High performance asymmetric supercapacitors using electrospun copper oxide nanowires anode Baiju Vidyadharan, Izan Izwan Misnon, Jamil Ismail, Mashitah M. Yusoff, Rajan Jose ⇑ Nanostructured Renewable Energy Materials Laboratory, Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang, 26300 Kuantan, Malaysia article info Article history: Received 18 January 2015 Received in revised form 28 January 2015 Accepted 30 January 2015 Available online 7 February 2015 Keywords: Electrochemical energy storage Asymmetric supercapacitor Renewable energy Metal oxide semiconductors Batteries One-dimensional nanostructures abstract We have fabricated, for the first time, an asymmetric supercapacitor (ASC) employing pseudocapacitive copper oxide (CuO) as anode and electrochemical double layer capacitive commercial activated carbon (AC) as cathode. The CuO is in the form of nanowires of diameter 30–50 nm developed using an aqueous polymeric solution based electrospinning process. The ASC showed larger voltage window (V  1.6 V) and specific capacitance (C S  83 Fg 1 ) than a control symmetric electrochemical double layer capacitor (EDLC) (V  1.4 V; C S  33 Fg 1 ) fabricated using the AC. The ASC delivered specific energy densities (E S ) of 29.5, 23.5, 19.2 and 16.4 W h kg 1 at specific power densities (P S ) 800, 1500, 4000 and 8400 W kg 1 , respectively. The performance of ASC is much superior to the control EDLC, which delivered E S of 11, 10 and 8.8 W h kg 1 at P S 800, 1600 and 3900 W kg 1 , respectively. Owing to the larger abun- dance of copper in the earth’s crust and promising charge storage properties achieved herewith, the pres- ent ASC could be developed as a commercial electrical energy storage device. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Deployment of electrical energy derived from renewable sources such as solar, water and wind demands efficient storage system such as batteries and supercapacitors [1,2]. Supercapacitors have gained considerable attention in recent years as an energy storage device due to their high power densities, fast recharge capability and long cycle life. Supercapacitors are classified into electric double layer capacitors (EDLCs) and pseudocapacitors according to charge storage mechanisms [3–6]. The EDLCs store electrical energy via accumulation of electric charges at an electri- cal double layer formed at an electrode–electrolyte interface. Car- bons such as activated carbon, carbon nanotubes, and graphene are choices to build EDLCs; however, lower specific capacitance (C S ) and specific energy density (E S ) limit its application areas [7–9]. The pseudocapacitors provide several times larger C S and E S than EDLC through a faradic reaction at the electrode–electrolyte inter- face [10–12]. Transition metal oxides (TMO), layered materials, metal hydroxides, and conducting polymers show pseudocapaci- tance [9,13–18]. However, they suffer from a lower operating potential (V) window and E S , which restricts their applications. Therefore, improvement of the E S of the pseudocapacitors is crucial to meet the energy storage demands [19–21]. The E S could be increased by increasing the C S and the V of the device as they are related by [9,22,23] E S ¼ 1=2C S V 2 ð1Þ A large number of TMOs are reported with large theoretical C S -  3500 Fg 1 , a brief review of which is available in recent articles [24–26]. One of the main issues in the electrode selection for pseudocapacitors is to unite high C S , specific power density (P S ), and abundance for large scale production. Although materials such as Co 3 O 4 show large practical C S (3560 Fg 1 ) [27] they are rela- tively lower abundant (<20 ppm) in the earth crust. On the other hand, materials such as MnO 2 with theoretical C S (1370 Fg 1 ) are largely abundant (>2000 ppm) but their practical C S are rela- tively lower (<400 Fg 1 ) [24,26,28,29]. Therefore, materials of high abundance and large practical C S are inevitable for fabrication of high performance commercial devices. The other parameter for achieving higher E S is V (Eq. (1)). The V depends on the difference in electrochemical potentials of the elec- trode material and the electrolyte. Therefore, for a given pseudoc- apacitive electrode, the V could be widened by choosing an electrolyte with much different electrochemical potential than the electrode material [19,30,31]. Towards this end, high operating voltage is reported using organic electrolytes (2.7 V) and ionic liq- uids (3.5 V) using this approach [32]. However, high cost, toxicity, low conductivity, flammability, and stringent device fabrication requirements restrict their use in large scales. Otherwise, aqueous http://dx.doi.org/10.1016/j.jallcom.2015.01.278 0925-8388/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: rjose@ump.edu.my (R. Jose). Journal of Alloys and Compounds 633 (2015) 22–30 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom