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MICMAT 5371 No. of Pages 1, Model 5G

16 February 2012

Graphical abstract

Microporous and Mesoporous Materials 2012, &, &&&Electrochemical properties of carbon xerogel containing nitrogenin a carbon matrix

Mariana Botelho Barbosa, Jefferson Patrício Nascimento,Patrícia B. Martelli, Clascídia Aparecida Furtado,Nelcy Della Santina Mohallem, H.F. Gorgulho *

Highlights

n in thr densiThe ni

" Carbon xerogel containing nitrogexerogels were characterised by highetransfer kinetics were determined. "layer charging.

eir structure were prepared and used to prepare carbon paste electrodes. " N-doped carbonties of surface functional groups and increased basicity. " The specific capacitance and electrontrogen introduction on carbon xerogels increased the electron transfer rate and electrical double-

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Microporous and Mesoporous Materials xxx (2012) xxx–xxx

le

M

.e l

MICMAT 5371 No. of Pages 8, Model 5G

16 February 2012

Electrochemical properties of carbon xe

Mariana Botelho Barbosa a, Jefferson Patrício NascClascídia Aparecida Furtado b, Nelcy Della Santinaa (UFSJ) Universidade Federal de São João del Rei, Praça Dom Helvécio 74, CEP36300b

1

Contents lists availab

Microporous and

journal homepage: www

(CDTN) Laboratório de Química de Nanoestruturas, Centro de Desenvolvimento da Tecnc (UFMG) Universidade Federal de Minas Gerais, Av. Antônio Carlos, 6627 – 31270-901, B

23lect ir24al d r25ne a ix26To e27erog y28. Th g29ectr n30ic v e31ned -32roc33c.

34

t-seecl

s,-ds-t-l

nss-ne

60

61

62

63s64y65n66o67d68-69n70l71-72e73l74t75n76.,77h78-79e80-81-

a r t i c l e i n f o

Article history:Received 25 August 2011Received in revised form 31 January 2012Accepted 3 February 2012Available online xxxx

Keywords:Carbon xerogelN-doped carbon xerogelElectron transfer kinetics

a b s t r a c t

The characteristics of epotential use in analyticprecursor using melamivia the sol–gel process.chemistry, the carbon xand acid–base titrationscarbon xerogel paste eltransfer kinetics in cyclfunctional groups obtailayer charging and elect

1. Introduction

Carbon xerogels (CXs) are highly porous materials with a nework structure consisting of interconnected sphere-like nodulewhose sizes are regulated by the synthesis conditions [1,2]. Thmorphology and surface properties of these carbon particles artwo of the most important factors for their potential for specifiapplications, such as in electric double-layer capacitors, thermaand phononic insulators, chromatographic packing materialadsorbents, and catalyst supports [3,4]. CXs are prepared by evaporative drying and the pyrolysis of organic gels usually obtainefrom resorcinol–formaldehyde polycondensation in an aqueousolution. The polymerisation mechanism involves aromatic electrophilic substitution followed by a condensation reaction thaleads to crosslinking. Carbon gels can be N-doped using nitrogencontaining precursors as the organic monomers. Thus, the sol–gepolymerisation of phenolic resoles with urea or melamine has beeinvestigated in the preparation of N-doped CXs [5]. For these CXprepared from nitrogen-containing precursors, nitrogen atomcan be inserted at the edges and in the carbon rings of the graphene layer. X-ray photoelectron spectroscopy studies on nitrogefunctionalities in CXs have shown that the major constituents ar

present on the surfaces as pyrrolic and pyridinic groups [6,7].The nitrogen atom in the pyridinic groups contributes one p-electron to the aromatic p-system and has an isolated electron pair

1387-1811/$ - see front matter � 2012 Published by Elsevier Inc.doi:10.1016/j.micromeso.2012.02.005

⇑ Corresponding author. Tel./fax: +55 32 33792483.E-mail address: gorgulho@ufsj.edu.br (H.F. Gorgulho).

