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Stoeckli, Fritz
Nom
Stoeckli, Fritz
Affiliation principale
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Professeur.e émérite
Email
fritz.stoeckli@unine.ch
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Voici les éléments 1 - 10 sur 141
- PublicationAccès libreCherry stones as precursor of activated carbons for supercapacitors(2009)
; 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. - PublicationAccès libreContributions of hemicellulose, cellulose and lignin to the mass and the porous properties of chars and steam activated carbons from various lignocellulosic precursors(2009)
; Chambat, GérardIn this study, contributions of hemicellulose, cellulose and lignin to the mass and the porous properties of chars and activated carbons from various lignocellulosic materials were studied. A predictive calculation was established using the experimental results obtained for the three components separately to evaluate the carbonization and activation yields and their respective contributions to the chars and to the subsequent activated carbons of various precursors in term of weight fraction. These equations were validated. The results showed that lignin can be considering as being the major contributor of all chars and activated carbons. Besides, the evolution of the mean pore size versus the specific porous volume showed that each component contributes to the porosity of chars and activated carbons whatever is its weight contribution. - PublicationAccès libreCorrelation between heats of immersion and limiting capacitances in porous carbons(2008)
- PublicationAccès libreExtension of Dubinin’s Theory to Adsorption from Aqueous Solutions(2008)
; - PublicationAccès libreApplications of Immersion Calorimetry in Dubinin’s Theory and Electrochemistry(2008)
; Centeno, Teresa A.This study shows that immersion calorimetry is a useful technique which simplifies considerably the analysis of porosity and chemical nature of activated carbons. The characterization of activated carbons in the general theoretical framework of Dubinin's theory with its extensions to calorimetry and adsorption from solutions allows the identification of some key parameters for the performance of these materials in electrochemical capacitors. - PublicationAccès librePerformance of mesoporous carbons derived from poly(vinyl alcohol) in electrochemical capacitors(2008)
; 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. - PublicationAccès libreImprovement of the structural and chemical properties of a commercial activated carbon for its application in electrochemical capacitors(2008)
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). - PublicationAccès libreEDLC performance of carbide-derived carbons in aprotic and acidic electrolytes(2008)
; Centeno, Teresa A.This study shows that carbide-derived carbons (CDCs) with average pore size distributions around 0.9–1 nm and effective surface areas of 1300–1400 m2 g−1 provide electrochemical double-layer capacitors with high performances in both aqueous (2M H2SO4) and aprotic (1M (C2H5)4NBF4 in acetonitrile) electrolytes.
In the acidic electrolytic solution, the gravimetric capacitance at low current density (1 mA cm−2) can exceed 200 F g−1, whereas the volumetric capacitance reaches 90 F cm−3. In the aprotic electrolyte they reach 150 F g−1 and 60 F cm−3.
A detailed comparison of the capacitive behaviour of CDCs at high current density (up to 100 mA cm−2) with other microporous and mesoporous carbons indicates better rate capabilities for the present materials in both electrolytes. This is due to the high surface area, the accessible porosity and the relatively low oxygen content.
It also appears that the surface-related capacitances of the present CDCs in the aprotic electrolyte are in line with other carbons and show no anomalous behaviour. - PublicationAccès libreBimetallic Metal-Organic Chains, Networks and Frameworks (MOC's, MON's & MOF's) Based on Cyanides: Structure and Physical Properties(2008)
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Tlus thesis is divided into five Chapters. Chapter 1 gives an introduction to the subject of porous coordination polymers and cyano-bridged bimetallic assemblies and on the experimental methods used in this work. Chapter 2 is dedicated to the structural transformations and ferromagnetism (Section 2.2). The "sponge-like' behaviour of the molecular transformations, described in Sections 2.1 and 2.3. are accompanied by a colour change and it has been shown by in-situ X-ray powder diffraction and immersion calorimetry to be completely reversible. In Section 2.2 the two systems studied exhibit ferromagnetism. The first system shows three-dimensional ferromagnetic ordering at ca. 4K. and second shows a weak infra-chain ferromagnetic exchange, as a result of Jahn- Teller elongation in copper (II) ions.
Chapter 3 describes the studies in the field of immersion calorimetry and adsorption. In Section 3.1 new MOCBF’s are described, two of which show dynamic behaviour triggered by guest removal and inclusion. The dehydration processes, which involve changes in the structures, are reversible, and this was shown using powder X-ray diffraction (PXRD) methods. By a combination of the DSC. PXRD and immersion calorimetry it was possible to find the net heat of the transformation. A remarkable structural transformation, driven by solvent molecules, is presented in Section 3.2. It was found that the two types of reported networks show different behaviour upon drying, falling within the category of "recoverable collapsing" and "guest-induced reformation" frameworks. The methanol adsorption isotherm for the fully out-gassed compound indicates that it is a two step process. Immersion calorimetric studies with different solvents were carried out. By the gravimetric adsorptions studies it was shown that the synthetic strategy based on cyanide-bridged bimetallic assemblies is advantageous for the formation of flexible nanoporous materials.
Chapter 4 illustrates the different methods used for the construction of MOCBF s and describes their structural properties.
Chapter 5 describes the adsorption of morphine from aqueous solutions by various nanoporous carbons. It was shown than 99°'o of morphine can be adsorbed in 5 minutes from an aqueous solution by activated carbon PC94-11. with particles of less than 400 μm and an amount in excess by a factor three with respect to the actual saturation capacity. It was found that the main parameters for a speedy elimination are: an excess of carbon, small particles and good initial mixing of the solid with the solution. This specific study has also provided subsidiary information on the affinity coefficient for the adsorption of morphine from aqueous solutions, βs (morphine) = 1-37 ± 0.02. Tins new parameter, of relevance to pharmacology, will allow the prediction of the adsorption equilibrium of this molecule by activated carbons, by using the modified Dubimn- Radushkevich-Kaganer equation.
In conclusion, this work describes the synthesis and physical properties of new MOCBF" s which exhibit striking structural transformations. Powder X-Ray diffraction combined with adsorption and immersion studies provides insights into these transformations. - PublicationAccès libreCorrelation between capacitances of porous carbons in acidic and aprotic EDLC electrolytes(2007)