Procedimiento para la preparación de membranas de carbono para la separación de gases permanentes

Referencia OEPM: P9801024.-- Fecha de solicitud: 18/05/1998.-- Titular: Consejo Superior de Investigaciones Científicas (CSIC).Procedimiento para la preparación de membranas de carbono para la separación de gases permanentes. El procedimiento incluye las siguientes etapas principales: disolución de un polímero precursor en un líquido para la obtención de una disolución polimérica transparente, deposición de la disolución sobre un soporte de carbono, introducción del substrato recubierto homogéneamente en un líquido en el cual precipita, formándose la membrana polimérica. Su aplicación principal es para la separación de gases permanentes.Peer reviewe

Recycling of residues as precursors of carbons for supercapacitors

En: 1st Spanish National Conference on Advances in Materials Recycling and Eco – Energy Madrid, 12-13 November 2009.-- Editors: F. A. López, F. Puertas, F. J. Alguacil and A. Guerrero.-- 4 pages, 3 figures, 1 table.It is shown that industrial wastes such as apple pulp (generated in the cider production), cherry stones (from the industrial manufacture of Kirsh and jam) and PET (plastic vessels) can be recycled as activated carbons for electrode material in supercapacitors. These precursors allow obtaining carbons with large specific surface areas (up to 1200 m2g-1) and average pore sizes around 0.9-1.3 nm, which makes them accessible to electrolyte ions. These features lead to electrical capacitances at low current density as high as 230 F g-1 in 2M H2SO4 aqueous electrolyte and 120 F g-1 in the aprotic medium 1M (C2H5)4NBF4 / acetonitrile. Furthermore, high performance is also achieved at high current densities, which means that the activated carbons derived from residues compete well with commercial carbons used at present in supercapacitors.Peer reviewe

Large-scale conversion of helical-ribbon carbon nanofibers to a variety of graphene-related materials

Helical-ribbon carbon nanofibers produced on an industrial scale were successfully converted in highly functionalized graphene nanoplatelets by using a slight modification of the Hummers oxidation method. The duration of the oxidative process severely affected the interlayer spacing in the resulting nanoplatelets and, consequently, they showed very different exfoliation behavior. Therefore, it was possible to obtain a variety of graphene-related products through their ultrasonication or thermal treatments such as exfoliation-reduction by flash-pyrolysis in air at temperatures between 400 and 1000 °C or standard activation with CO2 at 800 °C. Detailed comparison of the functionalized carbon nanoplatelets, graphene oxides, reduced graphene oxides and activated carbon nanoplatelets reveals the wide spectrum of their properties with specific surface areas in the range of 4–500 m2 g−1, oxygen content from 38 to 5 wt% and different structural ordering. This study also underlines the impact of the structural, textural and chemical changes experienced by the carbon nanofibers along the various processes on the performance as supercapacitor electrodes. This preliminary study, based on cyclic voltammetry in 2 M H2SO4 aqueous electrolyte, is a summary of the strengths and weaknesses of the different graphene-related materials for this application. The helical-ribbon carbon nanofibers displayed only 10 F g−1, the capacitance of the functionalized graphene nanoplatelets greatly rose to 104 F g−1 with clear contributions from pseudocapacitance. Values around 100–120 F g−1 were found for the graphene oxides and activated graphene nanoplatelets although a marked resistive character is detected. Flash-pyrolysis at 1000 °C leads to lower capacitance (79 F g−1) but much quicker charge propagation. Among all these materials, the lower-cost functionalized graphene nanoplatelets displayed the better behavior for aqueous supercapacitors.Financial support from EU 7FP (Project Electrograph- 266391) and EU FP7- ICT-2013-FET-FGRAPHENE Flagship Project (Nr. 604391) is gratefully acknowledged.Peer reviewe

Correlation between capacitances of porous carbons in acidic and aprotic EDLC electrolytes

5 pages, 4 figures, 2 tables.-- Printed version published Jun 2007.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)4 NBF4 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.Peer reviewe

The volumetric capacitance of microporous carbons in organic electrolyte

The study of 26 microporous carbons with average pore widths Lo between 0.7 and 1.8 nm shows that the volume-related capacitance Cmi (F cm− 3) in the organic electrolyte (C2H5)4NBF4/acetonitrile is an inverse function of the average micropore width Lo. This means that for a given micropore volume Wo, the gravimetric capacitance C (F g− 1) is larger for carbons with smaller pores due to the higher surface to volume ratio. Furthermore, the increase of Cmi (F cm− 3) in smaller pores suggests that the surface-related capacitance C (F m− 2) should be relatively constant in slit-shaped micropores. This has important technological consequences for the design of carbon-based supercapacitors.Peer reviewe

