C A R B O N 4 8 ( 2 0 1 0 ) 1 0 5 6 – 1 0 6 3

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Bimodal, templated mesoporous carbons for capacitorapplications

Dustin Banham, Fangxia Feng, Jason Burt, Enam Alsrayheen, Viola Birss *

Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada T2N 1N4

A R T I C L E I N F O

Article history:

Received 28 July 2009

Accepted 16 November 2009

Available online 23 December 2009

0008-6223/$ - see front matter � 2009 Publisdoi:10.1016/j.carbon.2009.11.025

* Corresponding author: Fax: +1 403 289 9488E-mail address: birss@ucalgary.ca (V. Birs

A B S T R A C T

Several high capacitance ordered mesoporous carbon (OMC) materials, containing a bimo-

dal pore distribution, were synthesized directly using hexagonal mesoporous silicas (HMS)

as the template material. The HMS templates were formed using amine surfactants

(CnH2n+1NH2) with hydrophobic chain lengths containing 8–16 carbons (n = 8–16). These

HMS structures were found to have an interconnected wormhole structure, high textural

mesoporosity, a surface area ranging from 910 to 1370 m2/g, and a total pore volume of

1.09–1.83 cm3/g. Also, evidence for a change in structure from hexagonally ordered to lay-

ered (for surfactants of chain length with n > 12) was found. The resulting OMCs, formed

using sucrose as the carbon precursor, contain bimodal pores 1.6–1.8 and 3.3–3.9 nm in

diameter and have a very high surface area (980–1650 m2/g). The OMCs were evaluated as

electrode materials for electrochemical capacitors using cyclic voltammetry in 0.5 M H2SO4

solution, giving a tunable gravimetric capacitance that increased linearly with BET area

(and surfactant chain length), up to 260 F/g, among the highest yet reported for ordered car-

bon formed from an HMS templated precursor. All OMCs studied in this work displayed a

specific capacitance of �0.15 F/m2.

� 2009 Published by Elsevier Ltd.

1. Introduction

High surface area carbon materials have attracted significant

attention for many applications, including as a catalyst sup-

port in fuel cells, as well as in energy storage devices such

as batteries and electrochemical capacitors [EC] [1–4]. 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 bal-

ance the electrical charge on the solid surface, and is there-

fore 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 diam-

hed by Elsevier Ltd.

.s).

eter 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]. Pseudocapaci-

tance is related to the properties and surface density of re-

dox-active functional groups, such as quinone and

hydroquinone, which are rapidly oxidized/reduced as the po-

tential is cycled positively/negatively [3].

An important parameter in characterizing potential capac-

itor materials is the specific capacitance (F/real m2), which de-

pends 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 sur-

face area of the meso- and micropores, the properties of the

electrolyte (concentration), and the density of surface func-

tional groups. In comparison, gravimetric capacitance (F/g)

C A R B O N 4 8 ( 2 0 1 0 ) 1 0 5 6 – 1 0 6 3 1057

takes into account both double layer (surface area) and pseud-

ocapacitance (density of functional groups). These capaci-

tances are equally important in assessing the viability of a

new carbon material for use as an EC, and both will be dis-

cussed in this paper.

Until recently, research into carbon-based ECs has been fo-

cused on high surface area activated carbons [4]. These mate-

rials 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 car-

bon (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 syn-

thesized 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 worm-

hole structure, and high textural mesoporosity [8]. The thick

walls are particularly advantageous, as these walls become

the pores of the resulting templated carbon. OMCs synthe-

sized from HMS templates are therefore expected to have lar-

ger 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 com-

parison 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.

2. Experimental methods

2.1. HMS synthesis

The synthesis of mesoporous HMS was based on a procedure

reported previously [8]. In a typical preparation, the molar ra-

tio used was 1.0 TEOS: 0.3 surfactant (CnH2n+1NH2, with n vary-

ing 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 at room temperature for 12 h. The sur-

factant was removed by calcining at 650 �C for 4 h in air with a

heating rate of 10 �C/min. These SiO2 templates are coded as

HMS-n, where ‘n’ refers to the chain length of the surfactant

that was used.

