Influence of the Boron Precursor and Drying Method on Surface

Jan 24, 2014 - Two series of B-doped carbon gels were prepared by the polymerization of resorcinol and formaldehyde in water using either boric acid o...
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Influence of the Boron Precursor and Drying Method on Surface Properties and Electrochemical Behavior of Boron-Doped Carbon Gels Zulamita Zapata-Benabihe,† Carlos Moreno-Castilla,* and Francisco Carrasco-Marín Departamento de Química Inorgánica, Universidad de Granada, 18071 Granada, Spain S Supporting Information *

ABSTRACT: Two series of B-doped carbon gels were prepared by the polymerization of resorcinol and formaldehyde in water using either boric acid or phenyl boronic acid as dopants. Both organic hydrogels were dried by four methods: supercritical, freeze, microwave oven, and vacuum oven drying. The effects of the boron precursor and drying method on the surface characteristics were studied by N2 and CO2 adsorption at −196 and 0 °C, respectively, immersion calorimetry into benzene and water, temperature-programmed desorption coupled with mass spectrometry, X-ray photoelectron spectroscopy, and thermogravimetric analysis. Electrochemical characterization was carried out in a three-electrode cell, using Ag/AgCl as a reference electrode and a Pt wire as a counter electrode. The surface area obtained from immersion calorimetry into benzene was more realistic than that yielded by the Brunauer−Emmett−Teller (BET) equation. The hydrophobicity of the samples decreased linearly with a higher oxygen content. In addition, the oxygen content of the B-doped carbon gels increased linearly with a higher B content, and the interfacial or areal capacitance decreased linearly with a larger surface area. The capacitance was increased by B addition because of the pseudocapacitance effects of the higher oxygen content of the samples. The cryogel and vacuum-dried xerogel obtained from the boric acid series, Bc and Bv, respectively, showed the largest gravimetric and volumetric capacitances, around 140 F/g and 95 F/cm3, respectively.

1. INTRODUCTION Carbon gels are porous carbon materials with applications in adsorption, catalysis, and electrochemical energy storage devices1−4 because of the ability to modify their pore texture, surface area, surface chemistry, and shape by controlling the ingredients, drying method, and carbonization−activation temperature. These materials can also be readily doped with different elements2 to change their adsorptive and catalytic properties, electrical conductivity, and capacitance. Thus, dopant atoms can enhance the electrochemical doublelayer (EDL) capacitance that arises from the accumulation of ions on the carbon electrode/electrolyte interface via modifications in the electronic structure of the carbon electrode. Dopant atoms can also introduce pseudocapacitance phenomena through reversible reactions at the carbon electrode/electrolyte interface. Both effects lead to an increase in the doped electrode capacitance. Boron is the most widely used doping element in carbon materials. It can substitute carbon atoms at the trigonal sites, thereby lowering the Fermi level of the solid and modifying its electronic properties.5−8 Boron can also be present in B-doped carbons in the form of oxides, oxycarbides, and carbides, which can alter the physicochemical properties of the solids. In addition, low-level boron doping has catalytic effects on oxygen chemisorption on the carbon surface.9−12 These modifications can enhance the EDL capacitance and pseudocapacitance of Bdoped carbon electrodes. © 2014 American Chemical Society

We recently reported the preparation, surface characteristics, and electrochemical characteristics of carbon aerogels prepared using boric acid as the polymerization catalyst of resorcinol (R), pyrocathecol, and formaldehyde (F) mixtures.13 Results obtained showed that the influence of B on the capacitance was due to the pseudocapacitance induced by the increase in the O content with a higher B content. The objective of this study was to study the effect of the B dopant and drying method on the surface physics and chemistry of B-doped RF gels and their electrochemical behavior. Two dopants, boric and phenyl boronic acids, and four drying methods, i.e., supercritical, freeze, microwave oven, and vacuum oven drying, were used for this purpose.

