Mass transfer at gas evolving electrodes

To elucidate the mechanism of the mass transfer at a gas evolving electrode, the thickness of the diffusion layer δ has been determined as a function of the volume rate of the gas evolution ν, for both hydrogen and oxygen evolving electrodes in alkaline solution. The effect of electrode material, alkaline concentrations, roughness, position and disk diameter of the electrode, gas pressure and of temperature upon the δ/gn relation is given. A low absolute value of the slope of the log δ/log ν, of about 0.3 is found experimentally when no coalescence of gas bubbles occurs and a high one, that is about 0.9 when coalescence occurs frequently. It has been concluded from the experimental results that the mass transfer can be explained on the basis of the hydrodynamic model when no coalescence of gas bubbles occurs and on that of the penetration model when coalescence occurs frequently.     

The effect of electrolytically evolved gas bubbles on the thickness of the diffusion layer—II

In order to elucidate the mechanism determining the mass transfer at a gas evolving electrode, the thickness σ of the Nernst diffusion layer was determined as a function of the current density for both hydrogen and oxygen evolving on horizontal and vertical electrodes in acid as well as in alkaline solutions. Also the diameter of the bubbles and the diameter of the bubble street were determined.It was found that both σ and the slope h of the log σ/log i curve strongly depend on the nature of the gas evolved and on the nature of the electrolyte and that the position of the electrode has only a small effect on σ and on h.At i < 10 mA/cm² the diameter of free bubbles is practically independent of the current density. At i > 30 mA/cm² coalescence of bubbles occurs very often with exception of the hydrogen bubbles in alkaline solution.From the experimental results it was concluded that the mass transfer at a gas evolving electrode is determined by the flow of solution along the electrode surface, this flow being caused by detaching and rising bubbles and by coalescence of bubbles.     

Electrolytic resistance of solution layers at hydrogen and oxygen evolving electrodes in alkaline solution

During alkaline water electrolysis, additional energy losses occur owing the presence of bubbles in the solution, particularly close to both the gas-evolving electrodes.For both hydrogen and oxygen-evolving disc electrodes (diameters from 0.2 to 2.0 mm) in KOH solutions, the reduced increase in ohmic resistance, ΔR*, has been determined by the alternating current—impedance method.It has been found that, for hydrogen-evolving electrodes, log ΔR* = 1₁ + b log i, where the exponent b at 0.1 A cm^?2 < i < 5 A cm^?2 does not depend on the diameter, position and material of the electrode, pressure and temperature but does significantly depend on KOH concentration. The factor a₁, however, being dependent on the position, height and material of electrode, temperature and KOH concentration. ΔR* cannot be expressed for the oxygen-evolving electrode by a general equation, due to the coalescence behaviour of oxygen bubbles.Moreover, it has been established that the Bruggemann equation is useful to determine the ohmic resistance of a solution layer containing gas bubbles of different size at which each bubbles adheres to the electrode surface.     

Mass transfer and shear stress on a vertical electrode with gas evolution

The relationship between the mass transfer coefficient and the shear stress along the vertical electrode was investigated under electrolytic gas evolution, either oxygen or hydrogen, from alkaline solution containing ferrocyanide and ferricyanide ions. The shear rate obtained from the shear stress measurement was empirically correlated with the gas evolving current density, the electrode height and the liquid kinematic viscosity. The dimensionless correlation of the mass transfer coefficient with the shear stress under oxygen evolution condition agreed formally with the correlation of the mass transfer in turbulent-free convection. On the other hand, the experimental results under hydrogen evolution varied greatly from those under oxygen evolution.     

Mass transfer at horizontal gas-evolving electrodes

The effect of H₂ and O₂ evolution on the mass transfer coefficient of the reduction of K₃Fe(CN)₆ and the oxidation of K₄Fe(CN)₆ at nickel electrodes was studied up to 105 mA/cm². The relation between the rate gas evolution and the mass transfer coefficient was found to be: log K = a + 0·25 log V for a H₂-evolving horizontal electrode and log K = a + 0·4 log V for an O₂-evolving horizontal electrode.Comparison was made between mass transfer coefficients at horizontal and vertical gas-evolving electrodes. Mass transfer coefficients at horizontal electrodes are much higher than at vertical electrodes.     