Please cite this article in press as: M.B. Barbosa et al., Micropor. Mesopor. M

gel containing nitrogen in a carbon matrix

ento b, Patrícia B. Martelli a,ohallem c, H.F. Gorgulho a,⇑, São João del Rei, Minas Gerais, Brazilologia Nuclear, Belo Horizonte, Minas Gerais, Brazilelo Horizonte, Minas Gerais, Brazil

rodes based on N-doped carbon xerogels were investigated to clarify theevices. Carbon xerogels were prepared from a nitrogen-containing polymend urea as nitrogen sources that were incorporated into the polymer matrinvestigate the effects of nitrogen on their texture, morphology and surfacels were characterised by nitrogen adsorption, scanning electron microscope electrochemical performance of these materials was studied by preparinodes, which were used to determine the specific capacitance and electrooltammetric experiments. The results show that the high density of surfacfor N-doped carbon xerogels has a positive influence on electrical double

hemical kinetics.� 2012 Published by Elsevier In

within the plane of the ring. For pyrrolic groups, the nitrogen atomcontributes two p-electrons to the p-system, and a hydrogen atomis bound within the plane of the ring [8].

Originally, interest in the introduction of nitrogen into poroucarbons arose from the desire to induce surface basicity, therebenhancing the adsorption properties. Several reports have showthat the presence of nitrogen atoms in the carbon matrix can alsimprove the electrochemical capacitance of these materials in acielectrolytes by a reversible faradaic redox reaction known as pseudo-capacitance [9–11]. The surface modification of CXs by nitrogegroups can also facilitate their application in electroanalyticamethods. In this case, the electrode surface properties are of particular concern because the interaction between the sensor and thenvironment mainly occurs at the surface and the electrochemicaresponse depends on the analyte adsorption and charge transfer athe work electrode. Thus, the introduction of functional groups othe CX surface is helpful for improving the analyte adsorption, i.ethe chemical modification may initiate a chemical reaction witthe analyte and improve the electron transfer reaction. The disadvantage of the high background current generated by the presencof some pseudo-capacitance can be minimised by the use of an inert electrolyte, i.e., an electrolyte that decreases the surface capac

at SciVerse ScienceDirect

esoporous Materials

sevier .com/locate /micromeso

82itive effect. In addition, even in a neutral electrolyte, such as KCl,83the surface groups can affect the electrochemical surface area of84these materials although the increase in their wettability, due to85the increase in the number of hydrophilic polar sites. This point86is crucial for the application of these materials as electrodes in ana-87lytical devices [12,13]. Although several studies have evaluated the

ater. (2012), doi:10.1016/j.micromeso.2012.02.005

88 presence of nitrogen functional groups on the electrochemical89 capacitance of CXs, these effects were not fully explored with the90 aim of applying these materials as carbon paste electrodes in elec-91 trochemical sensors. Moreover, the correlation between the intro-92 duction of nitrogen groups on carbon surfaces and their electron93 transfer reactivity has not yet been investigated. This study is94 essential to better understand these materials and to expand their95 range of application as electrode materials. Thus, the aim of the96 present investigation was to evaluate the role of the textural97 properties and the surface chemistry of carbon paste electrodes98 prepared from CXs in the electrochemical behaviour of the elec-99 trodes in a neutral electrolyte. Urea and melamine were used as

100 nitrogen precursors to prepare a series of N-doped CXs that were101 used to obtain a carbon paste electrode (CPE). The performance102 of these electrodes is discussed in terms of their specific capaci-103 tance and electron transfer kinetics obtained from cyclic voltam-104 metric experiments.

105 2. Experimental

106 2.1. Sample preparation

107 Resorcinol (R) (Vetec, 99%), formaldehyde (F) (Aldrich, 37 wt.%108 in water, stabilized by 10–15 wt.% methanol), sodium carbonate109 (anhydrous, ACS, Fisher), melamine (M) (Sigma; Aldrich, 99.6%),110 and urea (U) (Vetec, 100%) were used as received. For the synthesis111 procedure, the following parameters were set: (i) resorcinol112 (9.09 g)/formaldehyde (13 mL) molar ratio R/F = 0.5, which is con-113 sidered the stoichiometric ratio; (ii) resorcinol (9.09 g)/melamine114 (5.26 g) molar ratio of R/M = 2.0 and (iii) resorcinol (9.09 g)/urea115 (4.96 g) molar ratio of R/U = 1.0. Sodium carbonate (Synth, 99.5%)116 was used as a catalyst, and two molar ratios of resorcinol/catalyst117 (R/C = 200 and R/C = 350) were used in the experiments. The syn-118 thesis procedure was developed using two water dilution volumes119 in the experiments (20 and 1000 mL) and fixing the ratios of the120 other components. For organic gel preparation, predetermined121 amounts of R and U were mixed in distilled water containing the122 catalyst at room temperature. Next, the solution was heated to123 approximately 358 K under stirring and transferred to an Erlen-124 meyer flask, which was sealed and preserved at 358 K in a125 Thermo-Water Bath (Tecnal) for 3 d. After incubation, the gels ob-126 tained were then dried in an oven at 373 K for three days. After127 drying, the gels were pyrolysed under nitrogen flow in a tubular128 oven. The heating program included the following sequential129 steps: (1) ramp at 2 K min�1 to 423 K and hold for 120 min, (2)130 ramp at 2 K min�1 to 573 K and hold for 60 min, (3) ramp at131 2 K min�1 to 873 K and hold for 60 min, and (4) ramp at 2 K min�1

