Vertically aligned ZnO nanorod core-polypyrrole conducting polymer sheath and nanotube arrays for electrochemical supercapacitor energy storage
© Sidhu and Rastogi; licensee Springer. 2014
Received: 30 May 2014
Accepted: 21 August 2014
Published: 31 August 2014
Nanocomposite electrodes having three-dimensional (3-D) nanoscale architecture comprising of vertically aligned ZnO nanorod array core-polypyrrole (PPy) conducting polymer sheath and the vertical PPy nanotube arrays have been investigated for supercapacitor energy storage. The electrodes in the ZnO nanorod core-PPy sheath structure are formed by preferential nucleation and deposition of PPy layer over hydrothermally synthesized vertical ZnO nanorod array by controlled pulsed current electropolymerization of pyrrole monomer under surfactant action. The vertical PPy nanotube arrays of different tube diameter are created by selective etching of the ZnO nanorod core in ammonia solution for different periods. Cyclic voltammetry studies show high areal-specific capacitance approximately 240 mF.cm-2 for open pore and approximately 180 mF.cm-2 for narrow 30-to-36-nm diameter PPy nanotube arrays attributed to intensive faradic processes arising from enhanced access of electrolyte ions through nanotube interior and exterior. Impedance spectroscopy studies show that capacitive response extends over larger frequency domain in electrodes with PPy nanotube structure. Simulation of Nyquist plots by electrical equivalent circuit modeling establishes that 3-D nanostructure is better represented by constant phase element which accounts for the inhomogeneous electrochemical redox processes. Charge-discharge studies at different current densities establish that kinetics of the redox process in PPy nanotube electrode is due to the limitation on electron transport rather than the diffusive process of electrolyte ions. The PPy nanotube electrodes show deep discharge capability with high coulomb efficiency and long-term charge-discharge cyclic studies show nondegrading performance of the specific areal capacitance tested for 5,000 cycles.
KeywordsZinc oxide nanorods Polypyrrole nanotubes 3-D nanostructures Supercapacitor Pulsed electrochemical polymerization Electrochemical energy storage Redox capacitance
Electrochemical energy storage in the ultracapacitor devices is emerging as a frontline technology for high-power applications ranging from modern portable electronics to electric automotive. A battery-supercapacitor hybrid energy system is a power source that can meet the peak power demands in camera flashes, pulsed lasers, and computer systems back-up as well as electric propulsion in diverse industrial and vehicular transport applications. Among the materials systems, structured carbons which store charges as an electric double layer (EDL) in liquid electrolyte medium are widely studied with a focus on overcoming the energy-density limitation . The materials systems which show capacitive function based on redox reactions are the insertion-type metal oxides and doped-conducting polymers capable of high energy-density storage [2, 3]. The conducting polymers, such as polypyrrole (PPy), poly(3,4 ethylenedioxythiophene) (PEDOT), and polyaniline (PANI) which undergo redox processes equivalent of doping and dedoping of electrolyte ions as means of energy storage are being aggressively studied. These polymers exhibit pseudocapacitance properties due to presence of charge transfer reactions. The other most widely studied materials are the metal oxides RuO2, MnO2, V2O5, NiO, and Co3O4 which show highly capacitive behavior due to reversible and fast surface redox reactions with electrolyte ions [2, 4].
In the recent years, conducting polymers with a nanoporous morphology and as nanocomposites with metal-oxides have emerged as the materials system of great potential for high energy-density storage. Electrodes based on these materials structured at the nanoscale enable many-fold enhancements of the electroactive surface and interface with electrolyte facilitating absorption, ingress, and diffusion of electrolyte ions which being the main energy storage units could lead to increased energy and power density of supercapacitor devices. The high surface area morphology in conducting polymers is attained by creating variations in its nanostructure like nanoporous , nanofibers [6, 7], nanowires , nanobelts , and by size-selective nanopores in the context of carbons . Most metal oxides are electrically resistive in character and the redox reactions here are limited to the surface regions. In order to accrue full advantage of their pseudocapacitive function, electrically conducting polypyrrole is encrusted by metal oxides [11, 12], PEDOT , CNT , and graphene [15, 16] in various nanostructured forms like nanoparticle [17, 18], nanotube [19, 20], and nanowire [21, 22] as supercapacitor electrode. In these materials systems, the nanostructure features are randomly distributed in the two-dimensional (2-D) film form mainly due to the preparatory methods.
