Ultralight, high-surface-area, multifunctional graphene-based aerogels from self-assembly of graphene oxide and resol

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Ultralight, high-surface-area, multifunctional graphene-based aerogels from self-assembly of graphene oxide and resol Yuqiang Qian a, Issam M. Ismail b, Andreas Stein a b

a,*

Department of Chemistry, University of Minnesota, Minneapolis, MN 55455, United States Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, United States

A R T I C L E I N F O

A B S T R A C T

Article history:

The self-assembly between graphene oxide sheets and resol-type phenolic prepolymers

Received 31 July 2013

was investigated as a method to form three-dimensional porous carbon objects with high

Accepted 30 October 2013

surface areas and low densities. After freeze-drying and subsequent pyrolysis of the

Available online 7 November 2013

assembled hydrogels, ultralight graphene/carbon composite aerogels with high surface areas and porosity, good conductivity, and well-defined bulk shape were obtained. By adjusting the amount of graphene oxide and resol in the precursor mixture, aerogels with a density as low as 3.2 mg/cm3 or a surface area as high as 1019 m2/g could be prepared. It is proposed that resol molecules are first adsorbed on the surface of graphene oxide sheets, and then the surface-coated sheets are crosslinked by the polymerization of resol prepolymers. The absorption performance was evaluated for the aerogel with the lowest density. Due to the high porosity, the aerogel displayed fast absorption rates for organic solvents as well as high absorption efficiencies. The high conductivity of the aerogels permits good performance as binderless monolithic electrodes for supercapacitors. Ó 2013 Elsevier Ltd. All rights reserved.

1.

Introduction

Graphene consists of two-dimensional, one-atom-thick sheets of sp2-hybridized carbon atoms and has attracted tremendous interest in a variety of fields, due to its outstanding electronic, thermal, and mechanical properties [1,2]. As such, fabrication of graphene into three-dimensional (3D) materials with high surface area and porosity that also maintain some of these properties is tremendously attractive to materials scientists and greatly desirable in advanced applications, such as absorption and energy storage. One approach to assemble graphene sheets into 3D bulk objects is to prepare them as aerogels. Aerogels are a class of light-weight, porous solids with large surface areas and pore volumes. They have been widely studied in various applications including thermal insulation,

absorption, and catalyst supports [3,4]. It was recently found that carbon aerogels composed of interconnected graphene sheets can be obtained by simple assembly of graphene oxide (GO) sheets [5–13]. Compared to traditional carbon aerogels which are typically polymer-derived glassy carbon [14], graphene-based aerogels permit the opportunity to achieve ultralow density, high surface area and porosity, combined with good conductivity for improved absorption, catalytic, and electrochemical performance [10,11,15–17]. Bi et al. reported the use of graphene aerogels obtained from hydrothermal assembly of GO as sorbent materials with an absorption capacity of 86 g/g for chloroform, much higher than that obtained with natural products and polymeric foams [18]. The absorption capabilities of the graphene aerogels can be easily restored by a simple heat treatment. Zhao et al. prepared N-doped graphene aerogels by hydrothermal assembly of GO in the

* Corresponding author. E-mail address: [email protected] (A. Stein). 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.10.082