2. EXPERIMENTAL SECTION B-Doped organic gels were prepared by the sol−gel polymerization reaction of R with F in water (W), using boric acid (sample B) or phenyl boronic acid (sample P) as catalysts (C) according to the recipes in Table 1. The amounts of C were adjusted to prepare both samples at a similar solution pH. The mixtures were stirred to obtain a homogeneous solution that was then cast into glass molds (45 cm length × 0.5 cm inner diameter). The molds were sealed, and the mixtures were cured for 5 days at different temperatures up to 80 °C. Received: December 4, 2013 Revised: January 17, 2014 Published: January 24, 2014 1716

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Table 1. Recipes (mol) of the B-Doped Organic Gelsa sample

R

F

C

pH

B P

0.112 0.112

0.224 0.224

6.94 × 10−3 H3BO3 4.57 × 10−3 C6H5B(OH)2

2.53 2.71

on graphite paper. The as-prepared electrodes were investigated by using a typical three-electrode cell, with Ag/AgCl as the reference electrode and Pt wire as the counter electrode. The three-electrode system is very useful for determining and comparing the specific capacitance of different materials and revealing the presence of pseudocapacitance phenomena.14 Cyclic voltammograms (CVs) were obtained at 0.5 mV/s, and the gravimetric capacitance, CCV (F/g), was calculated from these curves by eq 1

a

R, resorcinol; F, formaldehyde; and C, catalyst. Total water was 0.85 mol.

After the curing cycle, the hydrogel rods were cut into 5 mm pellets and divided into four portions that were dried under the different conditions. Thus, one portion was dried with supercritical CO2 after exchanging water with acetone and yielding an aerogel, as explained elsewhere.13 A second portion of the hydrogels was freeze-dried to obtain the corresponding cryogels, freezing the samples at −15 °C overnight and vacuum drying at around 10−3 mbar and −5 °C until a constant weight in a Labconco dryer. Xerogels were prepared from the third and fourth portions by drying the hydrogels in a microwave oven and vacuum oven, respectively. The microwave oven was operated at 384 W in a N2 atmosphere for periods of 1 min until a constant weight, while the vacuum oven was operated at 60 °C and 10 mbar also until a constant weight. Henceforth, the aerogels, cryogels, microwave-dried gels, and vacuum-oven-dried gels are designated by the addition of the letters a, c, mw, and v, respectively, to the gel name. Dried organic gels were pyrolyzed under N2 flow (300 cm3/min) at a heating rate of 1.5 °C/ min up to 900 °C with a soaking time of 5 h. Carbon gels were characterized by N2 and CO2 adsorption at −196 and 0 °C, respectively, immersion calorimetry into benzene and water, temperature-programmed desorption coupled with mass spectrometry (TPD−MS), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). Adsorption isotherms were measured with an Autosorb 1 from Quantachrome after outgassing samples overnight at 10−6 mbar and 110 °C. N2 adsorption isotherms were analyzed using the Brunauer− Emmett−Teller (BET) equation, yielding the surface area, SBET. The Dubinin−Radushkevich (DR) equation was applied to N2 and CO2 adsorption isotherms at −196 and 0 °C, respectively, to determine the micropore volume, W0, and mean width, L0. The mesopore volume, Vmeso, was calculated from the difference between the volume of nitrogen adsorbed at p/p0 of 0.95, V0.95, and the micropore volume, W0(N2). The mean mesopore size, dBJH, was determined by applying the Barrett−Joyner−Halenda (BJH) method to the desorption branch of the N2 adsorption−desorption isotherms. The particle density, ρ, was obtained by mercury pycnometry at atmospheric pressure. The enthalpy of immersion into benzene and water at 30 °C, ΔHB and ΔHW, respectively, was determined using a C80-D model Setaram calorimeter. Samples were previously outgassed overnight under a dynamic vacuum of 10−6 mbar at 120 °C. Measurements were carried out at least twice for each sample. TPD−MS was performed by heating samples up to 1000 °C at 10 °C/min and analyzing CO and CO2 evolved with a model Prisma mass spectrometer from Pfeiffer. The total oxygen content, OTPD, was calculated from the amount of CO and CO2 evolved. XPS was performed using an Escalab 200R system (VG Scientific Co.) equipped with a Mg Kα X-ray source (hγ = 1253.6 eV) and hemispherical electron analyzer. The internal standard peak for determining the binding energies (BEs) was the C1s peak at 284.6 eV for obtaining the number of components, position of the peaks, and peak areas. Oxygen and boron contents obtained with this technique, OXPS and BXPS, respectively, are the surface contents, because the depth of this technique is around 2−3 nm below the external surface. The total boron content, Btotal, was measured by weighing the residue left (as B2O3) after burning a portion of the samples in air at 800 °C using a TGA-50H model from Shimadzu. Electrochemical measurements were carried out in a VPM3 model Biologic multichannel potentiostat at room temperature using 1 M H2SO4 as the electrolyte. The working electrode was a homogeneous mixture of the finely ground carbon gel, acetylene black, and binder [polytetrafluoroethylene (PTFE)] at a mass ratio of 80:10:10, pasted

