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Nano Energy (2015) 15, 9–23

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Graphene-based nitrogen self-dopedhierarchical porous carbon aerogelsderived from chitosan for highperformance supercapacitorsPin Haoa,b, Zhenhuan Zhaoa, Yanhua Lenga, Jian Tiana,Yuanhua Sanga, Robert I. Boughtonc, C.P. Wongb,n,Hong Liua,d,n, Bin Yange,*

aState Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR ChinabSchool of Materials Science and Engineering, Georgia Tech, Atlanta, GA 30032, USAcDepartment of Physics and Astronomy, Green State University, Bowling Green, OH 43403, USAdBeijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100864, PR ChinaeDivision of Advanced Materials, High Technology R&D Center, Ministry of Science & Technology,Beijing 100044, PR China

Received 24 February 2015; accepted 28 February 2015Available online 9 April 2015

KEYWORDSGraphene-based car-bon aerogel;N-self doping;Hierarchical porousstructure;Chitosan;Supercapacitor

0.1016/j.nanoen.2lsevier Ltd. All rig

thors.: cpwong@cuhk.ed

AbstractGraphene-based nitrogen self-doped hierarchical porous carbon aerogels were synthesized forsupercapacitor electrode application by using chitosan as a raw material through a carefullycontrolled aerogel formation–carbonization–activation process. The as-synthesized N-dopedgraphene-based carbon aerogels contained both macropores and mesopores from the aerogelpreparation and carbonization process, and micropores from the chemical activation, con-firmed by TEM, SEM, BET, etc. Because chitosan is a nitrogen-containing renewable biopolymer,the carbon aerogel derived from chitosan in this work was N-self-doped. The carbonized carbonaerogel was composed of a graphene framework and amorphous carbon, and the ratio betweenthese two components was controlled by the activation temperature. With an increase inactivation temperature, the amorphous carbon was etched away gradually, and a stablegraphene portion remained to form a framework. Accordingly, the performance of thegraphene-based carbon aerogel as a supercapacitor varied with increasing activation tempera-ture. Electrochemical investigation measurements showed that the N-doped graphene-basedhierarchical porous carbon aerogel represents a good electrode candidate for construction of asolid symmetric supercapacitor, which displays a high specific capacitance of about 197 F g�1 ata current density of 0.2 A g�1. In addition, the solid state supercapacitor displayed excellent

015.02.035hts reserved.

u.hk (C.P. Wong), hongliu@sdu.edu.cn (H. Liu), yangb@htrdc.com (B. Yang).

P. Hao et al.10

cyclability with a capacitance retention of about 92.1% over 10,000 cycles. The excellentenergy storage ability of the chitosan-derived hierarchical graphene-based carbon aerogels isascribed to the high conductivity of the graphene framework with nitrogen doping and the highstorage ability of amorphous carbon with variable pore size and distribution.& 2015 Elsevier Ltd. All rights reserved.

Introduction

Supercapacitors, possessing a high power density, long life-time and excellent safety properties, have been widely usedin various areas, such as memory back-up, electric vehicles,power quality management, battery improvement andrenewable energy applications [1]. A number of differentmaterials have been selected to build high-performancesupercapacitors to obtain high energy storage capability,such as porous carbon, transition metal oxides, and conduct-ing polymers [2–6]. However, supercapacitors using metaloxides and conducting polymers exhibit poor stability andrate capability, despite their high energy density and specificcapacitance. As an alternative, activated carbon has beencommercially applied as a supercapacitor electrode materialbecause of its well-developed microstructure, high specificsurface area and relatively high packing density [7,8]. Atpresent, commercial porous activated carbon-based super-capacitors have two main bottlenecks: a low energy densityand high cost, which significantly limit its broad application.To enhance the energy density, many carbon materials, suchas activated carbon, carbon fibers, carbon nanotubes andgraphene, are usually coupled with transition metal oxidesand conducting polymers, or used as the anode material toassemble asymmetric supercapacitors such as battery-likesupercapacitors [9–19]. However, the comparatively poorelectrochemical performance of carbon limits the develop-ment of carbon-based supercapacitors. Another disadvantagefor the carbon-based electrode materials is that most ofthem are derived from petroleum-based chemical products,which introduces the high cost associated with a limitedresource because of the fossil fuel crisis looming in the verynear future.

Graphene, a single layer of carbon atoms in a hexagonallattice, which can be derived from graphite formation orobtained by a synthesis process, has been considered to be anoutstanding candidate for supercapacitor fabrication due toits stable thermal and mechanical properties, high specificsurface area, high electrical conductivity and stable chemicalproperties [7,20–24]. More importantly, the theoretical spe-cific capacitance of graphene is calculated to be 550 F g�1

with the entire utilization of the surface [25]. In fact,graphene has already exhibited its superiority as a super-capacitor electrode material. Because of its unique layeredstructure, various materials have been coupled with gra-phene to form a layer-by-layer structure, leading to theenhancement of both the specific capacitance and energydensity [26]. Recently, much research has been focused onenhancement of the electronic properties of graphenematerials through doping hetero-atoms, such as N and B[27–31]. For instance, nitrogen-doped graphene (NG) can be

synthesized through the thermal annealing of GO in a NH3

atmosphere, and the obtained NG showed improved excellentelectrochemical performance [22]. Unfortunately, most gra-phene nanostrucutres for supercapacitor applications areprepared from the chemical vapor deposition (CVD) methodor chemical oxidation/exfoliation from graphite. The synthesisof graphene using CVD is typically a high energy-consumptionprocess and the chemical method usually involves highly toxicmaterials. Neither of these methods can produce graphenewith a specially designed microstructure on a large-scale. Inaddition, it is difficult to prepare nitrogen-doped graphenewith uniform and high-concentration nitrogen doping whenusing NH3 as the N source. The high cost of raw materials,environmental destructiveness of preparation, complicatedmanufacturing and difficulty in obtaining graphene by theton level, largely limit its further applications in the market[32–34]. Therefore, researchers have turned their attention tousing green, reproducible biomass or its derivatives, and theuse of renewable materials to produce porous carbon andgraphene materials is critical to sustainable development andenvironmental protection [27,35–38].