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presence of pyrrole, and the product showed improved absorption capacity (480 g/g for chloroform) resulting from its low density (density = 2.1 mg/cm3) [11]. In addition, it also exhibited a high conductivity of 1200 S/m and superior capacitance of 484 F/g due to the N-doped graphene structure. The least dense graphene-based aerogel up to date, reported by Sun et al., was prepared by assembly of carbon nanotubes and giant GO sheets, had a density of 0.16 mg/cm3, and exhibited a very high absorption capacity of 568 g/g for chloroform [13]. Although the materials were ultralight, the surface areas of the above aerogels were all less than 300 m2/g, far less than the theoretical value of single-layered graphene (2600 m2/g). High-surface-area graphene-based aerogels were obtained with resorcinol and formaldehyde to provide crosslinks, the highest reported surface area being 763 m2/g for these systems, but their densities were all above 10 mg/cm3 [6,19]. Even though a number of methods have been reported for the preparation of graphene aerogels, it is still challenging to achieve ultralight graphene-based aerogels (<10 mg/cm3) with high surface areas (>1000 m2/g) and good bulk electrical conductivity (>10 S/m). Tunability of the physical properties of graphene-based aerogels is rarely studied, so it is of great interest to explore preparations of ultralight, high-surface-area graphene-based aerogels with controlled properties. Here we report the preparation of graphene/carbon composite aerogels with densities as low as 3.2 mg/cm3 or specific surface areas as high as 1019 m2/g from the assembly of graphene oxide and a resol-type phenolic resin, and control the properties of the resulting graphene/carbon composite aerogels by adjusting the amounts of GO and resol precursor. Use of resol resin as the mediating agent for the assembly of GO also offers better control of the shape of the aerogel because the volume of the original precursor mixture is nearly retained, and the aerogels replicate the interior shape of the reactor. We also propose an adsorption–assembly mechanism to explain the gelation mechanism of the GO–resol mixture and the difference in textural properties of the aerogels. Due to their highly porous structure, the aerogels have outstanding absorption efficiencies and can absorb hydrocarbons and chloroform up to 400 times their weight, which ranks among the best absorbing materials. The aerogels also display reasonably good electrochemical performance as binderless monolithic electrodes for supercapacitors with specific capacitance values of 99 and 80 F/g at current densities of 100 mA/g and 2 A/g, respectively.

2.

Experimental

2.1.

Materials

The following chemicals were used as received: natural graphite flakes (SP-1, 45 lm) from Bay Carbon, Inc.; NaNO3 and KMnO4, and formaldehyde (37%) from Fisher Scientific; H2SO4 and HCl from VWR International; H2O2 (30%) and KOH from Macron Fine Chemicals; phenol from Sigma–Aldrich.

2.2.

Synthesis of graphene oxide and phenolic resol resin

Graphene oxide was synthesized according to the Hummers method, as reported elsewhere [20,21]. The resulting slurry

was centrifuged and washed with 2 M HCl until it was SO24 free. The acid-washed GO was redispersed in water to form a brown dispersion. The pH of the dispersion was then adjusted to 5–6 with concentrated ammonia solution to promote exfoliation of GO, and unoxidized graphite impurities were removed by centrifugation at 4000 rpm. The pH of the resulting supernatant was adjusted back to 3, and the supernatant was dialyzed several times in deionized (DI) water until its pH became close to 5 and remained unchanged. The dispersion was then diluted to obtain a GO concentration of 1 mg/mL. Resol resin was prepared according to a literature procedure [22]. A clear orange solution with 50 wt.% of resol in ethanol was obtained for further use.

2.3.

Preparation of graphene/carbon composite aerogels

In a typical synthesis, 15 mL GO aqueous dispersion (1 mg/ mL) was adjusted to pH 3 using 0.1 M HCl, and then 0.30 g of resol solution (50 wt.% in ethanol) was added while stirring. The mixture was then transferred to a 23 mL autoclave with a Teflon liner and hydrothermally treated at 180 °C for 24 h. A graphene/phenolic polymer composite aerogel was obtained after freeze-drying of the resulting hydrogel and was denoted as G1P2 aerogel. A graphene/carbon composite aerogel was obtained after pyrolysis at 900 °C for 2 h in flowing nitrogen and was denoted as G1C2 aerogel. Aerogels with various precursor compositions were also prepared, and they were denoted as GxPy aerogels, where x is the concentration of GO dispersion in units of mg/mL, and y is the number of aliquots of 0.15 g of resol solution; these materials correspond to the GxCy aerogel after pyrolysis. Syntheses using only resol prepolymers or GO were conducted similarly. The freeze-dried and pyrolyzed products using only the resol prepolymers were denoted as PF–HT and PFC, respectively, where PF stands for phenol–formaldehyde polymer and HT stands for hydrothermal treatment. Freeze-dried and pyrolyzed products obtained using only GO were denoted as GO–HT and GS, respectively.

