Uniformly Embedded Metal Oxide Nanoparticles in VerticallyAligned Carbon Nanotube Forests as Pseudocapacitor Electrodes forEnhanced Energy StorageYingqi Jiang,*,† Pengbo Wang,†,‡ Xining Zang,† Yang Yang,†,§ Alina Kozinda,† and Liwei Lin†

†Berkeley Sensor and Actuator Center, Department of Mechanical Engineering, University of California, Berkeley, California 94720,United States‡Robotics and Microsystems Center, Soochow University, Suzhou 215021, China§Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

*S Supporting Information

ABSTRACT: Carbon nanotube (CNT) forests were grown directly on a silicon substrate using a Fe/Al/Mo stacking layerwhich functioned as both the catalyst material and subsequently a conductive current collecting layer in pseudocapacitorapplications. A vacuum-assisted, in situ electrodeposition process has been used to achieve the three-dimensionalfunctionalization of CNT forests with inserted nickel nanoparticles as pseudocapacitor electrodes. Experimental results haveshown the measured specific capacitance of 1.26 F/cm3, which is 5.7 times higher than pure CNT forest samples, and theoxidized nickel nanoparticle/CNT supercapacitor retained 94.2% of its initial capacitance after 10 000 cyclic voltammetry tests.

KEYWORDS: Vertically aligned carbon nanotube, nanoparticle, electrodeposition, metal oxide, pseudocapacitor, energy storage

A pseudocapacitor is an excellent candidate to bridge theperformance gap between supercapacitors and batteries.1,2

Specifically, conventional supercapacitors utilize high-surface-area electrodes to store electrical charges, and they haveadvantages over batteries in areas such as a fast charge/discharge rate (seconds vs hours), long life cycles (106 vs 103

cycles), as well as simple and stable structures (low cost andphysical nature vs complex and chemical nature).2 As such,supercapacitors have been widely used in pulse-powerapplications such as vehicle regenerative braking, camera LEDflash, battery loading buffer, and so forth. These performanceadvantages rely on the fact that the stored charges simply stayon the very surface of the electrode rather than penetrating intothe inside of the electrode as they do in rechargeable batteries.However, this mechanism inevitably leads to the bottleneck ofsupercapacitors’ developmentit has a relatively low energydensity as the electrode is not fully utilized. On the other hand,while batteries have high energy density thanks to the fullyusage of the electrode, it takes additional time for the storedcharge to enter and leave the electrode, resulting in a lowerpower density and significant electrode volume change duringcharge and discharge. To combine the high power density(=high surface area) of supercapacitors and the high energydensity (=inside storage of electrode) of batteries, a new combo

device called a pseudocapacitor has been designed.1 Pseudo-capacitors use a highly porous electrode to maintain high powerdensity as supercapacitors and also integrate functionalizationmaterials such as metal oxide or conducting polymer, where thecharges can be stored inside locally. In other words, the chargesin a pseudocapacitor are still stored around the two-dimensional surface just as the case of supercapacitors globally,but when examined closely, the charges are locally stored withinthe three-dimensional functionalization materials as the case ofbatteries. By controlling the relative ratio between the highlyporous material and the functionalization material in theelectrode, pseudocapacitors can behave as either super-capacitors (for no functionalization material) or batteries (forno highly porous material). While pseudocapacitors look like an“ideal” energy storage solution, there are two practicalconstraints: first, the porous and often mechanical fragileelectrode needs a conductive substrate to effectively collect thecurrent. Second, the functionalization materials need to bedistributed conformally on all surfaces both inside and outside

Received: March 12, 2013Revised: July 8, 2013

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the porous electrode. Agglomeration of functionalizationmaterial not only reduces the effective utilization of thefunctionalization material but also blocks transfer paths ofelectrolytic ions. In the worst case, the electrode may lose theinside surface area totally if the functionalization material sealsthe top surface of the electrode.Nanomaterials, particularly carbon nanotube (CNT), could

