Electrochimica Acta 203 (2016) 21–29

Three-dimensional hierarchical interwoven nitrogen-doped carbonnanotubes/CoxNi1-x-layered double hydroxides ultrathin nanosheetsfor high-performance supercapacitors

Jian Wua,b,1, Wei-Wei Liua,1, Yu-Xuan Wua, Ting-Cha Weia, Dongsheng Gengb, Jun Meib,Hao Liub,*, Woon-Ming Laua,b,*, Li-Min Liua,*aBeijing Computational Science Research Center, Beijing 100193, ChinabChengdu Green Energy and Green Manufacturing Technology R&D Center, Chengdu Development Center of Science and Technology of CAEP, Chengdu,Sichuan, 610207, China

A R T I C L E I N F O

Article history:Received 8 February 2016Received in revised form 28 March 2016Accepted 7 April 2016Available online 8 April 2016

Keywords:Layered double hydroxidesCarbon nanotubesNitrogen dopingSupercapacitorFirst-principles calculations

A B S T R A C T

Three-dimensional (3D) interwoven nitrogen-doped carbon nanotubes (N-CNTs)/CoxNi1-x-layereddouble hydroxides (CoxNi1-x-LDHs) ultrathin nanosheets on Ni foam have been rationally designedvia the combination of chemical vapor deposition and electrochemical deposition approaches. TheCoxNi1-x-LDHs nanosheets are uniformly distributed on the N-CNTs, which can not only serve as thestable frame to improve the specific surface area, but also can enhance the conductivity. The Ni foam/N-CNTs/Co0.5Ni0.5-LDHs nanosheets electrode displays a remarkable maximum capacitance (2170 F g�1 at1 A g�1 and 1.62 F cm�2 at 1 mA cm�2), excellent rate capability (80.9% specific capacitance retention at20 A g�1 and 75.8% areal capacitance retention at 30 mA cm�2) and good cycling stability. First-principlescalculations further reveal that the Co doping can effectively reduce the band gap of Ni(OH)2 and increasethe conductivity. This work demonstrates a facile synthesis strategy of 3D hierarchical Ni foam/N-CNTs/CoxNi1-x-LDHs nanosheets electrode with remarkable electrochemical properties, which can be used inenergy storage and conversion.

ã 2016 Published by Elsevier Ltd.

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1. Introduction

Supercapacitors or electrochemical capacitors, which possessfast charge/discharge rate, long durability and high power density,are very promising next-generation energy storage devices [1,2].Based on the different charge storage mechanisms, supercapaci-tors can be classified into two categories: electrical double-layercapacitors (EDLCs) and pseudocapacitors [3]. Pseudocapacitorsstore energy through fast surface redox reactions and show muchhigher capacitance and energy density than EDLCs which aredominated by electrostatic charge diffusion and accumulation atthe interface of the electrode/electrolyte [4,5]. Common materialsfor pseudocapacitors are conducting polymers, transitional-metaloxides and hydroxides [6–9]. Early research focused on the noble

* Corresponding authors.E-mail addresses: mliuhao@gmail.com (H. Liu), llau22@uwo.ca (W.-M. Lau),

limin.liu@csrc.ac.cn (L.-M. Liu).1 Jian Wu and Wei-Wei Liu contributed equally to this work.

http://dx.doi.org/10.1016/j.electacta.2016.04.0330013-4686/ã 2016 Published by Elsevier Ltd.

metal oxides (RuO2 and IrO2) as pseudocapacitor materials, but theextremely high cost limited their practical use. Therefore, it isnecessary to find alternative pseudocapacitors materials withsuperior electrochemical properties but at a low cost. Of thesealternative materials, the group of transitional metal-basedlayered hydroxides has been widely explored.

