Flexible energy storage devices based on carbon nanotube forests with built-in metal electrodes

Sensors and Actuators A 195 (2013) 224–230

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Flexible energy storage devices based on carbon nanotube forests with built-in metal electrodes Y. Jiang ∗,1 , A. Kozinda 1 , T. Chang, L. Lin Mechanical Engineering Department, Berkeley Sensor and Actuator Center, University of California at Berkeley, CA, USA

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Article history: Received 15 April 2012 Received in revised form 6 July 2012 Accepted 10 July 2012 Available online 17 July 2012 Keywords: Carbon nanotube Flexible device Bending test Supercapacitor Energy storage

a b s t r a c t Mechanically bendable energy storage devices have been demonstrated based on a lift-off and liquid densification process to construct carbon nanotube (CNT) forests with built-in bottom metal electrodes on top of a flexible substrate. The flexible CNT structure has been fabricated and tested as a supercapacitor electrode with the following salient features: (1) excellent transfer of charge from the aligned CNTs to the bottom contact metal layer, (2) a simple and straightforward fabrication process, and (3) easy integration with a variety of surfaces and topographies. Experimental results have shown that a 5 mm × 10 mm electrode with 40 ␮m-thick CNT forest and 50 nm-thick molybdenum bottom metal contact has been transferred from a silicon growth substrate onto a 200 ␮m-thick Al/ThermanoxTM plastic substrate. The attached film survived the bending of as large as 180◦ . A measured specific capacitance of 7.0 mF/cm2 has been achieved. Repeated mechanical bending tests followed by CV cyclic measurements have shown good device stability. As such, flexible energy storage devices composed of CNT forests with built-in metal electrodes could have broad applications in modern systems that demand components with adaptable shapes to fit into small form factors and ergonomic designs. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The demand for energy storage devices with high energy density and high power has continued to rise while their physical size inside electronic products has continued to shrink. Since it is desirable for energy storage devices to have adjustable shapes to fit into various form factors, energy storage devices made from flexible electrodes could be attractive. Over the past decades, there has been strong interest in the advanced development of energy storage devices such as batteries [1–3] and supercapacitors [4–7]. In the area of rechargeable batteries, a redox reaction occurs inside the electrode during each charge and discharge cycle, while in a supercapacitor, charge is stored at the interface of the electrode and electrolyte, which is known as the electro-chemical double layer (EDL) [4]. As a result, electroactive materials in a battery must go through large volumetric changes with every charge/discharge cycle to allow for the intercalation of ions in the electrodes. This reaction reduces the mechanical stability of the electrodes and slows down the energy transfer process. For example, charge and discharge processes in lithium ion batteries can deposit residuals inside the electrolyte which inhibits the ion transport process such that the internal

∗ Corresponding author at: 497 Cory Hall #1774, University of California at Berkeley, Berkeley, CA 94720-1774, USA. Tel.: +1 510 642 8983. E-mail address: [email protected] (Y. Jiang). 1 Y. Jiang and A. Kozinda contributed equally to the work presented in this paper. 0924-4247/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2012.07.007

resistance of the batteries could increase, in effect limiting the cyclic lifetime of the batteries and slowing down their charge/discharge speed [8]. In contrast, supercapacitors can release and store charge at a much faster rate in the electro-chemical double layer for operations up to millions of cycles without significant degradation while the typical lifetime of Li-ion batteries is only a few thousands cycles. Furthermore, it is possible to increase the energy storage capacity of supercapacitors without downgrading their high power density by using additional electrode materials such as metal oxide or polymers [9]. The basic structure of a supercapacitor contains two opposing electrodes with an ion-conducting electrolyte in between. Upon the application of an applied voltage, the electrolyte becomes polarized and ions inside the electrolyte migrate to the surface of the electrodes. An EDL is established at the interface between the electrode and electrolyte with a gap size of around one nanometer. If the total electrode area of a supercapacitor is increased, then the total possible charge that can be stored, or the capacitance, is also increased. Therefore, the energy storage capacity can be increased by using electrodes with large specific surface area. While activated carbon particles have been conventionally used as electrodes in supercapacitors, they are limited by relatively low effective pore area, high internal resistances between carbon particles, and low transport rate of ions through their microporous structure [7]. CNTs have outstanding electrical conductivity and mechanical strength and could be advantageous in supercapacitor applications. Particularly, CNT forests can be synthesized by several chemical vapor deposition

