Electrolytic cells near equilibrium

With no current through the electrolytic cell, it does not matter whether the electrodes are large or small the equilibrium potentials are the same. But with current flow, the current density and therefore the voltage drop and the polarization, will he much higher at the small electrode. An increased potential drop will occur in the constrictional current path near the small electrode, and in general the properties of the small electrode will dominate the results. The small electrode will be the electrode studied, often called the working electrode. It is a monopolar system, meaning that the effect is determined hy one electrode. The other electrode becomes the indifferent or neutral electrode. Note that this division is not true in potentiometry, electrode area is unimportant under no-current conditions. 

The technical process of copper refining is an important example of an electrolytic concentration cell. Two copper electrodes operate in the same Cu++-containing electrolyte, one as anode, the other as cathode. Clearly, at equilibrium, E - 0. The current flow at the anode will dissolve Cu as Cu++, raising the concentration there. On the other hand, at the cathode, Cu++ deposits as Cu. Consequently, near the cathode, the Cu++ concentration decreases. Thus, a cell voltage will establish that opposes the applied voltage and leads to a loss of energy. Note that the aim of the process is the... 

In the real non-equilibrium conditions of a present-day MCFC with very successful electrode reform, the cell electrode reaction, voracious for fuel, consumes the reformer product and favourably influences the reform process. The latter turns out to operate well at 600 °C, compared with about 800 °C in a fired reformer coupled, say, to much less voracious hydrogen separation and storage. In the practical SOFC, 1000 °C at the anode promotes excessively vigorous electrode reform, which leads to a local electrode cold spot. There are also stability considerations (Gardiner, 1996). Hence the contemporary movement towards lower SOFC temperatures, via new ceria electrolytes, and interconnect change from ceramic to steel. A PEFC near Tq, must have a combustion-operated 800 °C reformer, since a Tq electrochemical reform process does not exist in practice. 

The earlier sections of this chapter discuss the mixed electrode as the interaction of anodic and cathodic reactions at respective anodic and cathodic sites on a metal surface. The mixed electrode is described in terms of the effects of the sizes and distributions of the anodic and cathodic sites on the potential measured as a function of the position of a reference electrode in the adjacent electrolyte and on the distribution of corrosion rates over the surface. For a metal with fine dispersions of anodic and cathodic reactions occurring under Tafel polarization behavior, it is shown (Fig. 4.8) that a single mixed electrode potential, Ecorr, would be measured by a reference electrode at any position in the electrolyte. The counterpart of this mixed electrode potential is the equilibrium potential, E M (or E x), associated with a single half-cell reaction such as Cu in contact with Cu2+ ions under deaerated conditions. The forms of the anodic and cathodic branches of the experimental polarization curves for a single half-cell reaction under charge-transfer control are shown in Fig. 3.11. It is emphasized that the observed experimental curves are curved near i0 and become asymptotic to E M at very low values of the external current. In this section, the experimental polarization of mixed electrodes is interpreted in terms of the polarization parameters of the individual anodic and cathodic reactions establishing the mixed electrode. The interpretation then leads to determination of the corrosion potential, Ecorr, and to determination of the corrosion current density, icorr, from which the corrosion rate can be calculated. 

The equilibrium potential for a aingle cell, given by equation (11), for the cathodic and anodic reactions (5) and (8), is -406mV for a process gas containing 2000 ppm HgS and an anode product of pure sulfur vapor. To this must be added the overpotentials needed for both electrode reactions and ohmic loss. The electrode reactions have been studied in free electrolyte on graphite electrodes . Potential-step experiments showed very rapid kinetics, with exchange currents in both cathodic and anodic direction near 40 mA/cm . Cyclic voltammetry verified a catalytic reaction mechanism with disulfide as the electro-active species. At the cathode ... 

This form of corrosion occurs in the presence of stagnant corrosive solution near a hole, under a deposit, or any geometric shape that can form a crevice. It is also known as cavernous corrosion or underdeposit corrosion. This form of corrosion results from a concentration cell formed between the electrolyte within the crevice, which is oxygen starved, and the electrolyte present outside the crevice where plenty of oxygen is present. The metal within the crevice acts as anode, and the metal outside the crevice functions as the cathode. The difference in aeration produces a different equilibrium potential, given by the Nemst equation applied to the reaction... 

In valve-regulated batteries the sulfuric electrolyte is reacting with the electrodes, too, so that here also the same dilution of the electrolyte takes place. But in contrast to flooded batteries, because of the solidified electrolyte, the lead ions are hindered from diffusion into the cell. Thus the concentration of lead ions near the plate surface increases extremely, so that because of the solution equilibrium a further solution of lead is prevented. Therefore the electrolyte cannot become saturated with lead ions and no precipitation of lead dendrites takes place. 

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