Chitosan, a material derived from the chitin in crustaceancells, insect exoskeletons, and fungus cell walls, is the secondmost abundant and renewable biopolymer after cellulose [39].Because of good bio-compatibility and non-toxicity, chitosan hasbeen widely used in the field of biology and medicine. However,its application in electrochemistry is currently limited to thepreparation of solid polymer electrolytes. Because chitosan caneasily form a 3D hierarchical porous scaffold, chitosan-basedsolid polymer electrolytes are expected to enhance electro-chemical cyclability and rate capability [39–43]. For example,chitosan can easily be used to form gels with LiClO4 or ionicliquids in order to prepare solid electrolytes [44,45]. Becausechitosan molecules are easy to connect together to form a largearea polymer, it can form graphene-like materials during thecarbonization process. To the best of our knowledge, the onlyreport on the synthesis of chitosan-derived graphene-basedmaterials is by García and coworkers [27]. In their study, anultrathin chitosan film of 100 nm thickness was first prepared ona substrate and then carbonized at a high temperature toobtain a graphene film. However, this method is difficult toscale up, and the product film cannot be used as a super-capacitor electrode. Up to now, there are few reports on thesynthesis of chitosan-derived graphene-based aerogels forsupercapacitor electrode applications. In addition, chitosanconsists of a large number of amino-groups that can be usedto synthesize N-doped carbon aerogels with excellent super-capacitor performance.

In this report, we present a method of fabrication ofgraphene-based nitrogen self-doped hierarchical porous car-bon aerogels using the abundant natural biopolymer chitosan

11Graphene-based nitrogen self-doped hierarchical porous carbon aerogels derived from chitosan for high

as a raw material. The obtained hierarchical porous carbonaerogels consist of a graphene framework and nanoporousamorphous carbon. At a high activation temperature, a singlephase graphene framework built with a few layered grapheneflakes can be obtained. The optimized graphene-basedcarbon aerogels exhibit a high specific capacitance andoutstanding cyclability due to the high degree of graphitiza-tion, the high specific surface area, reasonable pore size anddistribution, and uniform nitrogen doping.

Experimental

Materials

Chitosan was purchased from Aladdin (deacetylation Z95%,viscosity 100–200 mPa·s). All other chemicals were reagentgrade and used without further purification. Deionized waterwas used throughout the experiments.

Material preparation

Chitosan-derived carbon aerogels with a hierarchical porousstructure were prepared by the following steps in sequence:(i) preparation of chitosan aerogels, (ii) carbonization ofchitosan aerogels, and (iii) activation of the carbon aerogels.

(i)

Chitosan aerogels were prepared by the freeze–dryingmethod. Here, 1 g of chitosan powder was dispersed in40 mL of deionized water, 1–2 v% of acetic acid solutionwas dropped into the system with vigorous stirring untila transparent chitosan solution with high viscosity wasobtained. After standing for 2 h, the obtained chitosansolution was then frozen to �80 1C for 12 h and driedusing a lyophilizer to form chitosan aerogels.

(ii)

The carbonization of the chitosan aerogels proceededin a tube furnace. The as-formed chitosan aerogelswere pyrolyzed at 800 1C for 3 h in flowing N2 at a rateof 3 1C min�1 to obtain carbonized aerogels.

(iii)

The KOH activation process is similar to the proceduredescribed in previous work [46]. Briefly, the obtainedcarbonized carbon aerogels were mixed with a KOH solutionwith a KOH/carbon aerogel weight ratio of 3:1. The mixturewas then dried at 110 1C to evaporate the water. Theimpregnated sample was heated up to a pre-designatedtemperature under N2 flow at a rate of 5 1Cmin�1 and heldfor 2 h. The activation temperature was selected as: 700 1C,800 1C, 900 1C and 1000 1C (samples were denoted as K700,K800, K900 and K1000, respectively). After activation, thesample was washed thoroughly with 1 M HCl solution anddeionized water. Finally, hierarchical porous carbon wasobtained by drying the sample at 70 1C overnight.

Structural characterization

A HITACHI S-4800 field-emission scanning electron micro-scope (FE-SEM) was used to characterize the morphology ofthe carbon aerogels. High-resolution transmission electronmicroscopic (HRTEM) images were obtained with a JEOL

JEM2100F microscope. Atomic force microscopy (AFM) wascarried out using a Bruker Dimension Icon with ScanAsyst.The chemical structure was characterized using a PHI 5000Versa Probe X-ray photoelectron spectroscope (XPS). Ramanspectra were acquired with a Jobin-Yvon HR 800 spectro-meter. The porosity characteristics of the porous carbonaerogels were obtained by N2 adsorption/desorption experi-ments at 77 K using ASAP 2020 V3.02 H. The specific surfacearea was measured using the Brunauer–Emmett–Teller (BET)method and the pore size distribution was calculated usingthe classical Barrett–Joyner–Halenda (BJH) model.

Electrochemical measurements

To obtain the electrochemical properties of the samples, both athree-electrode configuration and a two-electrode configura-tion were used. To prepare the electrodes used in the threeelectrode set-up following conventional optimization methods,the supercapacitor electrodes were made of activated carbonaerogel, acetylene black and poly-vinylidenefluoride (PVDF)[47]. Prior to preparation of the electrode, PVDF was addedinto an N-methyl pyrrolidinone (NMP) solution (0.01 g mL�1),and then carbon aerogel and acetylene black were added to theabove solution with a carbon aerogel/acetylene black/PVDFweight ratio of 8:1:1 to form the slurry. After stirring for 12 h,the mixture was pressed onto a nickel foil with a size of1 cm� 1 cm, and the prepared electrodes were dried at 60 1Covernight to remove NMP. The mass loading of the samplewithout activation, K700, K800 and K900 is 3.0, 2.2, 2.4 and2.7 mg, respectively. In a conventional three-electrode cell, aPt wire and a Ag/AgCl (saturated with KCl (aq)) were used asthe counter electrode and the reference electrode, respec-tively, and an aqueous solution of 6 M KOH was used as theelectrolyte. The performance evaluation of activated carbonaerogel as two symmetrical electrodes was carried out in a two-electrode configuration. Two-electrode set-up was built in astainless cell with a glassy fibrous paper as separator and carbonfiber paper as current collectors. The electrode can be obtainedthrough drying the above slurry in 55 1C for 20 min and thenrolling to the thin slice. After drying at 60 1C for 12 h to removeNMP, the slice was cut into circular films with the diameterabout 6 mm. The mass loading of the electrode is 2.8 mg.TEABF4 and 6 M KOH solution are used as the electrolyte. Toprepare the KOH/PVA gel electrolyte, 4.5 g of KOH was addedto 60 mL of deionized water, into which was added 6 g of PVApowder. The mixture was heated to 85 1C under stirring untilthe solution became clear. Two electrodes made by the abovemethod were immersed in the KOH/PVA solution for 5 min,keeping the foil without carbon materials above the solution,and then overlaying the two electrodes head-to-head until thegel solidified at room temperature. The mass loading of theelectrode is 2.0 mg.