Transitional metal-based layered hydroxides (M(OH)x, M = Ni,Co, Mn, Al, etc.) have captivated wide attention in lithium ionbatteries, dye-sensitized solar cells, catalysts, and particularlysupercapacitors owing to their high redox actively, sufficientinterspacing, low cost and environmental friendliness [10–12].Although the layered hydroxides exhibit high capacitance forsupercapacitors, their durability is poor due to their low electricalconductivity and severe polymorphic transformation during thecharge-discharge cycles [13,14]. Therefore, considerable work hasbeen devoted to improving the overall performance of super-capacitors mainly based on modifications of the nanostructure andcombinations of different conductive materials. For example,among numerous morphologies, two-dimensional (2D) ultrathinnanosheets (NSs) were proven to present improved

22 J. Wu et al. / Electrochimica Acta 203 (2016) 21–29

electrochemical performance because of their increased activesites, reduced diffusion path and improved electrical conductivity[15,16]. Recently, it was reported that the electroactive sites andthe electrical conductivity of a layered hydroxide could be furtherenhanced by a partial isomorphous substitution with othertransitional metal ions to form layered double hydroxides,products which are a family of layered anionic clays [17,18]. Forinstance, the NiCo-LDHs, through the incorporation of Co into Ni(OH)2, show improved electrochemical performances due to themore diversified valence interchange or charge hopping betweenCo and Ni cations [19,20]. To further increase the electron transfer,NiCo-LDHs are often directly grown on conductive substrates orcoated on other materials, such as carbon fiber paper [21], stainlesssteel sheet [22], Ni foam [23], Zn2SnO4 nanowires [24] and TiNnanotubes [25]. However, these strategies cannot provide bothhigh electronic conductivity and large surface areas, two factorswhich are important to improve supercapacitive performances.

An alternative substrate to the ones mentioned above is carbonnanotubes. Carbon nanotubes (CNTs), as the typical nanocarbonmodel, have large aspect ratio, excellent electronic conductivityand high activated surface area [26,27]. Results showed thatelectrochemical performances can be enhanced when CNTs areemployed as the conductive matrix to grow NiCo-LDHs [28,29].Substitutional heteroatom doping, especially nitrogen doping, hasbecome an extensive focus on various carbon materials to modifytheir electronic structure, chemical and other properties [30–32].Due to the additional p-electrons derived from N with excessivevalence electrons, nitrogen-doped carbon nanotubes (N-CNTs)show improved surface energy and surface reactivity than CNTswhile maintaining excellent electrical conductivity [33,34]. Mean-while, N-CNTs not only can provide a strong backbone forhomogeneous growth of electrode materials with enhancedsurface area, but also can immobilize their surface metallicparticles [35,36]. In our previous study, dene N-CNTs can be grownuniformly on Ni foam by a chemical vapor deposition method [36–38]. By entangling each other, the N-CNTs construct a 3Dconducting network with greatly enhanced surface area. Directlyused for Li-O2 batteries, the 3D N-CNTs/Ni foam exhibits highdischarge capacity and improved cycle performance [37]. Further-more, by electrodepositing materials homogeneously on thesubstrate, the binder-free electrodes show excellent performancein lithium ion batteries [38] and supercapacitor [36] applications.

Taking the above discussions into consideration, growingtransitional metal-based double layered hydroxides on the 3DN-CNTs/Ni foam is an effective way to create a large surface areaand high electrical conductivity for the supercapacitor electrode.Moreover, when the growth is achieved by using a electrodeposi-tion method, the process is simple and economical as well as easyto control. Herein, we report a facile way to design a novel 3D Nifoam/N-CNTs/CoxNi1-x-LDHs ultrathin nanosheets electrode by achemical vapor deposition method and followed by an electrode-position process. The CoxNi1-x-LDHs electrodes exhibit extremelyimproved capacitances than either Co or Ni hydroxide alone. Amaximum capacitance of 2170 F g�1 at 1 A g�1 and 1.62 F cm�2 at1 mA cm�2 can be reached; meanwhile, 1756 F g�1 (80.9% specificcapacitance retention) at 20 A g�1 and 1.23 F cm�2 (75.8% arealcapacitance retention) at 30 mA cm�2 can be maintained. Evenafter 5000 cycles at a high current density of 10 A g�1, the specificcapacitance of the 3D Ni foam/N-CNTs/Co0.5Ni0.5-LDHs is still1080 F g�1. Density functional theory (DFT) calculation reveals thatthe band gap of CoxNi1-x(OH)2 are reduced when partial nickel issubstituted with cobalt. The excellent electrochemical perfor-mance makes the as-prepared Ni foam/N-CNTs/Co0.5Ni0.5-LDHsNSs a viable electrode material for energy storage and conversionsystems.

