DOI: 10.1002/chem.201300157

Hybrid Hydrogels of Porous Graphene and Nickel Hydroxide as AdvancedSupercapacitor Materials

Sheng Chen,[a] Jingjing Duan,[a] Yonghong Tang,[b] and Shi Zhang Qiao*[a]

Introduction

Supercapacitors, also known as ultracapacitors or electro-chemical capacitors, are power devices capable of beingfully charged and discharged in seconds; thus they are po-tentially used in the applications requiring short-term powerboosts, such as emergency doors, hybrid electrical vehiclesand uninterrupted power sources.[1] The critical element of asupercapacitor is the material of its electrode, which directlydetermine their capacitances, delivery rates and stability. Be-cause traditional electrode materials, such as active carbons,hydrous RuO2 and polyaniline,[1a] do not meet the stringentrequirement of supercapacitors owning to their sluggishelectrode kinetics, high price or inferior electrochemical sta-bility, it is a major topic of interest to exploit efficient super-capacitor materials with high performance and low cost tocater the ever-increasing demands for sustainable energystorage.

Graphene is a state-of-the-art carbon material with excep-tional mechanical and electrical properties, along with ahuge surface area (theoretically �2630 m2 g�1),[2] suggestingit as an ideal component for developing high-performance

supercapacitors. Interestingly, the two-dimensional graphenesheets can self-assemble into macroscopic hydrogels struc-tures without involving any binders or linking agents.[3]

Apart from the intrinsic properties of graphene, such hydro-gel structures are featured by large out-of-plane pores, self-contained hydrophilic groups (such as carboxyl, epoxyl orhydroxyl groups) and conductive frameworks, thus they arepromising candidates as binder-free electrodes for superca-pacitors. Nevertheless, further exploiting the performance ofgraphene hydrogels meets an insurmountable challenge thatgenerally appears in many graphene-based systems, that is,the large aspect ratio of graphene sheets may compromisethe kinetics of processes occurring in electrodes. Graphenesheets are usually several micrometers in size with a highmechanical durability; therefore, electrolyte ions have to cir-cumvent large graphene sheets during the charging–dis-charging processes. The elongated ion transport path lengthsinevitably lead to impaired electrode kinetics. Moreover, be-cause their charging–discharging mechanism merely relieson physical adsorption and charges arrangement, the capaci-tance of graphene hydrogels still underperforms in compari-son with other electrode materials such as transition-metaloxides/hydroxides, which involve reversible faradic reactionsfor charge storage. It is desirable to further enhance both ki-netics and capacitances for graphene-hydrogels-based elec-trodes.

It is known that the performances of materials are signifi-cantly affected by their structures, especially their porestructures.[4] In graphene hydrogels, the partially restackingin graphene sheets can create large out-of-plane pores thatare beneficial for access to electrolytes. Interestingly, recentstudies reveal another kind of pores, that is, in-plane pores,

Abstract: Graphene-based hydrogelscan be used as supercapacitor electro-des because of their excellent conduc-tivity, their large surface area and theirhigh compatibility with electrolytes.Nevertheless, the large aspect ratio ofgraphene sheets limits the kinetics ofprocesses occurring in the electrode ofsupercapacitors. In this study, we haveintroduced in-plane and out-of-planepores into a graphene–nickel hydroxide(Ni(OH)2) hybrid hydrogel, which fa-cilitates charge and ion transport in the

electrode. Due to its optimised chemis-try and architecture, the hybrid elec-trode demonstrates excellent electro-chemical properties with a combinationof high charge storage capacitance, fastrate capability and stable cycling per-formance. Remarkably, the Ni(OH)2 inthe hybrid contributes a capacitance as

high as 3138.5 Fg�1, which is compara-ble to its theoretical capacitance, sug-gesting that such structure facilitateseffectively charge-transfer reactions inelectrodes. This work provides a facilepathway for tailoring the porosity ofgraphene-based materials for improvedperformances. Moreover, this work hasalso furthered our understanding in theeffect of pore and hydrogel structureson the electrochemical properties ofmaterials.

