Superficial area, electrode

Weaver [40] studied alternate cathode materials at 650 °C, finding several that performed well. Steady-state polarization on Ni, Co and Fe porous electrodes operating as cathodes in a MCFC, with a standard (Li/K)2 C03 tile is shown in Figs. 30-32. Note that the oxidant gas fed to these cathodes is, in normal MCFC operation, the fuel, composed of 32.5% H2, 17.5% COz, 17.5% H20, the balance N2. Polarizations were first taken with this clean gas where the only reaction can be Eq. (35). After steady-state was attained, 0.65% H2S was added and sufficient time allowed for the electrode to convert to the sulfides. After 24 hours, the outlet H2S reached the inlet level and polarizations were measured. Note in Figs. 30-32, that the performance with H2S is significantly improved over the clean gas. (The Ni sample was a commercial (Gould) MCFC electrode the Co and Fe were pressed from powders. Each gas was 8 sq cm in superficial area). The improvement is probably due to a catalytic mechanism involving sulfur interactions with the electrode, as, for Co ... 

A new approach to improve the performance of solar devices using natural pigments is to employ carbon nanotube (CNT)-based counter-electrodes. As previously reported, the excited dye transfers an electron to Ti02 and so it acquires a positive charge. Then, the cationic molecule subtracts an electron from the counterelectrode which is transported by the electrolyte. This reaction is usually catalyzed by means of conductive and electrocatalytically active species for triiodide reduction of carbon coatings. CNTs have a high superficial area, which represents a very... 

It is useful to define an active-area density a as the local fraction of the cathode area that is electroactive (i.e., the fraction that is not blocked by resist). This concept is illustrated in Fig. 2. In places where a large fraction of the electrode surface is blocked, a is low. Further, it is useful to define two different measures of current density. Let the active current density / act represent the current per unit of electroactive area (remembering that some fraction of the electrode area is blocked by resist). Let the superficial current density /gup represent the current per unit superficial area (including both the resist-covered and the electroactive portions). The relationship between /act and /gup is illustrated in Fig. 3. The physical basis for... 

Porous electrodes are used in fuel cells in order to maximize the interfacial area of the catalyst per unit superficial area. As described above, this is critical not only for lowering the total amount of catalyst needed for the fuel cell but also to increase the power that can be delivered per unit superficial area. The... 

The values of k, D, and a used in Equations 3, 12, and 4, lespeetively, are reduced within the porous electrode relative to their bulk values due to the tortuous path which the ions in solution must make around the solid particles or which the electrons must make around the electrolyte-fiUed pores. Generally, the effect of volume fraction e, and tortuosity of the phase of interest on property P in that phase is accounted for by the Bmggeman relation [82], P = EpPoo/T where x is the tortuosity and is usually related to the porosity by t = s . Because the flux N is defined with respect to superficial area as opposed to electrolyte area, D already gets multiplied once by e in the mass balance, so D = Dj ... 

Surface and Double-layer Properties Valette [19] has analyzed earlier experimental data on the inner-layer capacity at PZC for Ag(lll), Ag(lOO), and Ag(llO) surfaces in order to estimate the surface area and capacitance contributions of superficial defects for real electrodes, as compared to ideal faces. Considering the application of surface spectroscopy techniques to single-crystal Ag electrodes, one should note that anisotropy of the SHG response for metal electrode allows one to analyze and correlate its pattern with interfacial symmetries and its variations by changing nonlinear susceptibility and the surface structure. Early studies on Ag(lll) single-crystal electrodes have... 

Noise analysis obtained from microelectrochemical investigations of stainless steels under potentiostatic conditions revealed that the current noise, expressed as standard deviation a of the passive current, increases linearly with the size of the exposed area, whereas the pitting potential decreases.47 However, to complete the electrochemical studies and distinguish between repassivating superficial pits and penetrating ones, microscopic studies are highly desirable. The scanning reference electrode technique (SRET) should be an appropriate complementary tool.28... 

