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Nature Communications-2013-suger

ARTICLE

Received14May2013|Accepted8Nov2013|Published16Dec2013

DOI:10.1038/ncomms3905OPEN Three-dimensional strutted graphene grown

by substrate-free sugar blowing for

high-power-density supercapacitors

Xuebin Wang1,2,Yuanjian Zhang3,Chunyi Zhi4,Xi Wang1,Daiming T ang1,Yibin Xu5,Qunhong Weng1,6, Xiangfen Jiang1,2,Masanori Mitome1,Dmitri Golberg1,6&Yoshio Bando1,2

Three-dimensional graphene architectures in the macroworld can in principle maintain all the

extraordinary nanoscale properties of individual graphene?akes.However,current3D gra-

phene products suffer from poor electrical conductivity,low surface area and insuf?cient

mechanical strength/elasticity;the interconnected self-supported reproducible3D graphenes

remain unavailable.Here we report a sugar-blowing approach based on a polymeric

predecessor to synthesize a3D graphene bubble network.The bubble network consists of

mono-or few-layered graphitic membranes that are tightly glued,rigidly?xed and spatially

scaffolded by micrometre-scale graphitic struts.Such a topological con?guration provides

intimate structural interconnectivities,freeway for electron/phonon transports,huge acces-

sible surface area,as well as robust mechanical properties.The graphene network thus

overcomes the drawbacks of presently available3D graphene products and opens up a wide

horizon for diverse practical usages,for example,high-power high-energy electrochemical

capacitors,as highlighted in this work.

1World Premier International Center for Materials Nanoarchitectonics(WPI-MANA),National Institute for Materials Science(NIMS),Tsukuba3050044, Japan.2Department of Nano-Science and Nano-Engineering,Faculty of Science and Engineering,Waseda University,T okyo1698555,Japan.3School of Chemistry and Chemical Engineering,Southeast University,Nanjing211189,China.4Department of Physics and Materials Science,City University of Hong Kong,Hong Kong999077.5Environment and Energy Materials Division,NIMS,Tsukuba3050047,Japan.6Nanotube Group,NIMS,Tsukuba3050044, Japan.Correspondence and requests for materials should be addressed to X.B.W.

(email:Wangxb@fuji.waseda.jp)or to Y.B.(email:BANDO.Yoshio@nims.go.jp).

G raphene,as an outstanding representative of two-dimen-

sional(2D)crystals,offers unique physics and exciting

functionalities1.In addition to electronic and photonic applications featuring its‘2D crystal’character,graphene can be used in large quantities as an adsorbent,a support or an electrode featuring its‘new carbon’status2,3and as a component of diverse composite materials featuring its‘nano’quality4.Unfortunately,the poor intersheet connections between isolated graphene?akes as building blocks break the continuous pathway for electron/phonon transports and severely suppress the intrinsically high conductivity and mechanical strength of individual graphene?akes5.The unavoidable restacking and agglomeration caused by van der Waals forces-induced adhesion in standard graphene products diminishes the accessible surface area6.New three-dimensional (3D)graphenes are thus calling for the accomplishment of perfect sheet-to-sheet connectivity and substantial supportability towards high performance in the massive applications.

Many efforts have recently been made in relation to the3D graphenes.Pillared graphenes7,8,graphene hybrids using physical/ chemical assemblies on functional metals,oxides,polymers or carbon nanotube(CNT)connectors5,6,9–12,macroporous graphenes using self-assemblies13–15and graphene networks derived from direct-templated chemical vapour deposition(CVD)16,17have been proposed and studied.However,both graphene hybrids and macroporous graphenes still suffer from heterogeneous intersheet connections and high junction resistances.CVD-derived graphene networks have a good interconnection but do not yet have suf?cient mechanical support due to the softness of pristine graphene frameworks.Moreover,the template utilization in CVD methods limits the large-scale production and leads to a high product cost (commercial price,$50per cm3).These currently available graphene products are still far from meeting the feasible and promising applications.

Herein,inspired by an ancient food art of‘blown sugar’,we develop a sugar-blowing technique to grow a3D self-supported graphene product,named by us as strutted graphene(SG).SG consists of continuous graphitic membranes,which are homo-genously connected and spatially supported by the networks of micrometre-width graphitic struts.The high electrical conductiv-ity,surface area,mechanical strength and elasticity are thus simultaneously achieved in this SG.The facile low-cost high-throughput production with the sugar-blowing technique further enables its immediate practical utilizations.SG quality fully?ts the above-mentioned large-scale applications,for example, supercapacitors.We additionally con?rm the high power density of an SG-based supercapacitor due to such structural inter-connectivities,hierarchical porous channels,huge accessible surface area and high mechanical and chemical stabilities.

Results

Characterization and properties of SG.The obtained SG pro-duct reveals a foam-like architecture(Fig.1a)packed by large polyhedral bubbles.The average diameter of the bubbles is 186m m(Supplementary Fig.S1).The struts are de?ned as the borders where three/four bubbles merged18.Graphitic struts with an average width of3.5m m constitute the skeletons of the bubbles and graphitic membranes are tightly anchored onto them as facets.The graphitic facets are mainly distorted pentagons,or sometimes quadrangles,hexagons and heptagons on a side of SG (Fig.1b);this topology approaches a steady status having polygonal units of5.1edges per face in theory19.

The interconnected struts of SG are integrated with mem-branes to form a unique architecture(Fig.1c,d).The struts serve as connectors to conduct electrons/phonons,as?xers to resist the disintegration of graphitic membranes and as supporters to prevent agglomeration or restacking of individual graphitic membranes.SG thus has an overall high conductivity of 1Smà1due to such speci?c structural con?guration.The con?guration is also bene?cial for providing a smooth transfer of foreign mass within the bubble channels and for retaining the intrinsically large surface area of graphenes.