Mass transfer characteristics of electrochemical reactors employing gas evolving mesh electrodes

Mass transfer rates were measured at a single screen and a fixed bed of closely packed screens for the simultaneous cathodic reduction of K₃Fe(CN)₆ and anodic oxidation of K₄Fe(CN)₆ in alkaline solution with H₂ and O₂ evolution, respectively. Variables studied were gas discharge rate, number of screens per bed and position of the electrode (vertical and horizontal). For single screen electrodes, the mass transfer coefficient was related to the gas discharge rate by the equations: $$\begin{gathered} K = aV^{0.190} , for H_2 evolving electrodes, \hfill \\ K = aV^{0.469} , for O_2 evolving electrodes \hfill \\ \end{gathered} $$ . Electrode position was found to have no effect on the rate of mass transfer for single and multiscreen electrodes in the case of H₂ and O₂ evolution. Mass transfer coefficients were found to increase with an increasing number of screens per bed in the case of H₂ evolution, while in the case of O₂ evolution the mass transfer coefficient decreased with an increasing number of screens per bed. A mathematical model was formulated to account for the behaviour of the H₂ evolving electrode which, unlike the O₂ evolving electrode, did not obey the penetration model. Power consumption calculations have shown that the beneficial effect of mass transfer enhancement is outweighed by the increase in the voltage drop due to gas evolution in the bed electrode.     

The influence of electrode porosity and temperature on electrochemical gas evolution at platinum and rhodium

The influence of electrode porosity and temperature on the rate of electrochemical gas evolving processes (H₂, Cl₂, O₂) was investigated. The experiments were carried out at electrodes with small pores (3 nm) and at smooth electrodes. To understand the results the hydrogen evolution process was used for detailed investigations. It was shown that the pores are only effective if the gas evolving process is an irreversible one (as with oxygen). The pores do not operate in the case of hydrogen and chlorine evolution. An explanation of this different behaviour is given. The temperature dependencies of the overvoltages of the chlorine and hydrogen processes are in contrast. An increase of hydrogen overvoltage with rising temperature is not yet fully understood. It can be stated, however, that the effectiveness of the hydrogen transport from the electrode surface into the bulk of solution decreases with increasing temperature.     

Mass transfer at a rotating ring—cone electrode and its use to determine supersaturation of gas evolved

The rotating ring-cone electrode (rrce) is a useful electrode assembly to study electrochemical reactions, in particular, when gas bubble formation occurs. The aim of this study is to characterize the rate of mass transfer from the bulk to the cone of a rrce, to determine its collection efficiency, N, in the presence or absence of gas bubble formation, and to investigate its usefulness for determination of the oxygen supersaturation at an oxygen-evolving electrode. The mass transfer coefficient for the cone of a rrce increases linearly with increasing rate of oxygen evolution, while N declines sharply with increasing rate of oxygen evolution on the cone. In the absence of gas bubble formation N is determined by the rrce geometry factors except the cone angle. The rrce can be used successfully to determine the supersaturation concentration of oxygen on an oxygen-evolving electrode. The absorption of dissolved oxygen by bubbles during the transport of supersaturated solution from the cone to the ring will be, however, a rather complicating factor.     

Electrochemical synthesis of Cr(II) at carbon electrodes in acidic aqueous solutions

The electrochemical synthesis of Cr(II) has been investigated on a vitreous carbon rotating disc electrode and a graphite felt electrode using cyclic voltammetry, impedance spectroscopy and chronoamperometry. The results show that in 0.1 M Cr(III) + 0.5 M sulphuric acid and in 0.1 M Cr(III) + 1 M hydrochloric acid over an electrode potential range of –0.8 to 0.8 V vs SCE, the electrochemical reaction at carbon electrodes is essentially a surface process of proton adsorption and desorption, without significant hydrogen evolution and chromium(II) formation. At electrode potentials more negative than –0.8 V vs SCE, both hydrogen evolution and chromium(II) formation occurred simultaneously. At electrode potentials –0.8 to –1.2 V vs SCE, the electrochemical reduction of Cr(III) on carbon electrodes is controlled mainly by charge transfer rather than mass transport. Measurements on vitreous carbon and graphite felt electrodes in 1 M HCl, with and without 0.1 M CrCl₃, allowed the exchange current density and Tafel slope for hydrogen evolution, and for the reduction of Cr(III) to Cr(II), to be determined. The chromium(III) reduction on vitreous carbon and graphite electrodes can be predicted by the extended high field approximation of the Butler–Volmer equation, with a term reflecting the conversion rate of Cr(III) to Cr(II).     