Cyclic voltammetric measurements (CV), galvanostatic char-ging/discharging measurements (GCD) and electrochemicalimpedance spectroscopy (EIS) were performed by using a CHI660C Electrochemical Workstation (CH Instruments, China). Thespecific capacitance was calculated from CV curves collectedfrom three electrodes testing according to the equationC=

RIdt/mV, where I is the current, V is the working voltage

window, and m is the mass of the working electrodeactive material. The specific capacitance from galvanostatic

P. Hao et al.12

charging/discharging curves was calculated via C=It/mV.Where I is the discharge current, t is the discharge time, V isthe working voltage window, and m is the mass of the activematerial at each electrode. The specific capacitance derivedfrom the CV curve and galvanostatic testing through twoelectrodes testing was determined via C=4

RIdt/MV and

C=4It/MV, respectively, where M is the total mass of theactive material at the two electrodes. The energy density (E)and the power density (P) were calculated from galvanostaticcharge/discharge testing via E=(CV2)/2 and P=E/t, respec-tively. Where C is the specific capacitance from the twoelectrodes testing and t is the discharge time.

Results and discussion

The microstructure of the as-synthesized carbon aerogels atdifferent synthesis steps was characterized by SEM. Figure 1shows SEM images of the hierarchical porous carbon aerogelsafter chemical activation at 700–900 1C. For comparison, themicrostructures of the as-formed chitosan aerogel and thecarbon aerogel without activation were also observed by SEM(Figure S1). Before carbonization, the as-synthesized cylindri-cal chitosan aerogel possessed a porous structure with uniform

Figure. 1 Representative SEM images of N-doped carbon aerogels aK900. Inset in (a) is magnified image of (a).

3-dimensional connected channels (Figure S1a). Interestingly,after carbonization, the uniform 3-D connected channels in thechitosan aerogel were broken, and re-assembled into a porousscaffold with a new morphology. Most channel walls became flatnanosheets oriented along a certain direction that were stackedtogether, to form a squashed 3-D porous microstructure(Figure S1b). After activation at different temperatures, asshown in Figure 1a, d and g, the carbon aerogels still maintainthe 3-D network morphology. From the inset of Figure 1a, onecan see that carbon aerogel activated at 700 1C consists of a 3-Dporous structure, which is similar to the morphology of the as-carbonated carbon aerogel. However, the wall surfaces of theporous structure are reassembled with some small carbon flakes.The surface of the carbon flakes is relatively smooth withoutmacro- or meso-pores (Figure 1b). It is difficult to observe themicropores on the flake surface in the carbon aerogel because ofresolution limitations of the SEM (Figure 1c). With increase inactivation temperature, the surfaces of the channel wallsthroughout the porous carbon aerogels became very rough(Figure 1e and h). From fractured surfaces taken from samplesK800 and K900, one can see that the nanoflake surfaces and thewalls of the 3-D porous network are full of open and inter-connected pores (Figure 1f and i). The reason can be ascribed tothe more violent reaction between carbon and KOH at higher

ctivated at different temperatures: (a–c) K700, (d–f) K800, (g–i)

13Graphene-based nitrogen self-doped hierarchical porous carbon aerogels derived from chitosan for high

activation temperatures. Therefore, with chemical activation,a high specific surface area and a hierarchical porousstructure in the carbon aerogel are expected.

To further investigate the microporous structure of thesamples activated at different temperatures, TEM images weretaken on ground fragments of carbon aerogel samples activatedat different temperatures (Figure 2). As shown in Figure 2, withan increase in the activation temperature from 700 1C to900 1C, the aerogel samples show distinctively differentmorphologies. At 700 1C, the carbon aerogel shows a sheet-like microstructure (Figure 2a), and is less transparent underelectron beam irradiation, which indicates that the nanosheetthickness is quite large. In a high-resolution image (Figure 2b),the nanosheet exhibits the typical morphology of amorphouscarbon with numerous nanopores about 2–3 nm in size. Whenthe temperature is increased to 800 1C, most parts of thecarbon aerogel walls become much more thinly layered and areconnected with each other. Among the connected thin layers,some amorphous carbon particles of 20–30 nm in diameterdistribute themselves and become tightly attached to theconnected transparent carbon layers to form a carbon parti-cle–carbon layer hybrid structure (Figure 2c). The amorphous

Figure 2 TEM images of N-doped carbon aerogels activated at diffInset in (g) is the corresponding SAED pattern.

nanoparticles also possess nanopores of 2–3 nm in diameter(Figure 2d). Further increasing the activation temperature to900 1C, only a few of the nanoporous amorphous nanoparticlescan be found (Figure 2e and f). Figure 2g is the HRTEM image ofcarbon layers in Figure 2e. It is nice to see the lattice fringes ofcarbon layers. And from selected area electron diffraction(SAED) pattern (inset in Figure 2g), we can find that carbonatoms are in a hexagonal lattice, indicating the formation ofgraphene. When the activated temperature was raised to1000 1C, there is no carbon particle in the image (Figure S2).Based on the above observations, we propose that the con-nected transparent thin walls of the carbon aerogel activatedat temperatures above 800 1C are graphene layers. To confirmthis suggestion, the pieces of wall fragment ground from thecarbon aerogel activated at 800 and 900 1C were used todetermine the phase composition by using Raman spectrumanalysis and AFM methods, and the corresponding results areshown in Figure 3.

The Raman spectra of the samples prepared at variousactivation temperatures are shown in Figure 3. For compar-ison, the Raman results on carbon aerogel without activationare also displayed in Figure 3. It can be seen that all the

erent temperatures: (a and b) K700, (c and d) K800, (e–g) K900.