The integrated architecture further determines robust mechanics of SG,although the product possesses an ultralow density of3.0mg cmà3comparable to a carbon aerogel.The ultralight SG exhibits high elasticity under compression tests (Fig.1e);such behaviours are similar to a graphene-based cellular monolith20,cork and other elastomeric foams21.SG can recover even under a compression to80%(Supplementary Fig.S2).The highly reversible resilience without collapse indicates the self-supporting robust structural integrity.This is further con?rmed by a remarkable conductivity of0.6Smà1sustained after a heavy compression to80%.The robust structure contributes to high tolerance/endurance with a reliable large surface area to

10304050607080

2 (°)

Figure1|The3D bubble-like network of SG.(a)Scanning electron microscope(SEM)image and optical photo of a70-mg SG piece obtained at a heating rate of4°C minà1.The inset is a reconstructed topology corresponding to the region marked in red for the two connected decahedron–dodecahedron bubbles faced by eight pentagons and two quadrangles,and by eight pentagons,three quadrangles and one heptagon,respectively.(b)SEM image of the ?at backside of the SG product with regular arrangement of connected cells identi?ed by arti?cial quilt colours.(c,d)2D and3D optical photos of SG networks.(e)Photos of original,compressed and recovered states of a SG specimen.(f)XRD pro?le of SG.Scale bars,200m m(in a–d)and1cm(in e).

further mechanical processing in practical applications without requirements for an additional protection.

SG is a highly graphitized product.It exhibits high crystalline degree with a strong(002)peak locating at26.0°in the X-ray diffraction(XRD)pattern(Fig.1f).The broadening of this peak indicates the downsized dimensions of graphitic crystals along the [002]directions,which are estimated as2.7nm according to Scherrer equation.

The SG has hierarchical porous structures at different scales, that is,bubbles,ripples and concaves.The bubbles are circumscribed by graphitic membranes joined onto graphitic struts.The spacious graphitic membranes are not?at but are full of dune-like ripples with an average size of12nm(Fig.2a,b). Spontaneous and stress-induced thermodynamic relaxation of the polymer precursors in the present sugar blowing might account for these ripples,which are as fashionable as the bumps of a monolayer graphene with a1-nm amplitude attributed to intrinsic and/or impurity-caused thermodynamic?uctua-tions22,23.The ripples contribute to resisting the restacking of overlapped graphenes to keep the large surface area of graphene products24.In addition,some concaves with an average size of 4nm are attached on graphitic surfaces,which are encircled by (002)graphitic basal planes(Supplementary Fig.S3).Such surface defects might result from oxidation by the surface C–O groups or residual O2on amorphous carbon under high-temperature annealing.They introduce abundant edges,that is,transverse sections of graphitic basal layers,bene?cial to the speci?c adsorption and achieving a high surface area.

The diverse porous structures were further characterized by N2 adsorption/desorption tests(Fig.2c).SG behaves close to the type IV isotherm having four steep condensations on the adsorption part.The?rst three ones correspond to‘pores’with widths of4,7

and30nm according to Barret–Joyner–Halenda theory(Fig.2d). The‘pore’dimensions located at condensations1and2are just consistent with the above-mentioned concaves and ripples, respectively.The steep condensation3around0.95P/P0results from some high-curvature regions inside the bubble cavities of SG.This condensation clearly disappeared for the milled SG(M-SG)in which the bubble structures were destroyed by heavy milling.These three structures are attributed to high-order speci?c morphologies mainly relevant to graphitic membranes, which can be considered as‘mesopores’.The condensation4 relates with the unsaturated adsorption at macroporous bubble cavities of SG or secondary piled slits of M-SG.Pore distributions based on quenched solid density function theory are similar, except a slight downshift compared with the Barret–Joyner–Halenda results.The‘pores’on different scales are nested and may cause the temporary locking of liquid N2and delayed evaporation in the desorption isotherm,which thus resembles type H3hysteresis loops.

The‘mesoporous’structures and supporting effect of SG struts result in a high speci?c surface area(SSA)of1,005m2gà1.This is larger than for the unsupported graphene products,because the high intrinsic SSA of graphenes is always suppressed by severe agglomeration or?ake restacking25.By contrast,M-SG without such supporting effect has a much low SSA of 780m2gà1.Moreover,the contribution fraction of micropores to the SSA and speci?c pore volume of SG is minor,around zero, as evidenced by a t-plot analysis.The‘mesopores’contribute to 55%of SSA;macroporous bubbles and external surfaces contribute with a notable fraction of28%according to quenched solid density function theory(Supplementary Fig.S4 and Supplementary Table S1).As a result,the high SSA derived from‘mesopores’and macroporous structures can facilitate intense and quick charge storage/release in SG electrodes for supercapacitor devices.

The graphitic membranes of SG are mainly large-area, ultrathin,high-quality mono-/few-layered graphenes and some multilayered graphite sheets.A representative spacious mem-brane with a lateral size of B100m m is transferred on substrates (Fig.3a).Atomic force microscopy reveals its small thickness of 2.2nm,that is,six graphitic layers,as shown in Fig.3b.The mono/few layers of ultrathin graphene membranes are also depicted in Fig.3c.The small thickness is consistent with the XRD results.The sp2-hybridized level of such mono-/few-layered graphene membranes is nearly perfect;the fraction of sp2/(sp2tsp3),by analysing the electron energy-loss spectroscopy pro?le26, is99%,which well matches the standard graphite(Fig.3d). Raman spectrum further re?ects high graphitization degree of mono-/few-layered graphene membranes(Fig.3e).A strong2D peak with a narrow full width at a half maximum of48cmà1 features a well-graphitized double-layered graphene27.The high graphitization status of few-layered graphene membranes was also veri?ed by the electron diffraction and the high conductivity of20,000Smà1(Supplementary Fig.S5).This?gure is close to that of graphite and is higher than B7,000Smà1of a typical reduced graphene oxide(RGO,the only graphene product available in ton quantity).

