Gas-evolving Electrodes Research Articles

Mass transfer at gas evolving electrodes

A new convection—penetration model for mass transfer of indicator ions to a gas-evolving electrode with forced convection of solution is described theoretically. It has been found that the mass transfer of indicator ions to a gas-evolving electrode can be described well by the new model. The contribution of an oxygen bubble departing from the electrode surface to the mass transfer is much greater than the contribution of a departing hydrogen bubble.

Effect of operational parameters on gas evolution in electrolyte bulk: possibilities for lowering interelectrode resistance

Two facts form the basis of the present investigation. Firstly: generally, only a fraction of the total quantity of dissolved gas generated at a gas-evolving electrode is evolved in gaseous form directly at the electrode. Secondly: commonly, only a fraction of the complementary amount of dissolved gas reaching the bulk of electrolyte is evolved in the interelectrode gap. This paper analyses the conditions in which a major portion of the total amount of dissolved gas generated at the electrode remains in dissolved form when being withdrawn from the interelectrode gap. Increasing this amount results in lowering the effective resistivity of the gas—electrolyte dispersion.

Mechanism of mass transfer of indicator ions to an oxygen-evolving and a hydrogen-evolving electrode in alkaline solution

A new convection-penetration model for mass transfer of indicator ions to a gas-evolving electrode at forced convection of solution is described theoretically. To check this model, the mass transfer coefficients for ferricyanide ions and ferrocyanide ions to, respectively, a hydrogen-evolving and an oxygen-evolving electrode in 1 M KOH and at 298K and various bulk-solution flow velocities have been determined in the usual way. The average diameter, cross-section and volume of detached bubbles and the efficiency of gas bubble evolution, have been determined photographically, however. It has been found that the mass transfer of indicator ions to a gas-evolving electrode can be described well by the new model. The contribution of an oxygen bubble departing from the electrode surface to the mass transfer is much greater than the contribution of a departing hydrogen bubble.

Studies on gas-evolving electrodes: The concentration of dissolved gas in electrolyte bulk

The supersaturation of the electrolyte with dissolved gas forms the key condition of gas evolution at electrodes. Previous studies on the gas concentration in the boundary layer at the electrode are supplemented by an analysis of the gas concentration in the bulk of the electrolyte. It is found that the bulk is supersaturated, but the supersaturation is by some orders of magnitude smaller than at the electrode—electrolyte interface.

Effect of PTFE Coverage on the Performance of Gas Evolving Electrodes

The effect of nickel electrode partial coverage with polytetrafluoroethylene (PTFE) on the electrode performance for the hydrogen evolution reaction (HER) was investigated. We show that for low current densities the only effect of the electrode coverage is to decrease the effective area. For medium current densities (>10 mA/cm2), coverage with PTFE has an overall beneficial effect in the electrode performance even with the resulting decrease in electrode surface area. Finally, for high current densities, in a regime of violent bubble release (1 A/cm2), coverage effect is decreased but still significant.

The rate of gas evolution of electrodes—I. An estimate of the efficiency of gas evolution from the supersaturation of electrolyte adjacent to a gas-evolving electrode

The rate of gas evolution immediately at the electrode has a great effect on heat and mass transfer (of any substance) at gas-evolving electrodes. The question of how much of the gas generated in dissolved form is transformed into the gaseous phase of bubbles adhering to the electrode is studied on the basis of calculations of mass transfer of dissolved gas. Contrary to the established view of the matter it is found that only a fraction of the dissolved gas is transformed into bubbles at the electrode. This fraction, expressed as the efficiency of gas evolution, increases as the current density increases but is far smaller than unity in usual industrial current density values and remains different from unity in the whole range of nucleate gas evolution.

The rate of gas evolution at electrodes—II. An estimate of the efficiency of gas evolution on the basis of bubble growth data

In Part I [ Electrochim. Acta 29, 167 (1984)] it was found from concentration data that at gas-evolving electrodes only a fraction of the dissolved gas formed is transformed into bubbles adhering to the electrode. The finding being contrary to the established view is now checked and confirmed by use of an independent method, namely by an analysis of available bubble growth data. Furthermore, the existence of two concentration differences controlling mass transfer of dissolved gas to the bulk of electrolyte and to adhering bubbles, respectively, is confirmed. Evidence is given of their interdependence.

Bubble behaviour on and mass transfer to an oxygen-evolving transparent nickel electrode in alkaline solution

A theoretical description is given of a new model for mass transfer at gas-evolving electrodes where bubbles coalesce frequently. To check this model, both the thickness of Nernst diffusion layer and the bubble behaviour were determined for an oxygen-evolving transparent nickel electrode in KOH solutions under various electrolytic conditions. This electrode behaves like a gas-evolving electrode where bubbles coalesce frequently. It has been found that the mass transfer can be well described with the new model. Moreover, interesting results have been obtained on the degree of screening of electrode surface by adhered bubbles and the gas void fraction in a solution layer at the gas-evolving electrode.

Distribution of reaction in a cell with gas-evolving electrodes. I. Magnesium/silver chloride seawater battery

The effects of gas evolution on the distribution of reaction at high current densities have been studied experimentally in a model Mg-AgCl battery cell. Measurements of current, potential, pH and temperature were made at four vertical locations during discharge at applied pressures between 1–6 bar(g). Temperature and pressure were found to influence local current density with consequences for overall battery lifetime.