Keywords: electrochemistry · gra-phene · mesoporous materials ·nanostructures

[a] S. Chen, J. Duan, Prof. S. Zhang QiaoSchool of Chemical Engineering, The University of AdelaideAdelaide, SA 5005 (Australia)E-mail : s.qiao@adelaide.edu.au

[b] Dr. Y. TangCentre for NanoScale Science and Technology, andSchool of Computer Science, Engineering, and MathematicsFlinders University. Adelaide, SA 5042 (Australia)

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201300157.

� 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2013, 19, 7118 – 71247118

which can be generated on graphene sheets.[5] Both experi-ments and theoretical simulations suggest enhanced trans-port performances in graphene-based materials with in-plane pores.[5,6] For example, rational generation of in-planepores in a graphene film has considerably increased its lithi-um-ions storage capabilities and the ion diffusion kinetics.[6a]

Therefore, provided that the in-plane porosity could be in-troduced into graphene-hydrogel systems, the electrolyteions might be capable of directly going through graphenesheets for fast charge transfers, and reducing the ion trans-port path lengths between electrodes and electrolytes. Thus,the development of hierarchical porosity in graphene hydro-gels by combining both in-plane and out-of-plane poresshould be promising for achieving enhanced electrode kinet-ics in supercapacitors. To the best of our knowledge, the re-ports on graphene hydrogels with both in-plane and out-of-plane pores are rare.

Transition-metal oxides/hydroxides, such as Ni(OH)2, NiOand Co3O4, have been widely explored for producing super-capacitors with a high theoretical capacitance and cost effec-tiveness.[7] Nevertheless, its capacitance is severely compro-mised in bulk electrodes because they are too insulating tosupport effective charge-transfer reactions. Moreover, theircycling performances are inferior relative to other electrodematerials because of the gradual loss of electroactive speciesduring the operation of supercapacitors. Recent studies sug-gest that various carbon materials, such as active carbons,[8]

carbon nanotubes[9] and graphene,[10] can improve both thecapacitance and the cycling capability of Ni(OH)2 due totheir high electrical conductivity and chemical stability. Con-sidering the multiple advantages of porous graphene hydro-gels in comparison with other carbon materials, includingenhanced transport property, mechanical flexibility as wellas a number of residue oxygen-containing functional groups(�COOH and �OH), which are potential anchoring sites forin situ assembly of nanoparticles, it is worthwhile to exploitthe possibility of combining Ni(OH)2 and porous graphenehydrogels for a boosted performance as an advanced super-capacitor material.

Herein, we report the fabrication of a hierarchicallyporous graphene–Ni(OH)2 (hGN) hybrid hydrogel, withboth out-of-plane macropores and in-plane mesopores in itsstructure. The key aspect of this study is the use of a holeygraphene as a precursor, in addition to the intrinsic out-of-plane pores of the hydrogel, which can substantially improvethe accessible surface area and ion transport performance inelectrodes. The hybrid hydrogel exhibits excellent electro-chemical performances with high capacitance, excellent ratecapability and stability. Remarkably, Ni(OH)2, as the mainelectroactive species in electrodes, contributes a capacitancealmost approaching its theoretical value. The excellent per-formance of hGN originates from the synergistic effect ofporous graphene and Ni(OH)2, and the hierarchical porosityof the hybrid hydrogels.

Results and Discussion

The hGN was obtained by the hydrothermal treatment of amixture of holey graphene oxide and Ni ACHTUNGTRENNUNG(NO3)2, in which atransparent hG solution changed into the black hGN hydro-gel as illustrated in Figures 1 a and S1 in the Supporting In-formation. Scanning electron microscope (SEM) images(Figure 1 b) reveal the large out-of-plane macropores inhGN, ranging from several to tens of micrometers. This sce-nario is different from the preparation of previously report-ed graphene-based samples, such as graphene films or pow-ders,[1e, 11] where their out-of-plane pores are much smaller.Control experiments reveal that graphene alone withoutnickel salt also formed similar porous structure (Figure S1cin the Supporting Information), suggesting that the p–p re-stacking between graphene sheets is responsible for formingsuch kind of pores.

