High performance supercapacitor electrodes from electrospun nickel oxide nanowires

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High performance supercapacitor electrodes from electrospun nickeloxide nanowires

Baiju Vidhyadharan, Nurul Khayyriah Mohd Zain, Izan Izwan Misnon, Radhiyah Abd Aziz, Jamil Ismail,Mashitah M. Yusoff, Rajan Jose ⇑Nanostructured Renewable Energy Materials Laboratory, Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang, 26300 Pahang, Malaysia

a r t i c l e i n f o

Article history:Received 28 February 2014Accepted 28 April 2014Available online 9 May 2014

Keywords:Renewable energyElectrochemical energy storageBatteriesNanofabricationCeramic nanostructures

a b s t r a c t

Electrochemical energy storage using pseudocapacitive mode is under intense research owing to theirpotential in fabricating high performance renewable energy devices at a lower cost. In this paper we char-acterize nickel oxide (NiO) nanowires developed by electrospinning an aqueous polymeric solution con-taining nickel precursor for its application as a pseudocapacitors electrode. The wires are of diameter�50–70 nm containing densely packed cuboidal grains (�10–20 nm) with less degree of crystal defects.Electrochemical properties of the electrodes fabricated on a nickel foam substrates are evaluated by cyclicvoltammetry (CV) and charge–discharge cycling (CDC), and electrochemical impedance spectroscopy(EIS) techniques. The best performing electrode showed a specific capacitance (CS) of �670 Fg�1 with highcycling stability (�100%) for over 1000 cycles and Coulombic efficiency �98%. Lower electrochemicalequivalent resistance (�0.76 X), charge transfer resistance (�0.45 X), and charge relaxation time(43 ms) are observed which are attributed to the defect free nanowire morphology that give rise to thesuperior performance.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Electrochemical capacitors (supercapacitors) represent a classof energy storage devices characterized by high power density(P5 kW/kg), fast charging and discharging times (<30 s), long cyclelife (P105 cycles), low maintenance cost and excellent reversibility[1–3]. Fast charging and discharging make supercapacitors favor-able for high power applications such as hybrid/electric vehicles,portable electronics, and energy backup systems [4,5]. Supercapac-itors are classified into electric double layer capacitors (EDLCs) andpseudocapacitors according to charge storage mechanisms [6,7].The EDLCs store electrical energy via accumulation of electriccharges at an electrical double layer formed at an interfacebetween a porous solid electrode and an electrolyte. Carbon struc-tures such as activated carbon, carbon nanotubes, and grapheneare choices to build EDLCs. No electron transfer takes place acrossthe electrode interface during its operation in EDLCs; therefore,this charge storage process is non-faradic. The pseudocapacitorsinvolve faradic charge storage process facilitated by a redoxreaction at the electrode–electrolyte interface [8,9]. Ceramic nano-structures and conducting polymers show high pseudocapacitance

(Cp); a theoretical Cp equal to F/(DE �m), where F is the Faradayconstant, m is the molecular weight and DE is the redox potentialof the material, could be achieved by these materials [10]. A highspecific surface area of the electrode material to have a large elec-trode–electrolyte interface to enable efficient redox reaction, highelectrical conductivity for fast charging and discharging, and fasterion diffusivity are properties of ceramics and electrolytes requiredto achieve higher energy and power densities in pseudocapacitors.

Owing to its relative larger abundance, lower toxicity, and desir-able electrochemical properties nickel oxide (NiO) gained consider-able attention as a supercapacitor electrode. Moreover, theoreticalCp of NiO is �2573 Fg�1, considering its DE at �0.5 V, [11] which ishigher than that of widely studied hydrated ruthenium oxide(RuO2�nH2O) (�2200 Fg�1) and MnO2 (�1300 Fg�1). Many effortsare made to optimize NiO morphologies and it’s composite with car-bon structures/metal to enable high specific capacitance (CS) andcycling stability; a summary of which is in Table 1. The CS in the tableare taken from the galvanostatic charge–discharge cycling experi-ments (except two entries); however, the CS reported are from vary-ing experimental conditions of current density (mA cm�2) orspecific current (Ag�1) which make a direct comparison of theresults cumbersome. Experimental CS as high as 65% of the theoret-ical Cp is reported for electrodes employed NiO microflowerssynthesized by a soft chemical approach [12] as well as by electro-deposition technique [13].

http://dx.doi.org/10.1016/j.jallcom.2014.04.2110925-8388/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +60 9 5492451; fax: +60 9 5492766.E-mail address: [email protected] (R. Jose).