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forests are directly grown on a conductive metal layer atop a silicon growth substrate such that each as-grown CNT is electrically connected to the metal current collector; (2) both the CNT forest and the bottom metal collector are intact during the transfer process by means of lift-off as shown. The existence of the Mo layer, the bottom metal collector, physically reinforces the fragile CNT forest from cracking and, more importantly, eliminates the necessity of a post-transfer metal deposition process; and (3) CNTs are well aligned after the transfer process, which retains the structural advantages of low electrical contact resistance over disorderly assembled CNT networks [24]. Since the thickness of the metal contact layer is very thin (sub-micron range) and the CNT forests are flexible, the transfer process can be applied to a flexible substrate to allow possible mechanical deformation afterwards as illustrated. Furthermore, the lift-off process is conducted under an aqueous environment such that a liquid densification process occurs naturally as the liquid is dried to achieve high volumetric energy density for supercapacitor applications [25]. Fig. 1. Schematic illustration of the flexible supercapacitor electrode using a waterassisted, lift-off process. (Top) After the CNT forest–Mo film has been placed onto a flexible substrate. (Bottom) After the growth of CNTs on top of an oxidized silicon substrate and the separation from the substrate. Circular inset shows the electrical double layer effect upon charging of the CNT forest electrode in electrolyte.

(CVD) methods [10–12]. CNT forests have a vertically aligned conformation, which promotes the transfer of ions between individual CNTs [13]. As a result, CNT-based electrodes have been studied by various groups as supercapacitor electrodes [5–7,14,15]. Previously, various approaches to construct flexible CNT electrodes have been reported to meet various design needs in modern electronics. Since CNTs are grown at high temperatures, typically around 700 ◦ C, a transfer process is required to avoid damage to the flexible substrate, which is usually made of polymeric materials. At the same time, a conductive bottom current collector is highly desirable. The conductivity of CNT forests is anisotropic and affected by tube orientation. The conductivity parallel to the tubes (namely, perpendicular to the growth substrate) is about 60 times higher than the conductivity perpendicular to the tube axis (namely, parallel to the growth substrate) [16]. This is understandable since electrons mostly travel within individual tubes when flowing parallel to the tubes but must jump from tube to tube frequently when they flow in a perpendicular direction to the tubes, thus facing numerous contact resistances. By using an aligned CNT forest on a conductive metal contact, the electrode minimizes the system internal resistance, and, as a result, increases the maximum output power, shortens the charge and discharge time, and reduces internal loss. One approach to make CNTs on flexible substrates is to use printing or electrodeposition techniques to fabricate CNT networks [17–19]. For example, Kaempgen et al. have sprayed aqueous single-walled CNT (SWCNT) suspensions onto polyethylene-therephthalate (PET) substrates, and achieved randomly entangled CNT networks with thicknesses of 0.6 ␮m [17]. The CNT film has served as both the electrode and charge collector, but has the drawbacks of limited film thickness and high internal resistance. Direct transfer of as-grown CNT forests onto flexible substrates has also been demonstrated [20–22]. The typical fabrication process has several steps: (1) CNT forests are immersed into a polymer substrate on a hot plate, (2) the polymer is cured to fix the fragile and porous CNT network, and (3) the CNT forests are peeled off from the growth substrate and attached to the flexible substrate. Similar to the printing method, it is difficult to construct a current collector on the flexible substrate in the aforementioned process and a metal deposition process is typically applied afterwards to form the metal electrode [20]. This work herein, as illustrated in Fig. 1 [23], presents some unique advancements as compared to previous efforts: (1) CNT