Most recent research thrust in the conducting polymers and their nanocomposite with metal oxides is directed towards the electrodes with three-dimensional (3-D) nanoarchitecture such as vertically aligned nanotubes  and nanorods . These nanostructures have potential for the limiting electrolyte-ion diffusion problem by decreasing the ion diffusion paths and at the same time increasing the surface area for enhanced electrode-electrolyte interaction. In the past, randomly oriented conducting polymer nanotubes structures have been synthesized [16, 25, 26] for supercapacitor applications. However, the vertically oriented nanostructures, nanorods, and nanotubes have been mostly configured using the metal oxide templates . Such nanostructures have been created by more innovating nanoscale engineering methods like oxidative polymerization , electrochemical anodic oxidation , electrodeposition , and hydrothermal synthesis [31, 32]. Furthermore, by combining the redox conducting polymers with the well-known pseudocapacitive oxide like MnO2, forming the nanocomposites in the 3-D nanoarchitecture presents multiple advantages with enormous potential to outperform their 2-D counterparts. The composite 3-D nanostructure can be created by conformal deposition of redox-active conducting polymer, pseudocapacitive oxide layer, or their multilayer stacks over vertical nanostructures of TiO2, ZnO, or NiO serving as templates. The composite 3-D nanostructured electrodes have synergic contribution to specific capacitance based on their electroactive functions which boost energy density, and their nanoarchitecture have the ability to mitigate the ion diffusion limitation thereby enhancing the power density. In the past, 3-D nanotube polymers, PPy-PANI  polymer-metal oxides, TiO2-PPy [34, 35], ZnO-PPy , TiO2-NiO , and TiO2-V2O5 have been reported.
In this work, we investigate the characteristics of nanocomposite electrodes for supercapacitors having 3-D nanoscale architecture, the one comprising of vertically aligned zinc oxide nanorod arrays at the core with doped-polypyrrole conducting polymer sheath and the other vertical polypyrrole nanotubes arrays. Although polypyrrole in the doped state shows high electrical conductivity, the conversion between redox states is very slow due to the slow transportation of counter ions to balance the charge in the polymer structure . The vertical polypyrrole nanotube and sheath structure are likely to decrease the charge transfer reaction time and thus enhance the charge storage capabilities . Zinc oxide used prominently in various devices such as biosensors , light emitting diodes , organic solar cells , and spintronics [42, 43] is a biocompatible, highly stable, and less expensive material as compared to ruthenium oxide and therefore has a good potential for the electrochemical energy storage devices. Zinc oxide characterized as a wide band gap semiconductor with excellent chemical and physical properties can be easily transformed in various nanostructure forms like nanowire, nanoplatelets, and nanoneedles mostly as flat two-dimensional structures . In the context of using a ZnO template for a supercapacitor electrode in the 3-D architecture, we have fabricated vertically aligned ZnO nanorods by hydrothermal synthesis which exhibit specific electrical and optical properties . The nanocomposite electrode is formed by deposition of doped polypyrrole layer over ZnO nanorod at the core by the commercially viable, low cost solution-based pulsed current electropolymerization process . Pulsed current process allows depositing polypyrrole layer selectively with highly controlled thickness through the application of number of pulses. Only a few studies have been reported on electrodes with zinc oxide and polypyrrole composite films for supercapacitor energy storage devices perhaps since zinc oxide nanostructures may be resistive compared to conducting polymers . ZnO nanorods as template to create PPy nanotube structures with inlay of MnO2 and their energy storage behavior have been reported . In this work, the conducting doped polypyrrole supercapacitor electrodes in two different 3-D architectural forms, one having ZnO nanorod core-PPy sheath and the other vertical PPy nanotube array have 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 resistance elements is developed to explain the characteristics of electrode at various frequency ranges. Long-term charge-discharge tests are carried out to evaluate the cycle life of such electrodes in supercapacitor energy storage device. This paper reports the results of these studies.
Synthesis of ZnO nanorod array template
Polypyrrole conducting polymer electrodes in the two ZnO core-PPy sheath and PPy nanotube structural forms were fabricated over a ZnO nanorod template. The template, a vertically oriented two-dimensional array of ZnO nanorods, was formed over surface-activated 500-μm-thin graphite substrates by thermo-decomposition of zinc nitrate aqueous solution in the presence of hexamethylenetetramine in a wet chemical process . The surface of the graphite substrate is activated by depositing a 20-nm-thick ZnO seed layer that acts as nucleation centers for the growth of ZnO nanorods. This layer is formed by radio-frequency (RF) plasma sputtering from a stoichiometric ZnO target in the argon ambient at 50 mTorr pressure and 100-W RF power. The growth of ZnO nanorods is 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 MΩ deionized water. The surface-activated graphite substrate is vertically kept in this solution in the autoclavable glass bottle and held steady at 95°C for 8 hours. This hydrothermal procedure results in the dissociation of Zn(II)-amino complex resulting in ZnO which grows as ZnO nanorods . Post deposition, the graphite substrates are rinsed in deionized water to remove any residual precursor chemicals and dried in air.
