Capacitance properties of ordered porous carbon materials prepared by a templating procedure

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Abstract

The electrochemical performance of carbon materials with a highly ordered nanoporous structure is investigated in two-electrode supercapacitors. The materials were prepared by a templating procedure using a silica matrix (type MCM-48 or SBA-15) with an organized porosity in which carbon was inserted, either by chemical vapor decomposition of propylene or by impregnation with a sucrose solution followed by carbonisation. After the removal of silica, a micro-mesoporous carbon residue is recovered which displays an uniform pore size distribution. Such a well-defined nanostructure is interesting for a fundamental study of the double layer capacitance behavior. The performance of supercapacitors built with electrodes prepared from the templated carbon was tested in acidic, alkaline and organic electrolyte solutions. High values of capacitance in aqueous and organic media were obtained with a rectangular shape of the voltammograms over a wide range of scan rates indicating a quick charge propagation. Especially, the templated carbons prepared by the impregnation of sucrose in MCM-48 display high capacitance values due to the formation of an adequate micro-mesoporous network during their formation. A marked shift of capacitance drop at higher values of frequency is clearly observed for the materials rich in mesopores; the mesopores make easier the diffusion of the ions to the active surface.

Introduction

In recent years, a growing interest is devoted to the development of electrochemical capacitors (supercapacitors) for energy storage applications in electric vehicles and any device that needs high-pulse discharge profile (e.g. digital telecommunication systems). A unit cell of an electrochemical capacitor is based on the double layer capacitance at the solid/electrolyte interface of a high surface area material, generally an activated carbon. Activated carbons are prepared by activation of a carbon precursor, that does not allow the porous structure to be perfectly controlled [1], [2]. By the later method, it is quite difficult to adapt the pore size to that of solvated anions and cations in order to get an optimised performance of the supercapacitor. Additionally, a well-balanced micromesoporosity is preferable; the micropores determine the high surface area on which ions are adsorbed and the mesopores are essential for ions transportation [3]. Hence, for improving the electrochemical performance of supercapacitors, the optimal porous characteristics required for carbon must be better determined; therefore, it is of great interest to synthesize carbon materials with a well tailored porous structure.

Recently, several new synthesis routes have been proposed to prepare ordered nanoporous carbon materials, among which the templating process appears very promising [4], [5], [6], [7], [8], [9], [10]. The carbon preparation by this route is based on gas or liquid phase filling of the pores of an organised mesoporous silica material by the carbon precursor, followed, if necessary, by a carbonisation step. The ordered carbon material recovered subsequently by dissolution of the silica matrix in hydrofluoric acid (HF) corresponds to the negative replica of the template; the walls and the pores of silica become the pores and the walls of the templated carbon, respectively. As mentioned earlier in the literature, the characteristics of the carbon material (especially the specific surface area, the pore organisation, the pore size and the pore size distribution) are depending on the nature of the carbon precursor and silica template [11]. The electrochemical performance of templated carbon materials prepared by different processes was briefly mentioned in the literature [12], [13], [14] but to our knowledge the electrochemical behaviour has not been systematically correlated with the porous characteristics. Recently, we have demonstrated that the electrochemical properties are influenced by the kind of carbon precursor [15].

For this work, two types of organised mesoporous silica (MCM-48 and SBA-15), which display different crystallographic structure, size and shape of the pores, were selected. The pores of the silica template were filled either by insertion of pyrolytic carbon from the thermal decomposition of propylene or by an aqueous solution of sucrose, which after elimination of water requires a carbonisation step at 900 °C. The electrochemical performances of capacitor electrodes prepared from the various templated carbons have been determined and are tentatively correlated with their structural and microtextural characteristics.

Section snippets

Silica template

Two types of organised mesoporous silica (MCM-48 and SBA-15) were synthesized according to the procedures already described in the literature [16], [17], [18]. Some of their characteristics determined by X-ray diffraction and nitrogen adsorption at 77 K are reported in Table 1. The mesoporous silica material referred as MCM-48 displays a cubic structure, which consists of two independent and intricately interwoven networks of mesoporous channels. The structure of the material named SBA-15 is

Results and discussion

The nitrogen adsorption/desorption isotherms at 77 K of the carbons CS48 and CS15 confirm the formation of micropores during the carbonization [11], [9]. The total surface areas of both carbons are significantly higher than the surface area of the corresponding silica template (Table 1). Just an opposite tendency is observed in this table when propylene is used as carbon precursor. In the latter case, the pore volume of the templated carbon is consistent with an uniform pore filling, and the

Conclusion

Interesting carbon materials for supercapacitor electrodes were prepared by a templating procedure from highly ordered mesoporous silica materials of type MCM-48 or SBA-15. The perfectly controlled texture of the templated carbons allows a better understanding of the electrochemical performance. The highest capacitance values are reached with the carbons CSX showing the highest total surface area, the highest total pore volume and the more marked microporous character. When sucrose is used as a

Acknowledgements

Financial support from the French Ministry of Research (Action Concertée Incitative Nanosciences et Nanotechnologies project:NN060), the Centre National de la Recherche Scientifique (CNRS) (Programme Matériaux 2002-project N °41) and NATO Science for Peace project 973849 is gratefully acknowledged.

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