2.2. OMC synthesis

The preparation of the ordered mesoporous carbons (OMC)

also followed a procedure reported previously [3,5], except

that HMS (a wormhole structure) silica was used here, as op-

posed to SBA-15 (a hexagonal structure) [11]. One gram of cal-

cined HMS was added to a solution containing 1.25 g of

sucrose, 0.14 g of H2SO4, and 5 g of water and then the solu-

tion was mixed thoroughly, dried at 100 �C for 6 h, and then

dried at 160 �C for another 6 h. The resulting product was

added to a solution containing 0.8 g of sucrose, 0.09 g of

H2SO4, and 5 g of water, mixed thoroughly, and dried follow-

ing the same steps. Carbonization was conducted at 900 �Cfor 2 h under vacuum at a heating rate of 5 �C/min. 1.0 g of

carbon–HMS composite was refluxed with 100 ml of 2.5 wt.%

NaOH solution for 24 h, washed with ethanol, and dried at

120 �C for 12 h. The OMCs formed are labelled as OMC-n,

where ‘n’ refers to the chain length of the surfactant that

was used in synthesizing the HMS template.

2.3. Material characterization

X-ray powder diffraction (XRD) patterns were obtained using a

Rigaku Multiflex X-ray diffractometer (Department of Geol-

ogy, University of Calgary), using Cu Ka radiation

(k = 0.15406 nm). The operating conditions were 40 kV and

20 mA and the data were processed with Jade software (Ver-

sion 6.5).

Nitrogen adsorption–desorption isotherms were collected

at �196 �C using a Micromeritics Tristar 3000 analyzer. Before

analysis, the samples were out-gassed at 150 �C for 2 h in N2.

The specific surface area (SBET) was obtained using the BET

(Brunauer–Emmett–Teller) plot (0.05 < P/P0 < 0.30), where P is

the partial pressure of the adsorbate gas and P0 is the vapour

pressure of the adsorbate gas. The total pore volumes (Vp)

were calculated at P/P0 = 0.99, while the pore size distribution

curves were calculated from the desorption branch of the iso-

therm using the BJH (Barrett–Joyner–Halenda) mode. The wall

thickness (W) was obtained using the following relationship:

W = a0 � D, where a0 is the unit cell parameter obtained from

XRD analysis and D is the pore diameter.

2.4. Electrochemical evaluation of OMC properties

Cyclic voltammetry (CV) was carried out using a Solartron

1287 potentiostat and a three-electrode cell. The working

electrode (WE) was a 7 mm diameter glassy carbon rotating

disc electrode. The films used for testing the capacitance of

the OMC samples were prepared by dispersing 0.01 g of OMC

powder into 0.4 g of 1.0 wt.% Nafion/iso-propanol. This mix-

ture was then sonicated for 15 min to obtain an ink and

1058 C A R B O N 4 8 ( 2 0 1 0 ) 1 0 5 6 – 1 0 6 3

14 ll were pipetted onto the glassy carbon WE, resulting in a

carbon loading of �0.3 mg. The WE was then dried at room

temperature for 20 min. In preparing these samples, the mass

of the deposited 14 ll aliquot was measured (typically 11.0–

11.5 mg) on a Mettler Toledo AB204 analytical balance and

the amount of OMC deposited was then calculated. We have

found this to be a very accurate and reproducible approach,

with little variation observed between trials.

A reversible hydrogen electrode (RHE) was used as the ref-

erence electrode (RE) and Pt gauze as the counter electrode

(CE). All CVs (5–500 mV/s) were collected in deaerated 0.5 M

H2SO4 at 25 �C in a potential range of 0.05–1.1 V vs. RHE. For

comparison, an ink made using the same procedure as de-

scribed above, but using Vulcan XC-72R (VC-72R, CABOT) in-

stead of the OMC carbons, was also tested electrochemically.

Capacitance values of the carbon samples were deter-

mined from the cathodic charge passed by sweeping between

0.05 and 1.1 V. This method of determining capacitance was

deemed reliable, as the capacitance calculated for VC-72

matched very closely with reported literature values [12], as

well as with values obtained using impedance methods in

parallel work.

3. Results and discussion

3.1. Physicochemical properties of HMS templatematerials

HMS-8 and -10 templates (n values of 8 and 10, respectively),

both in the as-synthesized and calcined forms, give very sim-

ilar XRD patterns. Fig. 1 shows the XRD pattern for the cal-

cined 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 peaks, with the d-spacing of the second

peak being about half of the first. These differences may re-

flect the transformation of hexagonally packed HMS-8 and -

10 to a layered structure in the case of HMS-12 and -16 [13].

It is known that, for a surfactant carbon chain length

of P 14 atoms, the hydrophobic chains can become too bulky

to pack together into rod-like micelles, and as a result, the

surfactant may begin to form lamellar micelles [14].

Fig. 1 – XRD patterns of calcined HMS-8 to HMS-12.

The raw N2 isotherm data is given in Fig. 2. HMS-8-Ca

(where Ca means calcined) appears to give a Type I isotherm,

indicative of a microporous material [15]. For HMS-10-Ca and

HMS-12-Ca, the isotherms indicate a structure between Types

I and IV, as a sharp increase in adsorption is observed at par-

tial pressures of 0.15–0.25, but no clear hysteresis is observed.

This suggests that the pores of HMS-10-Ca and HMS-12-Ca are

just large enough to start allowing capillary condensation to

occur, which is responsible for the sharp increase in adsorp-

tion in this pressure range [15]. HMS-16-Ca is the only HMS

structure to display the classical characteristics of a Type IV

(mesoporous) isotherm in Fig. 2, with a distinct hysteresis

seen between the adsorption and desorption branches at P/

P0 � 0.35. These N2 isotherms therefore qualitatively show

that the pore size of the HMS structures increase in the order

of HMS-8-Ca < HMS-10-Ca < HMS-12-Ca < HMS-16-Ca, fully as

expected in relation to the length of the surfactant chains

used in their synthesis. Table 1 shows more precisely the pore

size distributions for the HMS structures, supporting the

qualitative implications of the N2 isotherms (Fig. 2). The pore

size and volume clearly increases with increasing n values.

Fig. 3 shows a plot of the HMS BET-measured pore size vs.

the calculated micelle sizes for the four surfactants. The lat-

ter were estimated using the normal carbon–carbon bond

length for saturated hydrocarbons (0.154 nm [16]) and also

taking into consideration the size of the NH2 group at the

end of the chain (0.26 nm [17]). The diameter of a single mi-

celle is assumed to be equal to the length of two fully ex-

tended surfactant chains. Fig. 3 shows that the measured

and predicted micelle (pore) sizes of the HMS structures are

linearly related. The fact that all of the measured pores are

slightly smaller (by �25%) than the predicted micelle size

can be attributed to the fact that the calculation assumes that

all of the surfactant tails are fully extended, which, in reality,

is unlikely. Fig. 3b also shows that a maximum appears in the

plot of HMS area vs. the number of carbons in the surfactant

(chain length), consistent with the results reported by Pinna-

vaia et al. [8] (Fig. 3b). However, the maximum surface area is

obtained for HMS formed using a n = 12 surfactant in our

work, while the maximum was seen for a surfactant with

an n value of 10 in Ref. [8].

Fig. 2 – N2 adsorption isotherms of HMS-8-Ca to HMS-16-Ca

template materials.

Table 1 – Summary of physical characteristics of the four silica templates used in preparing mesoporous carbons.

Silica sample HMS-8 HMS-10 HMS-12 HMS-16

Surfactant C8H17NH2 C10H21NH2 C12H25NH2 C16H33NH2

Expected micelle size (nm) 2.0 2.5 3.0 4.0Crystal size (nm) 8 ± 1 7 ± 1 10 ± 1 11 ± 1d100 (nm) 3.4 ± 0.2 3.5 ± 0.2 3.7, 1.8 ± 0.2 4.6, 2.3 ± 0.2Pore size (nm) 1.5 ± 0.2 1.7 ± 0.2 2.2 ± 0.2 3.1 ± 0.2Surface area (m2/g) 910 ± 10 1270 ± 130 1370 ± 130 1280 ± 130Wall thickness (nm) 2.4 ± 0.2 2.3 ± 0.2 2.1 ± 0.2 2.2 ± 0.2Vtotal (cm3/g)a 1.21 ± 0.1 1.34 ± 0.1 1.09 ± 0.1 1.83 ± 0.1Vmicro (cm3/g)a 0 0 0 0Vframe (cm3/g)a 0.47 0.63 0.79 1.30Vtextural (cm3/g)a 0.74 0.71 0.30 0.53Vt/Vf

a 1.57 1.13 0.38 0.41a0 (nm)a 3.93 ± 0.2 4.04 ± 0.2 4.33 ± 0.2 5.31 ± 0.2

Vmicro – total pore volume from micropores.

Vframe – total pore volume at P/P0 < �0.4.

Vtextural – total pore volume at P/P0 > �0.4.

Vt/Vf – ratio of textural to framework porosity.

a0 – distance between pores (center to center).a Vtotal – total pore volume determined at P/P0 = �0.99.

Fig. 3 – (a) Measured vs. predicted pore size of HMS

structures, and (b) surface area of HMS structures vs.

number of carbon atoms in surfactant (both our results and

those of Pinnavaia [8] are shown).

Fig. 4 – High angle (10–90o) XRD pattern of OMC-8 to OMC-16.

C A R B O N 4 8 ( 2 0 1 0 ) 1 0 5 6 – 1 0 6 3 1059

Table 1 summarizes the physicochemical properties of the

four HMS precursors used in this work. The surface areas and

total pore volumes are in the range of 910–1370 m2/g and 1.09–

1.83 cm3/g, respectively, while the wall thicknesses are

�2.3 nm in all four cases. The surface areas of our HMS sam-

ples trend well with those reported by Tanev and Pinnavaia

[8]. It can be seen that the framework porosity increases with

increasing n. The spacing between pores (a0), given in Table 1,

is very similar for HMS-8-Ca and HMS-10-Ca (both being

�4 nm). However, the spacing between pores shows a clear

increase for HMS-12-Ca, and reaches its maximum value for

HMS-16-Ca at 5.3 nm. These data, along with the trend in sur-

face area observed by us, and by Tanev and Pinnavaia [18],

suggest that the structure of the HMS template changes as

the surfactant length is increased beyond a chain length of

12 carbons, perhaps involving a transition from an ordered

rod-like to a lamellar morphology.

3.2. Physicochemical properties of OMC materials

All of the OMCs displayed similar high angle XRD patterns, as

shown in Fig. 4. It is known [19] that the use of aromatic car-

bon precursors will result in more graphitic carbon structures

than when non-aromatic precursors are used. However, even

with the use of sucrose as a carbon precursor in the present

work, the XRD peaks in Fig. 4 are narrower than what has

1060 C A R B O N 4 8 ( 2 0 1 0 ) 1 0 5 6 – 1 0 6 3

been reported previously for sucrose derived OMCs [20]. This

indicates the formation of relatively large graphite crystallites

using our approach. None of the low angle XRD patterns dis-

play clear peaks, suggesting that the hexagonal ordering of

the HMS templates was not imparted to the OMCs.

A selection of the N2 isotherms and pore size distributions

of the OMC structures is given in Fig. 5a and b, showing mes-

oporous characteristics. A new feature seen in this work is the

bimodal porosity of the OMCs. The smaller pore size (1.6–

1.8 nm, Table 2) corresponds roughly to the wall thickness

of the original HMS templates, formed after the sucrose is

infiltrated and then carbonized in the HMS pores and after

the SiO2 template is removed during the NaOH reflux. The

fact that the carbon pores are slightly smaller than the HMS

wall thickness may be due to the partial collapse of the car-

bon structure during NaOH refluxing.

The explanation for the presence of the larger pores is less

clear, but may be due to the fact that two sequential carbon

(sucrose) loading steps were used in our work. In the first infil-

tration step, it is likely that the aqueous sucrose solution pref-

erentially coated the hydrophilic HMS pore walls [15]. In the

second loading, after carbonization of the first aliquot of su-

crose on the pore walls, the pores may not have filled com-

pletely, as the carbonized walls would be less hydrophilic

than the original HMS surface. This sequence of steps could

Fig. 5 – (a) N2 adsorption isotherms and (b) pore size

distributions of OMC-8 to OMC-16.

then result in two pore sizes. The smaller pores (equal to

the HMS wall thickness) will arise from regions where the

pores were completely filled, while the larger ones would

have a size equivalent to the wall thickness plus any void

space left in the unfilled HMS pore. Work is presently under-

way to confirm this suggestion. The approximate distribution

of the two types of pores is 60–70% for the small pores, and

30–40% of the larger (double) sized pores.

