78 calvo e.g. exploring new routes in the synthesis of carbon xerogels for their application in...

3334r 2010 American Chemical Society pubs.acs.org/EF

Energy Fuels 2010, 24, 3334–3339 : DOI:10.1021/ef901465jPublished on Web 02/26/2010

Exploring New Routes in the Synthesis of Carbon Xerogels for Their Application in

Electric Double-Layer Capacitors†

E. G. Calvo, C. O. Ania, L. Zubizarreta, J. A. Men�endez, and A. Arenillas*

Instituto Nacional del Carb�on, Consejo Superior de Investigaciones Cientı́ficas (CSIC), Apartado 73, 33080 Oviedo, Spain

Received December 2, 2009. Revised Manuscript Received February 11, 2010

Resorcinol-formaldehyde carbon xerogels were prepared by means of two different synthesis methods:conventional (C) and microwave heating (MW). The influence of the heating method and the pH of theprecursor solution on the textural and chemical properties of the carbon xerogels was investigated. It wasfound that bymodifying the initial pH, it is possible to control the porosity of carbon xerogels independentof the heatingmethod used. The electrochemical performance of a selection of synthesized carbon xerogelsas electrode materials in electric double-layer capacitors was studied by cyclic voltammetry and charge/discharge experiments in an acidic medium (1MH2SO4). The electrochemical performance of the carbonxerogels was compared to that of an activated carbon commercialized for this application (Norit SuperDLC-50), and it can be seen that the carbon xerogels display similar specific capacitances to those of thecommercial carbon. Moreover, carbon xerogels have a good cycle durability after 18 000 galvanostaticcycles, with a drop in specific capacitance of around 10%. This excellent cycle durability, together with theattractive properties of carbon xerogels and the saving of time and energy achieved with microwave-assisted synthesis, would make resorcinol-formaldehyde carbon xerogels promising materials forapplications of an electric double-layer capacitor (EDLC).

1. Introduction

Carbon gels have been extensively studied over the past fewdecades because of their great potential and versatility.1,2 Theirtextural and structural characteristics can be controlled accord-ing to the synthesis and processing conditions; thus, the mainadvantage of carbon gels is the possibility of tailoring their pro-perties to fit specific applications.3,4 For this reason, carbon gelsare used in a wide range of applications, including catalysis,5,6

adsorption,7,8 and energy storage.9,10 In this last application,carbon gels are considered perfect materials for their use aselectrodes in energy storage devices because of their high specificsurface area and pore volume, low density, and high electricalconductivity.11 In addition, these materials can be obtained indiverse forms, such as monoliths, powder, films, microspheres,etc., which is another reason for their wide applicability.12

The synthesis of carbon gels from the polycondensation ofresorcinol and formaldehyde involves three steps: (i) gelsynthesis, where the stages of gelation and curing take place,(ii) drying of the gel saturated with solvent, and finally,(iii) carbonization of organic gel to obtain carbon gel. Withregard to the drying step, severalmethods can be used, each ofwhich producing materials with different characteristics.Nowadays, the most widely studied drying methods aresupercritical drying, freeze-drying, and evaporative drying.The carbon gels obtained are referred to as aerogels, cryogels,and xerogels, respectively.13 Among these methods, evapora-tive drying is the simplest and most rapid method.

Despite the advantages of carbon gels, the main obstacle totheir application at the industrial level is the long time requiredfor their production, because several days are needed for theprocesses of gelation, curing, and drying to be completed byconventional methods. It is therefore necessary to find alter-native techniques to reduce the synthesis time and makecarbon gels competitive with respect to other materials thatare used as electrodes in supercapacitors (i.e., activatedcarbons). In a previous work by our research group,14 micro-wave drying was used to obtain organic gels, resulting in aconsiderable savings of time and energy compared to conven-tional drying (i.e., using a conventional oven).However, it stilltook 3 days for the polycondensation of resorcinol andformaldehyde (i.e., gelation and curing steps) to take place.Kang et al.15 used microwave radiation during the stages ofgelation and aging in a microwave digestion system at 120 �C

†This paper has been designated for the special section Carbon for EnergyStorage and Environment Protection.

