Using Asphaltene Supermolecules Derived from Coal for thePreparation of Efficient Carbon Electrodes for SupercapacitorsWen-Hui Qu,† Yu-Bo Guo,† Wen-Zhong Shen,*,‡ and Wen-Cui Li*,†

†State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China‡State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Science, Taiyuan, 030001, China

*S Supporting Information

ABSTRACT: Asphaltene supermolecules extracted from coalconsist of highly condensed polyaromatic units and peripheralaliphatic chains, which is a natural source with high carbon content.In this study, we demonstrate that the asphaltene can be used as anideal supermolecular carbon precursor for the fabrication of carbonnanosheets by self-assembly via π−π and hydrogen bondinginteractions with a sheet-structure-directing agent of grapheneoxide. The overall thickness of the obtained asphaltene basedcarbon nanosheets can be tuned from 13 ± 3 to 41 ± 5 nm. Thesecarbon nanosheets show an electrical conductivity of ca. 450 S m−1.When they are used as electrode materials for supercapacitors, thecarbon nanosheets demonstrate a specific capacitance of 163 F g−1

even at a current density of 30 A g−1 tested in a three-electrodesystem, due to high electrically conductive networks and short diffusive paths. The maximum specific gravimetric capacitance andsurface area-normalized capacitance in two-electrode system are 191 F g−1 and 43 μF cm−2, respectively, indicating very highutilization of the available surface area. These results prove that asphaltene is a promising molecular precursor for the preparationof energy materials, further displaying an efficient route for staged conversion of coal that is abundant in nature.

1. INTRODUCTION

Carbon materials play a crucial role in electrochemical energy-storage systems due to their extraordinary physicochemicalproperties such as high surface area, good electricalconductivity, and excellent thermal and chemical stability.1−4

In order to achieve good electrochemical performance, thestructural design and synthesis of carbon materials in acontrolled manner is thus of great interest and driving theexploration of new carbon precursors for both fundamentalresearch and practical applications.5,6 The “classical” carbonprecursors, e.g. coal, coconut shell, or other complex biomass,are naturally abundant, low-cost and easily accessible and havebeen widely used to prepare carbon materials through thepyrolysis process mostly followed by an activation step.7

However, these “classical” precursors have disadvantages ofhigh impurity content and high polymerization degree whichcauses barriers to structural control at the molecular level andimprovement of electrochemical performance.The use of molecular precursors provides the opportunity to

exert control over the mesoscopic morphology of the finalcarbon materials by self-assembly of block copolymers, sugars,phenolic resin oligomers, etc.8,9 In our previous work,poly(benzoxazine-co-resol) was developed based on phenols,aldehydes, and diamines to prepare porous carbon monolithswith multiple-length-scale porosity (macro-, meso-, and micro-pores).10,11 However, molecular precursors are often expensiveor difficult to be synthesized.12 To obtain naturally abundant

and accessible molecular precursors, directly extracting themfrom classical carbon sources would be a sustainable strategy,and meanwhile achieve an efficient utilization of rawmaterials.13 For example, coal macromolecule unit typicallyconsists of more than two aromatic rings, which are coupled by“bridges” of aliphatic chains or heteroatoms.14,15 In addition tocovalent bridges, there are a significant number of noncovalentbonds such as electrostatic interactions, hydrogen bonds, andπ−π interactions between aromatic rings.16 By dissociatingthese abundant weak noncovalent bonds, polycyclic aromatichydrocarbons can be selectively extracted from coal. Due toenriched sp2-hybridized carbon species, polycyclic aromatichydrocarbons are considered to be a promising molecularprecursor for highly graphitized porous carbon.9 This propertymakes it promising to prepare porous carbons with excellentelectrical conductivity for use as electrode materials.17,18

In this report, an alkyl-decorated aromatic hydrocarbonderivative, so-called asphaltene, is extracted from coalliquefaction residues, which as the byproducts or wastes arerequired to be used as much as possible with the purpose ofimproving the coal utilization value. Although asphaltene hasbeen used as a carbon precursor in some previous reports,19,20

it is the first time asphaltene is used to prepare carbon

Received: May 20, 2016Revised: July 6, 2016Published: July 6, 2016

Article

pubs.acs.org/JPCC

© 2016 American Chemical Society 15105 DOI: 10.1021/acs.jpcc.6b05136J. Phys. Chem. C 2016, 120, 15105−15113

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nanosheets, as a supermolecular precursor, through solutionchemistry and self-assembly process. The asphaltene basedcarbon nanosheets (ACNs) used as electrode materials forsupercapacitors in a 6 M KOH electrolyte exhibit a highcapacitance of 214 F g−1 in a three-electrode system and canretain 163 F g−1 even at 30 A g−1, due to high electricallyconductive networks and short diffusive paths. In a symmetricalcapacitor, the specific gravimetric capacitance (Cg) and surfacearea-normalized capacitance (Csa) of 191 F g−1 and 43 μF cm−2,respectively, can be achieved. The synthesis not only presents astaged conversion approach of coal but also provides apromising precursor to prepare carbon materials for energyapplications.

2. EXPERIMENTAL SECTION2.1. Chemicals and Materials. Tetrahydrofuran and 1,6-

diaminohexane (99.0% AR) were supplied by SinopharmChemical Reagent Co., Ltd. Graphene oxide colloids used inthis work were prepared following a modified Hummersmethod.21,22 The graphene oxide colloids were dispersed indeionized water to obtain a specified concentration andsonicated at a power of 100 W for 4 h before use (KQ-100TDB, Kun Shan Ultrasonic Instruments Co., Ltd., China).The preparation procedure of asphaltene used in this study wasdescribed elsewhere.23,24 Briefly, the asphaltene was a productextracted from coal liquefaction residues by a Soxhlet extractorwith n-hexane and toluene as the extraction solvents.2.2. Sample Preparation. 2.2.1. Oxidation of Asphaltene.

Asphaltene (2 g) was oxidized in 10 M HNO3 (100 mL) underreflux at 70 °C for 16 h. The resulting brown suspension wasthen filtered through a 0.22 μm microporous membrane andwashed repeatedly with water until the decantate becameneutral. The obtained asphaltene oxide was dried at 90 °Covernight.2.2.2. Synthesis of Carbon Nanosheets. Typically, 0.2 g of

asphaltene oxide was first dispersed in 4 mL of tetrahydrofuranunder magnetic stirring at 25 °C. After completely dispersing, acertain amount of the graphene oxide water solution was addedto the above asphaltene oxide-tetrahydrofuran dispersion andstirred for ca. 5 min. Subsequently, 0.6 mL of 1,6-diaminohexane water solution (0.1 g mL−1) was quicklyinjected into the solution. The reaction mixture was stirred at25 °C for another 5−10 min. The homogeneous solution wasthen sealed and transferred to an oven at 90 °C. It gelled andsolidified within 4 h. This gel was cured for an additional 20 h.The gel was washed with distilled water to remove thetetrahydrofuran solvent and then freeze-dried for 48 h. Theobtained polymer prior to pyrolysis was denoted as ACN-P.The obtained product was pyrolyzed at 800 °C for 2 h under anitrogen atmosphere to obtain carbon nanosheets. By varyingthe mass ratio of graphene oxide to asphaltene oxide, differentACNs were prepared (denoted ACN-x, where x represents themass ratio of graphene oxide/asphaltene oxide). In allsyntheses, the mass ratio of asphaltene oxide to 1,6-diaminohexane was set to 10/3. For comparison, the sampleprepared without graphene oxide by the same procedure wasdenoted as ACN-0. Also a sample without asphaltene wassynthesized in the same process.2.3. Structure Characterization. Thermogravimetric

(TG) analysis was performed in N2 from 100 to 800 °C witha heating rate of 10 °C min−1 on an STA449 F3 Jupiterthermogravimetric analyzer (NETZSCH). The mass spectrawere acquired using a matrix assisted laser desorption/

