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Synthesis and Electrochemical Properties of Spin-Capable Carbon Nanotube Sheet/MnOx Composites for High-Performance Energy Storage Devices Jae-Hak Kim, Kyung H. Lee, Lawrence J. Overzet, and Gil S. Lee* Department of Electrical Engineering, University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080-3021, United States

bS Supporting Information ABSTRACT: Inspired by the high specific capacitances found using ultrathin films or nanoparticles of manganese oxides (MnOx), we have electrodeposited MnOx nanoparticles onto sheets of carbon nanotubes (CNT sheets). The resulting composites have high specific capacitances (Csp e 1250 F/g), high charge/discharge rate capabilities, and excellent cyclic stability. Both the Csp and rate capabilities are controlled by the average size of the MnOx nanoparticles on the CNTs. They are independent of the number of layers of CNT sheets used to form an electrode. The high-performance composites result from a synergistic combination of large surface area and good electron-transport capabilities of the MnOx nanoparticles with the good conductivity of the CNT sheets. Such composites can be used as electrodes for lithium batteries and supercapacitors. KEYWORDS: Energy storage, battery, supercapacitor, carbon nanotube sheet, manganese oxide, composite

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ncreasing research effort has been focused on the development of energy storage devices having high-performance and energy storage capability because portable electronics and hybrid vehicles are increasingly being used.1
3 This has led to widespread research on the synthesis and electrochemical properties of various metal oxides. These oxides can be used as electrode materials to improve the specific capacitance, long-term cycling stability, and charge/discharge rate capabilities in lithium-ion batteries and supercapacitors.4
8 Among the various possible metal oxides, manganese oxides (MnOx) have been investigated as promising electrode materials because they are less expensive and less environmentally toxic while still providing a large theoretical specific capacitance (Csp ∼1370 F/g).9
12 While large Csp has been observed, 700 to 1380 F/g, they have only been measured in systems that are not amenable to manufacturing. These systems consisted of either ultrathin films or nanosized particles on planar current collectors.12,13 The small characteristic size helps to overcome the poor electrical conductivity of MnOx.14 It also facilitates rapid diffusion of the cations involved in the charge/discharge cycling processes.15
17 This has inspired research on new electrode designs in which thin films and/or particles of MnOx could be coated on the surface of nanostructured substrates such as porous carbons18,19 and carbon nanotubes (CNTs).20
22 Recently, several nanostructured composites have been explored in which MnOx was combined with CNTs.20
22 This was done to enhance both the poor electrical conductivity of MnOx14 and its inferior cycling stability.15,23 The CNTs help to more effectively utilize the redox properties of MnOx and enhance the r 2011 American Chemical Society

electrochemical performance due to their outstanding electrical conductivity and superior mechanical properties.24 Our research has been on the synthesis and the electrochemical properties of sheets of CNTs integrated with MnOx nanoparticles. These composites were synthesized by electrodepositing MnOx on the CNTs of the CNT sheets. This process is more controllable, efficient, and reproducible than those used in previous reports such as creating functional groups on the sidewall of CNTs,21 dispersion of CNTs in a liquid medium,25,26 layer-by-layer assembly of charged CNTs,27 and using an alumina template to form coaxial MnO/CNT arrays.20 The synthesis of the CNT sheet/MnOx composites can be carried out using various (inexpensive) substrates because the CNT sheets can be pulled directly from CNT forests and placed on literally anything. The CNT sheets in the MnOx composites improve the electrical conductivity over that of MnOx itself because the parallel alignment of the CNTs in the sheets provides good conductive paths.28
30 Another important aspect is that these MnOx composites do not require binders (adhesives). Binders are often required between the metal oxide and current collector.31 Their presence is made unnecessary in our composite because the MnOx binds directly to the CNTs which make up the current collector. The CNT sheet can be directly connected to external circuits. Received: February 12, 2011 Revised: May 30, 2011 Published: June 10, 2011 2611

