Journal ofMaterials Chemistry A

PAPER

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In situ fabrication

aNovitas, Nanoelectronics center of excelle

Engineering, Nanyang Technological Univer

ntu.edu.sg; Tel: +65 67906783bCINTRA CNRS/NTU/THALES, Nanyang Tec

† Electronic supplementary informa10.1039/c4ta04023f

‡ These two authors contributed equally t

Cite this: DOI: 10.1039/c4ta04023f

Received 5th August 2014Accepted 17th September 2014

DOI: 10.1039/c4ta04023f

www.rsc.org/MaterialsA

This journal is © The Royal Society of

of three-dimensional, ultrathingraphite/carbon nanotube/NiO composite asbinder-free electrode for high-performance energystorage†

Wenwen Liu,‡a Congxiang Lu,‡ab Xingli Wang,a Kun Lianga and Beng Kang Tay*ab

Metal oxides have attracted considerable attention as promising electrode materials for energy storage, but

the use of metal oxides for electrodes still faces challenges such as attaining high capacity, good cycle

stability, and high-rated performance. Therefore, rational design of electrode architectures and

assembling metal oxides into desired structures to further enhance electrochemical performance is

necessary. Here, novel 3D electrode architectures consisting of 3D ultra-thin graphite film (UGF)/carbon

nanotubes (CNTs) uniformly covered by NiO nanosheets are successfully constructed by a chemical

vapor deposition and subsequent electrodeposition, which are directly used as bind-free electrodes for

supercapacitors and Li-ion batteries. In such composite structures, 3D UGF/CNTs serve as substrates for

NiO nanosheet decoration, and act as spacers to stabilize the composite structure, making the active

surfaces of NiO nanosheets accessible for electrolyte penetration and accommodating volume changes

during charge/discharge processes. As expected, 3D UGF/CNTs/NiO as electrode material for

supercapacitors showed high specific capacitance (750.8 F g�1 at current density of 1 A g�1), superior

rate performance (capacitance of 575.6 F g�1 at 10 A g�1) and excellent cycle stability (no decay after

3000 cycles). Moreover, the 3D CNTs/UGF/NiO composite also exhibited enhanced lithium storage

properties as anode materials for Li-ion batteries.

1. Introduction

In response to the rapid depletion of fossil fuels and environ-mental pollution, energy storage and conversion from alterna-tive clean-energy sources have been major challenges.1–4

Recently, rapidly increasing demands for energy storage ofconsumer electronic devices and electric vehicles have alsocalled for novel high-performance energy-storage devices.5,6

Therefore, development of new and highly efficient energy-storage devices is urgent to resolve or alleviate present energyand environmental problems and meet higher demands offuture systems.7–12 Among highly promising candidates, super-capacitors and Li-ion batteries (LIBs) have attracted muchattention due to excellent performance, and have also becomemajor targets of research and development.

Given that energy-storage device performance depends inti-mately on the properties of respective electrode materials,13,14

nce, School of Electrical and Electronic

sity, Singapore 639798. E-mail: ebktay@

hnological University, Singapore 637553

tion (ESI) available. See DOI:

o the current work.

Chemistry 2014

the latter have become increasingly important for new-genera-tion energy conversion and storage devices such as super-capacitors and LIBs. Among various electrode materials, metaloxides (such as NiO,15–17 Co3O4,18,19 SnO2,20,21 Fe2O3,22 Fe3O4,23

MnO2,24,25 and V2O526) have garnered a great deal of attention

because of high theoretical capacities and rich redox reactions.In particular, NiO has stood out as one of the most promisingmaterials due to many remarkable features such as high theo-retical capacities, high chemical/thermal stability, ready avail-ability, environmental friendliness, and low cost.27–30 However,the poor electrical conductivity of NiO limits its rate capabilityfor high-power performance and leads to real experimentalspecic capacitance far below its theoretical value. It is there-fore urgent and necessary to further boost the electrochemicalperformance of NiO.

To date, various strategies have emerged to fabricate NiO-based electrodes with improved electrochemical performance.One effective strategy is to fabricate nanostructured NiO mate-rials by adjusting morphology (nanowires, nanobelts, nano-tubes, nanosheets, nanoakes, nanospheres, nanoowers,nanocone arrays),31–37 specic surface area and pore sizedistribution.38,39 Nanostructured NiO can improve electro-chemical performance by enlarging the contact area betweenthe electrolyte and electrodes, as well as shorten diffusion

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distances, but this function is limited especially at high-ratedischarge and long cycling, probably because of low electricalconductivity, easy agglomeration and large volume changeduring the charge/discharge process. Another effective strategyis to improve electrical conductivity by combining NiO withcarbon materials (such as graphene, carbon nanotubes andacetylene black),40–42 and then enhance electrochemical perfor-mance. Although these two strategies can effectively improvethe electrochemical performance of NiO, there are drawbacks:obtained active materials must be mixed with binding addi-tives, and then coated onto a current collector. Adding polymerbinder not only increases the “dead volume” in electrodematerials, but also affects rate performance.