132 to 1073 K and hold for 120 min. The carbon xerogels obtained were133 re134 XU135 th136 th137 wa138 (n139 wi140 xe

1412.2. Textural properties

142Specific surface areas were determined by N2 adsorption in an143Quantachrome Autsorb-1 using the BET method. The total pore144volume was determined at a relative pressure of 0.99. The samples145were outgassed for 2 h at 393 K before analysis. All data analyses146were performed using the NovaWin V.10 � 1997–2007 Quanta-147chrome Instruments Software.

1482.3. Nitrogen determination

149The nitrogen content in the prepared N-doped CXs was evalu-150ated using a Flash EA-1112 Series CHNS-O analyzer from the Ther-151mo Electron Corporation. Acetonitrile was used as the standard.

1522.4. Acid–base titrations

153The acid–base properties of the samples were determined by154titration. The selective neutralisation of surface groups by equili-155bration with HCl and NaOH was performed by adding the carbon156samples (0.1000 g) to two separated bottles. Next, 20 mL of1570.0200 mol L�1 NaOH was transferred into one bottle, and 20 mL158of 0.0500 mol L�1 HCl was transferred into the other. The two bot-159tles were degassed with N2, closed, and shaken for 24 h. The solu-160tions were then filtered, and the amount of each reagent consumed161was determined by back-titration of the solution.

1622.5. pH at the point of zero charge (PZC)

163The determination of the PZC was based on the procedure of164mass titration described by Noh et al. [14]. Three aqueous solutions165with pH values of 3, 6 and 11, respectively, were prepared using166solutions of HNO3 (Synth 64–66%) and NaOH (Merck, 99%). KNO3

167(Dinamica, 99%) was used as the background electrolyte to main-168tain the ionic strength at 0.10 mol L�1. For each initial pH, six con-169tainers were filled with 50 mL of the solution and different170amounts of carbon sample (0.025 g, 0.05 g, 0.12 g, 0.25 g and1710.50 g) were added. The equilibrium pH was measured after 24 h.172The PZC value was obtained from the plateau in the plot of equilib-173rium pH versus mass fraction.

1742.6. Morphology

175Surface morphological observations were obtained by scanning176electron microscopy (SEM) using a Quanta 200 - FEG - FEI – 2006177microscope. The samples were attached to the metal sample holder178by adhesive carbon tape.

1792.7. Electrochemical characterisation

180

181lab182eq183of184pu185all186pr

R/M

––22

Vt =

2 M.B. Barbosa et al. / Microporous and Mesoporous Materials xxx (2012) xxx–xxx

MICMAT 5371 No. of Pages 8, Model 5G

16 February 2012

Pl

ferred to as XM when melamine was used as the precursor and aswhen the precursor was urea. The numbers (200 or 350) after

e letters represent the molar ratio R/C used in the xerogel syn-esis. The letter L is used in the sample designation when 1 Ls used as the water dilution volume. A sample of carbon xerogel

itrogen-free) was synthesised following the same procedure butthout using melamine or urea as the nitrogen precursor. Thisrogel was designated XP350.

Table 1Synthesis conditions and textural properties of the carbon xerogel samples.

