Effective electrode area

In order to extend the effective electrode area in principle three-dimensional electrodes are possible, for example, by using a packed particle bed, a sintered or foamed metal, or a graphite fiber felt. But the depth of the working electrode volume usually is only small (it is dependent on the ratio of the electrode and electrolyte conductivity, for example, [45]). 

When [W(CN)s] " was coimmobilized with BOD and poly(L-lysine) on carbon felt sheet of 1-mm thickness on an RDE, a current density of 17 mA/ cm was observed at 0.4 V and 4000 rpm in oxygen-saturated phosphate buffer, pH 7. The authors partially attribute the high current density to convective penetration of the oxygen-saturated solution within the porous carbon paper electrode. This assertion is justified by calculation of an effective electrode area based on the Levich equation that exceeds the projected area of the experimental electrode by 70%. ° This conclusion likely applies to any... 

We stipulate the electrode to be smooth (though not necessarily flat) and of constant area A. By smooth we mean that any undulations in the electrode surface should not exceed the thickness of the double layer. For an electrode that is less smooth than this, the concept of electrode area is somewhat vague and the effective electrode area may change with time. By prescribing a constant electrode area, we exclude one of the most practical electrodes the dropping mercury electrode treated in Chap. 5. 

The former observation is concerned with the effective electrode area. In the early part of drop life, its size is similar to that of the capillary orifice. A significant part of the drop is thus not in contact with the solution, a fact which qualitatively explains the lower observed currents. Also, close to the capillary surface, the diffusion process will be restricted, the so-called shielding effect. This is particularly pertinent with modern polarographic equipment where mechanical drop timers are often used in conjunction with short drop times. These problems have been discussed recently [59]. The following modification was proposed... 

Fe ] and of the effective electrode area. Similar considerations apply to the rate of Fe oxidation (anodic current), which is proportional to [Fe " ], electrode area, and the exponential of the potential. It is obvious from the schematic representation that an infinitesimal shift of the electrode potential from its equilibrium value will make the half-reaction proceed in either of the two... 

Values of n may be obtained by almost any electrochemical technique, provided that a number of other parameters are known as illustrated, for example, by the relations between n and i given in Eqs. (13), (61), (64), (74), and (88). However, it is the exception rather than the rule that all the necessary parameters are known for a particular experiment. This applies especially to the effective electrode area A and diffusion coefficient Da, which is known only for few substances in the solvent systems commonly used in electrochemical studies. A simple solution to this kind of problem is based on the quantitative comparison of, for instance, LSV and CA [273]. From Eqs. (13) and (64) it is easily seen that the ratio R = (ip/v )/(it ) is given by the simple expression of Eq. (98), since all other parameters cancel. 

Since we are providing an average current density, (j), we also need to rationalize the value of the effective electrode area (AR) ... 

From the comparison of the resistance Ri and the Q factor one can infer the effective electrode area as ... 

The metal/electrolyte interface area is called the electrode area (EA). EA may be the plane area or include a surface roughness or fractal factor. The interface area of the contact medium and the tissue is called the effective electrode area (EEA). Skin surface electrodes are often EEA > EA (Eigure 7.30). Electrolyte-fiiied glass micro-electrodes are often EEA < EA. 

The area of the skin wetted or in contact with the electrolyte solution is called the effective electrode area of the electrode. EEA is a dominating factor determining electrode/skin impedance. EEA may he much larger than the metal area in contact with the solution (EA), which determines the polarization impedance. The electrode is fixed with a tape ring outside the EEA. Electrolyte penetrating the tape area increases EEA, hut reduces the tape sticking area. Pressure on the electrode does not squeeze electrolyte out on the skin surface because of the rigid container construction. 

The perception of a current through human skin is dependent on frequency, current density, effective electrode area, and skin site/condition. Current duration also is a factor, in the case of DC determining the quantity of electricity and thereby the electrolytic effects according to Faraday s law (see Section 7.8). 

The effective electrode area of one monopolar cell is 0.21 m, which makes this electrolyzer very compact [100]. The individual, lightweight electrodes are readily handled without the need for lifting devices, allowing the electrolyzer to be rebuilt or refurbished by a small crew in a short time. 

In the 1970s, the idea of photoelectrochemical cells for light to electrical and/or chemical energy conversion attracted scientists in various fields as a novel energy conversion device. Photoetching has been widely used to modify the semiconductor surface and consequently improve the cell efficiency [1, 2]. This improvement is attributed to several effects, such as a decrease in reflection losses (3), an increase in effective electrode area, a removal of surface defects acting as recombination centers [4-7], and change in the chemical composition of surface [8, 9j. 

Noise analysis has been particularly fruitfiil in characterizing various aspects of hydrodynamics, as noted above for the specific case of corrosion processes. First of all, multiphase flows were investigated, either gas/water [78], solid/liquid [79, 80], oil/water [81] or oil/brine [82]. In these flows, fluctuations are due primarily either to fluctuations in transport rates to an electrode or to fluctuations in electrolyte resistance. If one phase preferentially wets the electrode, then there may be fluctuations due to variation in the effective electrode area. Each of these phenomena has a characteristic spectral signature. Turbulent flows close to a wall have been investigated by means of electrochemical noise by using electrochemical probes of various shapes, by measuring the power spectral density of the limiting diffusion current fluctuations [83-86],... 

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. 

Protruding (1C) t Capacitance increases relative to ideal due to increased effective electrode area. 

The maximum cell current, since this determines the product output from the cell and, hence, the number of cells which must be purchased. The cell current depends on the total effective electrode area and the current density. The latter, in single-phase systems is, in turn, determined by the solubility of the substrate and the mass transport conditions. Hence, it is a useful rule of thumb to remember that a solubility of 1-10% for the clectroactive species will be necessary to achieve a current density > 0.1 Acm, ... 

Effective electrode area is 2 x 0.21 m per electrode, which gives a very compact electrolyzer. The individual electrodes are readily handled without the need for lifting apparatus, which allows the electrolyzer to be rebuilt and refurbished in the minimum of time. 

MEA 25, 28, and 29 were tested using a 4cm effective electrode area. The current density from MEA 28 and 29 was higher at 40 °C (Figure 8.8a 270 and 180 mA/cw respectively) than with MEA 10 11 (Figure 8.4 38 and 60mA/cm respectively). This is due to optimized flow rates and catalysts selection on the anode with MEA 28 and 29. 

Figure 3.6. Schematic illustration of the effective electrode area in (a) a metal-coated, glass nano-ES emitter and (b) a metal capiUary ES emitter. The necessary length of the electrode, I, sufficient for all of reduced species R to contact the electrode surface on passage through the electrode and react forming oxidized species O (complete electrolysis) is also illustrated in part b. A metal wire in solution to make electric contact is shown as an alternative electrode arrangement to a conductive contact at the tip part (part a). (Adapted with permission from Ref 60. Copyright 2000, Elsevier.)...

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