True surface area

The table convincingly demonstrates how the unsuspected presence of micropores can lead to an erroneous value of the specific surface calculated from a Type II isotherm by application of the standard BET procedure. According to the foregoing analysis, the external specific surface of the solid is 114m g" the micropore volume (from the vertical separation of isotherms A and E) is 105 mm g but since the average pore width is not precisely known, the area of the micropore walls cannot be calculated. Thus the BET figure of 360m g calculated from isotherm E represents merely an apparent and not a true surface area. 

In uniform corrosion the superficial or geometrical area of the metal is used to evaluate both the anodic and cathodic current density, although it might appear to be more logical to take half of that area. However, surfaces are seldom smooth and the true surface area may be twice to three times that of the geometrical area (a cleaved crystal face or an electropolished single crystal would have a true surface area that approximates to its superficial area). It follows, therefore, that the true current density is smaller than the superficial current density, but whether the area used for calculating /, and... 

In lithium-ion batteries, with carbonaceous anodes, (7IK can be lowered by decreasing the true surface area of the carbon, using pure carbon and electrolyte, applying high current density at the beginning of the first charge, and using appropriate electrolyte combinations. 

Leikis, method for obtaining true surface area, 46... 

True surface area, origination of method for obtaining (Leikis), 46 Turnover numbers, for minority carriers, 494... 

Gext = K4 has passed the external circuit (the values of g xt are always referred to the unit surface area of the electrode) is shown in Fig. 10.9. From the slope of the curve one can determine the electrode s capacitance C. In this method there is no need for complex equipment it is very convenient for samples with large true surface areas (highly disperse deposits, powders, etc.). 

By using the valne Q = 220 J,C/cm, one can readily determine the true surface area of any compact or disperse sample of platinnm without adducing any other methods, simply by measuring the total amount of charge required to accomplish the potential shift. 

Porous electrodes are systems with distributed parameters, and any loss of efficiency is dne to the fact that different points within the electrode are not equally accessible to the electrode reaction. Concentration gradients and ohmic potential drops are possible in the electrolyte present in the pores. Hence, the local current density, i (referred to the unit of true surface area), is different at different depths x of the porous electrode. It is largest close to the outer surface (x = 0) and falls with increasing depth inside the electrode. 

Glassy carbon is obtained by specifically controlled thermal decomposition of certain carbonaceous materials. Because of its almost ideally smooth, glasslike surface, it is the favorite material for laboratory studies demanding an exact knowledge of the true surface area. The electrochemical and other properties of glassy carbon depend on the temperature at which it was produced. 

Although the true surface area increases with increasing exposure time, Figure 4 shows that the impact of heterogeneity on electrochemical measurements is established quickly and remains constant with time. 

If ordered structures involving aromatic polycyclic skeletons of the type studied here were present in coal, it would indicate that such structures would be permeated by methanol, and that surface area measurements based on methanol sorption would give values greater than the true surface area as that area is normally defined. 

Several attempts have been made to correlate the adsorptivity of hydrolyzable cations to the composition of the species in aqueous solution (1, 2, 20). In particular, the adsorption of thorium on silver halides indicated a very close relationship between the change in the amount of thorium adsorbed and the concentration of the hydrolyzed soluble species in solution (19). The major difficulty in this type of work is the lack of quantitative data on the hydrolysis of various metal ions. The other uncertainty is with regard to the knowledge of the true surface area of the adsorbent in aqueous solution. This latter information is needed if surface coverages are to be evaluated. 

The 9-parameter is also subject to uncertainty. It is likely that Whitby s (1978) estimates of 9 (Table 10.1) do not reflect the true surface area distribution, since they were based on the average size spectrum of aerosols and assumed spherical particles. Adsorption/desorption kinetics (Kamens et al., 1995 Rounds and Pankow, 1990,1993) and relative humidity (Goss and Eisenreich, 1997 Lee and Tsai, 1994 Pankow et al., 1993 Storey et al., 1995 Thibodeaux et al., 1991) influence the adsorption of POPs onto aerosols, and the Junge-Pankow model does not take these factors into account. 

Measurement of Electrode Area. Because of surface toughness, the real or true surface area of a solid electrode is greater than the projected or geometric area. However, if the electrode is polished to a smooth surface finish, this will be of no consequence in most voltammetric work. The depth of the depleted region around the electrode surface (the diffusion-layer thickness) is substantially larger than the characteristic dimensions of surface toughness for electrolysis times that are greater than 1 s. [The diffusion-layer thickness may be crudely approximated by the term (Dt)m, where D is the diffusion coefficient (cm2 s"1) and t is the time.]... 

If the latter explanation is correct, it follows that the value of As as derived by either BET or Langmuir analysis (In fact, many microporous solids do not give linear BET plots although their Langmuir plots may be linear over an appreciable range of p/p°) cannot be accepted as the true surface area of a microporous adsorbent. On the other hand, if the slope of the isotherm is not too low at higher p/p° and provided that capillary condensation is absent, it should (in principle) be possible to assess the external surface area from the multilayer region. 

The results of studies of copper surfaces by low-temperature adsorption isotherms may be summarized as follows. True surface areas of metallic specimens as small as 10 sq. cm. can be derived with a precision of 6% from low-temperature adsorption isotherms using vacuum microbalance techniques. This method is of special value in determining the average thickness of corrosion films formed by the reaction of gases or liquids with solids. The effect of progressive oxidation of a rough polycrystalline metal surface is to decrease the surface area to a point where the roughness factor approaches unity. 

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