Two-dimensional electrodes

Two-Dimensional Electrode Flow Cells. The simplest and least expensive cell design is the undivided parallel plate cell with electrolyte flow by some form of manifold. Electrical power is monopolar to the cell pack (72). An exploded view of the Foreman and Veatch cell is shown in Figure 7. Note that electrolyte flow is in series and that it is not easily adapted for divided cell operation. 

Two-dimensional (2D) separation systems in microfluidic assays, 26 970 Two-dimensional (2D) transistor strustures, in scaling to deep submicron dimensions, 22 256 Two-dimensional data searches, 6 6-8 Two-dimensional electrode flow cells, 9 664-665... 

The electrochemical reduction of nitrobenzene to produce p-aminophenol has attracted industrial interest for several decades. However, some limitations may be met in this process regarding overall reaction rate, selectivity and current efficiency using a two-dimensional electrode reactor. These restrictions are due to the organic electrode reaction rate being slow and to the low solubility of nitrobenzene in an aqueous solution. In this example, a packed bed electrode reactor (PBER), which has a large surface area and good mass transfer properties, was used in order to achieve a high selectivity and a reasonable reaction rate for the production of p-aminophenol. The reaction mechanism in an acid solution can be simplified as... 

The properties characteristic for electrochemical nonlinear phenomena are determined by the electrical properties of electrochemical systems, most importantly the potential drop across the electrochemical double layer at the working electrode (WE). Compared to the characteristic length scales of the patterns that develop, the extension of the double layer perpendicular to the electrode can be ignored.2 The potential drop across the double layer can therefore be lumped into one variable, DL, and the temporal evolution law of DL at every position r along the (in general two-dimensional) electrode electrolyte interface is the central equation of any electrochemical model describing pattern formation.3 It results from a local charge bal-... 

Although neither a wire nor a thin ribbon is a strict example of a WE supporting base geometry, we can expect that for both cases, i.e. essentially one-dimensional electrodes in a three-dimensional electrolyte, edge effects are much less pronounced than in the case of two-dimensional electrodes surrounded by an insulating plane. Hence, we can tentatively interpret the observations by assuming that the accelerated... 

In the present context, we are interested in how best to simulate electrochemical processes at a two-dimensional electrode. The flat disk, the UMDE, is taken as the only example, as the techniques that have been developed for it are the same as those for the other geometries. 

Packed bed electrode — A static three-dimensional - electrode consisting of a restrained bed of electronically conducting particles in continuous intimate contact. Packed Bed Electrodes (PBEs) present high electroactive area per unit electrode volume and moderately high -> mass transport characteristics (the limiting current at a PBE may exceed 100 times the one observed at a two-dimensional electrode of the same volume). 

In classic preparative electrochemistry and in most research work, two-dimensional electrodes have been used, partly because it is simpler to control the different parameters at such electrodes and partly because time-space yield is not a deciding factor for laboratory cells. 

Current and power densities achieved with electrodes using the direct electron transfer approach will be limited, however, because of the need to have intimate contact between the two-dimensional electrode surface and a coating monolayer of correctly oriented biocatalyst. The use of small redox molecules that can mediate electron transfer between the biocatalyst and the electrode surface offers an opportunity to improve output from biocatalytic electrodes, as three-dimensional films of biocatalysts may now be used. In addition the distance between the active site of the enzyme and the electrode surface is often too great to allow efficient direct electron transfer. In these cases the electron transfer rate is not effective because of the insulation of the redox active site by the surrounding protein. A redox mediator can shuttle electrons between the enzyme and the surface. In the example of redox mediated biocatalytic oxidation of a fuel, depicted in Fig. 12.3, the enzyme catalyzes the oxidation of the mediator... 

Two-dimensional electrodes. Two-dimensional electrodes appropriate for metal recycling are typically based on the tank electrolyser to enable ready removal of metal plated electrodes. The simplest cells are the vertical, plate or mesh, electrode in tank units where turbulence is provided by using either inert fluidised beds [8] (Chemelec Cell, BEWT Water Engineers Ltd.) or air agitation (Reconwin cell), in conjunction with electrolyte pumping (see Figure 11.3). 

Cells design with two- and three-dimensional electrodes have been also used for the oxidation of organic compounds. Among the two-dimensional electrodes, the cells containing dimensional stable anodes (DSA) and boron-... 

In most industrial electrochemical processes (such as in chlor-aUcali and metal electrowinning industries), the use of plate (two-dimensional) electrodes is quite adequate since the electroactive species are present at very high concentrations. Although the main advantages of two-dimensional electrodes are addressed as to their construction and operational simplicity, surface area and mass transfer limitations would be impeditive for their use in wastewater treatment since the typical metal concentrations are very low [2]. In order to overcome such disadvantages of two-dimensional electrodes, the development of three-dimensional electrodes was proposed. 

Three-dimensional electrodes show more problems associated with potential and current distribution than two-dimensional electrodes. The anisotropy of porous or particulate... 

FIGURE 5.2. Classification scheme based on current distribution, (a) Two-dimensional electrode (b) three-dimensional electrode (c) counter electrode (d) electrolyte flow. 

In the third, space-time yield, the size of the cell is directly proportional to a, the electrode area. Table 5.4 gives values of a for four typical designs (see Section 5.1). The effective electrode area for three-dimensional electrodes is smaller than what is shown in Table 5.4 nevertheless, it will be at least an order of magnitude larger than that for two-dimensional electrodes. In practical terms of cell selection (particularly for small-tonnage chemicals), small differences in cell volume are largely academic because cells are often dwarfed by separation equipment such as distillation columns. 

Another composite electrode is constructed by filling the voids of reticulated vitreous carbon with nonconductive epoxy to produce two-dimensional electrode materials (25). Reticulated vitreous carbon has been used as a three-dimensional highly porous electrode material and has been used in flowing systems as well as in thin slices as an optically transparent electrode. 

Concentric cells provide another method of maintaining a uniform and small interelectrode gap. The cell shown in Fig. 2.33(b) was developed as a simple annutar-flow tubular reactor for laboratory and pilot-scale organic electro-synthesis. While the space-time yield is relatively low (due to the use of essentially two-dimensional electrodes and a dead space within the inner electrode) the reactor is robust and a separator may be incorporated much more readily than for the stacked-disc cell The resulting reactor provides a convenient modular flow-through cell, which has been utilized for industrial-scale processes. 

A regular array of (essentially two-dimensional) electrodes, e.g. meshes or perforated plates. 

While two-dimensional electrodes are simpler to prepare, a growing number of detectors utilize porous electrodes, including reticulated vitreous carbon. Three-dimensional electrodes are particularly suited to coulometric cells. (Fig. 12.21a)... 

It has already been mentioned that three-dimensional, porous, electrodes offer particularly high values of the electroactive area per unit reactor volume whilst also giving a moderate increase in the mass transport coefficient The result is a significantly increased performance from a given volume of reactor, compared to two-dimensional electrode materials, due to the high value of fct s-This performance may be utilized in various ways including ... 

The performance of porous, three-dimensional electrodes is often difficult to predict and scale-up may be less predictable than for two-dimensional electrodes. 

---