Anodes real surface area

As was discussed in section 2.1.1, electrocapillarity measurements at mercury electrodes, which have well-defined and measurable areas, allow the double-layer capacitance, CDL, to be obtained as Fm-2. Bowden assumed that the overpotential change at the very beginning of the anodic run in H2-saturated solution was a measure of the double-layer capacity. The slope of the E vs. Q plot in this region was taken as giving 1/CDL, and this gave 2 x 10 5 F. He then assumed that, under these same conditions, the double-layer capacity, in Fm-2, of the mercury electrode is the same. This gave the real surface area of the electrode as 3.3cm 2, as opposed to its geometric area of I cm2. 

The surface state capacitance for t i CdTe-electrolyte interface is plotted as a function of electrode potential in Fig. 16 (the minimum was taken as the value at 0.2V NHE). The surface state capacitance decreases in the cathodic direction in the region -0.56 to -2.26V (NHE). Capacitance measurements at cathodic potentials less negative than -0.56V could not be carried out because of the onset of a C02 independent anodic dark current. Assuming (in consistence with other examples of pseudo capacitance behavior) that the capacitance-potential curve is symmetrical with respect to a maximum at -0.66V, the number of surface states was calculaed using the above equation. The number of surface states as a function of electrode potential, on the basis of this assumption, is shown in Fig. 17. Geometric area of the electrode was used to calculate the surface state density. Real surface area may be larger. 

Novel types of synthesis of modem electrocatalysts revealed that the properties of electrode materials can be affected by the controlled formation of nano-sized, finely dispersed, electrocatalyst particles. In the case of DSA, already the traditional preparation procedure involves the thermal decompositiOTi of the corresponding chlorides after dissolution in an appropriate solvent (usually a solvent of low viscosity, e.g., 2-propanol) [3], Recently, sol-gel synthesis was introduced for DSA preparatirMi, with the main effect being related to the increase in the real surface area of the anode [9,10], The effect is recognized as the geometric factor of increased electrocatalytic ability in addition to an electronic factor related to the chemical structure of electrocatalyst [2,4], which is essential for step (6). The geometric factor is important since the measure for the reaction rate is the current density, i.e., the current per surface area of the electrode available for the reaction. Thus, the reaction rate can be considerably increased by the application of nano-3D electrodes, which are porous systems with an extended real surface area. The polarization curves for the CER on... 

The ship has two propellers and two rudders. The propellers were made of nickel-aluminium-bronze alloy (NAB) and modelled as solid disks with a surface area equivalent to the real propellers. The shaft is made of carbon steel and the propellers and shafts were assumed to be uncoated because of turbulence engendered by propeller movement. The ship s hull and rudders are also made of carbon steel, which were coated to prevent corrosion. The ICCP system evaluated included four anodes and a centre controlled power supply. The half of ship BEM model was shown in figure 2. 

The processes in real corroding systems are obviously more complicated than represented by this model. Useful quantitative calculation of the distribution of current density, and hence corrosion rate along the surface, based on the polarization curves for the anodic and cathodic reactions and on the geometry of the anodic and cathodic sites is very complex. In principle, computer-based techniques can be used if exact polarization curves and the geometry of the anodic and cathodic areas are available. For most industrially important situations, this information is not available. Also, time-dependent factors, such as film formation, make quantitative calculations of long-time corrosion rates very uncertain. The theory underlying these calculations, however, has been useful in interpreting observations in research and in industrial situations. 

It is also important to realize that most textbooks present corrosion current data as current densities. The main reason for that is simple Current density is a direct characteristic of interfacial properties. Corrosion current density relates directly to the penetration rate of a metal. If one assumes that a metallic surface plays equivalently the role of an anode and that of a cathode, one can simply balance the current densities and be done with it. In real cases this is not so simple. The assumption that one surface is equivalently available for both processes is indeed too simplistic. The occurrence of localized corrosion is a manifest proof that the anodic surface area can be much smaller than the cathodic. Additionally, the size of the anodic area is often inversely related to the severity of corrosion problems The smaller the anodic area and the higher the ratio of the cathodic surface Sc to the anodic surface Sa, the more difficult it is to detect the problem. 

CI2 evolution reaction, 38 56 electrochemical desorption, 38 53-54 electrode kinetics, 38 55-56 factors that determine, 38 55 ketone reduction, 38 56-57 Langmuir adsorption isotherm, 38 52 recombination desorption, 38 53 surface reaction-order factor, 38 52 Temkin and Frumkin isotherm, 38 53 real-area factor, 38 57-58 regular heterogeneous catalysis, 38 10-16 anodic oxidation of ammonia, 38 13 binding energy quantification, 38 15-16 Haber-Bosch atrunonia synthesis, 38 12-13... 

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