*To whom correspondence should be addressed. E-mail: [email protected].(1) Lin, C.; Ritter, J. A. Carbon 1997, 35, 1271–1278.(2) Mahata, N.; Pereira, M. F. R.; Su�arez-Garcı́a, F.; Martı́nez-

Alonso, A.; Tasc�on, J. M. D.; Figueiredo, J. L. J. Colloid InterfaceSci. 2008, 324, 150–155.(3) Al-Muhtaseb, S. A.; Ritter, A. Adv. Mater. 2003, 15, 101–113.(4) Job, N.; Th�ery, A.; Pirard, R.; Marien, J.; Kocon, L.; Rouzaud,

J. N.; B�eguin, F.; Pirard, J. P. Carbon 2005, 43, 2481–2494.(5) Job, N.; Heinrichs, B.; Ferauche, F.; Noville, F.; Marien, J.;

Pirard, J. P. Catal. Today 2005, 102-103, 234–241.(6) Job, N.; Heinrichs, B.; Lambert, S.; Pirard, J. P.; Colomer, J. F.;

Vertruyen, B.; Marien, J. AIChE J. 2006, 52, 2663–2676.(7) Jayne, D.; Zhang, Y.; Shaker, H.; Erkey, C. Int. J. Hydrogen

Energy 2005, 30, 1287–1293.(8) Kang, Y. K.; Lee, B. I.; Lee, J. S. Carbon 2009, 47, 1171–1180.(9) Frackowiak, E.; B�eguin, F. Carbon 2001, 39, 937–950.(10) Fang, B.; Binder, L. J. Power Sources 2006, 163, 616–622.(11) Schmit, C.; Probstle,H.; Fricke, J. J.Non-Cryst. Solids 2001, 285,

277–282.(12) Al-Muhtaseb, S. A.; Ritter, A. Adv. Mater. 2003, 15, 101–104.

(13) Zubizarreta, L.; Arenillas, A.; Pirard, J. P.; Pis, J. J.; Job, N.Microporous Mesoporous Mater. 2008, 115, 480–490.

(14) Zubizarreta, L.; Arenillas, A.; Men�endez, J. A.; Pis, J. J.; Pirard,J. P.; Job, N. J. Non-Cryst. Solids 2008, 354, 4024–4026.

(15) Kang, K. Y.; Hong, S. J.; Lee, B. I.; Lee, J. S. Electrochem.Commun. 2008, 10, 1105–1108.

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Energy Fuels 2010, 24, 3334–3339 : DOI:10.1021/ef901465j Calvo et al.

for 1 h, but the drying stepwas performed in a vacuumoven at60 �C, with the resulting prolongation of synthesis time. Inaddition, these carbongelswere ammonia-assisted carbonizedto improve their properties for specific applications.

In the present work, microwave radiation was used in theglobal synthesis of organic xerogels; therefore, the gelation,aging, and drying of the samples were carried out in one singledevice.16 The carbon xerogels obtained bymicrowave heatingwere compared to conventionally synthesized carbon xerogelsby studying their textural and chemical characteristics. Theirelectrochemical performance as electrode materials in electricdouble-layer capacitors (EDLCs) was also investigated usinga selection of samples obtained by both heating methods.

2. Experimental Section

2.1. Synthesis of Organic and Carbon Xerogels. Aqueousorganic xerogels were synthesized by the polycondensation ofresorcinol (R) and formaldehyde (F) solubilized in water. Asodium hydroxide solution (1.0 M) was used as the basificationagent. Resorcinol (VWR International, 99%)was first dissolvedin deionized water in a sealed flask under magnetic stirring.Once a dissolution was obtained, formaldehyde (Aldrich,37 wt % in water, stabilized by 10.7 wt %methanol) was addedunder stirring. In all cases, the molar ratio of resorcinol/for-maldehyde (R/F) was kept at 0.5, and the dilution ratio, D (i.e.,the total solvent/reactant molar ratio) was fixed at 5.7. Fourdifferent organic xerogels were prepared, varying the pH of theinitial solution between 5.8 and 6.5, adjusted by the addition ofthe NaOH solution. In the case of the conventional synthesis (Cseries), the solutions were heated in an oven at 85 �C for 72 h forgelation and curing to take place and the organic gels were thendried by evaporation of the solvent at 150 �C for 24 h. Thesynthesis of organic xerogels by means of microwave heating(MW series) was carried out as follows: the solutions wereplaced in a microwave oven at 40 �C to undergo gelation.Afterward, the temperature was increased to 60 �C for curingand drying. The final drying of the samples was controlled byweight stabilization, and the time employed for the wholesynthesis was approximately 3-4 h. Because of the rapidity ofthe process in the microwave oven, it is only possible toappreciate the point at which the solution loses fluidity andgel formation occurs. However, the curing and drying steps arenot easy to distinguish.

