (kg m^—) - _a apparent density of the felt (kg m^–3) - _m residence time of the reservoir (s)     

Effect of thiourea,benzotriazole and 4,5-dithiaoctane-1,8-disulphonic acid on the kinetics of copper deposition from dilute acid sulphate solutions

The effects of thiourea (TU), benzotriazole (BTA) and 4,5-dithiaoctane-1,8-disulphonic acid (DTODSA) on the deposition of copper from dilute acid sulphate solutions have been studied using potential sweep techniques. Tafel slopes and exchange current densities were determined in the presence and absence of these organic additives. TU and BTA were found to inhibit the copper deposition reaction; increases in the BTA concentration gave a systematic lowering of the exchange current density, whilst TU behaved in a less predictable manner. For BTA and TU concentrations of 10^–5 mol dm^–3,j ₀ values of 0.0027 ± 0.0001 and 0.0028 ± 0.0002 mA cm^–2 were obtained compared to a value of 0.0083 ± 0.0003 mA cm^–2 for the additive free acid sulphate solution. In contrast, in the presence of DTODSA, an increased exchange current of 0.043 ± 0.0003 mA cm^–2 was observed. The presence of additives gave rise to measured Tafel slopes of –164, –180 and –190 mV for TU, BTA and DTODSA, respectively, compared to that of –120 mV for copper sulphate alone.List of symbols A electrode area (cm²) - b _C cathodic Tafel slope (mV) - c _B bulk concentration (mol cm^–3) - D Diffusion coefficient (cm² s^–1) - F Faraday constant (A s mol^–1) - I _L Limiting current (A) - j Current density (A cm^–2) - j _CT Charge transfer current density (A cm^–2) - j ₀ Exchange current density (A cm^–2) - k _L Mass transport coefficient (cm s^–1) - R Molar gas constant (J K^–1 mol^–1) - T Temperature (K) - z Number of electrons (dimensionless) Greek symbols _C Cathodic transfer coefficient (dimensionless) - Overpotential (V) - v Kinematic viscosity (cm² s^–1) - Rotation rate (rad s^–1)     

Studies of three-dimensional electrodes in the FMO1-LC laboratory electrolyser

A FMO1-LC parallel plate, laboratory electrochemical reactor has been modified by the incorporation of stationary, flow-by, three-dimensional electrodes which fill an electrolyte compartment. The performance of several electrode configurations including stacked nets, stacked expanded metal grids and a metal foam (all nickel) is compared by (i) determining the limiting currents for a mass transport controlled reaction, the reduction of ferricyanide in 1 m KOH and (ii) measuring the limiting currents for a kinetically controlled reaction, the oxidation of alcohols in aqueous base. It is shown that the combination of the data may be used to estimate the mass transfer coefficient, _L, and the specific electrode area, A _e, separately. It is also confirmed that the use of three dimensional electrodes leads to an increase in cell current by a factor up to one hundred. Finally, it is also shown that the FM01-LC reactor fitted with a nickel foam anode allows a convenient laboratory conversion of alcohols to carboxylic acids; these reactions are of synthetic interest but their application has previously been restricted by the low rate of conversion at planar nickel anodes.Nomenclature A _e electrode area per unit electrode volume (m²m^–3) - c bulk concentration of reactant (mol m^–3) - E electrode potential vs SCE (V) - E _1/2 half wave potential (V) - F Faraday constant (96 485 C mol^–1) - I current (A) - IL limiting current (A) - j _L limiting current density (A m^–2) - _L mass transfer coefficient (m s^–1) - n number of electrons transferred - p empirical constant in Equation 2 - P pressure drop over reactor (Pa) - R resistance between the tip of the Luggin capillary and the electrode surface () - q velocity exponent in Equation 2 - (interstitial) linear flow rate of electrolyte (ms^–1) - V _e volume of electrode (m³)     

Behaviour of a carbon felt flow by electrodes Part I: Mass transfer characteristics

The properties of a carbon felt electrode have been experimentally investigated with special attention to its possible application in the electrochemical recovery of heavy metals. The mass transfer process has been studied by means of the reduction of ferricyanide and cupric ions for a flow-by electrode operating under limiting current conditions. An empirical correlation between the Sherwood and Reynolds numbers has been used to compare the experimental data with those obtained by other authors for different porous electrodes.Notation a specific electrode area (m^–1) - a _v area per unit solid volume (m^–1) - C _in entering concentration of reacting species (kmol m^–3) - C _out exit concentration of reacting species (kmol m^–3) - d _f fibre diameter (m) - d _b hydraulic diameter of the felt fibres (m) - D diffusion coefficient (m² s^–1) - F Faraday number 96 487 (C mol^–1) - k _m mass transfer coefficient (m s^–1) - l_lim limiting current (A) - l length of the electrode (m) - L thickness of the electrode (m) - Q _L catholyte flow rate (m³ s^–1) - Re Reynolds numberRe=d _h u/v - Sh Sherwood numberSh=k _m d _h/D - u solution velocity in the empty cross-section (m s^–1) - X reaction conversion - z number of electrons in the electrochemical reaction Greek letters porosity of the felt - kinematic viscosity of the solution (m² s^–1) - _Rgq_A true and apparent density of the felt (kg m^–3)     