To transcend the abovementioned limitations of nano-structured NiO-based materials, selecting a proper approach,structure, and combination of materials is paramount.Recently, 3D self-supported nanoarchitecture electrodes haveattracted much attention owing to a combination of benets,including greater surface area in favor of high capacity, stablestructures to benet buffering for volume change andimproving the electric conductivity of electrodes, and loweraggregation and collapse in comparison to nanoparticles.43,44

Currently, the design and optimization of 3D nanostructures forhigh-performance supercapacitors and LIBs are hot topics.44–48

Herein, we present a new method to prepare the 3D nickelfoam/UGF/CNTs/NiO architecture used as electrode materialsfor supercapacitors and LIBs, respectively. Carbon nanotubes(CNTs) and ultra-thin graphite lm (UGF) constitute the twomost exotic classes of functional carbon materials representingone-dimensional (1D) and two-dimensional (2D) nano-structures. Apart from individual properties and applications, ahybrid structure based on UGF and CNTs can have signicanttechnological advantages. Therefore, it can be expected thatnovel 3D nickel foam/UGF/CNTs/NiO architecture electrodeswould show promising potential in energy storage devices suchas supercapacitors and LIBs. The electrochemical resultsdemonstrate that 3D nickel foam/UGF/CNTs/NiO architectureelectrode has superior electrochemical properties includinghigh capacity/capacitance, good cycling stability and excellentrate performance.

2. Experimental

All chemicals were in reagent grade and used without furtherpurication. The synthesis procedure of the novel 3D nickelfoam/UGF/CNTs/NiO composite is schematically illustrated inFig. 1.

Fabrication of CNTs on 3D nickel foam/UGF

3D nickel foam/UGF/CNTs were synthesized via a two-stepchemical vapor deposition (CVD) method with nickel foam asthe substrate and ethanol as the carbon source under atmo-spheric pressure. First, nickel foam with a diameter of 12 mmwas ultrasonically cleaned in acetone, isopropanol and de-ionized water. Subsequently, the nickel foam was transferredinto a quartz tube and kept at 1000 �C for 5 min to clean the

J. Mater. Chem. A

nickel surface under an Ar/H2 atmosphere, and then ethanolvapor was introduced into the quartz tube by bubbling Ar/H2

through ethanol liquid for 10 min. Aer cooling down to roomtemperature, nickel foam/UGF treated by O2 plasma wasmerged into ethanol solution containing 0.1 mol L�1 Ni(NO3)2for 5 min. Finally, CNTs were grown via CVD at 700 �C for10 min.

Synthesis of 3D nickel foam/UGF/CNTs/NiO composite

Prior to electrodeposition, the 3D nickel foam/UGF/CNTs weretreated by O2 plasma to change wettability (Fig. S1 ESI†). Theelectrochemical deposition of nickel hydroxide precursor wascarried out at�0.9 V in 5 mMNi(NO3)2$6H2O electrolyte using aSolartron (1287 + 1260) electrochemical workstation in a stan-dard three-electrode system, where the nickel foam/UGF/CNTs,a platinum plate (2 cm � 2 cm � 0.02 cm) and a saturatedcalomel electrode (SCE) were used as the working electrode, thecounterelectrode and the reference electrode, respectively. Aerelectrodeposition for 15 min, the sample was carefully rinsedseveral times with deionized water and dried at 60 �C in air.Subsequently, the sample was annealed at 300 �C for 2 h with aramping rate of 1 �Cmin�1 to transform the precursor into NiO.The entire fabrication process was clearly proved by the colorchange of nickel foam aer successive growth of UGF, CNTs andNiO (Fig. S2 ESI†). For comparison purposes, nickel foam/UGF/CNTs and NiO nanosheets on nickel foam/UGF (nickel foam/UGF/NiO) were also prepared under the same annealingtemperature in air.