2.4.

Characterization

Scanning electron microscopy (SEM) images were taken using a JEOL 6500 FEG–SEM with an accelerating voltage of 5 kV. Transmission electron microscopy (TEM) images were obtained on a FEI Tecnai T12 microscope using an accelerating voltage of 120 kV. Samples were sonicated in ethanol and then dropped onto a Cu grid. X-ray diffraction (XRD) patterns were acquired using a PANalytical X-Pert Pro MPD X-ray diffractometer equipped with a Co source (45 kV, 40 mA, ˚ ) and an X-Celerator detector. Small-angle X-ray k = 1.790 A scattering (SAXS) patterns were acquired using a Rigaku RU200BVH 2D SAXS instrument with a Cu X-ray source (45 kV, ˚ ) and a Siemens Hi-Star multi-wire area 40 mA, k = 1.542 A detector with a sample-to-detector distance of 70 cm. For atomic force microscopy (AFM) measurements, graphene oxide was spin-coated at a rate of 100 L/0.5 in.2 of substrate from a 0.5 mg/mL dispersion in water at 3000 rpm for 30 s on top of freshly cleaved V-5 muscovite, and then dried under vacuum overnight at room temperature. The AFM imaging was performed in contact mode to record GO sheet thickness

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using a Digital Nanoscope III Multi-mode instrument. AFM cantilever calibration was first performed using a HF-etched mica according to the procedure reported by Nagahara et al. [23], showing a z-measurement error of about 0.1 nm. An alpha 300R confocal Raman microscope equipped with a UHTS200 spectrometer and a DV401 CCD detector from WITec (Ulm, Germany) was employed to collect Raman spectra. An Ar-ion laser with a wavelength of 514.5 nm and a power of 6 mW was used for excitation. X-ray photoelectron spectroscopy (XPS) spectra were collected using an SSX-100 instrument (Surface Science Instruments) with a 200 W highthroughput bent quartz crystal monochromated Al Ka X-ray source, a hemispherical sector analyzer (HAS) and a resistive anode detector. The samples were mounted on the sample holder using double-sided carbon tape. The electrical conductivities of graphene/carbon composite aerogels were measured via the van der Pauw technique using a homemade four-point probe setup [24]. Each of the four copper wire probes was glued to a corner of 1-mm-thick aerogels using colloidal silver paste (Ted Pella, Inc.). The measurements were carried out on an Arbin battery-testing system after the silver paste had dried in air for 1 day.

2.5.

Electrochemical tests

Galvanostatic charge–discharge measurements were performed with a sandwich-type, symmetric cell as shown in Fig. 11(a), in which two 1-mm-thick aerogel blocks were used as monolithic electrodes, a porous cellulose filter paper as separator, and strips of Ni foam (MTI Corp.) as the current collectors. The cell was sandwiched between plastic substrates that were pressed together with a binder clip. The electrolyte was a 6.0 M KOH aqueous solution, and the cell was immersed in the electrolyte under static vacuum for 2 h. Electrochemical cycling was conducted on an Arbin battery-testing system with a voltage range from 0.001 to 0.8 V.

3.

Results and discussion

3.1. Morphologies and structures of G1P2 and G1C2 aerogels In graphene-based aerogels, the structural features of the aerogel walls originate from the sheet-structure of the GO