Growth and structural regulation of SG.The SG was synthe-sized through the controlled heating of glucose and NH4Cl (Fig.4a).In this process,a molten syrup was gradually poly-merized,whereas chemically released gases from NH4Cl blew glucose-derived polymers,such as melanoidin(Supplementary Fig.S6and Supplementary Note1),into numerous large bubbles, which features the sugar blowing.The bubble walls were gradu-ally thinned by the gas release and blowing,surface-tension-induced drainage of the polymer?uid out of the walls

and 900

Desorption-QSDFT

Adsorption-BJH

Adsorption-QSDFT

6000.9

S G

M-S G

1.0

(

–

)

300

0.0110

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Relative pressure P/P0Pore width (nm)

Figure2|Rippling few-layered graphene membranes and high surface area of SG.(a,b)Scanning electron microscope(SEM)images of a broken few-layered graphene membrane linked by its strut and the magni?ed surface with many ripples.Scale bars,10m m(in a),1m m(in b)and200nm (in inset of b).(c)N2adsorption/desorption isotherms of SG and M-SG, with a particularly enlarged inset at0.95P/P0.Numbers1–4indicate four condensations.(d)Corresponding pore-size distributions according to Barret–Joyner–Halenda(BJH)and quenched solid density function theory (QSDFT)calculation/simulation.

elimination of small molecules from the polymers.The polymer walls were subsequently graphitized into ultrathin graphitic membranes at a high temperature.

On the basis of the understanding of sugar blowing,we synthesized different SG products by regulating the heating rates (Fig.4b–d and Supplementary Fig.S7).Both cell size and strut width decrease with an increase of heating rate and the bubbles become more distorted at a faster heating rate.The ratio of cell perimeter to strut width re?ects the fraction of graphitic membranes in the entire foam and thus veri?es the quality of SG.The ratio is highest at a heating rate of 4°C min à1,indicating the optimized SG architecture;this is consistent with the SSA conclusive data from N 2adsorption (Fig.4e).The optimized SG has a topological structure of an 80-m m cell size and a 186-m m bubble diameter in average,as discussed above.The key point in the sugar blowing,that is,the match between ammonium salt decomposition and glucose polymerization,is crucial to produce the highest-quality SG,whereas the mis-matched ammonia-release of (NH 4)2SO 4,(NH 4)2CO 3or urea with the curing of glucose-derived polymers resulted in poor-quality products (Supplementary Fig.S8and Supplementary Note 2).The best-quality SG can display exciting 2D graphene properties and functionalities more and thus has been applied by us to construct SG-based supercapacitors.

SG-based supercapacitors .The speci?c topological con?guration of SG enables its high performance in an electrical double-layer capacitor (EDLC).A supercapacitor using SG obtained at 4°C min à1was examined by a two-electrode system in an H 2SO 4electrolyte.During constant-current discharging,the nearly linear voltage pro?les closely approach an ideal capacitor (Fig.5a).The capacitance is as high as 250F g à1at 1A g à1and slowly decreases to 130F g à1at a high current of 100A g à1(Fig.5b).While testing M-SG-based supercapacitors,the

initial

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π*σ*

320Figure 3|Ultrathin high-graphitized few-layered graphene membranes.(a )Optical image of a large few-layered graphene membrane on a SiO 2/Si substrate taken from SG obtained at a 4°C min à1heating rate.Scale bar,100m m.(b )Atomic force microscopy image of an individual few-layered graphene membrane (the inset is a cross-section height pro?le).Scale bar,5m m.(c )High-resolution transmission electron microscopy images of a one-to two-layered graphene (left),and a three-to four-layered graphene (right).Scale bar,5nm.(d ,e )Electron energy-loss spectroscopy pro?le and Raman spectrum of few-layered graphene

membranes.

Glucose Melanoidin Graphene

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1,000R a t i o o f c e l l -p e r i m e t e r t o s t r u t -w i d t h

Figure 4|Growth process and structural regulation of sugar-blowing production.(a )Scanning electron microscope (SEM)images of glucose (crystals marked in green)and NH 4Cl (marked in blue),which were subsequently transformed to melanoidin bubbles (marked in brown)under a browning reaction;these were ?nally converted into the SG containing graphitic membranes and struts (marked in gray).The arti?cial melanoidin macromolecule structure is proposed with features of indole and catechol according to Supplementary Note 1.The heating rate is 4°C min à1.Scale bar,50m m.(b –d )SEM images of the SG products grown at the heating rates of 1,20and 100°C min à1,respectively.Scale bars,200m m.(e )Changes of SSA and ratio of cell perimeter to strut width versus heating rates.

voltage drop is much larger than in the case of SG-based ones,which determines a large internal resistance of M-SG.M-SG electrodes only demonstrate 41F g à1at 100A g à1,much smaller ?gures than that of https://www.wendangku.net/doc/f34364367.html,mercial graphene platelets (GP)and activated carbon (AC)materials also exhibit weaker super-capacitor performances compared with SG.This was further con?rmed by the alternating-current tests (Fig.5c).At high fre-quencies,SG-based supercapacitors have a slower capacitance decay than those of M-SG ones,as indicated by the slopes of ?tting lines.The SG-based supercapacitors present high average-power density and energy density (Fig.5d).The high available average-power density of SG-based supercapacitors is close to an aluminum electrolytic capacitor;this is higher than for the EDLCs of CNT/graphene sponge 28,graphene hybrid 29,3D RGO hydrogel 30,31and a pseudocapacitor of 3D oxides 32.

