Electrolytic cells overvoltage

The various possible electrode reactions at the cathode and at the anode in electrolytic cells have been shown in Table 6.2. It has been pointed before that the outcome of an electrolytic process can be made on the basis of knowledge of electrode potentials and of overvoltages. The selection of the ion discharged depends on the following factors (i) the position of the metal or group in the electrochemical series (ii) the concentration and (iii) the nature of the electrode. Examples provided hereunder deliberate on these aspects. 

In an earlier note (p. 9) we mentioned the occurrence of overvoltage in an electrolytic cell (and overpotentials at single electrodes), which means that often the breakthrough of current requires an Uappl = Eiecomp r] V higher than Ehack calculated by the Nernst equation as this phenomenon is connected with activation energy and/or sluggishness of diffusion we shall treat the subject under the kinetic treatment of the theory of electrolysis (Section 3.2). 

The causes of overvoltage are very complex. Basically, the phenomenon is caused by difficulties in transferring electrons from the species in the solution to the atoms on the electrode across the electrode-solution interface. Because of this situation, %° values must be used cautiously in predicting the actual order of oxidation or reduction of species in an electrolytic cell. 

A selective hydrogenation of diacetyl to acetoin can also be carried out in an electrolytic cell using a cathode with a high hydrogen overvoltage [83], It is recommended to start with a solution consisting of... 

Understand the basis of an electrolytic cell describe the Downs cell for the production of Na, the chlor-alkali process and the importance of overvoltage for the production of CF, the electrorefining of Cu, and the use of cryolite in the production of Al know how water influences the products at the electrodes during electrolysis of aqueous salt solutions ( 21.7) (SP 21.8) (EPs 21.63-21.75,21.82)... 

A very interesting application of the classical current interruption technique has been reported by Lorenz and Eichkom [67], who showed that by adopting the galvanostatic configuration it is possible to evaluate the importance of the ohmic drop realistically, point by point, and obtain polarization curves with a trend very close to the ideal one. In fact, it can be experimentally demonstrated that the value of R, is not constant but is influenced by the mass transfer when the current flowing in the electrolytic cell is sufficiently high. In other words, it cannot be excluded a priori that the quantity R, depends on the electrode overvoltage. 

The degree of polarization of an electrode in an electrolytic cell is measured by the overvoltage, or overpo-te.ntial, 17, which is the difference between the actual electrode potential E and the ihermodynamic. or equilibrium, potential Thai is,... 

When the mass-transport process cannot meet the demand for reactant, the IK drop in Equation 22-16 becomes smaller than the theoretical value, and a diffusion overvoltage appears that just offsets the decrease in H. Here, we consider an electrolytic cell to which we apply a negative voltage to produce a reduction at the cathode. We assume that the anode is nonpolarized, Thus, with the appearance of concentration polarization, Equation 22-16 becomes... 

What characterizes an electrolytic cell What is an ampere When the current applied to an electrolytic ceU is multiplied by the time in seconds, what quantity is determined How is this quantity converted to moles of electrons required How are moles of electrons required converted to moles of metal plated out What does plating mean How do you predict the cathode and the anode half-reactions in an electrolytic cell Why is the electrolysis of molten salts much easier to predict in terms of what occurs at the anode and cathode than the electrolysis of aqueous dissolved salts What is overvoltage ... 

Based on Eq. (5) one can compare two electrocatalytic materials. For example, if particular electrode reaction takes place on two different electrode materials with same current density, more effective electrocatalyst requires lower overvoltage. If overvoltage is the same for both materials, higher current density will be measured at more efifeetive electrocatalyst. Now, let us consider the power (/ ) necessary for electrode reaction to take place in electrolytic cell, or power gained by some power source. In both cases general expression is ... 

---