In electrocatalysis, the activity of different electrocatalysts is usually expressed via the exchange current I0, and the specific activity, via the exchange current density, iQ (A cm-2), still often computed on the basis of the superficial electrode surface area. Only when the current is normalized using the true surface area of the electrode-electrolyte interface, the comparison between different electrocatalysts is truly meaningful. The determination of the true surface area of porous electrodes is discussed in Sect. 2.3.5. 

Thus, in the metal/YSZ systems of solid-state electrochemistry, AC-impedance spectroscopy provides concrete evidence for the formation of an effective electrochemical double layer over the entire gas-exposed electrode surface. The capacitance of this metal/gas double layer is of the order of 100-500 pF cm-2 of superficial electrode surface area and of the order 2-10 pF cm-2 when the electrode roughness is taken into account and, thus, the true metal/gas interface surface area is used, comparable to that corresponding to the metal/solid electrolyte double layer. Furthermore AC-impedance spectroscopy... 

The Oj-H cell reactor and the principle of the method for the oxygenation of hydrocarbons are demonstrated in Figure 1. A detailed description of the cell setup has been given elsewhere [13]. A silica-wool disk (thickness 2.0 mm and diameter 26 mm) impregnated with aqueous HjPO., (IM, 1 ml) as an electrolyte separates the anode and cathode compartments. The anode was prepared by a hot-press method from a mixture of graphite, Pt-black and Teflon powder. The cathodes were prepared from a mixture of carbon whiskers with various metal oxides or metal salts. The superficial surface area of both electrodes was 3.1 cm. ... 

Two main types of models for tubular lamps (the most widely used) will be described. There are lamps that produce an arc that emits radiation and, consequently, photons come out directly from such an arc. Emission is made by the whole lamp volume. We call this process Voluminal Emission. There are other types of lamps in which the discharged arc between electrodes induces an emission produced by some particular substance that has been coated on the lamp surface. We call this process Superficial Emission. Voluminal emission may be safely modeled as an isotropic emission in this case the specific intensity associated with each bundle of radiation originated in some element of volume of the lamp is independent of direction, and the associated emitted energy (per unit time and unit area) is also isotropic (Figure 6.6). On the other hand, it seems that superficial emission can be better modeled by a diffuse type of emission that is also known as one that follows the Lambert s cosine law of emission in this case the emitted intensity is independent of direction but the emitted energy depends on the surface orientation and follows the cosine law equation (Figure 6.7). The following assumptions are made (Irazoqui etal., 1973) ... 

In the first case the atom density in the surface remains constant at least as long as no restructuring occurs. The transfer of atoms from the bulk into the newly created surface increases the surface area. As the process is the same as for liquid electrodes, this may be termed surface energy a. In the review of Linford this contribution is called superficial work. ... 

It is current practice to impregnate the graphite, traditionally with linseed oil, although synthetic resins are also successful. The concept behind impregnation is to reduce the porosity and hence inhibit subsurface gas evolution or carbon oxidation which would initiate spalling and early anode failure. Electrode processes occur to a depth of 0-5 mm below the surface of the anode and the true current density can be shown to be only I/400th of the value indicated by the superficial geometrical area Acidity has been found to increase the wear rate and so has the presence of sulphate... 

Pt particles supported on high surface-area carbon substrates to form supported catalyst (abbreviated as Pt/C) is the most widely used ORR catalyst at the current state of technology. The surface characteristics of Pt particles supported on carbon are similar to that of Pt polycrystalline electrode surface, which can be observed by their surface CVs. However, the ORR activity of Pt/C is different from what can be observed for polycrystalline Pt surface. This difference is possibly due to some differences in the superficial structure, or a too strong adsorption of oxygenated species on very small particles. In addition, the ORR activity of carbon-supported Pt particles might also be affected by the electronic properties of Pt atoms from carbon. 

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