vertically aligned zno nanorod core-polypyrrole conducting polymer sheath and nanotube arrays for...

Sidhu and Rastogi Nanoscale Research Letters 2014, 9:453http://www.nanoscalereslett.com/content/9/1/453

NANO EXPRESS Open Access

Vertically aligned ZnO nanorod core-polypyrroleconducting polymer sheath and nanotube arraysfor electrochemical supercapacitor energy storageNavjot Kaur Sidhu and Alok C Rastogi*

Abstract

Nanocomposite electrodes having three-dimensional (3-D) nanoscale architecture comprising of vertically alignedZnO nanorod array core-polypyrrole (PPy) conducting polymer sheath and the vertical PPy nanotube arrays havebeen investigated for supercapacitor energy storage. The electrodes in the ZnO nanorod core-PPy sheath structureare formed by preferential nucleation and deposition of PPy layer over hydrothermally synthesized vertical ZnOnanorod array by controlled pulsed current electropolymerization of pyrrole monomer under surfactant action. Thevertical PPy nanotube arrays of different tube diameter are created by selective etching of the ZnO nanorod core inammonia solution for different periods. Cyclic voltammetry studies show high areal-specific capacitance approximately240 mF.cm−2 for open pore and approximately 180 mF.cm−2 for narrow 30-to-36-nm diameter PPy nanotube arraysattributed to intensive faradic processes arising from enhanced access of electrolyte ions through nanotube interiorand exterior. Impedance spectroscopy studies show that capacitive response extends over larger frequency domain inelectrodes with PPy nanotube structure. Simulation of Nyquist plots by electrical equivalent circuit modeling establishesthat 3-D nanostructure is better represented by constant phase element which accounts for the inhomogeneouselectrochemical redox processes. Charge-discharge studies at different current densities establish that kinetics of theredox process in PPy nanotube electrode is due to the limitation on electron transport rather than the diffusive processof electrolyte ions. The PPy nanotube electrodes show deep discharge capability with high coulomb efficiency andlong-term charge-discharge cyclic studies show nondegrading performance of the specific areal capacitance tested for5,000 cycles.

Keywords: Zinc oxide nanorods; Polypyrrole nanotubes; 3-D nanostructures; Supercapacitor; Pulsed electrochemicalpolymerization; Electrochemical energy storage; Redox capacitance

BackgroundElectrochemical energy storage in the ultracapacitordevices is emerging as a frontline technology for high-power applications ranging from modern portable elec-tronics to electric automotive. A battery-supercapacitorhybrid energy system is a power source that can meetthe peak power demands in camera flashes, pulsedlasers, and computer systems back-up as well as electricpropulsion in diverse industrial and vehicular transportapplications. Among the materials systems, structuredcarbons which store charges as an electric double layer

* Correspondence: [email protected] and Computer Engineering Department and Center forAutonomous Solar Power (CASP), Binghamton University, State University ofNew York, New York 13902, USA

© 2014 Sidhu and Rastogi; licensee Springer. TCommons Attribution License (http://creativecoreproduction in any medium, provided the orig

(EDL) in liquid electrolyte medium are widely studiedwith a focus on overcoming the energy-density limita-tion [1]. The materials systems which show capacitivefunction based on redox reactions are the insertion-typemetal oxides and doped-conducting polymers capable ofhigh energy-density storage [2,3]. The conducting poly-mers, such as polypyrrole (PPy), poly(3,4 ethylenediox-ythiophene) (PEDOT), and polyaniline (PANI) whichundergo redox processes equivalent of doping anddedoping of electrolyte ions as means of energy storageare being aggressively studied. These polymers exhibitpseudocapacitance properties due to presence of chargetransfer reactions. The other most widely studied mate-rials are the metal oxides RuO2, MnO2, V2O5, NiO, andCo3O4 which show highly capacitive behavior due to

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Sidhu and Rastogi Nanoscale Research Letters 2014, 9:453 of 16http://www.nanoscalereslett.com/content/9/1/453

reversible and fast surface redox reactions with electro-lyte ions [2,4].In the recent years, conducting polymers with a nano-

porous morphology and as nanocomposites with metal-oxides have emerged as the materials system of greatpotential for high energy-density storage. Electrodesbased on these materials structured at the nanoscale en-able many-fold enhancements of the electroactive sur-face and interface with electrolyte facilitating absorption,ingress, and diffusion of electrolyte ions which being themain energy storage units could lead to increased energyand power density of supercapacitor devices. The highsurface area morphology in conducting polymers isattained by creating variations in its nanostructure likenanoporous [5], nanofibers [6,7], nanowires [8], nano-belts [9], and by size-selective nanopores in the contextof carbons [10]. Most metal oxides are electrically resist-ive in character and the redox reactions here are limitedto the surface regions. In order to accrue full advantageof their pseudocapacitive function, electrically conduct-ing polypyrrole is encrusted by metal oxides [11,12],PEDOT [13], CNT [14], and graphene [15,16] in variousnanostructured forms like nanoparticle [17,18], nanotube[19,20], and nanowire [21,22] as supercapacitor electrode.In these materials systems, the nanostructure features arerandomly distributed in the two-dimensional (2-D) filmform mainly due to the preparatory methods.Most recent research thrust in the conducting

polymers and their nanocomposite with metal oxides isdirected towards the electrodes with three-dimensional(3-D) nanoarchitecture such as vertically aligned nano-tubes [23] and nanorods [24]. These nanostructures havepotential for the limiting electrolyte-ion diffusion prob-lem by decreasing the ion diffusion paths and at thesame time increasing the surface area for enhancedelectrode-electrolyte interaction. In the past, randomlyoriented conducting polymer nanotubes structures havebeen synthesized [16,25,26] for supercapacitor applica-tions. However, the vertically oriented nanostructures,nanorods, and nanotubes have been mostly configuredusing the metal oxide templates [27]. Such nanostruc-tures have been created by more innovating nanoscaleengineering methods like oxidative polymerization [28],electrochemical anodic oxidation [29], electrodeposition[30], and hydrothermal synthesis [31,32]. Furthermore,by combining the redox conducting polymers with thewell-known pseudocapacitive oxide like MnO2, formingthe nanocomposites in the 3-D nanoarchitecture pre-sents multiple advantages with enormous potential tooutperform their 2-D counterparts. The composite 3-Dnanostructure can be created by conformal deposition ofredox-active conducting polymer, pseudocapacitive oxidelayer, or their multilayer stacks over vertical nanostruc-tures of TiO2, ZnO, or NiO serving as templates. The

composite 3-D nanostructured electrodes have synergiccontribution to specific capacitance based on their elec-troactive functions which boost energy density, and theirnanoarchitecture have the ability to mitigate the ion dif-fusion limitation thereby enhancing the power density.In the past, 3-D nanotube polymers, PPy-PANI [33]polymer-metal oxides, TiO2-PPy [34,35], ZnO-PPy [36],TiO2-NiO [23], and TiO2-V2O5 [37] have been reported.In this work, we investigate the characteristics of