Journal of Alloys and Compounds 610 (2014) 143–150

Contents lists available at ScienceDirect

Journal of Alloys and Compounds

journal homepage: www.elsevier .com/locate / ja lcom


One of the preliminary requirements for a material to beselected as an electrode for a high performance supercapacitordevice is high electrical conductivity; and therefore, one-dimen-sional morphologies are attractive owing to their directionalcharge transport properties. Among the many nanowire formingtechniques, electrospinning represents a simple and versatile tech-nique for producing nanostructured materials as well as mem-branes for many engineering applications to be enabled such asfiltration, healthcare, and energy [14,15]. In the electrospinningtechnique, a polymeric solution, usually prepared by dissolvingpolymers in organic solvents, is injected through a syringe needleconnected to a power supply. A polymeric jet is initiated uponinjection of the solution that undergoes asymmetric bending dur-ing the passage between the injector and the collector [16]. Thisasymmetric bending increases the path length of the jet and allowsthe solvent to evaporate thereby producing solid continuous wireswith diameters ranging from nanometers to sub-micrometers on acollector surface. If the polymeric solution contains precursors forformation of an inorganic solid, then appropriate annealing pro-duces its continuous nanowires [17]. Electrospun hollow NiO

nanowires of diameter �100–120 nm are reported for supercapac-itor electrode that showed a CS of �336 Fg�1 at 5 mA cm�2 [18].However, evaporation of large volume of organic solvents duringelectrospinning have adverse environmental effects; this drawbackis overcome in ‘‘greener’’ electrospinning using aqueous polymericsolutions.

Herein, we report an aqueous polymeric solution based elec-trospinning procedure to synthesize NiO nanowires and their mor-phological, structural, and electrochemical characterization. TheNiO nanowires thus developed have 50% lower diameter �50–70 nm than previous attempts and exhibited superior supercapac-itive performance. The factors that affected the superior perfor-mance have been investigated and reported herewith.

2. Experimental details

2.1. Synthesis and characterization of NiO nanowires

Starting materials were nickel acetate tetrahydrate [Ni(CH3-

COO)2�4H2O; NiAc; 99%; Sigma Aldrich, USA] and polyvinyl alcohol

Table 1Summary of research on the use of NiO nanostructures and their composites with carbon structures/metals for application as a supercapacitor electrode.

Morphology Method of synthesis CS

(Fg�1)Stability Current

densityElectrolyte Ref.

Porous thin film Chemical bath deposition 104.2 No fading until 1500 cycles 1 mA cm�1 1 M NaOH [35]

Core/shell microspheres Lamellar template method 448 No fading until 500 cycles 0.5 Ag�1 5 M KOH [26]

Porous hollow spheres Chemical bath deposition 346 No fading until 2000 cycles 1 Ag�1 2 M KOH [27]

Nanowires of diameter �65 nm Hydrothermal 750 12% fading at 1000 cycles 1 mV s�1 0.1 M NaOH [36]

Microflowers Coordination of microflowers 1678 0.3% fading at 1000 cycles 0.625 Ag�1 3 wt% KOH [12]

Nanoporous Electro deposition 1305 No fading until 5000 cycles 1 Ag�1 1 M KOH [13]

Nanotubes Anodic aluminium oxide template 266 7% fading until 1000 cycles 0.1 Ag�1 6 M KOH [37]

Hollow NiO nanofibers Electrospinning 336 13% fading until1000 cycles 5 mA cm�1 6 M KOH [18]

Mesoporous nanoparticles EPE template 736 No data available 0.5 Ag�1 1 M KOH [38]

Microporous nanoparticles 533

Hexagonal nanoplates Surfactant self assembly 287 11.5% until 1000 cycle 1 Ag�1 6 M KOH [39]

