Journal of Alloys and Compounds 589 (2014) 364–371

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Journal of Alloys and Compounds

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Heterostructured Ni(OH)2–Co(OH)2 composites on 3D ordered Ni–Conanoparticles fabricated on microchannel plates for advanced miniaturesupercapacitor

0925-8388/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jallcom.2013.11.230

⇑ Corresponding author. Tel.: +86 21 54345160; fax: +86 21 54345119.E-mail address: lwwang@ee.ecnu.edu.cn (L. Wang).

Mai Li a, Shaohui Xu a, Yiping Zhu a, Pingxiong Yang a, Lianwei Wang a,⇑, Paul K. Chu b

a Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of Electronic Engineering, East China Normal University, 500 Dongchuan Road, Shanghai200241, PR Chinab Department of Physics and Material Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, PR China

a r t i c l e i n f o

Article history:Received 12 October 2013Received in revised form 28 November 2013Accepted 30 November 2013Available online 7 December 2013

Keywords:Electrochemical capacitorsElectrodepositionNi–Co alloysOrdered three-dimensional microchannelplatesNanoparticles

a b s t r a c t

Silicon microchannel plates (Si-MCPs) coated with a layer of nickel cobalt alloy (NCA) constitute an excel-lent substrate for miniature supercapacitors. Nanoscale Ni(OH)2–Co(OH)2 composite particles serving asthe active materials are electrodeposited on ordered three-dimensional (3D) NCA/Ni/Si-MCPs and theNi(OH)2–Co(OH)2 composites have different structures depending on the amount of Co(OH)2 in Ni(OH)2.The nickel hydroxide synthesized from a water–acetone 0.1 M Ni(NO3)2�6H2O solvent has a compactstructure, but that from a 0.1 M Ni(NO3)2�6H2O solvent containing 5% Co(NO3)2�6H2O is loosely packedwith nanoparticles and that from a 0.1 M Ni(NO3)2�6H2O solvent containing 10% Co(NO3)2�6H2O containsmany nanoparticles. Addition of Co(NO3)2�6H2O results in a smooth morphology. The smooth structure isalso observed from active materials produced in 20% and 30% Co(NO3)2�6H2O solvents. Five types of elec-trode materials are investigated from the perspective of electrochemical capacitors by conducting cyclicvoltammogram, galvanostatic charge–discharge measurements, and electrochemical impedance spec-troscopy. In this experiment, the highest specific capacitance of 7.8 F cm�2 is achieved in the samples pre-pared in 10% Co(NO3)2�6H2O at a discharge current density of 20 mA cm�2. It is much better than1.46 F cm�2 observed from previous attempts and the materials have excellent capacity retention.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

With potential depletion of fossil fuels and growing concernsabout air pollution and global warming, there are extensive re-searches on energy storage and alternative energy sources. Pseud-ocapacitors or electrochemical supercapacitors (ES) have attractedconsiderable attention in recent years because they have a highpower density, long cycle life, and fast charge discharge rate com-pared to batteries in addition to low maintenance cost [1,2]. Theseadvantages make ES increasingly important in applications such asmemory backup devices, day night storage, and uninterruptiblepower supplies for computers [3–5]. In fact, ES has been considereda key technology in future energy storage systems [6]. Research onsupercapacitors is presently divided into two categories that arebased primarily on the reaction mechanisms, namely nonfaradiccharge separation at the electrode/electrolyte interface as in elec-trical double layer capacitors (EDLCs) and pseudocapacitors origi-nating from the fast and reversible redox reactions at/near thesurface of the active materials similar to processes occurring in

batteries [7]. The most common EDLC materials are carbon materi-als which store energy by charge separation and exhibit a very highdegree of reversibility in repetitive charge–discharge cycling forover 5,00,000 cycles [7–9]. So far, the specific capacitance of carbonmaterials is around 200 F g�1 [8,9] which is low due to the limitedaccessibility of the carbon surface to electrolyte. Pseudocapacitorsgenerally show relatively less cycling stability than EDLCs becauseof the faradic reaction mechanism. However, the specific capaci-tance of some transition metal oxides or hydroxide-based pseudoc-apacitive materials such as RuO2, MnO2, Ni(OH)2, and cobalt nickellayered double hydroxides [10–16] could be 10–100 times higherthan that of EDLCs [15] thereby significantly enhancing the energydensity of supercapacitors. In particular, layered metal hydroxidessuch as Co(OH)2 and Ni(OH)2 have drawn immense attention asalternative capacitor materials to the state-of-the-art amorphousRuO2 because of the high theoretical capacitance, unique electro-chemical properties, low cost, and environmental friendliness[12,13,16].

The highest specific capacitance achieved on a-Ni(OH)2–nickelfoam composite is 3152 F g�1 at a current density of 4 A g�1 [12]but the composite suffers from a significant capacitance loss ofabout 50% of the initial capacitance after 300 cycles. In addition,

M. Li et al. / Journal of Alloys and Compounds 589 (2014) 364–371 365

the specific capacitance decreases to 280 F g�1 at a current densityof 16 A g�1. In 2007, Gupta and co-workers fabricated nanostruc-tured CoxNi1�x layered double hydroxides as electrode materialsfor redox-supercapacitors. The capacitive characteristics of the Cox-

Ni1�x LDHs in 1 M KOH electrolyte showed that Co0.72Ni0.28 LDHshad the highest specific capacitance value, 2104 F g�1. While theydo not measured the other important parameters e.g. cycle perfor-mance [13]. Recently, Martins synthesis a kind of stabilized a-NiCo(OH)2 nanomaterials for high performance device applicationfrom sol–gel nickel/cobalt mixed hydroxide nanoparticle precur-sors which demonstrate that the content of Co(OH)2 in Ni(OH)2

has great effect on the structure of materials [14]. Therefore, fur-ther systematic study the properties of these materials on ad-vanced three-dimensional substrate is extremely important.