nanocomposite electrodes for supercapacitors having3-D nanoscale architecture, the one comprising of verti-cally aligned zinc oxide nanorod arrays at the core withdoped-polypyrrole conducting polymer sheath and theother vertical polypyrrole nanotubes arrays. Althoughpolypyrrole in the doped state shows high electricalconductivity, the conversion between redox states is veryslow due to the slow transportation of counter ions tobalance the charge in the polymer structure [38]. Thevertical polypyrrole nanotube and sheath structure arelikely to decrease the charge transfer reaction time andthus enhance the charge storage capabilities [38]. Zincoxide used prominently in various devices such as bio-sensors [39], light emitting diodes [40], organic solarcells [41], and spintronics [42,43] is a biocompatible,highly stable, and less expensive material as compared toruthenium oxide and therefore has a good potential forthe electrochemical energy storage devices. Zinc oxidecharacterized as a wide band gap semiconductor withexcellent chemical and physical properties can be easilytransformed in various nanostructure forms like nano-wire, nanoplatelets, and nanoneedles mostly as flat two-dimensional structures [44]. In the context of using aZnO template for a supercapacitor electrode in the 3-Darchitecture, we have fabricated vertically aligned ZnOnanorods by hydrothermal synthesis which exhibit spe-cific electrical and optical properties [32]. The nano-composite electrode is formed by deposition of dopedpolypyrrole layer over ZnO nanorod at the core by thecommercially viable, low cost solution-based pulsedcurrent electropolymerization process [45]. Pulsed cur-rent process allows depositing polypyrrole layer select-ively with highly controlled thickness through theapplication of number of pulses. Only a few studies havebeen reported on electrodes with zinc oxide and poly-pyrrole composite films for supercapacitor energy stor-age devices perhaps since zinc oxide nanostructures maybe resistive compared to conducting polymers [46]. ZnOnanorods as template to create PPy nanotube structureswith inlay of MnO2 and their energy storage behaviorhave been reported [36]. In this work, the conductingdoped polypyrrole supercapacitor electrodes in two dif-ferent 3-D architectural forms, one having ZnO nanorodcore-PPy sheath and the other vertical PPy nanotube arrayhave been investigated. The electrochemical properties of


these electrodes were studied by impedance spectroscopy,cyclic voltammetry, and charge-discharge measurements.Randles circuit model with additional capacitance and re-sistance elements is developed to explain the characteris-tics of electrode at various frequency ranges. Long-termcharge-discharge tests are carried out to evaluate thecycle life of such electrodes in supercapacitor energystorage device. This paper reports the results of thesestudies.

MethodsSynthesis of ZnO nanorod array templatePolypyrrole conducting polymer electrodes in the twoZnO core-PPy sheath and PPy nanotube structural formswere fabricated over a ZnO nanorod template. The tem-plate, a vertically oriented two-dimensional array of ZnOnanorods, was formed over surface-activated 500-μm-thingraphite substrates by thermo-decomposition of zinc ni-trate aqueous solution in the presence of hexamethylene-tetramine in a wet chemical process [32]. The surface ofthe graphite substrate is activated by depositing a 20-nm-thick ZnO seed layer that acts as nucleation centers forthe growth of ZnO nanorods. This layer is formed byradio-frequency (RF) plasma sputtering from a stoichio-metric ZnO target in the argon ambient at 50 mTorr pres-sure and 100-W RF power. The growth of ZnO nanorodsis done in a solution of 0.03 M zinc nitrate hexahydrate(Sigma-Aldrich) and equimolar hexamethylenetetramine(HMT) (Sigma-Aldrich, St. Louis, MO, USA) in 18 MΩdeionized water. The surface-activated graphite substrateis vertically kept in this solution in the autoclavable glassbottle and held steady at 95°C for 8 hours. This hydrother-mal procedure results in the dissociation of Zn(II)-aminocomplex resulting in ZnO which grows as ZnO nanorods[47]. Post deposition, the graphite substrates are rinsed indeionized water to remove any residual precursor chemi-cals and dried in air.

Pulsed current electropolymerization of polypyrrolesheath and nanotube formationA uniform and conformal deposition of polypyrrole of acontrolled thickness over ZnO nanorods is essential forthe creation of PPy sheath and nanotube nanostructures.Polypyrrole has been deposited the past by variouschemical [48] and potentiostatic electropolymerization[49] methods. In this work, PPy deposition is done byelectropolymerization of pyrrole monomer in situ overZnO nanorods using the ultrashort multiple unipolarpulsed-current method reported earlier [45]. Electropoly-merization is accomplished in an electrolyte solutioncontaining 0.01 M pyrrole (Py) monomer (Sigma-Aldrich)and 0.1 M lithium perchlorate dopant ions in the presenceof 0.06 M sodium dodecyl sulfate (SDS) surfactant. Thepreparatory steps involve first the dissolution of pyrrole

monomer in the presence of SDS under continuous stir-ring in deionized water warmed to 40°C. Later, the ZnO-seed-layer-coated graphite substrate is dipped verticallyfor approximately 20 min in this solution which helps inthe wettability of the ZnO nanorods with pyrrole mono-mer. Before initiating the electropolymerization process,lithium perchlorate is added to form 0.1 M solution. Theformation of the polypyrrole sheath layer over ZnO nano-rods is carried out by pulsed current electropolymeriza-tion method in a two-electrode cell using a platinum (Pt)sheet as a counter and a reference electrode. In thismethod, multiple unipolar anodic ultrashort 10-ms-duration constant-current pulses of amplitude 4 mA.cm−2

are applied. Each of the pulses is interspaced by a current‘off ’ period of 100-ms duration. The electropolymerizationis initiated during the pulse ‘on’ period as the corre-sponding anodic potential exceeds the oxidation potentialof the pyrrole monomer. The current off period essen-tially helps create the equilibrium conditions in the vicin-ity of ZnO nanorods for deposition under homogenouspolymerization conditions. The number of current pulseseffectively controls the polypyrrole-layer thickness andusually approximately 5 to 10 k pulses were used to formfully covered PPy sheath over ZnO nanorods. In somecases 20 k pulses were also applied to form a thicker PPysheath.The PPy nanotube structure is obtained by etching

away the vertically aligned ZnO nanorod core in a 20%ammonia solution [36]. The ZnO etching process istypically initiated over ZnO nanorod tips due to rela-tively thin cladding of the electropolymerized polypyr-role. As a result, the PPy nanotube structure showsdependence on the etching time. In this work, etchingtimes of 2 and 4 h are used for the formation of PPynanotube arrays.