NiO nanotubes Hydrothermal 701 7% fade at the end of 1000cycles

0.5 Ag�1 KOH [40]

NiO nanosheet Surfactant template wet chemistry 1025 No data available 3 Ag�1 No dataavailable

[41]

Porous NiO nanowall arrays Direct synthesis 236 7% fading until 4000 cycles 13.35 Ag�1 No dataavailable

[42]

Mesoporous NiO Template synthesis 526 6% fading until 2000 cycles No dataavailable

No dataavailable

[43]

Co/NiO core shell nanowire Hydrogen reduction + chemical bath deposition 956 2% fading until 6000 cycle 2 Ag�1 No dataavailable

[44]

Ni-(Cu) electrodesposition 428 No fading until 5000 cycles No dataavailable

1 M KOH [13]

NiO-Carbon composite Hydrothermal 265 30% fading until 1000 cycles 0.25 Ag�1 6 M KOH [45]

Monolayer graphene/NiOnanosheet

Modified Hummer 525 5% fading until 1000 cycles 0.2 Ag�1 No dataavailable

[46]

3 D graphene on template for NiO Chemical vapor deposition 816 No data available 5 mV s�1 No dataavailable

[47]

Graphene sheet/porous NiO Electrophoteric deposition and chemical bathdeposition

400 No data available 2 Ag�1 No dataavailable

[48]

NiO coated Si nanowire 787.5 No data available 2.5 mA No dataavailable

[49]

NiO/Ni composite Simple reduction method 789 Stable even after 1000 0.5 Ag�1 No dataavailable

[50]

NiO/reduced graphene oxidecomposite

Homogenous co-precipitation 770 No data available 2 mV s�1 6 M KOH [51]

NiO/reduced graphene oxidecomposite

One pot chemical reduction method 461 No data available 0.21 Ag�1 6 M KOH [51]

144 B. Vidhyadharan et al. / Journal of Alloys and Compounds 610 (2014) 143–150


(PVA; Mw–95000, Merck). In a typical synthesis, NiAc (0.9 g) wasdissolved in 15 g aqueous PVA solution (7 wt% PVA in water) andstirred for 20 h at room temperature to form a homogeneous solu-tion. The as-prepared solution was electrospun using a commercialelectrospinning unit (Electroris, nanoLab, Malaysia) with a solutioninjection rate of 0.5 ml h�1 using a 21 G needle and at a potential of�24 kV. The solid fibers were collected at a distance �17 cm awayfrom the spinneret. The relative humidity of the electrospinningchamber was maintained at �30%. The as-spun fibers were cal-cined at 500 �C for 1 h in air.

Crystal structure of the material was studied by X-ray diffrac-tion (XRD) technique using Rigaku Miniflex II X-ray diffractometeremploying Cu Ka radiation (k = 1.5406 Å). Morphology and micro-structure of the materials were studied by scanning electronmicroscopic technique (7800F, FESEM, JEOL, USA). High resolutionlattice images and selected area diffraction patterns were obtainedusing transmission electron microscope (HRTEM) operating at200 kV (FEI, Tecnai G2). The BET surface area of the material wasmeasured using gas adsorption studies employing a micromeritics(Tristar 3000, USA) instrument in the nitrogen atmosphere.

2.2. Electrodes preparation and cyclic voltammetry studies

Supercapacitor electrodes were fabricated on pre-cleanednickel foam substrates. The nickel foam was cleaned by degreasingin acetone, etching in 1 M HCl for 15 min, and subsequently wash-ing in water and ethanol for 5 min each. The working electrode wasprepared by mixing the NiO nanowires with polyvinylidenefluo-ride (PVDF) (Sigma Aldrich, USA) and carbon black (Super P con-ductive, Alfa Aesar, UK) in the ratio 75:15:10. The above mixturewas stirred in N-methyl-2-pyrrolidinone for better homogeneity.The as-prepared slurry was then pasted on a nickel foam substrate(area�1 cm2) and dried in an oven at 60 �C for 24 h. The dried elec-trode was then pressed using a hydraulic press at a pressure of5 ton. Electrochemical properties of the electrodes were studiedby cyclic voltammetry (CV), galvanostatic charge–discharge

cycling, and electrochemical impedance spectroscopy in 6 M KOHelectrolyte. These electrochemical properties in three electrodeconfigurations were obtained at room temperature using a poten-tiostat galvanostat (PGSTAT M101, Metrohm Autolab B.V., TheNetherlands) employing NOVA 1.9 software. A platinum rod anda saturated Ag/AgCl electrode were used as the counter and the ref-erence electrodes, respectively.

