Carbon Nanotube−Nanocup Hybrid Structures for High Power Supercapacitor Applications Myung Gwan Hahm,* ,†,‡ Arava Leela Mohana Reddy, † Daniel P. Cole, § Monica Rivera, § Joseph A. Vento, ∥ Jaewook Nam, ⊥ Hyun Young Jung, # Young Lae Kim, ∇ Narayanan T. Narayanan, † Daniel P. Hashim, † Charudatta Galande, † Yung Joon Jung, # Mark Bundy, ○ Shashi Karna, ○ Pulickel M. Ajayan,* ,† and Robert Vajtai* ,† † Department of Mechanical Engineering and Materials Science, Rice University, Houston, Texas 77005, United States ‡ Research Center for Exotic Nanocarbons, Shinshu University, 4-17-1 Wakasato, Nagano-Shi, Nagano 380-8553, Japan § Motile Robotics, Inc., Joppa, Maryland 21085, United States ∥ Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, United States ⊥ School of Chemical Engineering, Sungkyunkwan University, 300 Cheongcheon-dong, Suwon, Gyeonggi-do, 440-746, Korea # Department of Mechanical and Industrial Engineering, Northeastern University, Boston, Massachusetts 02115, United States ∇ Department of Electrical and Computer Engineering, Northeastern University, Boston, Massachusetts 02115, United States ○ U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005, United States * S Supporting Information ABSTRACT: Here, we design and develop high-power electric double-layer capacitors (EDLCs) using carbon-based three dimensional (3-D) hybrid nanostructured electrodes. 3- D hybrid nanostructured electrodes consisting of vertically aligned carbon nanotubes (CNTs) on highly porous carbon nanocups (CNCs) were synthesized by a combination of anodization and chemical vapor deposition techniques. A 3-D electrode-based supercapacitor showed enhanced areal capacitance by accommodating more charges in a given footprint area than that of a conventional CNC-based device. KEYWORDS: Carbon nanotubes, carbon nanocups, energy storage, supercapacitor, high surface area E lectrochemical capacitors, also called supercapacitors or ultracapacitors, have attracted much attention in the automotive and consumer electronics industry due to their high capacitance, pulse power capabilities, and long cycle life. 1−4 Supercapacitors bridge the gap between batteries and dielectric capacitors; the capacitance of supercapacitors is several orders of magnitude higher than dielectric capacitors, and the peak power density is much higher than most batteries. 5−9 Although supercapacitors have high specific power, their low energy density restricts their use to supplementary power for battery-driven systems. In a supercapacitor, energy is stored in the form of charge separation between the double layer formed at the interface of the solid electrode surface and the liquid electrolyte. 1 Currently, highly porous carbon structures are the primary materials used in commercial electrochemical double layer capacitors (EDLCs). 1,10 The high surface area of the porous carbon electrodes increases the amount of interaction between the electrolyte ions and the electrode. In an effort to meet the ever-increasing demands of the consumer electronics industry, there is a significant amount of research focused on developing materials capable of accumulating more charge per unit area. Along these lines, several carbon nanostructured materials such as activated carbons, carbon nanotubes (multi- and single-walled CNTs), carbon nanowires, spherical carbon nanoparticles, and one- to few-layered graphene structures have been explored as the electrode materials in supercapacitors. 10−23 With the aim of developing a supercapacitor with high areal energy, we propose and demonstrate vertically aligned CNT−carbon nanocup (CNC) 3-D hybrid supercapacitors. CNT−CNC 3-D hybrid structures were grown by chemical vapor deposition (CVD) using a short channel anodized aluminum oxide (AAO) template. Detailed structural and electrochemical studies on the resulting CNT−CNC 3-D hybrid structures show significant improvement in the areal capacity over purely CNC-based supercapacitors. The vertically aligned CNT−CNC hybrid structure is fabricated by a multistep CVD technique. A schematic of the CNT−CNC 3-D hybrid fabrication approach can be found in Figure 1. First, precisely controlled short AAO nanochannels having a 10 3 −10 5 times smaller aspect ratio than conventional Received: July 24, 2012 Revised: September 14, 2012 Letter pubs.acs.org/NanoLett © XXXX American Chemical Society A dx.doi.org/10.1021/nl3027372 | Nano Lett. XXXX, XXX, XXX−XXX