Electrochemical characterization of supercapacitorelectrodesEfficacy of the ZnO nanorod core-PPy sheath and PPynanotube electrodes for the supercapacitor energy storagedevice application was analyzed by various electrochem-ical characterizations. These electrodes were characterizedby cyclic voltammetry (CV), alternating current (ac) im-pedance spectroscopy, galvanic charge-discharge, andlong-term cyclic tests in a three-electrode cell with Ptsheet as counter electrode and the potential referenced toa saturated Ag/AgCl electrode in an electrolyte compris-ing of an aqueous solution of 1 M lithium perchlorate.Cyclic voltammetry and galvanic charge-discharge mea-surements were carried out using Solartron electrochem-ical interface (Model 1287 from Solartron Analytical, OakRidge, TN, USA). In cyclic voltammetry, the flow of elec-tric current between the working electrode and Pt counterelectrode was recorded in the potential range −0.5 to +0.5

200 nm

A

100 nm

200 nm

B

Figure 1 SEM images of ZnO nanorod arrays grown on graphitesubstrate. (A) Image showing the microstructure of ZnO nanorodarrays. (B) Magnified image showing the top end of the rods withhexagonal facets.


V scanned at different rates between 5 to100 mV.s−1. Theareal-specific capacitance, Csv (F.cm−2), of the electrodeswas calculated using the relation,

Csv ¼ ia þ icj j2s

ð1Þ

where ia and ic are the absolute values of the anodic andcathodic current (mA.cm−2) of the electrode area and sis the scan rate (mV.s−1). The galvanic charge-dischargecharacteristics were measured at various current dens-ities, id, varying between 1, 2, and 3 mA.cm−2 in thepotential range of 0.05 to 0.5 V. In the discharge cycle,using the discharge time, Δt, and a correspondingchange in voltage, ΔV, excluding the IR voltage drop, theareal-specific capacitance Csd (F.cm−2) is calculated bythe relation,

Csd ¼ id⋅ΔtΔV

ð2Þ

The ac impedance measurements were carried out in atwo-electrode configuration in the frequency range 1mHz to 100 kHz with ac signal amplitude of 10 mVusing Solartron Impedance/Gain-Phase Analyzer (Model1260). Measured low-frequency imaginary impedance Z″provides estimate of the overall capacitance Ci usingthe relation Ci = 1/|ωZ″|. The Nyquist plots using theimpedance data were simulated using the equivalentelectrical model representing the electrochemical andelectrophysical attributes of the nanostructured ZnO-PPy electrode using ZPlot software (Scribner Associates,NC, USA) which provide the characteristic resistancesand various contributing factors to the overall electrodecapacitance.

Results and discussionMicrostructure of ZnO nanorod core-polypyrrole sheath,nanotube electrodesThe microstructure of ZnO nanorod arrays grown overgraphite substrates is shown by SEM micrograph inFigure 1A. These vertically grown ZnO nanorods arehomogeneously dispersed across the substrate surfaceand their average length dependent on the growth timeis typically approximately 2.2 to 2.5 μm. These rods ap-pear to have smooth surface texture and nearly uniformaverage diameter of approximately 60 nm. As shown inthe magnified image in Figure 1B and its inset, the topend of these rods have a hexagonal facet signifying theserods grow along the crystalline c-axis.In the formation of PPy sheath over ZnO nanorods, its

thickness is controlled by the number of pulsed currentcycles. Figure 2A shows the early steps of the pulsedpolymerization representing the formative stages of thegrowth of polypyrrole layer over ZnO nanorod arrays. It

shows that the polypyrrole layer consisting of smallcompact nodular features forms conformal to the ZnOnanorods across its entire length. The nodular surfacestructure of polypyrrole layer is due to congregation ofpyrrole monomer resulting from the action of SDS sur-factant [50]. Furthermore, there is no deposition of poly-pyrrole in the interrod space and the PPy sheath formspreferentially over ZnO nanorods due to pyrrole mono-mer incursion by the action of the SDS surfactant as dis-cussed later [50]. The inset shows a magnified view of aZnO nanorod at the core coated with PPy sheath havingoverall average diameter of approximately 110 nm.Figure 2B shows ZnO core-PPy shell structure after elec-tropolymerization has been accomplished for the full 10k unipolar pulsed current cycles. The average diameterof the ZnO-core-PPy shell grows to approximately 360nm which translates to approximately 150 nm averagethickness of the PPy layer as shown by the magnifiedview of the top of ZnO nanorods in the inset ofFigure 2B. At this growth stage, the inter-ZnO nanorodspace begins to fill due to the coalescence of PPy sheathformed over different ZnO nanorods in the array. Forthe creation of the freestanding PPy nanotube array, the

360 nm

200 nm 200 nm

200 nm 200 nm

A B

C D

Figure 2 SEM images of ZnO nanorod arrays coated with pulsed current polymerized PPy sheath. (A) Initial stage of PPy oligomers clusterdeposition, (B) ZnO core-PPy sheath structure after 10 k pulsed electropolymerization cycles, (C) PPy nanotube array after 2-h etch, and (D) openpore PPy nanotube array after 4-h etch.


ZnO nanorod in the core is etched away in 20% ammo-nia solution. Figure 2C shows the partial etched state ofthe ZnO core for 2 h which creates tubular holes of ap-proximately 30 to 36 nm in average diameter as shownin the inset of Figure 2C. At this stage, the PPy nanotubearrays still have in their interior a finite thickness ofZnO cladding. To remove the ZnO cladding, additionaletching was carried out. It was observed that after aprolonged etching for approximately 4 h, a completeremoval of the ZnO cladding was realized which resultedin the formation of a network of PPy nanotubular arraysas shown in the micrograph in Figure 2D. A magnifiedview in the inset shows PPy nanotubes of diameterapproximately 60 to 70 nm consistent with the typicaldiameter of the ZnO nanorod core. Figure 2D alsoshows that a large number of these PPy nanotubes sharea common sheath wall which had initially resulted fromthe PPy growth in the space between neighboring ZnOnanorods. This enables formation of the interconnectedopen network of freestanding PPy nanotube arrays. ThePPy nanotube diameter can be enhanced by formingthicker ZnO nanorod array core structure. However, thisreduces the effective thickness of PPy tubular sheath andhence the effective mass of PPy which is an active com-ponent for charge storage. On the other hand, increasingthickness of PPy by electropolymerization for longerpulsed current cycles excessively covers the top of theZnO nanorod arrays making it difficult to etch away theZnO core which prevented realization of PPy nanotubu-lar arrays. Figure 3 shows the ZnO core-PPy sheath

structure with the thicker PPy layer deposited using 20 kunipolar pulsed current cycles. This results in formationof thick conjoined PPy sheath with thickly deposited PPyover the top of ZnO nanorods (Figure 3A). Figure 3Bshows a cross-sectional view indicating the ZnO nano-rods could still be coated with PPy along its length. Theside panel in Figure 3C shows conjoined PPy sheath overZnO nanorods of average diameter approximately 985nm to 1 μm. Morphology of the thick PPy deposit is likenodules. Figure 3D shows the top view of the PPy coatedZnO nanorods tips. Figure 3E shows the same view afterammonia etching for 4 h. It is evident that such ZnOnanorod core-PPy sheath structure did not result in thePPy nanotube structure after etching. The evolution of thePPy sheath and nanotube structure is schematically shownin Figures 4A, B, C, D, E, F. The vertical ZnO nanorodarray (Figure 4A) is preferentially coated with PPy bypulsed electropolymerization process through surfac-tant action. Progressively, on continued pulsed currentpolymerization cycles, the PPy sheath thickness in-creases (Figure 4B) with possible merging of PPy sheathwalls (Figure 4C). Figures 4D, E, F show the evolutionof PPy nanotubes through etching of ZnO core startingat the nanorod tips which after short term etchingresults in the PPy nanotubes along with the invertedconical ZnO cladding (Figure 4D). The PPy nanotubearrays without the ZnO cladding are created by completeetching of ZnO for longer periods as depicted in Figure 4Ewith an open pore structure as shown in the top viewin Figure 4F.