3. Result and discussion

3.1. Morphology and Crystal structure of electrospun NiO nanowires

Fig. 1 shows the FESEM and HRTEM images of the annealednanowires. The as-prepared wires were uniform and had an aver-age diameter of �250 nm (Fig. 1A), which upon annealing reducedto �50–70 nm. i.e., the fibers showed a lowering of over 300% indiameter upon annealing (Fig. 1B). The diameter of these ‘‘green’’electrospun wires are�50% lower than that of the NiO wires devel-oped in the previous efforts using organic solvents (100–120 nm).Lower diameter of the fibers was further confirmed by HRTEManalysis (Fig. 1C). More TEM images are in the SupplementaryInformation S1. The bright field image in Fig. 1 (C) shows thatthe fibers are made of particles of size �10–20 nm with densepacking. The particles were of well faceted cuboidal morphologywith sharp edges (Fig. 1C). A selected area diffraction pattern(SAED) of a typical nanowire segment is in the in Fig. 1D. The SAEDpattern consists of diffraction spots oriented in typical polycrystal-line ring patterns which further confirm the high degree of particleorientation in the nanowires. The SAED pattern is indexed for aface centered cubic (space group: Fm�3m) structure with latticeconstant a = 4.18 Å. Fig. 1(E) shows a typical high resolution latticeimage of a typical particle; well ordered lattice fringes are clearlyvisible in HRTEM images thereby indicating that the nanowiresare well crystallized. More lattice images are in the SupplementaryInformation S2. The particles are observed to be free from crystal

Fig. 1. (A and B) the FESEM images of calcined NiO nanowires at different magnifications; (C) bright field image of a typical nanowire; (D) selected area electron diffractionpattern; (E) high resolution lattice image of a typical particle in the TEM sample.

B. Vidhyadharan et al. / Journal of Alloys and Compounds 610 (2014) 143–150 145


defects such as line and point defects; therefore, they could becalled as ‘‘highly crystalline’’.

The measured BET surface area of the NiO nanowires was�14 m2/g with a pore volume of �0.5 cm3/g, which are at lowerend for wires of diameter �50–70 nm. The observed relativelylower surface area is partly contributed by dense particle packingin the wires that reduced porosity despite the lower wire diameter.The Barrett–Joyner–Halenda (BJH) analysis showed a mean poresize of �11.5 nm because the dense particle packing resulted inlarger pores (see the TEM images in the Supporting InformationS2).

Fig. 2 shows the XRD pattern of the NiO nanowires refined usingthe PowderCell 2.3 program [19]. All the diffraction peaks corre-spond to the Fm�3m phase group; the lattice parameter obtainedfrom the diffraction pattern is 4.1716 Å, the value matches verywell with that reported for NiO (PDF card No. 10711179) and thatdetermined from SAED patterns. The XRD peaks showed linebroadening on account of the lowering of particle size; the crystal-lite size calculated using the Scherrer formula was �15 nm, whichis in good agreement with the particle size determined from theHRTEM micrographs. All these characterizations confirm that thesingle phase nanowires of NiO are obtained through the presentelectrospinning procedure.

