Phenomenon of overpotential

In principle, the 3.2 eV (309 kJ mol-1) electron donor/electron acceptor pairs in Ti02 should have more than enough energy to decompose water into hydrogen and oxygen (1.23 V), but the evolution of both O2 and H2 on TiC>2 surfaces is hindered by very high overpotentials. The phenomenon of overpotential is considered at length in Section 15.4, but for present pur-... 

It is true that this small window of 1.2 V is extendable in both potential directions, particularly on the positive side because the phenomenon of overpotential (Chapter 7) is especially strong there and the potentM that has to be applied to the positive electrode to get a significant current density may be as high as 1.8 V. Nevertheless, it has been felt for decades that systems were needed in which one could make the potential of electrodes more positive (thus releasing a greater power in oxidation) and also more negative (greater power of reduction) than is currently possible because of the solvent decomposition problem in aqueous solutions. 

Electrodics were not as exciting at the time as ionics—there was certainly no theory of such universality as thermodynamics, nor were the instruments available in the early part of this century capable of measuring potentials, currents and time intervals to the needed degree of accuracy. The few who were interested in the study of electrode processes could only laboriously measure current-potential curves, establish the experimental relationship between them, wonder why they do not fit thermodynamic calculations and speculate on the nature of the mysterious phenomenon of overpotential. 

In some cell types, especially those in which electrolysis generates gas at an electrode, the phenomenon of overvoltage may occur, which means that the voltage to be imposed must be higher than the emf plus an overvoltage the term overpotential must be strictly used for the single electrode. 

Now, any reader of this chapter will realize that, to use a phrase, this strikes a bell. We are drawing close to the birth of theoretical electrocatalysis and Butler s 1936 paper can be seen as a birthing of the explanation that in fact has gone into most parts of electrochemistry and affected particularly those that have financial consequences in industrial processes for a lesser overpotential translates into a fall in price of the product on sale. But it is most interesting to find out that even then, in 1936, there was still no explicit recognition of the connection in the phenomenon of the metal-hydrogen bonding in electrochemistry to catalysis in chemistry. 

The slight mathematical operation, which placed the overpotential in the exponential position and the current density on the line, was not yet normal. It was introduced by Agar, also of Cambridge University, in 1938 [7]. One had to wait 23 years before the connection to the well-known phenomenon of catalysis in chemistry became so obvious that, at last, the right name, electrocatalysis, was used in fuel cell work by Gmbb of the General Electric Company in a 1959 publication [8]. 

Anions of common strong acids, such as C104, S04, CF, NOa , etc. exhibit as a rule only weak complexing interactions, if any. Nevertheless, even weak complexation may be of importance in electrode kinetics if the complex ion undergoes electrode reaction more easily than the free metal ion, as is often the case, especially with chlorides. In such cases, the complex takes the role of an electroactive species, as already discussed for the hydroxo complexes. Thus, e.g., nickel can hardly be anodically dissolved at all if chloride ions are not present in the solution. In sulfate electrolytes, the oxidation product (some oxygen-containing species) forms a passive film and further dissolution is blocked soon after an anodic overpotential is imposed upon the electrode. The phenomenon of passivity is discussed elsewhere (cf. Volume 4). At this point, one should note that passivity is absent in the presence of chlorides. 

Corresponding to the charge in the potential of single electrodes which is related to their different overpotentials, a shift in the overall cell voltage is observed. Moreover, an increasing cell temperature can be noticed. Besides Joule-effect heat losses Wj, caused by voltage drops due to the internal resistance Rt (electrolyte, contact to the electrodes, etc.) of the cell, thermal losses WK (related to overpotentials) are the reason for this phenomenon. 

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). 

If current passes through an electrolytic cell, then the potential of each of the electrodes attains a value different from the equilibrium value that the electrode should have in the same system in the absence of current flow. This phenomenon is termed electrode polarization. When a single electrode reaction occurs at a given current density at the electrode, then the degree of polarization can be defined in terms of the over potential. The overpotential r) is equal to the electrode potential E under the given conditions minus the equilibrium electrode potential corresponding to the considered electrode reaction Ec ... 

The dissolution of zinc in a mineral acid is much faster when the zinc contains an admixture of copper. This is because the surface of the metal contains copper crystallites at which hydrogen evolution occurs with a much lower overpotential than at zinc (see Fig. 5.54C). The mixed potential is shifted to a more positive value, E mix, and the corrosion current increases. In this case the cathodic and anodic processes occur on separate surfaces. This phenomenon is termed corrosion of a chemically heterogeneous surface. In the solution an electric current flows between the cathodic and anodic domains which represent short-circuited electrodes of a galvanic cell. A. de la Rive assumed this to be the only kind of corrosion, calling these systems local cells. 

Polarization has various meanings and interpretations depending on the system under study. For an electrochemical reaction, this is the difference between actual electrode potential and reaction equilibrium potential. Anodic polarization is the shift of anode potential to the positive direction, and cathodic polarization is the shift of cathode potential to the negative direction. In an electrochemical production system driven with an external current source, polarization is a harmful phenomenon. It will increase the cell voltage and therefore production costs. A system that polarizes easily will not pass high currents even at high overpotentials. The reaction rates are therefore small. 

Along with the anode reaction, the so-called anode effect, a phenomenon often observed in fused salt electrolysis (see Chapter 4), may occur. In the present case, it may be due to a surface film of the type CVX formed on the anode material. This film on the one hand protects the carbon against destruction (and is the reason for high anodic overpotentials) in normal operation and, on the other hand, under more or less known conditions may block electron transfer completely. These conditions depend strongly on the electrolyte composition (purity) [46,47]. Additives, such as lithium fluoride, may be helpful in preventing the anode effect by wetting the electrode material. 

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