Morphology and structural characterization

Field-emission scanning electron microscope (FESEM, LEO1550 GEMINI) and transmission electron microscopy (TEM,Model JEM-2100) were used to characterize the morphology andstructure of the samples. To prepare the TEM samples, the as-prepared sample was rst scraped and sonicated in de-ionizedwater for 2 min, and then a droplet of the obtained suspensionwas dropped onto a TEM grid, followed by natural drying atambient conditions. The crystal structures of the samples wereanalyzed by powder X-ray diffraction (XRD) using a Bruker D8Advanced X-ray diffractometer with Cu-Ka radiation (l ¼0.15418 nm), recorded with the 2Q ranging from 15 to 70�. X-rayphotoelectron spectroscopy (XPS, Model PHI Quantera SXM)with an Al anode was used for determining the elementalchemical state of the sample. Raman spectra were collected witha WITEC CRM200 Raman System (532 nm laser, 2.54 eV, WITec,Germany).

Electrochemical characterization

Supercapacitor performance measurements. The super-capacitive performances of the 3D nickel foam/UGF/CNTs/NiOcomposite were carried out using a Solartron (1287 + 1260)electrochemical workstation at room temperature in a three-electrode system, whereas a slice of platinum and a standardcalomel electrode (SCE) served as the counter and referenceelectrodes, respectively. The cyclic voltammetry (CV) tests weremeasured with the potential window from 0 to 0.45 V (vs. SCE) at

This journal is © The Royal Society of Chemistry 2014

Fig. 1 Schematics of fabrication process for 3D nickel foam/UGF/CNTs/NiO composite: CVD growth of UGF on nickel foam (step 1), CVDgrowth of CNTs on nickel foam/UGF (step 2), and deposition and annealing treatment of nickel foam/UGF/CNTs/Ni(OH)2 precursor (step 3).

Fig. 2 SEM images of (a) 3D nickel foam/UGF; (b and c) 3D nickelfoam/UGF/CNTs network; and (e, f and g) 3D nickel foam/UGF/CNTs/NiO core–shell structure. The inset in (a) shows high-magnificationSEM image of (a). The inset in (b) shows a low-magnification SEMimage of (b). The inset in (d) shows the element (Ni, O, and C) mapping

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different scan rates varying from 10 to 100 mV s�1. The elec-trochemical impedance spectroscopy (EIS) plots were tested inthe frequency ranging from 100 kHz to 0.05 Hz at open circuitpotential with an AC perturbation of 5 mV. Galvanostaticcharge/discharge tests were measured with the constant currentdensity ranging from 0.5 to 10 A g�1. The experiments wereperformed at room temperature in 6.0 M KOH electrolytesolution. The specic capacitance was calculated from thedischarge curve by the formula:

C ¼ IDt

mDV(1)

The specic energy density and power density were dened,respectively, by

E ¼ CDV 2

7:2(2)

and

P ¼ E � 3600

Dt(3)

where I is the constant discharging current (A g�1), Dt is thedischarge time (s), DV is the potential window during thedischarge process aer the internal resistance drop (V),m is themass of the electrode material (g), C is the specic capacitance(F g�1), E is the energy density (W h kg�1), and P is the powerdensity (W kg�1).

Battery performance measurements. In order to investigatethe lithium storage performances of the sample, 3D nickelfoam/UGF/CNTs/NiO composite was directly assembled into CR2032-type half cells without any binders or conductive additives,while pure Li metal was used as the counterelectrode. Assemblytook place in a glovebox lled with pure Ar (Innovative Tech-nology, Inc.). The electrolyte was LiPF6 (1.0 M) dissolved inethylene carbonate and diethyl carbonate (EC–DEC, 1 : 1 byvolume). Charge–discharge capacities were measured via gal-vanostatic cycling the half cells over the potential range of 0.01–3.0 V (vs. Li/Li+) in a battery tester (NEWARE BTS-5 V, NewareTechnology Co., Ltd.). The CV test was performed using aSolartron (1287 + 1260) electrochemical workstation.

This journal is © The Royal Society of Chemistry 2014

3. Results and discussionsMorphological and structural characterization

Scanning electron microscopy (SEM) was used to illustrate thestructural and morphological properties of the as-grown nickelfoam/UGF, nickel foam/UGF/CNTs network, and nickel foam/UGF/CNTs/NiO core–shell nanocomposites, respectively. Fig. 2adisplays the representative SEM image of UGF on nickel foamwith the ripples and wrinkles generated from different thermalexpansions. Using UGF as the catalyst support for CVD growthof CNTs, a dense CNTs mesh was grown coaxially and uniformly

of marked area in (d).

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Fig. 3 TEM images of (a) UGF, (b) UGF/CNTs, and (c and d) UGF/CNTs/NiO composite. Corresponding (e) SAED pattern and (f) EDS spectrumof the UGF/CNTs/NiO composite. Inset in (b) shows a high-magnifi-cation TEM image of CNTs. Inset in (d) shows the corresponding high-magnification TEM images of marked area in (d).