For comparison, the data for VulcanTM carbon (VC) are also

included in Fig. 5 and Table 2. While VC is not an appropriate

choice for use in electrochemical capacitors, due to its rela-

tively small surface area, it is used in this work primarily to

compare the electrochemical responses of our bimodal, mes-

oporous carbons with that of a typical microporous carbon. It

can be seen that the pore size and surface area of VC are both

much smaller than for our OMCs. The structural properties of

various activated carbons and mesoporous carbons from the

literature are also given in Table 2. OMC-16 has an area

(1650 m2/g) approaching that of the previously reported

OMCs, while OMC-8, -10, and -12 are notably lower in area.

However, our OMCs are the only carbons to possess a bimodal

pore size distribution, with the exception of C-50-3/7-HT,

which required a special alteration of its synthesis to produce

this outcome. It has been shown that a bimodal pore size dis-

tribution, with interconnected pores, allows for better perfor-

mance of capacitor materials at high current densities [20].

3.3. Capacitive properties of mesoporous carbons

Fig. 6a shows the CVs of our OMCs, as well as of VC-72R for

comparison. All of the carbon samples display a set of peaks

centered at �0.6 V, typically attributed to the reduction/oxida-

tion of the quinone/hydroquinone redox pair [3]. For our

OMCs, the similarity in the peak potentials and in the relative

size of the redox peaks compared to the background double

layer capacitance indicates that all of the carbon samples

have similar surface properties. Capacitance data were ob-

tained from the CV charge (10 mV/s, 0.05–1.10 V), and when

divided by the mass of carbon in a 14 ll aliquot

(�2.5 · 10�4 g), this gives the gravimetric capacitance. This is

a common method used in the literature, as well as imped-

ance spectroscopy (EIS) [21,22] and galvanostatic charge/dis-

charge experiments [9]. While these techniques are also

very informative, the similarity between our measured capac-

itance for VC-72 and the EIS-determined values in the litera-

ture indicates that the CV approach can be trusted.

VC-72R shows a gravimetric capacitance of 21 F/g (Table 2),

very close to the values reported in the literature [12]. In com-

parison, the values for our OMCs are 130–260 F/g and a plot of

gravimetric capacitance (Table 2) vs. BET surface area (propor-

tional to the number of carbon atoms in the surfactant) is gi-

ven in Fig. 6b. This is expected, since as the surface area is

increased, the total double layer area is increased. However,

this correlation is rarely found in the literature, as most acti-

vated carbons studied previously contain primarily microp-

ores. These are often inaccessible to ions in solution, when

probed with electrochemical techniques, and are not observa-

ble when using methods based on the Kelvin equation to

interpret gas sorption data [23]. The fact that the density of

functional groups is not the same for different carbon sur-

Table 2 – Summary of physical and electrochemical properties of ordered mesoporous carbons.

Type of carbon Surfacearea (m2/g)

Pore diameter(nm)

Gravimetriccapacitance (F/g)

Specific capacitance(F/m2 real area)

References

VC-72 250 <2 22 0.09 [12]MSC25 1970 <2 200 0.10 [30]YA 2200 <2 118 0.05 [31]PX-21 3500 <2 190 0.05 [32]CS48 2000 2.7 170 0.09 [21]CS15 1470 3.1 144 0.10 [21]C-50-3/7-HT 2060 2.5, 5.5 187 0.09 [9,10]VC-72 222 ± 5 <2 21 ± 2 0.09 ± 0.01 This studyOMC-8 980 ± 10 1,8, 3.9 130 ± 7 0.14 ± 0.02 This studyOMC-10 1140 ± 11 1.7a 170 ± 9 0.15 ± 0.02 This studyOMC-12 1240 ± 12 1.8, 3.8 175 ± 9 0.14 ± 0.02 This studyOMC-16 1650 ± 17 1.6, 3.3 260 ± 12 0.16 ± 0.02 This studya The larger pore was not easily resolvable for OMC-10.

Fig. 6 – (a) Cyclic voltammograms (10 mV/s) of OMC-8 to

OMC-16 and VulcanTM carbon in a deaerated, 25 �C, 0.5 M

H2SO4 solution, and (b) resulting gravimetric capacitance vs.