3.2. Electrochemical properties of NiO nanowires

3.2.1. Cyclic voltammetryFig. 3 shows CV graph of the NiO nanowires measured in 6 M

KOH aqueous solution in the voltage window 0–0.5 V at differentscan rates between 1 and 30 mV s�1. The CV profiles show oxida-tion (anodic) and reduction (cathodic) events, which are character-istics of pseudocapacitors. The anodic peak in the CV profile shiftedtoward positive potentials with increase in the scan rate and thecathodic peak to the negative potential on account of the polariza-tion in the electrode material [20]. The anodic sweeps of the CVcurves are asymmetric with the corresponding cathodic sweepsindicating partial irreversibility in the redox process due to thepolarization and ohmic resistance during the faradic process [21].In pseudocapacitive materials, such as NiO, the scan rate (v) depen-dence of voltammetric current (i) is analyzed to determinewhether the capacitance originates from the surface redox reactionor from a bulk intercalation. i / m for surface redox reaction and

i /ffiffiffimp

for semi-infinite bulk intercalation [2]. We observed a goodfit for i /

ffiffiffimp

(inset of Fig. 3); and therefore, the capacitance isdominated from the bulk diffusion process.

The redox peaks are observed at all scan rates, which corre-sponds to the conversion between different oxidation/reductionstates of nickel according to the reaction [22,23].

NiOþ nOH� $ nNiOOHþ ð1� nÞNiOþ ne� ð1Þ

where n is the number of active sites accessed by the OH� ion. Themagnitude of n could be determined from the relation

n ¼ CsmDVF

ð2Þ

where m is molecular weight and DV is the potential window. Fig. 4correlates n with CS (Fg�1) as a function of scan rate. The CS of thesamples was estimated from the integrated charge from cathodicor anodic part of the CV data using the equation:

Cs ¼1

mmðE2 � E1Þ

Z E2

E1

iðEÞdE ð3Þ

where E1 and E2 are the cutoff potentials in the CV curves and i(E) isthe current at each potential, E2 � E1 is the potential window, m isthe mass of the active material, and v is the scan rate. The value of nfor the lowest scan rate (1 mV s�1) was calculated to be �29% forwhich a CS of �746 Fg�1 was obtained. i.e., only 29% of the activematerial composing 75 wt% of NiO, 10 wt% of PVDF, and 15 wt%super P involved in the electrochemical reaction gave the observedCS. In other words, remaining �71% of the active material was notavailable for the electrochemical reaction, which is because theobserved dense packing of the particles and the polymer binder(TEM image in the Fig. 1C and Supporting Information S1 and S2)hindered the electrolyte penetration to the interior of the particlesforming the wire. While comparing with the theoretical CP of NiO(�2573 Fg�1), the observed CS is �29% of it leading to conclude thatmajority of the available surface of the active material in the elec-trode contributed to the electrochemical reaction. The CS and nshowed exponential decay with increase in scan rate. Slower scanrates enable higher diffusion of hydroxyl ions into the NiO nano-wires thereby accessing a major fraction of the active site in thematerial and show high CS. The CS remained practically same forhigher scan rates (>5 mV s�1) primarily because surface redox isthe dominant mechanism at such conditions. i.e., at higherscan rates, the ions movement is limited only to the surfaces ofthe electrode material.

0

300

600

900

1200

1500

20 30 40 50 60 70 80-150

0150

(311

)

(200

)

(111

)

(220

)

Inte

nsty

(cps

)

Experimental Simulated

ΔΔI

Fig. 2. The XRD pattern of nickel oxide nanowires fitted to Fm�3m unit cell. Theexperimental and simulated intensity data (Ie and Is) are plotted as circle signs (o)and red solid lines, respectively, and DI = Ie–Is below. The tick marks indicate theposition of all possible Bragg reflections from the CIF file obtained from theCrystallography Open Database collection code COD: 1010093. (For interpretationof the references to color in this figure legend, the reader is referred to the webversion of this article.)

0.0 0.1 0.2 0.3 0.4 0.5 0.6-0.08

-0.04

0.00

0.04

0.08

1 2 3 4 5 60

30

60

90Experimental data Linear fit

Peak

Cur

rent

(mA

)

(Scan rate)1/2 ( mVs-1)1/2

Cur

rent

(A)

Potential (V) vs.Ag/AgCl

30 mVs-1

10 mVs-1

5 mVs-1

2 mVs-1

1 mVs-1

Fig. 3. The CV data of the NiO nanowire electrode in 6 M KOH aqueous solution atscan rates between 1 and 30 mV s�1 with respect to Ag/AgCl reference electrode.Inset: the variation of anodic peak current and square root of scan rate.