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precursor. GO was prepared using Hummers’ oxidation of graphite in this study. The introduction of oxygen-containing groups significantly increased the d-spacing of the basal plane from 0.34 nm for graphite to 0.75 nm for GO (Fig. S1 in Supplementary data). The resulting GO had a C:O ratio of 2.2:1 (Fig. S1), and therefore it was very hydrophilic and could be easily dispersed in water. As shown in Fig. 1, the GO dispersion used here contained GO sheets with sizes of several micrometers, and AFM analysis confirmed that these sheets were exfoliated with a layer thickness of 0.7 nm, in agreement with the d-spacing value of GO. The process to prepare the graphene/carbon composite aerogels is shown in Fig. 2. After hydrothermal treatment and subsequent drying, the aerogel G1P2 replicated the interior shape of the reactor, as indicated by the cylindrical monolith in Fig. 2(c). Pyrolysis of the phenolic polymer in the aerogel resulted in a conductive G1C2 aerogel without noticeable volume shrinkage. However, when only resol or GO were used as precursors, powdery products were obtained after hydrothermal treatment and freeze-drying, as seen in Fig. 2(a) for the resol product PF–HT and in Fig. 2(b) for GO–HT. Resol resin is a common precursor for microporous carbon spheres obtained by pyrolysis of phenolic polymer spheres that are formed by the condensation and crosslinking of resol resin [25–27]. In our study, microspheres can be observed in the SEM image in Fig. 3(a) for the sample containing only resol resin during the hydrothermal synthesis. Carbon spheres with diameters ranging from submicrometer to several micrometers were obtained after pyrolysis, which resulted in some volume shrinkage, as shown in Fig. 3(b). Under the same hydrothermal conditions, graphene oxide undergoes a dehydration process in which a conjugated graphene network is partially recovered [28–30]. Although a macroporous structure was formed for both GO–HT and its pyrolysis product, GS, as shown in Fig. 3(c) and (d), the reduction caused the graphene sheets to be restacked and aggregated, which resulted in a powdery product. Hydrothermal condensation of resol in the presence of graphene oxide produced a monolithic G1P2 aerogel with very open macropores and thin walls composed of phenolic resin/graphene composite sheets (Fig. 3(e)). From the observation that no spherical particles formed when resol and GO were present at the same time, we conclude that resol molecules coated the graphene sheets and then underwent a condensation reaction. After pyrolysis, the structure of the

Fig. 1 – (a) TEM and (b) AFM images of GO sheets. The inset in (b) shows the line profile to reflect the thickness of GO sheets. (A color version of this figure can be viewed online.)

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Fig. 2 – Top: scheme of the synthesis of graphene/carbon composite aerogel. Bottom: (a) PF–HT, (b) GO–HT, (c) G1P2 and (d) G1C2. The large divisions on the ruler are in units of cm. (A color version of this figure can be viewed online.)

Fig. 3 – SEM images of hydrothermal products before (left) and after (right) pyrolysis. (a) PF–HT, (b) PFC, (c) GO–HT, (d) GS, (e) G1P2 and (f) G1C2. Scale bar: 10 lm.

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G1C2 aerogel (Fig. 3(f)) was more open compared to G1P2 due to the pyrolysis of phenolic polymers. As the structure was supported by the assembled graphene sheets, there was no noticeable shrinkage of the structure, in contrast to the observed volume shrinkage for PF–HT spheres. Due to the abovementioned resin coating, the walls of the G1C2 aerogel became thicker than those of the GS powders, so in TEM images (Fig. 4), the wrinkles of the G1C2 aerogel walls are more discernible, compared to the thin sheets present in GS powders. The structure of the carbon products was analyzed by XRD and Raman spectroscopy. The PFC sample contained glassy carbon, as revealed by the broad d002 peak centered around 25.5° 2h in Fig. 5(a), which has been reported previously for resol-type carbons [31,32]. The peak broadening in this pattern was due to turbostratic ordering of small graphitic grains [31]. For GS, the d002 peak appeared at a higher 2h value and was sharper than that of the PFC sample, indicating closer packing of large graphene layers. The d002 spacing of GS was calculated to be 0.344 nm, very similar to that of graphite (0.335 nm) due to the recovery of the graphene structure from GO after pyrolysis. The d002 peak of G1C2 aerogel was the broadest among all the samples, and its peak value corresponded to 0.354 nm. In this material the graphene sheets were in a highly exfoliated state, and the glassy carbon component was highly dispersed on the graphene sheets.