ionization-time-of-flight (MALDI-TOF) instrument (WatersMALDI Micro MX, USA). The samples were prepared bydispersing 0.5 mg of asphaltene oxide in 1 mL oftetrahydrofuran. 1 μL of dispersion was transferred to astainless steel target plate and taken to dryness beforemeasurements. UV−vis absorption spectra were obtained ona UV−vis spectrophotometer (TECHCOMP, UV-2300), andthe wavelength range was 200−700 nm. Fourier transforminfrared spectroscopy (FT-IR) was performed with a Nicolet6700 (Thermo scientific Co., Ltd., USA) by averaging 64 scansin the 670−4000 cm−1 spectra range at 4 cm−1 resolution. Themorphology of the samples was characterized by FE-SEM(NOVA NanoSEM 450) and TEM (FEI Technai F30).Nitrogen sorption isotherms were measured with a Micro-meritics TriStar 3000 physisorption analyzer. The Brunauer−Emmett−Teller (BET) method was used to calculate thespecific surface areas (SBET). The electrical conductivity wasmeasured on a bulk sample by a four-point probe resistivitymeasurement instrument. Raman spectra were collected on aDXR Microscope Raman Spectrometer, using a 532 nm line ofKIMMON laser.

2.4. Electrochemical Measurements. A 6 M KOHaqueous solution was used as the electrolyte. The workingelectrode was prepared by mixing the active materials, PTFEand carbon black (mass ratio 80:10:10) in 7 mL ethanol,followed by ultrasonication for 20 min. A slurry of the mixturewas rolled into a film, cut into a plate and dried at 150 °C for 6h, followed by placing it on a nickel foam current collector. Theelectrode mass loading is 5−6 mg cm−1 and the thickness ofelectrode is ∼27 μm.The capacitive performance was tested on an electrochemical

workstation (CH Instruments Inc., Shanghai, China,CHI660D). Cyclic voltammetry (CV), galvanostatic charge−discharge cycling (GC), and electrochemical impedancespectroscopy (EIS) measurements were carried out at roomtemperature with a conventional two-electrode electrochemicalsetup, in which the active material served as the workingelectrode and a Pt plate and Hg/HgO oxide were used as thecounter electrode and reference electrode, respectively. The EISspectra were recorded in the range from 10 mHz to 0.1 MHzwith a signal amplitude of 5 mV. All of the electrochemicalmeasurements were carried out at room temperature.Specific capacitance (C) under three-electrode system was

calculated from the discharge curve based on the followingequation:

= Δ Δ−C I t V( )/3 electrode (1)

where I (A g−1) is the discharge current density based on themass of active material, Δt (s) is the discharge time, and ΔV(V) is the potential window from the end of the internalresistance (IR) drop to the end of a discharge process.A full cell was assembled with two symmetrical electrodes.

The stability measurement of the full cell was carried out withan Arbin BT2000 multichannel college station (Arbin Instru-ments USA). The specific capacitance (C2‑electrode) and full cellcapacitance (Ccell) were calculated from the discharge processafter 20 cycles’ activation, according to the equation below:

= Δ Δ−C I t V(4 )/2 electrode (2)

= −C C14cell 2 electrode (3)

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where I (A g−1) is the discharge current based on the total massof active material on anode and cathode and the definitions ofΔt (s) and ΔV (V) are as same as those in eq 1. The Warburgelement Z0 can be given by

ω ω= | |Z Z j B j( ) coth0 w (4)

where Zw is the bounded Warburg impedance, j is theimaginary unit (j = √−1), and ω the angular frequency (ϖ= 2πf, f being the frequency). Based on Z0, the diffusioncoefficient (D) is calculated by

=D R T n A C Z/( F )2 2 4 4 2s

20

2(5)

where R is gas constant (J mol−1 K−1), T is the absolutetemperature (K), n is the valency of the ion, F (C mol−1) is theFaraday constant, A (cm2) is the area of electrode, and Cs (molL−1) is the concentration of the electrolyte on the surface ofelectrode. Cs can be replaced by the bulk concentration of theelectrolyte because of the reversible formation process ofelectric double layer. The energy density (E) was obtained fromthe capacitance of supercapacitors:

= ×E C V /(2 3.6)cell2

(6)

where V (V) is the potential window. The power density (P) ofa full cell was determined by

= ΔP E t3600 / (7)

where Δt (s) is the discharge time.To measure the leakage current at the working potential, the

supercapacitors were first charged to 1 V and then the potentialwas kept at 1 V for 2 h, while the current flowing through thesupercapacitors was recorded. For self-discharge test, thesupercapacitor was first charged to 1 V, which was followedby a period of several hours at open circuit when thedependence of voltage on time was recorded.