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around the UGF skeleton (Fig. 2b). CNT diameters were mainlyin the range of 10–60 nm (Fig. 2c), while the length could reachtens of micrometers. Interestingly, the CNTs were distinctlyinterconnected with each other and self-assembled into 3Dnetwork architecture, which is benecial for the uniformdecoration of active material onto the CNTs. It should bementioned that the CNTs were still well-adhered to the nickelfoam/UGF substrate even aer being ultrasounded at 200 W for10 min (Fig. S3 ESI†), revealing the robust mechanical adhesionbetween the CNTs and UGF. Therefore, the material may be areliable binder-free integrator for the nickel foam/UGF and insitu-grown CNTs to serve as a composite current collector. Asshown in Fig. 2d, the surface of the 3D nickel foam/UGF/CNTsnetwork is homogeneously covered by NiO nanosheets, which isfurther conrmed by the uniform distribution of Ni, O and Ccomponents over all 3D nickel foam/UGF/CNTs/NiO nano-composites. The corresponding high-magnication images(Fig. 2e and f) reveal that each CNT is uniformly anchored andwrapped by thin NiO nanosheets, forming a CNTs/NiO core–shell structure. More importantly, a large void space betweenneighboring CNTs/NiO core–shell nanorods remained, which isadvantageous for facile access of electrolyte ions, especially at ahigh charge–discharge rate during electrochemical tests.

Extensive TEM analysis was carried out to further investigatethe microstructural feature of bare UGF, UGF/CNTs and UGF/CNTs/NiO composite. Herein, the TEM images of the above-mentioned three samples are peeled off from nickel foam. Asclearly evidenced by the structure at the edge, the UGF consistsof several layers of graphene (Fig. 3a), revealing its ultrathinbody. Fig. 3b shows typical TEM images of the UGF/CNTs. It canbe clearly observed that the root regions of CNTs directlyconnect to the UGF, assisting in the formation of strong adhe-sion between the CNTs and UGF. The high-resolution TEM(HRTEM) image inset in Fig. 3b shows that the CNTs con-structed by several tens of carbon layers are characteristicallymultiwalled CNTs.45 TEM images of the UGF/CNTs/NiOcomposite (Fig. 3c) exhibit that the CNTs are tightly bonded andtotally covered with ultrathin NiO nanosheets (Fig. 3c), forminga typical CNTs/NiO core–shell architecture, which agrees wellwith the SEM images. The HRTEM image (Fig. 3d) displays theperiodic fringes of UGF, CNTs and NiO nanosheets. The insetimages in Fig. 3d show that the adjacent lattice fringes are 0.20nm and 0.24 nm corresponding to the (111) and (200) latticespaces of spinel NiO,30,42 respectively. Moreover, the selected-area electron diffraction (SAED) pattern (Fig. 3e) exhibits well-dened diffraction rings, indicating the crystalline character-istics of the NiO phase in the UGF/CNTs/NiO composite. Theserings can be readily indexed to the (111), (200), (220), and (311)planes of the NiO,30 highly consistent with the XRD results.Additionally, the energy-dispersive X-ray spectrometry (EDS)spectrum (Fig. 3f) shows only peaks of C, Ni and O except for theCu signal originating from the copper grid, demonstrating thehigh purity of the UGF/CNTs/NiO composite.

XRD and Raman spectroscopy are well-known instrumentsfor characterizing the structure of samples. As shown in Fig. 4a,with the exception of the reections owing to nickel foam, allpeaks are indexed to the nal UGF/CNTs/NiO composite. The

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diffraction peaked around 26.5� is the characteristic peak ofcarbon (UGF/CNTs), while the diffraction peaks at 37.2�, 43.5�,and 62.3� matched well with the crystal planes of the (111), (002)and (312) of NiO (JCPDS card no. 04-0835) as reported previ-ously.37,49 The relative broad diffraction peaks reveal that NiOnanosheets in UGF/CNTs/NiO composite are of relatively lowcrystallinity. Moreover, no impurity peaks were found in theXRD pattern, indicating that the as-synthesized samples havehigh phase purity. Fig. 4b presents the Raman spectra of barenickel foam/UGF, nickel foam/UGF/CNTs and nickel foam/UGF/CNTs/NiO composite. For bare nickel foam/UGF, two prom-inent peaks at 1560 and 2700 cm�1 correspond to the charac-teristic G and 2D bands of defect-free UGF,42,50–53 respectively.Moreover, the integrated intensity ratio between the G and 2Dbands (IG/I2D) revealed that the UGF was composed of few layergraphene domains. In comparison with the Raman spectrum ofbare nickel foam/UGF, the Raman spectrum of the 3D nickelfoam/UGF/CNTs exhibited typical characteristic D, G and 2Dpeaks of multiwalled carbon nanotubes.52,53 Moreover, theRaman spectrum of nickel foam/UGF/CNTs/NiO composite notonly presents the characteristic D, G and 2D peaks of UGF andCNTs, but also shows several new peaks around 508 cm�1,which were ascribed to the characteristic modes of crystallineNi–O lattice vibrations in the 3D nickel foam/UGF/CNTs/NiOcomposite.42,54

