The assessment of surface areas in porous carbons by two model-independent techniques, the DR equation and DFT

The strong linear correlation observed between S-BET and the micropore volume of 190 carbons with pore widths between 0.5 and 1.8 nm confirms the unreliable character of S-BET, in spite of its frequent use. (It corresponds approximately to 2200-2300 m(2) per cm(3) of micropores, whatever their width). Alternative determinations of the surface area are therefore required. It is shown that two model-independent techniques (Kaneko's comparison plot for nitrogen and the enthalpies of immersion into aqueous solutions of phenol) and two model-dependent approaches (Dubinin's theory and DFT) lead to total surface areas which are in good agreement. Their average S-av is probably a reliable assessment of the total surface area. It is often in disagreement with S-BET, but a closer study of 42 well characterized microporous carbons, for which all four techniques are available, shows that the ratio S-BET/S-av increases linearly with the average pore width. This should be taken into consideration when surface-related properties (e.g., densities of chemical groups or adsorbed species, specific capacitances) are examined on the basis of a single determination and in particular on the BET technique. (C) 2010 Elsevier Ltd. All rights reserved.