Electrochimica Acta 367 (2021) 137226 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Direct growth of nickel cobalt layered double hydroxide on nickel foam via redox reaction between nitrate ion and ethanol for hybrid supercapacitors Thi Toan Nguyen a,1 , Debananda Mohapatra a,1 , Deivasigamani Ranjith Kumar a , Marjorie Baynosa b , Sumanta Sahoo a , Jintae Lee a , Jae-Jin Shim a,∗ a School of Chemical Engineering, Yeungnam University, Gyeongsan, Gyeongbuk 38541, Republic of Korea b Department of Chemical Engineering, University of the Philippines-Diliman, Diliman, Quezon City 1101, Philippines a r t i c l e i n f o Article history: Received 18 July 2020 Revised 28 September 2020 Accepted 3 October 2020 Available online 16 October 2020 Keywords: Ni–Co LDHs Binder-free Redox reaction High-performance Hybrid supercapacitor a b s t r a c t Binary combinations of transition metal-layered double hydroxides (LDHs) have attracted considerable attention for supercapacitor applications owing to their extraordinary charge storage, low internal resis- tance, and superior electrochemical stability. In this study, nickel–cobalt LDH (Ni–Co LDHs) nanosheets were grown hierarchically on the surface of Ni foam (NF@Ni–Co LDHs) using a simple solvothermal ap- proach without any binder elements. Interconnected nanosheets with a thickness of ~25 nm were grown uniformly on the nickel foam. The synergistic combination of Ni 2+ and Co 2+ /Co 3+ LDHs grown directly on nickel foam resulted in a remarkably high specific capacity of 1757 C g −1 (4392 F g −1 ) at a current density of 1 mA cm −2 (0.44 A g −1 ). Furthermore, a hybrid supercapacitor (HSC) device was fabricated us- ing activated carbon as the anode and NF@Ni–Co LDHs as the cathode. The hybrid device exhibited high energy densities of 51.1 Wh kg −1 (at a power density of 777 W kg −1 and a current density of 1.0 A g −1 ) and 11.8 Wh kg −1 (at a power density of 12.1 kW kg −1 and current density of 20 A g −1 ). The electro- chemical stability of the HSC device was 72.2% after 10,000 charge/discharge cycles at a current density of 10 mA cm −2 . The electrochemical performance of the Ni–Co LDHs nanosheets on Ni foam in a three- electrode system and as a hybrid device highlights its potential in energy storage applications. © 2020 Elsevier Ltd. All rights reserved. 1. Introduction Currently, supercapacitors are attracting considerable attention, not only in academia but also in practical energy storage device applications, such as backup power systems, electric vehicles, and portable electric devices [1–3], owing to their rapid energy deliv- ery, high power density, and long-life cycling behavior [4,5]. Tra- ditional electric double-layer capacitors (EDLCs) only store energy via electrostatic interactions, so they still have low energy den- sities compared to conventional batteries [6,7]. Hybrid superca- pacitors (HSCs), which consist of a combination of capacitive and battery-type electrodes, are currently under development to over- come this energy gap restriction. Capacitive electrodes with car- bonaceous materials, such as carbon nano-onions [8] or graphene [9], provide a high power density and long cycle life, whereas battery-type faradaic electrodes provide a high energy density. A ∗ Corresponding author. E-mail address: jjshim@yu.ac.kr (J.-J. Shim). 1 These authors contributed equally to this work. suitable combination of these two types of electrodes in terms of the charge balance can broaden the working potential window, re- sulting in higher capacity and energy density based on the for- mula, E = CV 2 /2 [6,7,10]. Commonly used battery-type materials are transition metal (Ni, Mn, Co, etc.) oxides and their respective hydroxides [11–13]. Among them, Ni and Co hydroxides have been studied extensively because of their attractive theoretical capacitance, low cost, vari- ous controlled morphologies, and easy fabrication techniques [2,3]. In particular, the synergistic effects between these two compo- nents brings together Ni and Co hydroxides as a comparatively better electrochemically performing electrode over individual ox- ide/hydroxide electrodes [14–16]. The partial replacement of Ni cations with Co cations can improve the conductivity, rate capac- ity, or stability restrictions of Ni hydroxide [17,18]. Remarkably, nickel–cobalt layered double hydroxides (Ni–Co LDHs), a deriva- tive of Ni and Co hydroxides, have attracted considerable interest. This compound has a lamellar structure with the general formula, [M 2+ 1- x M 3+ x (OH) 2 ] x + [A n− x / n ] x − mH 2 O, where M 2+ and M 3+ de- note di- and trivalent metal cations, respectively, and A n− indicates https://doi.org/10.1016/j.electacta.2020.137226 0013-4686/© 2020 Elsevier Ltd. All rights reserved.