Stoeckli, Fritz

2009, Cagnon, Benoît, Py, Xavier, Guillot, André, Stoeckli, Fritz, Chambat, Gérard

In 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.

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.

2007, Sevilla, M., Álvarez, S., Centeno, Teresa A., Fuertes, A. B., Stoeckli, Fritz

By analogy with other types of carbons, templated mesoporous carbons (TMCs) can be used as supercapacitors. Their contribution arises essentially from the double layer capacity formed on their surface, which corresponds to 0.14 F m⁻² in aqueous electrolytes such as H₂SO₄ and KOH and 0.06 F m⁻² for the aprotic medium (C₂H₅)₄NBF₄ in CH₃CN. In the case of a series of 27 TMCs, it appears that the effective surface area determined by independent techniques can be as high as 1500–1600 m² g⁻¹, and therefore exceeds the value obtained for many activated carbons (typically 900–1300 m² g⁻¹). On the other hand, the relatively low amount of surface oxygen in the present TMCs, as opposed to activated carbons, reduces the contribution of pseudo-capacitance effects and limits the gravimetric capacitance to 200–220 F g⁻¹ for aqueous electrolytes. In the case of non-aqueous electrolyte, it rarely exceeds 100 F g⁻¹.

It is also shown that the average mesopore diameter of these TMCs does not improve significantly the ionic mobility compared with typical activated carbons of pore-widths above 1.0–1.3 nm.

This study suggests that activated carbons remain the more promising candidates for supercapacitors with high performances.

2003, Fernandez, Elena, Gotovac, Suzana, Hugi-Cleary, Deirdre, López-Ramón, M. V., Stoeckli, Fritz

2006, Centeno, Teresa A., Stoeckli, Fritz

Twelve well-characterized activated carbons with average micropore widths between 0.7 and 2 nm, total surface areas of 378–1270 m²g⁻¹ and specific capacitances C up to 320 F g⁻¹ have been investigated, using H₂SO₄ 2 M as electrolyte. Some of the carbons have also been oxidized with (NH₄)2S₂O₈, which leads to specific oxygen contents between 0.4 and 7.1 μmol m⁻² of carbon surface area. It appears that C_o, the limiting capacitance at a current density of 1 mA cm⁻² of electrode surface, does not depend significantly on the oxygen content. An empirical equation is proposed to describe the decrease of C with increasing current density d (1–70 mA cm−2 of electrode surface), as a function of the oxygen content.

As suggested by different authors, C_o can be expressed as a sum of contributions from the external surface area S_e and the surface of the micropores S_mi. A closer investigation shows that C_o/S_mi increases with the pore size and reaches values as high as 0.250–0.270 F m⁻² for supermicropores. It is suggested that the volume W_o^* of the electrolyte found between the surface layers in pores wider than 0.7–0.8 nm contributes to C_o. However, this property is limited to microporosity, like the enthalpy of immersion of the carbons into benzene. The latter is also correlated to C_o, which provides a useful means to identify potential supercapacitors.