Rotating disk electrode.

In characterizing the effects of solution on corrosion, a rotating-disk electrode is convenient for such an assessment since the angular velocity (w) must influence the Kmiting current density. For forced convection under laminar flow, the Sherwood Number is applicable in the following form [9,62-63] 

Substituting eqs. (7.79), (7.104) and (7.105) into (7.103) and inserting the resultant expression into (7.87) along with 8 = r yields the current density, eq. (4.90), for kinetic studies 

In addition, Levich [9] derived an ejqtression for the diffusion layer as a function angular velocity or rotating speed as 

Other hydrodynamic cases for corrosion studies can be found elsewhere [10]. On the other hand, for a continuous metal removal from solution in electrowinning, rotating cylinders and disks are used as cathodes (Figme 7.6). The classical rotating-disk cathode is known as Weber s disk [25-26] having a diameter of 2a. The corresponding differential equation in cylindrical coordinates is free of the diffusivity and it is given by 

Recently, Michel et al. [169] carried out a numerical computation of the faradaic impedance of inlaid microdisk electrodes using a finite-element method and discussed the limitations of the Fleischmann and Pons [168] approach. Calculation of the functions I 4 and I 5 is shown in Exercise 4.8. 

Impedance may also be studied in the case of forced diffusion. The most important example of such a technique is a rotating disk electrode (RDE). In a RDE conditions a steady state is obtained and the observed current is time independent, leading to the Levich equation [17]. The general diffusion-convection equation written in cylindrical coordinates y, r, and q is [17] 

4 Impedance of the Faradaic Reactions in the Presence of Mass Transfer 

It is obvious that because of the cylindrical symmetry derivatives dCildcp = 0 and because Ci(0) is independent of the coordinate r, Eq. (4.120) reduces to 

To solve this equation, the velocity of the solution flowing toward the electrode must be known. This is a hydrodynamic problem involving a Navier-Stokes equation solved by von Karman and Cochrane assuming laminar flow [170]  

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Volt mmetiy. Diffusional effects, as embodied in equation 1, can be avoided by simply stirring the solution or rotating the electrode, eg, using the rotating disk electrode (RDE) at high rpm (3,7). The resultant concentration profiles then appear as shown in Figure 5. A time-independent Nernst diffusion layer having a thickness dictated by the laws of hydrodynamics is estabUshed. For the RDE,... 

Derive the Levich equation for the limiting current at the rotating disk electrode [based on combining equations (4-4) and (1-12)]. 

Rigid film approximation, 53 Rotating disk electrode, 111 Rotating ring disk electrode, 113 Ruthenium dioxide, 121... 

Theoretical treatment of polarographic curves for the calculation of values of jo has been described [65Hey, 66Hey], for an overview see [94Gal], a further evaluation procedure has been described [6801d]. Experimental details, in particular of solid electrodes in combination with a rotating disk electrode have been reported elsewhere [84Guy]. (Data obtained with this method are labelled PP.)... 

The rqjroducibility of polymer film formation is greatly improved by the spin coating technique where the polymer solution is applied by a microsyringe onto the center of a rapidly rotated disk electrode Rather thick films can be produced by repeated application of small volumes of stock solution. A thorough discussion and detailed experimental description of a reliable spin coating procedure was given recently... 

Fig. 3. Steady state concentration profiles of catalyst and substrate species in the film and diffusion layer for for various cases of redox catalysis at polymer-modified electrodes. Explanation of layers see bottom case (S + E) f film d diffusion layer b bulk solution i, limiting current at the rotating disk electrode other symbols have the same meaning as in Fig. 2 (from ref.

Cu9ln4 and Cu2Se. They performed electrodeposition potentiostatically at room temperature on Ti or Ni rotating disk electrodes from acidic, citrate-buffered solutions. It was shown that the formation of crystalline definite compounds is correlated with a slow surface process, which induced a plateau on the polarization curves. The use of citrate ions was found to shift the copper deposition potential in the negative direction, lower the plateau current, and slow down the interfacial reactions. 

In such systems the researcher can electrochemically clean and precondition the metal electrode before each run to provide an identical surface for the anodic and the cathodic half-reactions as well as for the catalytic reaction between them. Use of a rotating disk electrode/ckatalyst also allows surface- and diffusion-controlled processes to be easily distin-guished. ... 

Chronoamperometry Linear sweep Polarography Rotating disk electrode Faradaic impedance... 

FIGURE 4.6 Rotating-disk electrode (arrows in the space below the electrode indicate the directions of hqnid flow). 

At the rotating-disk electrode (RDE Fig. 4.6), it is the solid electrode and not the liqnid that is driven bnt from a hydrodynamic point of view this difference is nnim-portant. Liquid flows, which in the figure are shown by arrows, are generated in the solution when the electrode is rotated around its vertical axis. The liquid flow impinges on the electrode in the center of the rotating disk, then is diverted by centrifugal forces to the periphery. 