CCV =

∑ |I |Δt 2mΔV

(1)

where ∑|I|Δt is the area of the current (A) against time (s) curve, m is the mass of active material in the electrode (g), and ΔV is the potential window (V). Chronopotentiograms (CPs) were performed at current loading between 0.125 and 1 A/g in a potential interval of 0−0.75 V. The gravimetric capacitance from these measurements, CCP (F/g), was obtained by eq 2 CCP =

IdΔt mΔV

(2)

where Id is the discharge current (A), Δt is the discharge time (s), and ΔV is the potential interval (V). The equivalent series resistance (ESR) of the cell was calculated from the voltage drop at the beginning of the discharge side of the CPs.

3. RESULTS AND DISCUSSION Figure 1 depicts the N2 adsorption−desorption isotherms at −196 °C on all B-doped carbon gels obtained. They are type IV

Figure 1. N2 adsorption (open symbols)−desorption (closed symbols) isotherms at −196 °C on samples: (a) black squares, Bc; red diamonds, Bmw; (b) blue circles, Ba; green triangles, Bv; (c) black squares, Pc; red diamonds, Pmw; and (d) blue circles, Pa; green triangles, Pv.

and show a type H2 hysteresis cycle,15 indicating the presence of mesopores. Most of the isotherms have hysteresis at low relative pressures. Table 2 compiles the results obtained from these isotherms and from the CO2 adsorption isotherms at 0 °C. CO2 adsorption at 0 °C yields the volume of narrow micropores (below approximately 0.7 nm in width), whereas N2 adsorption at −196 °C yields the total micropore volume if there are no 1717

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Table 2. Porosity and Surface Area of the B-Doped Carbon Gels Ba

Bc

W0(N2) W0(CO2) L0(N2) L0(CO2) V0.95 Vmeso dBJH P

cm3/g cm3/g nm nm cm3/g cm3/g nm g/cm3

0.22 0.27 0.82 0.67 0.61 0.39 7.61 0.60

0.24 0.24 0.97 0.64 0.62 0.38 7.79 0.67

SBET −ΔHB Simm

m2/g J/g m2/g

580 74 649

591 74 649

Bmw Porosity 0.22 0.29 1.05 0.71 0.59 0.37 7.79 0.62 Surface Area 550 78 684

Bv

Pa

Pc

Pmw

Pv

0.23 0.29 1.08 0.67 0.59 0.36 7.86 0.67

0.23 0.26 0.78 0.63 0.52 0.29 3.37 0.71

0.03 0.22 1.39 0.52 0.27 0.24 4.78 0.67

0.22 0.25 0.84 0.59 0.53 0.31 4.76 0.67

0.03 0.25 1.49 0.67 0.28 0.25 4.85 0.74

568 80 701

592 83 728

77 47 412

564 76 667

78 34 298

Table 3. Amounts of CO and CO2 Evolved up to 1000 °C, Oxygen and Boron Contents, Immersion Enthalpy into Water, and HF of the B-Doped Carbon Gels CO CO2 OTPD OXPS Btotal BXPS −ΔHW HF

