Rotating ring-disk electrode steady-state

Hydrodynamic electrodes — are electrodes where a forced convection ensures a -> steady state -> mass transport to the electrode surface, and a -> finite diffusion (subentry of -> diffusion) regime applies. The most frequently used hydrodynamic electrodes are the -> rotating disk electrode, -> rotating ring disk electrode, -> wall-jet electrode, wall-tube electrode, channel electrode, etc. See also - flow-cells, -> hydrodynamic voltammetry, -> detectors. 

In the non-steady state, changes of stoichiometry in the bulk or at the oxide surface can be detected by comparison of transient total and partial ionic currents [32], Because of the stability of the surface charge at oxide electrodes at a given pH, oxidation of oxide surface cations under applied potential would produce simultaneous injection of protons into the solution or uptake of hydroxide ions by the surface, resulting in ionic transient currents [10]. It has also been observed that, after the applied potential is removed from the oxide electrode, the surface composition equilibrates slowly with the electrolyte, and proton (or hydroxide ion) fluxes across the Helmholtz layer can be detected with the rotating ring disk electrode in the potentiometric-pH mode [47]. This pseudo-capacitive process would also result in a drift of the electrode potential, but its interpretation may be difficult if the relative relaxation of the potential distribution in the oxide space charge and across the Helmholtz double layer is not known [48]. 

A regime of simultaneous dissolution has also been found for Cu—Ni alloys in acidic chloride solutions. Rotating ring-disk electrode studies revealed an apparent Tafel region of the alloy and component polarization curves with mixed mass transfer and kinetic rate control [44, 45]. For a CugoNiio alloy, the kinetic parameters again indicate a coupling of the copper and nickel partial currents under steady state conditions [44]. 

The most frequently used techniques for ORR catalysis studies are steady-state polarization, cyclic voltammetry, rotating disk electrode (RDE), and rotating ring-disk electrode (RRDE). 

Figure 6 Steady state rotating ring-disk voltammograms of (A) compound (42) (B) compound (43) and (C) a Ru-bridged polymer of (43) each adsorbed to a graphite working electrode. Disk current shows reduction of 02 while ring current reveals the presence of H202 simultaneously reoxidised at the ring anode poised at +1.0 V (reproduced with permission of the American Chemical Society from Acc. Chem. Res., 1997, 30, 437-444).

Although a major advantage of rotating disk electrode techniques, compared to stationary electrode methods, is the ability to make measurements at steady state without the need to consider the time of electrolysis, the observation of current transients at the disk or ring following a potential step can sometimes be of use in understanding an electrochemical system. For example the adsorption of a component. 

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