Electrochimica Acta 133 (2014) 522–528 Contents lists available at ScienceDirect Electrochimica Acta j our na l ho me pa g e: www.elsevier.com/locate/electacta High-performance asymmetric supercapacitors based on core/shell cobalt oxide/carbon nanowire arrays with enhanced electrochemical energy storage G.X. Pan a,∗ , X.H. Xia b , F. Cao a , J. Chen a,c , P.S. Tang a , Y.J. Zhang a , H.F. Chen a a Department of Chemistry, Huzhou Teachers College, Huzhou, 313000, China b Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore c College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 313000, China a r t i c l e i n f o Article history: Received 18 March 2014 Received in revised form 17 April 2014 Accepted 18 April 2014 Available online 24 April 2014 Keywords: Core/shell Nanowires Cobalt oxide Supercapacitor Carbon a b s t r a c t High-reactivity electrode materials are indispensible for developing high-performance electrochemical energy storage devices. Herein, we report self-supported core/shell Co 3 O 4 /C nanowire arrays by using hydrothermal synthesis and chemical vapor deposition methods. A uniform and thin carbon shell is coated on the surface of Co 3 O 4 nanowire forming core/shell nanowires with diameters of ∼100 nm. Asymmetric supercapacitors have been assembled with the core/shell Co 3 O 4 /C nanowire arrays as the positive electrode and activated carbon (AC) as the negative electrode. The core/shell Co 3 O 4 /C nanowire arrays exhibit a specific capacity of 116 mAh g -1 at the working current of 100 mA (4 A g -1 ), and a long cycle life along with ∼ 92% retention after 8000 cycles at 4 A g -1 , higher than the unmodified Co 3 O 4 nanowire arrays (81 mAh g -1 at 4 A g -1 ). The introduction of uniform carbon layer into the core/shell structure is favorable for the enhancement of supercapacitor due to the improved electrical conductivity and reaction kinetics. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Supercapacitors are an important kind of electrochemical energy storage devices, which can provide higher power density than batteries, and higher energy density than conventional dielec- tric capacitors [1,2]. Such outstanding properties make them as potential energy storage devices for transportation, hybrid vehi- cles and modern electronics. However, to date, supercapacitors still suffer from a low energy density, which limits their applications as a main power source [3,4]. For practical applications, advanced supercapacitors must be developed with higher operation voltage and higher energy density without sacrificing power delivery and cycle life in the future [5,6]. A promising way to increase cell voltage is to develop asym- metric supercapacitors, which consist of a battery-type Faradic electrode (positive electrode) and a capacitor-type electrode (negative electrode), offering the combined advantages of both supercapacitors (eg., high -rate and high cycle life) and advanced batteries (eg., high energy density) [1–4,7]. With the help of ∗ Corresponding author. Tel.: +86 572 232 1166. E-mail addresses: hipgxzjut@hotmail.com, pgxzjut@sohu.com (G.X. Pan). the above two kinds of electrodes, asymmetric supercapacitors can make full use of different potential windows provided by the electrodes to increase the operation voltage in the cell system, resulting in enhanced specific capacitance/capacity and significantly improved energy density [8]. For the capacitor-type electrode (negative electrode), activated carbon (AC) is the most widely used negative electrode material because of its high sur- face area, relatively good electrical conductivity and low-cost [9,10]. Now the main focus of the asymmetric supercapacitors is to develop advanced positive electrode materials, which include transition metal oxides/hydroxides [11–14], binary metal oxides [15–17], metal sulfides [18] and conducting polymers [19–21]. Over recent years, metal oxides have attracted increasing attention as electrode materials for supercapacitors due to their excellent redox reversibility and high capacity/capacitance. Tran- sition metal oxides (such as Co 3 O 4 , NiO, and MnO 2 ) are promising electrode materials to replace expensive RuO 2 and meet the requirement of practical application [22,23]. Nevertheless, the intrinsic low conductivity of these metal oxides severely limits their electrochemical performance. It is accepted that the supercapacitor performance of metal oxides is controlled by the electrochemical reactivity of active materials and kinetics of ion/electron of the elec- trodes. Among the modification research for metal oxides, most of http://dx.doi.org/10.1016/j.electacta.2014.04.097 0013-4686/© 2014 Elsevier Ltd. All rights reserved.