The assessment of surface areas in porous carbons by two model-independent techniques, the DR equation and DFT

The strong linear correlation observed between SBET and the micropore volume of 190 carbons with pore widths between 0.5 and 1.8 nm confirms the unreliable character of SBET, in spite of its frequent use. (It corresponds approximately to 2200–2300 m2 per cm3 of micropores, whatever their width). Alternative determinations of the surface area are therefore required. It is shown that two model-independent techniques (Kaneko’s comparison plot for nitrogen and the enthalpies of immersion into aqueous solutions of phenol) and two model-dependent approaches (Dubinin’s theory and DFT) lead to total surface areas which are in good agreement. Their average Sav is probably a reliable assessment of the total surface area. It is often in disagreement with SBET, but a closer study of 42 well characterized microporous carbons, for which all four techniques are available, shows that the ratio SBET/Sav increases linearly with the average pore width. This should be taken into consideration when surface-related properties (e.g., densities of chemical groups or adsorbed species, specific capacitances) are examined on the basis of a single determination and in particular on the BET technique.Peer reviewe

On the specific double-layer capacitance of activated carbons, in relation to their structural and chemical properties

Twelve well-characterized activated carbons with average micropore widths between 0.7 and 2 nm, total surface areas of 378–1270 m2 g−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 Co, 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 View the MathML sourceWo* of the electrolyte found between the surface layers in pores wider than 0.7–0.8 nm contributes to Co. However, this property is limited to microporosity, like the enthalpy of immersion of the carbons into benzene. The latter is also correlated to Co, which provides a useful means to identify potential supercapacitors.The authors wish to thank Professor C. Moreno-Castilla (University of Granada) for the gift of carbons of series AZ and BV.Peer reviewe

Surface-related capacitance of microporous carbons in aqueous and organic electrolytes

This paper examines the surface-related capacitance C/S of carbons with average micropore widths between 0.73 and 1.80 nm, using 1–2 M H2SO4, 6 M KOH and 1 M (C2H5)4NBF4 in acetonitrile as electrolytes. Following corrections for pseudocapacitance effects for the aqueous electrolytes and the use of an average surface area suggested by independent techniques, different from the BET area, it appears that C/S is practically independent of the micropore width. The analysis of the data with the help of recent models suggests that the dielectric constants ɛr of the different electrolytes may decrease with the pore size. It is surprising that the coexistence of two sets of values for the surface area of microporous carbons and its consequence on C/S have not received more attention in the past.Peer reviewe

The role of textural characteristics and oxygen-containing surface groups in the supercapacitor performances of activated carbons

It is suggested that the specific capacitance C0 of activated carbons at low current densities (d ∼ 1 mA cm−2) consists, to a good first approximation, of two contributions. For the H2SO4 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 CO2-generating surface groups and on 1/L0, the inverse of the average micropore width.The authors wish to thank Professor C. Moreno-Castilla (University of Granada) for the gift of carbons of series AZ and BV, Dr. M.J. Lázaro (Instituto de Carboquímica- CSIC) for further TPD measurements and Dr. M. Hahn (Paul Scherrer Institut Villigen, Switzerland) for preliminary results obtained with three-electrode cells and useful discussions.Peer reviewe

Optimization of the characterization of porous carbons for supercapacitors

The optimization of carbon-based supercapacitors is of great technological importance for electrical energy storage. This challenge stresses the relevance of current attempts to increase the surface-related capacitance C/S (F m−2) of carbons. We show that for microporous carbons this property is practically constant for pore widths between 0.7 and 1.8 nm. The study is based on two independent approaches: (i) the effective surface area accessible to the ions, which may differ considerably from the BET-surface and (ii) the volumetric capacitance in the micropores. One obtains 0.094 F m−2 in (C2H5)4NBF4/acetonitrile and approximately 0.100 F m−2 in aqueous H2SO4 after correcting for pseudo-capacitance effects. This pattern is supported by recent modelling which takes into account the solvent. It is also suggested that constant values of the surface based-capacitance may reflect a gradual decrease of the dielectric constant of the electrolytes in smaller micropores due to desolvation.Financial support to T.A.C. from EU 7FP (Project Electrograph- 266391) and MICINN (project MAT 2011-25198) is gratefully acknowledged. F.S. wishes to thank the University of Neuchâtel for its constant support beyond his retirement.Peer reviewe