Figure. 3 (a) Raman spectra of pristine and activated samples, (b) ratio of integrated intensities of D- and G-bands (ID/IG), (c) fullwidth at half maximum (FWHM) of D- and G-band peaks, AFM images of (d) K800 and (e) K900, (f) thickness of the samples vs. length.

P. Hao et al.14

samples display two distinct bands, indexed to the D band atabout 1350 cm�1 and the G band at about 1580 cm�1.Generally, the height of the D band represents the concen-tration of disordered carbon in the sample, while theintensity of the G peak indicates the concentration ofgraphitized carbon [48,49]. The intensity of the D band isobviously lower than that of the G peak for all the samples,indicating a high graphitization concentration in the carbonaerogels. This can be demonstrated by the intensity ratio(ID/IG) of the D and G bands, which reflects the ratio ofdisordered carbon and ordered graphitized carbon in thecarbon aerogels. The intensity ratio of disordered carbon toordered carbon (ID/IG) can be used to make an estimate ofdisordered carbon content in the carbon aerogel samples. Asshown in Figure 3b, the ID/IG ratio of the carbon aerogelwithout activation is about 0.83. With increase in theactivation temperature, the ID/IG ratio decreases. When theactivation temperature reaches 900 1C, the graphitizationratio decreases to 0.74, indicating a very high concentrationof ordered carbon in the activated carbon aerogel samples.The enhanced degree of graphitization indicates that theactivation process not only can be expected to create pores,as discussed above, but also can enhance the concentrationof graphitized carbon in the carbon aerogel. This structuralchange is also demonstrated by a drastic decrease in peakwidth (full-width at half maximum, FWHM) when comparingthe sample without activation to sample K900 (Figure 3c). Asshown in Figure 3c, with an increase in activation tempera-ture, the FWHM of both bands becomes smaller, indicating anincreased sharpness of the peaks due to the high degree ofgraphitization. Impressively, samples K800 and K900 not onlyshow the D band and the G band, but also display the 2D peak

at about 2700 cm�1, which is thought to be the character-istic peak of graphene [50,51]. The AFM results of K800 andK900 are showed in Figure 3d–f. As shown in Figure 3d,different brightness is displayed in the sample of K800,indicating different thickness. This is because that at theactivation temperature of 800 1C, the reaction between KOHand amorphous carbon became violent and most of amor-phous carbon was etched away, leaving some carbon particlesor small carbon pieces between the layers of graphene, andresulting in the different thickness. However, further increas-ing the activation temperature to 900 1C, the reaction wasmore violent, almost all the amorphous carbon was etchedaway and the major remaining component of the carbonaerogel is the graphene framework, resulting in the uniformbrightness in Figure 3e. From Figure 3f we can see that K800exhibits different thickness and the thickest is 13 nm. Thethickness of K900 is less than 1.5 nm, indicating about fourlayers of graphene. Therefore, in this study we conclude thatwhen the chitosan aerogel precursor was carbonized at800 1C, a part of the aerogel precursor in the walls of theaerogel was graphitized to become layered graphene, andother parts were carbonized to become amorphous carbon.These two carbon portions comprise the thick wall of thecarbon aerogel, in which the graphene nanosheets connecttogether, but are buried in solid amorphous carbon to form agraphene-based carbon aerogel. After activation by KOH athigh temperature, the solid amorphous carbon was etched toform a nanoporous structure, and the wall thicknessdecreased. With an increase in activation temperature, theamorphous carbon layer on the graphene-based carbonaerogel wall decreases, and the relative graphene contentincreases. This is why the samples activated at 800 and

15Graphene-based nitrogen self-doped hierarchical porous carbon aerogels derived from chitosan for high

900 1C look transparent under HRTEM, and the amorphouscarbon particles become fewer. When the activation tem-perature reached 1000 1C, most of the amorphous carbon wasetched away by KOH during the high temperature activationprocess, and the remaining part of the carbon aerogel wasmainly composed of graphene nanosheets. It should bepointed out that the formation temperature of graphene ismuch lower than the pyrolysis temperature of graphite or theepitaxial growth temperature on single crystal silicon carbide[52,53].

XPS was carried out to examine the chemical nature of thecarbon and the presence and types of nitrogen in the graphene-based carbon aerogels after activation. From the XPS results(Figure 4a), there are three peaks, C1s, N1s and O1s exhibitedby all the samples after activation. Thus, the XPS data givestrong evidence for the existence of nitrogen in the graphene-based carbon aerogels, which is expected when using chitosanas the carbon source. The high resolution C1s spectrum of K800was deconvolved and is shown in Figure 4b. The line represent-ing sp2 hybridized carbon was recorded at around 284.5 eV,whereas the sp3 hybridized state has a slightly higher bindingenergy at around 285.0 eV [54]. For comparison, the highresolution C1s spectra of the other samples are displayed inFigure S3. As shown in Figure S3a, the sample withoutactivation displays a broad sp2 shape, indicating a much lowerrelative content of sp2-hybridized carbon. After activation at700 1C and 800 1C, the sp2 peak displays a sharper shape andincreased intensity. Until activation at 900 1C, the sp3 peak isvery weak, but then the sp2 peak becomes very strong at900 1C, hence there is a high degree of graphitization, furtherdemonstrating the existence of the graphene structure. TableS1 displays the sp2/sp3 ratio of the samples. As shown in thistable, the ratio increases from 3.5 for the sample withoutactivation to 10.81 for sample K900, showing a much higherdegree of graphitization, in agreement with the Raman results.

Figure. 4 (a) XPS spectra of the samples, (b) high-resolution Cdesorption isotherms of the samples, (e) pore size distribution (cal

The peaks around 286.2, 286.6 and 288.1 eV correspond to C–N,C–O and C=O, respectively. The N1s XPS spectra of sample K800are shown in Figure 4c. The N1s peak can be deconvolved intothree components, indicating that the N atoms exist in threedifferent bonding situations in the graphene-based carbonaerogels. The peaks at binding energies of 398.4, 400.4 and402.2 eV correspond to pyridine nitrogen, pyridinium nitrogenin condensed polycycles and pyridine N-oxide, respectively[27,55–57]. Furthermore, the proportion of pyridine nitrogen,pyridinium nitrogen in condensed polycycles and pyridine N-oxide are quantified as 15.0, 72.3 and 12.6%, respectively.Overall, the XPS results agree with the Raman data, andconfirm the proposed structure of nitrogen-doped graphene-based carbon aerogels, which is favorable for the enhancementof supercapacitor performance.

