Cation Depletion Research Articles

The Mechanism of Demineralization at Carbon Electrodes

The results of coulometric and mass‐balance analyses were used to establish the mechanism of demineralization and regeneration at chemically treated carbon paste electrodes. The chemical treatment results in the formation of weak acid groups on many carbon surfaces. The acid groups are ionized in the cathodic cycle by the electrochemical generation of hydroxyl ion. Anionic groups thus produced result in depletion of cations from solution. During regeneration, the hydrogen which was adsorbed on the carbon during the demineralization cycle is oxidized, giving rise to acid conditions. The anionic groups are thus converted back to the acid form, and cations are released to the solution.

Elektrochemisch-morphologische studien zur erforschung des mechanismus der elektrokristallisation, fern vom anfangszustand

1.1. The current density at the transition of pyramids into cubic layers (indicator current density) is increased by using interrupted d.c. with augmented pulse frequency below a critical pulse time (e.g. 4 × 10−3 s). The shorter the pulse time the more incompletely the ad-atoms are incorporated during the pulse time. With the method of interrupted d.c. it is shown, that copper, far from initial state (layer thickness about 5 μ), is deposited by surface diffusion. Also in this case ad-atoms are incorporated more slowly than copper ions cross the double layer.2.2. The edge length of the two-dimensional nucleus can be determined by Frank from the variability of the whorl distance of corresponding growth spirals with current density. From the edge length results the oversaturation for spiral growth. Since oversaturation also equals the proportion of given current density to exchange current density, the exchange current may be determined at least from the whorl distance of the spirals. Values of j0 (e.g. 2·1 × 10−3 A/cm2) found with such morphological means are well confirmed under equal conditions of deposition by electrochemically determined values of j0 (e.g. 1.6 × 10−3 A/cm2).3.3. Cubic copper layers seem to originate by influence of inhibition (e.g. effect of H3O+ or of glycine), for below a critical value of inhibitor concentration only pyramids are deposited on copper with (100) texture within the investigated range of current density. Increasing oversaturation, i.e. current density, increases the angle of inclination of pyramid faces in the electrolyte free from inhibitors. On the other hand the angle diminishes by increased inhibition, i.e. decreased oversaturation. Hence in this case pyramids becoming more and more flat turn to cubic layers. Considering this behaviour one may with Frank interprete pyramids as growth spirals.In the current density range of cubic layers increased inhibitor concentration (e.g. H3O+ or glycine) enlarges the crystals up to a limiting value. Above the limiting concentration (of glycine) the mean diameter of grains is in reverse proportion to the inhibitor concentration. Corresponding to this relation the number of grains increases with the square of the inhibitor concentration.The observed insensitivity of the indicator current density to increased inhibition may be understood as consequence of the following compensation: the diffusion path of the ad-atoms (extended by inhibitor molecules blocking-up) will be shortened due to spontaneous nucleation. But if the diffusion paths are entirely blocked-up in the case of strong inhibition, the metal ions have to be deposited directly on the growth centres, i.e. surface diffusion of ad-atoms will be stopped.4.4. As opposed to the growing of pyramids apparently without nucleation the formation of the cubic layers seems to require nucleation in every case. In the process of layer growing the highest current density is believed to exist at the edge face of the step or in the neighbourhood of it. This hypothesis is supported morphologically by corrosion sites on the step faces, observed with the electron microscope, to which nearby metal accumulations are related. This phenomenon may be interpreted as secondary effects of short-circuited elements, of local concentration formed by switching off (or locally decreasing) the current. During electrolysis the highest local current density (with corresponding highest depletion of metal cations) will be concentrated on the (later) corrosion sites (anodes of the elements).

Welding by friction. (in Polish) : Jerzy Orlowski. Przeglad techniczny, v. 78, no. 17, Sept. 1957, p. 668–669

Full depletion of magnesium cations from lithium rich brines is vital for the successful crystallization of battery grade lithium salts in subsequent production steps. In a three compartment water electrolyser, we demonstrate the full abatement of Mg2+, with concomitant recovery of magnesium hydroxide. We have studied the influence of three operational variables, current density, temperature and forced mass transport in the coulombic efficiency, cell voltage between the electrodes, solid purity and lithium ions loss. Response surface methodology was used to model the system. Our results show that both in terms of energy efficiency, product purity and minimal Li+ loss it is best to work at very low current density, high flow rate, and high temperature (32 A m−2, 60 °C, and 18 L h−1). Working at higher current density, low temperature, and high flow rate (100 A m−2, 26.5 °C, and 18 L h−1) still guarantees a product of acceptable quality while keeping a moderate electrical consumption. The current density is the factor that affects results the most. High current densities are correlated with a faster nucleation, and smaller particle size, resulting in higher adsorption from other ions and co-precipitation of CaB by-products.