Nitrogen-Doped Carbon Nanotube Spherical Particles for Supercapacitor Applications: Emulsion-Assisted Compact Packing and Capacitance Enhancement Donghee Gueon and Jun Hyuk Moon* Department of Chemical and Biomolecular Engineering, Sogang University, 35 Baekbeom-ro, Mapo-gu, Seoul 121-742, Republic of Korea * S Supporting Information ABSTRACT: The combination of the control of CNT assembly density and the control of intrinsic carbon properties by doping can synergistically improve the supercapacitor performance of CNT-based electrodes. We prepared a dense-packed CNT spherical assembly via emulsion-assisted evaporation and subsequently conducted nitrogen (N) doping to make CNT-based supercapacitors. The assembly of CNT spherical particles is applied as the supercapacitor electrode. We control the N doping content and obtain a specific capacity of 215 F/g at a current density of 0.2 A/g, which is 3.1 times higher than that of the untreated sample. The enhancement stems from high pseudocapacitance and high electrical conductivity that result from the N doping of the CNT assembly. In a comparison of the specific capacitance of N-CNT spherical particles with that of the CNT films prepared by conventional solution-coating, we found that N-CNT samples display a capacitance that is 1.8 times higher, thus confirming the morphological advantage provided by the CNT packing and the hierarchical porous structure in the CNT particle assembly. Our approach allows a facile and high throughput production of high performance electrodes based on CNTs that are commercially available. Moreover, our approach can be extended to produce spherical particles consisting of other nanostructured carbon materials and their composites. KEYWORDS: carbon nanotubes, nitrogen doping, droplet confinement, pseudocapacitance, supercapacitors ■ INTRODUCTION Carbon nanotubes (CNTs) are nanostructured carbonaceous materials that are appealing for various energy devices, such as supercapacitors and lithium-ion batteries. 1-3 In addition to their high chemical stability and high electrical conductivity, CNTs have unique morphological characteristics: they possess an intrinsic central canal micropore, and the CNT assembly creates an interconnected meso-to-macroscale pore network among CNTs. These hierarchical porous structures effectively facilitate ion transport in high surface area CNT films and, thus, kinetic properties in their application. 4 However, most CNTs reveal relatively lower energy storage density than other porous carbon materials, including activated carbon and soft- or hard- templated porous carbons. 5,6 In supercapacitor applications, CNT films display the electrochemical capacitance of approximately 100 F/g, while porous carbon materials display a capacitance over 200 F/g. 5,6 One of the reasons for this low capacitance of CNT films is that the dense packing of fibrous CNTs is limited by entanglement. The liquid-evaporation- induced assembly of CNTs has been developed for this purpose. Densely aligned CNTs can be produced by the liquid- induced collapse of vertically aligned CNT arrays. 6,7 Our group utilized the packing of CNTs in a liquid droplet during the evaporation of the solvent. 4 Meanwhile, the recent improvement of supercapacitance performance in carbon-based electrodes has also been driven by introducing the doping of heteroatoms, such as nitrogen (N) 2,3,8,9 and boron (B). 10 The exact mechanism has not been elucidated, but most of the literature reports that the enhancement of electrochemical capacitance is related to various factors, including the enhancement of electrical conductivity and/or pseudocapacitive properties. 3,9-11 The Received: June 12, 2015 Accepted: August 24, 2015 Research Article www.acsami.org © XXXX American Chemical Society A DOI: 10.1021/acsami.5b05231 ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX Downloaded by UNIV OF PRINCE EDWARD ISLAND on September 6, 2015 | http://pubs.acs.org Publication Date (Web): September 1, 2015 | doi: 10.1021/acsami.5b05231