Carbon 41 (2003) 1049–1056

T he effect of silica template structure on the pore structure ofmesoporous carbons

*Sangjin Han, Kyu T. Lee, Seung M. Oh, Taeghwan HyeonSchool of Chemical Engineering, Seoul National University, Seoul 151-744, South Korea

Received 2 October 2002; received in revised form 28 November 2002; accepted 16 December 2002

Abstract

We have synthesized two kinds of mesoporous carbons using a spherical silica sol (SMC1 carbon) and an elongated silicasol (SMC3 carbon) as templates. Nitrogen isotherms and electrochemical experiments were performed to investigate theeffect of the silica template structure on the pore structure of the resulting mesoporous carbons. When carbons producedusing the same silica to resorcinol molar ratio were compared, both nitrogen isotherms and electrochemical studies revealedthat the SMC3 carbons exhibit simpler pore connectivity than SMC1 carbons. 2002 Elsevier Science Ltd. All rights reserved.

Keywords: A. Porous carbon; B. Carbonization; C. BET surface area; D. Electrochemical properties

1 . Introduction example, by adding surfactant which induces the formationof a doughnut-like shape, or by applying an electric field

Template synthetic approaches have been utilized exten- which results in a concaved disk-like shape [8,9].sively in the fabrication of various porous materials. Many For many years, activation processes (chemical ormesoporous materials have been synthesized using the physical methods) have been used to produce porousself-assembly of surfactants or block-copolymers structures carbon materials, most notable activated carbons [10,11].as templates [1,2]. Recently, different kinds of macro- However, the pore size and structure cannot be easilyporous materials have been fabricated using spherical controlled through the activation process. Various types ofsubmicrometer-sized templates [3,4], and several macro- inorganic templates have been used to design and controlporous inorganic and metallic materials with closed packed the pore structure of porous carbon materials. Kyotani andpore arrangements have been synthesized using monodis- co-workers reported the synthesis of microporous carbonperse polystyrene latex spheres [5]. Using colloidal crys- materials using zeolites as templates [12,13]. Our researchtals composed of closed-packed monodisperse silica ma- group and several other groups have synthesized differentterials as templates, macroporous carbons and polymers mesoporous carbon materials using mesoporous silicahave been synthesized. Zakhidov et al. [6] synthesized materials as templates [14–18]. Some of these mesoporousmacroporous carbons with close-packed|100 nm pores carbons exhibit excellent performance as electrode materi-using synthetic opals as templates and Mallouk and co- als in electrochemical double-layer capacitors (EDLC) andworkers [7] reported the synthesis of mesoporus polymers in fuel cells [19,20]. One major disadvantage in thewith a pore size of|50 nm using close-packed silica application of mesoporous silica materials to the fabrica-nanoparticles as templates. Using such spherically shaped tion of mesoporous carbon materials is the high productiontemplates, porous materials with spherical pore structures costs. We have synthesized mesoporous carbon materialshave been synthesized. Few research groups have reported using cheap commercial silica sols as templates. Mesopor-an ability to control the shape of porous materials by ous carbons, designated as SMC1 (silica-sol mediatedcoupling hydrodynamics and particle accumulation, for synthesized carbon one) carbons, with a wide range of pore

sizes, from 10 to 100 nm, have been fabricated usingLudox HS-40 aqueous silica sol as a template and re-*Corresponding author. Tel.:182-2-880-7150; fax:182-2-sorcinol-formaldehyde gel as a carbon precursor [21,22].880-7295.

E-mail address: thyeon@plaza.snu.ac.kr(T. Hyeon). We have also synthesized mesoporous carbons with uni-

0008-6223/02/$ – see front matter 2002 Elsevier Science Ltd. All rights reserved.doi:10.1016/S0008-6223(02)00439-6

1050 S. Han et al. / Carbon 41 (2003) 1049–1056

Table 1 2 . Experimental sectionThe physical properties of colloidal silica template solutions

2 .1. Preparation of SMC1 and SMC3 carbonsSample name SiO Density Primary Shape2

(wt.%) particlesize The overall synthetic procedures have been described in

the previous papers [21,22]. For the synthesis of the SMC1Ludox 30 1.22 8 nm Sphericaland SMC3 carbons, the aqueous silica sol solutions,SM-30 sol

