Stirring of the electrolyte

None of the set-ups discussed so far provides stirring of the electrolyte for bubble removal or for enhancement of the reaction rates. A standard set-up developed to study kinetic electrode processes is the rotating disc electrode [11]. The electrode is a small flat disc set in a vertical axle. The hydrodynamic flow pattern at the disc depends on rotation speed and can be calculated. An additional ring electrode set at a different potential provides information about reaction products such as, for example, hydrogen. However, because this set-up is designed to study kinetic processes and is usually equipped with a platinum disc, it becomes inconvenient if silicon samples of different geometries have to be mounted. 

One way to control the reactivity of the intermediates appearing is the electrolysis performance at high concentrations of substrate up to 90% of the electrolyte composition. Additionally, a good mass transport from the anode into bulk is important to avoid electrochemical incineration. For such preparative work a conceptually novel architecture is required. The two major issues are an electrolysis cell on the micro scale and avoiding nonemployed volumes by efficient stirring of the electrolyte. A micro electrolysis cell with volumes of 0.7-4 mL has been designed (Fig. 6). All components are made of Teflon or coated by this inert material. A cylindrical... 

The amount of cathodically reduced chlorine depends on the distance between the electrodes and rate of flow of the brine through the electrolyzer (i. e. on the intensity of stirring of the electrolyte). 

In order to accelerate the electrolysis, mass transfer is enhanced by means of stirring the solution, rotating or vibrating the electrode, sonication, etc. With a mercury pool electrode, the magnetic stirrer ensures both the cleaning of the electrode surface and stirring of the electrolyte. 

Electrolytic gas evolution is a significant and complicated phenomenon in most electrochemical processes and devices. In the Hall process for aluminum production, for example, bubbles evolved on the downward-facing carbon anodes stir the bath and resist the current, both of which directly affect the heat balance and the cell voltage. Bubbles appear as a result of primary electrode reactions in chlorine and water electrolysis, and as the result of side reactions in the charging of lead-acid batteries and some metal electrowinning. Stirring of the electrolyte by gas evolution is an important phenomenon in chlorate production. Electrolytically evolved bubbles have also been used in mineral flotation. Relatively few major electrochemical processes do not evolve gas. 

The effect of electrolysis conditions (stirring of the electrolyte and temperature) is illustrated in Fig. 3.10. It can be seen that a stirring of the electrolyte leads to total ohmic control because of the strong increase in the limiting diffusion current density which in turn leads to a decrease in concentration overpotential. 

Fig. 3.10 Current density-cell voltage dependencies of the system (—) Cu I 0.10 M CUSO4, 0.10 M H2SO4 I Cu (+) (a) the effect of stirring of the electrolyte and the effect of temperature of (b) 30 °C and (c) 40 °C. The inter-electrode distance 150 mm. The different distances between the edge of the electrode and the side wall of the cell are indicated in the diagrams (Reprinted liom Ref. [5] with permission from the Serbian Chemical Society)...

An advanced solution to the problem of decreasing the free mobility of the electrolyte in sealed batteries is its gel formation. By adding some 5-8 wt.% of pyrogenic silica to the electrolyte, a gel structure is formed due to the immense surface area (-200-300 m2 g ) of such silicas, which fixes the sulfuric acid solution molecules by van der Waals bonds within a lattice. These gels have thixotropic properties i.e., by mechanical stirring they can be liquefied and used to Filled into the... 

On examination of the electrocatalytic activity of metal nanoparticles in dark electrode processes, of significant interest is the appearance of the limiting current on the i,V curve for the Cu-modified Ti02 electrode, especially taking into account that it does not depend on the stirring of electrolyte and cannot be considered as a consequence of any diffusion limitation caused by the electrolyte solution. At the same time, this limiting current is very sensitive to the heating of the electrolyte as well as to the IR illumination of the electrode. 

General Procedure. The selected electrolyte solution was placed in the jacketed reaction flask which was held at constant temperature by the connected circulating water bath. The tip of a buret containing the cyclic ether was introduced through a stopper into the mouth of the reaction flask. With the electrolyte solution undergoing constant stirring the cyclic ether was delivered slowly from the buret into the reaction flask to the point of turbidity or precipitation, and the delivered volume of cyclic ether was noted. The densities of the cyclic ethers (dioxane and THF) and of the electrolyte solutions used had been determined previously the normalities of the electrolyte solutions were known. With these data, the volume measurement of each of the components of the mixture was... 

A 1.0 molar solution of copper sulfate is electrolyzed with an inert, e.g., platinum, anode and a copper cathode of 55 sq. cm. exposed area the current is maintained constant at 0.040 amp. If the electrolysis vessel contains 1 liter of solution and there is reasonable circulation of the electrolyte, without resort to stirring, estimate approximately how long electrolysis will proceed before hydrogen evolution commences. How much of the original copper will then have been deposited ... 

One of the simplest experiments employed in electrochemistry is chronoamperometry (CA). In CA, the electrode potential is changed abruptly from a potential with no current flow to a potential where the surface concentration of O becomes zero. If only O is initially present in the cell, and homogeneous kinetics, migration and convection can be disregarded (i.e., the supporting electrolyte is in 100-fold excess and there is no stirring of the solution), the experiment can be described by the following equations (Eqs. 34-38). 

---