Porous three-dimensional (3D) structures act as conductive net-works enabling access of ions and electrons to the active surfacesand produce better electrochemical response on the electrodes.Obviously, the structure and morphology of the electrode materialsare important from the perspective of the limitations imposed byionic and electronic transport. The ideal supercapacitors shouldpossess high power and energy densities, long cycle life, and highrate capability. However, most of nanostructured materials havedisadvantages mainly in terms of the intrinsic properties associ-ated with the slow kinetics, poor electrical conductivity, and weakmechanical stability. Apart from the kinetics issue, the surfaceshield is essential for maintaining the capacitance during long cy-cling because side reactions with electrolytes, structure collapse,and re-agglomeration into large grains may lead to high irrevers-ibility and finally poor cycle life [16,17]. Accordingly, in order toproduce high power and energy density and make the supercapac-itors with a long cycle life, nanoscale engineering is necessarywhen fabricating the capacitive materials by taking into accountthe morphology, pore size and distribution, redox active sites,and mass transport pathways. Recent reports and reviews have de-scribed electrodes consisting of three-dimensional (3D) interpene-trating structures that can provide a good solution to circumventthe poor ionic and electronic transport in electrode materials[18,19].

Here, the advantages has been combined by the Ni–Co alloynanostructures, 3D ordered porous structure, and controllablemanner of electrodeposition to fabricate capacitor-type electrode(as the power source) [19,20]. To achieve higher specific capaci-tance and structural stability, one promising route is to optimizethe 3D nanoarchitecture and hybridizeation of pseudocapacitiveoxides. Ni–Co nano-alloy modified three-dimensional ordered sili-con microchannel plates (Si-MCPs) instead of carbon materials orgrapheme are used as the conductive electrodes in the supercapac-itors to improve the surface area of the electrodes and cycling char-acteristics [21]. The nano-alloy contains the same elements as theactive materials (NixCo1�x) so that the active materials can begrown on the electrode surface. At the same time, standard silicontechnology and a 3D template are adopted to fabricate miniaturesupercapacitors having specific capacitance of 855.8 F g�1

(7.8 F cm�2) at 3 A g�1. Owing to the large specific surface area ofthe 3D structure, the experimental results are better than those ob-tained from ordinary electrodes such as nickel foam (NF) [22] ormetal deposited structure [23]. In addition, part of Ni(OH)2 be re-placed by Co(OH)2 in order to block the transformation of b-Ni(OH)2 to c-Ni(OH)2 to improve the stability and cycle character-istics of the supercapacitors [13,24].

2. Experimental details

All chemical reagents were AnalaR (AR) grade and used as received without fur-ther purification.

2.1. Preparation of current collector consisted of 3D ordered low-resistance Ni–Co alloy(NCA) nanoparticles

Before putting on the Ni–Co films, a nickel layer was electroless-plated on boththe outer surface and inner side walls of the Si-MCPs (Ni/Si-MCPs) to form the cur-rent collector as shown in Fig. 1. The Ni/Si-MCPs current collector was preparedaccording to the procedures described in previous studied [1,2]. The Ni/Si-MCPswere cut into small thin foils with an area of 0.8 � 0.8 cm2 and put in a buffer solu-tion of Triton X-100 for at least 2 min to increase the hydrophilicity. The Ni–Co filmwere electrodeposited on the Ni/Si-MCPs (NCA/Ni/Si-MCPs) in an aqueous electro-lyte containing 0.06 M NiCl2�6H2O, 0.04 M CoCl2�6H2O, and 0.5 M H3BO3 [21]. Theelectrolyte pH value was set to 5.7 by addition of NaOH. Electrodeposition was car-ried out at room temperature in a conventional three-electrode electrochemicalcell. A platinum (Pt) plate was used as the counter electrode and a saturated calo-mel electrode (SCE) served as the reference to which all the potentials were referredto. Simultaneous electrodeposition of cobalt and nickel was carried out by imposinga direct-wave cathodic current. The distance between the two electrodes was 1 cmand electrodeposition was performed at a constant current of 0.16 A cm�2 for 100 s.In order to determine the electrical properties of the Ni–Co alloy and exclude theimpact of the metal electrode on the active materials, four other electrodes werefabricated. The Ni/Si-MCPs were replaced with copper and a Ni–Co layer was elec-trodeposited on plane copper (NCA/Cu) to study the performance of the two-dimen-sional (2D) electrode. A porous nano-Ni film was electrodeposited on the Ni/Si-MCPs (Nano-Ni/Ni/Si-MCPs) in a standard two-electrode glass cell at 23 ± 1 �C con-taining an electrolyte of 0.06 M NiCl2�6H2O and 0.5 M H3BO3 with a pH of 5.7. Theother electrical parameters were the same as those used in the fabrication of theNCA/Ni/Si-MCPs.

2.2. Fabrication of heterostructured Ni(OH)2–Co(OH)2 composites with differentCo(OH)2 ratios on NCA/Ni/Si-MCPs

The NCA/Ni/Si-MCPs were put into a buffer solution of Triton X-100 for at least2 min to improve the hydrophilicity. Afterwards, the MCPs were dipped in the elec-troplating solution for the purpose of producing the nano-flakes. In order to inves-tigate the effects of different Co(OH)2 contents in the Ni(OH)2 capacitors on theactive materials morphology and electrical properties, the electrodes were fabri-cated from electrodeposition electrolyte contained different Co, Ni ion ratios were100:0, 100:5, 100:10, 100:20 and 100:30 respectively. For pure Ni(OH)2 electrode[pure-Ni(OH)2], the electrodeposition electrolyte contained 0.1 M Ni(NO3)2�6H2O,for 5% Co(OH)2 and 95 Ni(OH)2 [NiCo-20:1] electrode, the electrodeposition electro-lyte contained 0.1 M Ni(NO3)2�6H2O and 0.005 M Co(NO3)2�6H2O, [NiCo-10:1] 0.1 MNi(NO3)2�6H2O and 0.01 M Co(NO3)2�6H2O, [NiCo-10:2] 0.1 M Ni(NO3)2�6H2O and0.02 M Co(NO3)2�6H2O, as well as [NiCo-10:3] 0.1 M Ni(NO3)2�6H2O and 0.03 MCo(NO3)2�6H2O. All of the electrodes were fabricated in the electrolyte consisted1:1 volume ratio water–acetone [25]. The current density was 50 mA cm�2 andthe temperature was 23 ± 1 �C. After plating for 6 min, the active materials werewashed with de-ionized water several times. After drying at 60 ± 1 �C, copper wireswere connected to a copper sheet by tin solder and the copper sheet was glued ontothe MCPs by conductive silver paste (DAD-40). The size of the electrode was about0.5 cm2.

