Bimodal, templated mesoporous carbons for capacitor applications
Introduction
High surface area carbon materials have attracted significant attention for many applications, including as a catalyst support in fuel cells, as well as in energy storage devices such as batteries and electrochemical capacitors [EC] [1], [2], [3], [4]. The capacitance of these carbon materials arises from two sources, double layer capacitance and pseudocapacitance.
Double layer capacitance is a reflection of the number of ions building up along the carbon/electrolyte interface to balance the electrical charge on the solid surface, and is therefore directly proportional to the true carbon surface area. Recent work has shown that, in the case of carbon, double layer capacitance is also influenced by the pore size, with the capacitance (per real area) seen to increase as carbon materials containing pore sizes smaller than ∼2 nm in diameter are used. It has been suggested that this is due to the desolvation of counter ions in the narrow pore spacing, thus decreasing the effective thickness of the double layer and resulting in an increased capacitance [1], [2]. Pseudocapacitance is related to the properties and surface density of redox-active functional groups, such as quinone and hydroquinone, which are rapidly oxidized/reduced as the potential is cycled positively/negatively [3].
An important parameter in characterizing potential capacitor materials is the specific capacitance (F/real m2), which depends on a variety of factors, including pore size (micro/meso/macro), pore size distribution (unimodal/multimodal), pore length, pore shape (cylinder/slit/sphere), the specific surface area of the meso- and micropores, the properties of the electrolyte (concentration), and the density of surface functional groups. In comparison, gravimetric capacitance (F/g) takes into account both double layer (surface area) and pseudocapacitance (density of functional groups). These capacitances are equally important in assessing the viability of a new carbon material for use as an EC, and both will be discussed in this paper.
Until recently, research into carbon-based ECs has been focused on high surface area activated carbons [4]. These materials often have a broad pore size distribution, with the majority being classified as micropores (d < 2 nm). Micropores may be accessible to ions in solution at low current densities, but at higher current densities, ionic transport within the pores is hindered, and thus the full electrochemically active surface area is often not accessed [4].
The development of templated ordered mesoporous carbon (OMC) materials provides a solution to this problem [5]. Like activated carbons, they possess a high surface area, but their pores are larger (2 < d < 50 nm) and generally more accessible, even at high current densities, thus making OMCs an attractive alternative for use in ECs [4]. OMCs can be synthesized in several ways, including by silica colloid imprinting of resorcinol–formaldehyde resin [6], the use of mesophase pitch [6], [7], and using mesoporous silica templates [3], [6].
Compared to other mesoporous silicas (such as MCM-48 and SBA-15) that have been used previously as templates to form mesoporous carbons, HMS shows several promising characteristics, such as thick walls, an interconnected wormhole structure, and high textural mesoporosity [8]. The thick walls are particularly advantageous, as these walls become the pores of the resulting templated carbon. OMCs synthesized from HMS templates are therefore expected to have larger pore diameters than those synthesized from MCM-48 or SBA-15. In previous studies, OMCs were synthesized from an HMS template prepared using dodecylamine (C12H25NH2) as the templating agent [9], [10], giving pores in the range of 2–11 nm.
In the present work, SiO2 templates were synthesized using four different amines (CnH2n+1NH2, with a hydrophobic tail length containing 8–16 carbon atoms) as the templating agents. OMC materials were then prepared by impregnating the SiO2 templates with sucrose, followed by carbonization and etching out of the SiO2 with NaOH. These OMCs were evaluated electrochemically using cyclic voltammetry (CV), giving among the highest gravimetric capacitance values (∼260 F/g) yet reported for ordered carbon and also a very high specific capacitance (∼0.15 F/m2). It is also concluded, by comparison with literature data, that the specific capacitance (which depends on the density of surface functional groups) is closely correlated with the nature of the carbon precursor that is used in synthesizing the OMC powder.
Section snippets
HMS synthesis
The synthesis of mesoporous HMS was based on a procedure reported previously [8]. In a typical preparation, the molar ratio used was 1.0 TEOS: 0.3 surfactant (CnH2n+1NH2, with n varying between 8 and 16, 99%, Aldrich):9.1 EtOH:29.6 H2O. The surfactant was dissolved in the EtOH/H2O solution, and the TEOS was then added dropwise under stirring. The resulting mixture (pH 11) was stirred for 24 h at room temperature. The product was filtered, washed with 1000 ml of deionized water, and dried in air
Physicochemical properties of HMS template materials
HMS-8 and -10 templates (n values of 8 and 10, respectively), both in the as-synthesized and calcined forms, give very similar XRD patterns. Fig. 1 shows the XRD pattern for the calcined samples. The d-spacings, in the range of 3.4–4.6 nm, closely match the values reported by Pinnavaia et al. in their original work [8], and are indicative of short range hexagonal ordering (>1 nm) of the pores. However, the HMS templates prepared from the longer chain length surfactants (HMS-12 and -16) show two
Conclusions
Templated ordered mesoporous carbons (OMCs) containing bimodal pores, an interconnected wormhole structure, high textural mesoporosity, and a high content of surface oxygen functional species, were synthesized in this work using hexagonal mesoporous silica (HMS) [8] templates. These templates were formed from surfactants with chain lengths which varied from 8 to 16 carbons in length. The small angle XRD pattern of the HMS templates showed that they all possess short range order in their porous
Acknowledgements
We gratefully acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC), through both the Discovery and Strategic Project programs, as well as Ballard Power Systems, for the financial support of this work. We also thank the Alberta Ingenuity Fund for scholarship support of DB, and Dr. Josephine Hill (Chemical and Petroleum Engineering, University of Calgary) for providing access to BET analysis facilities.
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