Letter pubs.acs.org/NanoLett
Uniformly Embedded Metal Oxide Nanoparticles in Vertically Aligned Carbon Nanotube Forests as Pseudocapacitor Electrodes for Enhanced Energy Storage Yingqi 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 layer which functioned as both the catalyst material and subsequently a conductive current collecting layer in pseudocapacitor applications. A vacuum-assisted, in situ electrodeposition process has been used to achieve the three-dimensional functionalization of CNT forests with inserted nickel nanoparticles as pseudocapacitor electrodes. Experimental results have shown the measured specific capacitance of 1.26 F/cm3, which is 5.7 times higher than pure CNT forest samples, and the oxidized 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
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device called a pseudocapacitor has been designed.1 Pseudocapacitors use a highly porous electrode to maintain high power density as supercapacitors and also integrate functionalization materials such as metal oxide or conducting polymer, where the charges can be stored inside locally. In other words, the charges in a pseudocapacitor are still stored around the twodimensional surface just as the case of supercapacitors globally, but when examined closely, the charges are locally stored within the three-dimensional functionalization materials as the case of batteries. By controlling the relative ratio between the highly porous material and the functionalization material in the electrode, pseudocapacitors can behave as either supercapacitors (for no functionalization material) or batteries (for no highly porous material). While pseudocapacitors look like an “ideal” energy storage solution, there are two practical constraints: first, the porous and often mechanical fragile electrode needs a conductive substrate to effectively collect the current. Second, the functionalization materials need to be distributed conformally on all surfaces both inside and outside
pseudocapacitor is an excellent candidate to bridge the performance gap between supercapacitors and batteries.1,2 Specifically, conventional supercapacitors utilize high-surfacearea electrodes to store electrical charges, and they have advantages 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 and physical nature vs complex and chemical nature).2 As such, supercapacitors have been widely used in pulse-power applications such as vehicle regenerative braking, camera LED flash, battery loading buffer, and so forth. These performance advantages rely on the fact that the stored charges simply stay on the very surface of the electrode rather than penetrating into the inside of the electrode as they do in rechargeable batteries. However, this mechanism inevitably leads to the bottleneck of supercapacitors’ developmentit has a relatively low energy density as the electrode is not fully utilized. On the other hand, while batteries have high energy density thanks to the fully usage of the electrode, it takes additional time for the stored charge to enter and leave the electrode, resulting in a lower power density and significant electrode volume change during charge and discharge. To combine the high power density (=high surface area) of supercapacitors and the high energy density (=inside storage of electrode) of batteries, a new combo © XXXX American Chemical Society
Received: March 12, 2013 Revised: July 8, 2013
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Figure 1. (a) Schematic of the pseudocapacitor using CNT forests functionalized with oxidized nickel nanoparticles as the electrode. The storage and release of electrical charges occur by absorption and desorption of charges on the electrode surface as well as reduction and oxidization of nickel ions between 2+ and 3+ chemical states. (b−d) The scanning electron microscope (SEM) images of the cross sections of the nickel nanoparticleembedded 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 all of the cases. The scale bar in b−d is the same, 300 nm. (e) Relationship between nickel nanoparticle diameters versus deposition time. The diameters were estimated based on the SEM pictures.
material quality for mass production. CVD-made CNTs are already available on the market.7 As a result, there have been increasing studies of using CNTs and CNT forests for energy storage applications.8−10 Pushparaj et al. demonstrated a flexible supercapacitor using nanoporous cellulose paper embedded with CNT forest and electrolyte.8 They used a postgrowth deposition and transfer process to form the current collecting layer. Their units were demonstrated as energy storage devices including supercapacitors, Li-ion batteries, and hybrids. Kaempgen et al. fabricated a flexible and printable thin film supercapacitor using sprayed networks of single-walled CNTs (SWCNTs) and printable gel electrolyte.9 The SWCNT networks served as both the electrode and charge collectors. The performances of the devices showed very high energy and power densities which was comparable to other SWCNT-based supercapacitor devices. Zhang et al. made pseudocapacitor electrodes from CuO/CNT nanocomposite.10 The CNTs were mechanically mixed with the CuO nanobelts. The process resulted in a randomly oriented CNT network as a current collector. Their nanocomposite electrode delivered a specific capacitance 2.6 times higher than that of a pure SWCNT electrode.
the porous electrode. Agglomeration of functionalization material not only reduces the effective utilization of the functionalization material but also blocks transfer paths of electrolytic ions. In the worst case, the electrode may lose the inside surface area totally if the functionalization material seals the top surface of the electrode. Nanomaterials, particularly carbon nanotube (CNT), could play critical roles to improve the performances of energy storage devices.3 CNTs have well-known outstanding electrical and mechanical characteristics such as high surface area to volume ratio and intrinsically metallic for any CNT with larger than 2 nm diameters.4 Furthermore, CNTs have a unique feature compared to other nanowires: they can naturally grow into a densely packed yet vertically aligned array, often called CNT forests.5 Such a structure provides many benefits for energy storage applications. In CNT forests, each CNT individually and robustly contacts to the growth substrate and therefore minimize the contact resistance (assuming the substrate is conductive). The aligned architecture facilitates the movement of the electrolytic ions for improved exchange rate.6 The well-developed chemical vapor deposition (CVD) method produces CNT forests easily and precisely in term of diameter and length, resulting in uniform and consistent B
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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 inside view 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) nickel electrodeposition, respectively.