2. Experimental

2.1. Synthesis of N-CNTs on Ni foam

The N-CNTs on Ni foam were produced from a floating catalystchemical vapor deposition (FCCVD) process, as reported before[37,39]. In brief, the Ni foam (2 � 1 cm) was ultrasonically cleanedin acetone, 3 M HCl solution, deionized water (DI water), andethanol in sequence for 20 min each. The pretreated Ni foam as thesubstrate to prepare N-CNTs was placed into a horizontal quartztube system. Before the CVD growth of N-CNTs, argon gas wasintroduced into the horizontal quartz tube at a flow rate of800 sccm for 20 min to expel the air in the tube. Then, the furnacewas heated to 900�C at a rate of 30�C and the target temperaturewas kept for 15 min. In the constant temperature time, theethylene gas was employed as the carbon source, and the ferroceneand melamine vapor were brought into the tube as the catalyst andnitrogen source, respectively. After the growth of N-CNTs for15 min, the ethylene gas was turned off and the whole systemcooled down to room temperature in the flowing argon gas.

2.2. Synthesis of 3D Ni foam/N-CNTs/CoxNi1-x-LDHs NSs hierarchicalstructure

The 3D Ni foam/N-CNTs were employed as the scaffold for thegrowth of CoxNi1-x-LDHs via simple electrodeposition. During theelectrodeposition process, the above prepared 3D Ni foam/N-CNTs, asaturated calomel electrode (SCE) and a Pt foil were used as theworking, the reference and the counter electrodes, respectively. Acathodic electrodeposition was conducted at a constant cathodiccurrent of 2 mA cm�2 for 10 min at 25�C in a 2 mM Co(NO3)2�6H2Oand 2 mM Ni(NO3)2�6H2O mixed electrolyte with different Co2+/Ni2+

concentration ratios of 0:1, 1:2, 1:1, 2:1 and 1:0 using a CHI760Eelectrochemical workstation. After electrodeposition, the substrateswere taken off and rinsed several times with DI water, and finallydried at 60�C. On average, the mass loading of CoxNi1-x-LDHs is about0.90 mg cm�2.

2.3. Material Characterizations

The samples were characterizedby X-ray diffraction (XRD, D/max2200/PC, Rigaku, 40 kV, 20 mA, Cu Ka radiation, l = 1.5406 Å), ascanning electron microscopy (SEM, Hitachi S-5200) and a highresolution transmission electron microscope (HRTEM, FEI, TecnaiF20) equipped with energy dispersive X-ray spectroscopy elemen-tal mapping. The elemental information and the valence state ofthe samples were investigated by an X-ray photoelectronspectroscopy (XPS, ESCALAB 250Xi, Al Ka,150 W). The specificsurface area was obtained by N2 adsorption and desorptionisotherms at 77 K using the Brunauer-Emmett-Teller (BET) method(Autosorb-iQ).