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Nano Letters CNT sheets are pulled directly from multiwalled carbon nanotube (MWCNT) forests grown using Fe catalysts as described in a previous report.28 We formed “multisheets” by overlaying (or stacking) multiple individual sheets. In this report, we used either a stack of 10 or 50 sheets (“10-sheet” or “50sheet”) for forming the composite electrode. The 10- or 50-sheet could be placed on a substrate such as silicon, glass, or overhead transparency film for ease of fabrication (Figure 1A). The multisheet electrodes tested were approximately 1 cm wide and 1 cm long (Figure 1B). As shown in Figure 1C, the MWCNTs in each multisheet are aligned in the drawing direction, which can provide well-directed conductive paths. To investigate the electrochemical reactions of a MnSO4 aqueous solution on an as-formed 10-sheet, cyclic voltammetry

Figure 1. CNT multisheet electrode. (A) A 10-sheet laid on an overhead transparency film. (B) A schematic representation and image of a 10-sheet electrode. (C) CNT alignment in the electrode.

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was performed in a solution of MnSO4 (0.02 M) and Na2SO4 (0.2 M). As the potential is increased from 0 V (vs SCE), the anodic current starts to increase at 0.75 V and peaks at 1.1 V corresponding to the oxidation potential of Mn2þ ions (Supporting Information Figure S2). The electrochemical response on the 10-sheet is similar to that found using platinum and graphite.32,33 This indicates that the redox reactions of MnSO4 with the 10-sheet are similar to those with platinum or graphite and lead to the growth of MnOx. Given this, MnOx was electrodeposited on 10-sheet electrodes at a potential of 1.2 V for times between 5 and 180 s in the MnSO4 solution. The detailed preparation and method are described in the Supporting Information Section S1. The morphology and microstructure of the CNT sheet/ MnOx composites varied substantially as the electrodeposition time was increased (Figure 2). After 5 s of growth, flower-shaped MnOx nanoparticles having diameters ∼70 nm covered portions of the CNTs in the 10-sheet (Figure 2A). Growing the nanoparticles directly on the CNTs of the 10-sheet ensures that the MnOx particles make good electrical contacts with the CNTs. The fact that portions of the CNTs remain uncovered as can be clearly seen in Figure 2A, indicates that the growth may have been stopped too soon. When the growth time was increased to 15 s, the MnOx particles connected and uniformly coated the CNTs in the 10-sheet, forming coaxial MnOx/CNT wires (Figure 2B). The cross-section SEM image (Supporting Information Figure S3) shows that the MnOx covers CNTs fully across the 10-sheet, resulting in a ∼150 nm thick composite. After 30 s of growth, the MnOx coating increased in thickness and began to fill in the voids between adjacent CNTs (Figure 2C). After 60 s, a contiguous film was formed on the top surface of the 10-sheet electrode (Figure 2D). We note that the CNTs are covered by MnOx particles (Figure 2B,F), indicating a nanostructured morphology in which the electrochemically accessible MnOx particles are attached tightly on highly conductive CNT cores (Figure 2F). This nanostructure

Figure 2. Morphology and microstructures of CNT sheet/MnOx composites. SEM images of the MnOx composites deposited for 5 (A), 15 (B), 30 (C), and 180 s (D) along with a cross-sectional view of the 180 s-deposited composite (E). The CNTs are shown as bright white lightning wires in panel E. A TEM image of the 15 s-deposited composite (F). White arrows in panel F indicate CNTs covered by the MnOx. 2612

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Figure 3. Characterization of the composite deposited on a 10-sheet for 180 s. (A) Cyclic voltammogram of the composite with continuous cycling between 0 and 1 V for 30 cycles. The increase in cycle number is represented by the arrows in panel A. (B) Deconvoluted Mn 3s and O 1s XPS spectra and (C) Raman spectra; the as-deposited composite (Before) and its resulting composite after 30 cycles (After).