3. RESULTS AND DISCUSSION3.1. Preparation Principle and Procedure of the

Asphaltene-based Carbon Nanosheets. A typical proce-dure is presented in Scheme 1. The asphaltene was firstextracted and subsequently oxidized in a pretreatment step.The carbon nanosheets were prepared by using asphaltene as a

carbon precursor, a small amount of graphene oxide as a sheet-structure-directing agent, and 1,6-diaminohexane as a bridgingagent. The molecular precursors reorganize into a designedcarbon nanostructure through a self-assembly process andfollowed by a pyrolysis step. The Tyndall effect was observedwhen asphaltene oxide was dispersed in tetrahydrofuran(Figure S1a), as well as graphene oxide in water (FigureS1b), indicating that both dispersions are in a colloidal state. Ina tetrahydrofuran and water cosolvent, asphaltene oxide,graphene oxide, and 1,6-diaminohexane can accomplish atwo-dimensional “bottom-up” self-assembly via π−π andhydrogen bonding interactions.25 The synthesis follows theconversion from noncovalent bonding to covalent bonding. Anin situ polycondensation was ultimately achieved between theamino group of 1,6-diaminohexane and the carboxylic acidgroups of asphaltene oxide and graphene oxide. Aftercondensation and subsequent pyrolysis, carbon nanosheetswere obtained.The π−π interaction between graphene oxide and asphaltene

oxide can be demonstrated by UV−vis absorption as shown inFigure S2. The graphene oxide curve exhibits a broad peak at231 nm and a shoulder peak at 290 nm, corresponding to π−π*electron transition of aromatic C−C bond and n-π* electrontransition of CO bonds, respectively.26,27 The absorptionpeaks gradually shift from 231 to 222 nm with addingasphaltene oxide into graphene oxide dispersion, resultingfrom the increased π conjugation between graphene oxide andasphaltene oxide molecules. The π−π interaction could assistthe formation of the composite of graphene oxide andasphaltene oxide in the subsequent polymeric process.28,29

According to previous research, asphaltene is 0.1−1 nm insize,23,30 and the sizes of graphene oxide sheets are in the rangeof 500−3000 nm.31 The large size difference betweenasphaltene oxides and graphene oxide allows asphaltene oxidemolecules to assemble on the graphene oxide substrate. Themolecular weights of asphaltene oxides were evaluated based ona MALDI mass spectrum. The asphaltene oxide in thisexperiment has a widely polydisperse molecular weightdistribution of m/z 500−1000 as shown in Figure S3.The condensation reaction between asphaltene oxide and

graphene oxide was evidenced by FT-IR. Figure 1a shows FT-IR spectra of graphene oxide, asphaltene oxide, obtainedpolymer (denoted as ACN-P) and ACN after pyrolysis. TheFT-IR spectrum of ACN-P shows peaks at 2927 and 1346 cm−1

for aliphatic C−H, which are also observed for asphalteneoxide. The formation of an amide covalent bond is proved bythe movement of the CO electron cloud from 1730 to 1700cm−1.21 After pyrolysis, ACN shows peaks at 3400, 1560, and1172 cm−1 for O−H stretching and CN and C−N vibrations,respectively, indicating nitrogen/oxygen containing functionalgroups. This is coincident with element analysis results of ACN(C ≈ 91.5%, H ≈ 0.84%, O ≈ 6.66%, N ≈ 1.0%). UV−visabsorption and FT-IR results indicate that the supermolecularasphaltene oxide is capable of entering, through reactive groups,into further polymerization, thereby contributing as themolecular precursor to final polymer. Additionally, one samplewithout asphaltene was synthesized to demonstrate the reactionbetween graphene oxide and diamine. As shown in Figure S4the FT-IR spectrum, the peak observed at 1184 cm−1 can beassigned to C−N vibrations.21,32,33

To evaluate the pyrolysis behavior and the yield ofprecursors, TG tests of asphaltene oxide, graphene oxide andACN-P were measured from 100 to 800 °C under N2

Scheme 1. Illustration of Preparation Principle andProcedure of Asphaltene Derived Carbon Nanosheets

