Concentration overvoltage

The potential required to split water into and O, i.e., (E - E is equal to 1.229 V. Though the theoretical potential is 1.23 V for water electrolysis, in practice the actual water decomposition will occur only above 1.7 V. The extra potential, which is essential for the water decomposition, is called overpotential. Overvoltages are composed of activation or charge transfer overvoltage, concentration or diffusion or mass transfer overvoltage and resistance overvoltage. Overvoltage is evaluated mainly as a function of current and temperature (Viswanathan, 2006). 

The concentration changes at the electrode surface due to mass transport limitations are responsible for the concentration overvoltages. When a reduction process takes place (e.g. Zn + + 2e Zn), a concentration of the oxidized species at the electrode surface (Cox,e) lower than that in the bulk makes the current, at a given potential, lower than that in the absence of an ion diffusion limitation, and to achieve the same current value an overvoltage (concentration overvoltage) must be imposed. This concentration (or diffusion) overvoltage can be calculated from Eq. (16) ... 

Chlorine—hydrogen ha2ards associated with mercury cells result from mercury pump failures heavy-metal impurities, particularly those with very low hydrogen overvoltage, ie. Mo, Cr, W, Ni excessively low pH of feed brine low NaCl concentrations in feed brine and poor decomposer operation, which leads to high sodium amalgam concentrations in the cell. 

Concentration of capacitor banks at one point in a system may cause amplification of harmonics, selfexcitation of machines, overvoltages due to switching and unsatisfactory working of audiofrequency remote-control apparatus. To overcome this, a concentration of capacitor banks at one point must be avoided as far as possible. These may be installed at different locations in the system. 

Cathodic inhibitors are passivating inhibitors they raise the hydrogen overvoltage, thus impeding the reduction of hydrogen ions at the cathode. They are suitable for inhibiting sulfuric or phosphoric acids. Examples are benzotriazole (at low concentrations) and alkylamines. 

Other types of polarization are caused by specific features in the various steps of the electrochemical reaction that produce a potential shift relative to the effective equilibrium potential (i.e., that which already accounts for the prevailing values of surface concentrations). These types of polarization, which may differ in character, are jointfy termed activation polarization. The value of activation polarization is sometimes called the overvoltage (this term should be reserved for the complete ceU see Section 2.5.2). 

In addition to this, one distinguishes that there are two other sources of voltage difference. One has its origin within the electrolyte of the cell, and the other is referred to rate processes taking place at the electrodes. The former is called concentration polarization, while the latter is called 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. 

Over-the-Counter Drug Review, 18 685 Over/under scales, 26 252 Overvoltage, 12 206-207, 214 mass transport (concentration),... 

Concentration overvoltage (reaction overvoltage and diffusion overvoltage) If a... 

Then, a concentration gradient of hj lrated redox particles arises in the interfacial diffusion layer and the Fermi level EnitEix ). of redox particles at the interface becomes different from the Fermi level ensEDox) of redox particles outside the diffusion layer as shown in Fig. 8-9(c). The partial overvoltages t]h and iidiff are then given by Eqn. 8-32 ... 

For p-type electrodes, the cathodic current is carried at low overvoltages by the minority carriers (electrons) in the conduction band and is controlled at high overvoltages by the limiting current of electron diffusion the anodic current is carried by the mtqority carriers (holes) in the valence band and the concentration of interfacial holes increases with increasing anodic overvoltage until the Fermi level is pinned in the valence band at the electrode interface, where the anodic current finally becomes an electron injection current into the electrode. 

When electronic equilibrium is established in the space charge layer, the concentration of interfacial electrons is given by n, = n exp (- e A /k T) and the concentration of interfacial holes is given by Pt = p exp(e A lk T) n and p are the concentrations of electrons and holes, respectively, in the semiconductor interior. In general, the ionization of surface atoms (Eqn. 9-24) is in quasiequilibrium so that the concentration of surface ions depends on the overvoltage... 

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