2.4. Electrochemical measurements

Electrochemical measurements were carried out using theCHI760E electrochemical workstation in a three-electrode systemat room temperature. Ni foam/N-CNTs/CoxNi1-x-LDHs NSs was usedas the working electrode, a standard Hg/HgO electrode as thereference electrode and a Pt foil as the counter electrode. 2 M KOHsolution was used as the electrolyte. The cyclic voltammetry (CV)curves were measured at various scan rates (5–40 mV s�1) between0-0.70 V (vs. Hg/HgO), and the galvanostatic charge/discharge testswere performed with the potential window from 0 to 0.50 V (vs. Hg/HgO) at different current densities (1–20 A g�1). Meanwhile, the

Fig. 1. (a) XRD patterns of the CoxNi1-x-LDHs NSs (x = 0, 0.33, 0.5, 0.67, 1) grown on carbon fiber paper (CFP) to avoid the strong background substrate of Ni foam. (b) Crystalstructure of CoxNi1-x-LDHs NSs.–!>

J. Wu et al. / Electrochimica Acta 203 (2016) 21–29 23

galvanostatic charge/discharge tests were also performed with thepotential window from 0.05 to 0.55 V (vs. Hg/HgO) for the Nifoam/N-CNTs/Ni(OH)2 NSs. The electrochemical impedance spec-troscopy (EIS) measurement was performed in the frequency range0.01Hz–100 kHz with a amplitude of 5 mV at open circuit potential.The specific capacitance and areal capacitance were calculated from

Fig. 2. Low (a) and high (b) magnification SEM images of the N-CNTs on Ni foam. (c) TEMCo0.5Ni0.5-LDHs NSs on Ni foam. (f) TEM image of the N-CNTs/Co0.5Ni0.5-LDHs NSs. (g) HCo0.5Ni0.5-LDHs NSs.

the discharge curves by the equations:

C ¼ IDtmDV

or C ¼ IDtSDV

ð1Þ

where C is the specific capacitance (F g�1) or areal capacitance(F cm�2), I (A) is the discharge current, DV (V) is the potential range

image of the N-CNTs. Low (d) and high (e) magnification SEM images of the N-CNTs/AADF STEM image and (h) elemental mapping of C, N, Ni and Co on the N-CNTs/

24 J. Wu et al. / Electrochimica Acta 203 (2016) 21–29

within the discharge time Dt (s), S (cm2) is the area of the workingelectrode, m (g) is the mass of CoxNi1-x-LDHs NSs.

3. Results and discussion

The formation process of the 3D hierarchical interwoven N-CNTs/CoxNi1-x-LDHs NSs on Ni foam is illustrated as follows. First ofall, 3D interwoven N-CNTs were grown on Ni foam through a one-step CVD method. Secondly, ultrathin CoxNi1-x-LDHs NSs withdifferent Co2+/Ni2+ concentration ratios of 0:1, 1:2, 1:1, 2:1 and1:0 were deposited on the Ni foam/N-CNTs substrate via a simpleelectrodeposition method.

In the electrodeposition process, NO�3 will be reduced on the

cathodic surface, where OH� ions will be produced as the electriccurrent passes through the Co(NO3)2�6H2O and Ni(NO3)2�6H2Omixed electrolyte with different Co2+/Ni2+ concentration ratios.Due to the similar solubility product constant of Co(OH)2 and Ni(OH)2, the uniform precipitation of mixed Co/Ni hydroxide on thesurface of Ni foam/N-CNTs will be accompanied at the workingelectrode. Therefore, the whole electrodeposition process isexpressed as follows [14,40].

NO�3 þ 7H2O þ 8e� ! NHþ

4 þ 10OH� ð2Þ

xNi2þ þ 2xCo2þ þ 6xOH� ! NixCo2x OHð Þ6x ð3Þ

Fig. 3. XPS spectra of (a) Survey scan and (b) N 1s for the N-C

The XRD patterns of the as-prepared CoxNi1-x-LDHs NSs arepresented in Fig. 1a. The diffractions peaks at 2u value of 11.2�,33.5� and 59.3� can be identified, which correspond to therhombohedral phase of both Co(OH)2 and Ni(OH)2, and areassigned to the (003), (100) and (110) planes, respectively[41,42]. It was hard to distinguish between the two phases ofa-Co(OH)2 and a-Ni(OH)2 due to their similar structures and veryclose diffraction peaks. Therefore, the CoxNi1-x-LDHs sample can beidentified as a-CoxNi1-x-LDHs. Fig. 1b shows that nickel ions areoctahedrally surrounded by hydroxyls forming Ni(OH)6 octahedra,and partially substituted by cobalt ions in the CoxNi1-x-LDHssystem which maintains the hydrotalcite-like structure as thephase of a-Co(OH)2 or a-Ni(OH)2 [10,43,44].