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atmosphere and the TG curves were shown in Figure 1b. Forasphaltene oxide, a decomposition of oxygenated carbonspecies mainly occurred below 500 °C. Destruction ofmolecules appears at 400−550 °C, involving dealkylation ofaliphatic chains and dehydrogenation of naphthenic rings.34 Forgraphene oxide, a large mass loss around 200 °C is attributed topyrolysis of the labile oxygen-containing functional groups,yielding CO, CO2 and H2O. Up to 200 °C, the sluggish massloss corresponds to the removal of more stable oxygenfunctionalities.21,35 The TG curve of ACN-P is found todecay smoothly from 200 °C to the final temperature, which isconsistent with the pyrolysis behavior of polymeric precur-sors.36,37 The residual solid of ACN-P at 800 °C under N2atmosphere is 63 wt %, which comfirms the polymerization andcross-linking between asphaltene oxide, graphene oxide, anddiaminohexane. The high yield suggests that asphaltene canserve as a promising carbon precursor with high yield.3.2. Structural Characteristics of the Asphaltene-

Based Carbon Nanosheets. By varying the mass ratios ofgraphene oxide/asphaltene oxide, a series of ACNs sampleswith different thickness were synthesized and correspondinglydenoted ACN-x, where x represents the mass ratio of grapheneoxide/asphaltene oxide. As shown in Figure 2, the obtainedACNs were composed of thin carbon nanosheets with averagethicknesses from 13 ± 3 to 41 ± 5 nm. The thickness graduallyincreases as the mass ratios of graphene oxide/asphaltene oxidedecrease from 0.1 to 0.01. Once the mass ratio of grapheneoxide/asphaltene oxide decreases to 0.005, the carbon units ofACN-0.005 form a honeycomb framework with macropores asshown in Figure S5a,b. The sample of ACN-0 was prepared inthe absence of graphene oxide. It shows a coalesced structurewith large particles size of 5−10 μm as shown in Figure S5c,d.The results reveal the sheet-structure-directing function ofgraphene oxide in the formation of sheet structure and the massratio of graphene oxide/asphaltene oxide has a significantimpact on the morphology of the carbon products.The high resolution transmission electron microscope (HR-

TEM) image of ACN-0.02 shows the interconnected micro-pores and graphene ribbons (Figure 3a). The crystallitescompose of only several graphene layers; thus, they areconsidered as graphene ribbons.38 Abundant nanosizedgraphene ribbons formed in the asphaltene derived carbonare not yet found in other resin or biomass derived porouscarbons.39,40 The TEM image is based on a view from thesurface of carbon nanosheet. We believe that the formation of

graphene ribbon regions results from the large number of sp2

graphene-like carbon in asphaltene. The graphene-like structurecan enhance the conductivity of the obtained carbon. Themeasured conductivity of ACN-0.02 is 450 S·m−1, and theconductivity values of the other samples are in the same level. Itis higher than that of commercial activated carbon (100−300 S·m−1) but lower than the reported reduced graphene oxide(500−5000 S·m−1).41,42

Meanwhile, the Raman spectra of ACNs were collected andshown in Figure 3b. There are two broadening bands: the Dmode peak centered on ca. 1345 cm−1 and the G band at ca.1588 cm−1. The D peak resulting from phonon mode is broad

Figure 1. (a) FT-IR spectra of asphaltene oxide, graphene oxide, the obtained ACN-P, and ACN after pyrolysis and (b) TG curves under N2atmosphere of asphaltene oxide, graphene oxide, and the obtained ACN-P.

Figure 2. SEM images of the carbon nanosheets: (a, b) ACN-0.01, (c,d) ACN-0.02, and (e, f) ACN-0.1. The thin carbon nanosheets haveaverage thicknesses from 13 ± 3 to 41 ± 5 nm.

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and intense due to the reduction in size of the in-plane sp2

domain caused by extensive oxidation. The G peak originatesfrom the first order scattering of the E2g phonon of sp2 carbonhybridization.43,44 The graphitization degree of carbonmaterials can be evaluated by the intensity ratio of Ramanpeaks denoted as ID/IG, as listed in Table 1. The ID/IG values of