Figure 1a, which takes the advantages of both high surface area of aligned CNT forests and oxidized nickel nanoparticles for enhanced energy storage. During the charge/discharge cycle, the following redox reaction happens:15
However, there is limited progress on CNT forest-based pseudocapacitors because of two practical challenges. First of all, to make a CNT forest-based pseudocapacitor, there must be a current collecting layer under the CNT forest. However, it is well-known that CNT forests mostly grow on nonconductive substrates.11 The direct growth of CNT forest on conductive substrate is so far limited on special substrates, which may not be suitable for general applications. As such, CNT forests have to be transferred from the growth substrate to a second substrate. Because CNT forest is highly porous and fragile, the transfer process is typically very complex.8 Second, it is important to uniformly and thoroughly functionalize CNTs in the forest format. So far there are few reports on the functionalization of the 100 μm level and above thick CNT forests. Common deposition techniques encounter various limits including damaging the aligned structure12 and shallow deposition13 (the deposition only reaches several micrometers below the top surface). In contrast, electrodeposition has the advantages of low cost and easy scale-up; however, CNT’s intrinsic hydrophobicity14 prevents the solution from entering the inside of the CNT forest. We experimentally proved a conventional electrodeposition only deposited material on the very top surface of the CNT forests (see S1 in the Supporting Information). This work has addressed the aforementioned challenges by the direct growth of CNT forests on a unique Fe/Al/Mo conductive substrate and the conformal deposition of nickel nanoparticles within the forests. The functionalized CNT forest pseudocapacitor electrode using such techniques is illustrated in
Ni(OH)2 + OH− ⇔ NiO(OH) + H 2O + e−
(1)
As such, the storage and release of electrical charges occur not only by the absorption and desorption of charges on the CNT surface but also by the reduction and oxidization of nickel ions between 2+ and 3+ chemical states within the nanoparticles. Figure 1b−e shows the flexibility on tuning nanoparticle size using our functionalization technique. To build a CNT forest pseudocapacitor as proposed in Figure 1a, the first step is to choose proper substrate. The substrate has two essential roles: (1) to support the CNT growth during the CVD synthesis process; and (2) to act as the current collecting layer in the charge/discharge cycle of the pseudocapacitor application. On the one hand, to support the CNT growth, the substrate needs to have a balanced interaction with catalysts, mostly iron.16 The surface property of the substrate is critical in controlling the synthesis of CNT forests, without which there could be no CNT growth (too strong interaction) or the growth of much thicker carbon nanofiber and/or amorphous carbon (too weak interaction). The recognized reality is that CNTs grow predominantly on nonconducting substrates.11 On the other hand, the substrate should obviously be conductive as a good current collector. We have experimentally tested several common metals on top of a silicon substrate, including Ti, Cr, Ni, Al, and Mo as the growth C
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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 of charge 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: cyclic voltammetry curves at every 1000 cycles.
