Electrode electron-transfer

Several mechanisms have been proposed to explain the activation of carbon surfaces. These have Included the removal of surface contaminants that hinder electron transfer, an Increase In surface area due to ralcro-roughenlng or bulld-up of a thin porous layer, and an Increase In the concentrations of surface functional groups that mediate electron transfer. Electrode deactivation has been correlated with an unintentional Introduction of surface contaminants (15). Improved electrode responses have been observed to follow treatments which Increase the concentration of carbon-oxygen functional groups on the surface (7-8,16). In some cases, the latter were correlated with the presence of electrochemical surface waves (16-17). However, none of the above reports discuss other possible mechanisms of activation which could be responsible for the effects observed. 

Fig. 7.37. A typical Tafel line for a one electron-transfer electrode reaction, showing the exponential relationship at high overpotentials, which makes the relation between ii and log r linear.

Identity (150) only holds if the same step remains rate-controlling over the wide range of potentials involved. As seen in Sect. 4.1, this condition is not very realistic in complex multistep electron transfer electrode reactions. 

In this section, the current-potential curves of multi-electron transfer electrode reactions (with special emphasis on the case of a two-electron transfer process or EE mechanism) are analyzed for CSCV and CV. As in the case of single and double pulse potential techniques (discussed in Sects. 3.3 and 4.4, respectively), the equidiffusivity of all electro-active species is assumed, which avoids the consideration of the influence of comproportionation/disproportionation kinetics on the current corresponding to reversible electron transfers. A general treatment is presented and particular situations corresponding to planar and nonplanar diffusion and microelectrodes are discussed later. 

Fig. 1. An equivalent circuit of three transfer gates of a-Si H charge-coupled devices. A central transfer electrode represented by a capacitor is assumed to be in a high state and to store signal electrons. Transfer electrodes of both sides are assumed to be in a low state.

The difference between the standard potential of an electron-transfer electrode reaction in two solvents, AS , is accordingly... 

Though much research on the influence of the solvent on the rate of electrode reactions has been done in recent years the problem is still far from a profound understanding. The basic question is the role of the dynamic and energetic terms in the control of the kinetics of simple electron-transfer electrode reactions. To answer this question it is essential to have reliable kinetic data for analysis. Unfortunately some kinetic data are too low and should be redetermined, preferably using submicroelectrodes. 

Although a large wave slope is a clear indicator that a system is not showing clean reversible behavior, it does not necessarily imply that one has an electrode process controlled by the kinetics of electron transfer. Electrode reactions frequently include purely chemical processes away from the electrode surface. A system involving chemical complications of this kind can show a wave shape essentially identical with that expected for a simple electron transfer in the totally irreversible regime. For example, the reduction of nitrobenzene in aqueous solutions can lead, depending on the pH, to phenylhydroxy-lamine (32) ... 

The adiabatic redox reactions at electrodes were first considered by MARCUS /40a,145/ in a classical (semiclassical) framework. lEVICH, DOGONADZE and KUSNETSOV /146,147/, SGHMICKLER and VIELSTICH /169/ a.o. have developed a quantum theory for non-adiabatic electron transfer electrode reactions based on the oscillator-model. The complete quantum-mechanical treatment of the same model by CHRISTOV /37d,e/ comprises adiabatic and non-adiabatic redox reactions at electrodes. 

Fig. 6 Standard free-energy diagram for a two-step electron-transfer electrode reaction with two potential dependent transition states.

Modes of coupling chemical steps with electron transfer (electrode mechanisms)... 

Among the studies of carbaborane-based products of possible commercial interest were investigations concerning fuels,40a,b electron transfer electrodes, 41 g.l.c. applIcatlons,42a,b flammability of carbaborane-s11oxane polymers,43 ceramic composites44a,b and related materials,44c and neutron-capture therapy of cancers.1 45a-c... 

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