BET-obtained surface area of OMCs, with best-fit line was

passed through the origin.

C A R B O N 4 8 ( 2 0 1 0 ) 1 0 5 6 – 1 0 6 3 1061

faces can also impact on the relationship between gravimet-

ric capacitance and BET area [4,24,25]. Notably, the linearity

of Fig. 6b suggests that all of the carbon samples prepared

in this work have similar surface properties.

The gravimetric capacitance of OMC-16 is the highest ever

reported yet in the literature for any templated mesoporous

carbon [9,20,21,26]. Compared to CS48 [21], which is made

using a 3D cubic mesoporous silica template (MCM-48) and

CS15 [21] (prepared from 2D hexagonal SBA-15), OMC-16 has

been synthesized here using hexagonal mesoporous silica

(HMS) that has an interconnected wormhole structure, as well

as high textural mesoporosity. OMC-16 has the same desir-

able characteristics as does HMS, leading to greatly facilitated

mass/electron transport [10].

OMC-16 also shows a much higher gravimetric capaci-

tance than what has been reported for other carbons, such

as templated mesoporous carbons, carbon nanotubes, and

microporous carbons obtained from carbide-derived car-

bons[3,10,21,27–29]. For example, after the carbonization of

a mixture of polyvinyl alcohol with Mg citrate, the resulting

mesoporous carbon, having a pore size of 8 nm, gave a capac-

itance of 180 F/g [29]. CS48 (from MCM-48) and CS15 (from

SBA-15) showed gravimetric capacitances of 170 and 144 F/g,

respectively [21], while C-50-3/7-HT (bimodal pores 2.5 and

5.5 nm in diameter) gave a capacitance of 187 F/g [9]. It should

be noted that most of these previously reported gravimetric

capacitance values were obtained using much thicker layers

(e.g., pellets) than were used in the present study, thus poten-

tially leading to transport limitations. However, as these val-

ues were all obtained at low dV/dt rates, comparison

between these values and ours remains valid.

In the present work, the specific capacitance (Cs) of the

OMCs (Table 2) was calculated according to [25]:

CsðF=m2Þ ¼ CGðF=gÞSBETðm2Þ ð1Þ

where CG is the gravimetric capacitance and SBET is the sur-

face area, as determined from BET analysis. Differences in

specific capacitance may be due to differences in pore size

(smaller pores may not allow easy access to ions) or in the

type and number of surface functional groups (more func-

tional groups per unit area would give a higher pseudocapac-

itance). In the present work, all of the OMCs have a similar

pore size and were made from the same carbon precursor (su-

crose), so it is not surprising that they have very similar spe-

cific capacitances of �0.15 F/m2, consistent with the slope of

the plot in Fig. 6b.

Compared to VC-72R, all of the OMCs show much higher

gravimetric and specific capacitances (Table 2). Our best car-

bon is OMC-16, which has the highest gravimetric capaci-

tance at 258 F/g, and a specific capacitance of 0.16 F/m2.

OMC-16 also shows a higher gravimetric capacitance when

1062 C A R B O N 4 8 ( 2 0 1 0 ) 1 0 5 6 – 1 0 6 3

compared to literature-reported activated carbons (Table 2),

containing mostly micropores (<2 nm) and having a very high

surface area, such as MSC25 (1970 m2/g, 200 F/g) [30], YA

(2200 m2/g, 118 F/g) [31], and PX-21 (�3500 m2/g, 190 F/g) [32].

Although OMC-16 has a much lower surface area than these

activated carbons, the higher gravimetric capacitance can be

attributed to the higher concentration of surface functional

groups and the larger, interconnected pores, which facilitate

mass transport [20,26].

The reason for the very large capacitance of our OMCs is

assumed here to be related to our choice of sucrose as the

precursor. It has been demonstrated that similarly high spe-

cific capacitances have been observed when ultramicropores

(d < 0.7 nm) are present [1]. While we did not do DFT studies

to rule out the possible presence of ultramicropores, t-plot

analysis indicated a negligible volume of micropores.