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All pyrolyzed carbon products displayed two peaks around 1348 cm 1 and 1600 cm 1 in Raman spectra (Fig. 5(b)), which are characteristic of symmetry breakdown at the edge of graphene sheets (D band, 1348 cm 1) and the E2g vibrational mode of graphitic planes (G band, 1580–1600 cm 1), respectively [33]. For PFC with a glassy carbon structure, the peaks were broad and the ID/IG ratio was 0.96. For GS, although the XRD pattern indicated that the stacking of graphene layers was similar to that in graphite, its Raman spectrum showed a higher ID/IG ratio of 1.27, consistent with previous reports for chemically converted graphene materials [34,35]. The high ID/IG value of GS indicates that the graphene stacks contain a large amount of structural defects introduced during the preparation of graphene oxide by intensive oxidation [36] and subsequent loss of surface groups during reduction [35]. With both GO and resol resin in the precursor, G1C3 showed an ID/IG ratio of 1.15, in between those of GS and PFC, indicating the presence of both GO-derived and resol-derived carbon.

3.2.

Adsorption–assembly mechanism for gel formation

In order to investigate the mechanism for the self-assembly of GO and resol resin, we adjusted the amounts of these two components in the hydrothermal reaction to see how their concentrations affect gel formation. The results are summarized in Table 1, in which x is the concentration of GO in units

Fig. 4 – TEM images of GS (a and b) and G1C2 (c and d).

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Fig. 6 – A scheme of the adsorption–assembly mechanism for the formation of graphene-based aerogels with resol resin. (A color version of this figure can be viewed online.)

Fig. 5 – XRD patterns (a) and Raman spectra (b) for PFC, GS and G1C2. (A color version of this figure can be viewed online.)

of mg/mL and y is the number of aliquots of 75 mg of resol in the mixture. A black slurry was obtained after hydrothermal treatment when the amount of resol was reduced to 37.5 mg, which indicated that it was inadequate to form a strong interconnected network with the G1P0.5 composition. Although a completely gelled network was obtained for G1P1, which could absorb all the water in the mixture, for G2P1 with a higher GO concentration, the solid gel was not able to hold all the water; this could be an indication of a reduced surface hydrophicility for the composite sheets. Similar behavior was observed for G3P2 and G5P2. For the compositions above the diagonal line in Table 1, completely gelled networks were obtained.

Fig. 7 – SEM images of G1P1 (a) and G1P2 (b). Scale bar: 100 nm.

Table 1 – The gelation states of hydrothermal products with various amounts of GO and resol precursors.a x

1 2 3 5

y 0.5

1

2

3

Slurry N/A N/A N/A

Gel Gel and water N/A N/A

Gel Gel Gel and water Gel and water

Gel Gel N/A N/A

a x is the concentration of GO in units of mg/mL and y is the number of aliquots of 75 mg of resol in the mixture. Combinations marked with ‘‘N/A’’ were not synthesized and thus their gelation states are not available.

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With the above observations, we propose an adsorption– assembly mechanism as shown in Fig. 6 to explain the different behaviors for GO, resol and GO + resol during hydrothermal treatment. Under hydrothermal conditions, GO sheets undergo a self-reduction process by losing the oxygenated surface groups [28,29], and consequently are prone to aggregate due to the large p–p and van der Waals interactions between the sheets. Resol prepolymers, on the other hand, polymerize and crosslink, eventually precipitating out from water and forming spherical nanoparticles to minimize the surface energy. The nanoparticles aggregate into micrometer-sized particles due to further aging under hydrothermal conditions. When both GO and resol are present in the mixture, due to the large surface area of GO sheets, resol is adsorbed on the GO surface and forms hydrogen bonds with GO. The coated GO sheets have a relatively hydrophilic surface even after self-reduction under hydrothermal conditions. Similar to the modification of nanoparticles with organic functional groups to improve their dispersion in organic matrices [37–39], the surface coating on the graphene sheets mitigates their aggregation in water. Wang et al. also observed that the presence of resol lessens the aggregation of graphene