All of the organic gels prepared (i.e., the C and MW series)were pyrolyzed at 800 �C under nitrogen flow in a horizontaltube reactor according to a heating program described else-where.13 The carbon xerogels (carbonized organic gels) weredesignated as follows: C or MW, depending upon the heatingmechanism used, followed by the pH of the resorcinol-formaldehyde aqueous solution.

2.2. Evaluation of the Textural and Chemical Properties of the

Samples. The textural properties of the carbon xerogels wereevaluated by means of N2 adsorption-desorption isotherms at-196 �C and CO2 adsorption isotherms at 0 �C, in a Micro-meritics Tristar 3000. The micropore volume was evaluated byapplying the Dubinnin-Raduskevich equation to the CO2 (Wo-CO2) and N2 (Wo-N2) adsorption isotherms, and the total porevolume (VP) was calculated from the adsorbed volume atsaturation.17 The Brunauer-Emmett-Teller (BET) surfacearea (SBET) was estimated from the nitrogen adsorption iso-therms.18 If the samples aremicro/mesoporous, the pore volume

at saturation,Vp, matches the total pore volume of the material,VT. However, in the case ofmaterials withmacropores, the totalpore volume cannot be determined exclusively from the nitrogenadsorption data. For this kind ofmaterial, mercury porosimetrywas used to determine the total pore volume and the averagemesopore diameter, dm. Mercury porosimetry measurementswere performed using a Micromeritics AutoPore IV, and theyare limited to pores larger than 5.5 nm. The total pore volume,VT, was then estimated as the sum of VHg and Wo-N2.

The chemical properties of the samples were determined bymeans of elemental analysis and temperature-programmeddesorption (TPD) experiments. The C, N, H, and S wereevaluated on a LECO-CHNS-932 microanalyzer, and the oxy-gen content was determined directly using a LECO-TF-900furnace. To evaluate the amount and type of oxygen function-alities, TPD analyses in an automatic chemisorption analyzer,Micromeritics Autochem II, were carried out under an inertatmosphere (Ar) with a flow rate of 50 cm3 min-1. The sampleswere heated to 1000 �C at a rate of 10 �C min-1, and thedecomposition products (COandCO2)were identified bymeansof an online Onmistar Pfeiffer mass spectrometer.

2.3. Electrode Preparation. The carbon electrodes were pre-pared according to the following procedure. A slurry with thecarbon xerogel, binder, and carbon black was prepared inacetone and homogenized in a mortar. The mass ratio of thereagents was 70 wt % carbon xerogel, 25 wt % poly(vinylidenefluoride) (PVDF, Sigma-Aldrich), and 5 wt % carbon black.The electrodes were prepared in the form of pressed pellets, witha diameter of 10 mm. Afterward, the electrodes were dried in anoven at 60 �C for 2 h to remove the acetone. The mass of eachelectrode was around 10-15 mg. Before performing any mea-surement, the carbon xerogel electrodes were placed in an excessof 1 M H2SO4 electrolyte solution overnight, to ensure thecomplete wetting of the electrode porosity by the electrolyte.

2.4. ElectrochemicalMeasurements.All of the electrochemicaltests were carried out in a Biologic multichannel potentiostat atroom temperature, employing a solution 1 M H2SO4 as theelectrolyte. The capacitors were constructed using a TeflonSwagelok-type cell formed by two carbon electrodes of compar-able mass (10-15 mg), electrically isolated by a glassy fibrousseparator. The specific capacitance was evaluated by cyclicvoltammetry at different scan rates in a voltage range of 0.8 Vand by galvanostatic charge/discharge cycles at several currentdensities (50 and 500 mA g-1) within the same voltage window(0.8 V). The gravimetric capacitance (C) expressed in farads pergram of carbon material (F g-1) was estimated using voltam-metry at a scan rate of 2-100 mV s-1 and galvanostatic charge/discharge at current densities between 50 and 500 mA g-1. Toassess the long-term electrochemical stability of the samples,charge/discharge tests at a current load of 200 mA g-1 werecarried out up to 18 000 galvanostatic cycles. In addition, thecarbon xerogels were electrochemically characterized by meansof cyclic voltammetry tests at 2 mV s-1 in a Teflon three-electrode cell, using a carbon xerogel pellet as the workingelectrode, a graphite rod as the counter electrode, and Hg/Hg2SO4 as the reference electrode.

3. Results and Discussion

3.1. Textural and Chemical Characteristics of the Synthe-

sized Carbon Xerogels. The N2 adsorption-desorption iso-therms of the synthesized carbon xerogels are shown inFigure 1. The isothermsof the samples synthesizedwith an ini-tial pH of 6.5 (both conventional and microwave heating) area combination of type I and type IV according to the Bruna-uer, Deming, Deming, and Teller (BDDT) classification,19

(16) Arenillas, A.; Men�endez, J. A.; Zubizarreta, L.; Calvo, E. G.Patent ES-200930256, 2009.(17) Dubinnin, M. M. In Progress in Surface and Membrane Science;

Danielli, J. F., Rosenberg, M. D., Cadenhead, D., Eds.; Academic Press: NewYork, 1975; Vol. 9, pp 1-70.(18) Parra, J. B.; de Sousa, J. C.; Bansal, R. C.; Pis, J. J.; Pajares, J. A.

Adsorpt. Sci. Technol. 1995, 12, 51–66.(19) Brunauer, S.; Deming, L. S.; Deming, W. E.; Teller, E. J. Am.

Chem. Soc. 1940, 62, 1723–1732.

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corresponding to micro/mesoporous materials. In contrast,when the pH of the precursor solution was fixed at 5.8, theisotherms exhibit a large increase in the volume of nitrogenadsorbed at relative pressures above 0.8, indicating thepresence of a large volume of mesopores of larger sizescompared to the xerogels prepared at pH 6.5 and evenmacropores. The microporosity is similar in all cases accord-ing to theN2 adsorption at low relative pressure, whereas themesoporosity differs slightly. The hysteresis loop of the N2

adsorption-desorption isotherms is smaller in the case of thesamples obtained by microwave heating, indicating a lowervolume of mesopores. For comparison purposes, Figure 1also shows the nitrogen adsorption isotherm of the commer-cial activated carbon DLC-50. This sample presents a type-Iisotherm, characteristic of a microporous material.

Table 1 summarizes the main textural parameters of thetwo series of carbon xerogels (C and MW) evaluated fromthe gas adsorption data andmercury porosimetry. As can beseen from this table, it is possible to control the pore textureof the carbon xerogels by adjusting the pH of the initial solu-tion. Both the specific surface area and microporosity arebarely affected by the pH of the resorcinol-formaldehydeaqueous solution, unlike mesoporosity, which is stronglyinfluenced by it. As the initial pH increases, the total porevolume andmesopore size decrease, regardless of the heatingmechanism employed (i.e., VT of 1.72 and 0.83 nm forsamples C-5.8 and C-6.5 and VT of 0.91 and 0.44 nm forsamples MW-5.8 and MW-6.5, respectively). The textural

properties of the commercial activated carbon are alsoshown in Table 1.

As mentioned above, conventional and microwave heatinggenerate a similar microporosity. The micropore volumes ob-tained by applying the Dubinnin-Raduskevich method to thenitrogen and carbon dioxide adsorption isotherms are verysimilar in the C and MW series. In the case of mesoporosity,conventional heating produces higher mesopore volumes (i.e.,VT of 1.72 and 0.91 nm for samples C-5.8 and MW-5.8,respectively). However, it should be kept in mind that the useof a rapid and simple method for the preparation of carbonxerogels is ahugeadvantage, especially from thepoint of viewofscalingup theprocess.Moreover, it is necessary topointout thatthe microwavemethod has yet to be fully optimized and, in thiswork, is only presented as a very promising synthesis option.The elemental analysis data (Table 2) indicate that the carbonmaterials studiedconsistmainlyof carbon (between93and97%in the case of the carbon xerogels and 93% for the activatedcarbon) and small quantities of oxygen (2-5%).With regard tothe carbon xerogels, the oxygen content is higher in the samplesprepared by microwave heating (i.e., 2.6 and 5.4 wt % forsamples C-5.8 and MW-5.8, respectively), and this is in agree-ment with previous work.20 The higher oxygen content of thesamples treated in the microwave oven (MW-5.8 andMW-6.5)can be attributed to secondary reactions that occur during thesynthesis with microwave radiation, which produce highercross-linked organic xerogels withmore stable oxygen function-alities.21 A deep analysis of the oxygenated surface functional-ities of the xerogels was carried out byTPD. The correspondingCO and CO2 profiles are shown in Figure 2. It is generallyaccepted that, as a result of heat treatment, oxygen-containingfunctional groups decompose into CO and CO2. Thus, weightloss can be linked to the chemical functionalities present on thesurface. Evolution of CO2 between 200 and 600 �C is due todecomposition of labile groups (i.e., carboxylic acid), whereasphenols andotherbasic groups (quinonesandpyrone-like struc-tures) decompose as CO at temperatures higher than 600 �C.22Data shown in Figure 2 indicate that the two synthetic routesproduce analogous kinds of oxygen functionalities. In all cases,CO evolves in larger amounts thanCO2, indicating thatmost ofthe surface functionalities are non-labile groups. Moreover,according to the decomposition temperatures of the CO de-sorption profiles (see Figure 2a), they can be assignedmostly toquinones and phenol-type groups. The CO2 desorption peakcentered at about 200 �C is, in all cases, almost negligible,especially for sample DLC-50.

3.2. Electrochemical Characterization of the Samples.Elec-trochemical characterization of samples C-6.5,MW-6.5, andDLC-50 was carried out in a three-electrode configura-tion. Figure 3 shows the cyclic voltammograms recorded at

Figure 1.Nitrogen adsorption-desorption isotherms of the carbonxerogels obtained by conventional (C) and microwave heating(MW) and the commercial activated carbon, Norit Super DLC-50.

Table 1. Textural Properties of the Carbon Xerogels Studied

sampleSBET

(m2 g-1)Wo-N2

a

(cm3 g-1)Wo-CO2

a

(cm3 g-1)VT

b

(cm3 g-1)dm

c

(nm)

C-5.8 671 0.26 0.24 1.72 38C-6.5 677 0.26 0.22 0.83 9MW-5.8 648 0.25 0.25 0.91 24MW-6.5 594 0.22 0.22 0.44 9DLC-50 1612 0.56 0.28

aObtained by applying the Dubinnin-Raduskevichmethod to the N2

and CO2 adsorption isotherms. bTotal volume of pores estimated bymercury porosimetry and the Dubinnin-Raduskevich method: VT =VHg þ Wo-N2.

cAverage mesopore diameter calculated from mercuryporosimetry.

Table 2. Chemical Composition of the Synthesized Carbon Xerogels

and the Commercial Activated Carbon, Norit Super DLC-50

elemental analysis (wt %, dry ash-free basis)

sample C H O N

C-5.8 96.0 1.3 2.6 0.1C-6.5 96.9 1.2 1.7 0.2MW-5.8 92.7 1.7 5.4 0.2MW-6.5 95.2 1.4 3.2 0.2DLC-50 92.9 1.1 5.4 0.6

(20) Zubizarreta, L.; Arenillas, A.; Domı́nguez, A.; Men�endez, J. A.;Pis, J. J. J. Non-Cryst. Solids 2008, 354, 817–825.