enables the composites to have enhanced electrical conductivity as well as mechanical stability compared to MnOx by itself. On the other hand, the poorly conductive MnOx particles can also deposit between CNTs in the sheet. This impedes the electrical connections between CNTs and increases the sheet resistance of the multisheets. The sheet resistance is increased slightly from 100 to 120 Ω/sq and from 16 to 24 Ω/sq in the 10sheets and the 50-sheets, respectively, after MnOx is deposited for 30 s (Supporting Information Figure S4). Nevertheless, the multisheets still maintain good electrical conductivity.29,30 When MnOx is deposited on the 10-sheet for 180 s, the MnOx composite becomes ∼800 nm thick. The bottom ∼200 nm is composite with the CNT sheet, but the top ∼600 nm is just MnOx with MnOx whiskers on its surface (Figure 2D,E). This thick MnOx film forms from the MnOx particles on the CNTs as Mn ions incorporate onto the top surface faster. As a result, the MnOx particles connect to each other and become a contiguous film on the top surface before the bottom is filled. Once the contiguous film forms, Mn ions can no longer diffuse into the lower reaches of the CNT sheet and growth there stops even though MnOx continues to deposit on the top. This is confirmed by comparing the coating on the bottom of the 10-sheets (shown in Supporting Information Figure S5) with that on the top (Figure 2C,D). The MnOx on the bottom is much thinner than that on the top surface. The electrochemical properties of CNT sheet/MnOx composites deposited for times between 30 and 180 s were examined by cyclic voltammetry (CV) in 0.1 M Na2SO4 aqueous solution at a scan rate of 5 mV/s. As shown in Figure 3A and Supporting Information Figure S6, a broad and quasi-reversible anodic peak at 0.9
0.95 V was observed for all the MnOx composites. (The peaks shifted toward higher potential as the deposition time or MnOx thickness increased.) This indicates that the low oxidation state Mn ions in the composites are oxidized in the electrolyte at 0.9
0.95 V.13,34
37 The coupled cathodic peak also appeared at ∼0.8 V along with a smaller peak at ∼0.4 V. Both the anodic and cathodic peaks gradually decreased in subsequent CV cycles, forming rectangular shaped CV curves. It is expected that low

oxidation state Mn ions are either dissolved into the Na2SO4 electrolyte or transformed into insoluble high oxidation state MnOx during the CV cycling.13,36,38,39 After more than 15 cycles, the CV curves of the composites became stable and rectangular shaped, suggesting good reversibility and capacitive properties. To investigate these phenomena in more detail, the composites were examined by various methods. X-ray photoelectron spectroscopy (XPS) was used to determine the oxidation state of the MnOx deposited for 180 s (Figure 3B). The Mn 3s spectrum showed doublet peaks that result from the parallel spin coupling between the electrons in 3s and 3d orbitals.40,41 The splitting width of the doublet peaks can thus be used for estimating the oxidation state of the MnOx. The splitting width of the MnOx is 4.73 eV, indicating that its oxidation state is 4 on the basis of an approximately linear relationship between the Mn 3s splitting widths and the Mn oxidation states.40 The O 1s spectrum was also used to estimate the oxidation state of the MnOx.12 The O 1s spectrum was deconvoluted into three components corresponding to oxide (Mn
O
Mn), hydroxide (Mn
O
H), and water (H
O
H). The amount of MnO2 and MnOOH in the MnOx film was determined to be 52 and 19%, respectively. This corresponds to an oxidation state of 3.73, indicating that the composite may include both Mn3þ and Mn4þ ions. We note that the Mn 3s splitting widths shown here are slightly smaller than those in previous reports,12,40 when compared with the manganese oxides in the equivalent oxidation states estimated from the O 1s spectrum. After the 180 s composite is cycled 30 times, the splitting width of MnOx is decreased from 4.73 to 4.58 eV. In agreement, its oxidation state is increased from 3.73 to 3.81 when estimated from the O 1s spectrum, indicating that the accumulated cycles oxidize the as-deposited MnOx further. Similar to the 180 s-deposited composite, the composites deposited for 30 or 60 s also follow the same trend (Supporting Information Table S1). Raman spectra of the MnOx composites reveal three major bands at 500, 575, and 655 cm
1 (Figure 3C). Note that only the band at 655 cm
1 decreased significantly after 30 CV cycles and it did so for all the samples tested. Others have attributed the 2613

