No abstract

The study of 28 porous carbons shows that the specific capacitance in the electrolyte (C(2)H(5))(4)NBF(4)/acetonitrile is relatively constant between 0.7 and 15 nm (0.094 ± 0.011 F m(-2)). The increase in pores below 1 nm and the lower values between 1 and 2 nm reported earlier are not observed in the present work.

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 m 2 g −1and 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 H 2 SO 4 and 6M KOH) and around 150 F g −1 in 1M (C 2 H 5 ) 4 NBF 4 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 (C 2 H 5 ) 4 NBF 4 /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 (H 2 SO 4 and KOH).

a b s t r a c tIn 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.

It is suggested that the specific capacitance C 0 of activated carbons at low current densities (d ∼ 1 mA cm −2 ) consists, to a good first approximation, of two contributions. For the H 2 SO 4 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 CO 2 -generating surface groups and on 1/L 0 , the inverse of the average micropore width.