The constancy of the diffusion layer over the entire surface and thus the uniform current-density distribution are important features of rotating-disk electrodes. Electrodes of this kind are called electrodes with uniformly accessible surface. It is seen from the quantitative solution of the hydrodynamic problem (Levich, 1944) that for RDE to a first approximation... 

The values of that can be realized experimentally vary between 5 X 10 " cm/s (natural convection) and 2 X 10 cm/s (rotating-disk electrode at/= 10,000 rpm). Therefore, reactions for which 10 cm/s will remain reversible whatever the stirring intensity. Such reactions are called completely reversible ( very fast ). Reactions with 10 cm/s will always be irreversible and are called completely irreversible ( very slow ). In the region of intermediate values of the constant, the character of the reaction will depend on stirring conditions. With other values of a and of ratios idfJid,on the boundaries between the various regions of electrode operation will shift slightly, but the overall picture of the phenomena remains the same. 

FIGURE 12.5 Calculation of the kinetic current from experimental data obtained with a rotating-disk electrode. 

Figure 6.9 Cyclic voltammetry of a Pt(llO) rotating disk electrode in a CO-saturated 0.1 M HCIO4 solution, (a) Influence of the voltage scan rate, (b) Influence of the disk rotation rate.

Figure 6.10 Galvanostatic scans of a Pt(l 10) rotating disk electrode in a CO-satuiated 0.1 M HCIO4 solution at two different current scan rates (disk rotation rate 400rev/min). The insert shows the potential fluctuations observed at an apphed current density of 0.74 mA/cm (disk rotation rate 900 rev/min).

Bhzanac BB, Arenz M, Ross PN, Markovic NM. 2004b. Surface electrochemistry of CO on reconstructed gold single crystal surfaces studied by infrared reflection absorption spectroscopy and rotating disk electrode. J Am Chem Soc 126 10130-10141. 

Figure 8.9 Polarization curves for a PtSn/C catalyst recorded by a rotating disk electrode in 0.5 M H2SO4 saturated with either pure hydrogen, a H2/2% CO mixture, and pure CO (the arrow points to the onset of CO oxidation) at 60 °C with 1 mV/s and 2500 rev/min the dashed curve is the cyclic voltammogram (in arbitrary units) in an argon-purged solution at 60 °C with 50 mV/s. (Reprinted with permission from Aienz etal. [2005]. Copyright 2005. Elsevier.)...

Figure 8.12 Relationships between the catalytic properties and electronic structure of Pt3M alloys correlation between the specific activity for the oxygen reduction reaction measured experimentally by a rotating disk electrode on Pt3M surfaces in 0.1 M HCIO4 at 333 K and 1600 lev/min versus the li-band center position for (a) Pt-skin and (b) Pt-skeleton surfaces. (Reprinted with permission from Stamenkovic et al. [2007b]. Copyright 2007. Nature Pubhshing Group.)...

Figure 8.17 Activities of Pt(l 1 l)-wML Pd electrodes from rotating disk electrode measurements, with corresponding ball models (a) electro-oxidation of formic acid in 0.1 M HCIO4 ...

Screening Tests of Sputtered Pt Alloys by Rotating Disk Electrode... 

A standard rotating disk electrode (RDE) setup with a gas-tight Pyrex cell was used for the experiment on CO adsorption and the HOR. A Pt wire was used as counterelectrode. A reversible hydrogen electrode, RHE(t), kept at the same temperature as that of the cell (t, in °C), was used as the reference. All the electrode potentials in this chapter will be referenced to RHE(f). The electrolyte solution of 0.1 M HCIO4... 

Higuchi E, Uchida H, Watanahe M. 2005. Effect of loading level in platinum-dispersed carbon black electrocatalysts on oxygen reduction activity evaluated by rotating disk electrode. J Electroanal Chem 583 69-76. 

Gasteiger HA, Markovic NM, Ross PN. 1995. H2 and CO electrooxidation on well-characterized PL Ru, and Pt-Ru. 1. Rotating disk electrode studies of the pure gases including temperature effects. J Phys Chem 99 8290-8301. 

Schmidt TJ, Gasteiger HA, Stab GD, Urban PM, Kolb DM, Behm RJ. 1998. Characterization of high-surface area electrocatalysts using a rotating disk electrode configuration. J Electrochem Soc 145 2354-2358. 

Schmidt TJ, Gasteiger HA, Behm RJ. 1999b. Rotating disk electrode measurements on a high-surface area Pt/Vulcan carbon fuel cell catalyst. J Electrochem Soc 146 1296-1304. 

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