Geometric surface

When a battery produces current, the sites of current production are not uniformly distributed on the electrodes (45). The nonuniform current distribution lowers the expected performance from a battery system, and causes excessive heat evolution and low utilization of active materials. Two types of current distribution, primary and secondary, can be distinguished. The primary distribution is related to the current production based on the geometric surface area of the battery constmction. Secondary current distribution is related to current production sites inside the porous electrode itself. Most practical battery constmctions have nonuniform current distribution across the surface of the electrodes. This primary current distribution is governed by geometric factors such as height (or length) of the electrodes, the distance between the electrodes, the resistance of the anode and cathode stmctures by the resistance of the electrolyte and by the polarization resistance or hinderance of the electrode reaction processes. 

An effectiveness value greater than one indicates that the porous electrode is more effective than an electrode of the same geometric surface area, and that the reaction extends into the porous electrode stmcture. 

Fig. 9. Discharge and charging curves for a sintered iron electrode at a constant current of 0.2 A where the apparent geometrical surface area is 36 cm and porosity is 65%. A and B represent the discharging and charging regions, respectively. Overall electrode reactions, midpoint potentials, and, in parentheses, theoretical potentials at pH 15 ate Al, n-Fe + 2 OH Fe(OH)2 + 2, 0.88 V (1.03 V) B, Fe(OH)2 FeOOH + H+ +, 0.63 V (0.72 V) C,...

Catalytic Support Body Monolithic Honeycomb Unit. The terms substrate and brick are also used to describe the high geometric surface area material upon which the active coating material is placed. Monolithic honeycomb catalytic support material comes in both ceramic and metallic form. Both are used in automobile catalysts and each possesses unique properties. A common property is a high geometric surface area which is inert and does not react with the catalytic layer. 

The precious metal or metal oxide imparts high intrinsic activity, the carrier provides a stable, high surface area for catalyst dispersion, and the mechanical support gives a high geometric surface area for physical support and engineering design features (20). Only the correct combination of these... 

Liquid holdup is made up of a dynamic fraction, 0.03 to 0.25, and a stagnant fraction, 0.01 to 0.05. The high end of the stagnant fraction includes the hquid that partially fills the pores of the catalyst. The effective gas/liquid interface is 20 to 50 percent of the geometric surface of the particles, but it can approach 100 percent at high hquid loads with a consequent increase of reaction rate as the amount of wetted surface changes. 

The advanced all-metallic catalysts are believed to be formed by bonding active copper-nickel alloys onto stainless steel wires. Under the scanning electron microscope, it appears that the surface area may be more than twenty times the geometric surface area (42) ... 

The other important factor is a, the geometric surface area exposed to gas per volume of reactor, which depends on the void fraction and the dimension of the packing. The product of the transfer coefficient and the surface-to-volume ratio governs the rate of heat and mass transfer per... 

Control of emissions of CO, VOC, and NOj, is high on the agenda. Heterogeneous catalysis plays a key role and in most cases structured reactors, in particular monoliths, outperform packed beds because of (i) low pressure drop, (ii) flexibility in design for fast reactions, that is, thin catalytic layers with large geometric surface area are optimal, and (iii) attrition resistance [17]. For power plants the large flow... 

Studies performed on CdS [282, 283] have revealed the importance of the microstructure, i.e., crystal structure, crystallite size, and geometrical surface area, in both the control of band structure and the concentration and mobility of charges, in relation to the photocatalytic performance of the photocatalyst. It has been shown also that the solubility product of CdS colloids prepared from acetate buffer aqueous solutions of suitable precursors increases from 7.2x 10 for large particles to about 10 for small (< 2.5 nm) particle colloids, this increase invoking a positive shift on the cathodic corrosion potential [284]. 

The kinetics of this reaction are relatively slow (io from 10 to lO" A cm- referred to the geometric surface area, depending on the degree of dispersion of the platinum catalyst), which is the main cause of the high... 

GL 18] ]R 6a]]P 17/Using the same experimental conditions and catalysts with the same geometric surface area, the performance of micro-channel processing was compared with that of a fixed-bed reactor composed of short wires [17]. The conversion was 89% in the case of the fixed bed the micro channels gave a 58% yield. One possible explanation for this is phase separation, i.e. that some micro channels were filled with liquids only, and some with gas. This is unlikely to occur in a fixed bed. Another explanation is the difference in residence time between the two types of reactors, as the fixed bed had voids three times larger than the micro channel volume. It could not definitively be decided which of these explanations is correct. 

GL 18] [R 6a] [P 17] A sol-gel deposited catalyst used in a fixed-bed reactor gave higher conversion than a micro-channel catalyst impregnated on a porous alumina layer [17]. This was due to the higher geometric surface area of the sol-gel deposited catalyst. 

In industrial electrochemical cells (electrolyzers, batteries, fuel cells, and many others), porous metallic or nonmetallic electrodes are often used instead of compact nonporous electrodes. Porous electrodes have large trae areas, S, of the inner surface compared to their external geometric surface area S [i.e., large values of the formal roughness factors y = S /S (parameters yand are related as y = yt()]. Using porous electrodes, one can realize large currents at relatively low values of polarization. 

Figure 11.11 Linear cyclic voltammograms of carbon-supported nanosized Pt and Pt-Cr alloy catalysts with different atomic ratios (prepared using the carbonyl route [Yang et al., 2004]) recorded in 0.5 M HCIO4 saturated with pure oxygen at a scan rate of 5 mV s and a rotation speed of 2000 rev min Current densities are normalized to the geometric surface...

Of direct interest for biofuel cell applications are the reported reduction of O2 by multi-copper oxidases on carbon nanotube electrodes [Yan et al., 2006 Zheng et al., 2006] and the oxidation of H2 by hydrogenase covalently bound to carbon nanotubes [Alonso-Lomillo et al., 2007]. The hydrogenase/nanotube anode is extremely stable (>1 month), and shows 33-fold enhanced enzyme coverage compared with similarly treated graphite of the corresponding geometric surface area. A. vinosum... 

Comparing the above-mentioned results with the data of morphological survey of islet films of gold on a ZnO surface [116- 168] leads to an inference that the maximum of curve 1 is associated with changes in the geometric surface of the Au islet film as it grows, while the maximum at curve 2 is connected with changes in the mean size of microcrystals in the islet film. 

The charge density at any point is X, the sum being over species of both phases. Note that no geographical separation of components of the phases is required, anticipating that, in the real system, it may not be possible to divide the system such that all the components of phase A lie on one side of a geometrical surface and all the components of phase B on the other. It is necessary only to identify each component as belonging to one phase or the other. In a geographical separation, one would write... 

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