DuPont Ludox SM-30 and Nissan SNOWTEX-UP ,SNOWTEX 20 1.13 Width of 5–20 Elongated respectively, were used as templates. The aqueous silica-UP nm, and length silica sol sol solutions were purchased from Aldrich and the Nissan,

of 40–300 nmrespectively, and their characteristics are shown in Table 1.Ludox SM-30 contains spherical silica sol nanoparticleswith an average particle diameter of 8 nm, and SNOW-TEX-UP have the elongated silica sol with thickness

form pore sizes in the range 8–12 nm, designated as SMC2 ranging from 4 to 20 nm. The transmission electroncarbons, using surfactant stabilized silica sols as templates micrographs (TEM) of these two silica solutions are shown[23]. These SMC1 carbons have been shown to be good in Fig. 1. Resorcinol (99%, ACS reagent) and formalde-adsorbents for large molecules, such as dyes and humic hyde (37 wt.% aqueous solution. ACS reagent) wereacids. Recently, Li and Jaroniec reported upon the syn- polymerized in the presence of the silica solution tothesis of mesoporous carbons using mesophase pitch or generate silica/ resorcinol-formaldehyde (RF) gel compos-polyacrylonitrile as a carbon precursor and silica sol or ites. Carbonization of these silica/carbon-precursor com-silica gel as a template [24,25]. For many applications, posites followed by etching using aqueous NaOH solutionmost notably for electrode materials in electrochemical resulted in the formation of porous carbons. In a typicaldevices, the fabrication of porous carbon materials having synthesis, a 1:2 (molar ratio) mixture of resorcinol andwell-connected pores is very important [26,27]. In porous formaldehyde was added to Ludox SM-30 solution (formaterials, the characterization of pore connectivity is SMC1) or SNOWTEX-UP solution (for SMC3). Theextremely difficult. In this paper, we report on the fabrica- reaction mixture, without further pH adjustment, was curedtion of mesoporous carbons having well-connected pores at 858C for 5 days to form a silica/ resorcinol-formalde-using an elongated silica sol as a template, designated as hyde (RF) gel composite. For the carbonization, theSMC3 carbons. We also compared the pore structures of composite was heated under a nitrogen atmosphere fromSMC1 and SMC3 carbons using gas adsorption/desorption room temperature to 8508C at a heating rate of 58C/minstudies and electrochemical techniques. and held at this temperature for 3 h. The resulting silica–

Fig. 1. The TEM micrographs of silica sol template solutions. (a) Ludox SM-30 silica sol; (b) Nissan SNOWTEX-UP silica sol.

S. Han et al. / Carbon 41 (2003) 1049–1056 1051

carbon composite was stirred in a 10 M NaOH solution for 3 . Results and discussion5 h. The carbon materials were retrieved by filtration andwashed exhaustively with deionized water until the pH of 3 .1. Synthesis and characterization of SMC1 and SMC3the filtrate reached 7. carbons

2 .2. Characterization of the mesoporous carbons SMC1 and SMC3 carbons were fabricated using asimilar synthetic procedure and nearly identical reaction

Nitrogen adsorption and desorption isotherms were conditions, except the use of different silica templates. Themeasured at 77 K using a Micromeritics ASAP2000 pH of both reaction mixtures, after adding resorcinol andsystem. Total surface area and pore volumes were de- formaldehyde into silica solution, were about 8, and notermined using the BET (Brunauer–Emmett–Teller) equa- further pH adjustment was made. In both cases, highertion and the single point method, respectively. Pore silica contents resulted in increased surface area and porecharacteristics in the range 1.7–300 nm were analyzed volume in the resulting mesoporous carbons. When theusing the BJH (Barrett–Joyner–Halenda) method. The silica/ resorcinol molar ratio was varied from 0.77 to 1.16pore size distribution curve was obtained by analyzing the for the synthesis of SMC1 carbons, the BET surface area

2adsorption branch of the nitrogen isotherm using the BJH increased from 457 to 746 m /g, and the single point totalmethod.S is the surface area as calculated using the pore volume increased from 0.68 to 1.5. As reported in ourBET