play critical roles to improve the performances of energystorage devices.3 CNTs have well-known outstanding electricaland mechanical characteristics such as high surface area tovolume ratio and intrinsically metallic for any CNT with largerthan 2 nm diameters.4 Furthermore, CNTs have a uniquefeature compared to other nanowires: they can naturally growinto a densely packed yet vertically aligned array, often calledCNT forests.5 Such a structure provides many benefits forenergy storage applications. In CNT forests, each CNTindividually and robustly contacts to the growth substrate andtherefore minimize the contact resistance (assuming thesubstrate is conductive). The aligned architecture facilitatesthe movement of the electrolytic ions for improved exchangerate.6 The well-developed chemical vapor deposition (CVD)method produces CNT forests easily and precisely in term ofdiameter and length, resulting in uniform and consistent

material quality for mass production. CVD-made CNTs arealready available on the market.7 As a result, there have beenincreasing studies of using CNTs and CNT forests for energystorage applications.8−10 Pushparaj et al. demonstrated aflexible supercapacitor using nanoporous cellulose paperembedded with CNT forest and electrolyte.8 They used apostgrowth deposition and transfer process to form the currentcollecting layer. Their units were demonstrated as energystorage devices including supercapacitors, Li-ion batteries, andhybrids. Kaempgen et al. fabricated a flexible and printable thinfilm supercapacitor using sprayed networks of single-walledCNTs (SWCNTs) and printable gel electrolyte.9 The SWCNTnetworks served as both the electrode and charge collectors.The performances of the devices showed very high energy andpower densities which was comparable to other SWCNT-basedsupercapacitor devices. Zhang et al. made pseudocapacitorelectrodes from CuO/CNT nanocomposite.10 The CNTs weremechanically mixed with the CuO nanobelts. The processresulted in a randomly oriented CNT network as a currentcollector. Their nanocomposite electrode delivered a specificcapacitance 2.6 times higher than that of a pure SWCNTelectrode.

Figure 1. (a) Schematic of the pseudocapacitor using CNT forests functionalized with oxidized nickel nanoparticles as the electrode. The storageand release of electrical charges occur by absorption and desorption of charges on the electrode surface as well as reduction and oxidization of nickelions between 2+ and 3+ chemical states. (b−d) The scanning electron microscope (SEM) images of the cross sections of the nickel nanoparticle-embedded the CNT forests after the electrodeposition of 20 s (b), 2 min (c), and 8 min (d), respectively. The current density was 50 mA/cm2 for allof the cases. The scale bar in b−d is the same, 300 nm. (e) Relationship between nickel nanoparticle diameters versus deposition time. Thediameters were estimated based on the SEM pictures.

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However, there is limited progress on CNT forest-basedpseudocapacitors because of two practical challenges. First ofall, to make a CNT forest-based pseudocapacitor, there must bea current collecting layer under the CNT forest. However, it iswell-known that CNT forests mostly grow on nonconductivesubstrates.11 The direct growth of CNT forest on conductivesubstrate is so far limited on special substrates, which may notbe suitable for general applications. As such, CNT forests haveto be transferred from the growth substrate to a secondsubstrate. Because CNT forest is highly porous and fragile, thetransfer process is typically very complex.8 Second, it isimportant to uniformly and thoroughly functionalize CNTs inthe forest format. So far there are few reports on thefunctionalization of the 100 μm level and above thick CNTforests. Common deposition techniques encounter variouslimits including damaging the aligned structure12 and shallowdeposition13 (the deposition only reaches several micrometersbelow the top surface). In contrast, electrodeposition has theadvantages of low cost and easy scale-up; however, CNT’sintrinsic hydrophobicity14 prevents the solution from enteringthe inside of the CNT forest. We experimentally proved aconventional electrodeposition only deposited material on thevery top surface of the CNT forests (see S1 in the SupportingInformation).This work has addressed the aforementioned challenges by

the direct growth of CNT forests on a unique Fe/Al/Moconductive substrate and the conformal deposition of nickelnanoparticles within the forests. The functionalized CNT forestpseudocapacitor electrode using such techniques is illustrated in

Figure 1a, which takes the advantages of both high surface areaof aligned CNT forests and oxidized nickel nanoparticles forenhanced energy storage. During the charge/discharge cycle,the following redox reaction happens:15

+ ⇔ + +− −Ni(OH) OH NiO(OH) H O e2 2 (1)

As such, the storage and release of electrical charges occur notonly by the absorption and desorption of charges on the CNTsurface but also by the reduction and oxidization of nickel ionsbetween 2+ and 3+ chemical states within the nanoparticles.Figure 1b−e shows the flexibility on tuning nanoparticle sizeusing our functionalization technique.To build a CNT forest pseudocapacitor as proposed in

Figure 1a, the first step is to choose proper substrate. Thesubstrate has two essential roles: (1) to support the CNTgrowth during the CVD synthesis process; and (2) to act as thecurrent collecting layer in the charge/discharge cycle of thepseudocapacitor application. On the one hand, to support theCNT growth, the substrate needs to have a balanced interactionwith catalysts, mostly iron.16 The surface property of thesubstrate is critical in controlling the synthesis of CNT forests,without which there could be no CNT growth (too stronginteraction) or the growth of much thicker carbon nanofiberand/or amorphous carbon (too weak interaction). Therecognized reality is that CNTs grow predominantly onnonconducting substrates.11 On the other hand, the substrateshould obviously be conductive as a good current collector. Wehave experimentally tested several common metals on top of asilicon substrate, including Ti, Cr, Ni, Al, and Mo as the growth

Figure 2. Characterizations of a 150 μm high nickel nanoparticle-functionalized CNT forest (electrodeposition for 4 min@42 °C, 50 mA/cm2). (a)Top view of the functionalized CNT forest. (b−d) Cross sectional views (created by splitting the sample after functionalization, so indeed an insideview is shown) of the top, middle, and bottom regions, respectively. (e) The overall cross section view. The scale bars are 200 nm (a), 500 nm (b−d), and 20 μm (e). The nanoparticle size is estimated as 80 nm from the SEM pictures. (f) TEM of the nickel nanoparticles with a diameter of 30 nm(20 s@42 °C, 50 mA/cm2). (g) Energy dispersive X-ray (EDX) analysis of CNT forest samples before (upper) and after (lower) nickelelectrodeposition, respectively.

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supporting layer with a 5 nm thick Fe layer as the catalyst ontop.17 Only the Al layer was found to allow dense growth ofCNTs, and other metals resulted in sparse or no CNT growth(see S2 in the Supporting Information). However, the Al layerlost its original conductivity after the CNT growth process, andwe suspect that there might have been an insulating layer thatformed during the CVD process. The Mo layer also stood outin our tests and was observed to have increased conductivityafter the CNT growth possibly due to some annealing effect.Based on the observations and analysis, the combination of Al/Mo bilayer with catalyst Fe layer on top has been proposed asthe CNT growth supporting material. Experimental resultsshow that both dense CNT forest growth and good electricalconductivity on the substrate can be achieved with Fe, Al, andMo of 5, 10, and 50 nm in thickness, respectively. The contactresistance between the CNT forest and the Mo layer wascharacterized as 5 × 10−4 Ω·cm2.18 Our hypothesis is that,because the Al layer is so thin, the current could tunnel throughthe Al layer (despite Al becomes insulating during the CVDprocess) when the conductive Mo layer is underneath.The next challenge comes from the uniform functionalization

of the porous CNT forests. To do so, a vacuum wetting processis used to overcome high hydrophobicity of CNT forests.19 Adetailed wetting process can be found in S3 in the SupportingInformation. Basically, the CNT forest sample is loaded in avacuum chamber, and the pressure is pumped down. Then theCNT sample is covered with deionized water. Afterward, thesystem is vented, and the pressure difference immediatelybuilds up between the environmental atmosphere and thevacuum inside the CNT forest and push water into the CNTforests. After the wetting process, the sample is placed intonickel electrolyte for the electrodeposition process. Figure 1parts b, c, and d are the SEM (scanning electron microscope)images showing the cross section of CNT forest samples after20 s, 2 min, and 8 min of the electrodeposition process,

respectively. Uniformly distributed nanoparticles can be clearlyobserved, and the size of these nanoparticles increases as thedeposition time increases. Figure 1e is the measurement resultsof the diameter of the nanoparticles with respect to thedeposition time. Nanoparticles as small as 30 nm and as big as100 nm can be homogeneously deposited in the CNT forests.From another point of view, Figure 2a−e shows different