To give an in-depth understanding of the crystalline struc-ture of the 3D nickel foam/UGF/CNTs/NiO composite, X-ray

This journal is © The Royal Society of Chemistry 2014

Fig. 4 (a) XRD pattern and (b) Raman spectra of 3D nickel foam/UGF/CNTs/NiO composite. The inset in (b) shows corresponding Ramanspectrum of marked region in (b).

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photoelectron spectroscopy (XPS) measurements are under-taken to investigate the chemical state of elements in the as-synthesized products. The survey XPS spectrum (Fig. 5a)certies the presence of Ni, O, and C without other elements inthe composite, which is consistent with the above EDX and XRDresults. As shown in Fig. 5b, the peaks located at 852.2–867.6 eVwith a main peak and two satellite peaks, and the peaks located

Fig. 5 XPS spectra of 3D nickel foam/UGF/CNTs/NiO composite: (a) su

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at 870.3–884.8 eV with a main peak and a satellite peak wereattributed to the Ni 2p3/2 and Ni 2p1/2 spin–orbit levels ofNiO,55,56 respectively. Apart from these peaks, an additionalfeature peak appeared at 855.7 eV as a small shoulder, whichwas presumably due to the presence of a small quantity of Ni2O3

on the surface.55 Fig. 5c shows the deconvoluted C 1s spectrawith different oxygen containing functional groups, including

rvey scan, (b) Ni 2p, (c) C 1s, and (d) O 1s.

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the non-oxygenated C in C]C/C–C at 284.5 eV, the carbon inC–O/C–O–C at 285.6 eV, and the carbon in C]O/O–C]O at288.4 eV.42,57 Furthermore, there are two peaks at 529.4 eV and530.7 eV for the O 1s spectrum (Fig. 5d). According to previousreports, the binding energy peak at 529.4 eV was ascribed to thetypical metal–oxygen bond of Ni–O/C–O–Ni,42,57 and the bindingenergy peak at 530.7 eV was attributed to Ni–OH/C]O,42,57

including defective nickel oxide with hydroxyl groups absorbedon its surface. Based on the abovementioned SEM, TEM, XRD,and Raman results, the 3D nickel foam/UGF/CNTs/NiOcomposite with 1D quasi-aligned CNTs/NiO core–shell structurethat tightly connected with nickel foam/UGF backbones hasbeen successfully constructed, which is favorable for developinghighly conductive and high-performance electrodes.

Supercapacitor performance

Herein, as-prepared 3D nickel foam/UGF/CNTs/NiO compositewas directly used as electrode material without the introductionof any polymer binders, and its electrochemical performanceswere characterized by cyclic voltammetry (CV), galvanostaticcharge/discharge measurements, and electrochemical imped-ance spectroscopy (EIS). Fig. 6a shows the representative CVcurves of 3D nickel foam/UGF/CNTs/NiO composite with thepotential window of 0.0–0.45 V in 6.0 M KOH solution atdifferent scan rates. The CV curve shows a pair of strong redoxpeaks at 0.19 V and 0.33 V, indicating the pseudocapacitivebehavior of the 3D nickel foam/UGF/CNTs/NiO composite fromfaradaic redox reactions between NiO and NiOOH in alkalinesolution as follows:32,58,59

NiO + OH� 4 NiOOH + e� (4)

Moreover, along with the increase of scan rates from 10 to100 mV s�1, the anodic peaks shied positively while thecathodic peaks shied negatively, which were attributed to thepolarization of the electrode at the higher scan rates.60,61

Meanwhile, a linear dependence was established between thepeak currents (Ipa (anodic peak current) and Ipc (cathodic peakcurrent)) and square root of scan rates from 10 to 100 mV s�1

(Fig. 6b), revealing that the redox reaction of 3D nickel foam/UGF/CNTs/NiO composite in KOH electrolyte was mainly underthe diffusion-controlled step.62,63 In addition, as shown inFig. S4 (ESI†), the 3D nickel foam/UGF/CNTs/NiO compositeexhibited the largest area surrounded by CV curve among nickelfoam/UGF/CNTs, nickel foam/UGF/NiO composite, and nickelfoam/UGF/CNTs/NiO composite, indicating the limited contri-bution of pure nickel foam/UGF/CNTs to the capacitance of 3Dnickel foam/UGF/CNTs/NiO composite and signicantlyenhanced capacitance due to the unique 3D structure.