N2 adsorption/desorption isotherms are used to examinethe specific surface area and the variation of pore size andthe pore distribution in the samples. As illustrated inFigure 4d, all the activated samples display type IV nitrogenadsorption isotherms, indicating different pore sizes frommicro- to meso-pores. Major adsorption by the samplesoccurs at a low relative pressure of less than 0.1 and givesrise to an almost horizontal plateau at higher relativepressures, indicating high microporosity in the samples[58]. Sample K900 exhibits a small rise at 0.5–1.0 P/P0,showing the existence of mesopores. This result can befurther demonstrated by the pore size distribution calculatedfrom the BJH model. As shown in Figure 4e and f, all threesamples have both mesopores (2–50 nm) and micropores(o2 nm). The mesopores have a pore size of �2 nm,consistent with the HRTEM images, while the size of themicropores is typically centered at 0.6 nm. In fact, the idealEC electrode materials should have a hierarchical porousstructure containing macropores (larger than 50 nm) for ion-buffering reservoir, mesopores (2–50 nm) for ion transport

1s and (c) N1s XPS spectra of K800, (d) nitrogen adsorption/culated by using BJH model), (f) detailed view of (e).

P. Hao et al.16

and micropores (less than 2 nm) for enhancement of chargestorage. Moreover, it is also desirable that the porousstructure have an optimal variation in pore size and poresize distribution [59]. However, from Figure 4f, it is seen thatsample K700 has a large number of micropores with very fewmesopores, while sample K900 has a large number ofmesopores with relatively fewer micropores. Impressively,sample K800 has the ideal pore size and size distribution. Thedata on specific surface area and pore volume are listed inTable S2. After activation, the specific surface area isenhanced to 2435.2 m2 g�1. The specific surface area ofsample K800 is the highest with ideal pore volumes of micro-and meso-pores. However, samples K700 and K900 have thelowest pore volume of mesopores and micropores, respec-tively. From the above results, it is seen that the activationprocess and the activation temperature can greatly influencethe specific surface area and pore volume. Sample K800 hasthe most suitable pore size and size distribution, which isexpected to enhance its behavior as a capacitor.

In order to demonstrate the high specific surface area andthe hierarchical porous structure of the graphene-basedcarbon aerogels, an organic solvent adsorption experimentwas performed on sample K800. Figure S4a reveals therapidity of the process of cyclohexane adsorption labeledwith Sudan III dye on the water surface. Furthermore,sample K800 has an outstanding regeneration capacity afterbeing dried 10 h at 100 1C. As shown in Figure S4b, thesample maintained good adsorption after six adsorbing anddrying cycles due to the high specific surface area, open andinterconnected network, and stable hierarchical porousstructure. In addition, as shown in Figure S4c, we putsample K800 on a flower to illustrate the high ratio ofsurface area/weight that arises from the porous structure.

Based on the above experimental results, a formationmechanism of graphene-based carbon aerogels is proposed.

Scheme 1 Structure evolution in the preparation process: (a) ch(c) carbon products from carbonization, (d) and (e) porous carbon

Scheme 1 illustrates the formation of the chitosan aerogelprecursor and the structure evolution of the carbon aerogelduring the carbonization and activation process. As shown inScheme 1a, chitosan, poly(2-amino-2-deoxy-D-glucose), pos-sesses a chain structure with β(1-4) linkage [60]. Whenchitosan is dissolved in acetic acid, the chitosan molecule issurrounded by acetate molecules, which significantly prohibitthe formation of hydrogen bonds between the chitosan mole-cules, thus reducing the possibility of crystallization. In addi-tion, as shown in Scheme 1b, ketonic oxygen from the acetatemolecule can form a hydrogen bond with other chitosanmolecules, which helps to form a planar network consisting ofchitosan molecules and acetate molecules. During the quick-freezing and lyophilization process, the surface tension of thechitosan solution forces the chitosan molecules to arrangethemselves into film structures that connect together to forma 3-D structure. This is the reason why a 3-D chitosan aerogelwith smooth channel surfaces can easily be obtained. Duringcarbonization, the chitosan planar network decomposes to formcarbon, carbon dioxide and water. Some of the carbon atomsdecomposed from the chitosan planar networks connecttogether in situ to form graphene nanosheets, and the atomsdecomposed from disordered connected chitosan moleculesform amorphous carbon (Scheme 1c), to complete the forma-tion of the graphene-based carbon aerogel. The –NH2 groupdecomposes into elemental N and H. The H atoms react withthe OH atoms decomposed from –C–OH to form H2O. The Natoms remain in the carbon aerogel to form the N self-dopedcarbon aerogel. During the high temperature activation pro-cess, KOH molecules react with amorphous carbon, to formH2O, K2O and K2CO3, which can be removed during the postwashing process, leaving some pores in the amorphous carbonportion of the carbon aerogel wall [46,61]. KOH is thought toreact with amorphous carbon first, because the amorphouscarbon possesses higher energy compared with well crystalline

itosan, (b) 3D chitosan network when dissolved in acetic acid,networks activated at 800 1C and 1000 1C, respectively.

Figure. 5 (a–d) CV curves of porous N-doped graphene-based carbon aerogel electrodes at various scan rates in 6 M KOH aqueoussolution ((a) without activation; (b) K700; (c) K800; (d) K900), (e) CV curves of the four electrodes at potential scan rates of100 mV s�1, (f) specific capacitance of the four electrodes as a function of scan rate derived from (a–d).