locations of a 150 μm-high, air-dried CNT forest sample thathas gone through the vacuum wetting and electrodepositionprocess (4 min, 50 mA/cm2). It is clearly seen that the top(Figure 2b), middle (Figure 2c) and bottom (Figure 2d)portions of the CNT forest have been decorated with discretenickel nanoparticles of about 80 nm in diameter. Discretenanoparticles instead of continuous thin film have beendeposited because the deposition process favors defects onthe sidewalls of CNTs.20 The functionalization process is highlyflexible by changing the current density and/or electro-deposition time to fabricate nanoparticles ranging from tensof nanometers to over a few hundred nanometers (see S4 in theSupporting Information). For example, Figure 2f shows a TEMimage of small nanoparticles with only 30 nm in diametersynthesized within the CNT forests by reducing the depositiontime to 20 s. Energy dispersive X-ray (EDX) measurements inFigure 2g show that the as-grown CNT sample has carbon,silicon, Al, Mo, and Fe as expected. The result from the bottompart of Figure 2g is from a sample with nickel nanoparticleswithout the silicon substrate. It has strong signs of nickel andoxygen as nickel oxide is probably formed after the nickeldeposition process.The growth of CNT forests with conductive electrode began

with thermal oxidation (1000−2000 Å) of a silicon wafer toimprove the adhesion between metals and the substrate as wellas to provide an insulation layer between the CNT forest andthe silicon substrate. Then, Mo, Al, and Fe were deposited ontothe substrate in sequence using e-beam evaporation with a

Figure 3. Performance of a nickel-functionalized CNT forest pseudocapacitor. (a) Cyclic voltammetry curves with “CNT+Nickel” (green) and CNT(red) electrodes. The scanning rate was 100 mV/s, and the electrolyte was 0.1 M KOH. (b) Chronoamperometry results showing 100 cycles ofcharge and discharge curves. (c) Overlay of the first and last five cycles of b. (d) Capacity retention rate for 10 000 cycles of CV tests. Inset: cyclicvoltammetry curves at every 1000 cycles.

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thickness of 50 nm, 10 nm, and 5 nm, respectively. The waferwas then diced into rectangular pieces with the size of 5 mm ×10 mm for CNT growth. A thermal chemical vapor deposition(CVD) furnace (Lindberg/Blue M three-zone tube furnace,Thermo Electron Corp., Asheville, NC) was used to grow theCNT forest. During the growth, the furnace was first pumpedto vacuum and then heated up. The hydrogen constantly flewthrough the 2-in. quartz tube with a flow rate of 50 sccm untilthe furnace temperature reached the target of 720 °C.Immediately after the target temperature was reached, the gaswas switched to a mixture of carbon precursor, ethylene, andcarrying gas, hydrogen, with the flow rates of 90 and 611 sccm,respectively. A standard growth time of 10 min usually resultsin 150 μm high CNT forests. Finally the furnace was cooleddown to room temperature before unloading.For the vacuum wetting process, CNT samples were loosely

fixed onto a plastic dish with the help of double-side tapes andtransferred into a vacuum chamber. Air was pumped (Emerson,Motor Division, St. Louis, MO, USA) out the system for about1 h (stable pressure is about 3 mbar). Then deionized (DI)water was let in to cover the CNT sample. Pumping continuedfor about another 0.5 h until the degassing of DI water becamebarely visible. Afterward, the chamber was vented, and thewetting process immediately occurred because of the pressuredifference between the surrounding and the CNT forest. Beforeelectrodeposition, the DI water was replaced with the nickelelectrodeposition solution (nickel sulfamate RTU, product no.030175, Technic Inc.). During the electrodeposition, half of the5 mm × 10 mm CNT sample was immersed into the solution(therefore, the effective functionalization area is 5 mm × 5mm). A copper clamp is used to hold the sample above thesolution and, meanwhile, form the electrical contact. An inertstainless steel electrode is used as the counter electrode. Thetypical current density was 50 mA/cm2, and the deposition timeranged from 20 s to a few minutes.The functionalized CNT forest samples were tested as

pseudocapacitor electrodes using a typical three-electrodeelectrochemical setup: the CNT working electrode, an Ag/AgCl reference electrode, and a Pt wire counter electrode. Thethree electrodes are immerged into 0.1 M KOH aqueoussolution. The functionalized part of the CNT forest sample isdipped into the electrolyte. All of the measurements have beencarried out on a reference 600 Potentiostat (Gamry Instru-ments, Inc., Warminster, PA). For consistency and simplicity,the CNT forest samples to be discussed below were all fromthe same growth batch with a height of about 150 μm and thesame electrodeposition recipe (2 min, 50 mA/cm2), resulting innickel nanoparticles of about 60 nm in diameter. All of thecyclic voltammetry tests used a constant scanning rate (dV/dt)of 100 mV/s. Before the tests, nickel nanoparticles were

oxidized first into nickel hydroxide through cycling thepotential from −1.2 to +0.8 V 20 times.21