2. Design and fabrication The fabrication process is detailed in Fig. 2. It begins with the preparation of the CNT forest. A 50 nm-thick Mo film is evaporated using electron-beam evaporation onto a thermally oxidized silicon substrate, followed by the electron-beam evaporation of the catalyst (aluminum and iron, 10 nm and 5 nm thick, respectively). The reason Al (aluminum) is used here is that it prevents Fe (iron) from alloying with Mo (molybdenum), which would decrease the viability of Fe as a catalyst for CNT growth. Next, the chip is placed into a vacuum quartz tube furnace (Lindberg/Blue M® three-zone tube furnace, Thermo Electron Corp., Asheville, NC). The quartz tube is evacuated and thereafter purged with hydrogen, then raised to a temperature of 720 ◦ C under atmospheric pressure. Once the temperature is stabilized, hydrogen and ethylene (the carbon precursor gas) are flown through the quartz tube at volumetric flow rates possessing a ratio of approximately 7:1. During the ten-minute long chemical vapor deposition (CVD) growth process, the temperature and gas flow rates are held constant. Once the growth process is complete, the heating power of the furnace is turned off, after which the quartz tube is cooled to room temperature. Before unloading the forest samples, the remaining gases are evacuated, and the tube is purged again with argon. The result of this process is a vertically self-aligned CNT forest grown upon and contacted by the Mo, a metal current collecting layer. The contact resistance between the Mo current collector and CNT film is about 5 × 10−3  cm2 [26]. The transfer process begins at the third step. The CNT forest is mechanically pressed to create a preferred bending direction as illustrated [25]. This mechanical pressing step may be accomplished with the use of a planar surface, a roller, or another custom device to ensure that the pressing surface does not cause the stiction of the CNTs. Fourth, the specimen is submerged in deionized (DI) water until the CNT–Mo layer is detached from the substrate. The water-assisted lift-off process releases the CNT–Mo structure with little damage to the integrity of the electrical contacts as demonstrated in the supercapacitor characterization experiments to be presented in a later section. One reason for the detachment of the Mo layer is that no adhesion layers were applied during the film deposition process, and the adhesion force between the Mo and silicon oxide is poor. With the existence of water molecules, the adhesion force between Mo and silicon oxide is further weakened. For the specimen size of 5 mm × 10 mm, it takes about 96 h to complete the lift-off process. On the other hand, if an adhesion layer such as titanium is applied, the CNT forests will not detach from the silicon substrate. In the fifth step of the process, the CNT–Mo structure is picked up by a flexible substrate (ThermanoxTM film) in the

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Fig. 2. Fabrication steps of the flexible CNT electrode: (1) multiple metal layer deposition to prepare the CNT growth samples; (2) growth of vertically aligned CNT forest using the CVD method; (3) mechanical deformation of CNT forest; (4) release of CNT–Mo film by submerging the specimen in DI water; (5) lift-off of the film out of liquid onto a flexible substrate; and (6) densification of CNT forest during the drying process.

aqueous environment. Sixth, the flexible substrate with the CNT forest is air dried. Due to the pressing process in the third step, CNT forests will densify in a downward, vertical fashion under Van der Waals forces [27]. The flexible substrate could have a metal contact layer to allow easy electrical connection to the CNT forests. In the prototype experiments, ThermanoxTM , Kapton® , and various metal contact layers including gold, copper and aluminum have been demonstrated to work successfully. The CNT–Mo film has been found to attach to all of these surfaces and to remain securely in place after the process due to strong Van der Walls force. To maintain the integrity of the CNT–Mo film during the transfer process, a mechanical press prior to immersion in water is a necessary step. The mechanical force introduced in step 3 is to ensure the CNT forest does not randomly collapse when the liquid evaporates from it in step 6. Without such pre-treatment, namely directly drying the as-grown CNT forest, the tubes would clump together randomly as seen in Fig. 3(a). Even though successfully released from the growth substrate, the un-pressed CNT–Mo film contains the coalesced patterns during the liquid densification process. In contrast, the pressed film, as shown in Fig. 3(b), remains intact after the densification process because the CNT forests are guided to collapse vertically instead of laterally to form a nice continuous film.