The excellent performances of SG electrodes are determined by three factors (Fig.5e):(i)total SG porosity is 99.85%as calculated from its apparent density;bubble cavities take the most SG volume.The cavities are connected by breaches and opened cells to form pathways for the smooth ion migration;inside the cavities the channel resistance is notated as R B .This resistance is only 0.23O for SG electrodes,whereas it increases to 2.66O for M-SG electrodes (Supplementary Fig.S9,Supplementary Table S2,and Supplementary Note 3).This re?ects the large diffusion drag and the exhaustion of ions within the con?ned piled pores and a narrow neck-like channel of the M-SG.The capacitance decay at the high-frequency domain can be attributed to distributed charge storage in electrode pores (equivalent to resistor-capacitor networks in a transmission-line model)and/or ion diffusion control 33.The suppressed diffusion in M-SG electrodes thus causes the steep capacitance decay and a small capacitance at 103Hz.By contrast,the surfaces of SG can be readily accessed without a diffusion limit,which guarantees a large capacitance at extremely high currents/frequencies.(ii)A natural,nearly perfect electron freeway is provided by the interconnected struts and by joined graphitic membranes in SG.The inner contact resistance of SG electrodes thus is nearly zero,but it is 1.07O for M-SG electrodes owing to many intersheet contacts (Supplementary Table S2).Such large resistance results in a larger initial voltage drop and smaller available output power and energy;it also contributes to the earlier appearance of ‘knee’in?exion at a lower frequency for M-SG electrodes,because such

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Graphene–cellulose (ref. 29)RGO hydrogel (ref. 30)RGO hydrogel (ref. 31)

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F g –1)

0.23 ΩFigure 5|Supercapacitor performance of SG electrodes.(a )Nearly ideal capacitor behaviours of SG electrodes under galvanostatic discharges at marked currents.(b )Correlation of speci?c capacitances with discharge currents of SG electrodes compared with M-SG,GP,AC and Al-electrolytic capacitor (denoted as Al-electrolytic).(c )Equivalent capacitances calculated from the frequency response analysis based on a serial RC model.The straight lines are the ?tted ones as discussed in Supplementary Note 3.(d )Ragone plots of the average-power density versus energy density of supercapacitors.Previously reported 3D-graphene-based supercapacitors are plotted for comparison.The Al-electrolytic capacitor was operated in an organic system with a 25-V voltage.The blue vertical line is the theoretical upper bound of graphene–H 2SO 4system,that is,maximal capacitance 710F g à1(Supplementary T able S4).The points in square frames are a half of the calculated maximum powers according to the alternating-current effective series resistance (see equations 8and 9).(e )Equivalent circuits (left)and schematic models (right)showing easy ion diffusion and minimized inner contacts for an SG electrode compared with the M-SG one.R A and R B indicate ionic resistors of bulk electrolytes and electrolytes inside electrode pores,respectively.R C re?ects contact resistors within electrode materials (see more in Supplementary T able S2and Supplementary Note 3).

contact resistors are distributed and coupled with EDLC networks.The frequency at the‘knee’point is the upper limit for utilizing most of cavity and pore surfaces below which the capacitance approaches its intrinsic value.The SG electrodes oppositely have a higher‘knee’frequency(11Hz),a lower capacitance-cutoff impedance(B0.4O)and a higher characteristic frequency(9.6Hz;see Fig.5d and Supplementary Fig.S10).(iii)In M-SG,individual graphitic?akes may easily agglomerate and restack into multilayered structures.This results in inaccessible surfaces and the capacitance loss34,35.On the contrary,the graphitic membranes in SG are attached and rigidly ?xed on their partners,the strut networks.This avoids the agglomeration and leads to the highest capacitance.SG electrodes also perform better than the GP ones and much better than microporous AC ones due to the similar factors.Their power density and energy density are thus undoubtedly outstanding compared with other EDLCs.

The maximum power density is widely accepted as a fundamental property of supercapacitors.The maximum power density of SG,achieving893kW gà1at100A gà1,is further compared with other materials in Supplementary Fig.S11a and Supplementary Table S3,showing one-to-two orders of magni-tude higher values than those for all previously reported3D graphenes16,36–38.The high power density not only implies quick charging completed within a few seconds but also presumes the huge available energy output within the limited time.The superfast available charging process illuminates an amazing future in‘?ash charging’of portable electronics,quick startup of electric vehicles and electromagnetic launching of aircrafts.

