Ring electrodes current densities

Despite the importance of the ORR and long history of study, very little is known about the reaction mechanism.126,130,131 Mechanistic information has been derived almost exclusively from rotating disk electrode (RDE)131,132 and rotating ring disk electrode (RRDE)133-136,62,128 studies. The rotating electrode minimizes mass transfer effects and allows a kinetic current density to be extracted. In the RRDE setup, the ring surrounding the disk electrode detects species weakly adsorbed to the electrode that are ejected due to electrode rotation. The ORR reaction (eqn 4) is... 

In order to use a consistent terminology in the following chapters it is emphasized that the term hole will always refer to a defect electron, while an etched feature in the electrode will be designated as a pore if its depth exceeds its width or otherwise as an etch pit. All values of current density refer to the initial surface of the electrode exposed to the electrolyte, which is for example defined by the O-ring of the set-up. 

Fig. 62. Snapshots and space-time current densities measured with ring-arrays of Fe electrodes in the oscillatory region on the transport-limited plateau of the current-potential curve. (Reproduced with permission from Z. Fei, J. Green and J. L. Hudson, J. Phys. Chem. B103 (1999) 2178, (1999) American Chemical Society.)...

As general trend, it can be observed that for most of the studied phenolic compounds the anodic oxidation at BDD was very fast and, if the applied current density was sufficiently high, the kinetics of the process was under mass-transfer control and it was not appreciably dependent on the nature of the substituting groups in the aromatic ring, as it was reported at more usual electrode materials, like platinum (Torres et al. 2003). 

The rotating ring-disc electrode is perhaps the most useful extension of the idea of the rotating disc. We have already seen (cf. Eq. 32D) that the ring current has the same form as the disc current, namely that it is also proportional to the bulk concentration of the electroactive species C° and to the square root of the rotation rate. A numerical comparison of Eqs. 32D and 25D shows that the current density at the ring is higher than at the disc, indicating that some increase in analytical sensitivity may be attained. This, however, is not the... 

The coupling sites and yields in the anodic coupling of the diarylalkanes (XXII) are strongly dependent on the value of n, the position of substituents in the aryl ring, the choice of the electrolyte-electrode system, the pH of the medium, the current density, and the oxidation potential. 

Indirect electrolysis of allyl alcohols can be performed at a mercury electrode at a rather positive potential [—0.2 V (SCE)], and high current densities may be used (10-20 A dm ). The first step is a transformation of the alcohol to an iodide the double bond is shifted to the terminal position, even if it means breaking a conjugation with a phenyl ring [56]. A 20 1 mixture of 1-butene and 2-butene was obtained from crotyl alcohol ... 

In order to determine the kinetic parameters of the reaction corrected from diffusion phenomenon, a voltammetric investigation was pursued with a rotating platinum ring disk electrode. Figure 21.10 represents current density-potential curves obtained for different rotation speeds and for the positive variations of potential. It appears that the current density of the rotating disk increases with the rotation rate. 

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