Ring electrodes electrochemical reactions

The creation of nanostructured surfaces is one thing, the study of electrochemical reactions on such nanostructures is another one. Especially in electrocatalysis, where size effects on reactivity are often discussed, there have been attempts to use the tip of an STM as a detector electrode for reaction products from, say, catalytically active metal nanoclusters [84]. Flowever, such ring-disk-type approaches are questionable,... 

There are, however, obvious limitations. It is not possible to make a very small spherical electrode, because the leads that connect it to the circuit must be even much smaller lest they disturb the spherical geometry. Small disc or ring electrodes are more practicable, and have similar properties, but the mathematics becomes involved. Still, numerical and approximate explicit solutions for the current due to an electrochemical reaction at such electrodes have been obtained, and can be used for the evaluation of experimental data. In practice, ring electrodes with a radius of the order of 1 fxm can be fabricated, and rate constants of the order of a few cm s 1 be measured by recording currents in the steady state. The rate constants are obtained numerically by comparing the actual current with the diffusion-limited current. 

Recently, Mottola [98] reported a sensor based on the disk-ring principle previously developed by Kamin and Wilson [99], and Wang and Lin [100]. Unlike Mottola s design, its forerunners involved no stationary ring electrode or rotation of the reactor part in addition, their reactor/electrode was located at the cell bottom. In Mottola s assembly, a product of an enzyme-catalysed reaction at a bioreactor rotated at a constant speed was hydrodynamically transported to a stationary ring electrode, where it was electrochemically monitored. The sample was transported to the detection imit by an tm-... 

Both the ring-disk and thin-layer electrodes provide a convenient means for observing unstable intermediate products from electrochemical reactions. Quantitative evaluations of the lifetimes of these intermediates and of the products from such intermediates are readily evaluated by each of these methods.52 53... 

In some cases it is of interest to determine products formed at semiconductor electrodes. If redox reactions are involved this can be done by using a rotating ring disc electrode assembly (RRDE), which has proved to be a powerful tool for investigating electrochemical reactions at metal electrodes. The technique and corresponding results as obtained with metal electrodes have been reviewed by Bruckenstein and Miller [6] and by Pleskov et al. [7]. 

The collector channel electrode of the DCE performs a similar function to the ring electrode of the RRDE (see Sect. 2.4.2.2), electrochemically detecting products electrogenerated on the upstream electrode, in this case the generator channel electrode. Usually, the generator electrode is held at a potential that ensures mass transport-limited electrogeneration of product and the collector electrode is held at a potential to ensure mass transport-limited electrochemical reaction of this product. [Like the RRDE (see Sect. 2.4.2.2), when more than one product is formed, the collector electrode material and the potential can be chosen to selectively detect the product of choice.]... 

Mechanisms for the electrochemical processes at mercury electrodes in solutions of [Ni(cyclam)] + and CO2 have been proposed (see Scheme 5.1 ). Scheme 5.1 shows the formation of a carbon-bonded Ni(II) complex by reaction of CO2 with Ni(cyclam)+. The formation of such a complex is considered to be a fundamental step in the mechanism of the [Ni(cyclam)] +-catalyzed electrochemical reaction. The overall process for the transformation of CO2 into CO also involves inner-sphere reorganization. Scheme 5.1 includes the formation of sparingly soluble complex containing Ni(0), cyclam and CO which is a product of the reduction of [Ni(cyclam)] + under CO. Depositation of a precipitate of the Ni(0) complex on the mercury electrodes inhibits catalysis and removes the catalyst from the cycle. The potential at which the [Ni L-C02H] + intermediate (see lower left hand of Scheme 5.1) accepts electrons from the electrode. This potential is not affected by substitution on the cyclam ring, as shown by comparison of [Ni(cyclam)] + and [Ni(TMC)] " (TMC = tefra-iV-methylcyclam)... 

The difference between an RRDE and an RDE is the addition of the second working electrode in the form of a ring around the central disk-working electrode. As the disk and the ring electrodes are electrically isolated, both the disk and ring electrodes of an RRDE function as the separate two hydrodynamic electrodes. RRDE experiments are useful in studying multielectron processes, the kinetics of a slow electron transfer, adsorption/ desorption steps, and electrochemical reaction mechanisms, particularly for ORR. 

As the disk electrode is independent of the ring electrode, here we give the discussion about the electrochemical reaction occurring on the disk and the ring electrodes, respectively. 

Because the RRDE itself is a universal tool for the study of any electrochemical reaction rather than a specific tool only for a specific reaction, its collection efficiency should be its intrinsic property and independent of the reaction studied. Therefore, this collection efficiency should be only the function of the geometric sizes of the disk and ring electrodes such as ri, rz, and tr in Figure 6.2. To obtain the relationship between the collection efficiency (A/) and the RRDE geometric size r, rz, and Tr), that is, N=/(ri,r2,r3)), we still need to get the expressions of I d and 7r,R in Eqn (6.15). 

Ordinarily, the selectivity of ORR is determined by means of rotating disk electrode (RDE) or rotating ring-disk electrode (RRDE) voltammetry. The RDE is designed to boost the diffusion of an electroanalyte in conditions where an electrochemical reaction is limited by diffusion of the analyte to the electrode. In RRDE, a ring (often platinum) surrounds the disk electrode with an insulating material (usually Teflon) between them (Fig. 7.5). 

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