Although it is possible that some ultramicropores exist, the

results of the t-plot analysis suggest the contribution of ultra-

micropores to the total surface area is negligible and thus

should not have a significant effect on the high specific capac-

itance that we observe. We therefore believe that the large

specific capacitances we observed are due to a high density

of functional groups, as evidenced by the CVs shown in Fig 6a.

In comparison, C-50-3/7-HT [10], with its lower specific

capacitance, was synthesized using the identical method as

we used for OMC-16, but using furfuryl alcohol (32.6% oxygen)

instead of sucrose (51.4% oxygen). A high oxygen content

should increase both the gravimetric and specific capaci-

tance, as the oxygen-bound carbon surface sites will undergo

redox chemistry, increasing the pseudocapacitance of the car-

bon [3]. This assumption is supported by the fact that the ra-

tio of oxygen content in sucrose vs. furfuryl alcohol (�1.6)

roughly matches the ratio of the specific capacitance of our

OMCs to that of C-50-3/7-HT (�1.7).

To investigate how accessible the pores of these OMC sam-

ples are to the solution, their capacitance was measured at

various sweep rates ranging from 5 to 500 mV/s. It was found

(Fig. 7) that, compared to Vulcan carbon layers prepared iden-

tically and of the same thickness, the OMCs retain a larger

fraction of their capacitance at all sweep rates used in this

work. This shows that the larger pores of our OMCs (as com-

pared to Vulcan carbon) allow for faster ion transport of ions.

A similar observation that smaller pores lead to a more signif-

Fig. 7 – Fraction of total capacitance retained by OMC-(8–16)

and Vulcan carbon at sweep rates varying from 5 to 500 mV/s.

icant drop in capacitance than larger pore materials was also

reported by Xing et al. [33], who examined the capacitance

fade of relatively thick mesoporous carbon pellets at sweep

rates from 5 to 50 mV/s in a 30 wt.% KOH solution. While di-

rect comparison between the values obtained by Xing et al.

[33] and the values obtained in this work are not possible

due to the differences in electrode thickness, the same obser-

vation that smaller pore materials have faster capacity fade

than larger pore materials can be observed.

4. 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 hex-

agonal mesoporous silica (HMS) [8] templates. These tem-

plates 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 pos-

sess short range order in their porous structure, with a pore

size similar to what is predicted from the surfactant chain

length. A possible change in HMS structure, from hexagonally

packed to layered, was observed as the surfactant chain

length increased beyond 12 carbons.

These HMS templates were impregnated with sucrose, car-

bonized, and then the silica was etched out with NaOH, leav-

ing an ordered mesoporous carbon (OMC) structure. High

angle XRD analysis showed that the pore walls of the OMCs

are more graphitic, than other OMCs reported in the litera-

ture, while the low angle XRD results indicate that the OMCs

do not display order in their porous structure. However, the

OMCs do possess a very narrow pore size distribution, as

determined from gas sorption studies. Also, they were found

to be bimodal, with small pores ranging from 1.7 to 1.9 nm,

and larger pores ranging from 3.3 to 3.9 nm in diameter. The

bimodal nature of the OMC porosity was explained in terms

of partial filling of regions of the HMS template, carried out

in two successive steps.

These OMC materials were investigated using cyclic vol-

tammetry (CV) in deaerated 0.5 M H2SO4 solution at 10 mV/s

and 25 �C. The gravimetric capacitance, determined from

the CV charge passed over a fixed potential range, was found

to increase linearly with the size of the surfactant chain used

to form the OMS templates, allowing the gravimetric capaci-

tance to be easily tuned. We attribute this to the fact that

our OMCs have a high surface density of functional groups

that can undergo pseudocapacitive reactions, as demon-

strated by the large specific capacitance (0.15 F/m2) obtained.

Finally, our OMCs deliver a larger percentage of their charge

than does Vulcan carbon (a typical microporous carbon) at ra-

pid (500 mV/s) sweep rates. Overall, the OMCs developed here

are highly promising candidates for application in electro-

chemical capacitors.

Acknowledgements

We gratefully acknowledge the Natural Sciences and Engi-

neering Research Council of Canada (NSERC), through both

the Discovery and Strategic Project programs, as well as Bal-

C A R B O N 4 8 ( 2 0 1 0 ) 1 0 5 6 – 1 0 6 3 1063

lard 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 Engi-

neering, University of Calgary) for providing access to BET

analysis facilities.

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