With regard to the in-plane pores, because the hGN hy-drogel contains a large amount of water (�80 wt %), its po-rosity cannot be directly examined by nitrogen adsorption,which is only applicable to dried samples; thus we examineits freeze-dried counterpart by nitrogen adsorption to get arough idea about its porosity. Interestingly, its nitrogen ad-sorption resembles the type IV isotherm characteristic (Fig-ure 1 c), with the mesopores sizes covering the range fromseveral to tens of nanometres as estimated by the Barrett–Joyner–Halenda (BJH) method (Figure S2 a in the Support-ing Information);[12] its Brunauer–Emmett–Teller (BET) sur-face area is estimated to be 81 m2 g�1 and its pore volume is0.23 cm2 g�1.

Methlylene blue (MB) adsorption experiments (Figur-es 1 d and S2 b in the Supporting Information) were furtherconducted to provide additional insight into the surfaceareas of hGN. MB is a commonly used dye probe for evalu-ating the surface area of graphitic materials and each milli-gram of adsorbed MB occupies 2.45 m2 of the surfacearea.[1f, 13] The amount of the adsorbed MB was probed byUV/Vis spectroscopy (Figure S2b in the Supporting Infor-mation), which reveal that hGN possesses a surface area of602 m2 g�1, higher than its non-pores counterpart (GN,469 m2 g�1), and 3.7 times higher than its freeze-dried coun-terpart (d-hGN, 164 m2 g�1, Figure 1 d). The less MB adsorp-tion amount of hGN than that of individual hG(828.8 m2 g�1) suggests that Ni(OH)2 has occupied some sur-face parts in the hybrid structure. The different surface areavalues of freeze-dried hGN obtained by N2 adsorption(81 m2 g�1) and MB adsorption experiments (164 m2 g�1) areaccounted by their discrepant testing mechanisms. General-ly, the nitrogen adsorption experiment is based on the rever-sible adsorption of nitrogen molecules onto solid surfaces,whereas the MB experiment is taking advantage of the ad-sorption of the MB dye onto graphitic carbons. Therefore,they reveal different aspects of hGN, and thus demonstratediscrepant surface area values.

The definite evidence of the in-plane pores of hGN is ob-tained by its high-resolution transmission electron micro-scope (TEM) images (Figure 1 e), which show the presence

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of mesopores of several nanometres to tens of nanometreson the graphene sheets. Comparing this image with that ofGO or of holey graphene oxide (Figure S1 a and b in theSupporting Information), one can judge that these poresshould originate from rationally etching of graphene sheetsin the current procedure. The aforementioned nitrogen ad-sorption isotherms, MB adsorption experiments and TEMimages clearly show evidence of the in-plane and out-of-plane pores in hGN hydrogels structures, which may offerlarge accessible areas for effective charge storage, leading toenhanced electrode kinetics.

On the other hand, the TEM image in Figure 1 f revealsthat hGN is composed of sheet-like graphene and Ni(OH)2

particles with the size of tens of nanometres. Such an obser-vation is remarkably consistent with other analyses inFigure 2, including powder X-ray diffraction (XRD), Fouriertransform infrared (FTIR) spectra, thermogravimetric analy-

sis (TGA), Raman and X-rayphotoelectron spectra (XPS),where the plots of hGN con-tain features of both holey gra-phene and Ni(OH)2.

The XRD pattern of hG ex-hibits a pronounced peakaround 2q=208 due to the par-tially restacking of the gra-phene sheets in the hG hydro-gel structure. The XRD profileof hGN (Figure 2 a) is similarto that of pure Ni(OH)2

(JPCDS card no. 38-0715) witha relatively weakened (002)peak for hGN, revealing that itcontains graphene andNi(OH)2 as main components.