2.3. Characterization

The morphology and microstructure of the nickel and nickel/cobalt hydroxidethin films were examined by field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800, Japan) and the crystal structure was detected by X-ray diffrac-tion (XRD, Rigaku, RINT2000, Japan). Electrochemical measurements were per-formed on a three-electrode electrochemical working station (Shanghai ChenhuaCHI660D) with a saturated calomel electrode and platinum gauze electrode servingas the reference electrode and counter electrode, respectively. All the measure-ments were performed at room temperature in a 6 M KOH aqueous electrolyte. Inorder to determine the electrochemical properties and specific capacitance of theelectrode samples, CV scans were acquired from 0.4 to 0.6 V (vs. SCE) from bothsamples at different scanning rates. Charge–discharge cycle tests were conductedin the potential range between 0.1 and 0.4 V at different constant current densities.Electrochemical impedance spectroscopy was performed at the open circuit poten-tial in the frequency range from 10,000 to 0.01 Hz with an excitation signal of 5 mV.

3. Results and discussion

3.1. Characterizations of Ni–Co alloys nanoparticles current collector

This nano-structured thin metal layer which covers the Ni/Si-MCPs uniformly plays a crucial role in the performance of thecapacitor. Four samples were studied to determine and understandthe characteristiscs, namely electroless-plated nickel on Si-MCPs(Ni/Si-MCPs), electrodeposited Ni–Co film on nickel plate (NCA/

Fig. 1. (a) SEM images acquired from the ordered and large-area-ratio Si-MCPs; (b) electroless-plated nickel on Si-MCPs and the magnified image; (c) electrodeposited Ni filmon Ni/Si-MCPs; (d) cross-sectional SEM morphology of the Nano-Ni/Ni/Si-MCPs; (e) electrodeposited Ni–Co nanoparticle film on Ni/Si-MCPs and the magnified image; (f)cross-sectional SEM morphology of the NCA/Ni/Si-MCPs; (g) electrodeposited Ni–Co alloy on Ni; and (h) EDS spectrum of the NCA/Ni/Si-MCPs composites structure.

366 M. Li et al. / Journal of Alloys and Compounds 589 (2014) 364–371

Ni), electrodeposited Ni film on Ni/Si-MCPs (Nano-Ni/Ni/Si-MCPs),and electrodeposited Ni–Co film on Ni/Si-MCPs (NCA/Ni/Si-MCPs).The surface morphology of the Si-MCPs and Ni/Si-MCPs beforeand after electroless plating is characterized by SEM. As shown inFig. 1(a), before electroless plating, the microchannels have a depthof about 250 lm and size of 5 � 5 lm yielding an aspect ratio ofabout 50. It has a sandwich structure in which the parallel struc-ture becomes a porous lamellar one. Electroless deposition is em-ployed to deposit Ni on the Si-MCPs and Fig. 1(b) depicts the Ni/Si-MCPs structure after electroless deposition for 30 min revealingthat nickel particles cover the sidewall of the Si-MCPs. The magni-fied image shows that the nickel layer has a smooth surface whichis not conducive to the growth active substances. Hence, the sur-face area is increased and the resistance is reduced by electricallydepositing a metal layer on the Ni/Si-MCPs to improve the perfor-mance. Fig. 1(d) and (e) shows the morphology of the electroplatednickel layer on the surface and sidewall. Although the thick Nilayer has a small resistance of less than 1.5 X, the microstructurein the nickel increases the surface area of the MCPs. However,the distribution of the nickel is not uniform particularly on thesidewall of the microchannel. Hence, the formulation of the platingsolution is changed to prepare the Ni–Co alloy as the modifiedlayer, as shown in Fig. 1(e) and (f) [21]. Owing to the different sizesof the nickel particles, a nickel layer with many bumps and pits isformed and the bumpy surface provides nucleation centers to facil-itate deposition of the composites. As shown in the cross-sectionalSEM morphology of the NCA/Ni/Si-MCPs in Fig. 1(f), the alloy isdeposited uniformly onto the sidewall of the microchannel whichhas many nanoparticles thereby boding well for further depositionof active substances on the inner-sidewall of the NCA/Ni/Si-MCPs.Compared to the nickel plate or nickel thin films (Fig. 1(g)) fromwhich the active substance can be easily delaminated, the NCA/Ni/Si-MCPs constitute an ideal substrate to fabricate the active sub-stance firmly. The EDS spectrum in Fig. 1(h) reveals the presence ofCo and Ni in the composites which form the Ni–Co alloy.

Cyclic voltammogram (CV) and chronopotentiometry measure-ments are conducted to evaluate the specific capacitance and elec-trochemical properties of the current collector. The total activemass of the Si-MCPs structure with an area of 0.5 cm2 is around4 mg as determined by a microbalance with a sensitivity of0.001 mg. The amounts of metal on Nano-Ni/Ni/Si-MCPs, NCA/Ni/Si-MCPs, and NCA/Ni are 1.247 mg, 1.372 mg, and 1.154 mg, respec-tively. Although the quality of the active substance is about thesame, the shape of the CV curves (Fig. 2(a)) and charge and dis-

charge curve (Fig. 2(b)) (charge current 10 mA – discharge current10 mA) of the samples is different. The enclosed area NCA/Ni/Si-MCPs in the CV curve is larger than those of other samples and con-sistent with the long discharge time. It is speculated that the largercapacitance of NCA/Ni/Si-MCPs stems from the Faraday capacitancewhich is related to the surface of the materials in contact with thesolution [39]. The indirect evidence reveals the large surface areaon the Ni–Co alloy and so subsequent experiments are based onthe NCA/Ni/Si-MCPs electrode.