(21) Caddick, S. Tetrahedron 1995, 51, 10403–10432.(22) Otake, Y.; Jenkins, R. G. Carbon 1993, 31, 109–121.

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2mV s-1 and at two different potential ranges using the carbonmaterials as working electrode andHg/Hg2SO4 and a graphiterod as reference and counter electrodes, respectively.

When the carbon xerogels are exposed to polarizationbetween -600 and þ200 mV versus Hg/Hg2SO4, a purelycapacitive behavior is observed (reversible charge/dischargeof the double layer), which is confirmed by the quasi-rectangular shape of the voltammogram. If the potential isextended to more negative potentials (i.e., -1 V versus Hg/Hg2SO4), the response is different (Figure 3b). At potentialsbelow-800mVversusHg/Hg2SO4, there is a rapid rise in thecurrent related to the reduction of water and nascent hydro-gen starts to form. For the anodic sweep, the potential waslimited to þ300 mV versus Hg/Hg2SO4 because higheranodic values might cause oxidation of the materials.23

In contrast, the cyclic voltammograms of the commercialactivated carbon deviate from the rectangular shape andshow two wide reversible humps. These are caused by redoxtransfer reactions involving the oxygen-containing function-alities in the carbon matrix. This behavior is consistent with

the higher oxygen content of this activated carbon comparedto that of the xerogels obtained at pH 6.5. The appearance ofthe humps is even more pronounced when the potentialvalues are extended from -1000 to þ300 mV versus Hg/Hg2SO4 (Figure 3b). The position of the cathodic and anodicpeaks confirms the existence of the quinone/hydroquinonepair (reduction potential close to-0.15V versusHg/Hg2SO4

for carbons in various environments.23

3.3. Electrochemical Performance of theMaterials Studied.

The potential-time curves obtained by galvanostaticcharge/discharge of the electrochemical capacitors con-structed using the carbon xerogels and the commercial acti-vated carbon are presented in panels a and b of Figure 4. Asmentioned above, the charge/discharge tests were performedat several current loads (50-500 mA g-1), in a voltage rangeof 0.8 V, but only the curves obtained at 50 and 500 mA g-1

are included in this work. Similar to other authors that haveobserved the case of carbon aerogels24 and cryogels,25 the

Figure 2. (a) CO and (b) CO2 TPD profiles of the carbon materials studied.

Figure 3.Cyclic voltammograms in a three-electrode cell usingHg/Hg2SO4 and graphite as reference and counter electrodes, respectively. Scanrate = 2 mV s-1.

(23) Kinoshita, K. Carbon: Electrochemical and PhysicochemicalProperties; Wiley: New York, 1988; p 293.

(24) Li, J.; Wang, X.; Wang, Y.; Huang, Q.; Dai, C.; Gamboa, S.;Sebastian, P. J. J. Non-Cryst. Solids 2008, 354, 19–24.

(25) Seperih, S.; Garcı́a, B. B.; Zhang, Q.; Cao, G. Carbon 2009, 47,1436–1443.

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curves present a quasi-triangular shape, suggesting the ab-sence of pseudo-capacitance. This is in agreement with theresults of the electrochemical characterization of the carbonxerogels in the three-electrode configuration (Figure 3a).However, from the electrochemical characterization of theactivated carbon DLC-50, pseudo-capacitance was clearlyobserved (see panels a and b of Figure 3). It can also be seenfrom Figure 4a that the ohmic drop is relatively limited in allof the samples, which demonstrates the ability of quickcharge propagation in the synthesized carbon xerogels,similar to that of a commercial carbon. When the galvano-static charge/discharge tests were performed at higher cur-rent load (Figure 4b), the carbon xerogel C-6.5 shows anegligible ohmic drop, whereas the samples MW-6.5 andDLC-50 exhibit a higher ohmic drop than their counterpartsat 50 mA g-1.