To further examine supercapacitive performances, a series ofcharge–discharge measurements were performed on 3D nickelfoam/UGF/CNTs/NiO composite electrode at various charge–discharge current densities, as shown in Fig. 6c. Unlike the caseof electric double-layer capacitors with linear charge–dischargecharacteristics, the charge–discharge curves of 3D nickel foam/UGF/CNTs/NiO composite electrode exhibit the typical

J. Mater. Chem. A

pseudocapacitive behavior, which agrees well with the CVresults. On the basis of the discharge curves (Fig. 6d), thespecic capacitances of 3D nickel foam/UGF/CNTs/NiOcomposite electrode were 815.5, 750.8, 675.3, 620.7, 595.1, and575.6 F g�1 at the current densities of 0.5, 1, 2, 4, 7, and 10 A g�1,respectively. The result was comparable or better than those ofthe reported NiO-based composite materials, such as hierar-chical porous NiO nano/micro superstructures (710 F g�1 at 1 Ag�1),17 NiO nanosheet arrays on Ni foam (674.2 F g�1 at 1 Ag�1),28 3D NiO/graphene composite (555 F g�1 at 1 A g�1),29 3Dporous graphene macroassembly/NiO hybrid (646 F g�1 at 0.5 Ag�1),57 and 3D NiO/ultrathin derived graphene hybrid (425 F g�1

at 2 A g�1).61 Additionally, when the charge–discharge currentdensity was increased 20-fold from 0.5 to 10 A g�1, about 71% ofthe original capacitance was retained, demonstrating good rateperformance of the 3D nickel foam/UGF/CNTs/NiO compositeelectrode. As shown in the Ragone plot (Fig. 6e), the powerdensity increased from 100.6 W kg�1 to 2007.3 W kg�1 and theenergy density decreased from 18.1 W h kg�1 to 12.4 W h kg�1,respectively, as the discharge current density increased from 0.5A g�1 to 10 A g�1.

Since long cycling life is another very crucial parameter inevaluating electrode materials for supercapacitors, the cyclestability of 3D nickel foam/UGF/CNTs/NiO composite is inves-tigated by repeating the charge–discharge test at a currentdensity of 5 A g�1 in 6 M KOH solution (Fig. 6f). Notably, thespecic capacitance is retained at 660 F g�1 aer 3000 cycles,corresponding to 110% of its original value. The slight increaseof capacitance is ascribed to the activation effect of electro-chemical cycling, suggested by previous reports of other NiO-based electrode materials.49,63 The cycling performance of 3Dnickel foam/UGF/CNTs/NiO composite was better than previousreports of NiO-based composites.17,28,29,57 These encouragingresults show that the 3D nickel foam/UGF/CNTs/NiO compositehas high specic capacitance and excellent cycling stability,which can be used as electrode material in high-performancesupercapacitors.

Such intriguing capacitive performances of 3D nickel foam/UGF/CNTs/NiO composite can be largely attributed to thesuperior 3D conducting network constructed by UGF and CNTs,and the pseudocapacitive behavior of the active material (NiOnanosheets). First, the 3D nickel foam/UGF/CNTs networkoffers a large material/electrolyte contact area and a shortdiffusion distance for electrolyte ions as well as better electricalcontact between NiO and current collector. Second, as anexcellent host material, CNTs not only can provide a goodelectrical conducting path for charge transfer and redox kineticsof the deposited layer (NiO nanosheets), but also serve as theuniform nucleation sites for NiO and offer more electrochemi-cally active sites by effectively inhibiting the aggregation of NiO.Moreover, ultrathin NiO nanosheets uniformly deposited on theCNTs network can signicantly improve the kinetics and theelectrochemical utilization of NiO due to the extraordinarilyshortened ion diffusion and transport length (Fig. S5 ESI†). Inaddition, the good electrical conductivity of CNTs directlygrown in situ on the UGF with robust adhesion can ensure closecontact and effective electron transport between the charge

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Fig. 6 Electrochemical characterization of supercapacitive performances of 3D nickel foam/UGF/CNTs/NiO composite: (a) CV curves at variousscan rates ranging from 10 to 100 mV s�1; (b) corresponding plot of anodic and cathodic peak currents vs. square root of scan rates; (c) charge/discharge curves at various current densities; (d) corresponding capacitance as function of current density; (e) Ragone plot of estimated energydensity and power density at various charge/discharge current densities; and (f) cycling performance at current density of 5 A g�1. The inset in (f)shows CV curves of 1st, 1000th, 2000th and 3000th cycles.

