Hydrodynamic electrode materials

A reversible one-electron transfer process (19) is initially examined. For all forms of hydrodynamic electrode, material reaches the electrode via diffusion and convection. In the cases of the RDE and ChE under steady-state conditions, solutions to the mass transport equations are combined with the Nernst equation to obtain the reversible response shown in Fig. 26. A sigmoidal-shaped voltammogram is obtained, in contrast to the peak-shaped voltammetric response obtained in cyclic voltammetry. 

The first hydrodynamic electrode to be invented was the dropping mercury electrode [1]. It has a cyclic operation and can thus be considered only as quasi-steady-state its hydrodynamic character derives from drop growth. The principal advantage of a dropping electrode is that a fresh electrode surface is constantly exposed to the solution however, there are few electrode materials available and mathematical solution of the mass transport to the drop surface is complicated by the fact that the surface is expanding. 

The development of solid hydrodynamic electrodes, which have the advantage of fixed area and a wide range of available electrode materials, occurred rather later. This was due mainly to the lack of a theoretical description of the mass transport. Levich s work on mass transfer to electrodes, which was largely unknown to the non-Russian-speaking world except for the occasional indication, e.g. ref. 2, only became widely available with the publication in 1962 of the English translation of Physicochemical Hydrodynamics [3]. This dealt with many... 

Choice of electrode materials depends usually on potential range in the solvent being studied and available purity. In the case of hydrodynamic electrodes, we also have to consider carefully the ease of machining in order to conform to the shapes and forms required by the theoretical equations. 

Chapter 1 serves as an introduction to both volumes and is a survey of the fundamental principles of electrode kinetics. Chapter 2 deals with mass transport — how material gets to and from an electrode. Chapter 3 provides a review of linear sweep and cyclic voltammetry which constitutes an extensively used experimental technique in the field. Chapter 4 discusses a.c. and pulse methods which are a rich source of electrochemical information. Finally, Chapter 5 discusses the use of electrodes in which there is forced convection, the so-called hydrodynamic electrodes . 

Since the electrode has to be transparent, the electrode material is limited to thin films of metals or semiconductors deposited on a transparent substrate (for example a thin film of tin(IV) oxide or platinum on quartz) or to very fine grids of the electrode material, as shown in Fig. 9.13. The first of these two options is preferable, since the transmission coefficient is uniform and the electrode can be truly planar, and as such can be used as a hydrodynamic electrode, for example. The change in absorbance with time due to one of the reagents or products of the electrode reaction characterizes the mechanism. 

The most important selectivity parameter of electrodes for voltammetric sensors is the applied potential. Ideally, the electrode potentials of the redox couples would be sufficiently far apart for there to be no interference between different species. Unfortunately this is not the case, and it is necessary to look for greater selectivity. We can discriminate better between the different species present in solution through a correct choice of conditions for the study of the electrode reaction electrode material (Chapter 7) in some cases through surface modification use of hydrodynamic electrodes (Chapter 8) application of potential sweep... 

Working electrodes which have material reaching them by a form of forced convection are known as hydrodynamic electrodes. There is a wide range of hydrodynamic electrodes rotating-disc electrodes (Albery and Hitchman, 1971), in which the electrode rotates at a fixed frequency and sucks up material to its surface, and channel electrodes (Compton et al., 1993c), over which the electroactive species flows at a fixed volume flow rate, are the primary ones used in the work described in this review (Section 4). 

The aim of the ED step is to provide a compact, adherent, laterally uniform precursor film with the desired stoichiometry for commercial appHcations, lateral compositional uniformity needs to be guaranteed over large areas. The desired properties can only be obtained if the ED process is properly controlled. The nature of deposited layers depends on electrode material, precursor species in solution, applied potential program, hydrodynamic conditions, and temperature. Eor single metal systems, these factors are well understood. Eor deposition of multiple elements including a chalcogen, the ED process becomes substantially more complex. 