3.2. Structure characterization of the hybrid nanostructured Ni(OH)2–Co(OH)2 composite films

The structure of the nano-flaked composites film is detected byX-ray diffraction and the (XRD) patterns are displayed in Fig. 3. Be-cause of stray signals from other materials, the peaks from the Si-MCPs with large intensity are not shown completely here. The XRDspectrum of the Ni(OH)2/NCA/Ni/Si-MCPs without the depositedCo(OH)2 is shown and compared to NiCo-20:1, NiCo-10:1, NiCo-10:2, and NiCo-10:3, this sample has more diffraction peaks, sug-gesting successful deposition of Ni(OH)2–Co(OH)2 composites onthe substrate. The XRD pattern of pure-Ni(OH)2 exhibits the char-acteristics of the a-Ni(OH)2 at 35.86�, 39.35�, 41.78� and 59.64�according to JCPDS card no. 38-0715 [26].

With the amount of Co(OH)2 is increasing, some of the weak a-Ni(OH)2 peaks disappear, suggesting that the peaks of a-Ni(OH)2

are influenced by the peaks of Co(OH)2. From the XRD of Co(OH)2,the diffraction peak of 2h values at 34.1� as well as 60.7�, are char-acteristic ones belonging to the a-Co(OH)2 phase (PDF, card no. 46-0605) which is the prominent one [18]. The Ni(OH)2–Co(OH)2 com-posites consist of Ni(OH)2 and Co(OH)2 phases, but pure-Ni(OH)2

only shows the phase of a-Ni(OH)2. The Ni(OH)2–Co(OH)2 compos-ite with a high density leads to excellent electrochemical perfor-mance. The XRD diffraction patterns of NiCo-20:1, NiCo-10:1,NiCo-10:2, and NiCo-10:3 correspond to both Co(OH)2 andNi(OH)2. It is difficult to differentiate between the two phases sincethey have similar structures and their diffraction peaks are veryclose. Nonetheless, it is observed that the (100) and (110) peaksfrom the mixed materials broaden thus showing combined effectsof Ni(OH)2 and Co(OH)2 [13].

Fig. 4 shows the morphology of the heterostructured Ni(OH)2–Co(OH)2 composite thin film on the five samples revealed byfield-emission scanning electron microscopy (FE-SEM). As shownin Fig. 4(a), the sandwich-like MCPs with a large surface area pro-

Fig. 2. (a) CV curves of the four samples at a sweeping rate of 80 mV s�1 in 6 M KOH solution and (b) discharge curves of the four samples at a discharge current density of20 mA cm�2 (5 A g�1).

Fig. 3. XRD patterns of pure-Ni(OH)2, NiCo-20:1, NiCo-10:1, NiCo-10:2 and NiCo-10:3.

M. Li et al. / Journal of Alloys and Compounds 589 (2014) 364–371 367

vides more nucleation centers and also good support with highconductivity to decrease the contact resistance between the activematerials. After electrochemical deposition, a smooth and compactNi(OH)2 layer more than 300 nm thick is formed and the cracks be-tween active materials resulting from deposition can be observedin the magnified image in Fig. 4(a). Ni(OH)2 is deposited uniformlyonto the surface and sidewall of the microchannel to enhance the

Fig. 4. SEM images of the heterostructures: (a) top view of pure-Ni(OH)2; (b) top viewNiCo-10:3; (f) cross-sectional view of NiCo-20:1; (g) cross-sectional view of NiCo-10:1;

performance. However, the capacity of pure Ni(OH)2 is smallerthan NiCo-20:1 and NiCo-10:1, even though Ni(OH)2 has a rela-tively large theoretical value. This is probably due to the smoothsurface which prevents the electrolyte from full contact with thesample [39].

Fig. 4(b) shows that NiCo-20:1 has a complex microstructure onthe nanometer scale and the network-like structure includinginterconnected small nanoparticles exhibits an anisotropic mor-phology. Fig. 4(c) shows NiCo-10:1 has the same structure butmore nanoparticles on the surface. The unique structure plays akey role in the electrochemical accessibility of the electrolyteOH� to the active materials and fast diffusion in the redox phase[40]. It is also believed that this unique structure provides theimportant morphological foundation for the extraordinary highspecific capacitance [27]. Fig. 4(b) and (c) discloses the nanoparti-cle structure. As shown in the cross-sectional SEM morphology ofNiCo-20:1 in Fig. 4(f) and NiCo-10:1 in Fig. 4(g), there are poresconsisted of nanoparticles on the sidewall. After formation of thenano-rods, they may also be the nucleation centers for growth ofthe Ni(OH)2–Co(OH)2 composite pores. It is different from thenanoparticle structure on the surface. The alloy-coated Si-MCPsare covered evenly by nanoparticles by electrochemical deposition.Fig. 4(h) depicts the magnified image of the micro-porous NiCo-10:1 and the nanoparticles are around 500 nm in size.

As shown in Fig. 4(d) and (e), on the surface of the Ni–Co film,there is a dense layer with a smooth surface which has a different

of NiCo-20:1; (c) top view of NiCo-10:1; (d) top view of NiCo-10:2; (e) top view ofand (h) magnified image of a single micro-porous of NiCo-10:1.

368 M. Li et al. / Journal of Alloys and Compounds 589 (2014) 364–371

structure compare with those on NiCo-20:1 and NiCo-10:1. Thereare fewer nanoparticles on the surface and the specific surfaceareas on NiCo-10:2 and NiCo-10:3 diminish. The morphology isdifferent from that of the other active materials produced on theMCPs. The Ni–Co particles on the sidewall of the MCPs providenucleation impurities to gather the active materials. The nano-structured materials play important roles in the supercapacitancewhile also providing the unique nickel modified template with alarge surface area in a small footprint. The morphology of the sam-ple depends largely on the solvent of the precursor.