Pulsed current electropolymerization of polypyrrole sheath and nanotube formation
A uniform and conformal deposition of polypyrrole of a controlled thickness over ZnO nanorods is essential for the creation of PPy sheath and nanotube nanostructures. Polypyrrole has been deposited the past by various chemical  and potentiostatic electropolymerization  methods. In this work, PPy deposition is done by electropolymerization of pyrrole monomer in situ over ZnO nanorods using the ultrashort multiple unipolar pulsed-current method reported earlier . Electropolymerization is accomplished in an electrolyte solution containing 0.01 M pyrrole (Py) monomer (Sigma-Aldrich) and 0.1 M lithium perchlorate dopant ions in the presence of 0.06 M sodium dodecyl sulfate (SDS) surfactant. The preparatory steps involve first the dissolution of pyrrole monomer in the presence of SDS under continuous stirring in deionized water warmed to 40°C. Later, the ZnO-seed-layer-coated graphite substrate is dipped vertically for approximately 20 min in this solution which helps in the wettability of the ZnO nanorods with pyrrole monomer. Before initiating the electropolymerization process, lithium perchlorate is added to form 0.1 M solution. The formation of the polypyrrole sheath layer over ZnO nanorods is carried out by pulsed current electropolymerization method in a two-electrode cell using a platinum (Pt) sheet as a counter and a reference electrode. In this method, 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 electropolymerization is initiated during the pulse ‘on’ period as the corresponding anodic potential exceeds the oxidation potential of the pyrrole monomer. The current off period essentially helps create the equilibrium conditions in the vicinity of ZnO nanorods for deposition under homogenous polymerization conditions. The number of current pulses effectively controls the polypyrrole-layer thickness and usually approximately 5 to 10 k pulses were used to form fully covered PPy sheath over ZnO nanorods. In some cases 20 k pulses were also applied to form a thicker PPy sheath.
The PPy nanotube structure is obtained by etching away the vertically aligned ZnO nanorod core in a 20% ammonia solution . The ZnO etching process is typically initiated over ZnO nanorod tips due to relatively thin cladding of the electropolymerized polypyrrole. As a result, the PPy nanotube structure shows dependence on the etching time. In this work, etching times of 2 and 4 h are used for the formation of PPy nanotube arrays.
Electrochemical characterization of supercapacitor electrodes
The ac impedance measurements were carried out in a two-electrode configuration in the frequency range 1 mHz to 100 kHz with ac signal amplitude of 10 mV using Solartron Impedance/Gain-Phase Analyzer (Model 1260). Measured low-frequency imaginary impedance Z″ provides estimate of the overall capacitance Ci using the relation Ci = 1/|ωZ″|. The Nyquist plots using the impedance data were simulated using the equivalent electrical model representing the electrochemical and electrophysical attributes of the nanostructured ZnO-PPy electrode using ZPlot software (Scribner Associates, NC, USA) which provide the characteristic resistances and various contributing factors to the overall electrode capacitance.
Results and discussion
Microstructure of ZnO nanorod core-polypyrrole sheath, nanotube electrodes
Growth features of ZnO nanorod-PPy sheath and PPy nanotube arrays
The preferential nucleation and growth of polypyrrole across the ZnO nanorod length is significantly affected by the lack of access of the pyrrole monomer in deep crevices along the depth of ZnO nanorod array marked by the narrow and not-so-consistent interrod spacing typically varying between 120 to 250 nm. This is further aggravated by aqueous immiscibility of pyrrole monomer which inhibits wetting of ZnO rods which might inhibit formation of uniform polypyrrole sheath. In the present case, the use of SDS anionic surfactant mitigates this by transporting pyrrole monomer to the surface of ZnO nanorods. A possible model of electropolymerization growth of PPy sheath over ZnO nanorods in the presence of SDS surfactant is shown schematically in Figure 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 critical value 8 mM for the first micelles concentration (CMC-1) hence the SDS molecular chain containing 12 carbon alkyls with sulfate group at the end are in the extended state in the aqueous medium [54, 55]. The dodecyl alkyl molecular chain being hydrophobic orients away from water and this easily attaches on to the ZnO nanorod surface while the hydrophilic OSO3- group project outward into aqueous environment. The pyrrole monomers are hydrophobic in character and sparingly soluble in water. A large number of pyrrole monomers are able to preferentially disperse within the hydrophobic region created by attached dodecyl alkyl molecular chain over ZnO nanorod surface . This ensures uninhibited supply of the pyrrole monomer and dopant ClO4- anions across the exterior of ZnO nanorods  and consequently forming PPy layer over ZnO rods comprising of short-chain doped PPy oligomers by electronation-protonation-conjugation reaction described in Figure 5B. Spatially distributed deposition of PPy oligomers as clusters is evident in the nodule like the microstructure study shown in Figure 2A. The pyrrole monomer availability during current pulsed off time is no longer diffusion-rate limited and efficient incursion of pyrrole results in the increased electropolymerization rates. In the subsequent pulse cycles, the electropolymerization is reinitiated over new ZnO surface sites or over PPy coated surface as shown schematically in Figure 5C resulting in homogenous formation of the PPy sheath over ZnO nanorods after a certain number of current pulsed polymerization cycles.