The FTIR spectrum (Fig-ure 2 b) of hG shows the typi-cal characteristic of carbon ma-terials, mainly including C=Cstretching vibration around n=

1600 cm�1, whereas the FTIRspectrum of hGN containsboth vibrations of hG (C=C)and Ni(OH)2 (Ni�O). Thisanalysis is consistent with itsRaman spectrum (Figure 2 c),in which the D and G bandsaround n=1350 and 1580 cm�1

correspond to disordered andgraphitic carbon atoms, respec-tively. There is a decrease inthe D/G peak intensity ratio ofhGN in comparison with hG,indicating that Ni(OH)2 parti-cles grew preferentially on thedefect sites of the hG sheets.

Moreover, the XPS survey(Figure 2 d) suggests that hGN contains C, O and Ni as themain elements, and the corresponding high-resolution C1XPS spectrum (Figure 2 e) signifies a number of functionalgroups on the holey graphene sheets, such as hydroxyl (C�OH), epoxyl (C�O�C) and carboxyl (�COOH) groups. Aswith many other graphene-based materials,[10, 14] these func-tional groups interacted with Ni(OH)2 by various interac-tions, such as covalent chemical bonding, van der Waalsforces and hydrogen bonding, thereby enabling the directgrowth of Ni(OH)2 on the holey graphene sheets and lead-ing to the hybrid hydrogel structure. The direct growth ofNi(OH)2 on holey graphene sheets is believed to impart theeffective electros transport from Ni(OH)2 to the underlyinggraphene sheets, which helps to improve the overall electri-cal conductivity in electrodes.

To determine the actual mass ratio of each component inhGN, we conducted TGA analyses (Figure 2 f). The TGA

Figure 1. A) Optical images of the formation of hGN. B) SEM images of freeze-dried hGN showing large out-of-plane macropores. C) Nitrogen adsorption–desorption isotherm (expressed in [cm3 STP g�1]) for freeze-dried hGN. D) Surface areas of hGN probed by the methlylene blue (MB) adsorption. E and F) TEM imagesof hGN, showing that the hybrid contains holey graphene sheets and Ni(OH)2 particles.

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plot of hGN is different from that of hG due to the exis-tence of Ni(OH)2 in the hybrid material. The continuousweight loss of hGN from 200 to 500 8C originates from thephase transition of Ni(OH)2 into NiO. The weight loss ofhG, Ni(OH)2 and hGN is 4.1, 14.2 and 34.4 wt %, respective-ly. According to the weight loss, the proportion of hG andNi(OH)2 in the hGN composite was estimated, that is, themass ratio of hG/Ni(OH)2 in hGN is approximately two.

The electrochemical performances of hGN were exam-ined to understand the role of the hierarchical porosity inthe hybrid electrodes. Similar to previous reports,[15] the hy-drogels were deposited into nickel foams (NF) and directlyused as the working electrode without any binder or conduc-tive additives (such as carbon black) (Figure S3 in the Sup-porting Information). The cyclic voltagrammetry (CV) plots(Figure 3 a) revealed similar redox peaks for hGN, GN and

d-hGN corresponding to thereversible reaction ofNi(OH)2+OH�!NiO(OH)+-

H2O+e�. However, hGNshowed the highest specific ca-pacitance (Cs) of 1250.3 F g�1

in comparison with other sam-ples (Figures 3 b and S4 in theSupporting Information),which is also higher than holeygraphene–Ni(OH)2 hybrid hy-drogels with other Ni(OH)weight percentages (for exam-ple, 1044.3 F g�1 for graphene/Ni(OH)2 = 2:3, 950.6 Fg�1 forgraphene/Ni(OH)2 = 2:5). Re-markably, the Cs of hGN isalso competitive with othercarbon–Ni(OH)2 composites,such as active carbons–Ni(OH)2 or carbon nanotubes–Ni(OH)2. Considering that theCs of individual graphene (hG)is 280.3 Fg�1 and the graphene/Ni(OH)2 mass ratio is approxi-mately two in hGN, Ni(OH)2

in the hybrid hydrogel contrib-utes a Cs as high as3138.5 F g�1, comparable to thetheoretical capacitance ofNi(OH)2 (�3300 F g�1), whichmeans almost all the Ni(OH)2

active material took part infaradic reactions for chargestorage. A further insight intothe CV plots reveals nearlylinear relationships betweenthe anodic current densitiesand the scan rates (Fig-ure S4b–d in the SupportingInformation), suggesting an in-

terfacial control in the charging–discharging process in theelectrodes, probably due to the largely separated graphenesheets with both in-plane and out-of-plane pores, which arereadily accessible to electrolyte ions in the hGN hydrogels.