Sample % N R/C R/U

XP350 – 350 –XU200 1.4 ± 0.1 200 1XM200 4.7 ± 0.2 200 –XM200L 4.51 ± 0.06 200 –

R/C = Resorcinol/catalyst ratio; R/C = Resorcinol/urea ratio; SBET = BET surface area;

ease cite this article in press as: M.B. Barbosa et al., Micropor. Mesopor. Mater.

Electrochemical measurements were performed using an Auto-Electrochemical System (EcoChemie, Utrecht, The Netherlands)

uipped with PGSTAT-12 and GPES software. All substances wereanalytical grade and used without further purification. Waterrified with a Milli-Q apparatus (Millipore) was used to preparesolutions. The electrolytes were purged with purified nitrogen

ior to each experiment. A three-electrode cell was used in all

SBET m2g�1 Vt cm3g�1 Vm cm3g�1

655 0.82 0.13258 0.20 0.11295 0.17 0.10378 0.67 0.04

total pore volume and Vm = micropore volume.

(2012), doi:10.1016/j.micromeso.2012.02.005

187

188

189

190

191

192

193

194

195

196

197

198

199

200

201

202

M.B. Barbosa et al. / Microporous and Mesoporous Materials xxx (2012) xxx–xxx 3

MICMAT 5371 No. of Pages 8, Model 5G

16 February 2012

experiments. A platinum foil (1 cm2) was used as the counter elec-trode, whereas an Ag/AgCl electrode was used as the referenceelectrode. The working electrode was prepared by the homogeni-sation of carbon xerogel powder (60%) and mineral oil (40%) in amortar for 5 min. The proportions of mineral oil used in these elec-trode preparations (40%) were previously optimised in our group.The carbon paste (10 mg) was packed into a cavity (1 mm in diam-eter � 5 mm in depth) at the end of a glass capillary tube. Electricalcontact was provided by a platinum wire connected to the paste inthe inner hole of the tube to obtain the carbon xerogel paste elec-trode (CXPE). Graphite electrode (area = 2 m2 g�1, from NACIONALDE GRAFITE – www.grafite.com) was employed as comparisonstandard. Capacitance (C) studies were performed by cyclic vol-tammetry (CV) using 0.10 mol L�1 KCl as neutral electrolyte. Thescan rates ranged from 10 to 30 mV s�1. The gravimetric capaci-tance (Cg), which corresponds to the specific capacitance of the

Fig. 1. SEM micrographs of (a) XP350, (b)XU200, (c and d) XM200 at a magnification o250000� respectively.

Please cite this article in press as: M.B. Barbosa et al., Micropor. Mesopor. M

203carbon xerogel in the electrode, was expressed in lF g�1 and calcu-204lated using the following Eq. (1):205

C ¼ I0:25

v ; ð1Þ 207207

208where I0.25 is the constant current (lA) obtained at a potential of2090.25 V, which was evaluated from the linear part of the discharge210of the CV curves, and v is the scan rate (V s�1). The capacitance211was also expressed in lF m2 by using the BET area of the carbon212xerogel as a substitute for its mass. The values of CBET reflect the dis-213tribution of the electrochemical active phase on the surface of the214electrode. The interfacial properties of the CXPE electrode were also215investigated by CV using the redox system FeðCNÞ3

�

6 =FeðCNÞ4�

6 . The216experiments were performed at a scan rate of 10 mV s�1 over the217relevant potential range using 1.0 10�3 mol L�1 K3Fe(CN)6 in a2180.1 mol L�1 KCl electrolyte solution. The standard heterogeneous

f 800� and 200000� respectively, (e and f)XM200L at a magnification of 100000� and

ater. (2012), doi:10.1016/j.micromeso.2012.02.005

219 ch220 wa221 th222 so223 ro224 lea225 an226 da227 se

228 3.

229

230 CX231 po

232m233aft234Ta235ad236CX237fo238(SB

239ca240ni241pr242re243se244ac245m246als

lts.

s

FigXU

4 M.B. Barbosa et al. / Microporous and Mesoporous Materials xxx (2012) xxx–xxx

MICMAT 5371 No. of Pages 8, Model 5G

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Pl

arge transfer rate constant (K0) associated with the redox processs calculated from the peak-to-peak potential separation (DEp) of

e voltammogram, according to the method described by Nichol-n [15]. All electrochemical measurements were performed atom temperature (298 ± 5 K). Each experiment was performed atst in triplicate, and data values are represented by mean valuesd their standard deviations. For the graphical depiction of theta, each point represents a mean value with error bars that repre-nt the standard deviation.