Frictional pressure drop of pure and contaminated bubble-electrolyte dispersions in electrochemical reactors

An earlier equation for predicting the pressure drop in interelectrode gaps of gas-evolving electrodes and the connecting pipes is extended to contaminated electrolytes. Simplified design equations are given.

A Modified Constriction Model for the Resistivity of a Bubble Curtain on a Gas Evolving Electrode

At gas evolving electrodes, the bubble curtain at the electrode surface contributes to the voltaic inefficiencies of the electrolyzer. Consequently, the structure of the bubble curtain and its effect on electrolyte resistivity is of practical importance. In order to predict the resistance of the bubble curtain several models have been suggested in the literature. Three‐dimensional models provide an adequate prediction of the resistance of dilute bubble curtains with monolayer gas voidages less than 0.55. This paper proposes a two‐dimensional constriction model of a dense bubble curtain which is modified to account for a three‐dimensional dispersed phase within the monolayer. The resulting mathematical expression behaves properly in the limits of voidage and its prediction agrees well with the limited available data.

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, Δ R*, has been determined by the alternating current—impedance method. It has been found that, for hydrogen-evolving electrodes, log Δ R* = 1 1 + 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 a 1, however, being dependent on the position, height and material of electrode, temperature and KOH concentration. Δ R* 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.

The rate of hydrogen generation in the electrodeposition of metal powder at gas-evolving electrodes

A general equation is presented to predict the current efficiency of hydrogen generation that occurs simultaneously in the electrodeposition of metals at an elevated current density. The equation, which is based on a theoretical mass transfer equation, is compared with numerous experimental data from various workers.

Over-potential measurement at gas-evolving electrodes

Current—potential measurements at a gas-evolving anode and cathode in sulphuric acid, are reported at ambient temperatures in the current density range 3–250 mA.cm −2. In the steady state measurements, the effect of Luggin capillary diameter and its distance from the working electrode surface are given. Except at the highest current densities the offset distance is not too critical, and shielding due to the Luggin seems unimportant, which is ascribed to the fact that shielding effects are already operative due to presence of bubbles. In non-steady-state measurements, the ohmic drop component of the total voltage is isolated. Very considerable differences exist between hydrogen and oxygen under otherwise identical conditions. The difference between “make” and “break” transients is not large and often the reverse of what has been predicted. It is suggested that there is a significant resistive element due to a thin layer of bubbles too small to be observed with the naked eye. There is no evidence of any significant local temperatures rise at the metal solution interface.

The role of mass transfer in the kinetics of the electrodeposition of metal powder at gas-evolving electrodes

Mass transfer rates were measured for the electrodeposition of copper powder from acidified CuSO 4 at currents above the limiting value when simultaneous hydrogen evolution takes place. The variables studied were the current density, the CuSO 4 concentration and the position of the electrode. It was found that hydrogen evolution increases the mass transfer coefficient by a factor ranging from 1.75 to 6.75 depending on the prevailing conditions. The horizontal electrode gave higher mass transfer coefficients than the vertical electrode under the same conditions. The mass transfer coefficient decreased with increasing CuSO 4 concentration. Power consumption calculations showed that the higher the CuSO 4 concentration the lower was the power consumption at a given current density.

Bubble behaviour on and mass transfer to an oxygen-evolving transparent nickel electrode in alkaline solution

A theoretical description is given of a new model for mass transfer at gas-evolving electrodes where bubbles coalesce frequently. To check this model both the thickness of Nernst diffusion layer and the bubble behaviour were determined for an oxygen-evolving transparent nickel electrode in KOH solutions under various electrolytic conditions. This electrode behaves like a gas-evolving electrode where bubbles coalesce frequently. It has been found that the mass transfer can be well described with the new model. Moreover, interesting results have been obtained on the degree of screening of electrode surface by adhered bubbles and the gas void fraction in a solution layer at the gas-evolving electrode.

The role of electrode structure and surface texture in the performance of gas evolving electrodes

The literature on gas-evolving electrodes is reviewed. Cell voltage measurements are reported for an undivided cell in NaOH at 70° C in which the cathode was mild steel plate, mild steel mesh or composites of both these materials. The anode was Ni-plated mild steel sheet or mesh. Variation in the structure of one electrode, leaving the other unchanged allowed changes in overvoltage to be measured. It is shown that substantial voltage reductions can be obtained using multiplex electrode structures and that these are mainly explicable in terms of specific surface areas. Sheet electrodes are seen to be more efficient than mesh ones on this basis however. It is shown that sheet electrodes with very high specific surface areas show significantly higher overvoltages than electrodes of low specific area and this surprising result is interpreted in terms of bubble entrapment.

Mass transfer at horizontal gas-evolving electrodes

The effect of H 2 and O 2 evolution on the mass transfer coefficient of the reduction of K 3Fe(CN) 6 and the oxidation of K 4Fe(CN) 6 at nickel electrodes was studied up to 105 mA/cm 2. 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 H 2-evolving horizontal electrode and log K = a + 0·4 log V for an O 2-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.