Performance evaluation of Asymmetric Supercapacitor based on Cobalt manganite modified graphene nanoribbons Preety Ahuja, Vikrant Sahu, Sanjeev Kumar Ujjain, Raj Kishore Sharma *, Gurmeet Singh * Department of Chemistry, University of Delhi, Delhi 110007, India A R T I C L E I N F O Article history: Received 25 July 2014 Received in revised form 9 September 2014 Accepted 15 September 2014 Available online 18 September 2014 Keywords: Cobalt manganite graphene nanoribbon asymmetric supercapacitor energy density power density A B S T R A C T An aqueous, high potential asymmetric supercapacitor (ASC) is developed using cobalt manganite (CoMn 2 O 4 ) modified graphene nanoribbon (GNR). Combining CoMn 2 O 4 /GNR and GNR as different electrodes of supercapacitor in Na 2 SO 4 electrolyte, the ASC device exhibited 1.9 V wide potential window for charge storage along with significantly improved capacitive performance than symmetric counterpart. When cycled reversibly in 0-1.9 V, the ASC demonstrates high energy density 84.69 Wh Kg 1 (more than four times of the symmetric cell) at high power density 22 kW Kg 1 due to elegant synergism between different electrodes. Additionally, it exhibits long term stable cycling performance with 96% capacitance retention and  97.5% columbic efficiency. These outstanding results pave their way for promising applications in high energy storage systems. ã 2014 Elsevier Ltd. All rights reserved. 1. Introduction Demand of supercapacitor devices is increasing particularly from the applications that require high pulsed power density and long cycling life [1,2]. Bridging batteries and conventional capacitors in energy and power density, supercapacitors can be used as backup power sources in specialized applications like hybrid electric vehicles, industrial equipments, power back up, military devices and portable electronics [3,4]. Charge storage mechanism in supercapacitors is classified into two i.e. electrical double layer capacitance (EDLC), where electrical energy is stored in the form of adsorbed ions at electrode/electrolyte interface, and the pseudo-capacitance in which the redox transitions are utilized. Supercapacitor electrode employs three main classes of materials which include carbonaceous material, transition metal oxides and conducting polymers [4–6]. Commercially, most supercapacitor devices are based on EDL charge storage and therefore high surface area carbon materials have been extensively investigated. By virtue of large surface, significantly large EDL formation takes place and consequently these material possesses high power density however lack in energy density [7]. Enhancing energy density to meet the demands without forfeiting power delivery and cycle life is a major challenge due to limited operating potential [8] Recently, considerable research efforts have been devoted to various asymmetric supercapacitor (ASC) systems employing two different electrodes with comprehensive working potential range leading to notable improvement in energy density such as activated carbon (AC)//Ni(OH) 2 [9], carbon nanotube (CNT)// MnO 2 [10], RGO//MnO 2 /RGO [11], Graphene//MnO 2 [12]. However, these devices still have restricted performance in terms of sacrificed power density. Mostly utilized anode materials i.e. activated carbons have high hydrogen evolution potential leading to large negative potential range with low average capacitance resulting in imbalanced synergism [13]. Moreover, AC has limited accessibility of hydrated ions to the pores smaller than 0.5 nm (lesser than hydrated ions) retarding their relaxation time constant [14,15]. Instead, graphene nanoribbon (GNR), a quasi one dimensional form of graphene is employed for the first time as an anode material for high performace supercapacitors. To bring dynamic synergistic effect in ASC, it is important to choose the cathode with efficient kinetics of redox reactions. Kinetics may be boosted by functionalized electrode materials capable in offering high electronic and ionic conductivities, active charge storage sites and desired microstructures. However, it is not possible for a single material to exhibit all desired characteristics and therefore binary or ternary nanocomposite have been investigated [16,17]. Low electrical conductivity, poor cycling stability at high current densities and less mechanical stability of * Corresponding Authors. Tel.: +91 11 27666616. E-mail addresses: drrajksharma@yahoo.co.in (R.K. Sharma), gurmeet123@yahoo.com (G. Singh). http://dx.doi.org/10.1016/j.electacta.2014.09.039 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved. Electrochimica Acta 146 (2014) 429–436 Contents lists available at ScienceDirect Electrochimica Acta journa l home page : www.e lsevier.com/loca te/ele cta cta