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collecting substrates of nickel foam/UGF/CNTs and the NiOnanosheets, and avoid the addition of extra additives andconductive binders.

Li-ion battery performance

In addition to supercapacitive performance, the lithium storageproperties of the 3D nickel foam/UGF/CNTs/NiO compositewere also evaluated. Fig. 7a displays the CV curves for the rstthree cycles of the 3D nickel foam/UGF/CNTs/NiO compositeelectrode at a scan rate of 0.1 mV s�1 in the range of 0.01–3.0 V.In the rst cathodic scan curve, a shoulder at around 0.89 V andan intensive reduction peak centered at 0.51 V corresponded,respectively, to the initial reduction of NiO to metallic Ni

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nanoparticles, the formation of amorphous Li2O (NiO + 2Li+ +2e� / Ni + Li2O), and the formation of a partially reversiblesolid electrolyte interface (SEI) layer.30,36,37,64,65 However, thecathodic peak at 0.51 V shied to 0.98 V during subsequentcycles, which may be due to the drastic lithium driven, struc-tural, or textural modied cations.30,37 The two broad peakslocated at about 1.34 and 2.24 V in the rst anodic scan curvewere attributed to partial decomposition of the polymericcoating on the NiO surface, and decomposition of Li2O andoxidation of metallic nickel to NiO (Ni + Li2O / NiO + 2Li+ +2e�), respectively.30,36,37,64 Well-known mechanisms for theabovementioned processes are the reversible reaction of NiO +2Li + 2e�4Ni + Li2O and partial composition/decomposition of

J. Mater. Chem. A

Fig. 7 Lithium storage properties of 3D nickel foam/UGF/CNTs/NiO composite: (a) CV curves for first three cycles measured at scan rate of 0.1mV s�1 between 0.01 V and 3 V; (b) voltage capacity profiles at a current density of 100 mA g�1; (c) rate capabilities at different current rates; and(d) specific capacities and coulombic efficiency versus charge/discharge cycle number at current density of 200 mA g�1.

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the SEI layer.30,36 Moreover, the CV curves were stable and welloverlapped aer the second cycle, indicating high electro-chemical reversibility and good capacity retention for the 3Dnickel foam/UGF/CNTs/NiO composite electrode.

As shown in the initial discharge curve (Fig. 7b), the voltagerapidly falls from 3.0 to 1.1 V, followed by a long plateau regionat about 0.73 V, which is relevant to the reduction of NiO tometallic Ni and the formation of SEI layers. Moreover, thespecic plateau between 0.12 and 0.07 V is due to the insertionof lithium ions into UGF/CNTs.65,66 In the rst charge prole,there were three plateaus located at 0.1, 1.6 and 2.2 V, whichoriginated from the extraction of lithium ions from UGF/CNTs,the partial decomposition of the SEI layer, and the formation ofNiO from Ni, respectively.30,36,37,64–66 These results obtained fromthe rst charge–discharge curve were highly consistent withthose of CV. The initial discharge and charge capacities are1359.4 and 963.3 mA h g�1, respectively, and the irreversiblecapacity loss of 29.1% was mainly attributed to possible irre-versible processes such as electrolyte decomposition and inev-itable formation of a polymer-like SEI layer.30,36,37 It is worthnoting that the initial plateau and slope in the second andsubsequent cycles are less obvious than the rst cycle, alsosuggesting an irreversible capacity loss in the rst cycle.

High rate performance is another important precondition toapply as anode material in high-power LIBs. Therefore, the as-prepared 3D nickel foam/UGF/CNTs/NiO is examined withdifferent current densities and results are given in Fig. 7c. Thedischarge capacities of the 3D nickel foam/UGF/CNTs/NiO at100, 200, 500, 800, 1000, 1500, and 2000 mA g�1 are 1143.1,

