Rotating disk electrode electron transfer kinetics

The electrochemistry of titanium (fV) has been exanuned in acidic l-ethyl-3-methyUmidazolium chloride/AlClj ([EmimJCl/AlClj) in 1990 by Carlin et al. [180]. It was shown that titaifium is reduced to Ti(lll) and Ti(n) in two one-electron steps, both of which exhibit slow electron-transfer kinetics. Ten years later, Mukhopadhyay et al. smdied the deposition of Ti nanowires at room temperature from 0.24 M TiCl in the ionic liquid l-butyl-3-methylimidazohumbis((trifluoromethyl)sulfonyl)amide [181]. They found that up to six wires grow at constant potential over a period of about 20 min wires exhibit a narrow width distribution of 10 2 nm and have a length of more than 100 nm. The chemical and electrochemical behavior of titanium was examined in the Lewis acidic [EmimlCl/AlClj molten salt at 353.2 K. The electrodeposition of Al-Ti alloys at Cu rotating disk and wire electrodes was investigated by Tsuda et al. [55]. It was found that Al-Ti alloys which contain up to 19% (atomic fraction) titanium, could be electrodeposited from saturated solutions of Ti(II) in the... 

Table H. Heterogeneous electron-transfer kinetics at the platinum disk electrode at 298 K. All values determined from linear portions of Tafel plots at rates of rotation extrapolated to infinity. Reaction of Se and As too slow for observation...

For a research on the electrode reaction mechanism and kinetics, particularly those of oxygen reduction reaction (ORR) (O2 + 4H+ + 4e -> 2H2O in acidic solution, or 02 + 2H20 + 4e -> 40H in alkaline solution), it is necessary to design some tools that could control and determine the reactant transportation near the electrode surface and its effect on the electron-transfer kinetics. A popular method, called the rotating disk electrode (RDE) technique has heen widely used for this purpose, particularly for the ORR. 

Miller, B., Bellavance, M., and Bruckenstein, S. (1971) Application of isosurface concentration voltammetry at a rotating disk electrode to simple electron transfer kinetics. /. Electrochem. Soc., 118 (7), 1082-1089. 

As the field of electrochemical kinetics may be relatively unfamiliar to some readers, it is important to realize that the rate of an electrochemical process is the current. In transient techniques such as cyclic and pulse voltammetry, the current typically consists of a nonfaradaic component derived from capacitive charging of the ionic medium near the electrode and a faradaic component that corresponds to electron transfer between the electrode and the reactant. In a steady-state technique such as rotating-disk voltammetry the current is purely faradaic. The faradaic current is often limited by the rate of diffusion of the reactant to the electrode, but it is also possible that electron transfer between the electrode and the molecules at the surface is the slow step. In this latter case one can define the rate constant as ... 

An interesting study [52] of the protonation kinetics and equilibrium of radical cations and dications of three carotenoid derivatives involved cyclic voltammetry, rotating-disk electrolysis, and in situ controlled-potential electrochemical generation of the radical cations. Controlled-potential electrolysis in the EPR cavity was used to identify the electrode reactions in the cyclic volt-ammograms at which radical ions were generated. The concentrations of the radicals were determined from the EPR amplitudes, and the buildup and decay were used to estimate lifetimes of the species. To accomplish the correlation between the cyclic voltammetry and the formation of radical species, the relative current from cyclic voltammetry and the normalized EPR signal amplitude were plotted against potential. Electron transfer rates and the reaction mechanisms, EE or ECE, were determined from the electrochemical measurements. This study shows how nicely the various measurement techniques complement each other. 

A more detailed kinetic investigation of the Au/Bipy/cytochrome c system was carried out using the rotating ring-disk technique (12). It was found that rate constants for adsorption and desorption of the protein were 3 x cm sec" and 50 sec", respectively. The limiting first-order rate constant within the protein-electrode complex was determined as 50 sec", a reasonable value as compared to that of long-range electron transfer between or within proteins. 

A rotating platinum disk electrode was used for gathering data for the calculation of the kinetic constants. Tafel plots (5) were developed (Figure 2), from which the heterogeneous rate constants, k , and the electron-transfer coefficients, a, were determined. In all determinations of kinetic parameters, a blank voltammogram, run under identical conditions, was subtracted from the data to remove current due to background reactions or charging of the double layer. 

Koutecky—Levich plot, measured at different electrode rotating rates, if the O2 concentration, the O2 diffusion coefficient, and the solution kinetic viscosity are known. Here, we give the expression of apparent electron-transfer number of ORR as functions of both disk and ring currents, measured using the RRDE technique. 

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