Résultat de la recherche
Improvement of the structural and chemical properties of a commercial activated carbon for its application in electrochemical capacitors
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).
Dubinin's theory and its contribution to adsorption science
Dubinin's theory for the volume filling of micropores (TVFM), originally developed for the adsorption of single vapours by microporous solids such as activated carbons and Zeolites, has gradually been extended to other areas. They include immersion calorimetry, the adsorption of water vapour and of mixtures, as well as adsorption front aqueous solutions. Recent studies in the field of adsorption from aqueous solutions, by activated carbons, suggest that the principle of temperature invariance is fulfilled and in the case of phenolic compounds a modified DRK equation can be used to predict the adsorption equilibrium over a certain range of temperatures. Computer modelling of CO2 adsorption by carbons at 273 K leads to micropore distributions, which are in good agreement with those derived from other techniques. It also appears that the model isotherm, in single slit-shaped micropores can be fitted to the Hill-de Boer isotherm, in agreement with mathematical studies of the origin of the Dubinin-Astakhov equation.
Recent Developments in the Dubinin Equation
A combination of techniques—mainly immersion calorimetry and adsorption from the vapor phase—provides independent data for the assessment of micropore sizes and their distribution in relation to parameters E0 and n of the Dubinin-Astahhov (D-A) equation. It is shown that carbons described by the Dubinin-Radushkevich (D-R) equation (n = 2) present some heterogeneity, which can be expressed in terms of contributions from true molecular-sieve carbons with n = 3.
On the electrical double-layer capacitance of mesoporous templated carbons
On the mechanisms of phenol adsorption by carbons
The removal of phenol and related compounds from dilute aqueous solutions by activated carbons corresponds to the coating of the micropore walls and of the external surface by a monolayer. This process is described by an analog of the Dubinin—Radushkevich—Kaganer equation. On the other hand, as suggested by immersion calorimetry at 293 K, in the case of concentrated solutions, the mechanism corresponds to the volume filling of the micropores, as observed for the adsorption of phenol from the vapor phase. The equilibrium is described by the Dubinin—Astakhov equation. It follows that the removal of phenol from mixtures with water depends on the relative concentrations, and the limiting factor for adsorption is either the effective surface area of the carbon, or the micropore volume.
Co-adsorption of 1,2-dichloroethane and 1-bromo,2-chloroethane on zeolite ZSM-5 from the liquid and vapour phases, using the Myers-Prausnitz-Dubinin model
The adsorption/co-adsorption of 1,2-dichloroethane (DCA) and 1-bromo,2-chloroethane (BCA) from the vapour and the liquid phases by zeolite ZSM-5 at ambient temperature is reported, using Dubinin's theory and the recent Myers-Prausnitz-Dubinin (MPD) theory. For adsorption from the liquid phase, the activity coefficients in the adsorbed and the liquid states are the same and no selectivity is observed. This is confirmed by the absence of an excess enthalpy of immersion of ZSM-5 into the mixtures. Adsorption from the vapour phase proceeds in two stages, as indicated by double Dubinin-Astakhov (DA) plots. If one considers only the domain of high relative pressure, co-adsorption is described by the MPD theory. The agreement between calculated and experimental compositions of adsorbate can be improved by using, as a first and good approximation, the activity coefficients of the liquid–solid equilibrium at the same composition. A good agreement is also observed between the enthalpies of immersion calculated from the DA equation and the experimental integral heats of adsorption. However, for vapour mixtures ZSM-5 shows selectivity in favour of the more volatile compound, 1-bromo,2-chloroethane. This probably results from adsorption at low pressures and could be due to differences in kinetics.
On the correlation between micropore distribution obtained from molecular probes and from high resolution electron microscopy
By using immersion calorimetry with liquids of various molecular dimensions, the distributions of the micropore sizes have been obtained for a number of active carbons, for the range 0.3 to 0.75 nm. These carbons have also been examined by high-resolution electron microscopy, the results from both techniques being displayed in the form of histograms. The agreement between the two methods appears to be good and provides possible evidence of the reduced interaction radius of adsorbed molecules and of exfoliation.