Electrode potential, affecting

Suppose that the rate of an electrode reaction at the interface with holes involved is sufficiently high so that each hole approaching the electrode surface is consumed. Under such conditions the change in the electrode potential affects the hole photocurrent mainly through a change in the depletion layer thickness Lsc, which depends on [see Eq. (20a)]. If the light is absorbed weakly (a"1 Lsc + Lp), the photocurrent must, obviously, be proportional to the region thickness, from which the surface collects holes,... 

MORE FUNDAMENTAL MODELS OF ELECTRON TRANSFER 3.2.1 Why do changes in electrode potential affect k and kl Absolute rate theory... 

It is now assumed that consists of a chemical component and an electrical component and that it is only the latter that is affected by changing the electrode potential. The specific assumption is that... 

Laister and Benham have shown that under more arduous conditions (immersion for 6 months in sea-water) a minimum thickness of 0-025 mm of silver is required to protect steel, even when the silver is itself further protected by a thin rhodium coating. In similar circumstances brass was completely protected by 0 012 5 mm of silver. The use of an undercoating deposit of intermediate electrode potential is generally desirable when precious metal coatings are applied to more reactive base metals, e.g. steel, zinc alloys and aluminium, since otherwise corrosion at discontinuities in the coating will be accelerated by the high e.m.f. of the couple formed between the coating and the basis metal. The thickness of undercoat may have to be increased substantially above the values indicated if the basis metal is affected by special defects such as porosity. 

The most widely used reference electrode, due to its ease of preparation and constancy of potential, is the calomel electrode. A calomel half-cell is one in which mercury and calomel [mercury(I) chloride] are covered with potassium chloride solution of definite concentration this may be 0.1 M, 1M, or saturated. These electrodes are referred to as the decimolar, the molar and the saturated calomel electrode (S.C.E.) and have the potentials, relative to the standard hydrogen electrode at 25 °C, of 0.3358,0.2824 and 0.2444 volt. Of these electrodes the S.C.E. is most commonly used, largely because of the suppressive effect of saturated potassium chloride solution on liquid junction potentials. However, this electrode suffers from the drawback that its potential varies rapidly with alteration in temperature owing to changes in the solubility of potassium chloride, and restoration of a stable potential may be slow owing to the disturbance of the calomel-potassium chloride equilibrium. The potentials of the decimolar and molar electrodes are less affected by change in temperature and are to be preferred in cases where accurate values of electrode potentials are required. The electrode reaction is... 

A novel development of the use of ion-selective electrodes is the incorporation of a very thin ion-selective membrane (C) into a modified metal oxide semiconductor field effect transistor (A) which is encased in a non-conducting shield (B) (Fig. 15.4). When the membrane is placed in contact with a test solution containing an appropriate ion, a potential is developed, and this potential affects the current flowing through the transistor between terminals Tt and T2. 

Preliminary measurements with space-resolved PMC techniques have shown that PMC images can be obtained from nanostructured dye sensitization cells. They showed a chaotic distribution of PMC intensities that indicate that local inhomogeneities in the preparation of the nanostructured layer affect photoinduced electron injection. A comparison of photocurrent maps taken at different electrode potentials with corresponding PMC maps promises new insight into the function of this unconventional solar cell type. 

Finally, the electrode potential may affect the overall process by determining the state of oxidation of the electrode surface. It is well known that m aqueous solution a platinum electrode has a bare surface only over the narrow potential range from approximately -t-0-4 V to -tO-8 V versus N.H.E. at more cathodic potentials it is covered by adsorbed hydrogen atoms while at more anodic potentials it is covered by... 

The basic law of electron photoemission in solntions which links the photoemission current with the light s frequency and with electrode potential is described by Eq. (9.6) (the law of five halves). This eqnation mnst be defined somewhat more closely. As in the case of electrochemical reactions (see Section 14.2), not the fnll electrode potential E as shown in Eq. (9.6) is affecting the metal s electron work function in the solution bnt only a part E - / ) of this potential which is associated with the potential difference between the electrode and a point in the solntion jnst outside the electrode. Hence the basic law of electron photoemission into solntions should more correctly be written as... 

The surface concentration Cq Ajc in general depends on the electrode potential, and this can affect significantly the form of the i E) curves. In some situations this dependence can be eliminated and the potential dependence of the probability of the elementary reaction act can be studied (called corrected Tafel plots). This is, for example, in the presence of excess concentration of supporting electrolyte when the /i potential is very small and the surface concentration is practically independent of E. However, the current is then rather high and the measurements in a broad potential range are impossible due to diffusion limitations. One of the possibilities to overcome this difficulty consists of the attachment of the reactants to a spacer film adsorbed at the electrode surface. The measurements in a broad potential range give dependences of the type shown in Fig. 34.4. 

The value of the electric potential affecting the activation enthalpy of the electrode reaction is decreased by the difference in the electrical potential between the outer Helmholtz plane and the bulk of the solution, 2, so that the activation energies of the electrode reactions are not given by Eqs (5.2.10) and (5.2.18), but rather by the equations... 

The transfer coefficient a has a dual role (1) It determines the dependence of the current on the electrode potential. (2) It gives the variation of the Gibbs energy of activation with potential, and hence affects the temperature dependence of the current. If an experimental value for a is obtained from current-potential curves, its value should be independent of temperature. A small temperature dependence may arise from quantum effects (not treated here), but a strong dependence is not compatible with an outer-sphere mechanism. 

The latter discussion confirms the results of the potential dependence of the current in that the activation barrier for the hydrogen evolution reaction is, at least on copper and silver, not affected by the electrode potential. This behavior is, on the other hand, connected with the observation of straight lines in a Tafel plot. It would be premature to come up with a comprehensive model that would explain this behavior more experimental work is necessary to substantiate and quantify the effects for a larger variety of systems and reactions. A few aspects, however, should be pointed out. 

A first type of reaction that may affect the first electron transfer intermediate is its reduction (or oxidation) at the electrode. In most cases, the second electron transfer is energetically more costly than the first (for a discussion of exceptions to this rule, see Section 1.5). The two processes thus occur at successive values of the electrode potential. There is therefore no difficulty in preventing the occurrence of the second reaction by an appropriate adjustment of the electrode potential. 

The clean surface of metals in vacuum sustains a surface lattice transformation, as described in Sec. 6.1. Similarly, an interfadal lattice transformation takes place also on metal electrodes in aqueous solutions. In general, the interfadal lattice transformation of metal electrodes is affected by both the electrode potential and the ionic contact adsorption. 

Oxidant The oxidant composition and utilization are parameters that affect the cathode performance, as evident in Figure 2-3. Air, which contains -21% Oi, is the oxidant of choice for PAFCs. The use of air with -21% Oi instead of pure Oi results in a decrease in the current density of about a factor of three at constant electrode potential. The polarization at the cathode increases with an increase in Oi utilization. Experimental measurements (38) of the change in overpotential (Aric) at a PTFE-bonded porous electrode in 100% H3PO4 (191°C, atmospheric pressure) as a function of O2 utilization is plotted in Figure 5-4 in accordance with Equation (5-7) ... 

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