BET method.S is the cumulative adsorption surface previous paper, excessive silica content in the reactionBJH

area in the pore size of 1.7–300 nm as calculated by the mixtures resulted in a reduced surface area and poreBJH method.V is the single point total pore volume volume. In the case of relatively low silica content, theTOTAL

of pores&150 nm.V is the cumulative adsorption pore surface area and pore volume of the mesoporous carbonsBJH

volume in the pore size of 1.7–300 nm as calculated by the increased as silica content was increased, demonstratingBJH method. Mesoporosities were calculated from the that these silica nanostructures effectively function asratio of S to S . D is the average pore diameter template and pore-generators. Nearly non-porous carbonBJH BET BET

calculated from the equation, 43V /S . Transmis- material was generated in the absence of silica template,TOTAL BET

sion electron micrographs were obtained on a Jeol JEM- confirming the templating role of silica materials. Table 22000EXII at an accelerating voltage of 200 kV. Elemental shows the pore characteristics of SMC1 and SMC3analysis was performed using an EA 1110 elemental carbons. When SMC1 and SMC3 carbon materials pre-analyzer. pared using the same silica to resorcinol molar ratio were

compared, the surface area and pore volume of the SMC32 .3. Electrochemical studies carbons were higher than those of SMC1 carbons.

In many applications of carbon materials, pore connec-To prepare the composite electrodes containing SMC1 tivity is a very important characteristic and we compared

or SMC3, a mixture of SMC carbon 10 mg and polytetra- the pore structures of SMC1 and SMC3 carbons usingfluoroethylene (PTFE) binder 1 mg (10:1 weight ratio) was nitrogen adsorption/desorption measurements and electro-dispersed in isopropyl alcohol, and coated on to a stainless chemical experiments. Among the five different types of

2steel Exmet (131 cm ) that was used as the current physisorption isotherms proposed by Brunauer, mesopor-collector. The resulting electrode plate was pressed under 3 ous materials are frequently characterized by the type IVp.s.i. and dried under vacuum at 1208C for 12 h. Electro- isotherms. Type IV isotherms exhibit increased adsorbedchemical testing was performed using a three-electrode volume at high relative pressure,p /p , resulting from0

configuration. The electrolyte was aqueous 2.0 M H SO adsorption in the mesopores. These isotherms also show2 4

and a Pt flag and a saturated calomel electrode (SCE) were hysteresis loops occurring from the difference betweenused as counter and reference electrode, respectively. adsorption and desorption, and the shape and the broadnessCyclic voltammetry was performed using an EG&G PARC of these hysteresis is affected by various pore characteris-362 potentiostat in the potential range of 0.0–0.7 V (vs. tics. Jaroniec and co-workers reported that the broadness ofSCE) at a scan rate varying from 5 to 50 mV/s. Chronoam- these hysteresis loops is mainly affected by two factors:perometry was conducted to estimate the RC time constant pore size distribution and pore connectivity [28,29]. Weand ESR (equivalent series resistance) of the electrochemi- conducted the nitrogen adsorption/desorption isotherms ofcal system. For this measurement, a potential step (DE510 SMC1 and SMC3 carbons in order to elucidate the effectmV) was applied at 0.2 V and the resulting current of template structure on the pore connectivity of thetransient was fitted toI(t)5 (DE /R)exp(2t /t) to extract resulting carbon materials. Fig. 2 shows the pore sizethe value of ESR, andt. To obtain the resistance of solely distribution curves of SMC1 and SMC3 carbons as calcu-electrode materials, the conductivity of these materials lated from the adsorption branch of nitrogen isotherms.were measured by van der Pauw method. The resistance of Both SMC1 and SMC3 carbons exhibited broad pore sizeelectrolyte was calculated from the conductivity data. distributions ranging from 2 to 50 nm, indicating that some

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Table 2The pore characteristics of SMC1 and SMC3 carbons

Sample name S S V V Meso-BET BJH Total BJH2 2 3 3(m /g) (m /g) (cm /g) (cm /g) porosity

SMC1-0.77 457 239 0.68 0.60 0.52SMC1-1.16 626 437 1.08 1.01 0.70SMC1-1.54 746 592 1.50 1.45 0.79SMC3-0.77 520 352 0.94 0.88 0.68SMC3-1.16 747 596 1.47 1.42 0.80SMC3-1.54 788 653 1.98 1.99 0.83

