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Stoeckli, Fritz

Nom
Stoeckli, Fritz
Affiliation principale
Fonction
Professeur.e émérite
Email
fritz.stoeckli@unine.ch
Identifiants
https://libra.unine.ch/handle/123456789/471

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  • Publication
    Accès libre
    Cherry stones as precursor of activated carbons for supercapacitors
    (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.
  • Publication
    Accès libre
    On the determination of surface areas in activated carbons
    (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, Se and Smi, have been determined by a variety of independent techniques. It appears clearly that SBET and the real surface area Smi + Se are in agreement only for carbons with average pore widths Lo around 0.8–1.1 nm. Beyond, SBET increases rapidly and SBET− Se is practically the monolayer equivalent of the micropore volume Wo. This confirms that a characterization of surface properties based on SBET 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 ΔiH 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 Lo> 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.
  • Publication
    Métadonnées seulement
    Modelling of water adsorption by activated carbons: effects of microporous structure and oxygen content
    (2004)
    Slasli, A M
    ;
    Jorge, M
    ;
    Stoeckli, Fritz 
    ;
    Seaton, N A
    The present paper examines the adsorption of water by microporous carbons containing various amounts of surface oxygen and a smaller proportion of basic centres. The modelling of water adsorption for 293 and 310 K, using variable pore size distributions (PSD), confirms that the overall type IV isotherm is the sum of a type I isotherm associated with the specific interactions, and a type V isotherm reflecting the non-specific interactions. The principle of temperature invariance is followed by these isotherms, which indicates that modelling leads to the Dubinin-Astakhov equation. The present approach allows the prediction of water adsorption near room temperature, on the basis of the PSD and the density of oxygen present on the surface area of the micropores. It is assumed, to a first and good approximation, that the pores are slit-shaped and the oxygen distribution is random. (C) 2004 Elsevier Ltd. All rights reserved.
  • Publication
    Accès libre
    The effect of the carbonization/activation procedure on the microporous texture of the subsequent chars and active carbons
    (2003)
    Cagnon, Benoît
    ;
    Py, Xavier
    ;
    Guillot, André
    ;
    Stoeckli, Fritz 
    Chars obtained by carbonizing coconut shells at different intermediate heat treatment temperatures (IHTT) between 400 and 800 °C were activated at 800 °C in a stream of N2+H2O, following two distinct procedures. In the first procedure, activation follows directly the carbonization, whereas in the second procedure, the sample was first brought back to 25 °C and subsequently heated again to the activation temperature of 800 °C. The data for CO2 adsorption at 25 °C and N2 at −196 °C with immersion calorimetry confirms that the activated carbons derived from chars obtained at low IHTT and in two steps, present a “gate effect” for burn-offs <20% or 25%, otherwise, the final carbons present similar structural characteristics for higher burn-offs. It also appears that the evolution of the average pore width L0 with the micropore volume W0 follows a general pattern outlined early.
  • Publication
    Accès libre
    The characterization of microporosity in carbons with molecular sieve effects
    (2002)
    Stoeckli, Fritz 
    ;
    Slasli, Abdou
    ;
    Hugi-Cleary, Deirdre
    ;
    Guillot, André
    The apparent and the real micropore size distributions (PSDs) of molecular sieve carbons can be assessed by combining the adsorption of CO2 at 273 K with immersion calorimetry into liquids of increasing molecular dimensions. On the basis of model isotherms resulting from computer simulations, the adsorption of carbon dioxide, a relatively small probe, leads to the overall PSD of the carbon (essentially the internal micropore system). Immersion calorimetry, on the other hand, reveals the distribution of the pores accessible directly from the liquid phase, that is without constrictions. Liquid CS2 probes the same volume as CO2 and can be used as a reference. The paper describes the case of an industrial molecular sieve carbon obtained by blocking partly the entrance to a relatively broad micropore system, thus limiting its accessibility to molecules with diameters below 0.5–0.6 nm. It is shown how activation by steam at 900 °C removes the constrictions and leads to a gradual overlap of the two PSDs. The distribution of the pore widths on the surface, observed directly by scanning tunnelling microscopy, is also given.
  • Publication
    Métadonnées seulement
    Dubinin's theory and its contribution to adsorption science
    (2001)
    Stoeckli, Fritz 
    Dubinin's theory for the volume filling of micropores (TVFM), originally developed for the adsorption of single vapours by microporous solids such as activated carbons and Zeolites, has gradually been extended to other areas. They include immersion calorimetry, the adsorption of water vapour and of mixtures, as well as adsorption front aqueous solutions. Recent studies in the field of adsorption from aqueous solutions, by activated carbons, suggest that the principle of temperature invariance is fulfilled and in the case of phenolic compounds a modified DRK equation can be used to predict the adsorption equilibrium over a certain range of temperatures. Computer modelling of CO2 adsorption by carbons at 273 K leads to micropore distributions, which are in good agreement with those derived from other techniques. It also appears that the model isotherm, in single slit-shaped micropores can be fitted to the Hill-de Boer isotherm, in agreement with mathematical studies of the origin of the Dubinin-Astakhov equation.
  • Publication
    Accès libre
    On the mechanisms of phenol adsorption by carbons
    (2001)
    Stoeckli, Fritz 
    ;
    Hugi-Cleary, D.
    The removal of phenol and related compounds from dilute aqueous solutions by activated carbons corresponds to the coating of the micropore walls and of the external surface by a monolayer. This process is described by an analog of the Dubinin—Radushkevich—Kaganer equation. On the other hand, as suggested by immersion calorimetry at 293 K, in the case of concentrated solutions, the mechanism corresponds to the volume filling of the micropores, as observed for the adsorption of phenol from the vapor phase. The equilibrium is described by the Dubinin—Astakhov equation. It follows that the removal of phenol from mixtures with water depends on the relative concentrations, and the limiting factor for adsorption is either the effective surface area of the carbon, or the micropore volume.
  • Publication
    Accès libre
    The physical and chemical characterization of a sulphur-impregnated active carbon, by combined adsorption and immersion techniques
    (1991)
    Rebstein, Patrick
    ;
    Stoeckli, Fritz 
    A commercially available active carbon, impregnated with sulphur, has been characterized and compared to the original carbon by using adsorption and immersion techniques. The specific reaction of liquid CS2 with sulphur, following the preadsorption of various amounts of n-nonane, shows that the accessibility of sulphur decreases rapidly. It is also found that the impregnated carbon becomes hydrophilic but, in the case of SO2 adsorption, no change in adsorption capacity is detected.
  • Publication
    Accès libre
    Evolution of microporosity during activation of carbon
    (1991)
    Stoeckli, Fritz 
    ;
    Ballerini, Luca
    Various adsorption and immersion techniques and a recent model for micropore distributions have been used to assess quantitatively the evolution of the main properties of active carbon. The precursors used in this study were of vegetable and polymeric origin. One activation series based on natural coal was also included.