The fast ion diffusion in hGN was further confirmed byits high rate capability and low resistance. The hGN exhibitstypical pseudo-capacitive behaviours in its discharge curvesat different current densities (Figure 3 c), which is consistentwith the CV results. In comparison with other samples (Fig-ures 3 c and d as well as Figure S5 in the Supporting Infor-mation), hGN has the highest Cs value the range of the cur-rent density from 5 to 40 A g�1, with the most capacitanceretention (for example 50.1 % for hGN and 41.9 % for GN).In the electrical impedance spectra (EIS) (Figure 3 e), theWarburg-type line (the slope of the 458 portion of thecurve) of hGN is shorter than that of GN, suggesting a

Figure 2. Structural characterisation of hGN. A) XRD profiles of hGN (top) and hG (bottom). Note that theXRD intensity of hGN was doubled for reliable comparison. B) FTIR spectra of hGN (top) and hG (bottom).C) Raman spectra of hGN and hG. D) XPS survey of hGN. E) High-resolution XPS spectra C1s of hGN.F) TGA plots of hGN (black curve), hG (dark grey curve) and Ni(OH)2 (light grey curve).

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faster ion diffusion process. Also, the internal resistance ofhGN is less than that of GN, as indicated by the high fre-quency intercept along the x axis in the EIS plots. Becausethey have similar components and structures, the better per-formance of hGN can only be attributed to the additionalin-plane pores. Moreover, further EIS analyses show a de-creased internal resistance of hGN after freeze dry (Fig-ure S6 in the Supporting Information), which is explainedby more restacking in graphene sheets through p–p interac-tions during dehydration.

Furthermore, as expected, hGN exhibited excellent elec-trochemical stability during long time cycling, with a capaci-tance loss less than 5 % after 1000 cycles, which is in greatcontrast with 37.1 % capacitance loss for Ni(OH)2 (Figur-es 3 f and S7 in the Supporting Information), illustrating thathGN hydrogels are promising electrodes for supercapacitors.

The electrochemical experiments unambiguously suggestthat the porous hybrid hydrogels have huge advantages aselectrodes for energy storage devices. Firstly, the capaci-

tance of the hGN hydrogel isdramatically increased (Figu-ACHTUNGTRENNUNGres 3 a and b). The hierarchicalpores allow the electrolyte ionsto be easily accessible to thehybrid electrodes, which isbeneficial to accommodatingmore ions for the reversiblefaradic reactions of Ni(OH)2.The residue oxygen-containingfunctional groups (such as�COOH, C�O�C and �OH)in hGN are believed to playdual roles here. On the onehand, these hydrophilic groupscan further improve the com-patibility between the electro-des and the electrolyte, therebythe electroactive species, suchas Ni(OH)2, can be easily infil-trated by electrolyte ions forcharge-transfer reactions.Moreover, due to the presenceof these functional groups, thewater molecules can absorbonto the graphene sheetsthrough chemical adsorptionsor hydrogen bonding and soon, acting as a soft “spacer” toprevent the graphene sheetsfrom restacking through p–p

interactions, thereby resultingin the highly opened pores inthe electrodes and a high ca-pacitance. On the other hand,these groups can act as anchor-ing sites to interact with nano-particles, enabling Ni(OH)2 to

directly growth on the graphene hydrogels. These intimatebindings afford facile electron transports through Ni(OH)2

to graphene, and the conductive framework of hGN furtherfacilitates electrons to the current collectors, leading to lowelectrical resistances.