2 µm 2 µm 1 µm

2 µm 2 µm

A B C

D E

Figure 3 SEM images. (A) Thicker PPy deposited over ZnO nanorod array when electropolymerization was carried out for 20 k pulsed currentcycles, (B) cross-sectional view of PPy sheath coated along the ZnO rod length, and (C) conjoined view of PPy sheath over ZnO nanorods withaverage diameter of 985 nm. Top view of ZnO nanorod tips with thick PPy sheath (D) before etch and (E) after ammonia etch.


Growth features of ZnO nanorod-PPy sheath and PPynanotube arraysUnlike the two-dimensional flat conducting substrates inwhich case conventional direct current (dc) potentiostaticelectropolymerization of pyrrole can produce uniform

ZnOnanorod array

Ppy nanotube2 h-etch 4 h-etch

A B

D EFigure 4 Schematic representation of the evolution of the PPy sheathcore coated by PPy sheath (A-C) and formation of PPy nanotube array afte

thick polypyrrole film, over the semiconducting ZnOnanostructures, pulsed current electropolymerizationemployed in this work was found essential to obtainhomogeneous polypyrrole sheath. In order to create PPy3-D tubular nanostructures for energy storage action, it is

PpySheathlayer

narrow2 h etch

Ppy nanotubes top view

open-network4 h etch

C

Fand nanotube structure. Schematic representation of ZnO nanorodr 2- and 4-h etch (D-E). Top view (F).


essential (i) to form the PPy sheath in the high-conductivity anion doped state and (ii) to have the PPysheath of desired thickness coated uniformly over the en-tire length of the ZnO nanorod array at its core. The firstcriterion is largely met by anodic electropolymerization ofpyrrole monomer in the aqueous medium in the presenceof ClO4

2− anions derived from LiClO4 in the electrolyte.Various mechanisms of pyrrole electropolymerizationhave been proposed under potentiostatic condition [51,52].In the pulsed current electropolymerization process, thepolypyrrole growth over ZnO nanorod surface proceedsby concomitant reactions, anodic oxidation of pyrrolemonomer, and conjugation reaction with electrolyte(ClO4

−) anions as shown in Figure 5A. On application of acurrent pulse of magnitude 4 mA.cm−2, the pyrrole mono-mer species over the ZnO nanorod surface rapidly oxidizeby electron transfer at electrode resulting in the nucle-ation of significantly large number of Py½ �þ●

nþm cation radi-cals. By themselves, these are unstable but stabilize rapidly

A

B

Figure 5 Electropolymerization process of the polypyrrole growth ovand ClO4 conjugation. (B) Model of electropolymerization growth of PPy sh(C) homogenous growth of PPy sheath over ZnO nanorods after a numbe

on interaction with the nearest cation radicals to formshort chain oligomers by coupling and bond linkagewith the involvement of deprotonation (−2H+)n+m step[5,45,51,52]. A number of cation radicals Py½ �þ●

m at theinitiation step are also influenced by strongly interactingelectrolyte ClO4

− anions which result in conjugation ofPPy short chain oligomers deposited over ZnO nanorods[53]. The current pulse off time replenishes the Py-monomers at the ZnO nanorods by diffusion in theaqueous medium. The subsequent pulsed current cyclereinitiates the electropolymerization reaction at freshnucleation sites on ZnO nanorods by a similar processsequence thus providing a uniform coverage.The preferential nucleation and growth of polypyrrole

across the ZnO nanorod length is significantly affectedby the lack of access of the pyrrole monomer in deepcrevices along the depth of ZnO nanorod array markedby the narrow and not-so-consistent interrod spacingtypically varying between 120 to 250 nm. This is further

C

er ZnO nanorods. (A) Electrochemical polymerization of Py monomereath over ZnO nanorods in the presence of SDS surfactant andr of pulsed current cycles.

-0.50 -0.25 0.00 0.25 0.50

-3

-2

-1

0

1

2

3c

b

Scan Rate 10 mV.s-1

a: ZnO Core:Ppy Sheathb: Ppy Nanotube: 2h etchc: Ppy Nanotube: 4 h etch

Cur

rent

(m

A.c

m-2

)

Potential (Volts vs Ag/AgCl)

B

a

-0.50 -0.25 0.00 0.25 0.50

-2

-1

0

1

2

c

b


Cur

rent

(m

A.c

m-2

)

Scan Rate 5 mV.s-1

a

a: ZnO Core:Ppy Sheathb: Ppy Nanotube: 2h etchc: Ppy Nanotube: 4 h etch

A

Figure 6 Cyclic voltammetric plots of the electrode withnanostructured ZnO nanorod core-PPy sheath. PPy nanotubeafter etching away ZnO nanorods for 2 and 4 h measured at scanrates (A) 5 mV.s−1 and (B) 10 mV.s−1.


aggravated by aqueous immiscibility of pyrrole monomerwhich inhibits wetting of ZnO rods which might inhibitformation of uniform polypyrrole sheath. In the presentcase, the use of SDS anionic surfactant mitigates this bytransporting pyrrole monomer to the surface of ZnOnanorods. A possible model of electropolymerizationgrowth of PPy sheath over ZnO nanorods in thepresence of SDS surfactant is shown schematically inFigure 5B. The SDS ionizes into Na + cation and CH3

(CH2)11OSO3− anion in aqueous medium. The SDS

concentration used in this study is less than the criticalvalue 8 mM for the first micelles concentration (CMC-1)hence the SDS molecular chain containing 12 carbon al-kyls with sulfate group at the end are in the extended statein the aqueous medium [54,55]. The dodecyl alkyl mo-lecular chain being hydrophobic orients away from waterand this easily attaches on to the ZnO nanorod surfacewhile the hydrophilic OSO3

− group project outward intoaqueous environment. The pyrrole monomers are hydro-phobic in character and sparingly soluble in water. A largenumber of pyrrole monomers are able to preferentiallydisperse within the hydrophobic region created by at-tached dodecyl alkyl molecular chain over ZnO nanorodsurface [50]. This ensures uninhibited supply of the pyr-role monomer and dopant ClO4

− anions across the exter-ior of ZnO nanorods [55] and consequently forming PPylayer over ZnO rods comprising of short-chain doped PPyoligomers by electronation-protonation-conjugation reac-tion described in Figure 5B. Spatially distributed depos-ition of PPy oligomers as clusters is evident in the nodulelike the microstructure study shown in Figure 2A. Thepyrrole monomer availability during current pulsed offtime is no longer diffusion-rate limited and efficientincursion of pyrrole results in the increased electropoly-merization rates. In the subsequent pulse cycles, theelectropolymerization is reinitiated over new ZnO surfacesites or over PPy coated surface as shown schematically inFigure 5C resulting in homogenous formation of the PPysheath over ZnO nanorods after a certain number ofcurrent pulsed polymerization cycles.