The reflective backside of the released film and the bare silicon surface after the lift-off process further verifies that the released film is composed of both CNT and Mo layers. Fig. 3(c) shows that our release process can be readily applied to patterned CNT–Mo films as well. The comb finger structures in the picture are 400 ␮m in width. We are exploring some applications based on the released and patterned CNT film. Fig. 4 shows a digital photograph of one flexible electrode thus created with a CNT film supercapacitor electrode on an Al/ThermanoxTM film surface, being bent to different angles. For all the angles we tried, we have not observed breakage or delamination of the film, which is a good sign of adhesion force and film extension capacity. The CNT specimen in the image is 5 mm × 10 mm. 3. Results and discussion The prototype device has a CNT forest height of 40 ␮m as shown in the SEM image in Fig. 5(a). After the release and liquid densification process, the height is reduced to 1.1 ␮m as estimated from Fig. 5(b), which is a factor of approximately 37 in reduction. If the measured capacitance does not change before and after the densification process, a potential 37-fold increment in the specific

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Fig. 4. Digital photographs displaying the flexibility of the CNT–Mo film supercapacitor electrode on an Al/ThermanoxTM film surface, being bent at a tangent-to-tangent angle of (a) about 90◦ and (b) about 180◦ , respectively. The CNT–Mo electrode in the image is 5 mm × 10 mm.

Fig. 3. Digital photographs of the released CNT–Mo samples. (a) A released, asgrown CNT–Mo film (left) without the mechanical pressing step and showing random patterns of CNTs after the water-assisted release process. The silicon substrate (right) has a clean surface without the Mo film, which is fully detached from the silicon. (b) A released, as-grown CNT–Mo film with the mechanical pressing step (top), the original growth silicon substrate (center), and backside view of another CNT–Mo film (bottom). The reflective metal layer as viewed from the bottom side of the CNT–Mo film and the clean surface of the growth silicon substrate validate the successful release of the CNT–Mo film. (c) The released CNT–Mo layer patterned into comb drives fingers floatingly loosely above the growth substrate. The finger width is 400 ␮m.

volumetric capacity of the electrode could be achieved as demonstrated in our previous work [25]. Fig. 5(c) shows an SEM image of the CNT forests after 40 electrochemical cycling tests. There is no obvious change in the height and morphology of the CNT forests after cycling as shown. Three-electrode electrochemical tests have been conducted to characterize the electrode’s performance in supercapacitor applications. The electrolyte used was 0.1 M K2 SO4 , with an Ag/AgCl reference electrode and Pt counter electrode. The CNT electrode tested was 5 mm × 10 mm with 40 ␮m-high CNT–Mo film (before the releasing process) on a flexible Kapton® film having a copper film contact layer on top. The electrode’s effective contact area with the electrolyte was 5 mm × 5 mm. Fig. 6 shows the cyclic voltammetry curve of the electrode in which voltage was cycled from −0.7 V to 0.3 V at 100 mV/s. The specific capacitance of the device is calculated as 7.0 mF/cm2 using the equation: Csp =

I (dV/dt) × A

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Fig. 5. SEM photos of CNT forest after (a) growth, (b) release and densification in liquid, and (c) 40 electrochemical cycling tests. Scale bars are 10 ␮m, 1 ␮m, and 500 nm for (a), (b), and (c), respectively.