Fig. 2a shows a representative SEM image of a dense N-CNTsnetwork covering the surface of the Ni foam. A high magnificationSEM image in Fig. 2b reveals that these N-CNTs grow entangledwith each other and present a bamboo-like structure, forming a 3Dporous mesh. The curvature of the basal planes is furtherevidenced by the TEM image (Fig. 2c). Through a simple in situgrowth, ultrathin CoxNi1-x-LDHs NSs can uniformly cover theskeleton of the N-CNTs (Fig. 2d and 2e). Furthermore, the 3Dnetwork structure with large void spaces among theN-CNTs/CoxNi1-x-LDHs NSs on the Ni foam is still maintainedwithout collapse or aggregation (Fig. S1). The TEM image (Fig. 2f)shows that the gauze-like and crosslinked CoxNi1-x-LDHs NSs aregrafted throughout the longitudinal axis of the N-CNTs, showing atypical core-shell heterostructure. TEM results also demonstrated

NTs. (c) Co 2p and (d) Ni 2p for the Co0.5Ni0.5-LDHs NSs.

Fig. 4. Electrochemical characterizations of the Ni foam/N-CNTs/CoxNi1-x-LDHs NSs electrodes (x = 0, 0.33, 0.5, 0.67, 1): (a) Schematic diagram of these electrodes forsupercapacitor in KOH solution. (b) A comparison of CV curves at a constant scan rate of 20 mV s�1. (c) A comparison of charge and discharge curves at a current density of1 A g�1. (d) Specific capacitances as a function of current density. (e) Areal capacitances as a function of current density. (f) Impedance Nyquist curve of the 1st cycle.

Fig. 5. Cycling performance of the Ni foam/N-CNTs/CoxNi1-x-LDHs NSs electrodes(x = 0, 0.33, 0.5, 0.67, 1) at a high current density of 10 A g�1 for 5000 cycles.

J. Wu et al. / Electrochimica Acta 203 (2016) 21–29 25

that the thickness of the nanosheets is about several nanometers,and the selected area electron diffraction (SAED) indicates thepolycrystalline nature of the nanosheets (Fig. S2).

The high-angle annular dark-field scanning transmissionelectron microscopy (HAADF-STEM) was employed to analyzethe elemental distribution and the composition of theN-CNTs/CoxNi1-x-LDHs NSs (Fig. 2g). The elemental maps of C,N, Ni and Co (Fig. 2h) clearly demonstrate a well-definedcompositional profile of the N-CNTs/Co0.5Ni0.5-LDHs NSs core-shell architecture with the homogeneous distribution of the outershell of Co0.5Ni0.5-LDHs NSs.

X-ray photoelectron spectroscopy (XPS) was employed tocharacterize the elemental information and the valence state ofthe N-CNTs and the Co0.5Ni0.5-LDHs NSs. Three strong peaks for thesurvey spectrum of the N-CNTs centered at 285, 400 and 531 eV areattributed to the C1s, N1s and O1s, respectively (Fig. 3a). The N 1sspectrum of the N-CNTs shows four different peaks at 398.6,401.2 and 404.7 eV, corresponding to the pyridinic-like nitrogen,graphite-like nitrogen and nitrogen oxide, respectively (Fig. 3b)[32,39]. In the Co 2p XPS spectrum (Fig. 3c), the Co 2p3/2 and Co2p1/2 peaks are located at about 796.6 and 780.9 eV, suggesting thepresence of Co2+ in the sample, and two shakeup satellites aredenoted as “Sat.” [25]. As shown in Fig. 3d, the Ni 2p3/2 (855.6 eV)and Ni 2p1/2 (873.2 eV) peaks are fitted with characteristic Ni2+, andaccompanied with two shakeup satellites [45]. Calculated from theXPS result, the ratio of Co/Ni in the Co0.5Ni0.5-LDHs NSs sample isabout 0.50/0.54, which is very close to the concentration ratio ofthe mixed electrolyte for electrodeposition.

