Cathode overpotential

The electrolysis voltage between two electrodes is the summation of the equilibrium potential difference, anode overpotential, cathode overpotential, and ohmic potential drop of the aqueous solution as shown below (Scott 1995 Chen et al. 2002c) ... 

Many electro-organic rednction reactions are performed nsing an aqueous reaction medinm (the H atoms in water snpply the necessary hydrogen for the reaction) and a high hydrogen overpotential cathode snch as lead or cadmium to minimize nnwanted H2 gas evolution (from the electro-rednction... 

When commercial nickel is severely cold-worked, unlike mild steel, it does not corrode appreciably more rapidly in acids [2], which suggests that similar formation of low-overpotential cathodic sites derived from segregated, interstitial impurities does not occur. 

E° = cell reversible voltage Tiact,a = anode activation overpotential = cathode activation overpotential riohm,a = anode ohmic over potential Tiohm,c = cathode ohmic overpotential nohm,e = electrolyte ohmic overpotential... 

Anode ohmic loss or overpotential Cathode ohmic loss or overpotential Electrolyte ohmic loss or overpotential... 

If the initial concentration of Cu + is 1.00 X 10 M, for example, then the cathode s potential must be more negative than -1-0.105 V versus the SHE (-0.139 V versus the SCE) to achieve a quantitative reduction of Cu + to Cu. Note that at this potential H3O+ is not reduced to H2, maintaining a 100% current efficiency. Many of the published procedures for the controlled-potential coulometric analysis of Cu + call for potentials that are more negative than that shown for the reduction of H3O+ in Figure 11.21. Such potentials can be used, however, because the slow kinetics for reducing H3O+ results in a significant overpotential that shifts the potential of the H3O+/H2 redox couple to more negative potentials. 

Overvoltages for various types of chlor—alkali cells are given in Table 8. A typical example of the overvoltage effect is in the operation of a mercury cell where Hg is used as the cathode material. The overpotential of the H2 evolution reaction on Hg is high hence it is possible to form sodium amalgam without H2 generation, thereby eliminating the need for a separator in the cell. 

Fuel cell stack voltage varies with external load. During low current operation, the cathode s activation overpotential slows the reaction, and this reduces the voltage. At high power, there is a limitation on how quickly the various fluids can enter and... 

As is well known, high-purity zinc corrodes much less rapidly in dilute acids than commercial purity material in the latter instance, impurities (particularly copper and iron) are exposed on the surface of the zinc to give local cathodes with low hydrogen overpotentials this result is of practical significance only in the use of zinc for sacrificial anodes in cathodic protection or for anodes in dry cells. In neutral environments, where the cathodic... 

Thus, irrespective of r.ceii. a thermodynamic parameter, the rate will be controlled by the irreversibility of the reaction, which is reflected in the magnitudes of the anode and cathode overpotentials. 

Fig, 1.24 Tafel lines for a single exchange process. The following should be noted (a) linear f-log I curves are obtained only at overpotentials greater than 0-052 V (at less than 0-052 V E vs. i is linear) b) the extrapolated anodic and cathodic -log / curves intersect at tg the equilibrium exchange current density and (c) /, and the anodic and cathodic current densities... 

The relation between transport overpotential and current density for a cathodic reaction is given by... 

Fig. 1.31 Shape of cathodic polarisation curve when transport overpotential is rate controlling, (a) Effect of velocity on ( l and corrosion rate, (b) effect of concentration on tY and corrosion rate and (c) effect of position and slope of anodic curve (after Stern...

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