Discussion

The developed sugar-blowing technique can reliably produce SG using many common precursors such as glucose and kitchen-use sugar(Supplementary Fig.S12).It therefore provides a universal pathway and opens a new window for the creation of3D graphene analogues.In addition,it should be considered as a new member of the family of synthetic protocols of3D graphene.The facile sugar-blowing method stands out of many previous routes for making3D graphenes,such as hydrothermal method13,39,40,cryodesiccation15,41,breath-?gure method14and CVD16,17,42.The produced SG normally possesses larger surface area than graphene papers43,higher conductivity than widely studied RGO hydrogels and stronger mechanical endurance than CVD-derived graphene foams.Moreover,the high-yield sugar-blowing route makes the production of SG scalable,up to kilogram or even ton levels.Massive SG has been produced at a yield of16±5wt%related to a raw glucose following the designed sugar-blowing method.The product costs only$0.5per gram referring to our laboratory-scale synthesis,in contrary to much more expensive other3D graphene products.The abundant production of SG together with its unique structural characteristics enlightens its wide and promising potentials. The mono-/few-layered graphene membranes of SG have the high graphitization degree.During sugar blowing,the walls of sugar-derived polymer bubbles are thinned down to B20nm by the expansion of released gases and surface drainage.Meanwhile, the elastic,highly elastic and viscous interior stresses are induced along the polymer walls of bubbles.As both ultrathin thickness and interior stress are bene?cial to orient polymer molecules44, the present bubble walls possess highly oriented arrangement of polymer molecules.The graphitization on such thin-oriented polymeric predecessors is easier than the conventional graphitization of a bulk carbon resource or carbon?lms/?bres at micrometre level45,because the energy used for the parallel arrangement of(002)graphitic layers becomes much lower in case of an oriented predecessor46,47.Hence,the well-graphitized graphitic membranes are formed.The graphitization approach of sugar blowing is clearly different from all previous methods of graphene syntheses,such as diverse exfoliations48–50,CVD51, solvothermal,organic synthesis34,52and substrate-assistant/ catalytic graphitization53–55.It illuminates a new graphene synthesis mechanism calling for extensive researches.

The structural regulation of SG was realized through modifying heating rates.The heating rate acts through the matching relation between the gas release and polymer curing owing to kinetics factors,including decomposition of NH4Cl,diffusion of gas, nucleation and growth of bubbles.This control is consistent with the experience in traditional polymer foaming.Furthermore, some rheology courses such as drainage,rupture and coalescence of bubbles also in?uence the bubbling process.In the bubble growth,the volume expansion is forced by gas release.The applied energy trades with viscous?ow and increasing surface area of bubbles.More effectively utilized gas can cause a larger viscous deformation and thus larger bubbles,giving the same acting trend in line with the kinetic factors(Supplementary Note 2).In bubble dilation,the interior stress strongly depends on the heating rates.A higher heating rate can substantially lead to the higher elastic interior stresses.This results in stronger shrinkage/ distortion of bubbles when NH4Cl exhausts,as observed in Figs1a and4b–d.The drainage is caused by surface tension. Smaller curvature radius at Plateau borders results in the smaller pressure compared with bubble walls,and thus a Laplace force causes continuous drainage of the intralamellar?uid into the borders,that is,gradual thinning of bubble walls.This mass transfer depends on surface viscosity and time56.A slow heating process gives rise to more?uid?owing into borders and thus thicker struts,as shown in Figs1a and4b–d and Supplementary Fig.S7c.

One of the most important purposes to develop the3D graphene subdiscipline is to reach the high power density of its supercapacitors.For example,they can be combined with a battery for power pulse output with a low overall internal resistance and long lifetime.The power density of rechargeable batteries is normally o500W kgà1;micropore-type supercapa-citors have the power density of102–104W kgà1;thick-layer graphene supercapacitors have the power density of103–104W kgà1(ref.57).These relatively low power densities are limited by the block effect of narrow channels to ion migration and the resistance of discontinuous matrix to electron transport. The3D carbon-based supercapacitors,such as CNT networks58 and3D graphenes,do not have the above-mentioned disadvantages in theory and can have increased power density. Our SG undoubtedly satis?es the three main requirements for porous electrodes:(i)suf?ciently connected cavities/channels providing an electrolyte pathway to maximize ionic conductivity and to have a free access to the inner surface;(ii)the minimal contact resistance between blocks building the porous electrodes; and(iii)a large SSA and the robust mechanical structure.The SG thus nicely functions in supercapacitors with a high maximum power density close to106W kgà1resulting from the former two advantages.A relatively high usable energy density is also achieved by merging all three mentioned factors.In addition,a maximum volumetric power density of27kW Là1of our supercapacitors was achieved after appropriate compression of SG(denoted as C-SG,see Supplementary Fig.S11b,c).The volume of C-SG decreased after compression,whereas its electronic and ionic resistances did not increase much;these features determined a higher maximum volumetric power density than the SG ones59,60.Furthermore,these SG electrodes have no restrictions for the further scaled-up fabrication due to their interconnected architectures,whereas analogous characteristics of

normal graphene-based supercapacitors decay with scaling up the loading mass/thickness on an electrode57,61.

In summary,we have developed a sugar-blowing approach to effectively synthesize a3D graphene product,namely SG,which is made of graphitic membranes tightly connected and supported by robust graphitic microstruts.The developed reliable and scalable synthetic protocol is envisaged to become a general path for the synthesis of3D graphene analogues,especially meaningful BC x N graphenes.The produced abundant SG enables many graphene-related applications as supports,catalysts,sorbents,hydrogen reservoirs,gas sensors,air?lters,sound absorbers for substituting current porous carbons and as the nano-?llers of functional nanocomposites.The selected example,SG-based supercapaci-tors,exhibit very high power densities.The robust3D topology of SG further allows avoiding disintegration and performance decay in any scaled-up device fabrication process.

Methods

Synthesis of SG.Typically,10g glucose was mixed with10g ammonium salts (NH4Cl),which was then heated under a desired heating rate(4°C minà1was recommended)and?nally treated at1,350°C for3h under Ar atmosphere in a tube furnace(50cm length by5cm diameter).A black foam-like product,that is, SG,was collected.White granulated sugar(a kitchen-use sugar from Nissin-Sugar Co.)was also used to replace glucose to grow SG.