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1097.3, 998.5, 919.9, 867.5, 786.4, and 716.8 mA h g�1, respec-tively, demonstrating an excellent high rate performance. Itshould be noted that as long as the current rate reversed back tothe low current rate (100 mA g�1), the cell capacity recovered tothe original value, indicating that the 3D nickel foam/UGF/CNTs/NiO composite is tolerant of various charge and dischargecurrents. As shown in previous studies, high rate performanceis greatly dependent on rapid ionic and electronic diffusion andtransport.30,37,42,47 In our case, the 3D network structure provided3D electron-conducting channels within the electrode, andfacilitated the penetration and diffusion of electrolyte, leadingto fast lithium ion transport. In addition, the thin NiO nano-sheets signicantly shortened the diffusion distance betweenlithium ion and electron. Herein, it should be mentioned thatthe capacity contributed from the Li+ lithiation/delithiationwith pure nickel foam/UGF/CNTs was negligible, which can beclearly proved by its specic capacity value at different currentdensities (Fig. S6 ESI†).

Fig. 7d displays the cycling performance of the 3D nickelfoam/UGF/CNTs/NiO composite at a current density of 200 mAg�1. The electrode still maintained a specic capacity of 995.3mA h g�1 (100.9% of the initial capacity) at the current densityof 200 mA g�1 aer 100 cycles, which is larger or comparable tothe previously reported values for NiO-based nanomaterials,such as mesoporous NiO achieving a capacity of 680 mA h g�1

aer 50 cycles at 0.1 C,16 hollow NiO nanotubes with a capacityof 600 mA h g�1 at 200 mA g�1 aer 100 cycles,33 mesoporousNiO nanosheet networks with a capacity of 1043 mA h g�1 at 0.2C aer 80 cycles,37 NiO nanosheets/graphene composite with a

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capacity of 1000 mA h g�1 aer 50 cycles at 50 mA g�1,42 NiO onvertically aligned, multiwall carbon nanotube arrays with acapacity of 864 mA h g�1 at 0.2 C aer 50 cycles,64 NiO–Ni/natural graphite with a capacity of 532 mA h g�1 at 0.15 C aer100 cycles.65 Meanwhile, coulombic efficiency was close to 100%in the overall battery operation, indicating the excellent cyclestability of 3D nickel foam/UGF/CNTs/NiO composite electrode.In addition, it was interesting to note that the slight increaseand then reduction in specic capacity from 2nd cycle to 55thcycle may be due to reversible growth of a polymeric gel-likelayer originating from kinetically activated electrolyte degrada-tion and interfacial lithium storage, which is common for mostanode materials.30,36

Based on previously discussed electrochemical perfor-mances, the excellent lithium storage properties of the 3Dnickel foam/UGF/CNTs/NiO composite in terms of remarkablerate capability and stable cycling performance can be attributedto rationally designed nanostructure and composition. Speci-cally, the 3D network structure, created by CNTs/NiO core–shellnanorods growth on the 3D nickel foam/UGF, enabled sufficientelectrode/electrolyte contact area for high Li+ ion ux across theinterface and reduced Li+ ion diffusion length, which promotedthe electrochemical processes. Moreover, the local voidsbetween the CNTs/NiO core–shell nanorods could efficientlyaccommodate the volume change during repeated charge–discharge cycling, thus leading to improved cycling stability.

4. Conclusion

In summary, we present the rational design and fabrication ofan interesting 3D nickel foam/UGF/CNTs/NiO composite via atwo-step process comprised of chemical vapor deposition andsubsequent electrodeposition. The as-obtained 3D nickel foam/UGF/CNTs/NiO composite can be used as a binder-free elec-trode for supercapacitors and LIBs. The unique architectures,structures and morphologies have advantages such as excellentelectrical contact between active materials and currentcollector, numerous active reaction sites and a facile ion diffu-sion path provided by the large open spaces between neigh-boring CNTs/NiO core–shell nanorods, and signicantlyreinforced mechanical strength, enabling their promisingperformance in supercapacitors and LIBs. Serving as a free-standing electrode for supercapacitors, 3D nickel foam/UGF/CNTs/NiO composite exhibits a high capacitance of 750.8 F g�1

at the current density of 1 A g�1, an outstanding rate perfor-mance (capacitance of 575.6 F g�1 at 10 A g�1) and excellentcycling stability. Moreover, the 3D nickel foam/UGF/CNTs/NiOcomposite as anode material for LIBs shows a reversiblecapacity of 1143.1 mA h g�1 at a current density of 100 mA g�1,retaining 100.9% of the initial discharge capacity aer 100cycles at a current density of 200 mA g�1. In addition, thisstrategy may be extended to synthesize other transition metaloxides with 3D carbon-based nanoarchitectures, which are verypromising for use in high-performance supercapacitors, LIBs orcatalysts because of their unique structural features.

This journal is © The Royal Society of Chemistry 2014

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