Figure 3 shows the cyclic voltammetry (CV) curve (greenline) of a nickel-functionalized CNT pseudocapacitor electrode.As a comparison, the performance of an as-grown CNT sample(red line) is also included. The CNT samples of both caseshave the same exposed physical area (5 mm × 5 mm) in theelectrolyte. It is clearly observed that the introduction ofoxidized nickel nanoparticles results in an enlarged CV curve.The peak and valley of the functionalized CNT forests at 500mV and 0 mV correspond to the reduction and oxidationreactions of nickel hydroxide, respectively. According to theequation:

= =CqV

IV t

dd d /d (2)

the current gap (ΔI) between forward and backward sweeps ofthe CV curve is in proportion to the capacitance. Thefunctionalized CNT electrode has a much larger current gapthan the nonfunctionalized sample throughout the voltagerange. Particularly, the functionalized sample has a current gapof about 800 μA near V = 0 V, eight times higher than that ofthe as-grown CNT sample, which was about 100 μA. Based onthe circled area of the CV curve, we calculated the averagespecific capacitance of the nickel-functionalized CNT forestelectrode as 1.26 F/cm3, 5.7 times higher than the 0.22 F/cm3

of the pure CNT sample (see S5 in the Supporting Informationfor detailed derivations).Figure 3b shows the results from 100 cycles of

chronoamperometry (charge/discharge) tests. There was noapparent current amplitude degradation, indicating goodstability of the electrode. The good match between the firstand the last five cycles of the 100-cycle test in Figure 3c clearlyconfirms oxidized nickel nanoparticles did not deteriorateduring these tests. ITo study the size and the loaded weight ofthe active particles to the performance of supercapacitors, CVtests on samples with various nickel electrodeposition time havebeen studied (Figure S6 in the Supporting Information), and anupward trend of capacitance with respect to electrodepositiontime has been found due to increased surface areas.Figure 3d shows the capacity retention rate (normalized to

the initial capacitance) from 10 000 cycles of CV tests. It isobserved that capacitance increased during the first 1000 cyclespossibly due to nickel gradually involved in the redox chargestorage. Afterward, the capacitance gradually reduced but stillretained 94.2% of its initial capacity after 10 000 cycles.Another proof of the thorough functionalization is that the

CNT electrode preserved its original shape thanks to thereinforcement of nickel nanoparticles after the dehydrationprocess to take SEM photos as illustrated in Figure 4a. Incontrast, we experimentally demonstrated elsewhere that,

Figure 4. (a) SEM photo of CNT forest with embedded nickel nanoparticles. The scale bar is 10 μm. (b) The close-up SEM photo of a. The scalebar is 200 nm. (c) After 1000 cycles of CV tests. The scale bar is 200 nm.

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without the support of these nanoparticles, a 320 μm thickCNT forest either broke and shrank significantly in the lateraldirection or collapsed its thickness by 16 times to a merely 21μm thick CNT sheet (with the assistance of an initialmechanical deformation) because of the liquid zipping effect.22