Secondly, the hierarchical pores on the graphene sheetscan enhance the rate capability of the hybrid electrodes(Figures 3 c and d). Thanks to the in-plane porosity of hGN,electrolyte ions are less blocked by graphene sheets in elec-trodes. Especially in high charging–discharging rates, thehGN electrodes can achieve higher charge mobility than itsnon-pores counterpart (GN), resulting in an enhanced ratecapability. Moreover, the use of conductive graphene hydro-gel as a substrate for growth of Ni(OH)2 can impart theelectrodes with excellent conductivity. The low resistances inthe electrodes allow rapid and effective electrons transport,which may also contribute to high rate performances.

Thirdly, the hybrid structure is stable with a long term cy-cling (Figure 3 f). The capacitance loss of Ni(OH)2 electro-

Figure 3. Electrochemical characterisation of hGN. A and B) CV plots and corresponding capacitance (CS) ofhGN and other samples at 10 mV s�1 (d-hGN =dark grey curve, hGN = light grey curve, GN =black curve).C) Discharge plots of hGN at different current densities. D) Rate capability of hGN, GN and d-hGN. E) Elec-trical impedance spectra of hGN (*) and GN (*). F) Cycle life of hGN and Ni(OH)2.

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des is mainly caused by phase transformation and structuraldegradation, which leads to less utilisation of active materi-als. In hGN, Ni(OH)2 particles can be protected by the adja-cent graphene sheets, which offer high mechanical durabilityand surface areas to inhibit the degradation and agglomera-tion of Ni(OH)2 particles. Moreover, the 3D structure of thehybrid hydrogels can also improve the cycle life by provid-ing stable conductive networks with high mechanical rigidi-ty, which may afford elastomeric space to accommodatevolume changes.

Conclusion

In summary, we have prepared a new structure of a gra-phene–Ni(OH)2 hybrid hydrogel featured by its hierarchicalporosity. The in-plane and out-of-plane pores are beneficialfor the accessibility to electrolytes in electrodes, thereby sig-nificantly improving the electrochemical performances.Moreover, the holey graphene–Ni(OH)2 hybrid hydrogelcan be easily prepared without laborious operations, thuseasily extended to the preparation of many other graphene-based structures containing various oxides such as Co3O4,SiO2 and Cu2O for a broad range of technological applica-tions in adsorption, catalysis, photovoltaic and energy stor-age.

Experimental Section

Graphene oxide (GO) was prepared from natural graphite by themethod of Hummers,[16] which was exfoliated by ultrasonication(�30 min) to give a homogeneous aqueous dispersion (�2 mg mL�1). Inorder to prepare porous graphene oxide, the graphene oxide dispersion(100 mL) was vigorously stirred, while potassium permanganate(KMnO4, 500 mg) was introduced rapidly. The mixture was kept in a cov-ered beaker under ambient conditions for 2 h. Next, hydrochloric acid(HCl, 10 mL) and hydrogen peroxide (H2O2, 10 mL) were added conse-quently. The as-generated product was collected after reacting for 3 h,and then washed with de-ionised (DI) water and ethanol, respectively.The concentration of the as-prepared holey graphene oxide was deter-mined by comparing its UV/Vis absorbance to that of a GO solutionwith known concentration, and then dispersed in water again at a concen-tration of 2 mg mL�1 by ultrasonication (�30 min). The formation of in-plane pores on the graphene oxide sheets was confirmed by TEM images(Figures S1a and b in the Supporting Information).

To synthesise the hGN hybrid hydrogel, holey graphene oxide dispersion(1.5 mL) was firstly mixed with an aqueous solution of Ni ACHTUNGTRENNUNG(NO3)2 (30 mL,100 mg mL�1). Then a small amount of N-methyl-2-pyrrolidone (NMP)(0.2 mL) was added to the mixed solution to promote the hydrolysis ofNi ACHTUNGTRENNUNG(NO3)2 into Ni(OH)2, The mixture was then loaded into a Teflon-linedstainless steel autoclave and heated at 150 8C for 12 h. After being cooledto room temperature, the composite hydrogel was dialysed for two daysto remove remaining salts and impurities. For the comparison purpose,hGN was freeze-dried to create its dried counterpart (d-hGN). The gra-phene–Ni(OH)2 (GN) composite hydrogel was prepared through a simi-lar procedure by replacing holey graphene oxide with graphene oxide.Holey graphene (hG) hydrogel or Ni(OH)2were prepared similarly byusing only holey graphene oxide or Ni ACHTUNGTRENNUNG(NO3)2.

