Disk electrodes materials

Figure 1. Rotating ring-disk electrode. 1 disk electrode material, 2 ring electrode material, 3 insulator, 4 metal support, r, disk radius, T2 inner ring radius, outer ring radius, r overall radius of the tip.

Voltammetry is a term used to include all the methods that measure current-potential curves (voltammograms) at small indicator electrodes other than the DME [6], There are various types of voltammetric indicator electrodes, but disk electrodes, as in Fig. 5.17, are popular. The materials used for disk electrodes are platinum, gold, graphite, glassy carbon (GC), boron-doped diamond8, carbon paste, etc. and they can be modified in various ways. For electrode materials other than mercury, the potential windows are much wider on the positive side than for mercury. However, electrodes of stationary mercury-drop, mercury-film, and mercury-pool are also... 

As described in the introduction, submicrometer disk electrodes are extremely useful to probe local chemical events at the surface of a variety of substrates. However, when an electrode is placed close to a surface, the diffusion layer may extend from the microelectrode to the surface. Under these conditions, the equations developed for semi-infinite linear diffusion are no longer appropriate because the boundary conditions are no longer correct [97]. If the substrate is an insulator, the measured current will be lower than under conditions of semi-infinite linear diffusion, because the microelectrode and substrate both block free diffusion to the electrode. This phenomena is referred to as shielding. On the other hand, if the substrate is a conductor, the current will be enhanced if the couple examined is chemically stable. For example, a species that is reduced at the microelectrode can be oxidized at the conductor and then return to the microelectrode, a process referred to as feedback. This will occur even if the conductor is not electrically connected to a potentiostat, because the potential of the conductor will be the same as that of the solution. Both shielding and feedback are sensitive to the diameter of the insulating material surrounding the microelectrode surface, because this will affect the size and shape of the diffusion layer. When these concepts are taken into account, the use of scanning electrochemical microscopy can provide quantitative results. For example, with the use of a 30-nm conical electrode, diffusion coefficients have been measured inside a polymer film that is itself only 200 nm thick [98]. 

Mercury is the electrode material of choice for many electrochemical reductions and some unique oxidations (see Chap. 14). We have explored the use of both small mercury pools and amalgamated gold disks in thin-layer amperometry. Other workers have used pools in a capillary tube [7] and amalgamated platinum wire [8]. In 1979, Princeton Applied Research introduced a unique approach based on their model 303 static mercury drop electrode (see Sec. II.F). Our laboratories and MacCrehan et al. [9] have focused on the use of amalgamated gold disks. This approach results in an inexpensive, easily prepared, and mechanically rigid electrode that can be used in conventional thin-layer cells (Sec. II.C) of the type manufactured by Bioanalytical Systems. 

Shuman, M.S., Collins, B.J., Fitzgerald, RJ. and Olson, D.L. (1983) Distribution of stability constants and dissociation rate constants among binding sites on estuarine copper-organic complexes rotated disk electrode studies and an affinity spectrum analysis of ion-selective electrode and photometric data. In Aquatic and Terrestrial Humic Materials (eds Christman, R.F. and Gjessing, E.T.). Ann Arbor Science, Ann Arbor, MI, pp. 349-370. 

To get relevant information about active materials, the working electrode is made as similar as possible to the electrode of an operational device. However, current collectors are usually made with corrosion resistant materials, with good electronic conductivity, and no concern is taken about its relative mass. Materials such as gold, platinum, and vitreous carbon are commonly used. The active mass is usually tested in small amounts, mixed with electronically conducting materials, such as acetylene black, and a binder, such as poly vinylidene fluoride PVDF or polytetrafluoroethylene. The working electrode may be flat, with a 1 cm2 surface, for example, a rotating disk electrode (RDE), or a microcavity electrode, or any geometrical convenient electrode. 

A rotating ring disk electrode has been manufactured at TNO and it was successfully tested. Preliminary experiments with some old complexes (Tinnemans et al., 1984) showed that these materials had deteriorated over time. The new complexes were either not yet available in the pure state or were intended for use in anhydrous solvent (2 in MeCN), where stability problems occur during electrochemical operation. 

For many applications, it is necessary to use metal disks embedded in electrical insulating materials, e.g., rotating disk electrodes. During preparation of these electrodes the following facts need to be taken into account. 

Fig. 10.12. General principles of the SECM feedback mode. The UME, normally a disk electrode of radius r, is used to generate a redox mediator in its oxidised or reduced form (a reduction process is shown here) at a diffusion-controlled rate. As the UME approaches an insulating surface (a) diffusion of Ox to the electrode simply becomes hindered and the recorded limiting current is less than the steady-state value measured when the electrode is placed far from the surface, in the bulk of the solution, /( >). This effect becomes more pronounced as the tip/substrate separation, dKcm, is decreased. As the UME approaches a conducting surface (b) the original form of the redox mediator (Ox) can be regenerated at the substrate establishing a feedback cycle and an additional flux of material to the electrode.

The RDE consists of a disk electrode embedded in an insulating rod material as shown in Fig. 14 (44). The composite electrode can be rotated about its axis, which causes electrolyte to be drawn up against, and forced outwards across, the face of the metal disk electrode, as shown in Fig. 15 (45). The convection and diffusion equations that describe solution flow in this situation have been rigorously established making this experimental approach a powerful one for the study of the effects of forced convection of electrochemical reactions. 

Hydrodynamic boundary layer — is the region of fluid flow at or near a solid surface where the shear stresses are significantly different to those observed in bulk. The interaction between fluid and solid results in a retardation of the fluid flow which gives rise to a boundary layer of slower moving material. As the distance from the surface increases the fluid becomes less affected by these forces and the fluid velocity approaches the freestream velocity. The thickness of the boundary layer is commonly defined as the distance from the surface where the velocity is 99% of the freestream velocity. The hydrodynamic boundary layer is significant in electrochemical measurements whether the convection is forced or natural the effect of the size of the boundary layer has been studied using hydrodynamic measurements such as the rotating disk electrode [i] and - flow-cells [ii]. 

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