Overpotential Subject

The distribution of current (local rate of reaction) on an electrode surface is important in many appHcations. When surface overpotentials can also be neglected, the resulting current distribution is called primary. Primary current distributions depend on geometry only and are often highly nonuniform. If electrode kinetics is also considered, Laplace s equation stiU appHes but is subject to different boundary conditions. The resulting current distribution is called a secondary current distribution. Here, for linear kinetics the current distribution is characterized by the Wagner number, Wa, a dimensionless ratio of kinetic to ohmic resistance. 

Fig. 10.12 Schematic representation of a pipewall subject to cathodic protection (see text), t , = overpotential at x 7) = overpotential at x = 0 7)p,oi = overpotential at x = a/2 1, - current line at x = current density entering line at x / = current in line at x = 0 from one side of drain point (2/ = total current drain) a = distance between the drain points...

The present Section, which provides an outline of selected relevant topics in electrochemistry, is intended primarily as an introduction to aqueous corrosion for those readers whose basic training has not involved a study of electrochemistry. The scope of electrochemistry is enormous and cannot be treated adequately here, but there are now a number of excellent books on the subject, and it is hoped that this outline will serve to stimulate further study. The topics selected are as follows a) the nature of the electrified interface between the metal and the solution, (b) adsorption, (c) transfer of charge across the interface under equilibrium and non-equilibrium conditions, d) overpotential and the rate of an electrode reaction and (e) the hydrogen evolution reaction and hydrogen absorption by ferrous alloys. For reasons of space a number of important topics, such as the electrochemistry of electrolyte solutions, have been omitted. 

In order to relax 1 mol of compacted polymeric segments, the material has to be subjected to an anodic potential (E) higher than the oxidation potential (E0) of the conducting polymer (the starting oxidation potential of the nonstoichiometric compound in the absence of any conformational control). Since the relaxation-nucleation processes (Fig. 37) are faster the higher the anodic limit of a potential step from the same cathodic potential limit, we assume that the energy involved in this relaxation is proportional to the anodic overpotential (rj)... 

Subsequently this classically promoted Rh(Na)/YSZ catalyst is subject to electrochemical promotion via application of positive (IV) and negative (-1V) overpotential. The classically promoted catalyst performance is further dramatically enhanced especially in terms of rN2, rN20 and SN2 (Figs. 2.3, 8.66 and 8.67) and particularly with positive overpotentials. The resulting pN2 and PN20 values are on the order of 10 in the temperature range 240° to 360°C. Figures 2.3, 8.66 and 8.67 demonstrate two important facts ... 

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 electrochemical characterisation studies, discussed in the previous section, showed that a 40 at.% Ru electrode, when subjected to extended electrolysis or potential or current cycling in NaCl solutions and when the chlorine overpotential reaches 300-400mV, behaves like a fresh, low at.% Ru (about 5 at.%) electrode. This strongly suggests that Ru losses from the Ru/Ti oxide coating occur during electrolysis. To determine whether or not the Ru losses in failed anodes take place by uniform dissolution across the entire coating or whether only localised surface... 

It is usual to operate an aqueous-medium fuel cell under pressure at temperatures well in excess of the normal boiling point, as this gives higher reactant activities and lower kinetic barriers (overpotential and reactant diffusion rates). An alternative to reliance on catalytic reduction of overpotential is use of molten salt or solid electrolytes that can operate at much higher temperatures than can be reached with aqueous cells. The ultimate limitations of any fuel cell are the thermal and electrochemical stabilities of the electrode materials. Metals tend to dissolve in the electrolyte or to form electrically insulating oxide layers on the anode. Platinum is a good choice for aqueous acidic media, but it is expensive and subject to poisoning. 

It is now necessary to take a more unified view by considering situations in which the rate of the electrodic process at the interface is subject both to activation and to transport limitations. One refers to a combined activation-transport control of the electrodic reaction. Under such conditions, there will be, in addition to the overpotential T)c produced by the concentration change (from c° to c ) at the interface, an activation overpotential because the charge-transfer reaction is not at equilibrium. The total overpotential rj is the difference between the interfacial-potential difference... 

A net flow of electrons occurs across the metal/solution interface in a normal electrode reaction. The term electrocatalysis is applied to working electrodes that deliver large current densities for a given reaction at a fixed overpotential. A different, though indirectly related, effect is that in which catalytic events occur in a chemical reaction at the gas/solid interface, as they do in heterogeneous catalysis, though the arrangement is such that the interface is subject to a variation in potential and the rate depends upon it... 

Ni can be taken as the reference material against which all other materials should be evaluated. On the average, the operating overpotential of untreated Ni electrodes is about 0.4 V at 0.2 A cm-2 [5], Beyond Ni, we deal with activated cathodes , which in fact derive from the idea of activated anodes such as the DSA . By activated electrodes we mean that the surface has been subjected to some treatments aimed at increasing its catalytic activity. This can be a treatment which modifies the surface structure and the morphology of the base metal, but more often the treatment is aimed at coating the base metal with a more active material [31]. 

Going downhill, i.e., discharging the cell, is subject to similar thinking but of course reversed. The potential available at the terminals of a battery in discharge will be the thermodynamically reversible potential but now diminished by the sum of the overpotential and the IR drop. 

Therefore, measurements with an electrode of low hydrogen overpotential are subject to error due to mixed potential behavior. Using an electrolytic tin electrode, Grube and Schlecht found values of the formal potential to be — 0.411 V in 0.0015 M sulfuric acid and —0.386 to —0.373 V in 0.1 M sulfuric acid. 

The electrochemistry and surface chemistry of such UPD species has been the subject of several previous reviews [6, 7, 99, 100) and many original papers Ref 99 reviews, in thorough detail, electrocatalysis induced or modified by UPD metal adatoms which really change the intrinsic catalytic nature of the substrate metal surfaces. It is surprising, however, that very little work has been done until recently (cf Refs. 75, 101-106) on the adsorbed species that are the kinetically involved intermediates in overall Faradaic reactions proceeding continuously at appreciable net rates (or equivalent current densities), for example, in the reactions of H2, O2, and CI2 evolution and other processes such as O2 reduction (more work, relatively, has been done on that reaction) or H2 oxidation proceeding at appreciable overpotentials. Such intermediates are conveniently referred to as OPD species. 

All that the models of the class described above need is the conversion function that defines the relationship between the local values of the reacting gas concentration, overpotential, and the produced current. The subject of the microscopic theory is to justify this relationship. 

Additional parameters specified in the numerical model include the electrode exchange current densities and several gap electrical contact resistances. These quantities were determined empirically by comparing FLUENT predictions with stack performance data. The FLUENT model uses the electrode exchange current densities to quantify the magnitude of the activation overpotentials via a Butler-Volmer equation [1], A radiation heat transfer boundary condition was applied around the periphery of the model to simulate the thermal conditions of our experimental stack, situated in a high-temperature electrically heated radiant furnace. The edges ofthe numerical model are treated as a small surface in a large enclosure with an effective emissivity of 1.0, subjected to a radiant temperature of 1 103 K, equal to the gas-inlet temperatures. 

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