To evaluate the performance of the Ni foam/N-CNTs/CoxNi1-x-LDHs NSs electrodes, the electrochemical characterizations werecarried out as described in the experimental section. Fig. S4 showsa set of cyclic voltammogram (CV) curves with different scan ratesin a potential window of 0 to 0.7 V. When x = 0, the CV curvesdisplay a pair of redox peaks (Fig. S4a), which originates from the

faradaic redox reaction Ni2+/Ni3+ with OH�: Ni(OH)2 + OH- $NiOOH +H2O + e� [46,47]. For Co(OH)2 (x = 1), multiple redox peaksare obtained (Fig. S4e), corresponding to the valence state changesbetween Co2+ and Co3+: Co(OH)2 + OH� $ CoOOH +H2O + e�, andbetween Co3+ and Co4+: CoOOH + OH� $ CoO2 + H2O + e� [48,49].When the proportion of cobalt element in the CoxNi1-x-LDHs NSs(x = 0.33, 0.5, 0.67) system was varied, the CV curves present twopairs of anodic peaks and cathodic peaks, indicating a pseudoca-pacitive behavior of the binary hydroxides (Fig. S4b-4d). Acomparison of CV curves at a scan rate of 20 mV s�1 shown inFig. 4b demonstrates that the Co0.5Ni0.5-LDHs NSs exhibit thelargest area, suggesting the highest capacitance.

Fig. 6. Structure and corresponding band structure and total density of states of (a � c) Ni(OH)2, (d � f) Co0.33Ni0.67(OH)2, (g � i) Co0.5Ni0.5(OH)2, (j � l) Co0.67Ni0.33(OH)2,(m � o) Co(OH)2, respectively. Ef, shown by dashed lines. The green lines and the red lines of the band structure and the DOSs correspond to the alpha states (spin-up) and thebeta states (spin-down), respectively.

26 J. Wu et al. / Electrochimica Acta 203 (2016) 21–29

J. Wu et al. / Electrochimica Acta 203 (2016) 21–29 27

As a contrast, the galvanostatic charge-discharge (GCD) curvesof these electrodes at a constant current density of 1 A g�1 and1 mA cm�2 within potential window of 0-0.5 V are shown in Fig. 4cand Fig. S7, respectively. All the GCD curves exhibit obviousplateaus and high symmetry with small IR drop, indicating theexistence of faradaic processes and good electrical and ionicconductivity of the Ni foam/N-CNTs/CoxNi1-x-LDHs NSs. Fig. S3ashows that the Ni foam/N-CNTs/Co0.5Ni0.5-LDHs NS electrode with10 min electrodeposition time for Co0.5Ni0.5-LDHs NS exhibits thelongest charge and discharge curves. Calculated from the dischargecurves, the specific capacitances based on the active Co0.5Ni0.5-LDHs NSs are 1920, 2170 and 1707 F g�1 for 5, 10 and 15 minelectrodeposition time, respectively (Fig. S3b). Therefore, all theCoxNi1-x-LDHs NSs samples are electrodeposited for 10 min, andthe specific capacitances are 1238, 1749, 1910 and 661 F g�1 forx = 0, 0.33, 0.67, 1, respectively (Fig. 4c and d). Furthermore, theareal capacitances of these electrodes turn out to be 1.10, 1.27, 1.62,1.42, 0.56 F cm�2 for x = 0, 0.33, 0.5, 0.67, 1 at 1 mA cm�2,respectively (Fig. 4e and Fig. S6). Apparently, the appropriatesubstitution of nickel with cobalt can improve the capacitances ofthe hydroxide and the optimal proportion of Ni/Co is 1/1. Ingeneral, the high capacitances of the Ni foam/N-CNTs/CoxNi1-x-LDHs NSs electrode are superior or comparable to that of otherreported layered double hydroxides based ones [25,41,50–57].