ACNs are generally in range of 1.95 to 2.54, suggesting afeature of amorphous carbons.45 The wide peak at ca. 2680 to2750 cm−1 are denoted as 2D, related to sp2 carbon, which is inagreement with the graphene layers observed by the HR-TEM.46 This could be ascribed to the plane polyaromatic unitsin asphaltene raw,47,48 which is hardly observed for the otherpolymer or resin carbon precursors.The porosity of the ACNs was measured by using N2

sorption technique. As shown in Figure 3c, the N2 sorptionisotherms of all samples are essentially of type I, indicating

microporous characteristics with an average pore size below 2nm. ACN-0.005 presents a specific surface area of 671 m2 g−1,while that of ACN-0 is only 491 m2 g−1 (Table 1). The addedsmall amount of graphene oxide can restrain asphaltene oxidefrom the stacking and ensure the pores on asphaltene oxidederived carbon are accessible, which is supposed to explain theincreased surface area of ACN-0.005.49 When the grapheneoxide continues to increase, specific surface area and micro-porous volume of samples gradually decrease. The similarphenomenon was observed in our previous work.37,50 It is likelybecause that graphene oxide does not contribute surface areabut only plays a structure-directing role.

3.3. Electrochemical Performance of the Asphaltene-Derived Carbon Nanosheets. Encouraged by the attractivestructural characteristics discussed above, ACNs are expected toachieve advanced performances as electrode materials forsupercapacitors. The capacitive behavior of ACNs was firstinvestigated in 6 M KOH electrolyte under a three-electrodesystem. Figure 4 show the cyclic voltammetry (CV) curves andgalvanostatic charge−discharge cycling (GC) curves of arepresentative sample ACN-0.02 at different scan rates andcurrent densities. The other CV and GC curves of the restACNs samples are shown in Figure S6−S9. Specific capacitanceof ACNs at various current densities ranging from 1 to 30 A g−1

are shown in Figure S10.For the representative sample ACN-0.02, CV curve at 100

mV·s−1 (Figure 4a) still retains a quasi-rectangular shape,indicating a fast charge transport and ion diffusion. As GCcurves shown in Figure 4b, the charge−discharge profiles have

Figure 3. (a) TEM image of ACN-0.02. The graphene ribbons structure was noted with yellow box and enlarged in the inset image. (b) Ramanspectra of the carbon nanosheets derived from asphaltene and (c) N2 sorption isotherms of ACNs. The isotherms of samples ACN-0.005, ACN-0.01,ACN-0, and ACN-0.02 are vertically offset by 120, 105, 80, and 35 cm3 g−1 STP, respectively.

Table 1. Structural Parameters of the ACNs

sample SBET/m2 g−1 Vtotal

a/cm3 g−1 Vmicb/cm3 g−1 ID/IG

c

ACN-0 491 0.19 0.15 2.54ACN-0.005 671 0.26 0.20 2.45ACN-0.01 566 0.27 0.19 2.18ACN-0.02 448 0.19 0.14 2.16ACN-0.1 393 0.19 0.12 1.95

aTotal pore volumes (Vtotal) were calculated from the amount ofnitrogen adsorbed at a relative pressure, P/P0 of 0.95. bMicroporevolume (Vmic) determined by the t -plot method. cID/IG was theintegral area ratio of D peaks and G peaks after multiple peak fitting.

Figure 4. (a) CV curves of ACN-0.02 at scan rates of 5, 10, 20, 50, and 100 mV s−1 and (b) GC curves of ACN-0.02 at current densities of 1, 2, 5, 10,20, and 30 A g−1. Inset is the enlarged GC curves at current densities of 10, 20, and 30 A g−1.

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quasi-linear shape indicating a capacitve storage and goodcharge propagation. The Coulombic efficiency of ACN-0.02 is97−100%. From the Figure 4b inset, the IR drops are 0.032,0.067, and 0.15 Ω, for 10, 20, and 30 A g−1, respectively,indicating good conductivity of this sample. A slight deviationfrom a straight line in GC discharge curve at a low currentdensity of 1 A g−1 demonstrates some Faradaic processes(Figure 4b) and the pseudocapacitance is estimated as 93 F g−1,which could be ascribed to nitrogen/oxygen doping andhydrogen storage as certified in many other reports.51−53 Theother sheet-structure ACNs samples show similar capacitivebehavior except for the sample ACN-0 with the coalescedstructure (Figure S6−S9). When the current density increasesto 30 A g−1, all thin carbon nanosheets remain high capacitanceretention ratios of 69% 76%, and 75% for ACN-0.01, ACN-0.02, and ACN-0.1, respectively. For the representative sampleACN-0.02, the Cg remains 163 F g−1 even at 30 A g−1,highlighting an excellent suitability for high-rate operation. Asprovided in Table S1, a comparison indicates that the ACN-0.02 is superior to commercial carbons and comparable tomany reported carbon nanosheet electrodes at high rates. Thethin carbon nanosheets exhibit both high specific capacitanceand good rate performance, due to their proper sheet thicknessand good electrical conductivity. This suggests that ions can notonly approach the spaces of carbon nanosheets but also diffusefast during charge and discharge processes.The electrochemical performance of ACN-0.02 has also been