Characterizations of SG.The morphologies of SG were visualized by scanning electron microscopy(Hitachi S-4800),high-resolution transmission electron microscopy(JEOL JEM-3000F),atomic force microscopy(JEOL JSPM-5200)and optical microscopy(Keyence VHX-900).The structures were characterized by electron diffraction and energy-loss spectroscopy(attachments of JEOL JEM-3100FEF with an Omega Filter),XRD(Rigaku Ultima III with Cu K a radiation), Raman spectroscopy(a Kr-Ar ion laser at514nm produced by Spectra-Physics Beamlok2060-RS laser combining Symphony CCD-1LS detection system)and Fourier transform infrared spectroscopy(Thermo Nicolet4700).The thermal analysis was done using thermogravimetric and derivative thermogravimetric (Rigaku Thermo plus TG8120)methods,as well as differential scanning calori-metry(Rigaku Thermo plus EVO DSC8230).The nitrogen adsorption–desorption measurements were carried out at a liquid nitrogen temperature on a Quanta-chrome Autosorb-1.Brunauer–Emmett–Teller surface area was estimated over a relative pressure range of0.05–0.3.The conductivity of individual few-layered graphene membranes was analysed within a?eld effect transistor con?guration using a semiconductor parameter analyser(Keithley4200-SCS).

Supercapacitors based on SG.The symmetric supercapacitors were fabricated using two electrodes made of diverse1.0mg active materials:SG,M-SG,com-mercial GP(from ACS Material LLC)and AC(charcoal activated,Merck Che-micals Co.),respectively.SG-based electrodes were made through placing SG foam-like blocks with designed shape onto steel collectors,to which surfaces a thin-layered binder of polyvinylidene?uoride was in advance applied;tetra-

?uoroethylene were further used to?x the SG blocks to collectors.If a1MPa pressure force was applied to the SG/collectors when drying them,the C-SG electrodes were thus prepared.The other active materials were tested here for comparison purposes(SG was in advance milled into powders in a mortar as a control denoted as M-SG).They were typically mixed with tetra?uoroethylene by 10:1ratio in water solutions;the concentrated mixtures were then pasted onto collectors and dried at60°C.The electrochemical examinations of the super-capacitors,including chronopotentiometry,cyclic voltammetry and electro-chemical impedance spectrometry were conducted in a1M H2SO4electrolyte by an electrochemical workstation(Solartron1280B).The galvanostatic discharge at 100A gà1of M-SG supercapacitors was measured by an electrochemical analyser (HJ1001SD8,OKUTO DENKO Co.).An Al-electrolytic capacitor of25-V/4.7-m F (Su’scon Co.)was also tested for comparison,in which two Al foils were16mg in weight and the dry packaging materials were140mg.

The gravimetric capacitance normalized on one electrode(C g)was calculated from galvanostatic constant-current discharging measurements in two-electrode systems using:

C g?

4I cons

med V=d tT

e1T

where I cons is the constant current in discharging(or steady current in cyclic voltammetry curves),m is the total mass of both electrodes and d V/d t is the slope of discharge curve.The current density is normalized with respect to the mass loaded on an electrode.The direct-current equivalent internal resistance of the cell can be obtained in continuous charging/discharging by:

R inter?

V drop

2I cons

e2T

where V drop is the initial Ohmic voltage drop of the cell at the beginning of the discharge,which is obtained through extrapolating the voltage–time curve back to zero time;such calculated apparent equivalent internal resistance may be larger than the true value at a low current due to the additional contribution of self-discharge to voltage drop.This is further used for the calculation of discharge energy densities of two electrodes E s,average-power densities of two electrodes P s and maximum constant-current discharge power densities of two electrodes P s,max-DC by using the formulas33,62:

E s?

C geV maxàI cons R interT2

e3T

P s?

I conseV maxàI cons R interT

E s

e4T

P s;maxàDC?

V2openàcircuit

4mR inter

E V

max

4mR inter

e5T

where V open-circuit is the open-circuit voltage of the supercapacitor after being completely charged to1V;V max is the applied voltage of the cell at maximum,1V for an aqueous system,here the difference between open circuit voltage and maximal voltage is small and ignored according to our measurements;and D t is the discharge time of the cell.The similar calculations are normalized to the volumes of active materials again.The volumetric energy densities of two electrodes E V and maximum volumetric power densities of two electrodes P V,max are determined according to:

E V?

E se6T

P V;max?

P s;maxàDCe7Twhere V AM is the volume of active materials on both electrodes.The effective series resistance R ESR of the cell,determined from the x-intercept of the Nyquist plots of frequency analysis,is also used to predict the maximum power densities of two electrodes and corresponding energy densities of two electrodes by:

P s;maxàAC?

V2max

4mR ESR

e8T

E s;at maximum power?

C g V2max

e9T

Note that the points(E s,at maximum power,1/2P s,max-AC)were plotted into Fig.5d to match the de?nition of average-power density of the vertical axis.The alternating-current equivalent capacitance normalized on one electrode(C eq)at frequency f was calculated from the imaginary component of the impedance(Z00)of the cell using a series-RC circuit model by:

C eq?

à1

:e10T

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Acknowledgements

We thank Professors R.S.Ruoff,T.Osaka,K.Kuroda,T.Chikyow,T.Momma and Y.Yamauchi for valuable comments.X.B.W.is grateful to Drs M.Hu,T.Z.Zhan and W.Yi for the discussions on experimental technologies,and to Drs T.Y.Zhai,L.Li,H.B. Zeng,X.S.Fang,M.S.Wang,X.L.Wei,S.M.Chen,D.Q.Liu and A.Pakdel for the helps. X.B.W.also thanks MANA foundry and MANA technical support staff for the tech-nological assistance during lithography and scanning electron microscopy analyses. Grateful thanks are due to Drs A.Nukui,N.Kawamoto,I.Yamada and Ms.Y.Hirai for the laboratory maintenance.Financial support from WPI-MANA is gratefully acknowledged.