Distribution of mass transfer over a 0.5-m-tall hydrogen-evolving electrode

In general, technical vertical electrolysers for the production of chlorine, hydrogen and oxygen are high and have a short cathode-anode distance. Up to the present, few results on mass transfer to gas-evolving electrodes in these industrial cells have been published. The mass transfer experiments were carried out in a divided cell for a vertical hydrogen-evolving platinum electrode in a solution containing 1 M KOH, 0.1 M KCN and 0.008 M AgCN, where the Ag(CN) ₂ ^– complex ion was used as the indicator ion. The platinum electrode was divided into 20 segments, each with a height of 24 mm and a width of 20 mm. Subsequent segments were separated by a 1-mm-thick Perspex layer. The height of the platinum electrode was 0.50 m. It has been found that the Ag/Ag(CN) ₂ ^– redox couple in an alkaline cyanide solution is very useful in determining mass transfer to a hydrogen-evolving electrode. When no hydrogen bubbles are formed at the working electrode, the mass transfer coefficient decreases at a decreasing rate as the distance from the leading edge of the working electrode increases. For a hydrogen-evolving electrode, however, it has been found that the mass transfer coefficient to the topmost 0.40 m of the working electrode is practically constant. For the entrance part of the working electrode, about 0.10 m in length, the dependence of the mass transfer coefficient on the distance to the leading edge of the electrode is complicated. The mass transfer coefficient and the mass transfer enhancement factor for the topmost 0.40 m of the hydrogen-evolving working electrode are given by complex correlations as a function of the current density required for hydrogen evolution and of the flow rate of solution. The effect of viscosity increase of the solution, caused by addition of a polymer, has been investigated. It has been found that the mass transfer coefficient decreases with increasing viscosity of solution in both cases, namely with and without gas bubble evolution, and that the mass transfer enhancement factor is independent of the viscosity of solution.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.     

Bubble behaviour during oxygen and hydrogen evolution at transparent electrodes in KOH solution

For oxygen and hydrogen evolving transparent nickel electrodes in KOH solutions, parameters characterizing the behaviour of bubbles which are adhered to the electrode surface during gas evolution, have been determined in dependence on current density, i, velocity of solution flow, v, pressure, p, temperature, T, and concentration of KOH. Based on experimental data a new basic bubble parameter, J, has been introduced, which accounts for the bubble behaviour. It has been found that J = a₁i^h₁ and J/(J₀?J) = a₂v^h₂ where J₀ = J at v = 0 ms^?1 and a₁,a₂h₁ and h₂ are empirical constants; some of these depend on nature of gas evolved. Moreover, the parameter J is almost proportional to the KOH concentration, increases in a decreasing rate with increasing pressure and increases linearly with the reciprocal of the absolute temperature.     

Effect of gas sparging on mass transfer in zinc electrolytes

The effect of sparging on mass transfer is reported for zinc electrolytes containing antimony and antimony-free electrolytes. Comparative results with non-sparged electrolytes show, an enhancement in mass transfer. In the sparged electrolyte, the mass transfer coefficients,K _Zn, increase with increasing current density, antimony additions, and sulphuric acid concentration. The deposition morphology is consistent with the mass transfer results. A relationship between the mass transfer coefficients for sparged and non-sparged systems is obtained. The relationship correlates satisfactorily with the data and provides a quantitative method for determining the degree of enhancement in mass transfer coefficients due to sparging. The correlation which best represents the mass transfer data for sparged zinc electrolytes is $$Sh = 105(ReSc)^{0.23} $$ whereSh, Re, andSc are the Sherwood, Reynolds, and Schmidt numbers, respectively. The correlation represents the case where sparging is applied to a gas evolving electrode, hydrogen in this case.     