respectively. Uniformly distributed nanoparticles can be clearly observed, and the size of these nanoparticles increases as the deposition time increases. Figure 1e is the measurement results of the diameter of the nanoparticles with respect to the deposition time. Nanoparticles as small as 30 nm and as big as 100 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 that has gone through the vacuum wetting and electrodeposition process (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 discrete nickel nanoparticles of about 80 nm in diameter. Discrete nanoparticles instead of continuous thin film have been deposited because the deposition process favors defects on the sidewalls of CNTs.20 The functionalization process is highly flexible by changing the current density and/or electrodeposition time to fabricate nanoparticles ranging from tens of nanometers to over a few hundred nanometers (see S4 in the Supporting Information). For example, Figure 2f shows a TEM image of small nanoparticles with only 30 nm in diameter synthesized within the CNT forests by reducing the deposition time to 20 s. Energy dispersive X-ray (EDX) measurements in Figure 2g show that the as-grown CNT sample has carbon, silicon, Al, Mo, and Fe as expected. The result from the bottom part of Figure 2g is from a sample with nickel nanoparticles without the silicon substrate. It has strong signs of nickel and oxygen as nickel oxide is probably formed after the nickel deposition process. The growth of CNT forests with conductive electrode began with thermal oxidation (1000−2000 Å) of a silicon wafer to improve the adhesion between metals and the substrate as well as to provide an insulation layer between the CNT forest and the silicon substrate. Then, Mo, Al, and Fe were deposited onto the substrate in sequence using e-beam evaporation with a
supporting layer with a 5 nm thick Fe layer as the catalyst on top.17 Only the Al layer was found to allow dense growth of CNTs, and other metals resulted in sparse or no CNT growth (see S2 in the Supporting Information). However, the Al layer lost its original conductivity after the CNT growth process, and we suspect that there might have been an insulating layer that formed during the CVD process. The Mo layer also stood out in our tests and was observed to have increased conductivity after 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 as the CNT growth supporting material. Experimental results show that both dense CNT forest growth and good electrical conductivity on the substrate can be achieved with Fe, Al, and Mo of 5, 10, and 50 nm in thickness, respectively. The contact resistance between the CNT forest and the Mo layer was characterized as 5 × 10−4 Ω·cm2.18 Our hypothesis is that, because the Al layer is so thin, the current could tunnel through the Al layer (despite Al becomes insulating during the CVD process) 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 process is used to overcome high hydrophobicity of CNT forests.19 A detailed wetting process can be found in S3 in the Supporting Information. Basically, the CNT forest sample is loaded in a vacuum chamber, and the pressure is pumped down. Then the CNT sample is covered with deionized water. Afterward, the system is vented, and the pressure difference immediately builds up between the environmental atmosphere and the vacuum inside the CNT forest and push water into the CNT forests. After the wetting process, the sample is placed into nickel electrolyte for the electrodeposition process. Figure 1 parts b, c, and d are the SEM (scanning electron microscope) images showing the cross section of CNT forest samples after 20 s, 2 min, and 8 min of the electrodeposition process, D
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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 scale bar is 200 nm. (c) After 1000 cycles of CV tests. The scale bar is 200 nm.
oxidized first into nickel hydroxide through cycling the potential from −1.2 to +0.8 V 20 times.21 Figure 3 shows the cyclic voltammetry (CV) curve (green line) 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 cases have the same exposed physical area (5 mm × 5 mm) in the electrolyte. It is clearly observed that the introduction of oxidized nickel nanoparticles results in an enlarged CV curve. The peak and valley of the functionalized CNT forests at 500 mV and 0 mV correspond to the reduction and oxidation reactions of nickel hydroxide, respectively. According to the equation:
thickness of 50 nm, 10 nm, and 5 nm, respectively. The wafer was 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 the CNT forest. During the growth, the furnace was first pumped to vacuum and then heated up. The hydrogen constantly flew through the 2-in. quartz tube with a flow rate of 50 sccm until the furnace temperature reached the target of 720 °C. Immediately after the target temperature was reached, the gas was switched to a mixture of carbon precursor, ethylene, and carrying gas, hydrogen, with the flow rates of 90 and 611 sccm, respectively. A standard growth time of 10 min usually results in 150 μm high CNT forests. Finally the furnace was cooled down 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 and transferred into a vacuum chamber. Air was pumped (Emerson, Motor Division, St. Louis, MO, USA) out the system for about 1 h (stable pressure is about 3 mbar). Then deionized (DI) water was let in to cover the CNT sample. Pumping continued for about another 0.5 h until the degassing of DI water became barely visible. Afterward, the chamber was vented, and the wetting process immediately occurred because of the pressure difference between the surrounding and the CNT forest. Before electrodeposition, the DI water was replaced with the nickel electrodeposition solution (nickel sulfamate RTU, product no. 030175, Technic Inc.). During the electrodeposition, half of the 5 mm × 10 mm CNT sample was immersed into the solution (therefore, the effective functionalization area is 5 mm × 5 mm). A copper clamp is used to hold the sample above the solution and, meanwhile, form the electrical contact. An inert stainless steel electrode is used as the counter electrode. The typical current density was 50 mA/cm2, and the deposition time ranged from 20 s to a few minutes. The functionalized CNT forest samples were tested as pseudocapacitor electrodes using a typical three-electrode electrochemical setup: the CNT working electrode, an Ag/ AgCl reference electrode, and a Pt wire counter electrode. The three electrodes are immerged into 0.1 M KOH aqueous solution. The functionalized part of the CNT forest sample is dipped into the electrolyte. All of the measurements have been carried out on a reference 600 Potentiostat (Gamry Instruments, Inc., Warminster, PA). For consistency and simplicity, the CNT forest samples to be discussed below were all from the same growth batch with a height of about 150 μm and the same electrodeposition recipe (2 min, 50 mA/cm2), resulting in nickel nanoparticles of about 60 nm in diameter. All of the cyclic voltammetry tests used a constant scanning rate (dV/dt) of 100 mV/s. Before the tests, nickel nanoparticles were