3.3. Electrochemical characterization

Typical CV curves obtained at various scan rates of this batch ofsamples are displayed in Fig. 5. As the scanning rate increases from2 to 120 mV s�1 [28], the peak current becomes larger and the oxi-dation and reduction peaks are quite apparent in the CV curves andthe peak shape is similar. However, the peak potential shifts to theanodic and cathodic directions, respectively, because of an increas-ing involvement of polarization at high scanning rates [29]. The CVof the double-layered capacitance normally approaches the idealrectangular shape, but the CV obtained from the composite pseu-

Fig. 5. Cyclic voltammetry curves of the Ni(OH)2–Co(OH)2 composites electrode with difNiCo-10:1; (d) NiCo-10:2; (e) NiCo-10:3; and (f) variation of the specific and interfacial caThe concentration of KOH in the electrolyte is 6.0 M.

do-capacitance is quite different. As shown in Fig. 5, as the scan-ning rate is increased, the peak currents are proportional to thesquare root of the scanning rates, which implies that the electrodeshave good electrochemical performance.

In the reverse scanning direction, the current is almost instan-taneous suggesting that in the CV curve, there is a small anglealong the horizontal axis, indicating that the electrodes have smal-ler impedance. Furthermore, it is apparent that NiCo-10:1 shownin Fig. 4(c) has a larger area under the same current–potential con-ditions compared to other samples, suggesting that NiCo-10:1 hasa relative larger specific capacitance (SC) and capacitive behavior.The nanoparticles in NiCo-10:1 provide the reaction sites basedon the complex nanostructured and network structure. More redoxreactions are expected on NiCo-10:1 because of the larger specificsurface area, more reaction sites, and better electrical conductancethan other samples.

The Ni(OH)2 and CoxNi1�x electrodes show very strong redoxpeaks due to the following Faradaic reactions of Co(OH)2 andNi(OH)2 [30]:

CoðOHÞ2 þ OH� () CoOOHþH2Oþ e�; ð1Þ

CoOOHþ OH� () CoO2 þH2Oþ e�; and ð2Þ

ferent Ni–Co ratios at different scanning rates: (a) pure-Ni(OH)2; (b) NiCo-20:1; (c)pacitances of the Ni(OH)2–Co(OH)2 composite electrodes at different scanning rates.

M. Li et al. / Journal of Alloys and Compounds 589 (2014) 364–371 369

NiðOHÞ2 þ OH� () NiOOHþH2Oþ e�: ð3Þ

The CV curves show shifts in the redox peaks as the composi-tions of CoxNi1�x are varied, as shown in Fig. 5. The oxidation andreduction peaks of the pure-Ni(OH)2 are 0.4 V and 0.15 V at a scan-ning rate 2 mV s�1, respectively, whereas for NiCo-10:1, the peaksare at 0.26 V and 0.1 V, respectively. The two pairs of visible redoxpeaks in the CV curves in Fig. 5 confirm the reactions shown, sug-gesting that instead of a pure electrical double-layer capacitance,the measured pseudo-capacitance is dominated by a redox mech-anism. It should be emphasized that the anodic peak potential, CVchange, and cathodic peak potential shift in the anodic and catho-dic directions with increasing sweeping rate and the capacitancedecreases. The observation is consistent with that from chronopo-tentiometry. By comparing the CV curves, the oxidation peaks ofthe samples at sweeping rates of 120 mV s�1 and 60 mV s�1 arenot obvious probably because the compact structure does not fullyreact in the 6 M KOH solution and has a low reaction rate.

The specific capacitance can be calculated from the CV curvesusing Eq. (4) [31]:

Cf ¼Z

idt� �

ðADVÞ�1; ð4Þ

where Cf is the electrode specific capacitance, i is the instantaneouscurrent, A is the footprint area of the entire electrode, and DV is po-tential voltage window. The specific capacitance can be calculatedfrom the CV curves. The change in the capacitance with scanningrates is illustrated in Fig. 5(f) which shows that NiCo-10:1 has thebest capacitance characteristics which are consistent with chrono-potentiometry. The CV curve shows the pseudocapacitive behaviorwith the capacitance obtained for 10% Co(OH)2 in Ni(OH)2 beingslightly higher than the previously obtained value by the samemethod (Fig. 5f). The effects of this phenomenon on the capacitanceand the results obtained by electrochemical impedance spectros-copy (EIS) will be described in subsequent sections in this paper.

Fig. 6 displays the discharge curves of the composite electrodein 6 M KOH at 20 mA cm�2 charge–discharge current density inthe potential range between �0.1 and 0.4 V from the five samples.The shape of the charge–discharge curves shows mainly pseudo-capacitance instead of pure double-layer capacitance and the re-sults are consistent with the CV data. Chronopotentiometry is arecommended method to determine the capacitance of superca-pacitors according to Eq. (5) [32]:

Cf ¼ ðIDtÞðADEÞ�1; ð5Þ

where I is the constant discharge current, Dt is the discharge time,DE is the potential drop during the discharge process, and A is thearea of the working electrode immersed in KOH.

Fig. 6. First charge (20 mA cm�2)–discharge (20 mA cm�2) curves: (a) NiCo-10:1;(b) NiCo-20:1; (c) pure-Ni(OH)2; (d) NiCo-10:2; and (e) NiCo-10:3.

