Electrodes rotation rate limits

The objective of the mass transport lab is to explore the effect of controlled hydrodynamics on the rate at which a mass transport controlled electrochemical reaction occurs on a steel electrode in aqueous sodium chloride solution. The experimental results will be compared to those predicted from the Levich equation. The system chosen for this experiment is the cathodic reduction of oxygen at a steel electrode in neutral 0.6 M NaCl solution. The diffusion-limited cathodic current density will be calculated at various rotating disk electrode rotation rates and compared to the cathodic polarization curve generated at the same rotation rate. 

Figure 4-12. Catalytic voltammetry of Paracoccus pantotrophus nitrate reductase (NarGH) adsorbed as a film on a PGE electrode at pH 6. (A) Increasing the electrode rotation rate from 0 to 3000 rpm removes the mass transport limitation of the catalytic response in 50 pM NO3 . (B) The enzyme s greater rate of chlorate reduction compared to nitrate reduction is reflected in greater distortion of the waveform through dispersion of sluggish interfacial electron transfer rates (see also Fig. 4-4C). Scan rate 10 mV s. Adapted from ref. 64. with permission.

The anodic limiting current in lithium salt solutions is determined by the diffusion of the solvated electrons to the electrode. This was quantitatively established by the measurements taken on rotating disc electrodes and also by galvanostatic measurements In fact, as seen from Fig. 8, the limiting current density is proportional to the square root of the disc electrode rotation rate. This, in accordance with the rotat-... 

The limiting current of the first wave fij is independent of the electrode rotation rate this current is a kinetic one. It has been proposed that the dissociation... 

Figure 6. Coexistence of stationary behavior (between 0 and 1.1s) and oscillations (from 2.4 s on) during the reduction of S20 at a rotating Pt electrode, rotation rate f = 20 Hz. Between points a and b the system was perturbed by enhancing the rotation frequency to about 150 Hz, triggering the transition from the fixed point to the limit cycle. (After Wolf etal. with the kind permission of VCH Publishers.)...

The first direct electrochemical study of this chicken liver SO was a brief report by Elliott et al The enzyme adsorbed on a pyrolytic graphite electrode showed a non-turnover response at 80 mV vs. NHE, which was attributed to the heme cofactor (Eigure 5.11, inset). A catalytic wave was identified in the presence of sulfite (Eigure 5.11) and the shape and amplitude were independent of electrode rotation rate. This reflects that the enzyme-sulfite reaction is rate limiting. Erom the maximum limiting current (4 ) the turnover number k2 (eqn (5.2b)) was found to be around 4 s which was significantly... 

In order to get the current—potential relationship on the RDE, particularly the expression of limiting current density as the function of the electrode rotating rate and the reactant concentration, Pick s second law has to be used to give the equations of reactant concentration change with time at the steady-state situation of diffusion—convection. When the surface concentration of oxidant reaches zero during the reaction at the steady-state situation, the concentration distribution within the diffusion—convection layer is not changing with time anymore, meaning that the diffusion rate is... 

When using RDE technique for electrochemical measurements, several cautions should be paid attention, including the iR drop between the RDE surface and the reference electrode tip, and the limitations of electrode rotating rate. 

As we discussed above, the RDE theory is based on the convection kinetics of the electrolyte solution. If the electrode rotating rate is too small, meaning that the solution flow rate is too slow, it will be difficult to establish the meaningful diffusion-convection layer near the electrode surface. In order to make meaningful measurement, there is a rough formula that can be used to obtain the limit of electrode rotating rate (wiow) ... 

In a linear potential sweep experiment performed on a RDE, the potential of the working electrode is scanned from a potential where no reaction occurs to a potential that causes a reaction to occur. A limiting current is achieved when the overpotential is high enough so that the reaction rate is determined by the mass transport rate of the reactant at a given electrode rotation rate. The surface concentration of the reactant drops to zero, and a steady mass transport profile is attained as C/L, where L is the diffusion layer thickness. At a fixed electrode rotation rate, L does not change, and thus C/L does not change. Therefore, a steady-state diffusion-controlled current is achieved, described by the Levich equation ... 

The film diffusion limiting current density and the adsorption limiting current density are both independent of disk electrode rotation rates and applied potential (E), thus it is impossible to dissociate them and Eq. 9.29 can be written ... 

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