Carbon Nanotube Hybrid Coaxial Arrays for

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J. Phys. Chem. C 2010, 114, 658–663

Multisegmented Au-MnO2/Carbon Nanotube Hybrid Coaxial Arrays for High-Power Supercapacitor Applications Arava Leela Mohana Reddy,† Manikoth M. Shaijumon,† Sanketh R. Gowda,‡ and Pulickel M. Ajayan*,† Department of Mechanical Engineering & Materials Science, and Department of Chemical and Biomolecular Engineering, Rice UniVersity, 6100 Main Street, Houston, Texas 77005 ReceiVed: September 9, 2009; ReVised Manuscript ReceiVed: NoVember 4, 2009

The present work reports on synthesis and supercapacitor applications of multisegmented Au-MnO2/carbon nanotube (CNT) coaxial arrays. Multisegmented Au-MnO2/CNT coaxial arrays are fabricated inside porous alumina templates using a combination of electrodeposition, infiltration, and chemical vapor deposition methods. CNTs serve as an alternative additive for improving the electrical conductivity of the manganese oxide electrodes, in addition to its active electrode characteristics. The well-adhered interface between Au and MnO2/ CNT hybrid segments leads to nanoscale electrical contacts between the electrode and current collectors. Electrochemical studies have been performed using cyclic voltammetry, galvanostatic charge-discharge, and impedance spectroscopy measurements. The results demonstrate that MnO2/CNT hybrid coaxial arrays are efficient electrodes for supercapacitor applications. Au-segmented MnO2/CNT hybrid coaxial electrodes showed further improvement in specific capacitance, energy, and power densities of a supercapacitor. 1. Introduction In response to the depletion of fossil fuels, sustainable and renewable resources have become a primary focus of the major world powers and scientific community. There has been great interest in developing and refining more efficient energy storage devices. One such device, the supercapacitor, also known as the electrochemical capacitor, stores energy using either ion adsorption (electrochemical double-layer capacitors) or fast surface redox reactions (pseudocapacitors).1 Because of greater energy density than those of conventional capacitors and greater power density than that of batteries, supercapacitors have kindled the interests of researchers in the field of energy storage.2,3 As a result, supercapacitors have become an attractive power solution for an increasing number of applications. However, substantial improvement in the performance to meet higher requirements in the future has been a challenge, and the efforts are widely scattered. Various transition metal oxides, such as RuO2, Fe2O3, Co3O4, NiO, SnO2, MnO2, and so forth, have been investigated as possible electrode materials for high-power electrochemical pseudocapacitors.4–8 The energy storage mechanism with these oxides is mainly based on fast faradaic redox reactions, which occur between the oxide and the electrolyte giving rise to the so-called pseudocapacitance.9 Among various transition metal oxides, manganese oxide is one of the most promising pseudocapacitor electrode materials with respect to its specific capacitance, environmental compatibility, and cost effectiveness.8,10–17 However, because of its poor electrical conductivity and cyclic stability, manganese oxide is still limited to potential applications. On the other hand, various carbonaceous materials, such as mesoporous carbon, activated carbons, carbon fibers, and carbon nanotubes (CNTs) have been studied as electrodes for electrochemical double layer capacitors (EDLCs).18–22 CNTs, because * To whom correspondence should be addressed. E-mail: [email protected]. † Department of Mechanical Engineering & Materials Science. ‡ Department of Chemical and Biomolecular Engineering.

