Synthetic approach from polypyrrole nanotubes to nitrogen doped pyrolyzed carbon nanotubes for asymmetric supercapacitors Deepak P. Dubal a, * , Nilesh R. Chodankar b , Zahilia Caban-Huertas a , Franciele Wolfart c , Marcio Vidotti c , Rudolf Holze d , Chandrakant D. Lokhande b , Pedro Gomez-Romero a, ** a Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, CampusUAB, Bellaterra, 08193 Barcelona, Spain b Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, M.S, India c Grupo de Pesquisa em Macromol eculas e Interfaces, Departamento de Química, Universidade Federal do Paran a, CP 19081, 81531-980 Curitiba, PR, Brazil d Technische Universit€ at Chemnitz, Institut für Chemie, AG Elektrochemie, D-09107 Chemnitz, Germany highlights  Synthesis of 1D PPy nanotubes.  Carbonization of PPy nanotubes to prepare N doped carbon nanotubes (N-CNTs).  Fabrication of asymmetric supercapacitor with PPy-NTs//N-CNTs.  High performance asymmetric device. article info Article history: Received 5 August 2015 Accepted 20 January 2016 Available online 30 January 2016 abstract Pseudocapacitive materials are highly capable to achieve high energy density integrated with high power electrostatic capacitive materials. However, finding a suitable electrostatic capacitive material to inte- grate with pseudocapacitive material in order to achieve high energy density with good rate capability is still a challenge. Herein, we are providing a novel synthetic approach starting from the synthesis of polypyrrole nanotubes (PPy-NTs) and ending up at the carbonization of PPy-NTs to obtain N-doped carbon nanotubes (N-CNTs). With highly porous nature of PPy-NTs and great graphitic texture with copious heteroatom functionalities, N-CNTs significantly promoted the faradic pseudo-capacitors, demonstrating high single-electrode capacitance over 332 F/g (PPy-NTs) and 228 F/g (N-CNTs) in 1 M H 2 SO 4 aqueous solution. Further, a novel asymmetric supercapacitor with PPy-NTs as positive and N-CNTs as negative electrode has been fabricated. This PPy-NTs//N-CNTs cell effectively provides high operation voltage (1.4 V) and hence high energy density over 28.95 W h/kg (0.41 mW h/cm 3 ) with a high power density of 7.75 kW/kg (113 mW/cm 3 ) and cyclic stability of 89.98% after 2000 cycles. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Supercapacitors with exclusive advantages over lithium ion batteries such as high power density and long cycling life are emerging as attractive electrochemical energy storage devices for various applications [1]. However, low energy density of super- capacitors hinders their practical applications. Intensive efforts have been dedicated to enhance their energy density to make it comparable to that of Li-ion batteries. Among supercapacitor electrode materials, pseudocapacitive transition-metal oxides (MnO 2 , IrO 2 , RuO 2 etc.), polyoxometalates and electronically con- ducting polymers (polyaniline, polypyrrole etc.) based on faradic redox charge storage have attracted significant attention because of their higher energy density than those of electrical double-layer capacitive carbon materials [2]. Recently, there are two promising and effective approaches are commonly adopted to improve the energy density of super- capacitors according to the equation (E ¼ 0.5 CV 2 ), one is increasing the capacitance of device which is possible by fabricating different * Corresponding author. ** Corresponding author. E-mail addresses: dubaldeepak2@gmail.com (D.P. Dubal), pedro.gomez@cin2.es (P. Gomez-Romero). Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour http://dx.doi.org/10.1016/j.jpowsour.2016.01.074 0378-7753/© 2016 Elsevier B.V. All rights reserved. Journal of Power Sources 308 (2016) 158e165