17Graphene-based nitrogen self-doped hierarchical porous carbon aerogels derived from chitosan for high

grapheme parts. In fact, the obtained carbon samples atdifferent activation temperatures contain different contentof amorphous carbon. According to Raman and XPS results,K700 sample has the highest content of amorphous carbon,which is followed by K800 and K900 sample. At the loweractivation temperature at 700 1C, the reaction between KOHand amorphous carbon is slight, leaving more amorphouscarbon on the surface of graphene. At the optimum activationtemperature, most of amorphous carbon is etched away,leaving the highly graphitized graphene layer and nanoporousamorphous carbon particles between or on the graphene layers

(Scheme 1d). This hybrid graphene-based carbon aerogel,possessing the high charge storability of nanoporous carbon,and the high conductivity of a graphene network, is expectedto display superior supercapacitor performance. When theactivation temperature is increased further, almost all theamorphous carbon is etched away, and the major remainingcomponent of the carbon aerogel is the graphene framework(Scheme 1e).

To evaluate the electrochemical performance of porousgraphene-based carbon aerogels, cyclic voltammetric (CV) testswere carried out using a three-electrode configuration in an

Figure. 6 Nyquist plots showing imaginary part versus realpart of impedance in 6 M KOH aqueous solution. Inset magnifiesthe data in high-frequency region showing series resistance inhigh frequency region and charge transfer resistance in mediumfrequency region.

P. Hao et al.18

aqueous solution of 6 M KOH, and the results are depicted inFigure 5. The CV scanning was performed in the voltage windowof �1 to 0 V for all the electrodes. As shown in Figure 5a, theCV curves of the carbon aerogels without activation exhibit atriangular shape, indicating poor supercapacitive behavior.After activation at different temperatures, the CV curves ofthe three samples exhibit an approximately rectangular shapeeven at a scan rate of 200 mV s�1, indicating near-idealcapacitive behavior (Figure 5b–d). Figure 5e displays the CVcurves of the four electrodes at a scan rate of 100 mV s�1. InFigure 5e, one can see that the current density of sample K800is the highest compared with the values exhibited by the othersamples. Figure 5f summarizes the gravimetric specific capaci-tance of the samples as a result of CV testing. It is found thatthe unactivated carbon aerogels and sample K700 had a verylow specific capacitance, and the specific capacitancedecreased to less than 100 F g�1 when the scan rate wasincreased to 200 mV s�1, indicating that the unactivatedcarbon aerogel and sample K700 not only have poor electro-chemical performance, but also exhibit undesirable ratecapability. Sample K800 has the highest capacitance amongall samples at the same scan rate. For instance, sample K800possesses a specific capacitance of about 291.8 F g�1 at2 mV s�1, far higher than the specific capacitance of samplesK700 (245.5 F g�1) and K900 (204.8 F g�1). Moreover, thecapacitance showed only a slight decrease to 214.5 F g�1

when the scan rate was increased to 200 mV s�1, suggestinggood rate capability for a K800-based supercapacitor. Nota-bly, the specific capacitance value of 291.8 F g�1 for K800 ishigher than the reported values for pure graphene electrodesin aqueous electrolyte (about 70–250 F g�1), [62–64]graphene-based frameworks (166 F g�1), [65] graphene/car-bon composites (150–210 F g�1), [7,66] and N-doped gra-phene (150–250 F g�1) [22,67] electrodes used insupercapacitors. In addition, sample K800 also shows bettersupercapacitive performance compared to electrodes madefrom various other carbon-based materials using other bio-polymers as reported in the literature. Bichat et al., reportedobtaining high oxygen content nanotextured carbon usingseaweed as the raw material, and the optimal specificcapacitance of the obtained carbon electrodes was 206 F g�1

in 6 M KOH solution [35]. Subramanian et al., also obtainedporous carbon after KOH activation using banana fiber as theraw material [6]. However, the electrochemical performancewas so poor that its specific capacitance was only 66 F g�1 at5 mV s�1, which is 4 times lower than that of sample K800.

The electrochemical impedance spectrum (EIS) was alsoused to compare the electrochemical properties of thesamples, and is shown in Figure 6. The series resistanceand charge transfer resistance of the electrodes can bederived from the Nyquist plot. Figure 6 shows the impe-dance curves obtained for different samples at differentfrequencies ranging from 0.01 Hz to 100 kHz. For an idealsupercapacitor electrode, the series resistance and chargetransfer resistance should be as small as possible. It is foundthat the sample K800 electrode displayed a relatively lowequivalent series resistance (ESR) of about 0.71 Ω, which islower than the as-synthesized carbon aerogels (0.78 Ω,) andsample K700 (0.72 Ω,), but higher than that of sample K900(0.59 Ω,). This result can be explained by examining thedisordered carbon content in graphene-based carbon aero-gel samples. When they are carbonized at 700 1C, there is a

large amount of amorphous carbon that can bury thegraphene layers, indicated by a high amorphous carboncontent. After activation by KOH, part of the amorphouscarbon was etched away, while the other part of theamorphous carbon and highly graphitized graphene layersremain in the aerogel. With an increase in activationtemperature, the relative content of graphene increases,leading to good conductivity after activation, which isconsistent with the Raman and XPS results. Compared withthe other samples, the K800 electrode displayed thesmallest charge transfer resistance of about 0.68 Ω becauseof the optimized manipulation of pore size and distribution.This result is in accordance with the results of the CV scansin which the sample K800 electrode shows the best super-capacitor performance. Hence, it is believed that theelectrochemical performance of as-prepared carbon aero-gels can be improved dramatically by the activationprocess.

The above results clearly reveal that sample K800 exhibitssignificantly improved electrochemical performance comparedwith samples K700 and K900. We summarize the significantfactors in order to elucidate the important aspects of highcapacitance, excellent rate capability and cycling performanceachieved in sample K800 as the following. First, K800 has a highdegree of graphitization, indicating high conductivity, whichensures rapid charge transport. Second, K800 possesses anultrahigh pore volume of 1.1 cm3 g�1 while maintaining the3D hierarchical porous nanostructure of graphene with a con-siderably large specific surface area of 2435.2 m2 g�1. It isknown that the capacitive behavior of graphene-based nanos-tructures used in supercapacitors is highly dependent on boththe accessible specific surface area and the pore structure. Thegraphene network ensures good conductivity. At the same time,the porous amorphous carbon enhances charge transport andstorage. However, samples K700 and K900 have a lowergraphene content and amorphous carbon content, respectively,leading to inferior electrochemical performance when comparedto sample K800. Impressively, sample K800 has the optimumcontent of both graphene and amorphous porous carbon, leadingto outstanding capacitive behavior. Additionally, sample K800