where Csp is the specific capacitance of the CNT supercapacitor normalized to the electrode area, I is the sweeping current, about 174 ␮A (half of the current difference at −0.2 V between forward and backward sweeping curve in Fig. 6), and dV/dt is the scanning rate of voltage, 100 mV/s in this work, and A is the effective physical area of the CNT electrode, 5 mm × 5 mm. This capacitance could be further increased by increasing the height of the CNT forest, as well as adding functional materials. For instance, it has been shown that coating a CNT forest with a low pressure CVD

amorphous silicon layer increases the total surface area available in the supercapacitor electrode, and boosts the capacitance [28]. Fig. 7 shows a typical charge/discharge cycles from chronopotentiometry scan of a CNT–Mo film attached on an Al/ThermanoxTM plastic substrate. Fig. 8 shows the measured cycling performance of the flexible supercapacitor electrode over 100 charge/discharge cycles, normalized to its average capacitance. Capacitance values are

Fig. 6. Cyclic voltammetry curve of the densified, flexible CNT–Mo film electrode cycled from −0.7 V to 0.3 V at 100 mV/s in 0.1 M K2 SO4 electrolyte. An Ag/AgCl reference electrode and Pt counter electrode were used in the experiments.

Fig. 7. Chronopotentiometry Scan of a CNT–Mo film attached on Al/ThermanoxTM plastic substrate. The period of sanning is 8 s and the current in the charging stage is set to 1 mA and not controlled at the discharging stage.

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delamination from the substrate was observed. After the CNT–Mo film has been lifted out with the flexible substrate and air-dried, it is impossible to remove the film from the substrate in air without completely scraping (and hence destroying) the film. All this is accomplished without the use of any adhesive material between the film and substrate. The strong adhesion between the CNT–Mo film and substrate is likely due to strong Van der Waals forces. 4. Conclusions

Fig. 8. Cycling performance of the flexible electrode measured over 100 cycles, normalized to its average capacitance. Capacitance values are represented by the voltage value of −0.2 V in measured cyclic voltammetry curves.

represented by the voltage value of −0.2 V in measured cyclic voltammetry curves. It is observed that the standard deviation in capacitance from the 100 cycles is only 3% and the variations can be due to the noises in the measurement machine and the environment. Since one application of the CNT–Mo film electrode could be a mechanically flexible energy storage device, the change in resistance (if any) after bending is also investigated. The bending tests were performed by holding the flexible electrode between two fingers of each hand, and bending the sample first to 20◦ from side to side then allowing the sample to unbend to its flattened state. This return to the original shape was inherent to the flexible substrate to which the CNT–Mo film was transferred. When the plastic substrate returned to 0◦ , the attached CNT–Mo film did as well. The bending-unbending motion was then repeated 100 times. Fig. 9 summarizes measured electrode resistance. The graph shows the resistance normalized to the original resistance of the electrode before any bending has occurred. The targeted amount of bending was kept constant throughout the examination, and resistance values were taken using a two-probe method. The electrode resistance remained essentially stable over the course of this test with a range of 199 ± 4 . The electrical contact may move slightly between measurements, which may give rise to the slight variation in resistance measured. The adhesion between the film and substrate appears to be strong. For example, during the aqueous electrochemical tests, no

Fig. 9. Electrode resistance vs. bending cycle of the flexible electrode, normalized to the original resistance before the sample had been bent. One bending cycle is comprised of one bend to 20◦ , as illustrated in the inset, and then one release to 0◦ .

