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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
    Performance of mesoporous carbons derived from poly(vinyl alcohol) in electrochemical capacitors
    (2008)
    Fernández, J. A.
    ;
    Morishita, T.
    ;
    Toyoda, M.
    ;
    Inagaki, M.
    ;
    Stoeckli, Fritz 
    ;
    Centeno, Teresa A.
    The present work shows that mesoporous materials obtained by the carbonization of mixtures of poly(vinyl alcohol) with magnesium citrate are very promising candidates for electrodes in supercapacitors. Their high performance arises essentially from a double-layer mechanism through the extent of the total surface area and one obtains at low current density (1 mA cm−2) values as high as 180 F g−1 in aqueous 2 M H2SO4 electrolyte and around 100 F g−1 in 1 M (C2H5)4NBF4 in acetonitrile. Moreover, in most cases the specific capacitance is reduced only by 15% at 100 mA cm−2, as opposed to many other types of carbons which display much higher reductions.

    This study suggests that these novel carbons could be potentially more advantageous as electrodes in electrochemical capacitors than templated mesoporous carbons.
  • Publication
    Accès libre
    Improvement of the structural and chemical properties of a commercial activated carbon for its application in electrochemical capacitors
    (2008)
    Lota, G.
    ;
    Centeno, Teresa A.
    ;
    Frackowiak, E.
    ;
    Stoeckli, Fritz 
    The present paper shows that the performance of an inexpensive activated carbon used in electrochemical capacitors can be significantly enhanced by a simple treatment with KOH at 850 °C. The changes in the specific surface area, as well as in the surface chemistry, lead to high capacitance values, which provide a noticeable energy density.
    The KOH-treatment of a commercial activated carbon leads to highly pure carbons with effective surface areas in the range of 1300–1500 m2 g−1 and gravimetric capacitances as high as three times that of the raw carbon.
    For re-activated carbons, one obtains at low current density (50 mA g−1) values of 200 F g−1 in aqueous electrolytes (1M H2SO4 and 6M KOH) and around 150 F g−1 in 1M (C2H5)4NBF4 in acetonitrile. Furthermore, the resulting carbons present an enhanced and stable performance for high charge/discharge load in organic and aqueous media.
    This work confirms the possibilities offered by immersion calorimetry on its own for the prediction of the specific capacitance of carbons in (C2H5)4NBF4/acetonitrile. On the other hand, it also shows the limitations of this technique to assess, with a good accuracy, the suitability of a carbon to be used as capacitor electrodes operating in aqueous electrolytes (H2SO4 and KOH).
  • Publication
    Accès libre
    Correlation between capacitances of porous carbons in acidic and aprotic EDLC electrolytes
    (2007)
    Centeno, Teresa A.
    ;
    Hahn, M.
    ;
    Fernández, J. A.
    ;
    Kötz, R.
    ;
    Stoeckli, Fritz 
    A study based on a total of 41 nanoporous carbons shows that there exists a good correlation between the limiting gravimetric capacitances Co at low current densities j (1 mA cm −2 ) measured in aprotic (1 M (C2H5)4NBF4 in acetonitrile) and in acidic (2 M aqueous H2SO4) electrolytes. The comparison of the surface-related capacitances (F m −2 ) of well characterized samples with the amount of thermodesorbed CO suggests a strong contribution of CO generating surface groups to charge storage in the acidic electrolyte, but a negligible contribution in the aprotic medium. It also appears that the decrease of the capacitance with current density is similar in both electrolytes. This confirms that the average micropore width and the CO2 generating surface groups are the main factors which limit the ionic mobility in both electrolytes.
  • Publication
    Accès libre
    The role of textural characteristics and oxygen-containing surface groups in the supercapacitor performances of activated carbons
    (2006)
    Centeno, Teresa A.
    ;
    Stoeckli, Fritz 
    It is suggested that the specific capacitance C0 of activated carbons at low current densities (d ~ 1 mA cm−2) consists, to a good first approximation, of two contributions. For the H2SO4 electrolyte they correspond to approximately 0.080 F m−2 from the total accessible surface area and an additional pseudo-capacitance of 63 F mmol−1 from the surface species generating CO in thermally programmed desorption (TPD). The new correlation proposed here is an alternative to Shi's earlier approach which considered contributions from the microporous and the external surface areas. Furthermore, it appears that the variation of the specific capacitance C at high current densities d (up to 100–150 mA cm−2) depends essentially on the CO2-generating surface groups and on 1/L0, the inverse of the average micropore width.