of their outstanding electrical properties apart from their high chemical stability, high aspect ratio, strong mechanical strength, and high activated surface area, are attractive electrode materials in energy storage devices, such as electrochemical capacitors, fuel cells, and lithium batteries.22–29 In spite of having ideal properties, CNT-based supercapacitors do not meet the expected performance.30 A composite material having high conductive CNT and high capacitive metal oxides, in particular MnO2/CNT composite, is of great interest because of its dual charge storage mechanisms. An EDLC rules high conductive CNT, while the manganese dioxide electrode shows a pseudocapacitive behavior involving MnIII/MnIV redox processes.31 Brousse et. al showed the concept of an hybrid capacitor with a negative AC electrode and a positive MnO2 electrode working with a mild aqueous electrolyte.32,33 Coaxial nanowires of MnO2/PEDOT synthesized by the coelectrodeposition method have been shown to be promising electrode materials of a supercapacitor.34 Coaxial nanowires/ nanotubes will lead to multiple functionalities by combining the physical properties of different materials. In order to build supercapacitors with improved capacitance and power capabilities, coaxial nanowires/nanotubes of multiple materials with specific electrochemical and physiochemical properties need to be fabricated. Recently, we have demonstrated improved Li storage properties of coaxial arrays of MnO2/CNT electrodes.35 Furthermore, high contact resistance between the electrode and the current collector is one of the major issues of supercapacitors that limits achieving high power capabilities.36–39 In previous work we reported a technique to fabricate ultrahigh power supercapacitors by using multisegmented CNT/gold nanowire (AuNW) hybrid structures as electrodes.40 Both the electrode (CNTs) and the current collector (AuNW) are integrated into a single nanostructured wire, thus resulting in excellent performance of supercapacitors with a very high power density of 48 kW kg-1, which is much higher than the reported values for CNT-based supercapacitors.35 In the present work, we fabricate Au segmented MnO2/CNT coaxial arrays and study their

10.1021/jp908739q  2010 American Chemical Society Published on Web 12/02/2009

Supercapacitor Application of Au-MnO2/CNT Coaxial Array

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Figure 1. Schematic diagram showing the fabrication of Au-MnO2/CNT hybrid coaxial nanotube arrays inside an AAO template using a combination of electrodeposition, vacuum infiltration, and CVD techniques. A thin layer of silver (∼100 nm) was sputter coated to act as current collector for the electrodes.

application as high performance electrodes of a supercapacitor. Au-MnO2/CNT coaxial electrode array configuration will lead to (i) improved electronic conductivity due to the presence of CNT, (ii) homogeneous electrochemical accessibility and high ionic conductivity by avoiding agglomerative binder and other conductive additives, (iii) well directed one-dimensional (1D) conductive paths due to perfect coaxial alignment, (iv) dual storage mechanism (EDLC due to CNTs and pseudocapacitance due to MnO2), and (v) low contact resistance between electrode and the current collector due to the presence of nanoscale Au segments. 2. Experimental Section Synthesis of Au-MnO2/CNT Coaxial Arrays. The experimental procedure for growing Au-MnO2/CNT coaxial arrays is shown in Figure 1. First, a layer of Ag coating (about 100 nm) was thermally evaporated onto one side of the anodic aluminum oxide (AAO; Whatman, Anodisc 25) template to serve as the working electrode for the consequent electrochemical deposition. The electrodeposition was carried out to deposit Au into the nanopores, using the standard three-electrode potentiostat system (AUTOLAB PGSTAT 302N potentiostat/galvanostat). Ag/AgCl reference electrode and platinum wire counter electrode were used for the process. The length of the metal nanowire can be controlled by varying the deposition time. After the electrodeposition of AuNWs, MnO2 nanotubes were fabricated by simple vacuum infiltration of manganese nitrate solution (0.5 mL; 0.2 M) into Au-electrodeposited AAO template. The upper side of the membrane was then polished gently using a Kimwipe tissue. After air drying, the MnO2 infiltrated alumina membrane was annealed in air at 300 °C for 10 h. To prepare Au-MnO2/CNT coaxial arrays, the above prepared Au-MnO2-nanotube-filled alumina membrane was placed in a tubular furnace with the membrane pores mounted horizontally and heated to 650 °C in argon flow (100 sccm). A mixture of C2H2 and Ar (20% C2H2 and 80% Ar; 20 sccm) was flown for 1 h and then the furnace was cooled to room temperature in Ar flow. Au-MnO2/CNT coaxial arrays were recovered after dissolution of the alumina templates in 3 M NaOH aqueous solution, followed by washing with water. The presence of an evaporated metal film prevents