A mechanically flexible CNT supercapacitor electrode has been demonstrated using a water-assisted lift-off and densification process. After the synthesis of the CNT forests, mechanical pressing is employed to guide the bending of CNTs in a specific downward direction during the later densification process. The water-assisted lift-off process releases the CNT film along with its intrinsic metal current collector from the growth substrate. The released film is then transferred on to a flexible substrate. The prototype experimental results show that the CNT film is densified by 37 times (the height is reduced by 37 times) after the above process. The prototype supercapacitor was measured to have a specific capacitance of 7.0 mF/cm2 and the device capacitance was stable over 100 charge/discharge cycles. To test the mechanical stability, the film on the flexible substrate underwent over 100 bending cycles and showed no sign of degradation. This flexible CNT electrode exhibits the following three features: (1) each CNT has a natural contact to its as-fabricated currentcollecting metal layer; (2) CNTs and the bottom metal layer are intact during the water-assisted lift-off process to the flexible substrate; and (3) the in situ liquid evaporation and densification process naturally occurs to increase the energy density. This thin CNT supercapacitor films have the potential to be applied to various surfaces in a variety of manufacturing processes, such as in roll-up electronics and modern electronic gadgets. Acknowledgement This project is supported in part by the Center of Integrated Nanoelectromechanical Systems (COINs) under NSF grant 0832819. References [1] G. Pistoia, Batteries for Portable Devices, Elsevier Science B.V, Amsterdam, The Netherlands, 2005. [2] S.W. Lee1, N. Yabuuchi, B.M. Gallant, S. Chen, B. Kim, P.T. Hammond, Y. ShaoHorn, High-power lithium batteries from functionalized carbon-nanotube electrodes, Nature Nanotechnology 5 (2010) 531–537. [3] D.T. Welna, L. Qu, B.E. Taylor, L. Dai, M.F. Durstock, Vertically aligned carbon nanotube electrodes for lithium-ion batteries, Journal of Power Sources 196 (2011) 1455–1460. [4] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum Publishers, New York, USA, 1999. [5] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nature Materials 7 (2008) 845–854. [6] E. Frackowiak, F. Beguin, Carbon materials for the electrochemical storage of energy in capacitors, Carbon 39 (2001) 937–950. [7] A.G. Pandolfo, A.F. Hollenkamp, Carbon properties and their role in supercapacitors, Journal of Power Sources 157 (2006) 11–27. [8] http://en.wikipedia.org/wiki/Lithium-ion battery. [9] B.E. Conway, V. Birss, J. Wojtowicz, The role and utilization of pseudocapacitance for energy storage by supercapacitors, Journal of Power Sources 66 (1997) 1–14. [10] M. Endo, M.S. Strano, P.M. Ajayan, Potential applications of carbon nanotubes, in: A. Jorio, G. Dresselhaus, M.S. Dresselhaus (Eds.), Carbon Nanotubes: Synthesis, Structure, Properties and Applications, Springer-Verlag, Berlin, 2008, pp. 13–62. [11] A.M. Cassell, J.A. Raymakers, J. Kong, H. Dai, Large scale CVD synthesis of single-walled carbon nanotubes, Journal of Physical Chemistry B 103 (1999) 6484–6492. [12] K. Hata, D.N. Futaba, K. Mizuno, T. Namai, M. Yumura, S. Iijima, Water-assisted highly efficient synthesis of impurity-free single-walled carbon nanotubes, Science 306 (2004) 1362–1364.