The differences between the diffusion-controlled conditions in cyclic voltammetry and the hydrodynamic conditions in HPLC detection frequently result, in the case of oxidations, in the optimum potential being shifted slightly to more positive values. Anderson et al. [7], have attempted to quantify the difference between the optimum potential for operation under hydrodynamic conditions and the peak and half-peak potentials obtained from cyclic voltammetry. Although this may have application where well-resolved cyclic voltammograms are obtained, in the authors experience many compounds that are less easily oxidized do not show well-resolved cyclic voltammograms at glassy carbon. (The method requires that the same electrode material... 

Overpolentials at each electrode due to charge transfer (and, hence, on the electrode material) or mass transport edects (and, hence, on the local hydrodynamics). 

One of these is the "shape change" phenomenon, in which the location of the electrodeposit is not the same as that of the discharge (deplating) process. Thus, upon cycling, the electrode metal is preferentially transferred to new locations. For the most part, this is a problem of current distribution and hydrodynamics rather than being a materials issue, therefore it will not be discussed further here. 

Convection. This is the physical movement of the solution in which the electroactive material is dissolved. In practice, convection arises from two causes, i.e. from deliberate movement of the solution, e.g. by mechanical stirring (sometimes called hydrodynamic control, see Chapter 7) or, alternatively, convection is induced when the amount of charge passed through an electrode causes localized heating of the solution in contact with it. The convective stirring in such instances occurs since the density p of most solvents depends on their temperature typically, p increases as the temperature decreases. 

The rotating disc electrode is constructed from a solid material, usually glassy carbon, platinum or gold. It is rotated at constant speed to maintain the hydrodynamic characteristics of the electrode-solution interface. The counter electrode and reference electrode are both stationary. A slow linear potential sweep is applied and the current response registered. Both oxidation and reduction processes can be examined. The curve of current response versus electrode potential is equivalent to a polarographic wave. The plateau current is proportional to substrate concentration and also depends on the rotation speed, which governs the substrate mass transport coefficient. The current-voltage response for a reversible process follows Equation 1.17. For an irreversible process this follows Equation 1.18 where the mass transfer coefficient is proportional to the square root of the disc rotation speed. 

All the electrode kinetic methodology described until now has assumed a steady state (or quasi-steady state in the case of the DME). Many techniques at stationary electrodes involve perturbation of the potential or current in combination with forced convection, this offers new possibilities in the evaluation of a wider range of kinetic parameters. Additionally, we have the possibility of modulating the material flux, the technique of hydrodynamic modulation which has been applied at rotating electrodes. Unfortunately, the mathematical solution of the convective-diffusion equation is considerably more complex and usually has to be performed numerically. 

Since the electrochemical reduction or oxidation of a molecule occurs at the electrode-solution interface, molecules dissolved in solution in an electrochemical cell must be transported to the electrode for this process to occur. Consequently, the transport of molecules from the bulk liquid phase of the cell to the electrode surface is a key aspect of electrochemical techniques. This movement of material in an electrochemical cell is called mass transport. Three modes of mass transport are important in electrochemical techniques hydrodynamics, migration, and diffusion. 

In electrophoresis an electric field is applied to a sample causing charged dispersed droplets, bubbles, or particles, and any attached material or liquid to move towards the oppositely charged electrode. Their electrophoretic velocity is measured at a location in the sample cell where the electric field gradient is known. This has to be done at carefully selected planes within the cell because the cell walls become charged as well, causing electro-osmotic flow of the bulk liquid inside the cell. From hydrodynamics it is found that there are planes in the cell where the net flow of bulk liquid is zero, the stationary levels, at which the true electrophoretic velocity of the particles can be measured. 

A mathematical description of an electrochemical system should take into account species fluxes, material conservation, current flow, electroneutrality, hydrodynamic conditions, and electrode kinetics. While rigorous equations governing the system can frequently be identified, the simultaneous solution of all the equations is not generally feasible. To obtain a solution to the governing equations, we must make a number of approximations. In the previous section we considered the mathematical description of electrode kinetics. In this section we shall assume that the system is mass-transport limited and that electrode kinetics can be ignored. 

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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