Result and discussion

The experimental parameters used in this work to prepare thes were based on those used in previous reports in which the pro-rtions of each reagent were exhaustively discussed [1,2,5,8]. The

Table 2Acidic and basic group concentrations obtained by titration and PZC resu

Sample Acidic groupsmmol g�1

Basic groupmmol g�1

XP350 0.01 ± 0.02 0.07 ± 0.01XU200 0.06 ± 0.01 0.39 ± 0.02XM200 0.07 ± 0.04 0.41 ± 0.01XM200L 0.24 ± 0.01 0.70 ± 0.01

* Basic group concentration normalized by the area BET.

0.003

-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6-0.003

-0.002

-0.001

0.000

0.001

0.002

25 mV s-1

20 mV s-1

15 mV s-1

30 mV s-1

10 mV s-1

Cur

rent

(μA

)

Potential vs. Ag/AgCl (V)

(a)

Cur

rent

(μA

)

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

-0.06

-0.04

-0.02

0.00

0.02

0.04

30 mV s-1

25 mV s-1

20 mV s-1

15 mV s-1

10 mV s-1

Cur

rent

(μ A

)

Potential vs. Ag/AgCl (V)

(c)

Cur

rent

(μΑ

)

. 2. Cyclic voltammograms obtained in 0.1 mol L�1 KCl electrolyte solution at the scan ra200; (c) XM200 and (d) XM200L.

ease cite this article in press as: M.B. Barbosa et al., Micropor. Mesopor. Mater.

olar ratio R/C = 200 used to prepare the N-doped CXs was chosener preliminary tests were performed to obtain the initial pH 6.5.ble 1 provides the quantitative results derived from nitrogensorption isotherms and the N content obtained for the differents. The introduction of a nitrogen precursor into the resorcinol–

rmaldehyde polymerisation reduced the specific surface areaET) and the total pore volume (Vt). This behaviour was probably

used by the greater degree of shrinkage of the gel prepared fromtrogen-containing precursors with respect to the standardocedure and may provide a denser structure for the CXs. Similarsults were previously reported by Pérez-Cadenas et al., who ob-rved that the N-doped CXs feature very narrow micropores tocommodate a molecular monolayer of N2 (0.36 nm), whichay lead to an underestimation of the SBET values of these materi-

[7]. The influence of this synthesis variable is particularly ob-

Density of basicgroups * lmol m�2

PZC

0.10 ± 0.02 9.1 ± 0.21.51 ± 0.06 9.50 ± 0.031.40 ± 0.04 10.0 ± 0.041.85 ± 0.02 9.65 ± 0.05

0.03

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

-0.03

-0.02

-0.01

0.00

0.01

0.02

30 mV s-1

25 mV s-1

20 mV s-1

15 mV s-1

10 mV s-1

Potential vs. Ag/AgCl (V)

(b)

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

-0.2

-0.1

0.0

0.1

0.2

30 mV s-1

25 mV s-1

20 mV s-1

15 mV s-1

10 mV s-1

Potential vs. Ag/AgCl (V)

(d)

tes 10, 15, 20, 25 and 30 mV s�1 for the electrodes prepared from: (a) XP350; (b)

(2012), doi:10.1016/j.micromeso.2012.02.005

247 s248 e249 t250 -251 -252

253 d254 -255 s,256 -257 f258 l259 t260 r261 e262 e263 -264 )265 o266 o267 t268 -269 s270 e271 e272

273

274d275-276r277

278s279d280t281t-282].283e284h285r286r287-288ic289ir290g291e292e293-294-295

296-297-298d

M.B. Barbosa et al. / Microporous and Mesoporous Materials xxx (2012) xxx–xxx 5

MICMAT 5371 No. of Pages 8, Model 5G

16 February 2012

served in the Vt values, which decrease markedly for the sampleXM200 and XU200. In the case of sample XM200L, the higher valuobserved for Vt and SBET suggests that the increase in the diluenconcentration during the polycondensation of the sol–gel melamine-resorcinol–formaldehyde can help to preserve its threedimensional network and mesoporosity.

