Stoeckli, Fritz

2009, Olivares-Marín, M., Fernández, J. A., Lázaro, M. J., Fernández-González, C., Macías-García, A., Gómez-Serrano, V., Stoeckli, Fritz, Centeno, Teresa A.

It is shown that cherry stones-wastes can be recycled as activated carbons for electrode material in supercapacitors. KOH-activation of this precursor at 800–900 °C is an efficient process to obtain carbons with large specific surface areas (1100–1300 m² g⁻¹), average pore sizes around 0.9–1.3 nm, which makes them accessible to electrolyte ions, and conductivities between 1 and 2 S cm⁻¹. These features lead to capacitances at low current density as high as 230 F g⁻¹ in 2 M H₂SO₄ aqueous electrolyte and 120 F g⁻¹ in the aprotic medium 1 M (C₂H₅)₄NBF₄/acetonitrile. Furthermore, high performance is also achieved at high current densities, which means that this type of materials competes well with commercial carbons used at present in supercapacitors.

2005, Stoeckli, Fritz, Centeno, Teresa A.

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− Se 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 Dubinin’s theory, shows that Δ_iH consists of three contributions, namely from the interactions with the micropore walls (−0.136 J m⁻²), the external surface (−0.114 J m⁻²), and from the volume W^*_o of liquid found between the surface layers in the micropores (−141 J cm⁻³). 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.