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Figure 4. Cyclic voltammograms of the composites deposited for 15 (red), 30 (blue), 60 (green), and 180 s (brown) at 5 (A) and 50 mV/s (B). The CV of a 10-sheet (black) is included in panel A. The charge/discharge curves for the composites at 1 (C) and 5 A/g (D). The iR drops are represented by dotted lines in panel C and D.

bands at 500, 575, and 655 cm
1 to MnO2,42
44 however, the band at 655 cm
1 overlaps a Raman band for Mn3O4.45,46 We therefore surmise that the as-deposited MnOx is a mixture of manganese oxides. The Mn3O4 on the composite surface is either oxidized to insoluble higher oxidation state manganese oxides36,46 or dissolved into the Na2SO4 electrolyte during the cycling.13,37
39 This results in the reduction of the quasi-reversible redox peaks and a more rectangular shaped CV curve. In addition, this increases the overall oxidation state of the MnOx in the composite after 30 cycles and is consistent with the XPS results. Unfortunately, we do not yet know whether the Mn3O4 oxidizes or dissolves. The electrolyte solution remains transparent after the CV cycling (no changes are visible) and we could not observe any clear changes in surface morphology of the composite after the CV cycling either (Supporting Information Figure S7). This still needs to be investigated as time permits. Additional analysis results can be found in the Supporting Information Section S2. CNT sheet/MnOx composites deposited for times between 15 and 180 s were examined by CV or galvanostatic charge/ discharge cycling in 0.1 M Na2SO4 aqueous solution in order to investigate the electrochemical properties depending on the amount of MnOx. The cyclic voltammograms were compared after 15 cycles. The CV curves at 5 mV/s display ideal capacitive behaviors with symmetrical rectangular shapes for all the composites (Figure 4A). This proves that the CNT sheets in the composites serve as good current collectors. In agreement with the CV results, the charge/discharge curves for all the composites (Figure 4C) show good symmetry and fairly linear slopes between 0 and 1 V at a current density of 1 A/g (based on the mass of MnOx), indicating good capacitive behaviors. The average specific capacitances (Csp) are determined to be 510, 445, 225, and 195 F/g for the deposition times of 15, 30, 60, and 180 s at 1A/g, respectively. We note that the CV curve of an uncoated 10-sheet exhibits a very narrow loop,47 indicating that the MnOx is mainly responsible for the Csp of the composites. The current response is negligible on the insulating carbon tape

(covering the copper tape) used here, indicating that the tape surface is inert electrochemically (Supporting Information Figure S8). As the deposition time is increased from 30 to 60 s, the Csp decreases from 445 to 225 F/g and the voltammetric current density greatly decreases too. These results are consistent with the suggestions of prior authors12,44,48 that the electrochemical redox reactions occur only in a thin volume at the surface of the MnOx film rather than in the bulk. The nanosized MnOx particles in the composite deposited for less than 30 s have a larger active surface area than that of the contiguous film deposited for more than 60 s even though the 60 s film has more bulk volume. This increased MnOx surface area provides more accessible sites for the intercalation and/or absorption of the Naþ and Hþ ions12,15
17 in the 30 s film than the 60 s film. This, in turn induces the higher voltammetric current density and Csp. As the scan rate was increased, the CV curves of the MnOx composites deposited for a longer time became distorted from rectangular shape. For example, when the scan rate is 50 mV/s (Figure 4B), the CV curves were badly distorted for the 60 and 180 s composites. Their initial potential drops (iR drops) that occur during the discharge are obviously increased from 60 and 95 mV at 1 A/g to 260 and 390 mV at 5 A/g, respectively (Figure 4D). Prior reports have noted that the bulk volume of thick MnOx films is a poor electron conductor,12,34,49 but electron conduction is necessary for the redox transitions of interfacial manganese oxide species.12,13,17,50 Electrons must travel from the interfacial oxides to the conductive CNT sheets and back again during charge/discharge cycling. As a result, an increased bulk volume slows the current response on voltage reversal at each end potential, resulting in the distorted shape at high scan rates and the large iR drop at high current densities. This is what is observed for the thick MnOx films. On the other hand, the CV curves of the composites deposited for 15 and 30 s exhibit symmetrical rectangular shapes even at a scan rate of 50 mV/s (Figure 4B). Their charge/discharge curves also show good 2614