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sheets by reducing their van der Waals interactions [40]. Meanwhile, polymerization and crosslinking of resol prepolymers take place on the GO surface. This drives the formation of an interconnected network of resol/graphene sheets because of crosslinking reactions at the interface between overlapping sheets. Due to the exfoliated morphology of the graphene-type sheets and their large hydroxyl-rich surfaces, a hydrogel is formed which holds all the water present in the mixture. According to the proposed mechanism, resol prepolymers help to prevent the stacking of GO sheets and promote the formation of a 3D network. So, compared to G1P1, reducing the amount of resol prepolymers to G1P0.5 prevents the full coverage of GO surfaces and effective linking between overlapping sheets. Similarly, increasing the amount of GO to G2P1 also results in insufficient coverage of GO, so the exposed surfaces undergo self-dehydration and restack. Since the concentration of GO sheets is increased, an interconnected network can still be formed, but the hydrogel cannot hold all the water because of the lower fraction of hydroxylrich surfaces. The proposed adsorption–assembly mechanism is also consistent with SEM observations. The SEM image of

Fig. 8 – High-resolution TEM images of (a and b) G1C3 and (c and d) GS sheets. (a) and (c) are side-views to show the sheet edge, and the inset figures show the (red) line profiles to calculate the average d-spacing of the graphene layers. (b) and (d) are top-down views to show the sheet surface texture. The black arrow in (a) points out the amorphous, porous carbon coating on the graphene layer. Scale bar: 5 nm. (A color version of this figure can be viewed online.)

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G1P1 shows a grainy topology (Fig. 7(a)), revealing incomplete surface coverage. Such a patchy coating behavior was not observed for G1P2 (Fig. 7(b)), indicating almost complete surface coverage in that sample. This adsorption–assembly mechanism was further supported by TEM images of the graphene/carbon composite aerogels. The images in Fig. 8(a) and (c) show side views of the G1C3 and GS sheets, respectively. It can be seen that for G1C3, the graphene layers at the center are surrounded by layers of amorphous, porous carbon, which originate from the pyrolysis of adsorbed resol polymers on the graphene sheets. For GS, although the layered structure can be easily seen, no amorphous layer was observed. The comparison between the top-down view images of G1C3 and GS in Fig. 8(b) and (d) also reveals that the resulting porous carbon layers are present in G1C3 sheets, but are absent in GS sheets.

3.3. Morphology and properties of GxCy graphene/carbon composite aerogels All the GxCy graphene/carbon composite aerogels showed a similar foam-like morphology (Fig. 9). With the least amount of precursors, G1C1 contained the largest macropores and a very open structure. With an increasing amount of resol precursor (from Fig. 9(a) to (c)), the structure became denser with smaller macropores, which could be due to a greater extent of exfoliation of graphene sheets facilitated by the increased amount of resol precursors, according to the proposed mechanism. Comparing panels (b)–(d) in Fig. 9, with increased GO concentration, the resulting aerogels also became much denser because of the higher concentration of graphene-like sheets present in the structure. The density, textural properties, and bulk conductivity of the GxCy graphene/carbon composite aerogels are summarized in Table 2. All the aerogels can be classified as ultralight materials as they have densities less than 10 mg/cm3. The G1C1 aerogel, which was obtained from the least amount of precursor, has a density of 3.2 mg/cm3, the lightest among all synthesized samples and one of the lightest graphenebased aerogels that have been reported. The nitrogen-sorption isotherms of graphene/carbon composite aerogels have Type IV adsorption hysteresis loops (Fig. 10), which indicate the presence of mesoporosity. Since the proposed mechanism relies on surface adsorption of resol prepolymers, the mesoporosity is believed to result from the pyrolysis of the PF polymer, forming textural mesopores. When the GO concentration was fixed at 1 mg/mL, more resol prepolymers in the precursor resulted in an increase of BET surface area, and a decrease of pore volume and pore radius. Interestingly, when the amount of resol was fixed at 0.15 g, a higher concentration of GO dispersion resulted in the opposite trends for the above textural properties. This can be explained by the adsorption–assembly mechanism. Resol prepolymers adsorbed on the GO surface formed a very thin layer of phenol–formaldehyde polymers, which was subsequently converted to a very thin layer of high-surface-area porous carbon coated on the large graphene surface, and the surface porosity was affected by the density of condensed phenolic polymers. Therefore a higher resol:GO ratio in the precursor resulted in a higher surface area, as well as a smal-