XRD analysis was performed on a Rigaku MiniFlex 600 diffractometeroperation at 40 KV and 15 mA with CoKa radiation. Raman spectra wererecorded on an iHR550 Raman microscope from HORIBA scientific

with a 532 nm solid laser as excitation source. TGA was performed on aTGA/SETARAM thermogravimetric analyser operating from 50 to700 8C at a heating rate of 10 8C·min�1 under a N2 flow. XPS was record-ed on an Axis Ultra (Kratos Analytical, UK) XPS spectrometer equip-ped with an AlKa source (1486.6 eV). FTIR spectra were recorded on aNicolet 6700 spectrometer. UV/Vis spectra were obtained by using aSHIMADZU UV-2600 spectrophotometer. Morphologies of the as-ob-tained products were observed on a TEM (Philips CM-100) and SEM(XL-30).

MB adsorption experiments were used to estimate the accessible surfaceareas of samples.[1f,13] Specifically, the samples were put into a MB solu-tion in ethanol (0.1 mg mL�1) and were left at 25 8C for 48 h to allow theaccessible surface of the samples to be maximally covered by the MBmolecules. The amount of the adsorbed MB was calculated from thechange of the concentration of MB in the solution by UV/Vis spectrosco-py.

Moreover, BET surface area and pore volumes were evaluated by usingnitrogen adsorption-desorption isotherms measured at 77 K on a TriStarII 3020 Micrometrics apparatus. The BET specific surface area was calcu-lated by using the adsorption data at a relative pressure range of P/P0 =

0.05–0.30. Pore size distribution was derived from the adsorption branchby using the BJH method. The total pore volume was estimated from theamounts adsorbed at a relative pressure (P/P0) of 0.99.

Electrochemical tests were conducted on a three-electrode cell configura-tion at room temperature. The working electrodes were fabricated by di-rectly depositing hydrogels on NF. Specifically, a piece of NF sheet wasfirstly cleaned by ultrasonication in acetone, ethanol and DI water, re-spectively. Next, the NF was immersed into the reaction solution, fol-lowed by sonication for 30 min to ensure the successful filling of themixed solution into the micropores of the NFs. Then the mixture was hy-drothermally treated at 150 8C for 12 h and dialysed for two days. Themass of the active electrode material was in a range of approximately1.5–3.6 mg. Before each electrochemical test, the prepared electrode wassoaked overnight in a 1 m KOH solution. Electrochemical characterisa-tion was carried out by using a three-electrode cell with 1m KOH aque-ous solution as the electrolyte. Platinum foil and a Ag/AgCl electrodewere used as the counter and reference electrodes, respectively. All elec-trochemical measurements were conducted on a CHI 760B electrochemi-cal workstation (Shanghai CH Instrument Company, China).

The average specific capacitance (Cs) was evaluated from the cyclic vol-tammetric (CV) curves according to Equation (1).

Cs ¼1

mv Vf � Vi

� �ZVf

Vi

IðVÞdV ð1Þ

where m is the mass of the active electrode material, n is the scan rate, Vf

and Vi are the potential limits of the voltammetric curve and I(V) is thevoltammetric current. The Cs values obtained from discharge curves werecalculated according to Cs = I/[m ACHTUNGTRENNUNG(dV/dt)] by using the discharge curves,where I is the constant discharge current, m is the mass of the samplesand dV/dt can be obtained from the slope of the discharge curve given bythe instrument.

EIS were recorded under the following conditions: AC voltage ampli-tude=5 mV, frequency range =105-0.1 Hz and open circuit.

Acknowledgements

This work is financially supported by the Australian Research Council(ARC) through the Discovery Project programs (DP1095861, DP0987969and DP130104459).

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Received: January 16, 2013Published online: March 28, 2013

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