mmol/g mmol/g wt % wt % wt % wt % J/g

Ba

Bc

Bmw

Bv

Pa

Pc

Pmw

Pv

0.61 0.78 3.5 9.0 2.1 0.3 59 0.20

0.86 0.26 2.2 6.7 1.7 0.6 33 0.55

0.90 0.42 2.8 8.2 1.2 0.7 51 0.35

0.85 0.32 2.4 7.4 1.2 0.5 37 0.54

0.39 0.10 0.9 6.4 0.6 0.2 22 0.73

1.18 1.20 5.7 17.6 2.4 1.3 94 −1.00

0.51 0.14 1.3 7.4 1.0 0.6 32 0.58

1.34 1.39 6.6 16.5 2.4 1.1 66 −0.94

molecules can access all micropores because of the much higher temperature (30 °C) at which immersion takes place. In addition, narrow micropores do not allow for a single molecular layer of N2 to be accommodated on each micropore wall, whereas benzene molecules interact with both micropore walls in the immersion method, giving a higher Simm value. Therefore, in micropores that are narrow or have restricted entrances, the BET method will yield an unrealistic value of the surface area.13,19,21 The drying method has a greater influence on the Simm value of samples from series P than those from series B. Thus, Simm varies between 300 and 728 m2/g in series P but between 650 and 700 m2/g in series B, with carbon xerogel Pc showing the smallest surface area and carbon aerogel Pa showing the largest surface area. Results of the surface chemistry characterization of the Bdoped carbon gels are compiled in Table 3. In all samples, OTPD < OXPS, indicating that the surface oxygen concentration is higher than the bulk concentration. This is typical of many carbon materials and results from oxygen chemisorption on the external surface after exposure to the atmosphere. The deconvolution of the O1s X-ray photoelectron (XP) spectra of the samples (see the Supporting Information) gives two peaks at 531.6 and 533.1 eV, corresponding to double CO bonds and single C−OH bonds,22 respectively. The relative surface concentrations of these functionalities are between 30 and 40 and between 70 and 60%, respectively. Hence, functionalities, such as carboxyl, hydroquinone, and phenol, predominate in the B-doped carbon gels. The hydrophobicity (HF) of the samples is compiled in Table 3 and was obtained from ΔHB and ΔHW (Tables 2 and 3) by applying eq 3.23

micropores that are very narrow or have constricted entrances.16,17 Results obtained with cryogel Bc show that W0(N2) = W0(CO2), indicating equal accessibility to both adsorbates. However, W0(N2) < W0(CO2) in the remaining B-doped carbon gels, more markedly in the case of Pc and Pv, because of the presence in these samples of very narrow micropores or micropore constrictions. The drying method has practically no influence on the micropore volumes of the B-doped carbon gels obtained, except in the case of Pc and Pv, with close SBET values that vary between around 560 and 590 m2/g. On the other hand, the mesopore volume is higher and the dBJH is wider in samples from series B than in samples from series P. The drying method, in each series of samples, also has practically no effect on the mesoporosity of the samples. The enthalpy of immersion into benzene can be converted into a surface area value by following the method proposed by Denoyel et al.18 and used by other authors.19−21 The enthalpy of immersion is assumed to be proportional to the surface area accessible to the wetting liquid, because the benzene molecule has no specific interactions with surface groups. A surface area value, Simm, is obtained by taking into account the enthalpy of immersion into benzene of a non-porous graphitized carbon black,18 −0.114 J/m2. Results compiled in Table 2 show that Simm > SBET in all samples. The difference between the two surface area values is much higher in samples Pc and Pv. The minimal dimensions of N2 (0.36 nm) and benzene (0.37 nm) are almost the same;20 therefore, both molecules should have access to a similar slitshaped micropore range. However, the SBET can be underestimated with respect to the Simm, because of the restricted diffusion of N2 at −196 °C in very narrow micropores or in those with constrictions at their entrance, whereas benzene 1718

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Article

ΔHW ΔHB

(3)

The immersion enthalpy into benzene depends upon the surface area of the carbon accessible to benzene molecules. In contrast, immersion enthalpy into water depends upon specific and non-specific interactions.24 Panels a and b of Figure 2 show a very good linear relationship between HF and both OXPS and OTPD, with

Figure 3. (a) Relationship between the surface oxygen and surface boron contents. Symbols: red circles, samples from this work; green squares, samples from ref 13. Reproduced with permission from Elsevier. (b) Relationship between the total oxygen and total boron contents.