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Figure 5. Cyclic voltammograms of the composites deposited on the 50-sheets (blue) and the 10-sheets (red) for 15 (A) and 30 s (B) at 100 and 200 mV/s. The charge/discharge curves at 10 A/g (C) and the Csp (D) for the composites deposited on the 50-sheets (blue) and the 10-sheets (red) for 15, 30, and 60 s. The iR drops are represented by dotted lines in panel C. The Csp was also included for the composite deposited on the 10-sheet (red) for 5 s in panel D. (E,F) Cycling stability at 2 A/g and Ragone plot of the 10-sheet deposited for 5 and 15 s and the 50-sheet deposited for 30 s.

symmetry and linear slope having small iR drops at a current density of 5 A/g (Figure 4D). The result is a Csp of 445 and 400 F/g, respectively (∼10% reduction when compared to the Csp at 1 A/g). To improve the electrochemical properties further by increasing the number of MnOx particles on the inner CNTs and reducing the sheet resistance of the multisheets, the number of CNT sheets was increased from 10 to 50. The MnOx was electrodeposited on the 50-sheet electrode for times between 15 and 60 s. The same amount of charge per unit area used to deposit MnOx on the 10-sheet electrode was used to deposit MnOx on the 50-sheet electrode in order to load the same amount of MnOx. As shown in Supporting Information Figure S9, the MnOx particles deposited on the 50-sheet are a little smaller in size than those on the 10-sheet in analogous conditions, forming more open pores on the surface of the 50-sheet electrode. This means that MnOx particles are spread further apart in the inner CNTs of the 50-sheet than on the 10-sheet electrode. All the CV curves of both the 50-sheet and the 10-sheet display symmetrical rectangular shapes at 5 mV/s (not shown here). In addition, the CV curves of both the 50-sheet and 10-sheet deposited for 15 s exhibit the same symmetrical rectangular shapes at scan rates of either 100 or 200 mV/s (Figure 5A). Their charge/discharge curves also show the same small iR drops even at a current density of 10 A/g (Figure 5C) and they exhibit almost the same Csp of 435 F/g (only a ∼15% reduction compared to the Csp at 1 A/g). Unlike the 15 s composites, slight differences are observed between the composites deposited for 30 s on the 50-sheet and the 10-sheet electrodes when tested at high scan rates and at high current densities. The 50-sheet has a more rectangular shape than the 10-sheet composite at 100 and 200 mV/s (Figure 5B). The iR drop of the 50-sheet is smaller than that of the 10-sheet at 10 A/g, resulting in a higher Csp by 10% for the 50-sheet. (The 60 s deposits on both the 50-sheet and 10-sheet electrodes result in highly distorted CV curves.) We note that Csp appears to have little or no dependence on the number of layers in the electrode.

