Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem Improving the symmetry of asymmetric supercapacitors using battery-type positive electrodes and activated carbon negative electrodes by mass and charge balance Syam G. Krishnan a , Midhun Harilal a , Bhupender Pal a , Izan Izwan Misnon a , Chelladurai Karuppiah b , Chun-Chen Yang b , Rajan Jose a,⁎ a Nanostructured Renewable Energy Materials Laboratory, Faculty of Industrial Sciences and Technology, Universiti Malaysia Pahang, 26300 Kuantan, Malaysia b Battery Research Center of Green Energy, Ming Chi University of Technology, New Taipei City, Taiwan ARTICLE INFO Keywords: Asymmetric supercapacitors Magnesium cobaltite Manganese cobaltite Charge storage ABSTRACT Asymmetric supercapacitors (ASCs) are routinely fabricated using battery-type electrode materials as a positive electrode and electrochemical double layer materials as a negative electrode; the mass-loading in the electrodes is determined by assuming both to be capacitive charge storage materials. This protocol is erroneous as the cyclic voltammograms and galvanostatic charge-discharge curves of the resulting devices showed dissimilarity in the stored charges of the two electrodes and battery-type behaviors, respectively. Herein, we show by employing two choices of battery-type electrodes as positive electrodes and commercial activated carbon as negative electrode in 3 M LiOH electrolyte that equal mass loading in both electrodes leads to supercapacitive charge storage. The positive electrode to negative electrode mass ratio is varied from 0.75 to 1.5 in a mass interval of 0.25 which includes a mass ratio of the conventional method. The electrochemical studies of the fabricated ASCs show that the charge storage capabilities depend on the electrode mass. Electrochemical impedance spectroscopy studies show that the equal mass ratio has low series and charge transfer resistances and wider frequency dispersion of capacitance. 1. Introduction Electrical charge (energy) storage devices, such as batteries and su- percapacitors, have been a topic of the intense investigation recently because of the widespread use of portable electronic devices and emer- gence of electric vehicles. One of the critical issues faced by the electric vehicle industry is the weight of the lithium ion batteries (LIBs) em- ployed in them; the specific power of the LIBs are rather low (0.5–1 kW kg -1 ) despite their high specific energy (150–200 Wh kg -1 ), therefore, huge battery stack is required to achieve a required power capability. Besides, their cycle life (< 10 3 cycles) and coulombic effi- ciency (< 50%) are rather low. Supercapacitors, on the other hand, offer much impressive specific power (2–10 kW kg -1 ) and cycle life (~10 5 cycles) with high coulombic efficiency (> 95%) but their specific energy is rather low (~5 Wh kg -1 ). Towards this end, asymmetric su- percapacitors (ASCs), which utilizes a battery-type electrode such as Co 3 O 4 [1,2], CuO [3,4], NiO [5,6], MCo 2 O 4 (M = Mg, Mn, Cu, Ni) [7–10] or pseudocapacitor electrode such as RuO 2 [11,12] and MnO 2 [13,14] as the positive electrode and a supercapacitor electrode as the negative electrode, could be a potential remedy to increase the charge storage capabilities of supercapacitors similar to batteries while main- taining their high power capabilities. The asymmetry of ASCs arise from two sources: (i) the charge sto- rage processes are different in the two electrodes and (ii) different amounts of charges are stored in the negative and the positive electrode [15]. The charge storage in the battery-type electrode is through deep intercalation [16,17] and would store an order of magnitude more charge than a supercapacitor electrode; whereas, in the latter, the charge storage is through reversible ion adsorption at an electrode – electrolyte interface [18]. If the intercalation in the battery-type is limited to few monolayers, they are termed pseudocapacitors; their storage capability lies between that of batteries and supercapacitors. One of the crucial issues of the fabrication of an asymmetric su- percapacitor is the mass-loading on the electrodes such that both electrodes store an equal amount of charges [15]. Currently adopted procedure for the electrode mass calculation assumes an ASC as equivalent to a series combination of two capacitors (electrodes). Theoretically, the reciprocal of the effective capacitance of ASCs is http://dx.doi.org/10.1016/j.jelechem.2017.10.029 Received 23 August 2017; Received in revised form 13 October 2017; Accepted 14 October 2017 ⁎ Corresponding author. E-mail address: rjose@ump.edu.my (R. Jose). Journal of Electroanalytical Chemistry 805 (2017) 126–132 Available online 16 October 2017 1572-6657/ © 2017 Elsevier B.V. All rights reserved. MARK