The ratio between the charge stored during the oxidation andthe reduction events, which is obtained from the respective inte-grated area under the CV curve, is a measure of Coulombic effi-ciency (g) of the system. The g of the present electrospun NiOnanowires is 97% in the 6 M KOH electrolyte.

To isolate the supercapacitive performance of the NiO nano-wires from the Ni foam substrate, the background capacitance ofNi foam was evaluated from the area under the CV curve. The areaof the CV curve without using the NiO nanowires was�1.3 � 10�4 cm2, which is only �0.01% of the total area in the pres-ence of it (see Supporting Information S3), thereby suggesting thatcontribution from the Ni foam is negligible in our experiment.

3.2.2. Galvanostatic charge/discharge studiesThe supercapacitive performances of the NiO nanowire elec-

trodes were evaluated from the galvanostatic charge–dischargecurves in the voltage range of 0–0.4 V in 6 M KOH electrolyte.Fig. 5 shows the first three charge discharge curves of the NiOnanowire electrode at a galvanostatic current density of 1 Ag�1 in6 M KOH. The potential drop during the discharge process, gener-ally caused by the internal resistance and incomplete Faradic reac-tion of the device, was rather low (�10 mV) indicating idealcharge–discharge cycling in the present NiO nanowires. This low-ering of internal resistance could be attributed to the high electri-cal conductivity offered by less-defective particles forming the

electrospun nanowires (Fig. 1D) [24] and high ionic conductivityof the 6 M KOH electrolyte used in this work [4].

Fig. 6 shows the galvanostatic discharge curves at various cur-rent densities from which usually the CS is calculated reliably.The discharge curve is observed to be a combination of three pro-cesses, viz. (i) a fast initial potential drop followed by (ii) a slowpotential decay, and (iii) a faster voltage drop corresponding toEDLC. The first two sections are assigned to reduction of Ni3+ toNi2+as observed from the CVs. The clear non-linear shape of thedischarge curves (Fig. 6) and the deviation from rectangular shapeof the CV (Fig. 3) reveal that the major contribution of CS of the NiOelectrode material originates from Faradic reactions.

The CS was calculated from the charge–discharge curves usingthe relation

Cs ¼It

mDVð4Þ

where I, t, m and DV are applied current, time, active mass, andpotential range of the charging and discharging events, respectively.Fig. 7 shows the CS calculated from galvanostatic discharge curvesas a function of specific current density. The CS decreased withincreasing current density similar to that observed in the CV mea-surements (Fig. 4). The total CS has contribution from Cp and EDLC.The Cp calculated from the charging (Fig. 5) and discharging((Figs. 5 and 6) to be �76%, which is generated in the potential

0 4 8 12 16 20 24 28 32

375

450

525

600

675

750

n

Data: CsModel: ExpDec1

Chi2/DoF = 569.51575R2 = 0.98509

y0 403.22706 ±24.85864A1 392.1541 ±34.74575t1 5.72289 ±1.33055

Experimental CS

Fitted CS

Scan rate (mV.s-1)

CS

(Fg-1

)

12

16

20

24

28

32

n (%

)Fig. 4. Variation in specific capacitance of NiO nanowire electrode (solid circles)and fraction of surface involved in the electrochemical reaction (solid stars)calculated from the CV data. The trend line is an exponential decay of the specificcapacitance, whose constants are given in the inset.

0 500 1000 1500

0.0

0.1

0.2

0.3

0.4

Pot

enti

al(V

) vs

.Ag/

AgC

l

Time (s)

Fig. 5. The first three charge–discharge curves of the NiO nanowire electrode in 6 MKOH aqueous solution at a galvanostatic current density of 1 Ag�1.

Fig. 6. The discharge curves of the NiO nanowire electrode at different currentdensities in 6 M KOH aqueous solution.