C=
dq I = dV dV /dt
(2)
the current gap (ΔI) between forward and backward sweeps of the CV curve is in proportion to the capacitance. The functionalized CNT electrode has a much larger current gap than the nonfunctionalized sample throughout the voltage range. Particularly, the functionalized sample has a current gap of about 800 μA near V = 0 V, eight times higher than that of the as-grown CNT sample, which was about 100 μA. Based on the circled area of the CV curve, we calculated the average specific capacitance of the nickel-functionalized CNT forest electrode 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 Information for detailed derivations). Figure 3b shows the results from 100 cycles of chronoamperometry (charge/discharge) tests. There was no apparent current amplitude degradation, indicating good stability of the electrode. The good match between the first and the last five cycles of the 100-cycle test in Figure 3c clearly confirms oxidized nickel nanoparticles did not deteriorate during these tests. ITo study the size and the loaded weight of the active particles to the performance of supercapacitors, CV tests on samples with various nickel electrodeposition time have been studied (Figure S6 in the Supporting Information), and an upward trend of capacitance with respect to electrodeposition time 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 is observed that capacitance increased during the first 1000 cycles possibly due to nickel gradually involved in the redox charge storage. Afterward, the capacitance gradually reduced but still retained 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 the reinforcement of nickel nanoparticles after the dehydration process to take SEM photos as illustrated in Figure 4a. In contrast, we experimentally demonstrated elsewhere that, E
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without the support of these nanoparticles, a 320 μm thick CNT forest either broke and shrank significantly in the lateral direction or collapsed its thickness by 16 times to a merely 21 μm thick CNT sheet (with the assistance of an initial mechanical deformation) because of the liquid zipping effect.22 In either case, there was no way that the shape could be preserved as well as Figure 4a. Figure 4b is the close-up SEM image showing nanoparticles are intact with the CNT forest after the drying process. Furthermore, after the 1000 cyclic test was conducted, the SEM picture of nickel nanoparticles was taken at a slightly different angle as shown in Figure 4c. It is observed that the morphology of these nanoparticles looked similar before and after the cyclic tests, and there was no noticeable change like redistribution, falloff, or size change of nickel nanoparticles. As a final note, unlike other approaches where special elements and complex processes are often involved, the techniques presented in this work are low cost. Our CNT growth method still uses the most common thermal CVD process. The substrate elements of Mo and Al are abundant and cheap. The vacuum wetting step does not add any material cost. The wet chemical electrodeposition has already been widely used in industry. In summary, this paper addresses the issues of the direct growth of CNT forest on a conductive electrode and the feasibility of the three-dimensional functionalization of CNT forests. The Fe/Al/Mo metallic stacking layer has been proposed and demonstrated to grow dense CNT forests with excellent conductivity. The vacuum wetting-assisted electrodeposition method has overcome the hydrophobicity issue of CNT forests and uniformly functionalized the 150 μm thick CNT forest. Using the functionalized CNT forests as the electrode of a pseudocapacitor, an enhanced energy density of 1.26 F/cm3 has been achieved, which is 5.7 times higher than the pure CNT forest electrode. The experimental results further confirmed that the oxidized nickel nanoparticles and the CNT forests have exhibited only 5.8% of degradation after 10 000 cyclic voltammetry tests. Looking beyond, low-cost methodologies presented in this work could advance the integration of CNT forests with devices on silicon-based substrate. The functionalization could greatly reinforce CNT forest from material perspective and broaden the potential applications of CNT forest such as various chemical/biological sensors and synthesis of nanoparticles. These results compare well in some of the very recent nanomaterial-based supercapacitor publications.23−25
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
Details for experimental procedures, additional functionalization results, and device performance calculation. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This project is supported in part by the DARPA N/MEMS Fundamentals Program and Siemens Inc. F
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(21) Deo, R. P.; Lawrence, N. S.; Wang, J. Electrochemical Detection of Amino Acids at Carbon Nanotube and Nickel−Carbon Nanotube Modified Electrodes. Analyst 2004, 129 (11), 1076−1081. (22) Jiang, Y.; Lin, L. A Two-Stage, Self-Aligned Vertical Densification Process for As-Grown CNT Forests in Supercapacitor Applications. Sens. Actuators, A 2012, 188, 261−267. (23) Zhou, C.; Zhang, Y.; Li, Y.; Liu, J. Construction of HighCapacitance 3D CoO@Polypyrrole Nanowire Array Electrode for Aqueous 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 of Graphene/MnO2 Nanostructured Electrodes by Conductive Wrapping. 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 Nanoflake Electrode by Spontaneous Electrochemical Pre-Intercalation for HighPerformance Supercapacitor. Sci. Rep. 2013, 3, 1718.
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