The morphology of these samples was studied using SEM, anthe results are shown in Fig. 1. As described in the literature, carbon xerogels are formed from interconnected sphere-like particlewith mesopores and macropores existing between these microspheres to form a three-dimensional structure. The SEM image oXP350 (Fig. 1a) shows a morphology that is similar to a typicaCX derived from a classical resorcinol–formaldehyde system thahas been described by different authors [16–18]. However, foXU200, although the texture still seems to be interconnected, thnodules appear to have disappeared to form a fused structur(Fig. 1b). In the case of XM200, the surface morphology is composed of larger particles (the diameter is approximately 5 lmformed by an assembly of fused spheres (Fig. 1c) that seems tbe very compacted (Fig. 1d). Although the pores are generally tosmall to be clearly visible by this technique, the more compacmorphology and the loss of most of the three-dimensional structure for XU200 and XM200 can explain the lower Vt and SBET valueobserved for these samples compared with XP350. However, thSEM image obtained for XM200L shows a nanometric structur

where the microspheres (with a diameter of approximately50 nm) are very well separated. The higher Vt and SBET values ob-

5 10 15 20 25 30 350

1

2

3

4

5

6

7

8

Scan rate (mV s-1)

Scan rate (mV s-1)

Cg(μ

F g-1

)

0.000

0.005

0.010

0.015

0.020

0.025

0.030

CB

ET (μ F m

-2)

(a)

5 10 15 20 25 30 350

10

20

30

40

50

60

70

80

90

100

Cg(μ

F g-1

)

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

CB

ET ( μF m

-2)

(c)

Fig. 3. Specific capacitance values expressed as Cg (lF g�1) and CBET (lF m�2), as functXM200L.

Please cite this article in press as: M.B. Barbosa et al., Micropor. Mesopor. M

tained for this sample compared with other nitrogen-modifieCXs can probably be attributed to the well-developed interconnectivity among the much delimited spherical particles observed foXM200L.

The different textural properties obtained for the prepared CXin this study were expected because the surface morphology anpore structure of these carbon materials are essentially dependenon the polymerisation reaction conditions and the gel-drying treament employed, as has been reported in the literature [2,5,16–18However, the surface acid–base characteristics of these CXs aralso an important parameter that has a great influence on botthe electrostatic and non-electrostatic interactions among othespecies in aqueous solutions and can be considered the main factoin improving the diversity of these materials’ applications. The results of this study are summarised in Table 2. Pronounced basproperties were observed for N-doped CXs as evidenced by thehigher PZC values and concentration of basic groups. Considerinthat the CXs were pyrolised at 1073 K under nitrogen flow, thlower proportion of acid groups observed in Table 2 for thessamples is expected. The differences in the basic group concentrations among the N-doped CXs can be better understood by considering the densities of the basic groups, which were obtained fromthe basic group concentration normalised by the SBET value. A lower density reflects a higher dispersion of the groups. Thus, the dispersions of basic groups are comparable between XU200 an

299XM200, although the dispersion was appreciably higher for300XM200L, which is probably a result of its higher Vt and SBET values.

Scan rate (mV s-1)5 10 15 20 25 30 35

45

48

51

54

57

60

63

66

69

Cg (

μF g

-1)

0.00

0.05

0.10

0.15

0.20

0.25

CB

ET (μF m

-2)

(b)

5 10 15 20 25 30 35400

425

450

475

500

525

550

575

Scan rate (mV s-1)

Cg( μ

F g-1

)

-30

-25

-20

-15

-10

-5

0

5

10

CB

ET (μF m

-2)

(d)

ion of the scan rate. Electrodes prepared from: (a) XP350; (b) XU200; (c) XM200 and (d)

ater. (2012), doi:10.1016/j.micromeso.2012.02.005

301 This result can be attributed to the fact that the number of basic302 sites increased proportionally to the total pore volume, as evi-303 denced by the equivalent values observed for the ratio between304 the number of basic groups of XM200L/XM200 = 3.42 and the total305 pore volumes of XM200L/XM200 = 3.94.306 The results of the CV characterisation of all CXs samples in307 0.1 mol L�1 KCl are now addressed. Fig. 2 shows the CVs obtained308 at scan rates of 10–30 mV s�1. It is worth noting that the observed309 background current from the voltammograms not exceed 0.02 lA,310 which favours the application of these materials in electroanalysis.311 The distorted shape observed for the CVs is characteristic of carbon312 materials and reflects the influence of the ohmic resistance from313 ele314 lay315 sh316 iou317 pe318 m319 as320 ta321 ele322 ch323 fo324 ar325 to