According to the chronopotentiometry of this batch of samples,the SC values of pure-Ni(OH)2 is 4.748 F cm�2, NiCo-20:1 is6.16 F cm2, NiCo-10:1 is 7.8 F cm2, NiCo-10:2 is 4.724 F cm�2, andNiCo-10:3 is 4.084 F cm�2 for a charging-discharging current den-sity of 20 mA cm�2. In our experiments, the capacity of the capac-itor changes with the Co(OH)2 content in Ni(OH)2 and 10% Co(OH)2

has the best capacitance of 7.8 F cm�2 at a discharge current of20 mA cm�2. A small amount of Co(OH)2 increases the capacitancebut excessively high Co(OH)2 may destroy the capacity of thecapacitor. By co-depositing a certain amount of Co(OH)2 in Ni(OH)2

and activating in KOH, Co(OH)2 is converted into b-Co(OH)2 thoughthe dissolution–deposition process subsequently depositing on theNi(OH)2 particle surface. b-Co(OH)2 is transformed into b-CoOOHwhich favors the subsequent charging process. b-CoOOH whichhas good electron conductivity increases the depth of dischargeon the electrode as well as the active materials utilization and dis-charge potential [33].

The total weight of the composite/Ni–Co/Ni/Si-MCPs structurewith an area of 0.5 cm2 is around 4 mg, whereas those of pure-Ni(OH)2, NiCo-20:1, NiCo-10:1, NiCo-10:2, and NiCo-10:3 are3.124, 4.152, 4.557, 3.924, and 3.752 mg, respectively. Since theweight of the nickel coated Si-MCPs is fixed and the electrodepos-ition time is also the same, the small mass difference can be attrib-uted to the morphology difference. The capacitance values of pure-Ni(OH)2, NiCo-20:1, NiCo-10:1, NiCo-10:2, and NiCo-10:3 are759.92 F g�1, 741.81 F g�1, 782.83 F g�1, 601.94 F g�1, and544.24 F g�1, respectively at a discharge current density of20 mA cm�2. The converted capacitance per unit mass at low ratesis close to the commonly reported results for Ni(OH)2 as superca-pacitors [12–16]. Even the smallest capacitance of NiCo-10:3 ob-tained at a discharge current density of 10 mA cm�2 is about544.24 F g�1 and it is comparable to that of many other pure elec-trical double layer electrochemical capacitors [34–36].

Since our three dimensional structure electrode is based on thesilicon process, which makes the calibration of the electrode muchmore complex than plate structure based on metal. In this content,the cycle performance of our capacitors were obtained as accurateas possible. In order to demonstrate the electrochemical stability ofthe nano-flaked Co(OH)2 electrode materials, the CV characteristicsare measured in 6 M KOH at a sweeping rate of 120 mV s�1. Asshown in Fig. 7, the specific capacitance calculated from Eq. (5) de-creases with cycle numbers. In the first 2500 cycles, the calculatedcapacitance losses of pure-Ni(OH)2, NiCo-20:1, NiCo-10:1, NiCo-10:2, NiCo-10:3 are 5.4%, 3.14%, 5.36%, 1.3%, and 7.5% respectively,whereas in the last 2500 cycles, they are 6.4%, 3.3%, 8.4%, 7.6%, and7.7%, respectively, thereby demonstrating good stability in longcharging-discharging cycles. It has been proposed that a smallersurface area and degradation of the active materials are responsi-ble for the capacitance loss in long charging–discharging [37,38].The samples after 5000 cycles are carefully weighed and no obvi-ous weight loss is found. The capacitance reduction may originatefrom slow oxidation of a-Ni(OH)2 to b-Ni(OH)2 and c-Ni(OH)2 be-cause a-Ni(OH)2 is more stable than b-Ni(OH)2 and c-Ni(OH)2 un-der alkaline conditions. As shown in Fig. 7, NiCo-10:1 is less stablethan NiCo-20:1 possibly because NiCo-10:1 has more nanoparti-cles with more active sites which can detach from the substratecausing larger consumption in the electrochemical reaction. Theelectrode with 5% Co(OH)2 has good long-term electrochemicalstability and after repetitive charging it discharging and does notdegrade significantly.

3.4. Evaluation of the overall capacitive performance of hybridNi(OH)2–Co(OH)2 composites structure

Another important aspect of a supercapacitor electrode is theimpedance spectra. The measurements are carried out on the com-

Fig. 7. Long cycling performance of the composite Ni–Co/Ni/Si-MCPs at a sweepingrate of 120 mV s�1.

Table 1Fitted results of important parameters in the equivalent circuit.

Samples R1 (X) R2 (X) R3 (X) CPE1�n* CPE2�n CPE3�n

Pure-Ni(OH)2 1.253 2.026 8.535 0.185 0.724 0.852NiCo-20:1 1.300 1.643 3.468 0.412 0.602 0.817NiCo-10:1 1.375 0.507 1.055 1.042 0.917 0.899NiCo-10:2 1.125 0.630 1.158 0.322 0.876 0.688NiCo-10:3 1.275 0.797 2.051 0.205 0.312 0.576

* CPE1�n represents the exponential parameter of constant phase element.

370 M. Li et al. / Journal of Alloys and Compounds 589 (2014) 364–371

posite electrodes with an excitation signal of 5 mV and the repre-sentative results are shown in the dotted line in Fig. 8 where Z0 andZ00 are the real and imaginary parts of the impedance, respectively.Since our electrode is a pseudocapacitance structure, the equiva-lent circuit inset in the upper right corner of Fig. 8 is selected tofit the impedance spectra by complex nonlinear least square(CNLS) fitting. The results are shown in the solid line in Fig. 8[39,40]. The impedance spectra can be fitted well and the parame-ters are shown in Table 1. In the equivalent circuit, a constantphase element (CPE) component is introduced [40] and CPE1 andCPE2 represent the double-layer and Faradaic capacitance that var-ies with frequencies, respectively. This modification from a purecapacitance behavior can be explained by distribution effects[42] and porosity [43] in the samples. Owing to the influence ofthe 3D structure of MCPs on mass transport, CPE3 represents theWarburg impedance.