Controlled electrochemical gas bubble release from electrodes entirely and partially covered with hydrophobic materials

This paper deals with an experimental study on millimetre-size electrochemically evolved hydrogen bubbles. A method to generate gas bubbles controlled in number, size at detachment and place on a flat electrode is reported. Partially wetted composite islands are implemented on a polished metal substrate. As long as the island size is lower than a limit depending on its wettability, only one bubble spreads on the island and its size at detachment is controlled by the island perimeter. The composite, a metal–polytetrafluoroethylene (Ni–PTFE), is obtained by an electrochemical co-deposition process. On the contrary to predictions of available models for co-deposition, at current densities beyond Ni²⁺ limiting current density, the mass ratio of PTFE in the deposit strongly increases. A mechanism is proposed to describe co-deposition when hydrogen bubbles are co-evolved. The observation of gas evolution on fully hydrophobic electrodes highlights the fact that bubbles growth rate on such electrodes differs from growth rates when bubble growth is controlled by mass transport of dissolved gas. The more a bubble grows by coalescence the more its foot expands on the electrode the bigger its size at detachment. This triple line creeping mechanism explains why, when attached bubbles coalesce many times before detaching, their size at detachment increases with current density.     

Properties and applications of electrodes covered with a porous diaphragm— 1. Mass transfer investigation

The use of porous coverings on counter electrodes is proposed for controlling the mass transfer rates to the electrode and therefore the extent of possible loss reactions. An experimental study was undertaken to determine the mass transfer rates to electrodes with and without coverings of porous materials (porous PVC, porous-polyethylene, non-woven polypropylene). Additionally the effect of simultaneous gas evolution was investigated. The electrochemical method, based on the measurement of the limiting current for ferricyanide reduction, was employed for most experiments with the exception of those in which gassing at the covered electrode occurred. In the latter case the mass transfer rates were calculated from the concentration change observed during the batch electrolysis with recirculation of Ce(IV) (reduction to Ce(III)).The results have been correlated for the case of the non-woven material type E 1583 with no simultaneous gassing according to

where Sh, Sc and Re are the Sherwood and Schmidt and Reynolds numbers, respectively and l, d_p and ? are the covering thickness, pore diameter and porosity, respectively.Reductions by a factor of 100 in mass transfer rate at the mass transfer surface are easily attainable through the use of porous coverings. Interestingly, the results obtained at covered gassing electrodes, showed that the mass transfer rate decreases with increasing gassing rate. This is the inverse behaviour to that obtained at uncovered gassing electrodes.The implications on the current efficiency for operation of a differential diaphragm-less electrolysis cell in which loss reactions can occur at the counter electrode are discussed. The use of porous electrode coverings on the counter electrode in diaphragm-less cells is a technique which opens up new applications for these simpler cells.     

Reduction of metal ions in dilute solutions using a GBC-reactor Part II: Theoretical model for the hydrogen oxidation in a gas diffusion electrode at relatively low current densities

The GBC-reactor is based on the combination of a gas diffusion anode and a porous cathode. A theoretical model for gas diffusion electrode, valid at relatively low current densities, is derived. This is based on the pseudohomogeneous film model including an approximation of the Volmer–Tafel mechanism for the hydrogen oxidation kinetics. Results show a severe mass transfer limitation of the hydrogen oxidation reaction inside the active layer of the gas diffusion electrode, even at low current densities. Empirical formulae are given to estimate whether leakage of dissolved hydrogen gas into the bulk electrolyte occurs at specific process conditions. A simplified version of the model, the reactive plane approximation, is presented.     