evaluated in a symmetrical two-electrode system. Measured in apotential range of 0−1 V, the CV curves remain quasi-rectangular shapes even at 200 mV s−1, as shown in Figure 5a.Based on GC curves in Figure 5b, the specific capacitance ofACN-0.02 is 191 F g−1 at 0.1 A g−1, which is well retained at125 F g−1 when the current rate is increased to 30 A g−1. Basedon the mass loading (∼5 mg cm−2) and thickness (∼27 μm) of

one electrode, the carbon nanosheet electrode exhibits adensity of 1.85 g cm−3. This electrode could show potentially avolumetric capacitance of 84 F cm−3, which is superior to thatof most low-density carbon materials.54 The calculated Csa ofACN-0.02 is 43 μF cm−2 at a current density of 0.1 A g−1,which is much higher than the theoretical EDL capacitance(15−25 μF cm−2),55,56 implying an efficient utilization ofsurface area and a contribution of heteroatom doping. Thevalue of Csa is also comparable to that of B or N dopinggraphene-based composites (38 and 22 uF cm−2, respec-tively),57,58 hierarchical porous carbon fiber (38 uF cm−2),59

and graphitic carbon fiber based composites (59 uF cm−2).60

Long-term cycling performance of ACN-0.02 supercapacitorwas studied at a constant current density of 2 A g−1. As shownin Figure 5c, no capacitance decay is observed after consecutive15 000 cycles. The specific capacitance gradually increases inthe first 500 cycles. The initial increase is probably attributed toan activation process of the electrode material, i.e. gradualwetting of the electrolyte deep inside the electrode material. Asimilar phenomenon was also observed in other carbonelectrodes.61,62 When the cycles went up to 5000 cycles, theabnormal increased capacitive effects may be attributed to thereduction of oxygen groups. A similar phenomenon was alsoobserved in the other carbon electrode.63,64 In Figure 5c, inset,the GC curves in the first ten cycles and last ten cycles are bothalmost isosceles triangles, which illustrates the enhanced idealcapacitive characteristic during the cycling process. Theseresults denote that highly reversible electrostatic adsorption anddesorption of electrolyte ions occur on the electrode surface,indicating an infinite lifetime for the repetitive charge−discharge cycling. This suggests that the asphaltene derivedcarbon nanosheet possesses superior electrochemical stabilitywith long cycle life, making it a promising candidate for long-term energy storage devices.

Figure 5. Capacitive performance of ACN-0.02 in a two-electrode system: (a) CV curves at a scan rates of 5, 10, 20, 50, 100, and 200 mV s−1 and (b)GC curves at current densities of 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, and 30 A g−1. Inset is the enlarged GC curves at current densities of 5, 10, 20, and 30 Ag−1. (c) The capacitance retention of the supercapacitor in 15 000 cycles’ charge−discharge at a current density of 2 A g−1. Inset is the comparison ofGC curves in the first ten cycles and last ten cycles of the long-term cycling process.

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The kinetic feature of the ion diffusion in nanosheet wasfurther investigated in two-electrode system by using electro-chemical impedance spectroscopy (EIS). Figure 6a presents theNyquist plots and electric equivalent circuit model of theACNs. Except for ACN-0, all samples exhibit a straight lineapproaching 90° in the low frequency, representing an idealcapacitive behavior of the electrode. The enlarged plots of thesesamples in the high-frequency are shown in Figure 6b. Theseries resistor (Rs) represents the sum resistance of theelectrodes, electrolyte, and separator. The Rs values of ACNsupercapacitors are in range of 0.63 to 1.19 Ω. The Rs in three-electrode system can represent the real conductivity of thecarbon materials. As shown in Figure S11, Rs decreases from0.61 to 0.27 Ω with the graphene content of samples increases.Table 2 shows the fitting value of charge transfer resistance

