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

The strong linear correlation observed between SBET and the micropore volume of 190 carbons with pore widths between 0.5 and 1.8nm confirms the unreliable character of SBET, in spite of its frequent use. (It corresponds approximately to 2200–2300m2percm3 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 Sav is probably a reliable assessment of the total surface area. It is often in disagreement with SBET, but a closer study of 42 well characterized microporous carbons, for which all four techniques are available, shows that the ratio SBET/Sav 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.