Cyclic voltammetry study
Electrochemical impedance spectroscopy data obtained from actual Nyquist plots
Rs(Ω . cm2)
Rct(Ω . cm2)
W(Ω . cm2)
ZnO nanorod core-PPy sheath
Narrow PPy nanotube (2-h etch)
Open PPy nanotube (4-h etch)
The measured charge transfer resistance, RCT, is 8.2 and 7.2 Ω cm2, respectively, for 2- and 4-h etched PPy nanotube structured electrodes, which is not much different from that of the unetched ZnO nanorod core-PPy sheath structured electrode. It is obvious that extent of anion conjugation reaction in the PPy nanotube sheath in response to the electron transfer action is not much affected as the ZnO core is etched away. A more significant effect of the PPy nanotube sheath is seen in the Warburg impedance values. The intercept of extrapolation of the low-frequency impedance on the x-axis gives resistance RCT + W, where W is the Warburg impedance. As shown in Table 1, W equals 20.2 Ω.cm2 for unetched ZnO nanorods core-PPy sheath electrode and decreases to 8.4 and 5.4 Ω.cm2 for the PPy nanotube structure realized after 2- and 4-h etching, respectively.
Characteristic resistance and capacitive parameters estimated by fitting of Nyquist plots
ZnO nanorod core-PPy sheath
Q = 0.025 p = 0.55
Q = 0.03 p = 0.61
Q = 0.012 p = 0.75
Narrow PPy nanotube (2-h etch)
Q = 0.0006 p = 0.87
Q = 0.036 p = 0.74
Q = 0.065 p = 0.44
Open PPy nanotube (4-h etch)
Q = 0.04 p = 0.61
Q = 0.04 p = 0.76
Q = 0.389 p = 0.42
The Z″ parameter calculated from the simulated exponent p and CPE Q values and the measured Z″ values
ZnO nanorod core-PPy sheath
Narrow PPy nanotube (2-h etch)
Open PPy nanotube (4-h etch)
Charge-discharge curves and stability analysis
Specific areal capacitance and ESR of nanostructured electrodes calculated from charge-discharge curves measured at 1 mA.cm -2
ZnO nanorod core-PPy sheath
Narrow PPy nanotube (2-h etch)
Open PPy nanotube (4-h etch)
Electrodes in the three-dimensional nanoscale architecture studied in this work in the form of vertically aligned ZnO nanorod PPy sheath and PPy nanotube show considerable potential for high energy-density storage in a supercapacitor device. These nanostructures are formed by depositing a sheath of PPy over vertical ZnO nanorod arrays by controlled pulsed current electropolymerization and by selective etching of the ZnO nanorod core. Based on the cyclic voltammetry data, electrode with open interconnected PPy nanotube array structure shows high areal-specific capacitance of approximately 240 mF.cm-2 attributed to realization of enhanced access to electrolyte ions. The observed scan rate dependence of the current has been interpreted as delayed response time of faradic reaction nonsynchronous with faster scan rate, which could possibly have boosted capacitance density further. Slow redox processes are shown to be due to limitation of electron transfer across the length of vertical PPy nanotube arrays rather than the diffusive transport of electrolyte ions. Managing this limitation could possibly enhance the specific capacitance and thus energy storage ability further.
NKS is presently a PhD student at the Electrical and Computer Engineering Department at the State University of New York, Binghamton. ACR is Associate Professor at the Electrical and Computer Engineering Department and Associate Director of the Center for Autonomous Solar Power (CASP) at the State University of New York, Binghamton.
This work was supported by the National Science Foundation (NSF) grant in aid project 1318202, the Partnership for Innovation in Electrochemical Energy Storage, and the Office of Naval Research (ONR) under contract N00014-11-1-0658 which are gratefully acknowledged.
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