have the largest BXPS (around 1 wt %) and Btotal (2.4 wt %) among all of the B-doped carbon gels, explaining why these two samples also have the largest oxygen content. The B1s XP spectra (see the Supporting Information) were difficult to deconvolute because of the low B content of the samples. Carbon gels show different peaks in the region between 187 and 195 eV, which can be assigned to B−O bonds in boron oxycarbide moieties and boron oxides25−28 and to different boron carbides (around 189 eV) and substituted B in the carbon lattice (between 187 and 189 eV).25,27,28 Figure 4 depicts typical CVs at 0.5 mV/s and CPs at 0.125 mA/g obtained with B-doped carbon gels. The CVs have a quasi-rectangular shape, and the CPs are triangular, indicating that all samples behave as ideal EDL capacitors. CVs also show redox humps between 0.3 and 0.4 V versus Ag/AgCl associated with quinone/hydroquinone redox processes,29−31 which induce pseudocapacitance. Table 4 compiles the gravimetric capacitances obtained from the above curves by applying eqs 1 and 2. The CCV and CCP values for each sample are fairly close, and samples Bc and Bv have the highest CCP value, around 140 F/g. The CCP value decreases as the current load increases between 0.125 and 1 A/ g (see the Supporting Information). The capacitance retention at the highest current load, RCP (Table 4), is between 70 and 80%. The ESR values (Table 4) range between 2.3 and 4.0 mΩ, with the vacuum-dried xerogels, Bv and Pv, having the lowest ESR value in each series. CCP and Simm values were used to calculate the interfacial or areal capacitance, ICCP (Table 4). Simm was used in this calculation because it gives a more realistic value of the surface area of the B-doped carbon gels. Most of these values are in fairly good agreement with the interfacial capacitance of a clean graphite surface,32,33 20 μF/cm2, and with the value range between 20 and 30 μF/cm2 reported for different carbons,29,30 indicating a good accessibility to the microporosity of the carbon gels.

Figure 2. Relationships between the hydrophobicity of the B-doped carbon gels and their (a) surface and (b) total oxygen contents.

correlation coefficients of 0.992 and 0.936, respectively. The decrease in HF with an increased oxygen content is attributable to the increased number of polar oxygen functionalities, which improve the wettability of the carbon surface and can facilitate EDL formation during the electrochemical measurements. The drying method influences the oxygen content of the samples, especially those from series P, with Pc and Pv having the highest OTPD and OXPS values. In all B-doped carbon gels, Btotal > BXPS. The B content of the carbon gels depends upon the drying method used. Thus, the B-doped carbon aerogels Ba and Pa show the lowest BXPS content within their respective series. This is attributable to a loss of superficial B during the exchange of water with acetone in the hydrogels before their supercritical drying. This was previously reported13 in B-doped carbon aerogels prepared with boric acid as the dopant, and the present study demonstrates the same phenomenon with phenyl boronic acid as the dopant, as confirmed by analysis of the acetone in which the hydrogels were immersed. Experimental and theoretical studies have reported9−12 that boron doping of carbons at a low concentration leads to catalytic effects on oxygen chemisorption. The relationship between OXPS and BXPS is depicted in Figure 3a, which also includes data from the literature on other B-doped carbon aerogels obtained at 900 °C.13 This figure shows a very good agreement between both parameters, with an increase in OXPS with higher BXPS. Figure 3b depicts a similar relationship between OTPD and Btotal. The cryogel Pc and the xerogel Pv 1719

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Figure 4. (a) CVs at 0.5 mV/s and (b) CPs at 0.125 A/g in 1 M H2SO4 for B-doped carbon gels from series B: blue circles, Ba; black squares, Bc; red diamonds, Bmw; and green triangles, Bv. (c) CVs at 0.5 mV/s and (d) CPs at 0.125 A/g in 1 M H2SO4 for B-doped carbon gels from series P: blue circles, Pa; black squares, Pc; red diamonds, Pmw; and green triangles, Pv.