230

Y. Jiang et al. / Sensors and Actuators A 195 (2013) 224–230

[13] H. Zhang, G. Cao, Y. Yang, Electrochemical properties of ultra-long, aligned, carbon nanotube array electrode in organic electrolyte, Journal of Power Sources 172 (2007) 476–480. [14] C. Peng, S. Zhang, D. Jewell, G.Z. Chen, Carbon nanotube and conducting polymer composites for supercapacitors, Progress in Natural Science 18 (2008) 777–788. [15] K.H. An, W.S. Kim, Y.S. Park, Y.C. Choi, S.M. Lee, D.C. Chung, D.J. Bae, S.C. Lim, Y.H. Lee, Supercapacitors using single-walled carbon nanotube electrodes, Advanced Materials 13 (2001) 497–500. [16] A.E. Aliev, C. Guthy, M. Zhang, S. Fang, A.A. Zakhidov, J.E. Fischer, R.H. Baughman, Thermal transport in MWCNT sheets and yarns, Carbon 45 (2007) 2880–2888. [17] M. Kaempgen, C.K. Chan, J. Ma, Y. Cui, G. Gruner, Printable thin film supercapacitors using single-walled carbon nanotubes, Nano Letters 9 (2009) 1872–1876. [18] P. Chen, G. Shen, S. Sukcharoenchoke, C. Zhou, Flexible and transparent supercapacitor based on In2 O3 nanowire/carbon nanotube heterogeneous films, Applied Physics Letters 94 (2009) 043113. [19] Y. Fang, J. Liu, D.J. Yu, J.P. Wicksted, K. Kalkan, C. Ozge Topal, B.N. Flanders, J. Wu, J. Li, Self-supported supercapacitor membranes: polypyrrole-coated carbon nanotube networks enabled by pulsed electrodeposition, Journal of Power Sources 195 (2010) 674–679. [20] V.L. Pushparaj, M.M. Shaijumon, A. Kumar, S. Murugesan, L.J. Ci, R. Vajtai, R.J. Linhardt, O. Nalamasu, P.M. Ajayan, Flexible energy storage devices based on nanocomposite paper, Proceedings of the National Academy of Sciences 104 (2007) 13574–13577. [21] T.Y. Tsai, C.Y. Lee, N.H. Tai, W.H. Tuan, Transfer of patterned vertically aligned carbon nanotubes onto plastic substrates for flexible electronics and field emission devices, Applied Physics Letters 95 (2009) 013107. [22] E.B. Sansom, D. Rinderknecht, M. Gharib, Controlled partial embedding of carbon nanotubes within flexible transparent layers, Nanotechnology 19 (2008) 035302. [23] A. Kozinda, Y. Jiang, L. Lin, Flexible energy storage devices based on liftoff of CNT films, in: Proceedings of IEEE MEMS 2012, Paris, France, January 29–February 2, 2012, pp. 1233–1236. [24] H. Zhang, G. Cao, Y. Yang, Z. Gu, Comparison between electrochemical properties of aligned carbon nanotube array and entangled carbon nanotube electrodes, Journal of the Electrochemical Society 155 (2008) K19–K22. [25] Y. Jiang, L. Lin, A two-stage, self-aligned vertical densification process for asgrown CNT forests in supercapacitor applications, Sensors and Actuators A: Physical (2012), http://dx.doi.org/10.1016/j.sna.2012.04.012.

[26] Y. Jiang, P. Wang, L. Lin, Characterizations of contact and sheet resistances of vertically aligned carbon nanotube forest with intrinsic bottom contacts, Nanotechnology 21 (2011) 365704. [27] D.N. Futaba, K. Hata, T. Yamada, T. Hiraoka, Y. Hayamizu, Y. Kakudate, O. Tanaike, H. Hatori, M. Yumura, S. Iijima, Shape-engineerable and highly densely packed single-walled carbon nanotubes and their application as supercapacitor electrodes, Nature Materials 5 (2006) 987–994. [28] A. Kozinda, Y. Jiang, L. Lin, Amorphous silicon-coated CNT forest for energy storage applications, in: Proceedings of IEEE Transduers’11 Conference, Beijing, China, June 5–9, 2011, pp. 723–726.

Biographies Yingqi Jiang is a PhD student under the supervision of professor Liwei Lin in mechanical engineering at University of California, Berkeley. His PhD research focused on carbon nanotube-based energy storage, particularly MEMS supercapacitor. Before his PhD, he got bachelor and master degrees with major of electronic science and technology in Tsinghua University, China. Alina Kozinda is a PhD candidate working as a graduate student researcher under the advisement of professor Liwei Lin in the Mechanical Engineering Department at the University of California, Berkeley. Her research focuses on energy storage devices produced with carbon nanotubes, namely supercapacitors and lithium ion batteries. Before her graduate school career, she received a BS in both mechanical engineering and mathematics from the University of Florida, and worked as an undergraduate researcher in microtribology under professor W. Gregory Sawyer. Tiffany Chang is an undergraduate student under the supervision of Alina Kozinda at University of California, Berkeley. Her thesis focuses on potential substrates for low-cost flexible supercapacitors. She is currently an environmental science major with an emphasis on energy and resources. Liwei Lin is a professor in Department of Mechanical Engineering at UC Berkeley and co-director of Berkeley Sensor and Actuator Center (BSAC). His research interests include MEMS, nanotechnology, energy, and biomedical applications.