the Au-MnO2/CNT coaxial arrays from collapsing after the removal of templates. For comparison, MnO2/CNT coaxial arrays were also prepared without Au heterojunction. Materials Characterization. The morphology of the prepared CNTs, MnO2 nanotubes, and MnO2/CNT coaxial nanotubes was characterized by scanning electron microscopy (SEM; FEI Quanta 400 ESEM FEG). Fabrication of Supercapacitor Electrodes and Its Electrochemical Measurements. A supercapacitor test cell was fabricated with two Au-MnO2/CNT coaxial array electrodes separated by a thin filter paper in 0.1 M Na2SO4 aqueous solution. The electrodes, which were thin flexible films (∼50 µm), were separated by a Whatman filter paper and were sandwiched in a Swagelok type stainless steel (SS) cell. The electrochemical properties and capacitance measurements of supercapacitor electrodes were studied in a two-electrode system by cyclic voltammetry (CV), galvanostatic charge-discharge, and impedance spectroscopy using an AUTOLAB PGSTAT 302N potentiostat/galvanostat. 3. Results and Discussion The crystallinity of the coaxial nanotubes was studied using powder X-ray diffraction (XRD), as detailed in a previous study.35 In brief, XRD patterns of the MnO2/CNT coaxial nanotubes crystallizes to tetragonal symmetry with an additional peak at 26.4° indicates the (002) plane of hexagonal graphite structure. Since there is high chance for manganese oxide to show multiple phases after the chemical vapor deposition (CVD) process, however, in the present study, a high inert atmosphere was maintained, and that could be the reason the tetragonal phase of MnO2 was retained. SEM images of Au-MnO2/CNT hybrid nanostructures clearly show uniform coaxial nanostructure (Figure 2). The two segments can be seen clearly in the image due to different contrast of gold and MnO2/CNT from low- and high-resolution SEM images (Figure 2a,b). The multisegmented electrode arrays enable an effective contact between electrode and current collector. The coaxial nature such as MnO2 shell and CNT core can be seen from Figure 2c,d. The structure of coaxial nanotubes such as shell thickness and nanotube length could be easily

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Figure 2. Textural characterization of Au-MnO2/CNT hybrid coaxial nanotubes. (a) Low- and (b) high-resolution SEM images showing Ausegmented MnO2/CNT. (c) Low- and (d) high-resolution SEM images showing MnO2 shell and CNT core.

controlled by varying the infiltration time, which will enable us to tune the electrochemical properties of the coaxial nanotubes. In the present study, the presence of Au segments has superior advantages in terms of electrical conductivity from electrode to the current collector, and the presence of a CNT core improves the electrical conductivity, mechanical stability, and electrochemical performance. Also, high-temperature annealing results in high crystallinity of the coaxial nanostructure. The growth process of the coaxial nanotubes follows a typical template synthesis, wherein the manganese precursor initially combines with the template by impregnation and results in nucleation and growth following annealing treatment. This forms the MnO2 shell. The CVD process allows CNTs to grow in the inner cores left by the MnO2 shells. Upon removal of templates, gold-segmented coaxial nanotubes with CNT cores and MnO2 shells are obtained. CV is considered as an ideal method for characterizing the capacitive behavior of any material. A large magnitude of current and a rectangular type of voltammogram, symmetric in the anodic and cathodic directions, are the indications of ideal capacitive nature of any electrode material. CV response of MnO2/CNT and Au-MnO2/CNT coaxial electrode arrays carried out at a scan rate of 10 mV/s in the potential range of 0-0.7 V using 0.1 M Na2SO4 aqueous electrolyte solution are shown in Figure 3. The lack of symmetry in the CV curves of coaxial nanotube electrodes is probably due to combined double-layer and pseudocapacitive contribution to the total capacitance. However, the response of both MnO2/CNT and Au-MnO2/CNT coaxial electrode arrays are very close to ideal pseudocapacitive behavior without showing any polarization. This behavior indicates the effective utilization of electrode material by an

Figure 3. Cyclic voltammograms of a supercapacitor cell having MnO2/CNT and Au-MnO2/CNT hybrid coaxial nanotube electrodes, at a scan rate of 10 mV s-1 in 0.1 M Na2SO4 aqueous electrolyte.

electrolyte, resulting in better ionic diffusion, predominantly due to the well-spaced nanostructured coaxial geometry. The increase in area of the CV curve for Au-MnO2/CNT coaxial electrodes when compared to MnO2/CNT coaxial electrodes indicates an enhancement of the specific capacitance. Since the measurements are made on symmetric assemblies of materials, by the basic circuit relationship for series capacitors, what is measured is actually 1/2 of the capacitance of the freestanding electrode. The specific capacitance has been obtained from the CV curve according to the following equation:

Supercapacitor Application of Au-MnO2/CNT Coaxial Array

Csp )

i sm

(F/g)

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(1)

where i is the average cathodic current, s is the potential sweep rate, and m is the total mass of electrode (including positive and negative electrodes). Figure 4 shows the galvanostatic charge-discharge behavior of MnO2/CNT and Au-MnO2/CNT coaxial array electrodes with an applied constant current density of 6.6 A/g (considering the mass of both positive and negative electrodes) in the potential range between 0 and 0.7 V. The charge-discharge curves show a slight curvature, indicating the pseudocapacitive contribution along with the double layer contribution. However, the absence of ohmic drop in the discharge curve for the Au-MnO2/CNT electrode clearly indicates improved contact between the electrode and current collector. The specific capacitance has been evaluated from the charge-discharge curves, according to the following equation:

Csp )

I (dV/dt)

(F/g)

(2)

where I is the applied current density in amperes per gram. The calculated specific capacitances were 44 and 68 F/g, respectively for MnO2/CNT and Au-MnO2/CNT coaxial array electrodes. Thus the specific capacitance for MnO2/CNT and Au-MnO2/CNT electrodes, which is in the same ball park compared to the reported values for MnO2 electrodes (∼36 F/g) in the pioneer work of Cottineau et al.8 Further, the normalized capacitances to geometric area of the MnO2/CNT and Au-MnO2/ CNT coaxial array electrodes are calculated to be 69 and 106 F/cm2, respectively. The initial enhancement of the specific capacitance of MnO2/CNT coaxial electrodes over CNTs can be attributed to the presence of the MnO2 shell over the CNT core structure.40 Such a coaxial combination of CNTs and metal oxide nanowires leads to dual storage mechanism of EDLC (due to CNTs) and pseudocapacitance (due to MnO2). Further enhancement in the specific capacitance with Au-MnO2/CNT coaxial array electrodes is due to the presence of Au segments, which provides an easy path for electron transfer between current collector and active material. Both MnO2/CNT and Au-MnO2/ CNT coaxial array electrodes were shown to possess good stability over 1000 charge-discharge tests (Figure 5a). As observed using SEM, there is no considerable morphological change of MnO2/CNT coaxial electrodes even after 1000 charge-discharge cycles (Figure 5b).

Figure 4. Galvanostatic charge-discharge behavior of supercapacitor cell having MnO2/CNT and Au-MnO2/CNT hybrid coaxial nanotube electrodes, at an applied constant current of 10 mA in 0.1 M Na2SO4 aqueous electrolyte.

Figure 5. (a) Specific capacitance versus cycle number plots of supercapacitors having symmetric assembly of MnO2/CNT and AuMnO2/CNT electrodes. (b) SEM image of MnO2/CNT electrode materials after 1000 cycles of charge/discharge.

Figure 6 presents complex-plane electrochemical impedance spectra (EIS) of MnO2/CNT and Au-MnO2/CNT coaxial array electrodes. EIS measurements were carried out at a dc bias of 0 V with a sinusoidal signal of 10 mV over the frequency range from 50 kHz and 10 mHz. A sharp increase of the imaginary part of EIS at lower frequency is due to capacitive behavior of the electrode, where a semicircular loop at higher frequencies is due to charge-transfer resistance of the electrode. For an ideal double-layer capacitor, the impedance plot should be a vertical line, parallel to the imaginary axis, which is generally observed in the case of CNT electrodes.24,40 An inclined behavior of MnO2/CNT and Au-MnO2/CNT coaxial array electrodes is due to pore size distribution. Equivalent series resistance (ESR) of 7.3 and 2.5 Ω cm2 are measured for MnO2/CNT and Au-MnO2/ CNT coaxial array electrodes, respectively (electrode area 0.9 cm2). The nanoscale contact between the electrode and current collector in the case of Au-MnO2/CNT coaxial array electrodes resulted in low ESR. The maximum power density (Pmax) of the supercapacitor can be calculated according to the following equation:

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Pmax )

V2 4(ESR)mtot

(W kg-1)

Reddy et al.