0 5 10 15 20200

300

400

500

600

700Data: Experimental CsModel: ExpDec1

Chi2/DoF = 414.05386R2 = 0.9926

y0 254.39806 ±21.18496A1 501.75166 ±31.15482t1 5.06384 ±0.91199

CS

(Fg-1

)

Current density (Ag-1)

Experimental Cs Fit

Fig. 7. Specific capacitance of nanowire electrode calculated from discharge curves.The trend line is an exponential decay of the specific capacitance, whose constantsare given in the inset.



range of 0.2–0.4 V corresponding to the oxidation/reduction of Ni2+/Ni3+ [22] consistent with other reports (Table 1). Therefore, EDLCcontributes 24% of the total CS. The CS evaluated from the dischargecurve is �670 and 430 Fg�1 in 6 M KOH at current densities of 1 and5Ag�1, respectively. These values are 30% higher than that reportedfor electrospun NiO developed using organic solvents, [18] whichcould be attributed to the 50% lower diameter of the ‘‘green’’ nano-wires (50–70 nm; Fig. 1) compared to the other [18]. A 50% lower-ing in wire diameter increases the effective surface area by 400%;the gain in surface area thereby achieved contributed to theenhanced CS. The relationship between surface area, pore size, andCS is well documented [25] in literature. All pores in a nanocrystal-line material, although they increase the surface area, do not con-tribute to the CS [25]. Those pores with diameter larger than thesize of the solvated electrolyte ion only contribute to the CS. Sizeof the hydrated OH� ion in the KOH electrolyte is 0.113 nm whichis much smaller than the pore size in the present NiO nanowire(11.5 nm). These larger pores provide clear insertion/exertion path-ways thereby increasing the pseudocapacitive properties as notedbefore. The contribution of CS from the substrate was studied in6 M KOH from the discharge curves without using the NiO nano-wires (see Supplementary Information S3). The discharge timewas 5 s for Ni foam alone whereas it was 268 s using NiO nano-wires; thereby demonstrating that capacitive contribution fromthe substrate could be neglected.

Effects of the high crystallinity and clear insertion/exertionpathways of the electrospun NiO nanowires are more evident inthe cycling stability of the electrodes fabricated in this study.Fig. 8 shows the stability of electrochemical cycling of the NiOnanowires. The electrodes showed good cycling behaviour with�100 % retention in CS at the end of 1000 cycles thereby shapelycontrasting the advantage of the present nanowires compared toprevious electrospun nanowires obtained using organic solvents[18] that marked >13% capacitance loss at similar conditions. How-ever, a capacity loss of 11% was recorded for the present NiO wiresfor 3000 cycles when cycled at a rate of 5 Ag�1 (SupplementaryInformation S4), which could be attributed to the partial irrevers-ibility of the current material system as observed in the CV curves.Nevertheless, compared to the literature values, a loss of 11% in3000 cycles is a superior performance [26,27]. The electrode wasalso physically stable; no peeling-off of the electrode material fromnickel form was observed even after 3000 cycles. The g calculatedfrom ratio of the discharging to the charging times during thecharge–discharge cycling was �98% and remind practically sameduring cycling.

3.2.3. Electrochemical impedance spectroscopyElectrochemical impedance spectroscopy (EIS) measurements

were carried out to determine the charge kinetic properties ofthe device associated with the above supercapacitive parameters.Fig. 9 shows the Nyquist plot of the NiO nanowire electrode deter-mined by electrochemical impedance spectroscopy (EIS) in the fre-quency range 100 kHz–0.01 Hz at open circuit potential (0.32 V) in6 M KOH. The EIS spectra of a supercapacitor device usuallydivided into three segments following three processes; (i) the bulkresistance of the device, or ESR, at high frequency (>1 kHz); (ii)capacitive effects at high and intermediate frequencies(>0.1 kHz); and (iii) Warburg diffusion resulting from the fre-quency dependence of ion diffusion/ transport in the electrolyteat low frequencies (<0.1 Hz) [28,29]. To elucidate the individualprocesses and associated impedance parameters, an electricalequivalent circuit comprising series and parallel combinations ofESR, electrical double layer capacitance (Cdl), charge transferimpedance (Rct), frequency dependent Warburg impedance (Zw),-internal resistance of NiO (Rin) and pseudocapacitance (Cp) is con-figured (bottom inset of Fig. 9) [30,31]. The ESR is a combination of(i) electrolyte resistance, (ii) intrinsic resistance of the electroactive material, and (iii) the contact resistance between the activematerial and the current collector that determines the high fre-quency off-set of the EIS spectrum [32]. The value of ESR deter-mined from the high frequency off-set of the EIS spectra is�0.76 X which is at the lower ends of the values reported forNiO based supercapacitors. The lower value of ESR could be attrib-uted to the high crystallinity of the present NiO nanowires in addi-tion to the one-dimensional morphology and ultrafine wirediameter. As the power density of a supercapacitor is equal toV2/4RSm, where V is the charging potential, a low ESR is preferredfor high power density. This low ESR could be correlated to the fastcharging/discharging displayed in Fig. 5 that would improve thepower density of the resulting device.