Mass transfer at carbon fibre electrodes

Mass transfer at carbon fibre electrodes has been studied using the mass transfer controlled reduction of potassium hexacyanoferrate(III) to potassium hexacyanoferrate(II). Different geometrical configurations have been assessed in a flow-by mode, namely bundles of loose fibres with liquid flow parallel to the fibres, carbon cloth with flow parallel to the cloth and carbon felt with liquid flow through the felt. For comparison, mass transfer rates at a single fibre have been measured; the experimental data fit the correlationSh=7Re ^0.4. The same correlation can be used as a first approximation for felts. Mass transfer for fibre bundles and cloth under comparable conditions is much lower owing to channelling.Nomenclature c reactant concentration (mol m^–3) - c ₀ reactant concentration atx=0 (mol m^–3) - c _L reactant concentration atx=L (mol m^–3) - d fibre diameter (m) - D diffusion coefficient (m² s^–1) - F Faraday number (96 487 C) - h depth of the electrode (m) - i current density (A m^–2) - I current (A) - k mass transfer coefficient (m s^–1) - L length of the electrode (m) - n number of electrons - S specific surface area (m² m^–3) - u (superficial) velocity (m s^–1) - V _R reactor volume (m³) - w width of electrode (m) - x distance in flow direction (m) - current efficiency - electrode efficiency - characteristic length (m) - v kinematic viscosity (m² s^–1) - _s ⁿ normalized space velocity (m³ m^–3s^–1) - Re Reynolds number (ud/v) - Sh Sherwood number (kd/D) - Sc Schmidt number (v/D)     

Pressure dependence of limiting current density of sparking during electrochemical drilling

The cathodic current density used in electrochemical drilling can be increased only up to a certain value, above which current oscillations, sparking and acoustic phenomena appear, whereby the cathode can be damaged. The limiting current density for sparking, j _s, depends on the rate of flow and properties of the electrolyte and on the hydrostatic pressure. Values of j _s were measured for metal capillaries provided with external insulation in the turbulent flow regime in the range of Reynolds numbers from 2 300 up to 30 000 and at hydrostatic pressures ranging from 0.12 to 1.1 MPa. A simple heat generation model is proposed and the limiting current densities for sparking (868 experiments) are correlated with a criterion equation enabling the calculation of j _s.List of symbols c _pE specific heat of electrolyte (J kg^–1 K^–1) - d ₁ inner diameter of the cathode (m) - d ₂ outer diameter of the cathode (m) - I current (A) - I _s limiting current for sparking (A) - j current density (Am^–2) - j _s limiting current density for sparking (Am^–2) KT constant - K _T constant - L characteristic length (m) - N _u Nusselt number - p pressure (Pa) - p ₀ reference atmospheric pressure (Pa) - P exponent - P _r Prandtl number - q exponent - q heat flux (W m^–2) - R exponent - Re Reynolds number - _E linear electrolyte velocity (m s^–1) Greek symbols - heat transfer coefficient (W m^–2 K^–1) - temperature difference (K) - _E electrolyte conductivity (^–1 m^–1) - _E electrolyte thermal conductivity (Wm^–1 K^–1) - µ_E electrolyte viscosity (kgm^–1 s^–1) - _E electrolyte density (kg m^–3)     

Cathodic reduction of hypochlorite during reduction of dilute sodium chloride solution

