Electrodes double

Electro-conductivity measurements were performed on the above systems using the double-electrode method described by Sheiko [325], and are reported in [324],... 

In this section, we demonstrate the real ORR activities (apparent rate constant per real active surface area, fe pp) and P(H202) at bulk Pt and nanosized Pt catalysts dispersed on carbon black (Pt/CB) with dp,= 1.6 + 0.4, 2.6 + 0.7, and 4.8 1.0 nm in the practical temperature range 30-110 °C [Yano et al., 2006b]. The use of a channel flow double-electrode (CFDE) cell allowed us to evaluate fe pp and P(H202) precisely. 

Wakabayashi N, Takeichi M, Itagaki M, Uchida M, Watanabe M. 2(X)5a. Temperature-dependence of oxygen reduction activity at a platinum electrode in an acidic electrolyte solution investigated with a channel flow double electrode. J Electroanal Chem 574 339-346. 

A combined electrode is a double electrode system consisting of a clast- electrode. md. 1 calomel electrode combined in one probe. 

Double electrodes are particularly useful in kinetic studies. Intermediates produced on the generator (upstream) electrode are transported to the downstream electrode where they react further. This is useful for the study of short-lived species, the quantity reaching the second... 

It will be seen that the form of the expression for N0 is quite general for double electrodes. 

The rotating ring—disc electrode (RRDE) is probably the most well-known and widely used double electrode. It was invented by Frumkin and Nekrasov [26] in 1959. The ring is concentric with the disc with an insulating gap between them. An approximate solution for the steady-state collection efficiency N0 was derived by Ivanov and Levich [27]. An exact analytical solution, making the assumption that radial diffusion can be neglected with respect to radial convection, was obtained by Albery and Bruckenstein [28, 29]. We follow a similar, but simplified, argument below. 

The important point about eqn. (41) is that it is dependent solely on the electrode geometry. This simplifies kinetic studies with double electrodes considerably. There have been numerous experimental verifications of N0 for different systems. Digital simulation also shows very good agreement with the analytical solution [31]. 

A double electrode is clearly a convenient tool for the investigation of multistep processes. Its application to the study of multistep electron transfer is the subject of the next section. 

Double electrodes are invaluable in the elucidation of multistep electron transfers. Applications have been almost exclusively at the RRDE. 

Whilst it is clearly the case that double electrodes offer a unique way of studying these reactions, in many cases information can be extracted from single electrodes much of the theoretical basis was derived before the use of double electrodes became widespread. The reaction scheme may be written as... 

For complex mechanisms such as ECE or other schemes involving at least two electron transfer steps with interposed chemical reactions, double electrodes offer a unique probe for the determination of kinetic parameters. Convection from upstream to downstream electrodes allows the study of fast homogeneous processes. The general reaction scheme for an ECE mechanism can be written... 

Fig. 14. Diffusion layer titration curve at a double electrode, when...

It is interesting that mathematical solution proceeds in a very similar fashion to that for N0 any double electrode system where the convective-diffusion equation can be reduced to the form... 

At a double electrode, such as the rotating ring—disc electrode, a potential step at the disc will produce a ring current transient, the form of which is affected only by Faradaic current components at the disc. This fact can be very useful in separating Faradaic and non-Faradaic processes. 

This point can be illustrated in a concrete manner by a consideration of the double electrode-pair system ... 

Thus, under equilibrium conditions, the emf of the double electrode-pair system is determined solely by electric potential differences developed at the two liquid junctions that involve KC1 salt bridges. The two Ej may differ because of the effect of soil colloids. Thus the fact that this emf can develop is known as the suspension effect.40 Only ionic transport processes across the liquid junctions need be taken into account in order to evaluate E. Ionic transport processes across the semipermeable membrane between the suspension and the solution are not germane. Moreover, since neither Ej nor Ej can be calculated by strictly thermodynamic methods, the interpretation of E must be made in terms of specific models of ionic transport across salt bridges contacting suspensions and solutions. Thus the relation between E and the behavior of ions in soil suspensions is not direct. 

The steady-state collection efficiency, N0, of the double electrode is given by... 

Tube double electrode (TDE) Channel double electrode (CDE) ... 

Instead of presenting the equations for these cases, which can be consulted in the literature1, we consider a mechanism where the double electrode is very valuable, that is in the ECE mechanism, the chemical step being of first or second order18. Schematically for a double electrode, one has... 

DOUBLE-ELECTRODE PROCESS (DOUBLE-GRADED PROCESS)... 

Although SECM operates on broadly similar principles to other double electrode methods, the SECM geometry offers several advantages, in terms of the range of kinetics (especially the upper limit) that can be studied and the precision with which measurements can be made, as discussed below. 

Distinguishing between ECE and DISP1 mechanisms by many conventional electrochemical methods is difficult. However, in common with generation-collection measurements at other double electrode geometries (39) and earlier applications of double potential step transient methods (40,41), generation-collection and feedback measurements with SECM have been shown to allow an unequivocal mechanistic assignment (7). Moreover, SECM allows the measurement of larger rate constants for Eq. (55) than the aforementioned techniques. 

The use of a second downstream electrode to monitor chemical fluxes at the working electrode is proving to be an important technique for the investigation of electrode mechanisms. This is particularly true for electrodes which have a more complicated structure than a simple metallic surface. Examples are modified electrodes, oxide electrodes, or enzyme electrodes. For these more complex systems, the separate measurement of the fluxes at the electrolyte-electrode interface provides unique and valuable information. Double electrodes can be constructed for all three hydrodynamic systems. A crucial parameter for such a double electrode is the collection efficiency, N, which, in the steady state, relates the flux of material detected as a limiting current on the downstream electrode to the flux of material generated on the upstream electrode. The collection efficiency is a function of the geometry of the electrode and is given for all three systems by [4, 9]... 

One advantage of the wall-jet system is that one can include a packed-bed electrode just upstream of the jet, thus making a packed bed wall-jet electrode (PBWJE) [5-7]. This is a valuable double electrode system in that a packed-bed electrode can achieve complete turnover of a reactant. We have verified that theory and experiment are in good agreement for the collection efficiency by the wall-jet electrode of material generated on the bed [6]. 

Fig. 37. Principle behind double electrode operation under steady-state conditions. A proportion of the species B, electrogenerated upstream, can be lost before detection downstream via diffusion or reaction in solution. The collection efficiency, N, provides information about these two processes. Where the product of the homogeneous reaction, P, is electroactive, it too can be detected downstream.

Fig. 41. A-O 1/3 relationship for the reduction of oxidized ADMA at the downstream electrode in a double electrode cell with geometry as given in the legend to Fig. 40. From ref. 125.

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