The complex-plane impedance plots of each sample consist of ahigh-frequency component and low-frequency component from10,000 Hz to 0.01 Hz. The impedance behavior in the high fre-quency region is characterized of the oxide–electrolyte interfacedue to discontinuity in the charge transfer process at the solidoxide/liquid electrolyte interface. This is a result of the differencein the conductivity between the solid oxide (electronic conductiv-ity) and aqueous electrolyte phase (ionic conductivity). The imped-ance behavior in this region also involves resistance from theFaradaic redox processes associated with the surface phenomenaof the porous composites electrode. Specifically, in the high fre-quency range and at the point intersecting the real axis, the inter-nal resistance values (which is equal to R1) of the composites

Fig. 8. Nyquist plots of hybrid Ni(OH)2–Co(OH)2 composite electrodes in 6 M KOHsolution of pure-Ni(OH)2, NiCo-20:1, NiCo-10:1, NiCo-10:2, and NiCo-10:3. Theequivalent circuit is used to fit the Nyquist plots.

electrodes are 1.253 X, 1.300 X, 1.375 X, 1.125 X, and 1.275 Xfor pure-Ni(OH)2, NiCo-20:1, NiCo-10:1, NiCo-10:2, and NiCo-10:3 respectively. R1 of those samples are similar and NiCo-10:1is a little larger. It can be explained by that as the solvent is chan-ged, the compact nano-flakes reduce the diffusivity of the electro-lyte ions in the pores.

A Faradic charge transfer resistance, R2, representing depositionor desorption of an electroactive species is parallel to the double-layer capacitance CPE1. The Faradic charge-transfer resistance(R2) corresponds to the semicircle in the high-frequency range re-lated to the surface properties of the electrode. Table 1 shows thatNiCo-10:1 has a R2 value of 0.507 X which is much smaller thanthe R2 values of other samples. As the double-layer capacitanceCPE1, we extract CPE1�n as the exponential parameter of CPE1 to as-sess the capacity of the double layer capacitance [41]. According toTable 1, the CPE1�n values of the composite electrodes are 0.185,0.412, 1.042, 0.322, and 0.205 for pure-Ni(OH)2, NiCo-20:1, NiCo-10:1, NiCo-10:2 and NiCo-10:3 respectively. The rich nanoparticlestructure of NiCo-10:1 has a larger double layer capacitance whichmay originate from the porous structure of the sample that canfully contact the solution [44].

The capacitance is a pseudo capacitance and so R3 and CPE2 arekey performance indicators. The Faradic or chemical resistance, R3,in the circuit corresponds to the reciprocal of the potential-depen-dent rate of the process. It enables CPE2 to be omitted or combinedto form an overall product of the reaction [39]. That is to say, R3 re-flects the difficulty of the capacitor to carry out chemical reactionand CPE2�n reflects the pseudo-capacitance. According to Table 1,the R3 values of the composite electrodes are 8.535 X, 3.468 X,1.055 X, 1.158 X, and 2.051 X for pure-Ni(OH)2, NiCo-20:1,NiCo-10:1, NiCo-10:2, and NiCo-10:3, respectively, suggesting thatsample (3) has more active materials and can easily react with theelectrolyte. Careful comparison of the index of CPE2�n shows thatsample (3) has a CPE2�n of 0.917 that is larger than those ofpure-Ni(OH)2 [0.724], NiCo-20:1 [0.602], NiCo-10:2 [0.876] andNiCo-10:3 [0.312]. It is consistent with the results obtained fromother tests.

On the other hand, the linear parts of the impedance plots atlower frequencies correspond to the interfacial diffusive resistance.The process designated as CPE3 in the circuit in Fig. 6(d) describesthe diffusive resistance of OH� ions in the composite electrodepores [40]. As the solvent is changed, there is a gradual change inthe linearity, especially NiCo-10:1. The Nyquist plot manifests asnearly a vertical line along the imaginary axis. Furthermore, theslope of the impedance plots of NiCo-10:1 is 0.899 which is largerthan those of pure-Ni(OH)2 [0.852], NiCo-20:1 [0.817], NiCo-10:2[0.688] and NiCo-10:3 [0.576] at low frequencies. This indicatesthat the special complex microstructure with flakes and particlesin NiCo-10:1 enables faster ion diffusion through the channel ofthe MCPs and increases the slope [41,44,45].

4. Conclusions

The effects and mechanism of improved nickel hydroxidecoated NCA/Ni/Si-MCPs fabricated by electrochemical depositionare investigated. The structure with a nanoparticle Ni(OH)2–

M. Li et al. / Journal of Alloys and Compounds 589 (2014) 364–371 371

Co(OH)2 composite film exhibits much higher specific capacitancethan the relatively compact composite films. The enhancement canbe attributed to the regular nano-structure consisting of mesop-ores and faster ion diffusion in the pores. In this experiment, thesamples prepared in 10% Co(NO3)2�6H2O, the highest specificcapacitance of 7.8 F cm�2 is attained at a discharge current densityof 20 mA cm�2 and it has the good electrochemical stability up to5000 cycles. The materials can be upscaled to the mass productionof miniature supercapacitors.

Acknowledgements

This work was jointly supported by Shanghai Natural SciencesFoundation No. 11ZR1411000, Shanghai Fundamental Key ProjectUnder Contract Numbers of 11JC1403700 and 10JC1404600,PCSIRT, and China NSFC Grant Nos. 61176108, 60990312 and61076060. The work was also supported by Hong Kong ResearchGrants Council (RGC) and General Research Funds (GRF) Nos. CityU112510 and 112212.

References

[1] H.L. Wang, H.S. Casalongue, Y.Y. Liang, H.J. Dai, J. Am. Chem. Soc. 132 (2010)7472–7477.

[2] H.L. Wang, H.S. Casalongue, Y. Wang, S.Q. Chen, Q. Liu, G.X. Wang, J. AlloysComp. 582 (2014) 522–527.

[3] J.R. Miller, P. Simon, Science 321 (2008) 651–652.[4] C. Liu, F. Li, L.-P. Ma, H.-M. Cheng, Adv. Mater. 22 (2010) E28–E62.[5] B.E. Conway, W.G. Pell, J. Solid State Electrochem. 7 (2003) 637–644.[6] Z. Wang, C.Y. Ma, H.L. Wang, Z.H. Liu, Z.P. Hao, J. Alloys Comp. 552 (2013) 486–

491.[7] H.T. Zhang, X. Zhang, D.C. Zhang, X.Z. Sun, H. Lin, C.H. Wang, Y.W. Ma, J. Phys.