Cyclic voltammetry studyFigure 6A, B shows a set of CV plots recorded at slowscan rates of 5 and 10 mV.s−1 comparing the electro-chemical performance of the ZnO nanorod core-PPysheath electrode with the PPy nanotube structured elec-trodes obtained by etching ZnO nanorods for 2 and 4 h,respectively. All CV plots are nearly rectangular inshape, symmetrical across the zero current axis, and donot show any oxidation-reduction peaks demonstratinghighly capacitive behavior. The current in the PPy nano-tube electrodes is significantly higher in comparison toZnO nanorod-PPy sheath electrode and it further in-creases for the electrode with an open pore PPy

nanotube structure obtained after 4-h etching as shownby the microstructure in Figure 2D. It is apparent thatPPy nanotube electrode structure offers improved accessto the ions through the tube interior in addition toexterior regions which are accessed equally by all threeelectrodes. Figure 7A, B shows CV plots measured atscan rates of 5 to 100 mV.s−1 for the PPy nanotube elec-trodes obtained after etching of ZnO nanorods at the corefor 2 and 4 h, respectively. The increase in the currentwith the scan rate indicates the kinetics of the faradicprocess and the electronic-ionic transport at the PPynanotube-electrolyte interface. It is easy to observe fromFigure 7 that more open PPy nanotube electrodes after4-h ZnO etch show higher anodic and cathodic currentat every scan rate as compared to the 2-h etchedelectrodes in the same potential window. Although bothelectrodes showed good charge propagation capabilities,

-0.50 -0.25 0.00 0.25 0.50

-8

-4

0

4

8


Cur

rent

(m

A.c

m-2

)

5 mV.s-1

10 mV.s-1

20 mV.s-1

50 mV.s-1

100 mV.s-1

Ppy Nanotube 2h Etched

A

-0.50 -0.25 0.00 0.25 0.50

-12

-8

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0

4

8

12


Ppy Nano Tube 4h Etched

5 mV.s-1

10 mV.s-1

20 mV.s-1

50 mV.s-1

100 mV.s-1

Cur

rent

(m

A.c

m2 )

B

Figure 7 Cyclic voltammetric plots of PPy nanotube electrodesmeasured at different scan rates. (A) 2-h etch and (B) 4-h etch.

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35

1.0

2.0

3.0

4.0

5.0

6.0

7.0

Ppy Nanotube: 2 h Etched

Peak

Cur

rent

Den

sity

, Jpc

(A.c

m-2

)

b

a

(Scan Rate, )1/2 (V.s -1)1/2

Ppy Nanotube: 4 h Etched

Figure 8 Randles-Sevcik plots of PPy nanotube electrodes after2- and 4-h etching of ZnO nanorod core.

0 2 40

50

100

150

200

250

100 mV.s-1

50 mV.s-1

20 mV.s-1

10 mV.s-1

Spe

cific

cap

acita

cne

(mF

.cm

-2)

ZnO Core Etching Time (hour)

5 mV.s-1

Figure 9 Specific areal capacitance at different scan rates for ZnOnanorod core-PPy sheath PPy and PPy nanotube electrodes.


the difference in the current density of the electrodes isattributed to the structural changes due to etching. TheCV plots show that though rectangular shape is nearlypreserved as the scan rate is increased until 50 mV.s−1, ageneral trend is a progressively narrower and slightlyoblique-angled CV plot for scan rate of ≥50 mV.s−1. Thefactors responsible for such a behavior are the contactresistance and the delayed response time of the faradicreactions nonsynchronous with the faster scan whichotherwise would have boosted the total capacitance.The growth in current density of the PPy nanotube

electrodes with the increasing scan rate as shown inFigure 7 is reflective of the dissimilarities in terms of theporosity of the nanotube structure and improved per-formance of the 4-h etched PPy nanotube electrode. Therise in the cathodic peak current density JPC with scanrate, ν, follows the Randles-Sevcik equation,

JPC ¼ 0:6104n1:5Fc

ffiffiffiffiffiffiffiFDRT

r⋅

ffiffiffiffiffiffiν

p ð3Þ

where F is the Faraday number and R is the ideal gasconstant. The active specie concentration in electrolyteis denoted by c, and the number of electron-involved re-duction processes by n. The parameter D represents theapparent charge transfer coefficient by diffusion. A linearplot of the current JPC vs

ffiffiffiν

pshown in Figure 8 for 2-

and 4-h ZnO core etched PPy nanotube electrodes sug-gests that according to the Randles-Sevcik formulationthe charge transport process is diffusive-controlled.Figure 8 shows that compared to the 2-h etched elec-trode, 4-h etched PPy nanotube electrode has a higherslope which suggests that in this electrode the electrolyteions are more easily accessible due to the presence ofhigher diffusivity paths through interconnected nano-tubes and therefore have improved ability to storecharges. This is consistent with the microstructure studyas well as the results of the CV analysis. The measured

Figure 10 Nyquist plots of actual data and fitted spectrum ofZnO nanorod core-PPy sheath electrode. Inset shows expandedview in the high- and mid-frequency region.


D values were found to be 7.27 × 10−8 and 1.09 × 10−7

cm2.s−1 for the PPy nanotube structure formed after 2-and 4-h etching, respectively, which is at least an orderof magnitude higher than for the PPy films in 2-D por-ous structure [45]. These data show that homogenoustransport dynamics of charge-compensating anions inthe electrolyte is generally fast for 3-D PPy nanotubesespecially for open interconnected PPy nanotubes formedafter 4-h etch.Specific capacitance CSV calculated from the CV plots

using Equation 1 at different scan rates is plotted inFigure 9 for both ZnO nanorod core-PPy sheath and PPynanotube electrodes represented by 0-, 2-, and 4-h ZnOcore etch times. The true faradic specific capacitance dueto redox processes measured at low scan rates increasesdramatically when the PPy nanostructure transforms fromcore-sheath to nanotube. Thus, ion diffusion process inPPy nanotube structure is kinetically faster. At higher scanrates (≥50 mV.s−1), the specific capacitance on structuretransformation shows moderate increase at best for elec-trode with open pore PPy nanotube structure obtainedafter a 4-h ZnO core etch. Limiting kinetics for ion diffu-sion is the same for PPy sheath and nanotube structures.

