Resistance overvoltage

Rapid reinsertion of the capacitors as soon as the fault coiiditions are removed is an important requirement to retain the stability of the system. This can be achieved with the use of an additional ZnO. non-linear resistance (ZnO being the latest in this field compared to SiC. which was used earlier), across the capacitor banks (Figure 26.12). Generally, the ZnO resistor will be adequate to dampen the fault current without initiating the spark gap. and w ill limit the overvoltage across the capacitors. It will also permit automatic reinsertion of the capacitors as soon as the fault conditions are removed without causing a delay. The spark gap will serve as a backup to the ZnO resistor in the event of very severe faults. 

With very high overvoltages, Eq. (24-82) is no longer applicable because of resistance polarization. It is assumed approximately that ... 

Tafel slope (Napieran loop) transfer coefficient diffusion layer thickness dielectric constant, relative electric field constant = 8.85 x 10 F cm overvoltage, polarization ohmic voltage drop, resistance polarization specific conductance, conductivity electrochemical potential of material X,... 

It has been suggested that the role of nickel (as NiAlj) is to provide sites of low hydrogen overvoltage, where cathodically liberated hydrogen may be liberated without disrupting the protective oxide . The distribution of such sites is apparently critical however, since high corrosion resistance is associated with a fine dispersion of the second phase, while the electronic conductivity of the film is probably also important . 

Rhodium and iridium have a resistance to anodic corrosion comparable with that of platinum, and are more resistant to the influence of alternating currents. A platinum-iridium alloy, in the form of a coating on titanium, is preferred to pure platinum for the production of chlorine from brine , due to its improved corrosion resistance and lower overvoltage. 

Major barriers are finding efficient light-absorbing materials and catalysts with corrosion-resistance properties in water and energy levels that match the reduction and oxidation half-reactions to reduce overvoltage losses. Major issues... 

Overvoltage Due to Electrolyte and Cell Separator Resistance The different overvoltages, discussed above, influence the correlation between electrode... 

Cathode Iron is a very cheap cathode material with a relatively low hydrogen overvoltage (e.g. [25]). It is of interest for industrial applications. In order to avoid corrosion during interruption of the current, stainless steel may be suitable, especially in laboratory cells where the increased electrical resistance and hydrogen overvoltage are irrelevant. 

As the resistance of mass transport increases, a diffusion overvoltage becomes significant and the total overvoltage t) is distributed to both interfadal ovmvoltage i1h and difiusion overvoltage T)di r as stressed in Eqn. 8-31 ... 

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). 

However, under working conditions, with a current densityj, the cell voltage E(j) becomes smaller than the equilibrium cell voltage eq, as the result of three limiting factors (i) the overvoltages Tia and T],- at both electrodes due to a rather low reaction rate of the electrochemical reactions involved (T] is deflned as the difference between the working electrode potential and the equilibrium potential , so that i = ( + T]), (ii) the ohmic drop J both in the electrolyte and interface resistances e and (iii) mass transfer limitations for reactants and products (Figure 1.2). ... 

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