Sample name, SMC1-X (X5SiO /resorcinol molar ratio).S is the surface area calculated by the BET method;S is the cumulative2 BET BJH

adsorption surface area (pores 1.7–300 nm) calculated by the BJH method;V is the single point total pore volume (pores,150 nm);VTotal BJH

is the cumulative adsorption pore volume calculated by the BJH method (pores, 1.7–300 nm). Mesoporosity is defined as the ratio ofS /S .BJH BET

aggregation of primary silica particles had occurred during capacitor (EDLC) utilizes the double-layer formed atthe synthesis. The isotherm results demonstrate that the electrode/electrolyte interface where electric charges arebroadness of the pore size distribution is not affected by accumulated on the electrode surface and ions with oppo-the structure of silica template. The peak pore size of site charge on the electrolyte side [30]. For the goodSMC1 was centered at|8 nm, which matched very well performance, EDLC electrodes should have high electricalwith the average diameter of the primary nanoparticles of conductivity, a high surface area for charge accumulation,Ludox SM30 silica sol. The peak pore dimension of SMC3 and a sufficiently large pore diameter and good porecarbons center at|15 nm, and this agrees well with the connectivity for electrolyte wetting and rapid ionic mo-average thickness of the elongated silica sol. For both tions. The equivalent circuit for EDLC electrodes can becarbons, as the relative amount of silica template was represented as a serial combination of equivalent seriesincreased, the pore size distribution became narrow. The resistance (ESR) and double-layer capacitance [31]. Thenormalized nitrogen adsorption/desorption isotherms of shape of cyclic voltammograms recorded with EDLCthe SMC1 and SMC3 carbons are shown in Fig. 3. The electrodes is primarily affected by theRC time constantshapes of the hysteresis loops of these two kinds of (t5R3C, whereR is ESR andC is the capacitance) of thecarbons were found to be very different. When the electrode system [30,31]. Whent50, for ideal EDLChysteresis loops of SMC1 and SMC3 carbons produced performance, a scan-rate-independent rectangular profile isusing the same silica template content were compared, expected. However, when the case oft±0, the currentSMC3 carbon exhibited a narrower hysteresis loop than contains a transient part, which varies exponentially with aSMC1 carbon and the desorption branch of the SMC3 RC time constant, and a steady-state current. Ast becomescarbon decreased more sharply. As we described above, higher, the transient part lasts longer, and takes longer timethe broadness of the pore size distribution of these two to charge the capacitor, which results in a collapse of thekinds carbons were not affected by the silica template rectangular current profile. To investigate capacitance andstructure. Consequently, the main factor affecting the cyclovoltammograms shape, Cyclovoltammetry (CV) ex-shape of hysteresis loops seems to be the pore connectivi- periments were conducted. Fig. 4 shows capacitance versusty. The narrower the hysteresis loop and the steeper voltage profiles that were derived by dividing the currentdecrease of the desorption branch of SMC3 carbon than by the scan rate and the mass of materials. For scan ratesthose of SMC1 carbon seems to be resulted from the of 5 and 10 mV/s, both carbon materials exhibit nearlysimpler pore connectivity of SMC3 carbons. This result rectangular shape cyclovoltammograms, demonstrating thecan be explained by the well-interconnected elongated possible EDLC application of the materials. When the scanshape of the SNOWTEX-UP silica template compared to rate was increased to 50 mV/s, almost collapsed cyclovol-the spherical shape of Ludox SM30 silica sol. tammograms were obtained. The difference between the

cyclovoltammograms of the SMC1 and SMC3 carbonsprepared using the same silica to resorcinol molar ratio

3 .2. Electrochemical characterization of SMC1 and was not distinct, because both the capacitance and the ESRSMC3 carbons affected the RC time constant.

In order to estimate the ESR and theRC time constant,Electrochemical double-layer capacitance studies were two carbon electrodes were measured using chronoam-

conducted on the SMC1 and SMC3 carbons to investigate perometry. Exponentially decaying current transients weretheir pore connectivities. Electrochemical double-layer fitted using the equation,

S. Han et al. / Carbon 41 (2003) 1049–1056 1053

Fig. 2. The pore size distributions calculated by adsorption branch of the corresponding nitrogen isotherms using (a) SMC1-0.77, (b)SMC3-0.77, (c) SMC1-1.16, (d) SMC3-1.16, (e) SMC1-1.54, (f) SMC3-1.54.