(3)

where V is the cell voltage, mtot is the mass of total active material in the symmetric supercapacitor (including positive and negative electrodes) and ESR is the equivalent series resistance, which can be obtained from the x-intercept of the impedance plot. A supercapacitor device having MnO2/CNT coaxial array electrodes gives a maximum power density of 11 kW kg-1, which falls in the range of CNT electrodes. However, Au-MnO2/ CNT coaxial array electrodes give a maximum power density of 33 kW kg-1, which is much higher than the reported values for CNT-based supercapacitor devices.37,41 The less power density of Au-MnO2/CNT over CNT/AuNW can be understood from the obtained ESR of these electrode materials.40 The AuMnO2/CNT electrodes showed ESR of 2.5 Ω, whereas CNT/ AuNW showed an ESR of 480 mΩ. The maximum energy density (Emax) of the supercapacitror can be calculated by using the following equation:

Emax

1 C V2 2 sp ) 3.6

(Wh kg-1)

(4)

where Csp is the specific capacitance of the supercapacitor, and V is the cell voltage, with the total weight of the active material in both electrodes being considered. A maximum energy density of 4.5 Wh kg-1 has been obtained for Au-MnO2/CNT coaxial array electrodes, while an energy density of 2.9 Wh kg-1 has been obtained for MnO2/CNT coaxial array electrodes. Energy density of MnO2/CNT electrodes is comparable to the real energy density of the MnO/ MnO2 cell (3.3 Wh/kg) reported in the pioneer work of Cottineau et al.8 The large improvement in power density and energy density for Au-MnO2/CNT coaxial array electrodes compared to MnO2/CNT coaxial array electrodes could be attributed to low contact resistance arising from the well-adhered interface between the MnO2/CNT coaxial array and AuNW junctions. Here, each coaxial nanotube is electrically connected to the AuNW current collector, so that all the nanotubes contribute to the capacitance there by improving the rate capabilities.

We expect that the use of coaxial nanotubes with dual charge storage mechanisms has enhanced the total specific capacitance. The nanosized and porous nature of the MnO2 shell allows fast redox processes, leading to pseudocapacitive behavior. In addition, the highly electrical conductive CNT core facilitates electron transport to the MnO2 shell, which has low conductivity that can limit its charge/discharge rate. Moreover, the presence of CNT in the core of hybrid nanotubes will also act as additional storage sites by forming EDLC, leading to a dual mechanism of storage and thereby resulting in an improved capacitance. Furthermore, the presence of Au segments in AuMnO2/CNT coaxial array electrodes influences the overall performance of the supercapacitor by reducing the interface resistance between the current collector and the active material. Thus, improved performance of coaxial nanotube electrodes is attributed to improved conductivity by providing highly conductive CNT in the inner core of MnO2 nanotubes, homogeneous electrochemical accessibility and high ionic conductivity by avoiding the agglomerative binder, well-directed conductive paths due to perfect coaxial alignment, and dual-charge storage mechanism of EDLC and pseudocapacitance. 4. Conclusion In summary, we have synthesized Au-MnO2/CNT hybrid coaxial nanotube arrays by combination of electrodeposition, vacuum infiltration, and CVD techniques using porous alumina templates. The coaxial hybrid structure formed by the highly conductive CNT core offers enhanced electronic transport to the MnO2 shell and well adhered interface between Au and MnO2/CNT segments resulting in nanoscale contact with each electrode and the current collector, leading to very low contact resistance. The Au-MnO2/CNT hybrid coaxial nanotube electrodes showed excellent electrochemical performance with a maximum specific capacitance of 68 F/g, a power density of 33 kW kg-1, and an energy density of 4.5 Wh kg-1. The coaxial combination of CNTs and metal oxide nanowires leads to a dual storage mechanism of EDLC and pseudocapacitance. The new device configuration involves a simple electrode fabrication procedure and also helps in efficient packing. Acknowledgment. The authors acknowledge funding support from the Army Research Office and the Hartley Family Foundation. References and Notes

Figure 6. Complex-plane impedance spectra of supercapacitor cell having MnO2/CNT and Au-MnO2/CNT hybrid coaxial nanotube electrodes, measured at an AC amplitude of 10 mV, in 0.1 M Na2SO4 aqueous electrolyte. Inset shows an enlarged scale.

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