On the determination of surface areas in activated carbons

Abstract

The paper examines the validity of two approaches frequently used to determine surface areas in activated carbons, namely the BET method and the use of immersion calorimetry. The study is based on 21 well characterized carbons, whose external and microporous surface areas, S e and S mi , have been determined by a variety of independent techniques. It appears clearly that S BET and the real surface area S mi + S e are in agreement only for carbons with average pore widths L o around 0.8-1.1 nm. Beyond, S BET increases rapidly and S BET À S e is practically the monolayer equivalent of the micropore volume W o . This confirms that a characterization of surface properties based on S BET is, a priori, not reliable. The study of the enthalpy of immersion of the carbons into benzene at 293 K, based on DubininÕs theory, shows that D i H consists of three contributions, namely from the interactions with the micropore walls (À0.136 J m À2 ), the external surface (À0.114 J m À2 ), and from the volume W Ã o of liquid found between the surface layers in the micropores (À141 J cm À3 ). It appears that for carbons where L o > 1 nm, the real surface area cannot be determined in a reliable way from the enthalpy of immersion and a specific heat of wetting alone.

Performance of templated mesoporous carbons in supercapacitors

“…As described in detail elsewhere [3,33,35], values of Dubinin's characteristic energy E 0 below 15-16 kJ mol −1 suggest the absence of any significant microporosity in TMCs such as CZ16-HT, CZ34-HT, CZ71-HT and C-25-HT ( Table 1). The domain of 15-16 kJ mol −1 itself corresponds to transition pores between supermicorpores and mesopores in which often no capillary condensation occurs.…”

“…Therefore, different techniques were used [3,[33][34][35]. The solids were characterized by the standard techniques based on N 2 adsorption-desorption isotherms at 77 K (Micromeritics ASAP 2010) and immersion calorimetry at 293 K (Tian-Calvet-type calorimeter).…”

“…The domain of 15-16 kJ mol −1 itself corresponds to transition pores between supermicorpores and mesopores in which often no capillary condensation occurs. On the other hand, values of E 0 above 17 kJ mol −1 reflect the presence of micropores (The higher E 0 , the smaller the micropore size [3,35]). It should be pointed out that the contribution of microporosity to the overall porosity is highest for the three carbons of series CG (around 45%), with an average width around 1.4 nm (E 0 ∼ 19 J mol −1 ).…”

Correlation between capacitances of porous carbons in acidic and aprotic EDLC electrolytes

“…As discussed in detail [6], a reliable determination of the surface areas of the carbons is essential for the study of EDLC properties in carbons. It appears that the relevant quantity is the total surface area S tot , which is the sum of the micropore surface area and the external surface area.…”

“…All the samples have been characterized within the framework of Dubinin's theory [6] by standard techniques based on vapour adsorption, typically with nitrogen at 77 K and/or benzene near 293-298 K, and by immersion calorimetry (293 K) into liquids with various molecular dimensions. In some cases, the characterization also included the analysis of the CO 2 isotherm by Monte Carlo simulations, which leads to the pore size distribution.…”