Fig. 9 – SEM images of graphene/carbon composite aerogels with different precursor compositions. (a) G1C1, (b) G1C2, (c) G1C3, (d) G2C2 and (e) G3C2. Scale bar: 10 lm. ler pore volume and average pore radius. With the highest resol:GO ratio used in this study, the G1C3 aerogel exhibited a surface area of 1019 m2/g, much higher than those of PFC (442 m2/g) or GS (584 m2/g). The textural mesopores formed by the pyrolysis of resol polymer on the graphene surface contribute to the high surface area of G1C3. Besides TEM observations in Fig. 8(a) and (b), the presence of mesostructure in G1C3 was also confirmed by SAXS results (see Supplementary data).

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Table 2 – Physical properties of GxCy aerogels, PFC and GS powders. Sample

Densitya (mg/cm3)

SBETb (m2/g)

Vtotalb (cm3/g)

Pore radiusb (nm)

Conductivityc (S/m)

G1C1 G1C2 G1C3 G2C2 G3C2 PFC GS

3.2 6.0 7.9 7.2 8.4 nd nd

640 872 1019 577 352 442 584

2.92 2.54 1.56 1.97 1.34 0.25 2.27

9.14 5.83 3.13 6.82 7.63 1.15 7.78

6.15 6.56 13.1 8.31 10 nd nd

nd = not determined. The density was determined by the weighed mass of aerogel divided by the measured volume. b The BET specific surface area, pore volume, and average pore radius were determined from N2 sorption analysis. c The conductivity was measured by the van der Pauw method [24]. a

Fig. 10 – N2 sorption isotherms of G1Cy (a) and GxC2 (b) aerogels. The isotherm of PFC is shown for comparison. (A color version of this figure can be viewed online.)

3.4. Sorption properties of the graphene/carbon composite aerogels Aerogels have a macroporous structure with open pores and large pore volumes, and these properties endow aerogels with good absorbing properties. The graphene-based aerogels prepared in this study have densities as low as 3.2 mg/cm3, so 99.8% of the aerogel can be filled with air (assuming the carbon density is 2.0 g/cm3), which is of great advantage for absorption applications. To evaluate the absorption capability of the aerogels, we cut a block of G1C1 aerogels with a volume

of 0.8 cm3 and then immersed it into a container with reddyed dodecane floating on water. The mass of dodecane was 150 times the mass of the aerogel, and within 2 s, all the dodecane was absorbed by the aerogel. The absorbing ability for different solvents was also tested by immersing an aerogel block into each solvent for at least 10 min. The solvent-filled aerogels were taken out, wiped gently, and then weighed. The absorption capacities were calculated as the ratio of the mass of absorbed organic solvent to the mass of aerogel (MOS/MAG) or the ratio of the volume of the absorbed organic solvent to the mass of aerogel (VOS/MAG). The results are summarized in Table 3. It is worth noting that the weight-based absorption capacity was greatly affected by the density of the solvent, so a volume-based absorption capacity was also calculated to obtain information on solvent wettability. As can be seen from Table 3, the aerogels can absorb organic solvents with 200–400 times their original mass, depending on the density of the solvents. These values are much higher than those for polymeric foams (5–25 g/g) and other high-density graphene-based aerogels (<200 g/g) [9,18] and ultralight carbon nanotube or nanofiber aerogels (<200 g/g) [41,42]. The absorption performance is comparable to other ultralight graphene aerogels with densities less than 3 mg/cm3 [11,13]. It is also interesting to see that the volumebased capacity for toluene and nitrobenzene is higher than that for dodecane and chloroform, because the aromatic solvents have higher affinity for graphene-based aerogels and wet them better than the aliphatic solvents. The high VOS/ MAG value of pump oil is due to the fact that its high viscosity slowed down the effluence when the aerogel was taken out of the solvent.