19Graphene-based nitrogen self-doped hierarchical porous carbon aerogels derived from chitosan for high

has a homogeneous nitrogen distribution using chitosan in therole of both the carbon and nitrogen sources, which differs fromnitrogen-doped graphene prepared using NH3 gas as the nitrogensource, in which the reaction mainly occurs on the exposedsurfaces of the carbon nanostructures, thereby resulting ininhomogeneous and comparatively low nitrogen doping[22,68,69]. As is well known, nitrogen doping can enhance theelectrochemical performance of a supercapacitor. According tothe calculation of the binding energy between the potassium

Figure. 7 (a) CV curves of K800 in a KOH/PVA solid electrolyte, (bfrom (a), (c) charge–discharge curves of K800 at different currentcurrent density derived from (c), (e) cycling performance and capaplots of K800.

ion and nitrogen configurations at different positions ofnitrogen-doped graphene, the pyridine N and pyridinium N,which constitute major parts of the graphene-based carbonaerogel have a larger binding energy with the potassium ion.This larger binding energy results in a larger number of ionsthat can be accommodated on the electrode surface even fora given electrode surface area, leading to an increase incapacitance [26,31]. Moreover, N species on the carbonsurface could lead to the pseudocapacitive interaction

) specific capacitance of K800 as a function of scan rate deriveddensities, and (d) specific capacitance of K800 as a function ofcitance retention at 1 A g�1 over 10,000 cycles and (f) Ragone

P. Hao et al.20

between the ions of electrolytes and the N-containingfunctional groups, and then generate high pseudocapacitance[70,71]. In addition, the oxygen functional groups in carbonmaterials have been reported that can also improve thecapacitive behavior [72–74]. It is well accepted that there aremainly two roles of the oxygen groups in carbon materials.The first one is to provide more available sites for ionadsorption in the micropores of carbon aerogels because ofthe iondipole attraction, which can generate an excessspecific double layer capacitance due to the local changesof electronic charge density [74]. The second role of oxygenis to provide redox activity, which is responsible for theenhancement of the overall capacitance for supecapacitors[72,73].

In order to further identify the eletrochemical perfor-mance of the K800 sample, CV tests were carried out using atwo-electrode configuration both in organic and 6 M KOHaqueous electrolytes and the results are depicted in FigureS5. As shown in Figure S5, K800 possesses a specificcapacitance of about 233.8 F g�1 at 2 mV s�1, lower thanthe specific capacitance of K800 in the three-electrode set-up. This can be explained that in a three-electrode set-up,the potential value that shown on the X-axis of the CVcurves is equal to the potential applied to the workingelectrode. However, for a symmetrical two-electrode cell,the sum of the potential differences applied to eachelectrode is equal to the value shown on the X-axis of theCV curves. Therefore, for a given potential range on the X-axis of the CV, the potential applied to the workingelectrode in a three-electrode cell is much higher thanthe one applied to the each electrode in a two electrodecell, and this results in an increase of the specific capaci-tance [75]. In addition, taking into account the equivalentcircuit of a two-electrode supercapacitor, the two electro-des are in series, the total capacitance of the supercapa-citor will be determined by the smaller of the two electrode[76]. When using TEABF4 as the electrolyte, the specificcapacitance of K800 at 2 mV s�1 decreases to 135.9 F g�1. Itis found that the specific capacitance of the symmetricsupercapacitor in aqueous electrolyte is much higher thanthat in organic electrolyte.

As is well known, the potential range and ionic conductiv-ity of the electrolyte play important roles in supercapacitorbehavior. Liquid electrolytes are widely used in manyresearches because of their higher ionic conductivity com-pared with organic electrolytes. However, there are alsomany existing shortcomings in the liquid electrolytes such aslow potential range, corrosion, leakage and explosions.Therefore, many studies have replaced liquid electrolyteswith solid electrolytes in the supercapacitor. The solidelectrolytes have several advantages over liquid ones, whichinclude easy handling, increased safety, flexibility in packing,etc. Therefore, a two-electrode symmetrical supercapacitordevice was assembled to characterize the electrochemicalstorage capability of sample K800 in a KOH/PVA solidelectrolyte. As shown in Figure 7a, cyclic voltammetry showsrectangular and symmetric curves from 0 to 1 V at variousscan rates ranging from 2 to 200 mV s�1, which is character-istic of ideal supercapacitor behavior with good rate perfor-mance. Even at a fast scan rate of 200 mV s�1, only a littledistortion can be observed, and the highest specific capaci-tance reaches a value of 192 F g�1 at 2 mV s�1 (Figure 7b),

which is slightly lower than the value obtained by using 6 MKOH liquid electrolyte because of the slower ion diffusion inKOH/PVA solid state electrolyte. The galvanostatic charge–discharge curves at various current densities are shown inFigure 7c. All the curves are nearly-isosceles triangles andshow good symmetry, quick current–voltage response andnearly linear slope, indicating high coulombic efficiency andstandard double layer capacitor behavior [77]. Outstandingspecific capacitance values calculated from the charge/discharge curves are shown in Figure 7d. The specificcapacitance at 0.2 A g�1 is about 197 F g�1 and still remainsat about 108 F g�1 even with a discharge current density ashigh as 10 A g�1. The specific capacitance of the sampledecreases slightly with an increase in the charge/dischargerate, additionally suggesting good rate capability. The vol-tage drop at the initiation of the discharge is extremely small(0.005 V) even at the high current density of 10 A g�1,indicating a very low equivalent series resistance (ESR) inthe symmetric supercapacitor. Stability testing was con-ducted under a constant charge and discharge currentdensity of 1 A g�1 for 10,000 cycles. Figure 7e shows thecycling stability of the symmetric electrodes. The capaci-tance retention was about 92.1% with a small decrease from165.2 to 152.1 F g�1 after 10,000 cycles, indicating theexcellent cycling stability of K800. For comparison, thecycling performance of graphene without nitrogen doping isalso displayed in Figure S6. As shown in Figure S6, thecapacitance retention was about 82.3% after 10,000 cycles,which is much lower than K800. This is because that thenitrogen groups in chitosan molecules are uniformly dis-persed and bound to graphene planes, so they are highlyreversible and provide considerable stability during thecycling process [78]. In addition, as shown in Figure 7f, theall solid-state supercapacitor displayed excellent electroche-mical performance with both very high energy and powerdensities. The energy density can reach as high as 27.4 W hkg�1 at a power density of 0.4 kW kg�1 and 15 W h kg�1 at apower density of 20 kW kg�1. Although the electrochemicalperformance of the K800 sample was not the most out-standing compared with a few individual cases prepared frommineral materials, petroleum or organic reagents, therequirement of highly toxic organic reagents and mineralmaterials which are expected to be depleted in the nearfuture in these other cases can discourage their use inelectrode preparation. Thus, the presented work is highlypromising if one can consider the availability of chitosan as abiopolymer and the ease of the preparation method.