(Rct), Warburg element (Z0), and diffusion coefficient (D) of

samples in the two-electrode system. ACN-0.1 with highestcontent of graphene oxide has the lowest Rct values of 0.07 Ω,indicating a fast charge transfer. The charge transfer resistanceof the supercapacitors increases with the thickness of ACNsincreasing and the amount of graphene oxide decreasing. TheRct of ACN-0 (10.51 Ω) exhibits 2.5−155 times as large asthose of nanosheet samples synthesized by adding graphene

oxide. Due to the short ion diffusion paths and goodconductivity, the nanosheets samples show not only lower Rctbut also higher diffusion coefficient compared to ACN-0 andACN-0.005. Z0 presents the diffusion impedance for 1D lineardiffusion. The Z0 and D of the ions in porous electrode werecalculated based on eqs 4 and 5 in the Experimental Section. Aslisted in Table 2, with the thickness of ACNs decreasing from41 ± 5 nm (ACN-0.01) to 13 ± 3 nm (ACN-0.1), the Dincreases successively from 1.18 × 106 to 5.13 × 106 cm2 s−1,implying an accelerated ion diffusion rate. This coincides withthe relationship between thickness and rate capability asdiscussed earlier.For practical applications, leakage current and self-discharge

characteristic of electrode materials are indispensable factors toconsider. The charging currents (Figure 6c) reached to stablevalues of 0.0068 mA g−1 at the end of the time. As shown inFigure 6d, the voltage decays and keeps at 0.60 V after 2 h,which means a self-discharge rate of 40%. The resultsdemonstrate that the supercapacitor exhibits low leakagecurrent and self-discharge characteristics. The supercapacitorachieves a maximum power density of 22.0 kW kg−1, as theRange plot shown in Figure S12, indicating high powerperformance and rate capability of this carbon nanosheet.

4. CONCLUSION

Using asphaltene as a carbon-rich molecular precursor, carbonnanosheets with controllable thicknesses have been prepared.This synthesis strategy demonstrates not only a novel carbonprecursor for the preparation of a carbon nanosheet but also anupgrade utilization of naturally abundant coal. Excellentcapacitive performance was achieved by using this carbonnanosheet as a supercapacitor electrode material. Theunprecedented surface area-normalized capacitance and rateperformance can be attributed to a short ion diffusion path and

Figure 6. (a) Nyquist plots of ACNs in two-electrode system. Inset is the electric equivalent circuit model of these samples. (b) The enlarged highfrequency range of the samples synthesized by adding graphene oxide and (c) leakage current curves and (d) self-discharge curves of thesupercapacitors charged at 1 V for an aqueous electrolyte, and kept for 2 h.

Table 2. Fitting Values of Rct, Z0, and D in two-electrode EISof ACNs

sample thickness/nm Rct/Ω Z0/Ω s−1/2 D × 106/cm2 s−1

ACN-0 10.51 44.63 4.77 × 10−3

ACN-0.005 4.09 2.84 1.18ACN-0.01 41 ± 5 1.83 2.16 2.03ACN-0.02 32 ± 3 0.17 1.47 4.39ACN-0.1 13 ± 3 0.07 1.36 5.13

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good electrical conductivity. Ion/charge transfer kineticsimproved by the superior microstructure leads to an efficientutilization of the surface area. Carbon nanosheets with variousthicknesses serve as a typical model for research on iondiffusion kinetics. These materials also have scope for uses inalternative applications such as catalyst supports, waterpurification, gas adsorption, and separation.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpcc.6b05136.

Figures S1−S5, containing structural characteristics ofsamples and their raw material; Figure S5−S12,containing electrochemical performance of the asphal-tene-derived carbon; Table S1, the capacitance ofreported carbon nanosheet and commercial activecarbon. (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*Fax: +86-351-4053091. E-mail: shenwzh2000@yahoo.com.*Fax: +86-411-84986355. E-mail: wencuili@dlut.edu.cn.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe project was supported by National Natural ScienceFoundation of China (Nos. 21376047 and U1303192) andthe Foundation of State Key Laboratory of Coal Conversion(Grant No. J14-15-904).

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