326co327so328in329vo330of331qu332XP333its334in335pa336pl337ob338cre339th340ele341ob342fro343pr344de345ge346tro347do348

349tro350ch

-

-

Cur

rent

(μA

)C

urre

nt (

μ A)

Fig(d)

6 M.B. Barbosa et al. / Microporous and Mesoporous Materials xxx (2012) xxx–xxx

MICMAT 5371 No. of Pages 8, Model 5G

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Pl

ctrolyte motion in carbon pores on the mechanism of double-er formation [19]. Thus, the voltammograms display a tilted

ape rather than the rectangular shape typical of capacitor behav-r, which suggests a decline in charge propagation. No visible

aks arising from redox reactions were observed in the voltam-ograms at the applied sweep potential, and it is possible tosume that only a capacitive current is present. Thus, the capaci-nce observed is derived from the double layer at the electrode/ctrolyte interface and is proportional to the effective electro-

emical surface area of these CXPEs. The specific capacitancesr these electrodes, expressed as Cg (lF g�1) and CBET (lF m�2),e depicted in Fig. 3 as a function of the scan rate. It is interestingnote that the Cg and CBET values for all CXPEs are approximately

-0.3 0.0 0.3 0.6-0.6

-0.3

0.0

0.3

0.6

Cur

rent

(μA)

Potential vs. Ag/AgCl (V)

(a)

0.0

0.3

0.6

Cur

rent

(μA

)

-0.3 0.0 0.3 0.6-0.6

-0.3

Potential vs Ag/AgCl (V)

(c)

. 4. Cyclic voltammograms for CXPEs obtained in 0.1 � 10�3 mol L�1 K3Fe(CN)6 at scan raXM200L.

ease cite this article in press as: M.B. Barbosa et al., Micropor. Mesopor. Mater.

nstant over the applied scan rate range, which indicates that thelvated ions from the electrolyte can diffuse sufficiently quicklyto the mesoporous structure of the CXs regardless of the appliedltage sweep rate. The ordered pore structure and large pore sizethese materials can help to maintain the capacitance duringick charge–discharge operations. The electrode prepared from350 has comparatively the lowest Cg and CBET values despitelarge surface area. The Cg and CBET values increase in the follow-

g order: XP350 < XU200 < XM200 < XM200L. This result is, inrt, attributed to the increase in the nitrogen content in the sam-es, as shown in Table 1. However the morphology of the spherestained after gel pyrolysis also plays an important role in the in-ase in the effective electrochemical surface area by improving

e electrolyte diffusion into the pores, as was observed for thectrode prepared from XM200L. The highest values of Cg and CBET

tained for XM200L (�450 lF g�1 and �1.20 lF m�2) may resultm a combination of factors that include the material’s textural

operties and its higher density of functional groups. Indeed, asscribed in the introduction of this report, the oxygen- and nitro-n-containing surface functional groups can provide a large elec-chemical surface area for fast transport of ions to achieve highuble-layer capacitance in aqueous electrolytes.To determine the electron-transfer properties of these elec-des, K3Fe(CN)6 was introduced into the electrolyte as an electro-

emical probe. The one-electron transfer for the redox system

-0.3 0.0 0.3 0.60.6

0.3

0.0

0.3

0.6

Potential vs. Ag/AgCl (V)

(b)

0.0

0.3

0.6

-0.3 0.0 0.3 0.6-0.6

-0.3

Potential vs. Ag/AgCl (V)

(d)

te of 10 mV s�1. Electrodes prepared from: (a) XP350; (b) XU200; (c) XM200 and

(2012), doi:10.1016/j.micromeso.2012.02.005

351 FeðCNÞ3�

6 =FeðCNÞ4�

6 has been assumed to follow an outer-sphere352 electron-transfer mechanism, which means that its behaviour is353 strongly influenced by the nature of the electrode surface [20,21].354 The voltammetric response of the CXPEs in a solution of355 0.1 � 10�3 mol L�1 K3Fe(CN)6 in 0.1 mol KCl is shown in Fig. 4.356 For all samples, the resulting voltammetric profile shows a large357 separation between the anodic and cathodic peak potentials, as is358 expected for a quasi-reversible redox reaction. Despite the asym-359 metric shape of the CVs, the ratio between the anodic and cathodic360 area was found to be approximately 1.0, indicating that both361 FeðCNÞ3

