This journal is c the Owner Societies 2013 Phys. Chem. Chem. Phys., 2013, 15, 4799--4803 4799

Cite this: Phys. Chem.Chem.Phys.,2013,15, 4799

Graphene ultracapacitors: structural impacts†

Weixin Song,a Xiaobo Ji,*a Wentao Deng,a Qiyuan Chen,a Chen Shena andCraig E. Banks*b

The structural effects of graphene on the electrochemical properties of graphene-based ultracapacitors

are investigated for the first time, where the competitive impacts resulting from the edge content,

specific surface area, edge/basal defects, oxygen-containing groups and metal oxides/surfactant

impurities are taken into consideration, demonstrating that not one element, but all are responsible for

the final behavior of graphene-based ultracapacitors. This work will be of wide importance to research

producing graphene-based energy storage/generation devices.

Graphene has been considered favorable as a new carbon materialfor significantly enhanced energy storage1–6 such as its reporteduse in rechargeable lithium-ion batteries,7,8 supercapacitors9–11

and fuel cell devices.12 Graphene-based ultracapacitors areextremely attractive as energy densities can be improved withoutsacrificing power density,13 where materials play a critical role inenergy storage devices.14–17 Mechanical exfoliation,18 epitaxialgrowth,19 chemical vapor deposition,3,20 and oxidation-exfoliationreduction of graphite,21,22 to name just a few processes, have beenreported to fabricate graphene to be used in the aforementioneddevices. Notably, graphene exhibits an electrical conductivity ofB64 mS cm�1, approximately 60 times more than that ofSWCNTs,23 and has an extremely high theoretical surface area(2675 m2 g�1),24 twice as large as that of CNTs (1315 m2 g�1),25

showing promise in many applications. Moreover, the unusualband structure of graphene could treat the charge carriers initself as massless Dirac fermions or mimic relativistic parti-cles2,18,26 to present a unique electronic quality. Also it shouldbe noted that the fast heterogeneous electron-transfer rate onthe order of 0.01 cm�1 has been reported at the edges ofgraphene sheets with combination of the defects along thebasal plane.27

Significantly, the intrinsic capacitance of single-layer graphenewas reported to be 21 mF cm�2, equal to 561 F g�1, if thetheoretical surface area (2675 m2 g�1) could be used sufficiently,

setting the upper limit for electrical double layer capacitance for allcarbon-based materials.24 High capacitances of 135 and 99 F g�1

in an aqueous and an organic electrolyte, respectively, have beenachieved using chemically modified graphene, depending on theflexible graphene sheets rather than the rigid porous structure ofactivated carbon to provide the large specific surface area (SSA) ofgraphene.25,28 Meunier and Ajayan et al. showed that the specificcapacitances for a 2D in-plane design of chemical vapor deposition(CVD) graphene and reduced multilayer graphene oxide electrodeswere greatly improved to 80 and 394 mF cm�2.29 However, a varietyof graphene-based materials derived from graphene oxide (GO)exhibited unsatisfactory electrochemical behavior,30 mainly attri-buted to the restacking of graphene sheets during processing thatresulted from the sheet-to-sheet van der Waals interaction.Recently, no-restacking curved graphene,31 solvated graphene32

and laser-scribed graphene9 have been obtained, showing anenhanced performance of 154, 215 and 265 F g�1, respectively.Furthermore, Ruoff et al. demonstrated that the super-highsurface area (3100 m2 g�1) was obtained for graphene oxide viaactivation with KOH assisted by microwaves,33 which is signifi-cantly higher than its theoretical surface area, presenting highvalues of gravimetric capacitance (150–200 F g�1). It wassuggested that the enhanced electrochemical performance ofgraphene was impacted significantly by its 2D environment andthe edge plane of graphene played a more important role thanthe basal plane, considering that the electron transfer kineticsof the edge plane were overwhelmingly dominant over that ofthe basal plane.34 Interestingly, the oxygen-functional groupsat the edges of graphene sheets have caused a controversialelectrochemical influence,10,35 not only on the heterogeneouselectron transfer rate but also on the adsorption/desorption ofmolecules, resulting from the electrochemical reaction.25,36

Previously, we have compared graphene and grapheneoxides for utilization as ultracapacitor electrodes, indicatingthat the former exhibits a larger capacitance than the latter,10,37

a Key Laboratory of Resources Chemistry of Nonferrous Metals,

Ministry of Education, College of Chemistry and Chemical Engineering,

Central South University, Changsha, 410083, China. E-mail: xji@csu.edu.cn;

Fax: +86731 88879616b Faculty of Science and Engineering, School of Science and the Environment,

Division of Chemistry and Environmental Science, Manchester Metropolitan

University, Chester Street, Manchester M1 5GD, Lancs, UK.