Rate capability is another key factor of electrochemical capacitorsfor practical applications. The GCD curves behavior of thesehydroxides is performed at various current densities from 1 to20 A g�1 (Fig. S5) and from 1 to 20 mA cm�2 (Fig. S6). The specificcapacitances and the areal capacitances of these hydroxides as afunction of current densities are shown in Fig. 4d and e, respectively.Noticeably, all of the samples exhibit excellent rate capability. For theNi foam/N-CNTs/Co0.5Ni0.5-LDHs NSs, the specific capacitance isretained at 80.9% (from 2170 to 1756 F g�1) as the GCD rate changesfrom 1 to 20 A g�1. Moreover, a high areal capacitance of 1.23 F cm�2

at 30 mA cm�2 with 75.8% retention can be maintained. It is to benoted that the contrastive capacitances in Fig. 4d and e and thecontrastive cycle stability in Fig. 5 for the Ni foam/N-CNTs/Ni(OH)2NSs are obtained in the more appropriate voltage range of 0.05-0.55 V instead of 0-0.50 V (Fig. S8). This is because when theelectrochemical tests were performed with the potential windowfrom 0 to 0.50 V for the Ni foam/N-CNTs/Ni(OH)2 NSs, both thespecific capacitance and areal capacitance decreased sharply as afunction of current density. Furthermore, the specific capacitancewas reducedto 83 F g�1 from 850 F g�1at a current densityof 10 A g�1

for only 1000 cycles.The long cycle stability of the Ni foam/N-CNTs/CoxNi1-x-LDHs NSs

electrodes were investigated at a high current density of 10 A g�1 for5000 charging-discharging measurements, as shown in Fig. 5. Thespecific capacitance of the Ni foam/N-CNTs/Co0.5Ni0.5-LDHs NSselectrode is still 1080 F g�1. Compared with the other four electrodes,although the cycle stability of the Co0.5Ni0.5-LDHs NSs electrode isnot the best, its high capacitances and remarkable rate capabilitymake it promising for practical applications.

For comparison, Co0.5Ni0.5-LDHs NSs were electrodeposited onNi foam with the similar active mass loading. Fig. S9a shows theSEM images of dense and crosslinked nanosheets on the Ni foamsurface. A high magnification SEM image in Fig. S9b reveals that thethickness of the nanosheets is thicker than that on N-CNTs. The BETsurface area of the Ni foam/Co0.5Ni0.5-LDHs and Ni foam/N-CNTs/Co0.5Ni0.5-LDHs were measured to be 1.20 and 3.16 m2g�1 based onthe mass of the whole electrode, respectively (Fig. S9c). Further-more, the Ni foam/Co0.5Ni0.5-LDHs NS electrode shows lowercapacitance (1136 F g�1 at 1 A g�1 and 0.77 F cm�2 at 1 mA cm�2),poorer rate capability (55.9% specific capacitance retention at20 A g�1 and 34.4% areal capacitance retention at 30 mA cm�2) andshorter cycle stability (46.3% specific capacitance retention at

10 A g�1 for only 1000 cycles) than that of the Ni foam/N-CNTs/Co0.5Ni0.5-LDHs NS electrode (Fig. S10).

Fig. S11 shows the typical SEM images of the Ni foam/N-CNTs/Co0.5Ni0.5-LDHs NSs electrode after 5000 cycles at a current densityof 10 A g�1. The 3D network structure and the crosslinkednanosheets morphology are well maintained, except for a smallvolume expansion of the layered structures. The corresponding CVcurves of these electrodes with different scan rates after5000 cycles are demonstrated by the curves in Fig. S12 whichexhibit similar redox peaks as those of the initial cycles.Electrochemical impedance spectroscopy (EIS) was further carriedout to investigate the electrochemical performance characteristics.All the electrodes display similar EIS profiles with superiorconductivity before and after 5000 cycles (Fig. S13).