Investigation of the hydrogen evolution reaction at a 10 wt% palladium-dispersed carbon electrode using electrochemical impedance spectroscopy

The hydrogen evolution reaction (h.e.r.) at a 10 wt % palladium-dispersed carbon (Pd/C) electrode in 0.1 m NaOH solution has been investigated with reference to that on carbon (Vulcan XC-72) and palladium foil electrodes by analysing the a.c.-impedance spectra combined with cyclic voltammograms. From the coincidence of the maximum charge transfer resistances and the minimum hydrogen evolution resistances for the h.e.r. at the respective electrode potential for the Pd/C, carbon and Pd foil electrodes, it is suggested that the h.e.r. at the Pd/C electrode takes place along with the absorption and diffusion of hydrogen above –1.10 V vs SCE, whereas the former dominates over the latter below –1.10V vs SCE. In the case of the Pd foil electrode the transition of absorption and diffusion to evolution occurs at –0.96V vs SCE. In contrast to the Pd/C and Pd foil electrodes the h.e.r. occurs strongly at the carbon electrode below –1.20V vs SCE. The hydrogen evolution overpotential on the Pd/C electrode is decreased by 0.10 V in comparison to the carbon electrode due to the larger electrochemical active area of the finely dispersed Pd particles.     

Ohmic resistance of solution in a vertical gas-evolving cell

Vertical electrolysers with a narrow cell gap between a gas-evolving electrode and a membrane or diaphragm are used to produce industrial gases. Generally, the local current density decreases with height in the cell. Electrolyses are carried out with a KOH solution in a tall vertical divided rectangular cell with two gas-evolving electrodes. Either the hydrogen or the oxygen bubbles containing solution from the divided cell are passed through a small measuring cell. Ohmic resistance experiments are carried out in the small measuring cell with a gas-evolving electrode and a gas diffusion electrode, on which no gas bubbles are evolved. The effect of various parameters, viz. current density, solution flow rate and temperature, on the ohmic resistance of solution in the measuring cell are determined. It is found that the normalized ohmic resistance of the solution in the measuring cell during electrolysis increases with current density and with the gas voidage in the bulk of solution, decreases with increasing solution flow rate and is practically independent of temperature at 25 to 60 OC. Moreover, it is found that for an oxygen evolving electrode in a solution containing only oxygen bubbles, as well as for a hydrogen evolving electrode in a solution containing only hydrogen bubbles, the normalized resistance of the solution between the gas-evolving electrode and the nongas evolving electrode is given by a relatively simple empirical relation. A relation is derived describing the gas voidage in the solution as a function of the distance from the gas-evolving electrode in the presence and the absence of gas bubbles in the bulk solution.     

Electrocatalysis and the oxygen evolution reaction

The oxygen evolution reaction has been investigated on a number of electrodes which are electrocatalysts for the chlorine evolution reaction, by making measurements in NaClO₄ solution. Steady state current—potential and impedance—potential measurements, obtained using automated equipment, have been used as the preferred experimental method. Analysis of the impedance has been undertaken by means of curve fitting, and the resulting parameter curves of the double-layer capacity, the charge transfer resistance, and the electrolyte ohmic resistance displayed as a function of potential. Some speculation is made about the interpretation of the parameter curves. The ohmic loss parameter, R_w, is used to correct the potentials to the true values. In comparison to some other gas evolution reactions, such as hydrogen and chlorine, R_w does not depend strongly on potential. The corrected log i—E curve is also considered. It is suggested that a more complete picture of the electrical performance of these electrodes can be obtained by using these electrochemical methods.     

Transfer of mass or heat to an electrode in the region of hydrogen evolution—II.: Experimental verification of mass and heat transfer equations

Radioactive mercury, ²⁰³Hg, was deposited both on smooth Pt and amalgamated Pt electrodes in 1 N KOH, in the region of hydrogen bubble evolution. The relation found between the cd for deposition of mercury, i_R, and for evolution of hydrogen, i_G, is in accord with the equation derived previously[1] for systems with a surface incompletely covered with hydrogen bubbles. The heat transfer coefficient on a platinum electrode located on the wall of a rectangular channel was measured under defined hydrodynamic conditions during hydrogen evolution in 0·5 N KOH. The measured dependence of the heat flow and cd of hydrogen evolution is in accord with the relationship obtained earlier[1] for systems with fully covered surface by the evolved gas bubbles.