The discharge behaviour of a supercapacitor electrodes couldalso be evaluated from the relaxation time constant (s), which isdefined as s = 1/2pf where f is the peak frequency of the imaginarypart of the complex capacitance [33]. See Supplementary Informa-tion S5 for calculation of the complex capacitance and associateddiscussion. The value of s is calculated to be 43 ms for the NiOnanowires, which is within the range for a high power densitysupercapacitor.

Origin of the high CS could be evaluated from the first semicirclein the EIS spectra (Inset of Fig. 9), representing the redox reaction

0 200 400 600 800 1000

96

98

100

102

Cs r

eten

tion

(%)

Cycle number

Fig. 8. Dependence of the discharge specific capacitance as a function of charge–discharge cycle numbers. The charge–discharge tests were performed at 5 Ag�1 in6 M KOH aqueous solution.

Fig. 9. Nyquist plot for NiO nanowire electrode at open circuit potential. Insets anexpanded high frequency region showing the ESR; equivalent electrical circuitmodel for impedance analysis.



between Ni2+ and Ni3+, followed by a linear portion extended in themiddle and low frequency regions. The diameter of the semicircle isa quantification of Rct of the redox reactions which in the presentcase is comparatively smaller. The Rct of the electrodes employedthe NiO nanowires is calculated to be �0.45 X thereby indicatingimproved ionic conduction and electrolyte diffusion through thepores of the electrode material that lead to increased capacitance[34].

4. Conclusions

In conclusion, we have shown that NiO nanowires obtained byelectrospinning an aqueous polymer solution containing a Ni pre-cursor have lower diameter and less-density of crystalline defectscompared to the ones developed using organic solvents. The‘‘greener’’ NiO nanowires thus developed demonstrated beneficialelectrochemical properties for fabrication of an energy storagedevice under supercapacitive mode. The device based on the presentNiO showed�30% higher CS than that has been achieved in the pre-vious efforts using electrospun NiO nanotubes on account of theirlower diameter and high degree of crystallinity achieved in thiswork. Furthermore, the high degree of crystallinity improved thecycling stability and Coulombic efficiency; devices showed capacityretention of�100% at the end of 1000 cycles and gave�98% Coulom-bic efficiency. Electrochemical impedance spectroscopy measure-ments showed an equivalent electrical resistance as low as 0.76 X,which explains the relatively high performance in the one-dimen-sional NiO structures. Our results demonstrate that ‘‘green’’ electro-spun NiO nanowires could be a choice of supercapacitor electrodematerial characterized by high CS and related properties.

Acknowledgements

This work was supported by Ministry of Higher Education(MOHE), Malaysia under Exploratory Research Grant Scheme(RDU110103) and Fundamental Research Grant Scheme (RDU110602) on energy storage devices; and Malaysian TechnologicalUniversities Network (MTUN) Grant on nanowires of metal oxidesemiconductors (RDU 121210).

Appendix A. Supplementary material

(S1) More TEM and (S2) lattice images, (S3) CV and dischargecurve of the Ni foam substrate in 6M KOH; (S4) Capacity retentionbetween 1001 and 3000 cycles; (S5) Real and imaginary parts ofthe complex capacitance as a function of frequency of the NiOwires. Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.jallcom.2014.04.211.

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High performance supercapacitor electrodes from electrospun nickel oxide nanowires

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