E-mail: c.banks@mmu.ac.uk; Fax: +44(0)1612476831

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cp50516b

Received 21st December 2012,Accepted 6th February 2013

DOI: 10.1039/c3cp50516b

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4800 Phys. Chem. Chem. Phys., 2013, 15, 4799--4803 This journal is c the Owner Societies 2013

resulting from the decrease of the oxygen content. We alsonoted that the physicoelectrochemical properties of CNTs aredue to the edge-plane-like defects at the open ends of carbonnanotubes and the hole defects in the tube walls,38–40 whilst theresidual catalyst metal nanoparticles present even after washingmay provide a facile electrochemical reaction,38,41 and theinfluence of oxygenated species on the correlation betweensurface structure and electron transfer reactivity needs to beconsidered.38 Herein, the aim of this current work is to inves-tigate the structural effects of graphene on the electrochemicalproperties of graphene-based ultracapacitors, as well asimprove the understanding of the effective mechanism ofcapacitance behavior of the surface area (edge/basal plane/defects), oxygen groups and residual impurities of graphenesheets produced from the preparation process.

Fig. 1(a) shows the transmission electron microscopy (TEM)images of GO and the perfect continuous morphology withindividual GO sheets observed. Following chemical activationwith 7 M KOH solution, the amorphous GO, called AGO, shows acorrugated surface as observed in scanning electron microscopy(SEM) images (Fig. S1, ESI†) with a dense 3D pore structure anddefects along with highly curved and predominantly atom-thick walls, further corroborated by the high-resolution TEM(HR-TEM) of AGO (Fig. 1b), play a significant role in increasingthe specific surface area (SSA). It is well known that theactivation with alkali (KOH, etc.) has been extensively used to

generate porous structures and high SSA of carbon-basedmaterials, and improved porosity and enhanced supercapacitorperformance were obtained with the activation process, whichis shown below:33,42

6KOH + 2C - 2K + 3H2 + 2K2CO3

Typically, for carbon electrodes, there are two graphite planes:the basal plane, which is an exposed hexagonal surface that isparallel to the graphite layer, and the edge plane, which is wherethe surface is cut perpendicular to the graphite layer as shown onthe top of Fig. 2. As shown in Fig. 2, the GO prepared by theHummers method43 was chemically activated to AGO in situationA (SA) first, with smaller GO fragments and a larger ratio of theedge plane to the basal plane, which mainly underwent situationB (SB) to form the real AGO, with more pore defects and higherSSA. Furthermore, the approximate monocrystalline charac-teristic of GO was transformed to polycrystalline after activationas AGO, as observed from the diffraction rings in Fig. S2 (ESI†),demonstrating that the activated defects are capable enough toalter the crystalline structure of graphene oxides.

The pristine three-dimensional framework of graphite andthe intrinsic characteristics of GO have been changed in AGOwith more defects in the basal and edge planes, leading to anincreasing content of oxygen functional groups. To verify the

Fig. 1 TEM of (a) GO and (b) AGO.

Fig. 2 Schematic of the graphite, following oxidation/exfoliation to grapheneoxide and KOH-activation to amorphous graphene oxide with modified surfacestructure.

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controversy whether the oxygen-containing groups such as thecarbonyl group could decrease the electrochemical perfor-mance of the graphene sheets, which has been reported pre-viously by our group,10 the Reduced GO, called RGO, and theReduced AGO, called RAGO, were fabricated via chemicalreduction by hydrazine monohydrate to investigate the relativeelectrochemical performance (see the experimental section inthe ESI†). The X-ray photoelectron spectra (XPS) of GO, AGO,RGO, RAGO are shown in Fig. 3, and the corresponding C and Oatom ratios listed in Table 1 display the decreasing oxygencontent of 19.43% and 21.16% when the GO and AGO werereduced to RGO and RAGO, respectively. The oxygen content ofAGO and RAGO was higher than that of unactivated GO andRGO, on account of the improved defects with more oxygenfunctional groups during the chemical activation process, inaddition to that generated by chemical reduction.44,45

Also, the Brunauer–Emmett–Teller (BET) SSAs of GO, AGO andRGO were calculated from N2 adsorption isotherms (Fig. S3, ESI†)and are listed in Table 1, with only 9.42 m2 g�1 increase from GOto RGO due to the graphene agglomeration in reduction22 andboth of the low SSA values could also be attributed to the largegraphene sheet plane.46 Surprisingly, the SSA of AGO was15.8 times larger than that of GO on account of the activationeffect producing more pores/edges and small graphene fragments,and the SSA of RAGO might be thus inferred to exhibit a smallincrease compared to that of AGO, reasonably, when referring tothe SSA values of GO and RGO. Notably, as a key characteristic ofgraphene, the SSA could be effectively obtained by the freezedrying method as described in the experimental section.