In either case, there was no way that the shape could bepreserved as well as Figure 4a. Figure 4b is the close-up SEMimage showing nanoparticles are intact with the CNT forestafter the drying process. Furthermore, after the 1000 cyclic testwas conducted, the SEM picture of nickel nanoparticles wastaken at a slightly different angle as shown in Figure 4c. It isobserved that the morphology of these nanoparticles lookedsimilar before and after the cyclic tests, and there was nonoticeable change like redistribution, falloff, or size change ofnickel nanoparticles.As a final note, unlike other approaches where special

elements and complex processes are often involved, thetechniques presented in this work are low cost. Our CNTgrowth method still uses the most common thermal CVDprocess. The substrate elements of Mo and Al are abundant andcheap. The vacuum wetting step does not add any material cost.The wet chemical electrodeposition has already been widelyused in industry.In summary, this paper addresses the issues of the direct

growth of CNT forest on a conductive electrode and thefeasibility of the three-dimensional functionalization of CNTforests. The Fe/Al/Mo metallic stacking layer has beenproposed and demonstrated to grow dense CNT forests withexcellent conductivity. The vacuum wetting-assisted electro-deposition method has overcome the hydrophobicity issue ofCNT forests and uniformly functionalized the 150 μm thickCNT forest. Using the functionalized CNT forests as theelectrode of a pseudocapacitor, an enhanced energy density of1.26 F/cm3 has been achieved, which is 5.7 times higher thanthe pure CNT forest electrode. The experimental resultsfurther confirmed that the oxidized nickel nanoparticles and theCNT forests have exhibited only 5.8% of degradation after 10000 cyclic voltammetry tests. Looking beyond, low-costmethodologies presented in this work could advance theintegration of CNT forests with devices on silicon-basedsubstrate. The functionalization could greatly reinforce CNTforest from material perspective and broaden the potentialapplications of CNT forest such as various chemical/biologicalsensors and synthesis of nanoparticles. These results comparewell in some of the very recent nanomaterial-based super-capacitor publications.23−25

■ ASSOCIATED CONTENT*S Supporting InformationDetails for experimental procedures, additional functionaliza-tion results, and device performance calculation. This materialis available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: jiangyq99@gmail.com.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis project is supported in part by the DARPA N/MEMSFundamentals Program and Siemens Inc.