Author contributions

X.B.W.designed the concept of SG and carried out its synthesis and characterization Y.J.Z.,X.W.and D.M.T.assisted the thickness analysis of mono-/few-layered graphenes or attended the discussion about the electrochemistry tests.X.B.W.fabricated and examined SG-based supercapacitor devices.All the authors discussed the results.X.B.W. analysed all the data and wrote the manuscript.D.G.revised the manuscript.D.G.and Y.B.oversaw the whole project.

Additional information

Supplementary Information accompanies this paper at https://www.wendangku.net/doc/f34364367.html,/ naturecommunications

Competing?nancial interests:X.B.W.,Y.B.and D.G.are co-inventors on an unpub-lished patent application owned by NIMS on the basis of the work reported in this paper. All other authors declare no competing?nancial interests.

Reprints and permission information is available online at https://www.wendangku.net/doc/f34364367.html,/ reprintsandpermissions/

How to cite this article:Wang,X.et al.Three-dimensional strutted graphene grown by substrate-free sugar blowing for high-power-density https://www.wendangku.net/doc/f34364367.html,m.4:2905 doi:10.1038/ncomms3905(2013).

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Supplementary Figure S1. Morphology and size of SG grown at the heating rate of 4 o C/min. (a ) Whole SEM image of SG. The red arrow marks the same region as of Fig. 1a. (b ) Histogram of the bubble diameter based on a statistical method of averaged chord length in assumption that the bubbles are spheres 63, which suggests an average value of 186 μm. The line is a Gaussian fitted curve.

C o u n t s

Bubble Diameter (μm)

Supplementary Figure S2. Reversible mechanical compression and low resistance of SG. (a ) Stress-strain curves of two compression-release cycles of a SG elastomer. The material behaves similar to many elastomeric foams with three typical ranges of deformations: nearly linear elastic bending, relatively smooth elastic buckling, and a high-stress densification. The mechanical strength of SG does not deteriorate under a heavy compression of 80%. The highly compressible SG elastomers thus have robust mechanical tolerance helpful in making practical electrode materials. (b ) Resistance change of a SG foam after compression-release cycles. The SG resistance only increases by 40% after 1st cycle due to the robust structures; the SG sustains its low resistance all the way up to the 20th cycle indicating the well-kept interconnectivity for electron transport. The conductance performance starts deteriorating during 20th -50th cycles.

S t r e s s (k P a )

Strain (%)

R e s i s t a n c e a f t e r C o m p r e s s i o n T e s t s (Ω)

Compression Cycles

Supplementary Figure S3. TEM image of a few-layered graphene membrane within SG obtained at a 4 o C/min heating rate. The inset is the sketch of a concave-like structure within

graphene membranes of SG corresponding to the marked region.

0102030

C u m u l a t i v e P o r e V o l u m e (c m 3/g )

Pore Width (nm)

24.4

Condensations

Supplementary Figure S4. QSDFT cumulative pore volume corresponding to the isotherm of SG in Fig. 2c. The marked three condensations reflect the contributions of different-scale “pores” to the specific pore volume and SSA of SG; the details are listed in Supplementary Table S1. Pore volume and SSA data result from the same topology of SG, and thus lead to a similar conclusion, i.e. “mesopores” and macropores are the main components of pore volume and SSA, as discussed in the main text.

Supplementary Figure S5. Highly-graphitized graphene membranes of SG. (a ) TEM image of a spacious individual few-layered graphene membrane on copper/carbon grids. (b ) HRTEM image of a few-layered graphene membrane showing well defined hexagonal honeycomb structure. (c ) Electron diffraction patterns of a few-layered graphene membrane. Twelve bright discrete spots compose the set of {110} diffractions of a well-ordered graphitic layer. (d ) I ~V curve of an individual few-layered graphene with thickness of 2 nm based on a device assembled in field effect transistor configuration which conductance was measured to be 48 μS.

Drain Voltage (V)

D r a i n C u r r e n t (A )

a d

a b

Supplementary Figure S6. Pyrolysis process of a glucose-NH4Cl mixture at a heating rate of 4 o C/min:(a) DSC and DTG profiles, (b) FTIR spectra; note that (NH

)2CO3was added instead of

NH4Cl to form glucosylamine to simplify the spectra. Briefly, the pyrolysis of glucose together with NH4Cl follows a Mailard browning process which occurs every day while baking cakes64. The detailed analysis on such reaction process of “sugar blowing” is discussed in Supplementary Note 1.

Supplementary Figure S7. Statistical analysis of different SG structures controlled in “sugar blowing”. (a ) A typical individual cell consisting of polygonal struts and a supported few-layered graphene membrane/wall. The intersections of three bubbles are called as Plateau borders which are actually the struts. The SG structure is defined by the cell size and strut width . (b-d ) Statistics of the cell size, strut width and ratio of cell perimeter to strut width counted from SEM images of Figs. 1a & 4b-d, respectively. The lines are Gaussian fitting curves. Green arrows denote the shift of distribution peaks following the sequence of low-to-high heating rates. Because of the distortion of bubbles, the ratio of cell perimeter to strut width can more accurately characterize the products. These ratios indicate an optimized SG, which was obtained at a 4 o C/min heating rate, with a topological structure of an 80-μm cell size, a 3.5-μm strut width and a 186-μm bubble diameter in average.

c a d

Supplementary Figure S8. Thermal analysis of glucose-NH 4Cl mixture. (a ) TG of a glucose-NH 4Cl mixture compared with pure NH 4Cl and glucose. (b ) Three possible S extents (varying with uncertain initial points a M I ) representing the size of bubbles obtained at the marked heating rates; the detailed discussion about the function S is presented in Supplementary Note 2.