The electron transfer properties of the as-resulted foursamples were estimated using the redox probe potassium

ferrocyanide in 0.1 M potassium chloride with a modifiedglassy carbon electrode (GC) and the cyclic voltammetric pro-files are shown in Fig. S4 (ESI†), from which the peak-to-peakpotential separation (DEp) values are obtained and listed inTable 1. It is clear that slower electron kinetics of all fourgraphene samples are observed when compared with the bareGC, demonstrating effective blocking of underlying electrontransfer due to the large basal and low edge plane content ofgraphene oxide sheets,34 while the electron transfer from/to agraphene sheet takes place on the edges of the sheets with avalue close to zero of the basal plane.47,48

Also as Pumera et al. have pointed out, the impurities left ingraphene, mainly arising from various preparation processes,including metal oxides or surfactants can affect electrochemicalproperties.49 It should be noted that the large DEp (164 mV) ofGO reflected a slower electron transfer34,37 than RGO (DEp = 103 mV)with a relatively low oxygen content, as the increasing amountof oxygen-containing groups on GO decreased the hetero-geneous electron transfer between the carbon-based materialand ferro/ferricyanide.37 Also, the electron transfer of AGO(DEp = 115 mV) is faster than that of GO, which is mainlyattributed to the activated higher SSA and more edges thanbefore, though the more oxygen-containing groups and defectson AGO have blocked the transfer to some extent. Moreover, thecomparative and high transfer capability of RGO and RAGO(DEp = 109 mV) demonstrated that the biggest influence onelectron kinetics is mainly from the edges created in activationand reduction processes, while the 5.8% oxygen content differ-ence between them had a relatively inconspicuous impact.

To reduce sheet agglomeration and increase the edge planecontent for the purpose of exerting the optimal properties inelectrochemical tests, the as-prepared materials were sonicatedin solution and dropped onto the nickel foams, followed bypressing, to fabricate the working electrode. The cyclic voltam-metric response in Fig. 4(a) displays the comparable capacitivecharacteristics of the four samples, the results of which corre-sponded to the listed capacities in Table 1 calculated fromFig. 4(b) showing nearly rectangular shapes of the cyclicvoltammetry curves except for that of GO, indicating that anefficient electrochemical double layer capacitive behavior hasbeen established.

The specific capacitances (Cm) of the four samples werecalculated based on the formula: Cm = IDt/DVm, where I is thedischarge current, m is the mass of the active materials, DV isthe potential drop during discharge, Dt is the total dischargetime, and the simultaneously obtained values are listed inTable 1. A prominent improvement of Cm can be observed forthe RGO and RAGO obtained by the reduction of GO and AGO,respectively, because of the oxygen-containing groups causingan adverse decrease in capacitance which supports our pre-vious conclusions.10 Interestingly, the SSA of AGO was almost15 times higher than that of GO, which was ascribed to theamorphous surface abundant in defects and edges, giving animproved but less than 15 times larger capacitance than that ofGO (75 F g�1 vs. 16 F g�1), further proving that the oxygen-containing groups have decreased the capacitance while the

Fig. 3 XPS of GO, AGO, RGO and ARGO.

Table 1 Measured properties of GO, AGO, RGO, RAGO

Sample GO AGO RGO RAGO

Carbon atom ratio (%) 69.21 62.52 85.36 81.58Oxygen atom ratio (%) 28.93 36.43 9.5 15.27SSA (m2 g�1) 22.04 347.89 31.46 —DEp (mV) 164 115 103 109Specific capacitance (F g�1) 16.3 75.0 104.4 164.3

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oxygen content of AGO was nearly 10% more than that of GO.Thus, the results that the oxygen functional groups couldproduce increased capacitance35 have ignored the influencesof other factors of SSA, edge defects, etc., for example, asproduced by plasma etching.50 Simultaneously, the decreasingcapacitance effect of oxygen-containing groups could be con-sidered to be inferior to the increasing effect of SSA, while thecompatible results could be observed between RGO and RAGO,as well, corresponding to the results from the electron transferanalyzed above. Moreover, the Cm of RAGO (B164 F g�1) in theNa2SO4 electrolyte could give a comparative result with theactivated microwave exfoliated GO (B165 F g�1) with a SSA of2400 m2 g�1 in the 1-butyl-3-methyl-imidazolium tetrafluoro-borate electrolyte.33 The summarized impact factors affectingthe capacitance of graphene-based material are presented inTable 2, based on the results from experimental comparison ofelectron kinetics and capacitance. Here, we propose that the

specific surface area, the ratio of the edge plane to the basalplane,10,34 defects involving edge defects and pore defects,chemical impurities of metal oxides and surfactant,51,52 oxygen-functional groups10,35 and the underlying supporting electrode34

are all key factors that influence the electrochemistry of grapheneultracapacitors and needed to be considered and controlled.

As can be observed from inspection of Scheme 1, the edgecontent, specific surface area, edge defects and metal oxideimpurities could have a positive impact to optimize the electro-chemical properties, while the basal/pore defects, oxygen-containing groups and surfactant impurities could have a negativeimpact on the properties. Consequently, in order to study thecapacitive behavior of graphene, the competitive structuraleffects described above should be routinely explored in detail.

Acknowledgements

Financial support from the National Natural Science Founda-tion of China (No. 51134007, 21003161, 21250110060), CentralSouth University Annual Mittal-Founded Innovation Project(11MX10) and Fundamental Research Funds for the CentralUniversities of Central South University (2011ssxt086, CL12137)is greatly appreciated.

Notes and references

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Surfactant Negative

Scheme 1 The structure of amorphous graphene oxide.

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