Impedance spectroscopyElectrochemical impedance spectroscopy (EIS) techniqueis extensively used to elucidate the electrical characteristics

Table 1 Electrochemical impedance spectroscopy data obtain

Components Rs (Ω.cm2) Rct (Ω.c

ZnO nanorod core-PPy sheath 0 5.8

Narrow PPy nanotube (2-h etch) 0 8.2

Open PPy nanotube (4-h etch) 1 7.2

of the electrode material and its interface with the sup-porting electrolyte. Frequency response of the real and im-aginary impedance of the pseudocapacitive ZnO nanorodcore-PPy sheath electrode with 1 M lithium perchlorateelectrolyte was studied. Impedance of the electrode is acomplex quantity and the extracted data are plotted as real(Z′) versus imaginary (Z″) impedance representing theNyquist plot. Figure 10 shows the Nyquist plot of the as-deposited ZnO nanorod core-PPy sheath electrode in thefrequency domain 0.1 MHz to 0.01 Hz and the inset showsexpanded view in the high- and mid-frequency region.The capacitive component is reflected in the rapidly in-creasing imaginary impedance (Z″) at lower frequencies.The high-frequency real impedance (Z′) characterizes thebulk electrode and interfacial resistive properties of theelectrode-electrolyte system. These parameters calculatedfrom the impedance plots are shown in Table 1. Instead ofthe characteristic whole semicircle, the high-frequencyNyquist plot degenerated into an arc segment. This sug-gests that contribution to the bulk electrode-electrolyte re-sistance is mainly from the ZnO-PPy interface barrier dueto polarization effect of the nanostructured electrode andnegligible electrolyte resistance. The mid-frequency cutoffof this arc on real impedance Z′-axis gives the chargetransfer resistance, RCT, of 6 Ω.cm2 resulting from thekinetically-controlled electron transfer and anion conjuga-tion reaction in the PPy sheath layer. In progression fromthe mid- (0.41 kHz) to low-frequency range, a knee fre-quency of 0.032 Hz is identified indicating the onset of thecapacitive impedance. The slow rising impedance in thisfrequency range is reflective of ion adsorption through theporous structure of the PPy sheath as well as along thelength of ZnO nanorods. The capacitive impedance (Z″)shows a shift along more resistive Z′ values which is causedby the limitation on the rate of ion migration. Beyond theknee frequency, however, the system response is highlycapacitive. The low-frequency areal-capacitance density, CF,is determined from the Nyquist plot as 107 mF.cm−2.Figure 11A, B shows the Nyquist plots of the PPy

nanotube structure obtained after etching ZnO core for2 and 4 h, respectively, as described by the SEM study inFigure 2C, D. The major effect of such structural changeappears in the shift of the knee frequency to higher fre-quency values. After 2-h etching with narrow (33 ± 3 nm)PPy nanotube opening and after 4-h etching with openpore interconnected PPy nanotube formation the re-corded shifts in knee frequency are 0.16 and 1.07 Hz,

ed from actual Nyquist plots

m2) W (Ω.cm2) Ci (mf.cm-2) Ci (f.g-1)

20.4 107.3 74

8.4 84.2 58

5.4 83 57.2

A

B

Figure 11 Nyquist plots of actual data and fitted spectrum ofPPy nanotube electrodes obtained after etching ZnO core.(A) 2 h and (B) 4 h.

0.01 0.1 1 10 100

0.0

0.2

0.4

0.6

0.8

1.0

Pha

se A

ngle

(de

g)

C'/C

0

-80

-60

-40

-20

0

4 h

4 h

2 h

2 h Ppy Nanotube Electrode

Frequency (Hz)Figure 12 Frequency dependence of areal-specific capacitanceto dc capacitance and phase angle variation for PPy nanotubeelectrodes.

RS

Rct

Rnr CPEdl

CPEW CPEnr

Figure 13 Equivalent electric circuit model used for simulationof Nyquist plots.


respectively, compared to the knee frequency of 0.032 Hzfor unetched ZnO nanorod-PPy sheath structured elec-trode. This shift is significant. Simultaneously, the low-frequency impedance Z″ shows a systematic shift to lowervalues on the real impedance axis. Considering that kneefrequency defines the upper frequency limit of the resist-ive behavior and a capacitive one at lower than kneefrequencies, it is inferred that the PPy nanotube sheathstructure is more capacitive in nature. Furthermore, forthe unetched ZnO nanorod core-PPy sheath electrodes,the capacitance at knee the frequency is approximately0.68CF of the overall capacitance CF. Correspondingvalues for the 2- and 4-h etched PPy nanotube electrodesare 0.61CF and 0.22CF, respectively. These data suggestthat over a substantive frequency range the impedance ofthe PPy nanotube electrode is capacitive in nature. Clearly,the frequency domain of ion diffusion region which resis-tively contributes to impedance, commonly known as theWarburg resistance, has shrunk in PPy nanotubes after

2-h etching and more significantly in the open intercon-nected PPy nanotube structure obtained after 4-h etchingof ZnO nanorods. It is obvious from the microstructurestudy of Figure 2C, D and schematic representation inFigure 4 that the interconnected network of 60 to 70 nmdiameter open PPy nanotubes facilitates rapid and morepervasive ingression of anions from electrolyte as fre-quency decreases below the knee frequency. As frequencydecreases, electrolyte ions by diffusion are accessible tomore and deeper porous surface of the PPy nanotube ar-rays. The frequency response of the impedance is modeledin terms of complex capacitance C(ω) =C′(ω) − jC″(ω) todescribe the capacitance behavior of the electrodes [56].Here, C′(ω) is the real part of capacitance representing theenergy storage component and C″(ω) the imaginary partrepresents the resistive losses in the storage process. Thereal capacitance is computed according to equation C′(ω) =[−Z″(ω)]/[ω|Z(ω)|2]. Figure 12 shows variation of C′/C0

as a function of frequency, where C0 is dc capacitance[57]. As the frequency decreases, C′ sharply increasesbelow and above 1 Hz, the capacitance is practically non-existent. Figure 12 also shows phase angle variation withfrequency. The low-frequency phase angle shows a plateauat −65° for PPy nanotube sheath electrode after 4-h

Table 2 Characteristic resistance and capacitive parameters estimated by fitting of Nyquist plots

Components CPEdl (mMho, p) Rct (Ω) CPEw (mMho, p) Rnr (Ω) CPEnr (mMho, p)

ZnO nanorod core-PPy sheath Q = 0.025 p = 0.55 21.24 Q = 0.03 p = 0.61 6 Q = 0.012 p = 0.75

Narrow PPy nanotube (2-h etch) Q = 0.0006 p = 0.87 18 Q = 0.036 p = 0.74 28 Q = 0.065 p = 0.44

Open PPy nanotube (4-h etch) Q = 0.04 p = 0.61 16 Q = 0.04 p = 0.76 20 Q = 0.389 p = 0.42

0.1 1 10 100 1000

1

10

-Z''

(O

hm.c

m2 )

Frequency (Hz)

ZnO Nanorod core-Ppy Sheath Narrow Ppy Nanotube Open Ppy Nanotube

Figure 14 The log-log Bode plot of measured data for ZnOnanorod core-PPy sheath and PPy-nanotube electrodes atvarious frequencies.