Figure 5. (a) Relationships between the interfacial capacitance and surface area. Symbols: red squares, samples from this work; green squares, samples from ref 34. Reproduced with permission from Elsevier. (b) Relationships between the interfacial capacitance and the areal oxygen content.

Table 4. Gravimetric Capacitance, CCV, at 0.5 mV/sa sample

CCV (F/g)

CCP (F/g)

RCP (%)

ICCPb (μF/cm2)

VCCP (F/cm3)

ESR (mΩ)

Ba Bc Bmw Bv Pa Pc Pmw Pv

130 150 139 159 105 111 149 126

114 143 126 142 102 105 130 113

73 70 78 64 80 83 76 83

18 22 18 20 14 26 20 38

68 96 78 95 67 64 87 84

2.7 2.6 3.0 2.3 4.0 3.5 2.9 2.7

64 and 96 F/cm3, with Bc and Bv also showing the highest VCCP value. These values are higher than the volumetric capacitance generally required for practical applications,36 making these samples suitable for use in small-volume devices.

4. CONCLUSION The drying method and boron precursor had practically no influence on the micropore volumes and BET surface area, except in the case of the cryogel Pc and the vacuum-dried xerogel Pv. Samples prepared with boric acid had a higher mesopore volume and wider mean mesopore size than samples prepared with phenyl boronic acid. However, the drying method had practically no influence on the mesoporosity in either series. Surface area obtained by immersion calorimetry into benzene at 30 °C was more realistic than the area obtained by applying the BET equation to N2 adsorption isotherms at −196 °C. The B content depended upon the drying method used. The hydrophobicity of samples decreased linearly with greater surface oxygen and total oxygen contents. In addition, the surface and total oxygen contents increased with greater surface B and total B contents, respectively. The interfacial or areal capacitance of the B-doped carbon gels decreased with an increase in the surface area obtained from immersion calorimetry into benzene, because of the higher number of basal plane sites versus edge sites with an expansion of the surface area. The interfacial capacitance also depended upon the oxygen content. Thus, the influence of B on the capacitance was due to the pseudocapacitance induced by the increase in surface oxygen functionalities of the samples. The cryogel and vacuum-dried xerogel obtained from the boric acid series, Bc and Bv, respectively, showed the largest gravimetric and volumetric capacitances, around 140 F/g and 95 F/cm3, respectively.

a

CCP, ICCP, and VCCP, gravimetric, interfacial, and volumetric capacitances from chronopotentiometry, respectively, at 0.125 A/g; RCP, capacitance retention at 1 A/g; and ESR, equivalent series resistance. bSimm was used to calculate ICCP.

Figure 5a depicts the relationship between ICCP and Simm for B-doped carbon gels. This figure also contains data for Ndoped carbon xerogels that show a similar relationship but using the micropore surface area, Smic.34 The plot shows a very good agreement (correlation coefficient of 0.927) between ICCP and surface area. The decrease in ICCP with a larger surface area can be explained by the much lower EDL capacitance on graphite basal planes versus edges.3,29,35 A rise in the Simm increases the proportion of surface sites on basal planes on the walls of the slit-shaped micropores versus edge sites mainly on the external surface, reducing the interfacial capacitance. Figure 5b shows that ICCP tends to increase with a higher areal O XPS concentration, because of the increase in pseudocapacitance effects produced by the surface oxygen functionalities, enhancing the total capacitance. Volumetric capacitance values, VCCP, obtained from the CCP and particle density are compiled in Table 4 and range between 1720

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ASSOCIATED CONTENT

S Supporting Information *

XPS profiles of the C1s (Figure S1) and B1s (Figure S2) regions for B-doped carbon gels and variation of the gravimetric capacitance CCP with the current load for B-doped carbon gels (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +34-958-243-323. Fax: +34-958-248-526. E-mail: [email protected]. Present Address

́ Zulamita Zapata-Benabihe: Facultad de Ingenieriá Quimica, ́ Colombia. Universidad Pontificia Bolivariana, 050031 Medellin, †

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financed by the Junta de Andalucia,́ Spain. Zulamita Zapata-Benabihe acknowledges a predoctoral fellowship from Colciencias, Colombia.

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REFERENCES

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