I(t)5 (DE /R) exp(2t /t), carbon materials. Table 3 summarizes the electrochemicalcharacteristics of the SMC1 and SMC3 carbons. TheRC

to estimate theRC time constant (t), and ESR. The ESR is time constant (t) and ESR were determined by theprimarily composed of the electrode resistance (R ), chronoamperometric experiment and the capacitance waselectrode

the bulk electrolyte resistance (R ) and the electrolyte calculated from the cyclovoltammogram. When carbonbulk

resistance in the pores (R ), in other words, ESR5 materials prepared using the same silica to resorcinolpore

R 1R 1R [19]. So, R could be obtained molar ratio were compared,R of SMC3 carbon waselectrode bulk pore pore pore

by subtractingR and R from ESR. As a result, found to be smaller than that of SMC1 carbon. Thisbulk electrode

the value of R can be used as an indirect tool to demonstrates the simpler pore connectivity of SMC3pore

estimate the pore connectivity of the SMC1 and SMC3 carbon, which is prepared using elongated silica sol as

1054 S. Han et al. / Carbon 41 (2003) 1049–1056

Fig. 3. The normalized N adsorption/desorption isotherms in the region of P/P.0.5. (a) SMC1-0.77, (b) SMC3-0.77, (c) SMC1-1.16, (d)2 0

SMC3-1.16, (e) SMC1-1.54, (f) SMC3-1.54. These graphs were obtained by the absolute adsorbed volume converting into the relativeadsorbed volume.

template, and confirms the nitrogen adsorption/desorption 4 . Conclusionresults. R decreases as the relative amount of silicapore

template increases for each carbon series, showing that We synthesized mesoporous carbons using two differenthigh contents of silica template induce silica aggregation, kinds of silica templates; a spherical silica sol (Ludoxwhich results in the simpler pore connectivity of the SM30) and an elongated silica sol (Nissan SNOWTEX-carbon materials. UP). When the same molar ratio of SiO to resorcinol was2

S. Han et al. / Carbon 41 (2003) 1049–1056 1055

Fig. 4. The cyclic voltammetry graphs of SMC1-1.54 (dotted line) and SMC3-1.54 (solid line) in 2.0 M H SO solution at scan rates of (a) 52 4

mV/s and (b) 10 mV/s.

Table 3The electrochemical characterization of SMC1 and SMC3 carbons

Sample name ESR RC time C R R RCV electrode bulk pore2 2 2 2(V cm ) constant (s) (F/g) (V cm ) (V cm ) (V cm )

SMC1-0.77 18.18 4.42 59 0.044 1.46 16.676SMC1-1.16 16.66 5.10 70 0.024 1.46 15.176SMC1-1.54 11.76 4.86 96 0.028 1.46 10.272SMC3-0.77 11.76 3.64 69 0.034 1.46 10.266SMC3-1.16 10.52 4.39 89 0.034 1.46 9.026SMC3-1.54 8.00 3.01 87 0.015 1.46 6.525

The ESR and RC time constant were calculated from the chronoamperometry.C was calculated from the plateau value in theCV

cyclovoltammetry. The electrode resistance was calculated from the conductivity data obtained by the four-probe van der Pauw method. The21bulk electrolyte resistance was calculated from the conductivity (0.68 S cm for 2.0 M H SO ). The electrolyte resistance in the pores was2 4

calculated by subtracting theR and R from the ESR.electrode bulk

applied in the synthesis, the surface area and pore volume chemical measurements can be applied to characterize theof the SMC3 carbons were found to be higher than those pore structure of porous carbon materials.of the SMC1 carbons. The maximum peak of the pore sizedistribution of SMC1 and SMC3 carbons were centered at8 and 15 nm, respectively, which matched the average A cknowledgementsdiameters of the Ludox SM-30 silica sol and the averagewidth of the SNOWTEX-UP silica sol. The gap of the We are grateful to the Korea Research Foundationhysteresis loop in the nitrogen adsorption/desorption iso- (KRF-2001-041-D00172) for the financial support.therms was much narrower for the SMC3 carbons than forthe SMC1 carbons, demonstrating the simpler pore con-nectivity of the former carbons. Electrochemical double R eferenceslayer capacitance studies revealed thatR of the SMC3pore

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