Stoeckli, Fritz

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.

2008, Stoeckli, Fritz, 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.

2008, Fernández, J. A., Arulepp, M., Leis, J., Stoeckli, Fritz, 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.

2006, Stoeckli, Fritz, Centeno, Teresa A.

2008, Centeno, Teresa A., Fernández, J. A., Stoeckli, Fritz

Based on more than 80 carbons, the paper shows that immersion calorimetry into benzene, water and carbon tetrachloride can be used to assess with a good accuracy the limiting capacitance Co at low current densities in both acidic (2 M H₂SO₄) and aprotic (1M tetraethyl ammonium tetrafluoroborate in acetonitrile) electrolytic solutions. The enthalpies of immersion ΔiH(C₆H₆) and ΔiH(H₂O) provide information on Co-acidic, where both the surface area and the oxygen content play a role. On the other hand, in the case of the organic electrolyte the oxygen content has only a small influence and Co-aprotic is directly related to ΔiH(C₆H₆) and ΔiH(CCl₄). Carbon tetrachloride has a critical dimension (0.65 nm), which is close to the size of the (C₂H₅)₄N⁺ ion (0.68 nm) and therefore ΔiH(CCl₄) provides better information in the case of carbons with small micropores. The advantage of this approach lies in the fact that immersion calorimetry, in itself a useful tool for the structural and the chemical characterization of carbons, can also be used to evaluate directly the gravimetric capacitances of these solids at low current densities.Based on more than 80 carbons, the paper shows that immersion calorimetry into benzene, water and carbon tetrachloride can be used to assess with a good accuracy the limiting capacitance Co at low current densities in both acidic (2 M H₂SO₄) and aprotic (1M tetraethyl ammonium tetrafluoroborate in acetonitrile) electrolytic solutions. The enthalpies of immersion ΔiH(C₆H₆) and ΔiH(H₂O) provide information on Co-acidic, where both the surface area and the oxygen content play a role. On the other hand, in the case of the organic electrolyte the oxygen content has only a small influence and Co-aprotic is directly related to ΔiH(C₆H₆) and ΔiH(CCl₄). Carbon tetrachloride has a critical dimension (0.65 nm), which is close to the size of the (C₂H₅)₄N⁺ ion (0.68 nm) and therefore ΔiH(CCl₄) provides better information in the case of carbons with small micropores. The advantage of this approach lies in the fact that immersion calorimetry, in itself a useful tool for the structural and the chemical characterization of carbons, can also be used to evaluate directly the gravimetric capacitances of these solids at low current densities.

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.

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.

2008, Stoeckli, Fritz, Nevskaia Dascha M., Castillejos-Lopez Eva, Centeno, Teresa A.

Adsorption of sparingly soluble organics from aqueous solutions, by activated carbons, can be described within the framework of Dubinin's theory by using a modified Dubinin-Radushkevich-Kaganer (DRK) equequation, where relative pressures are replaced by relative concentrations. With respect to the descriptions based on the Langmuir model and similar expressions, this approach has the advantage that it allows predictions on the basis of simple physico-chemical properties of the solid and of the adsorbate. Preliminary experiments indicate that in the case of dilute binary mixtures, the model of independent coadsorption, based on the DRK equation, applies. However, more experimental evidence is needed to confirm this potentially very useful approach in filtration technology.

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).

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.