Since the substitution of nickel with cobalt has strong influenceon the electrochemical performance of the CoxNi1-x-LDHs NSselectrodes (x = 0, 0.33, 0.5, 0.67, 1), DFT calculations were employedto investigate the electronic structure of these hydroxides. Fig. 6and Fig. S14 show the crystal structure, the corresponding bandstructure and total and partial density of states of Ni(OH)2,Co0.33Ni0.67(OH)2, Co0.5Ni0.5(OH)2, Co0.67Ni0.33(OH)2, Co(OH)2, re-spectively. Based on the calculation with GGA + U, the pristine Ni(OH)2 exhibits a typical semiconductor properties with a band gapof 3.43 eV. With increasing concentration of cobalt, the band gapgradually reduced to 3.13 eV for Co0.33Ni0.67(OH)2, 3.06 eV forCo0.5Ni0.5(OH)2, 3.01 eV for Co0.67Ni0.33(OH)2. In the band of Co(OH)2, there is a local density of states mainly derived from Co witha band gap of 3.06 eV.

The excellent electrochemical performance of the Ni foam/N-CNTs/Co0.5Ni0.5-LDHs NSs benefits from the following merits. Onthe one hand, the current collector of the 3D Ni foam/N-CNTsbenefits from its large void space, large surface area and highelectrical conductivity. Meanwhile, the entangled N-CNTs canprovide strong skeletons for uniform growth of Co0.5Ni0.5-LDHsand good electrical conducting pathways for ion and electrontransportation of Co0.5Ni0.5-LDHs NSs. On the other hand, theultrathin characteristic of the nanosheets gives rise to large surfacearea, promoting the contact areas between the electrode andelectrolyte which facilitate ion and electron diffusion. Moreover,the mesopores structure between the interconnected nanosheetsallows easy electrolyte penetration and provides a large void spaceto endure the volume expansion. In addition, the direct growth ofCo0.5Ni0.5-LDHs NSs on N-CNTs enables good mechanical adhesionand improves the contact resistance, thus eliminating the use ofpolymer binder and conducting additives.

4. Conclusion

In summary, a facile, low-cost and scalable route has beendeveloped to design and fabricate 3D hierarchical Ni foam/N-CNTs/CoxNi1-x-LDHs NSs which exhibits excellent supercapacitiveperformances for supercapacitor applications. Results show thatthe double hydroxides outperform either the Co or Ni hydroxidealone. The Ni foam/N-CNTs/Co0.5Ni0.5-LDHs NSs electrode pos-sesses the maximum specific capacitance of 2170 F g�1 at 1 A g�1

and areal capacitance of 1.62 F cm�2 at 1 mA cm�2, and maintains aremarkable value of 1756 F g�1 (80.9% retention) at 20 A g�1 and1.23 F cm�2 (75.8% retention) at 30 mA cm�2, respectively. Theexcellent electrochemical performance of the Ni foam/N-CNTs/CoxNi1-x-LDHs NSs is attributed to the 3D entangled N-CNTsprepared on Ni foam and the ultrathin and interconnectednanosheets morphology. These results demonstrate that the 3Dhierarchical architectures could have potential for energy storageapplications.

28 J. Wu et al. / Electrochimica Acta 203 (2016) 21–29

Acknowledgments

This work was supported by the National High TechnologyResearch and Development Program of China (863Program)(No.2013AA050905), the National Natural Science Foundation ofChina (No.51222212), the LPMT, CAEP (No.ZZ13007 and ZZ14004),the Synergistic Innovative Joint Foundation of CAEP-SCU (No.XTCX2014007) and the Science and Technology Foundation ofChina Academy of Engineering Physics (No.2015B0302073 and2013A030214). Theoretical research work was supported by aTianhe-2JK computing time award at the Beijing ComputationalScience Research Center (CSRC). We are grateful to Margaret Yauand Xichuan Liu for their valuable discussions and assistance inmeasurements.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.electacta.2016.04.033.

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