0.1

0.20.30.40.50.3 0.5

B u b b l e -S i z e F u n c t i o n , S

1 o

C/min

4 o

C/min 20 o

C/min

= 0.65

M a s s L o s s

Temperature (o

Supplementary Figure S9.

(a data of an Al electrolytic capacitor is obtained at 4 o C/min was

examined in a two-electrode system because depends on the SSA of an

electrode material 65,66. CV curves of SG capacitive behavior, i.e. more close to a rectangle than M-SG (milled SG), C-SG (compressed SG, which was made through applying 1 MPa pressure force when drying SG electrodes), GP, and AC ones, which again indicates their smaller ESR. (b ) The correlation of specific capacitances versus scan rates. SG electrodes demonstrate higher specific capacitances both at low and high scan rates than others, and a slower scan-rate-dependent capacitance decay. The capacitance decay results from sieving effect on different-size pores and a possible diffusion influence. Microporous “concaves” and surface-attached “mesopores” continuously become inaccessible along with increasing scan rates because of distributed charge storage 67,68; and the penetration depth in certain pores (i.e. responsive surfaces) decreases with

Scan Rate (mV/s)

S p e c i f i c C a p a c i t a n c e

(F /g )

Frequency (Hz)

-Z '' (Ω)

Z' (Ω)

Z '' (Ω)

0.00.20.40.60.8 1.0

-300

-200-1000100200300 GP AC

Al-electrolytic

Cell Voltage (V)

S p e c i f i c C a p a c i t a n c e (F /g )

SG M-SG C-SG

c d

scan rates or current density69. The slower capacitance decay reflects the excellent configuration of largely available connected cavities of the SG for free ion transports. In contrary, conventional AC and GP have their intrinsic drawback, i.e.smaller capacitance at a high current/scan rate, because AC surfaces are embedded within micropores and GP surfaces locate within piled slits due to the agglomeration. This further supports the capacitance versus current discussions related to Fig. 5b. (c,d) Nyquist plots and corresponding imaginary components of impedance of SG, M-SG and C-SG electrodes, with the “knee” frequencies of 11, 4.5 and 8.6 Hz respectively. The inset is a magnified portion. Standard-polished plane-Pt-disk electrodes with a diameter of 3 mm are additionally tested for comparison; 100 times downscaled values are shown. Impedance-frequency fitting is done using a constant phase element for low frequencies and Equation S7 for high frequencies. The fitting lines are drawn in (c), (d) and Fig. 5c. The details are discussed in Supplementary Table S2 and Note 3.

Supplementary Figure S10. High electrochemical performance of SG-based supercapacitors. (a ) Capacitance versus absolute magnitude of the impedance. The data of the Al electrolytic capacitor is amplified by 100 times. C ~|Z | diagram visually shows more discriminating plots than capacitance versus frequency. There exhibits sharp capacitance cutoffs which are caused by the porous characteristics (original or secondary piled pores). As the impedance decreases below the inflexion, stored energy quickly becomes inaccessible. The SG electrodes can last until very small impedance, which is beneficial to achieve a 100% efficiency and a maximum available output. (b ) Corresponding Bode plots. Because of the minimized electronic and ionic resistances of the SG electrode, its characteristic frequency at a phase angle of -45° is 9.6 Hz. Although it is smaller than a supercapacitor of vertically oriented graphene walls 70, it is envisaged that our electrode is a thick-layer one which has no decay in further scaled-up fabrication (ref. 57). (c ) Quick charging and slow discharging of a SG-based supercapacitor. The high-power and relatively high-energy SG-based supercapacitors can operate under quick charging and slow discharging. It was quickly charged for only 2.4 s, which supplied a small consuming current for a decently 10-fold long time of 212 s. This simulates a flash-charged power supply, which is promising for the flash charging of portable electronics.

C e l l V o l t a g e (V )

Time (s)

E q u i v a l e n t C a p a c i t a n c e (

F /g )

|Z| (Ω)

P h a s e A n g l e (o )

Frequency (Hz)

a b

Supplementary Figure S11. A survey of different EDL-type supercapacitors based on 3D graphenes. (a ) A survey of maximum power density versus energy density of aqueous EDL-type supercapacitors based on 3D graphenes; the plots of SG, C-SG and M-SG supercapacitors were calculated by Equations 3 and 5. The details of other supercapacitors are listed in Supplementary Table S3. (b ) Galvanostatic discharge plots of C-SG at marked currents and the correlation of specific capacitances with discharge currents. (c ) Ragone plots of maximum volumetric power density versus volumetric energy density of supercapacitors according to Equations 6-7. When compressing the SG electrodes into C-SG ones, the thickness of active materials becomes to ca. 2% of the original value with the same area. The density is estimated as 0.1 g/cm 3. Although both the resistances resulting from

101010101010

M a x i m u m P o w e r D e n s i t y (W /L )

Energy Density (Wh/L)

10-1

100101102

101

102

103

104

106

M a x i m u m P o w e r D e n s i t y (W /k g )

Energy Density (Wh/kg)

C e l l V o l t a g e (V )

Charge Removed (mAh)