Overpotential, electrolytic processes

V) is actually required to electrolyze water when platinum electrodes are used. Much contemporary research on electrochemical cells involves attempts to reduce the overpotential and hence to increase the efficiency of electrolytic processes. 

In order to obtain a definite breakthrough of current across an electrode, a potential in excess of its equilibrium potential must be applied any such excess potential is called an overpotential. If it concerns an ideal polarizable electrode, i.e., an electrode whose surface acts as an ideal catalyst in the electrolytic process, then the overpotential can be considered merely as a diffusion overpotential (nD) and yields (cf., Section 3.1) a real diffusion current. Often, however, the electrode surface is not ideal, which means that the purely chemical reaction concerned has a free enthalpy barrier especially at low current density, where the ion diffusion control of the electrolytic conversion becomes less pronounced, the thermal activation energy (AG°) plays an appreciable role, so that, once the activated complex is reached at the maximum of the enthalpy barrier, only a fraction a (the transfer coefficient) of the electrical energy difference nF(E ml - E ) = nFtjt is used for conversion. 

The said ratio is enhanced, too, by using higher temperatures, for instance, near the boiling point. The reason is that chemical reactions usually are much more accelerated by temperature compared with electrochemical reactions (the activation energy of an electrochemical reaction is lowered by the overpotential applied, thus lowering the temperature dependence of the rate constant [24, p. 166]). In the case of a flow system, that is, different locations for the electrolytic process and the chemical sequence, the first location is cooled by cooling the inlet brine, while the higher temperature needed for a quicker chemical reaction is produced by the ohmic losses within the cells and the heat of the reactions. 

The usual operating voltage of a fluorine cell is 8.5-10.5 V at 1-2 kAm-2. The high voltage is due to - in comparison to other electrolytic processes - relatively low electrolyte conductivity, long current paths, and especially due to the high anodic overpotential mentioned. The specific energy consumption so amounts to about 14000 kWh/t F2. This is about four times the theoretical value. 

Powders of both academically and technologically important metals, such as silver, lead, cadmium, cobalt, nickel, and iron, were produced by the electrolytic processes. The constant regimes of electrolysis and the regime of pulsating overpotential (PO) were used for the electrochemical synthesis of powders of these metals. Morphologies of such obtained powders were characterized by the scanning electron microscopy and optical microscopy, as well as by X-ray diffraction techniques. 

During the electrolytic process, current flows at a fine rate, but overpotential drop emerges due to connectors and wiring system, and electrolyte electrical resistance at the cathode-electrolyte interface. Also, overpotential may occur when concentration polarization is due to mass transfer by diffusion [1]. 

Electrochemical technology, with the exception of corrosion, certain metal finishing processes, batteries and other power sources, is coDcerned with carrying out reactions where the free energy is positive (],e. .- is negative) and, indeed, an advantage of electrolytic methods is that they may be used to drive very unfavourable reactions, e.g. the electrolysis of molten NaCl to sodium metal and chlorine. In electrolytic processes the overpotentials and iR terms... 

In any electrolytic process a part of the energy supplied is used in overcoming the internal resistance of the cell. If this is Rfl and the current passing is I A, the corresponding voltage drop through the solution is AE = IR V. In addition, there will be a concentration overpotential (q.v.) at each electrode. 

Concentration overpotential is one of the causes of overvoltage. It is present in all electrolytic processes, but can be studied without the complication of other effects by considering a cell in which two identical copper electrodes dip into a solution of a copper salt. In this symmetrical arrangement identical potentials are set up at the two electrodes, and if these are connected no current will flow the system is in equilibrium. If now the electrodes are connected to an external source of e.m.f., however small, copper ions will be discharged at the cathode, and copper ions will pass into solution at the anode. The net chemical effect is nil, and for very small currents the energy supplied by the external source has only to overcome the resistance of the solution to the movement of the ions carrying current through the electrolyte. 

In any electrolytic process ions are used up or are generated at the electrodes, and concentration changes occur at the electrode surfaces, leading to concentration overpotential (q.v.). The concentration change near the electrode sets up a concentration gradient, and diffusion of electrolyte into the depleted surface layer (or from a more concentrated surface layer) supplements ionic migration in providing conditions in which electrolysis can continue (cf. transport number),... 

Diffusion overpotential. When high current densities j exist at electrodes (at the boundary to the electrolyte), an impoverishment of the reacting substances is possible. In this case the reaction kinetics are determined only by diffusion processes through this zone, the so-called Nernst layer. Without dealing with the derivation in detail, the following formula is obtained for the diffusion overpotential that occurs (with as the maximum current density) ... 

Reaction overpotential. Both overpotentials mentioned above are normally of higher importance than the reaction overpotential. It may happen sometimes, however, that other phenomena, which occur in the electrolyte or during electrode processes, such as adsorption and desorption, are the speed-limiting factors. Crystallization overpotential. This exists as a result of the inhibited intercalation of metal ions into their lattice. This process is of fundamental importance when secondary batteries are charged, especially during metal deposition on the negative side. 

Figure 18 shows the dependence of the activation barrier for film nucleation on the electrode potential. The activation barrier, which at the equilibrium film-formation potential E, depends only on the surface tension and electric field, is seen to decrease with increasing anodic potential, and an overpotential of a few tenths of a volt is required for the activation energy to decrease to the order of kBT. However, for some metals such as iron,30,31 in the passivation process metal dissolution takes place simultaneously with film formation, and kinetic factors such as the rate of metal dissolution and the accumulation of ions in the diffusion layer of the electrolyte on the metal surface have to be taken into account, requiring a more refined treatment. 

However, under working conditions, with a current density j, the cell voltage E(j) decreases greatly as the result of three limiting factors the charge transfer overpotentials r]a,act and Pc,act at the two electrodes due to slow kinetics of the electrochemical processes (p, is defined as the difference between the working electrode potential ( j), and the equilibrium potential eq,i). the ohmic drop Rf. j, with the ohmic resistance of the electrolyte and interface, and the mass transfer limitations for reactants and products. The cell voltage can thus be expressed as... 

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