�

6 andFeðCNÞ4�

6 are stable in solution. It may be possible to362 study the kinetics of the electrode reaction using these conditions363 of quasi-reversibility, under which the anodic/cathodic peak sepa-364 ration, DEp, is associated with the electron-transfer kinetics across365 the electrode/solution interface [19]. Thus, the DEp decreases if the366 electron transfer in the electrode is improved. On the contrary, if367 the electrochemical response results in a higher resistance, a high-368 er DEp is observed. The perfect situation is observed for a Nernstian369 system, for which the separation of the peak potentials is close to370 59 mV at 298 K (electron number = 1). The DEp values were intro-371 duced into the working curve described by Nicholson to obtain372 the transfer parameter, w, and then the standard heterogeneous373 charge transfer rate constant (Ko) [15]. Table 3 summarises the val-374 ues of DEp and Ko obtained from the CVs at a scan rate of375 10 mV s�1. It is important to notes that the estimated Ko values376 are of the same order of magnitude as those found by previous377 works for glassy carbon electrode (4.8 � 10�3 cm s�1 obtained for378 10 mV s�1) and higher than the ones obtained for the graphite elec-379 trode produced in this work [22]. In addition, DEp was lower for the380 CXPEs prepared from the N-doped CXs than for XP350, and higher381 Ko values were obtained as a result of increasing the electron trans-382 fer rate. This behaviour can be attributed to the increase in the383 density of surface functional groups observed for the N-doped384 CXs, which not only enhances FeðCNÞ3

�=4�

6 adsorption onto the elec-385 trode surface but also improves the wettability of the aqueous386 electrolyte, i.e., its hydrophilicity.

387 4. Conclusions

388 Carbon xerogels containing nitrogen within their structure were389 prepared by using melamine and urea as nitrogen components390 incorporated into the polymer matrix via the sol–gel process. The

391nitrogen adsorption results show that the SBET and Vt values392decrease after nitrogen is introduced into the CX preparations.393However, N-doped CXs were characterised by higher densities of394surface functional groups and increased basicity. The values of Cg

395and CBET were positively affected by the higher concentration of396surface groups obtained for the N-doped CXs. Similarly, the vol-397tammetric response of the CXPEs in a solution of 0.1 � 10�3 mol L�1

398K3Fe(CN)6 showed a direct correlation between the DEp reduction399and the introduction of nitrogen into the CXs. The Ko values ob-400tained for the CXPEs are of the same order as those observed for401glassy carbon electrodes, which are materials that are commonly402used in electroanalytical devices.403In summary, this study shows that N-doped CXs exhibit excel-404lent electrochemical activity with the advantage of having an ac-405tive surface able to improve the analyte adsorption. Our results406provide useful information for expanding the application of CXs407as electrode materials for electrochemical sensors.

408Acknowledgements

409This research was supported by the Brazilian Agencies FAP-410EMIG, FINEP, CNPq, CAPES, Nacional de GRAFITE (www.gra-411fite.com), Brazilian Institute of Nanotechnology (INCT) and412Microscopy Center of UFMG.

413References

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450

Table 3The standard heterogeneous charge transfer rate constant (K0) and the peakto peak potential separation (DEp) obtained from CVs experiments using theredox system FeðCNÞ3

�

6 =FeðCNÞ4�

6 .

Sample DEp mV 10�3 K0 cm s�1

XP350 118 ± 7 1.0 ± 0.2XU200 101 ± 4 1.2 ± 0.4XM200 103 ± 4 1.3 ± 0.4XM200L 95 ± 1 2.0 ± 0.1Graphite* 141 ± 1 0.7 ± 0.2

* Electrode prepared with 40% of mineral oil.

M.B. Barbosa et al. / Microporous and Mesoporous Materials xxx (2012) xxx–xxx 7

MICMAT 5371 No. of Pages 8, Model 5G

16 February 2012

Please cite this article in press as: M.B. Barbosa et al., Micropor. Mesopor. Mater. (2012), doi:10.1016/j.micromeso.2012.02.005