etching which indicates a capacitor-like behavior thoughnot yet an ideal one for which phase angle should be closerto −90°. Compared to the nonplateau behavior and lowphase angle of −40° observed in the unetched ZnO nano-rod core-PPy sheath electrode, the PPy nanotube electrodeshows considerably improved capacitor behavior.The measured charge transfer resistance, RCT, is 8.2

and 7.2 Ω cm2, respectively, for 2- and 4-h etched PPynanotube structured electrodes, which is not much dif-ferent from that of the unetched ZnO nanorod core-PPysheath structured electrode. It is obvious that extent ofanion conjugation reaction in the PPy nanotube sheathin response to the electron transfer action is not muchaffected as the ZnO core is etched away. A more signifi-cant effect of the PPy nanotube sheath is seen in theWarburg impedance values. The intercept of extrapola-tion of the low-frequency impedance on the x-axis givesresistance RCT +W, where W is the Warburg impedance.As shown in Table 1, W equals 20.2 Ω.cm2 for unetchedZnO nanorods core-PPy sheath electrode and decreasesto 8.4 and 5.4 Ω.cm2 for the PPy nanotube structurerealized after 2- and 4-h etching, respectively.The impedance parameters of the complex ZnO nano-

rod core-PPy sheath electrode system were analyzed byequivalent circuit modeling. Nyquist plots are simulatedusing the equivalent circuit shown in Figure 13 and thecomponent parameters were derived that provide closestfit at each frequency point [58]. Electrical equivalence ofthe physical electrochemical entities is marked inFigure 13. The equivalent circuit model includes solu-tion resistance RS, charge transfer resistance RCT repre-senting the electrode kinetics, and Warburg elementCPEW representing the resistance encountered in diffu-sion and access of ions within nanoporous electrodestructure. The inclusion of the constant phase elementCPEdl instead of the conventional purely capacitiveelement Cdl is to account for the dispersive behavior ofthe capacitance arising from the charge accumulationlayer at the ZnO nanorods exposed to the electrolytethrough pores in the PPy sheath and nanostructure ofthe electrode. Similarly, CPEnr is the capacitive elementwhich characterizes the pseudocapacitance property ofthe nanotubular PPy-anion conjugation. The nanostruc-ture resistance, Rnr, is representative of the electrontransport resistance due to narrow (approximately 60nm diameter) vertically long (approximately 2.2 μm)ZnO nanorods and Cnr its electrochemical capacitance

[59]. The continuous lines in the Nyquist plots inFigures 10 and 11 are the results of the fitting based onthis model. Excellent fit is observed over the entire fre-quency range. Various electrical resistance and capacitiveparameters estimated by fitting of Nyquist plots aresummarized in Table 2.The constant phase element (CPE) instead of the cap-

acitor in the equivalent circuit above is justified in orderto more appropriately account for the heterogeneities in-cluding the surface roughness, porosity, and variation inthe PPy thickness arising from the nanostructured na-ture of the ZnO-PPy electrode. The long, vertical, anddispersed 3-D ZnO nanorod core-PPy sheath (nanotube)nanostructure has a diverse aspect ratio relative to a flat2-D electrode structure and therefore differently impactsthe ion diffusion kinetics. This gives rise to the distrib-uted time constants simulating the capacitance disper-sion which is better represented by the RC networkcomprising of nanostructure resistance, Rnr, and the con-stant phase element, CPEnr [60]. The CPEnr impedanceis given as, [61].

Z″CPEnr ¼1

jωð Þp⋅Q ð4Þ

where exponent p represents dispersive nature of timeconstant, since with p = 1, the impedance Z″ is purelycapacitive characterized by a single time constant and

Table 3 The Z″ parameter calculated from the simulated exponent p and CPE Q values and the measured Z″ values

Nanostructure electrode Simulated Z 0 0CPEnrmhos:cm−2s−p Measured Z 0 0

CPEnrmhos:cm−2s−p Percentage deviation

ZnO nanorod core-PPy sheath 11.8 7.7 34.7

Narrow PPy nanotube (2-h etch) 11.2 7.15 36.1

Open PPy nanotube (4-h etch) 9.59 6.50 32.2


the parameter Q is equivalent to a capacitance, while forp < 1 parameter Q is basically a CPE with units Mho.cm−2.Figure 14 shows the log-log plot of measured Z″ versusfrequency data for ZnO nanorod core-PPy sheath and PPynanotube electrodes which is a straight line according toabove Equation 4, and the slope corresponds to exponent,p. The Z″ calculated from the simulated exponent p andCPE Q values according to above equation and measuredZ″ values are compared in Table 3. The low-frequencymeasured impedance, Z″, shows a deviation by a constantfactor from the simulated CPEnr impedance which isattributed to Cnr which has weak frequency dispersion butcontributes to the measured Z″ values. The exponent p inthe simulation of the impedance due to the Warburgelement has the values 0.5 < p < 1 (Table 2). A p = 0.5usually defines the CPEW due to Warburg impedance.The higher simulated p values are interpreted as diffusion-based resistance signifying the pseudocapacitive nature ofelectrodes.

0 25 50 75 1000.0

0.1

0.2

0.3

0.4

0.5

Pot

entia

l (V

)

Time (sec)

1 mA cm-2

2 mA cm-2

3 mA cm-2

Ppy Nanotube 2h Etched

0 25 50 75 100 1250.0

0.1

0.2

0.3

0.4

0.5

Pot

entia

l (V

)

Time (sec)

a: Zno nanorod core-Ppy sheathb: Narrow Ppy nanotube sheathc: Open Ppy nanotube sheath

A

C

(a) (b) (c)

Figure 15 Charge-discharge characteristics. (A) ZnO nanorod core PPymeasured at a constant current density of 1 mA.cm−2. Charge-discharge charcore-PPy sheath, (C) PPy nanotube 2-h etch, and (D) PPy nanotube 4-h etch.

Charge-discharge curves and stability analysisGalvanostatic charge-discharge performance of the ZnOnanorod core-PPy sheath and PPy nanotube electrodeswas studied at various current densities from 1 to 3 mA.cm−2 in the voltage range 0.05 to 0.5 V. Figure 15Ashows charge-discharge curves measured at 1 mA.cm−2

for the PPy nanotube electrode obtained by a 2- and 4-hetch and its comparison with that of the ZnO nanorodcore-PPy sheath electrode. The discharge curves arenearly linear for all the three electrode structures indi-cating highly capacitive character. The areal-capacitancedensity Csd of the electrodes was calculated from thecharge-discharge curves at 1 mA.cm−2 using Equation 2and the results are presented in Table 4. The electrodewith PPy open nanotube structure is higher than theelectrode with ZnO nanorod core-PPy sheath structure.This suggests that electrolyte ions are able to accessthrough nanotubes and can intercalate better with PPytaking advantage of the exposed nanotube surface. A

0 25 50 75 100 125 1500.0

0.1

0.2

0.3

0.4

0.5Ppy Nanotube 4h Etched

1 mA cm-2

2 mA cm-2

3 mA cm-2

Time (sec)

Pot

entia

l (V

)

0 20 40 60 800.0

0.1

0.2

0.3

0.4

0.5

Pot

entia

l (V

)