■ REFERENCES(1) Conway, B. E.; Birss, V.; Wojtowicz, J. The Role and Utilizationof Pseudocapacitance for Energy Storage by Supercapacitors. J. PowerSources 1997, 66, 1−14.(2) Conway, B. E. Electrochemical Supercapacitors: ScientificFundamentals and Technological Applications; Kluwer Academic/Plenum Publishers: New York, 1999.(3) Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors.Nat. Mater. 2008, 7, 845−854.(4) Kane, C. L.; Mele, E. J. Size, Shape, and Low Energy ElectronicStructure of Carbon Nanotubes. Phys. Rev. Lett. 1997, 78 (10), 1932−1935.(5) Fan, S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.;Cassell, A. M.; Dai, H. Self-Oriented Regular Arrays of CarbonNanotubes and Their Field Emission Properties. Science 1999, 283,512−514.(6) Zhang, H.; Cao, G.; Yang, Y.; Gu, Z. Comparison BetweenElectrochemical Properties of Aligned Carbon Nanotube Array andEntangled Carbon Nanotube Electrodes. J. Electrochem. Soc. 2008, 155(2), K19−K22.(7) Product page of CVD-made Carbon Nanotubes. http://www.sigmaaldrich.com/materials-science/material-science-products.html?TablePage=16376687; accessed March 15, 2013.(8) Pushparaj, V. L.; Shaijumon, M. M.; Kumar, A.; Murugesan, S.;Ci, L. J.; Vajtai, R.; Linhardt, R. J.; Nalamasu, O.; Ajayan, P. M. FlexibleEnergy Storage Devices Based on Nanocomposite Paper. Proc. Natl.Acad. Sci. U.S.A. 2007, 104, 13574−13577.(9) Kaempgen, M.; Chan, C. K.; Ma, J.; Cui, Y.; Gruner, G. PrintableThin Film Supercapacitors Using Single-Walled Carbon Nanotubes.Nano Lett. 2009, 9 (5), 1872−1876.(10) Zhang, X.; Shi, W.; Zhu, J.; Kharistal, D. J.; Zhao, W.; Lalia, B.S.; Hng, H. H.; Yan, Q. High-Power and High-Energy-Density FlexiblePseudocapacitor Electrodes Made from Porous CuO Nanobelts andSingle-Walled Carbon Nanotubes. ACS Nano 2011, 5 (3), 2013−2019.(11) Talapatra, S.; Kar, S.; Pal, S. K.; Vajtai, R.; Ci, L.; Victor, P.;Shaijumon, M. M.; Kaur, S.; Nalamasu, O.; Ajayan, P. M. DirectGrowth of Aligned Carbon Nanotubes on Bulk Metals. Nat.Nanotechnol. 2006, 1, 112−116.(12) Ma, S.; Ahn, K.; Lee, E.; Oh, K.; Kim, K. Synthesis andCharacterization of Manganese Dioxide Spontaneously Coated onCarbon Nanotubes. Carbon 2007, 45 (2), 375−382.(13) Fang, W.; Chyan, O.; Sun, C.; Wu, C.; Chen, C.; Chen, K.;Chen, L.; Huang, J. Arrayed CNx NT−RuO2 NanocompositesDirectly Grown on Ti-buffered Si Substrate for SupercapacitorApplications. Electrochem. Commun. 2007, 9 (2), 239−244.(14) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.;Amaratunga, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K.Superhydrophobic Carbon Nanotube Forests. Nano Lett. 2003, 3 (12),1701−1705.(15) Patil, U. M.; Gurav, K. V.; Fulari, V. J.; Lokhande, C. D.; Joo, O.S. Characterization of Honeycomb-like “β-Ni(OH)2” Thin FilmsSynthesized by Chemical Bath Deposition Method and TheirSupercapacitor Application. J. Power Sources 2009, 188 (1), 338−342.(16) Gohier, A.; Ewels, C. P.; Minea, T. M.; Djouadi, M. A. CarbonNanotube Growth Mechanism Switches from Tip- to Base-growthwith Decreasing Catalyst Particle Size. Carbon 2008, 46, 1331−1338.(17) Jiang, Y.; Zhou, Q.; Lin, L. Planar MEMS Supercapacitor UsingCarbon Nanotube Forests. IEEE Int. Conf. Micro Electro Mech. Syst.,Tech. Dig., 22nd 2009, 587−590.(18) Jiang, Y.; Wang, P.; Lin, L. Characterizations of Contact andSheet Resistances of Vertically Aligned Carbon Nanotube Forest withIntrinsic Bottom Contacts. Nanotechnology 2011, 22, 365704.(19) Jiang, Y.; Wang, P.; Zhang, J.; Li, W.; Lin, L. 3D SupercapacitorUsing Nickel Electroplated Vertical Aligned Carbon Nanotube ArrayElectrode. IEEE Int. Conf. Micro Electro Mech. Syst., Tech. Dig., 23nd2010, 1171−1174.(20) Fan, Y.; Goldsmith, B. R.; Collins, P. G. Identifying andCounting Point Defects in Carbon Nanotubes. Nat. Mater. 2005, 4,906−911.

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dx.doi.org/10.1021/nl400921p | Nano Lett. XXXX, XXX, XXX−XXXF

(21) Deo, R. P.; Lawrence, N. S.; Wang, J. Electrochemical Detectionof Amino Acids at Carbon Nanotube and Nickel−Carbon NanotubeModified Electrodes. Analyst 2004, 129 (11), 1076−1081.(22) Jiang, Y.; Lin, L. A Two-Stage, Self-Aligned VerticalDensification Process for As-Grown CNT Forests in SupercapacitorApplications. Sens. Actuators, A 2012, 188, 261−267.(23) Zhou, C.; Zhang, Y.; Li, Y.; Liu, J. Construction of High-Capacitance 3D CoO@Polypyrrole Nanowire Array Electrode forAqueous Asymmetric Supercapacitor. Nano Lett. 2013, 13 (5), 2078−2085.(24) Yu, G.; Hu, L.; Liu, N.; Wang, H.; Vosgueritchian, M.; Yang, Y.;Cui, Y.; Bao, Z. Enhancing the Supercapacitor Performance ofGraphene/MnO2 Nanostructured Electrodes by Conductive Wrap-ping. Nano Lett. 2011, 11 (10), 4438−4442.(25) Mai, L.; Li, H.; Zhao, Y.; Xu, L.; Xu, X.; Luo, Y.; Zhang, Z.; Ke,W.; Niu, C.; Zhang, Q. Fast Ionic Diffusion-Enabled NanoflakeElectrode by Spontaneous Electrochemical Pre-Intercalation for High-Performance Supercapacitor. Sci. Rep. 2013, 3, 1718.

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