Chem. B 117 (2013) 1616–1627.[8] C. Masarapu, H.F. Zeng, K.H. Hung, B.Q. Wei, ACS Nano 3 (2009) 2199–2206.[9] L.F. Chen, X.D. Zhang, H.W. Liang, M.G. Kong, Q.F. Guan, P. Chen, Z.-Y. Wu, S.-H.

Yu, ACS Nano 6 (2012) 7092–7102.[10] M.X. Liao, Y.F. Liu, Z.H. Hu, Q. Yu, J. Alloys Comp. 562 (2013) 106–110.[11] X. Lang, A. Hirata, T. Fujita, M.W. Chen, Nanotechnology 6 (2011) 232–236.[12] G.-W. Yang, C.-L. Xu, H.-L. Li, Chem. Commun. 48 (2008) 6537–6539.[13] V. Gupta, S. Gupta, N. Miura, J. Power Sour. 175 (2008) 680–685.[14] P.R. Martins, A.L.A. Parussulo, S.H. Toma, M.A. Rocha, H.E. Toma, K. Araki, J.

Power Sour. 218 (2012) 1–4.

[15] D.M. Luo, Y.P. Li, J.L. Liu, H.B. Feng, D. Qian, S.J. Peng, J.B. Jiang, J. Alloys Comp.581 (2013) 303–307.

[16] B.G. Choi, M.H. Yang, S.C. Jung, K.G. Lee, J.G. Kim, H.S. Park, T.J. Park, S.B. Lee, Y.-K. Han, Y.S. Huh, ACS Nano 7 (2013) 2453–2460.

[17] R. Liu, J. Duay, S.B. Lee, Chem. Commun. 47 (2011) 1384–1404.[18] J.Y. Yao, P. Xiao, Y.H. Zhang, M. Zhan, F. Yang, J. Alloys Comp. 583 (2014) 366–

371.[19] M. Li, S.H. Xu, T. Liu, F. Wang, P.X. Yang, L.W. Wang, P.K. Chu, J. Mater. Chem. A

1 (2013) 532–540.[20] J. Yan, Z.J. Fan, W. Sun, G.Q. Ning, W. Tong, Q. Zhang, R.F. Zhang, L.J. Zhi, F. Wei,

Adv. Funct. Mater. 22 (2012) 2632–2641.[21] R.P. Silva, S. Eugénio, T.M. Silva, M.J. Carmezim, M.F. Montemor, J. Phys. Chem.

C 116 (2012) 22425–22431.[22] Z. Tang, C.H. Tang, H.A. Gong, Adv. Funct. Mater. 22 (2012) 1272–1278.[23] J. Benson, S. Boukhalfa, A. Magasinski, A. Kvit, G. Yushin, ACS Nano 6 (2011)

118–125.[24] G.P. Wang, L. Zhang, J. Kim, J.J. Zhang, J. Power Sour. 217 (2012) 554–561.[25] Z.J. Yu, Y. Dai, W. Chen, J. Chem. Soc. 57 (2010) 423–428.[26] Y.M. Wang, D.D. Zhao, Y.Q. Zhao, C.L. Xu, H.L. Li, RSC Adv. 2 (2012) 1074–1082.[27] L. Cao, L.B. Kong, Y.Y. Liang, H.L. Li, Chem. Commun. (2004) 1646–1647.[28] D.C. Yang, G.W. Meng, C.H. Zhu, X.G. Zhu, Chem. Commun. (2009) 7110–7112.[29] W.G. Zhang, W.Q. Jiang, L.M. Yu, Z.Z. Fu, W. Xia, M.L. Yang, Int. J. Hydrogen

Energy 34 (2009) 473–480.[30] Y.Y. Liang, S.J. Bao, H.L. Li, J. Solid State Electrochem. 11 (2007) 571–576.[31] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845–854.[32] M. Inagaki, H. Konno, O. Tanaike, J. Power Sour. 195 (2010) 7880–7903.[33] Y.G. Tang, W.L. Li, Ni–MH Battery, Chemical Industry Press, 2007. pp. 100.[34] C.M. Niu, E.K. Sichel, R. Hoch, D. Moy, H. Tennent, Appl. Phys. Lett. 70 (1997)

1480–1482.[35] J. Gamby, P.L. Taberna, P. Simon, J.F. Fauvarque, M. Chesneau, J. Power Sour.

101 (2001) 109–116.[36] D. Lozano-Castello, D. Cazorla-Amoros, A. Linares-Solano, S. Shiraishi, H.

Kurihara, A. Oya, Carbon 41 (2003) 1765–1775.[37] C.Z. Yuan, X.G. Zhang, L.H. Su, B. Gao, L.F. Shen, J. Mater. Chem. 19 (2009)

5772–5777.[38] G.X. Hu, C.X. Li, H. Gong, J. Power Sour. 195 (2010) 6977–6981.[39] B. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and

Technological Applications (POD), Kluwer Academic/Plenum, New York, 1999.[40] M.E. Orazem, B. Tribollet, Electrochemical Impedance Spectroscopy,

Wiley.com, 2011. pp. 233–236.[41] S.K. Meher, G. Ranga Rao, J. Phys. Chem. C 115 (2011) 15646–15654.[42] G.J. Brug, A.L.G. Van Den Eeden, M. Sluyters-Rehbach, J.H. Sluyter, J.

Electroanal. Chem. 176 (1984) 275–295.[43] L.M. Gassa, J.R. Vilche, M. Ebert, K. Juttner, W.J. Lorenz, J. Appl. Electrochem. 20

(1990) 677–685.[44] T.Y. Wei, C.H. Chen, K.H. Chang, S.Y. Lu, C.C. Hu, Chem. Mater. 21 (2009) 3228–

3233.[45] E. Frackowiak, F. Beguin, Carbon 39 (2001) 937–950.