Time (sec)

ZnO Nanorod Core-Ppy Sheath

1mA cm-2

2mA cm-2

3mA cm-2

D

B

sheath electrode and PPy nanotube electrodes after 2-h and 4-h etchacteristics measured at different current densities for (B) ZnO nanorod

Table 4 Specific areal capacitance and ESR of nanostructuredelectrodes calculated from charge-discharge curvesmeasured at 1 mA.cm−2

Nanostructure electrode Csd (mF.cm−2) ESR (Ω.cm2)

ZnO nanorod core-PPy sheath 131.22 40.5

Narrow PPy nanotube (2-h etch) 132.28 25.08

Open PPy nanotube (4-h etch) 141.09 32.09

1 2 380

90

100

110

120

130

140

150

Spe

cific

Cap

cita

nce

mF

.cm

-2

Charge-Discharge Current density mA.cm-2

Ppy Nanotube 4 h etchedPpy Nanotube 2 h etcehd ZnO Nanorod Core-Ppy Shell

Figure 16 The specific capacitances of the ZnO nanostructuredelectrodes plotted as a function of charge-discharge currentdensity.

Figure 17 Long-term charge-discharge cycle tests for PPynanotube 4-h etched electrode showing discharge capacitancedensity and ESR variation.


small IR drop at the start of the discharge cycle is no-ticed in each of these electrodes which is due to equiva-lent series resistance (ESR) arising from contacts andinternal electrode cell resistance. Typical ESR values arein the range 25 to 40 Ω.cm2 as shown in Table 4. Theinternal resistance contribution to the ESR originatesfrom the core and the thin ZnO seed layer at the graph-ite substrate. In the charge (doping) cycle extraction andin the dedoping (discharge) cycle, the ingress of elec-trons from and into the PPy tubular shell has a percola-tion path through ZnO rods which offers a finiteresistance. This is evident from the significantly lowerESR values in the electrodes with the partial dissolutionof ZnO core with narrow opened PPy nanotubes ob-tained after a 2-h etch. A slight increase ESR is observedin the electrodes with open PPy nanotube structure. Thecontact between PPy sheath over ZnO nanorods and theothers in the vicinity is minimal at best as the sheaththickness is on the average less than the inter-ZnOnanorod spacing (see Figures 2, 3 and 4). After completedissolution of ZnO, the finite contact resistance betweenthe freestanding PPy nanotube sheaths is responsible forincrease in ESR. The effect of charge-discharge currentdensity on the charge-discharge characteristics for eachof these electrodes in ZnO nanorod core-PPy sheath PPynanotube structures is shown in Figure 15B, C, D whichfollows a similar trend as discussed in the context ofFigure 15A. The specific capacitances of these electrodeswere calculated at different constant current density andthe results are plotted in Figure 16 as a function ofdischarge current density. In the case of PPy nanotubeelectrodes, a decrease in the specific capacitance withincreasing discharge current is observed. This suggeststhat the redox process is kinetically dependent on theionic diffusion at the PPy nanotube-electrolyte interfaceeven though the nanotubes have an unabated access tothe ions as evident from the increased specific capaci-tance of the electrode with open PPy nanotube structureover the one having narrow PPy nanotube structure.The nearly constant specific capacitance of the ZnOnanorod core-PPy sheath electrode with increasing dis-charge current density is suggestive of faster redox kinet-ics at the interface. These observations suggest that theredox process in the PPy nanotube electrodes is due tolimitation on electron transport rather than the diffusive

access of electrolyte dopant ions to the PPy in the nano-tube structure. The electron transport is facilitatedthrough ZnO nanorods in close contact with graphitesubstrates. In the case of PPy nanotubes, electron trans-port can only take place through the PPy nanotube alongits length. Since anion conjugation (doping) is in re-sponse to the electron extraction in spite of unimpededaccess to electrolyte anions, the doping process is lim-ited by electron transport. The reduction in the specificcapacitance in PPy nanotubes at higher charge currentand the increase in specific capacitance of 3-D ZnOnanorod PPy sheath structure electrode with the in-crease in charging current as observed in Figure 16 areexplicable on this basis.


Cycling testThe cycling stability of the open PPy nanotube electrodewas investigated at a constant charge-discharge currentdensity of 1 mA.cm−2 for a continuous 5,000 cycles.Figure 17 shows the effect on the discharge capacitancedensity as a function of the number of charge-dischargecycle. The overall change in the discharge capacitance isonly <12% indicative of highly stable redox performanceand electrochemical stability of the PPy nanotube elec-trode. This stability arises from unimpeded access of theelectrolyte ions through diffusive transport across to alarge fraction of the PPy polymer surface due to the 3-Dnanotube structure in the redox process. Furthermore,the PPy nanotube electrodes do not show physical orchemical degradation during cycling. This is borne outfrom the ESR data, which remains on the average nearlyconstant during cycling tests for 5,000 cycles.

ConclusionsElectrodes in the three-dimensional nanoscale architec-ture studied in this work in the form of vertically alignedZnO nanorod PPy sheath and PPy nanotube show con-siderable potential for high energy-density storage in asupercapacitor device. These nanostructures are formedby depositing a sheath of PPy over vertical ZnO nanorodarrays by controlled pulsed current electropolymeriza-tion and by selective etching of the ZnO nanorod core.Based on the cyclic voltammetry data, electrode withopen interconnected PPy nanotube array structureshows high areal-specific capacitance of approximately240 mF.cm−2 attributed to realization of enhanced accessto electrolyte ions. The observed scan rate dependenceof the current has been interpreted as delayed responsetime of faradic reaction nonsynchronous with faster scanrate, which could possibly have boosted capacitancedensity further. Slow redox processes are shown to bedue to limitation of electron transfer across the length ofvertical PPy nanotube arrays rather than the diffusivetransport of electrolyte ions. Managing this limitationcould possibly enhance the specific capacitance and thusenergy storage ability further.

Competing interestsThe authors declare that they have no competing interests.

Authors' contributionsNKS carried out the experiment and data analysis. ACR guided the study andhelped in data interpretation. Both authors read and approved the finalmanuscript.

Authors' informationNKS is presently a PhD student at the Electrical and Computer EngineeringDepartment at the State University of New York, Binghamton. ACR isAssociate Professor at the Electrical and Computer Engineering Departmentand Associate Director of the Center for Autonomous Solar Power (CASP) atthe State University of New York, Binghamton.

AcknowledgementsThis work was supported by the National Science Foundation (NSF) grant in aidproject 1318202, the Partnership for Innovation in Electrochemical Energy Storage,and the Office of Naval Research (ONR) under contract N00014-11-1-0658 whichare gratefully acknowledged.

Received: 30 May 2014 Accepted: 21 August 2014Published: 31 August 2014

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doi:10.1186/1556-276X-9-453Cite this article as: Sidhu and Rastogi: Vertically aligned ZnO nanorodcore-polypyrrole conducting polymer sheath and nanotube arrays forelectrochemical supercapacitor energy storage. Nanoscale Research Letters2014 9:453.

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