Stoeckli, Fritz

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.

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).

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.