Increasing from 10 to 50 sheets made essentially no difference in Csp. We found that Csp does have a significant dependence on the deposition time and therefore the size of the MnOx particles. Csp increased from 225 to 445 F/g as the deposition time decreased from 60 to 30 s on the 10-sheet electrode. On the other hand, Csp was not significantly improved when the number of CNT sheets was increased by a factor of 5 even though the electrical conductivity increased commensurately. We thus believe that both the Csp and the charge/discharge rate capability are enhanced by controlling the size of MnOx particles.12,13,51 We compared the electrochemical properties of composites deposited on the 10-sheet electrode for 5 and 15 s in order to more fully understand how the MnOx particle size impacts the electrochemical responses. As mentioned above, while the MnOx deposited for 15 s forms coaxial MnOx/CNT wires, the 5 s deposit forms individual nanosized particles. The 5 s composite shows symmetrical rectangular shapes at 100 and 200 mV/s, and good symmetry and linear slopes with respect to charging and discharging at 10 A/g (Supporting Information Figure S10). In addition, the Csp is determined to be 1250, 1110, 960, and 960 F/ g at 1, 2, 5, and 10 A/g, respectively, which is two times larger than the Csp for the 15 s composite (Figure 5D). This phenomenon is not clearly understood. We believe that the large Csp and rate capability result from the individual MnOx nanoparticles which have a large active surface area, short diffusion paths for cations, and the ability to quickly transport electrons to the CNTs due to small dimensions of the MnOx particles. These still need to be investigated further. We note that the mass of the 5 s-deposited composite consists of 25% for MnOx and 75% for MWCNTs. This means that the gravimetric efficiency of the composite electrodes can be improved by decreasing the mass of MWCNTs. This can be achieved by decreasing the number of CNT walls or using single-walled metallic CNTs. Figure 5E shows the long cycle stability of the composites deposited for 5, 15, and 30 s. The initial Csp is maintained without loss even after 500 charge/discharge cycles for the composites 2615

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Nano Letters deposited for 15 and 30 s. On the other hand, the Csp of the 5 s composite gradually increases from 1110 to 1235 F/g during initial 50 cycles, and then decreases to 1060 F/g (4.5% decay) after 250 cycles. The decreased Csp is maintained without further loss even after 500 cycles. This phenomenon is not clearly understood. This still need to be investigated further. The Ragone plot (Figure 5F) shows high power
energy characteristics of the composites deposited for 5, 15, and 30 s. The 5 s composite has higher power density and energy density than the others, having energy densities of 135 and 49 Wh/kg at power densities of 1.0 and 17.4 kW/kg, respectively. These values are not only much higher than those of both current Li-ion batteries and electrochemical capacitors in Ragone plot,52 but also comparable to those of the nanostructured MnOx composites.53,54 In summary, CNT sheet/MnOx composites were synthesized by electrodepositing MnOx on CNT multisheets. The as-formed MnOx is a poorly crystallized MnO2 including a minor Mn3O4 component. The electrochemical properties of the composites depend on loading amount and morphology of MnOx. When nanosized MnOx particles are formed on the multisheets, the Csp of the composites has high values of 500
1250 F/g along with high charge/discharge rate capabilities. Contiguous MnOx films have a lower Csp of ∼200 F/g. In particular, both the Csp and the rate capability were enhanced by controlling the size of the MnOx particles rather than the number of layers in the CNT multisheets. The best performance composites result from a synergistic combination of large surface area and good electron-transport capabilities of the nanosized MnOx particles with the high conductivity of CNT sheets. This suggests that CNT sheet/ MnOx composites having nanosized particles are promising as electrode materials for lithium batteries and supercapacitors. We are currently studying the integration structures in which the composites can be overlaid to tens of micrometers thickness as a commercial approach for lithium batteries.

’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental details, a table, additional figures, and additional references. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: 972-883-6839. Telephone: 214-405-1216.

’ ACKNOWLEDGMENT The authors thank DMS Co., Ltd. for donating the CVD chamber and Y. J. Chabal for help with using facilities at the Laboratory for Surface and Nanostructure Modification. ’ REFERENCES (1) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J.-M.; Schalkwijk, W. V. Nanostructured materials for advanced energy conversion and storage devices. Nat. Mater. 2005, 4, 366–377. (2) Liu, C.; Li, F.; Ma, L.-P.; Cheng, H.-M. Advanced Materials for Energy Storage. Adv. Mater. 2010, 22, E28–E62. (3) Liu, R.; Duay, J.; Lee, S. B. Heterogeneous nanostructured electrode materials for electrochemical energy storage. Chem. Commun. 2011, 47, 1384–1404.

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