3.5. Electrochemical composite aerogels

performance

of

graphene/carbon

The interconnected porosity and conductivity of the aerogels are also preferred in electrochemical applications, such as electrode materials for supercapacitors or batteries. Electrochemical cells can be assembled with pieces of aerogels cut from the original monoliths. As illustrated in Fig. 11, two 1mm-thick aerogel pieces were assembled into a symmetric supercapacitor cell with cellulose paper as the separator and nickel foam as current collectors. The assembly was secured with a binder clip so that the aerogel and the current

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4.

Conclusions

We have demonstrated the preparation of ultralight graphene/carbon composite aerogels with high surface areas and porosity, good conductivity, and well-defined bulk shape by assembly of graphene oxide and resol-type phenolic prepolymers. The properties of the aerogels could be easily controlled by adjusting the amount of GO and resol in the precursor mixture, and aerogels with a density as low as 3.2 mg/cm3 (G1C1) or a surface area as high as 1019 m2/g (G1C3) could be prepared. Based on the observed structures and properties of the aerogels, we have proposed an adsorption–assembly mechanism for the assembly of GO sheets and resol prepolymers under hydrothermal conditions. Due to its high porosity, the G1C1 aerogel is a fast sorbent of organic solvents with high absorption efficiencies, capable of absorbing 400 times its weight of chloroform. The aerogels also displayed good conductivity and good supercapacitive performance as binderless monolithic electrodes with a capacitance of 99 F/g at 100 mA/g.

Acknowledgments

Fig. 11 – (a) Scheme of the assembled supercapacitor cell. (b) The capacitance of the G1C3 aerogel at various current densities. (A color version of this figure can be viewed online.)

The authors thank Dr. Xueyi Zhang for discussions and assistance with the TEM work. This work was supported by the University of Minnesota Initiative for Renewable Energy and the Environment (IREE). Portions of this work were carried out at the University of Minnesota Characterization Facility, which receives partial support from the NSF through the MRSEC, ERC, MRI, and NNIN programs.

Table 3 – Absorption capacities of G1C1 aerogel for organic solvents.

Appendix A. Supplementary data

Organic solvent Density Absorption capacity of solvent M /M (g/g) V /M (mL/g) OS AG OS AG (g/mL) Dodecane Chloroform Toluene Nitrobenzene Pump oil

0.749 1.483 0.866 1.205 0.87

216 400 279 370 273

288 270 322 307 314

collector were in close contact. A capacitance of 99 F/g was achieved at a current density of 100 mA/g, and gradually the capacitance dropped to 93 F/g after 10 cycles. The capacitance decreased to 80 F/g at the higher current density of 2 A/g. The capacitance of our aerogels is lower than that of previously synthesized aerogels [11,15,19,43], and one reason is that we used a monolithic electrode, with no added polymeric binder or conductive additive. The other reason is that during the assembly of the cell, the structure of the aerogel was damaged when pressurized by the binder clip. It is speculated that some pieces of graphene sheets lost contact with the conductive matrix and thus failed to contribute to the capacitance. Still, the capacitance drop from 200 mA/g to 2 A/g was only 10%, which is similar to or better than in previous reports [19,43], and after returning to 200 mA/g, the capacitance recovered to 89 F/g, indicating high capacitance retention and good stability after about 10 cycles.

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2013.10.082.

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