Nyquist plots of the all solid-state supercapacitor wereobtained over the range of 0.01 Hz to 100 kHz, as shown inFigure S7 with an expanded view provided in the inset. TheERS of sample K700 is 0.74 Ω, and a more vertical straightline is a typical response for a hierarchical porous structurein the low frequency region, indicating faster ion diffusionin sample K800.

Conclusion

In summary, graphene-based hierarchical porous nitrogen self-doped carbon aerogels with outstanding performance havebeen produced by the carbonization of chitosan aerogeland activation with KOH. The resulting graphene-based

21Graphene-based nitrogen self-doped hierarchical porous carbon aerogels derived from chitosan for high

carbon aerogels have macro-, meso-, and micro-hierarchicalporous structures and possess an extremely high specificsurface area of 2435.2 m2 g�1. The specific capacitancecalculated from charge–discharge measurements using an allsolid-state symmetric supercapacitor was about 197 F g�1 at0.2 A g�1 with excellent capacitance retention of �92.1% over10,000 cycles. The energy density reached as high as 27.4 Wh kg�1 at a power density of 0.4 kW kg�1 and 15 W h kg�1 at apower density of 20 kW kg�1. The outstanding capacitivebehavior is attributed to the unique features of graphene-based carbon aerogels, including a high degree of graphitiza-tion, high specific surface area and pore volume, reasonablepore size and pore size distribution, and uniform nitrogendoping. The results obtained demonstrate the possibility forlarge-scale production of hierarchical porous carbon andgraphene materials by choosing a low-cost biopolymer, i.e.chitosan, as the raw material.

Acknowledgements

The authors are thankful for funding from the NationalNatural Science Foundation of China (Grant no. 51372142),Innovation Research Group (IRG: 51321091) and the “100Talents Program” of the Chinese Academy of Sciences.Thanks for the support from the “thousands talents“ programfor pioneer researcher and his innovation team, China.

Appendix A. Supporting information

Supplementary data associated with this article can befound in the online version at http://dx.doi.org/10.1016/j.nanoen.2015.02.035.

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Pin Hao She received her B.S. and Masterdegree in the department of MaterialsScience and Engineering in 2008 and 2011from the Shaanxi University of Science andTechnology, China. She is now pursuing thePh.D. degree in the department of StateKey Laboratory of Crystal Materials atShandong University (China). She is nowunited training by Georgia Institute of Tech-nology. Her research interests include

energy storage and conversion, photocatalysis, etc.

Zhenhuan Zhao He is currently pursuing hisPhD at State Key Laboratory of CrystalMaterials in Shandong University. Hisresearch is focused on nanostructured semi-conductors for photocatalytical applicationand porous carbon materials for electroche-mical energy storage using renewable nat-ural sources.

Yanhua Leng He obtained his B.Sc. degreefrom Shandong University in 2012. Cur-rently, he is pursuing his M.Sc. degree inState Key Laboratory of Crystal Materials,Shandong University, China. His research ismainly focused on the first-principle calcu-lation and crystal growth simulation.

Dr. Jian Tian He obtained his B.Sc. degreefrom Beijing University of Chemical Technol-ogy in 2008. He received his doctor degree inthe department of State Key Laboratory ofCrystal Materials at Shandong University in2014. His research interest in his Ph.D. studyis mainly focused on preparation and modifi-cations of one-dimensional TiO2 nanostruc-tured surface heterostructures for photo-catalysis applications.

Dr. Yuanhua Sang He obtained hisB.S. degrees at Shandong University inChina in 2007 and completed his Ph.D. withProf. Hong Liu at Shandong University inJuly 2012. Now, he works as a lecturer inState Key Laboratory of Crystal Materials,Shandong University, China. His researchinterests are structure and property inves-tigation of inorganic crystal materials withneutron and X-ray diffraction, functional

materials, and materials for solar light conversion.

Robert I. Boughton He is a professor in thedepartment of Physics and Astronomy atGreen State University, His research inter-ests mainly focus on the measurement ofelectron transport properties at LHe temps,Nanomaterials.

Prof. C. P. Wong, the Dean of Engineeringof the Chinese University of Hong Kong, is aworld-renowned scholar in Electronic Engi-neering and a member of the US NationalAcademy of Engineering. He is on a no-payleave from the Georgia Institute of Technol-ogy where he is a Regents’ Professor andCharles Smithgall Institute Endowed Chair inthe School of Materials Science and Engi-neering. Professor Wong has published

23Graphene-based nitrogen self-doped hierarchical porous carbon aerogels derived from chitosan for high

widely with over 900 technical papers. He has yielded fruitfulresearch results and holds over 50 US patents. Professor Wong isconsidered an industry legend and has made significant contribu-tions to the industry by pioneering new materials, which funda-mentally changed the semiconductor packaging technology.

Prof. Hong Liu is a professor in State KeyLaboratory of Crystal Materials, ShandongUniversity, and adjunct professor in BeijingInstitute of Nanoenergy and Nanosystem,Chinese Academy of Science. He receivedhis PhD degree in 2001 from ShandongUniversity. He has published over 200 refer-eed papers, and over 30 patents. In 2009,he was awarded as Distinguished YoungScholar by National Natural Science Founda-

tion of China. In 2012, he is awarded as Professor of HundredTalents Program of CAS at Beijing Institute of Nanoenergy andNanosystem. His current research is focused mainly on nonlinearcrystal growth, chemical processing of nanomaterials for energyrelated applications including photocatalysis, energy storage andconversion, tissue engineering, especially the interaction betweenstem cell and nanostrucuture of biomaterials.