Rotating disc electrode mass transfer control

Oxygen is reduced at the rotating disc electrode and the potential of the concentric ring is set such that the oxidation of hydrogen peroxide is under mass transfer control. Plots are then made of the inverse of the observed collection efficiency versus the inverse of the square route of the rotation speed (in accordance with normal ROE theory). Linear plots are usually observed. Extrapolating to infinite rotation speed allows the prediction of the maximum possible collection efficiency. This is the maximum amount of hydrogen peroxide produced. In terms of the two possible reduction routes (equations 4 and 5 above) this gives the parameter x which is defined as. 

The analogy in the mass transport effects in electrode reaction and homogeneous second-order fast reactions in solution becomes clear. In electrode kinetics, however, the charge-transfer rate coefficient can be externally varied over many orders of magnitude through the electrode potential and kd can be controlled by means of hydrodynamic electrodes. For instance the mass transport rate coefficient, kd, for a rotating disc electrode at the maximum practical rotation speed of 10 000 per min is approximately 2 x 10... 

As may be anticipated, the rotating ring-disc electrode may be more useful for the study of this reaction type. The ring should be set at the potential where the mass-transfer controlled back reaction B — A occurs. The deviation of the measured collection efficiency N = — Iring/ Idisc (cf. section 8.3 in chapter 2) from the value N found in the absence of the chemical step allows the rate constant kf to be determined. For electrode systems with very thin rings and small gaps a fairly simple equation has been suggested [60] ... 

The experimental cell is controlled by a potentiostat/galvanostat, which is also coupled with a frequency response analyzer for EIS measurements. The potentiostat (connected to a computer) measures the WE potential ( ) with respect to the RE, and the current (/) through the CE. The resistor (> 1 Gf2) is internal to the potentiostat and prevents current flow in the RE. The electrochemical cell shown in Figure 3.4(a) can also be used with rotating disc electrodes (RDEs), with the addition of an RDE rotor/controUer. RDE-based experiments do not necessarily mimic the hydrodynamic conditions of CMP, because the fluid velocity prohle at the surface of an RDE (Bard, 2001) is different from that expected for a CMP pad (Thakurta et al., 2002). Nevertheless, certain details of the CMP-related reaction kinetics and the effects of convective mass transfer on such reactions can be examined using RDEs. 

The hydrodynamically modulated rotating disc electrode The idea of this technique [15-17] is to allow the separation of kinetic and mass transport controlled components of a measured current (either in the region of mixed electron transfer/mass transport control or in situations where there are two competing electrode reactions, one mass transport controlled and the other kinetically controlled, (e.g. solvent decomposition) by using a sinusoidal modulation of the rotation rate and employing a phase sensitive detection method to measure the perturbation of the current. Only the mass transport limited component of the current will respond to the modulation. 

The rotating disc electrode (RDE) is the classical hydrodynamic electroanalytical technique used to limit the diffusion layer thickness. However, readers should also consider alternative controlled flow methods including the channel flow cell (38), the wall pipe and wall jet configurations (39). Forced convection has several advantages which include (1) the rapid establishment of a high rate of steady-state mass transport and (2) easily and reproducibly controlled convection over a wide range of mass transfer coefficients. There are also drawbacks (1) in many instances, the construction of electrodes and cells is not easy and (2) the theoretical treatment requires the determination of the solution flow velocity profiles (as functions of rotation rate, viscosities and densities) and of the electrochemical problem very few cases yield exact solutions. 

In a typical experiment with semiconductor-liquid junctions, one of the most important experimental problems is the differentiation between reactions that involve chemical changes at the semiconductor electrode (corrosion with insoluble products) and chemical changes in the electrolyte that might be subject to mass transfer limitations. The technique of Rotating Ring Disc Electrode (RRDE) (17-19) provides an opportunity to differentiate between these two types of reactions under controlled hydrodynamic conditions. In its simplestform, the metallic ring is isolated... 

---