To evaluate the degree of dependency of specific capaci-tance on the scan rate, cyclic voltammetry tests were re-corded at several scan rates, between 2 and 100 mV s-1

(Figure 5), using samples C-6.5, MW-6.5, and DLC-50 aselectrode materials. At a low scan rate, the commercialactivated carbon displays a higher specific capacitance (i.e.,190 F g-1) than the synthesized carbon xerogels (i.e., 155 and113F g-1 for samplesC-6.5 andMW-6.5, respectively), but itmust be remembered that the activated carbon was com-pared to non-activated samples. In fact, the activated carbonhas about twice as much surface area and micropore volume

as the xerogels (Table 1). Another fact that stands out is thevariation of specific capacitance with the scan rate that islower when the carbon xerogel C-6.5 was used as theelectrode material, which could be related to its highernarrow mesopore volume compared to the other samplesstudied.26,27 As the scan rate increases, the ions of theelectrolyte have a shorter time to spread into the smallermicropores, so that narrow mesopores acquire a greater rolein the process of charge storage. In summary, the carbonxerogels prepared appear to have promising electrochemicalproperties for their use as electrodes in supercapacitors.

A factor that seriously limits the application in electro-chemical devices of high-surface area carbon materials is thelong-term stability of the capacitor.28,29 To confirm the goodperformance of the carbon xerogels synthesized in this work,we tested their stability using galvanostatic long-term cy-cling. Figure 6 shows the variation of the specific capacitancevalues with the number of galvanostatic cycles carried out at200 mA g-1 in a voltage range of 0.8 V. The carbon xerogelsstudied showed excellent cycle durability, because the capa-citance fading after 18 000 cycles was very low for both

Figure 4.Galvanostatic charge/discharge cycles for carbonmaterials C-6.5,MW-6.5, andDLC-50 performed at a current load of (a) 50mAg-1

and (b) 500 mA g-1. Voltage window = 0.8 V.

Figure 5. Specific capacitance, C (F g-1), for the carbon xerogelsand the activated carbon, Norit Super DLC-50, as a function of thescan rate (mV s-1). The electrolyte used was 1 M H2SO4, and thevoltage range was 0-0.8 V.

Figure 6. Variation of the specific capacitance with the number ofgalvanostatic cycles performed for samples C-6.5 and MW-6.5.Current density, 200 mA g-1; voltage window, 0-0.8 V.

(26) Li, W.; Reichenauer, J.; Fricke, J. Carbon 2002, 40, 2955–2959.(27) Pandolfo, A. G.; Hollenkamp, A. F. J. Power Sources 2006, 157,

11–27.(28) Portet, C.; Taberna, P. L.; Simon, P.; Flahaut, E.; Laberty-

Robert, C. Electrochim. Acta 2005, 50, 4174–4181.(29) Khomenko, V.; Raymundo-Pi~nero, E.; B�eguin, F. J. Power

Sources 2008, 177, 643–651.

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materials, representing a fall of around 12% in the case ofconventional heating and only 6% in the case of microwaveradiation. Similar results has been reported by other authorsusing an activated carbon as the electrode material.28,29

4. Conclusions

In this work, an ultra-fast and low-cost microwave-assistedheating method was used to prepare resorcinol-formalde-hyde aqueous gels. The textural, structural, and electroche-mical properties of the carbon xerogels obtained were com-pared to those of conventionally synthesized carbon xerogels.The results show that microwave radiation produces carbonxerogels with comparable characteristics to those of conven-tional heating but with an enormous savings of time and

energy (i.e., synthesis time of 3-4 h under microwave radia-tion compared to several days by conventional heating).Although it is necessary to optimize this novel method toobtain carbon xerogels with a better textural developmentand electrochemical performance, the use of microwaveradiation to synthesize carbon xerogels leads to promises ofmore rapid progress toward their implementation on anindustrial scale.

Acknowledgment. The support from the Government of Prin-cipado de Asturias PCTI (IB09-00201) and the Ministerio deCiencia e Innovaci�on (MAT2008-00217/MAT) is greatly ac-knowledged. E. G. Calvo also acknowledges a predoctoralresearch grant from Fundaci�on para el Fomento en Asturias dela Investigaci�on Cientı́fica Aplicada y la Tecnologı́a (FICYT).