Sodium chloride solutions of concentration 15 and 30 g dm^–3 were electrolysed in a flow-through electrolyser with a titanium/TiO)₂/RuO₂ anode at current densities 1059–4237 A m^–2. The current yield for the reduction of hypochlorite on a stainless steel cathode was found to be 13–32% at 7 g dm^–3 NaClO, in agreement with that calculated on the basis of the Stephan-Vogt theory. Migration of ions was taken into account, the diameter of hydrogen bubbles was set equal to 0.04 mm and the coverage of the electrode with the bubbles was estimated as = 0.897. The results of calculations show that the reduction rate of hypochlorite at low NaCl concentrations is lowered by migration. Literature data for the reduction of hypochlorite are in accord with the current yield calculated on the basis of the Stephan-Vogt theory using = 0.787 and = 0.949.List of symbols C _i ^o concentration of species i in the bulk (mol m^–3) - C _i ^s concentration of species i at the cathode surface (mol m^–3) - d _B bubble diameter (m) - D _e equivalent diameter (characteristic dimension) (m) - D _i diffusion coefficient of species i (m² s^–1) - f _G gas evolution efficiency - F Faraday constant (96 487 C mol^–1) - j total current density (Am^–2) - j _B current density for gas evolution (Am^–2) - j _{c, lim} limiting current density for cathodic reduction of ClO^– (A m^–2) - j _{c, r} critical current density (A m^–2) - L length of electrode (m) - M migration correction factor - n _B number of electrons exchanged in gas evolution - n _ClO ^– number of electrons exchanged in reduction of ClO^– - N _i flux of species i (mol m^–2 s^–1) - Q charge passed (C) - P _t total gas pressure (Pa) - Re Reynolds number (Equation 14) - Re _B Reynolds number (Equation 17) - Sc Schmidt number (Equation 13) - Sh Sherwood number (Equation 12) - Sh _B Sherwood number (Equation 15) - T absolute temperature (K) - u _i mobility of ion i (m² s^–1 V^–1) - _B fictitions linear velocity of gas formation (ms^–1) - _el rate of electrolyte flow (ms^–1) - V volume of the electrolyte in the system (m³) - V _{H 2} content of hydrogen in gas phase (%) - V _{O 2} content of oxygen in gas phase (%) - y _i current yield (differential) for production of species i (%) - y _r current yield (differential) for reduction of ClO^– and ClO ₃ ^– (%) - Y _ClO–,r current yield (differential) for reduction of CIO^– (%) - Y _i integral current yield for production of species i (%) - z _i charge number of ion i Greek symbols thickness of Nernst diffusion layer (m) - _c thickness of convective diffusion layer (m) - _B thickness of diffusion layer controlled by gas evolution (m) - dynamic viscosity (m² s^–1) - time (s) - coverage of electrode surface with gas bubbles - Galvani potential (V) - correction function (Equation 11)     

Mass transfer to percolated porous materials in a small-scale cell operating by self-pumping

The paper deals with an experimental electrochemical study of mass transfer to porous nickel materials (felt, foams) in a small-scale laboratory cell functioning in a self-pumping mode. The liquid flow through a disc of the porous material is induced by the rotation of a solid circular disc. The cell is simple and is useful for laboratory studies of materials for porous electrodes and also for small-scale synthesis using such materials. The work examines separately the mass transfer to the rotating disc and to the porous disc. Empirical correlations of the experimental data are given.Nomenclature a _e specific surface area (per unit of total volume of electrode) (m^–1) - C ₀ entering concentration of ferricyanide ions (mol m^–3) - D molecular diffusion coefficient of ferricyanide (m² s^–1) - e thickness of the sheet of material (m) - F Faraday number (C mol^–1) - g acceleration due to gravity (m s^–2) - h distance between the discs (m) - I _L limiting current (A) - 736-1 mean mass transfer coefficient (m s^–1) - N roating velocity (rev min^–1) - Q _v volumetric electrolyte flow rate (m³ s^–1) - R radius of the solid disc (m) - R _c inner radius of the cell (m) - R _i radius of the porous disc (m) - Re _h Reynolds number based onh (=h²/) - Re _R Reynolds number based onR (=R²/) - S _c Schmidt number - Sh _h Sherwood number based onh (=k _d h/D) - Sh _r Sherwood number based onR (=k _d R/D) - mean electrolyte velocity (m s^–1) - V electrode volume (m³) - X conversion - electrolyte density (kg m^–3) - _e number of electrons in the electrochemical reaction - kinematic viscosity (m² s^–1) - angular velocity (s^–1) - ₀ minimum angular velocity (s^–1)     

Meniscus shape and lateral wetting at the hanging meniscus rotating disc (HMRD) electrode

The hanging meniscus rotating disc (HMRD) electrode is a configuration in which a cylinder of the electrode material is used without an insulating mantle. We have recently shown that the hydrodynamic behaviour of the HMRD is similar to that of the conventional rotating disc electrode and that this configuration can also be used to study the kinetics of simple charge transfer reactions. In this paper experimental data on the change of meniscus shape upon meniscus height and rotation for different electrode materials are presented and analysed in relation to lateral wetting and stability.List of symbols A electrode area (cm²) - C ₀ ^* bulk concentration (mol cm^–3) - D ₀ diffusion coefficient (cm²s^–1) - f force on a cylinder supporting a hanging meniscus (dyn) - F Faraday (96 500 Cmol^–1) - g gravitational acceleration (cm s^–2) - h height (cm) - h _m meniscus height (cm) - h ₀ critical meniscus height (cm) - i total current (A) - i _L limiting current (A) - i _max kinetic current (A) - k proportionality constant (cm^–1) - K dimensionless constant - n number of electrons exchanged - R _eff effective radius of the electrode (cm) - R ₀ geometric radius of the electrode (cm) - V volume of the meniscus above the general level of the liquid surface (cm³) Greek letters ₀ thickness of hydrodynamic boundary layer (cm) - surface tension (dyn cm^–1) - kinematic viscosity (cm²s^–1) - density difference between the liquid and its surrounding fluid (gcm^–3) - _C normal contact angle - _L local contact angle 0_L + 90° - electrode rotation rate (s^–1)