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In this work, a three-dimensional (3D) porous hybrid nickel/aluminum layered double hydroxide (Ni/ Al-LDH)-carbon cloth (CC), the working electrode without binders or conductive additions for supercapacitor, was successfully synthesized via facile one-step hydrothermal method. The asobtained Ni/Al-LDH/CC sample exhibited good charge storage performance (the specific capacitance was up to 359 F/g at a current density of 0.3 A/g), as well as superior cycling stability (5.9% capacitance increase after 3000 cycles at 1.0 A/g). Furthermore, an asymmetric supercapacitor, Ni/ Al-LDH/CC as positive electrode and activated carbon (AC) as negative electrode (Ni/Al-LDH/CC// AC), achieved a high energy density (20.9 Wh/kg vs. the power density 262.5 W/kg) and good cycle lifetime (83.9% retention of the initial value after 3000 cycle tests at a current density of 0.5 A/g). The unique 3D porous structure and binder-free electrode display great potential in supercapacitors.

I. INTRODUCTION

The demands for renewable sources and efficient energy storage devices of people have been increasingly urgent, due to the rapid development of global economy, a sharp drop in fossil energy reserves and the ever increasing environmental problems. Supercapacitors (SCs, also known as electrochemical capacitors or ultracapacitors), with high power density, long cycle life, fast charge/discharge rates, high stability, and low maintenance cost, have attracted extensive attention as a novel type of energy storage device,1 wherein the properties of electrode materials are the key to determine the performance of supercapacitors. Currently, there are several types of electrode materials for supercapacitors as follows: carbonaceous materials (carbon nanotubes, activated carbon, graphene, carbon fabrics),2–4 transition metal oxides (RuO2, NiCo2O4, MnO2, Co3O4)5–8/ hydroxides (Ni(OH)2, Co(OH)2)9–11 and conducting polymers (polyaniline, polythiophene, polypyrrole).12,13 Layered double hydroxides (LDHs), the lamellar compounds, consist of the interaction between the positively charged host layers and interlayer anions by noncovalent bonds. Its hydrotalcite-like structure can be characterized by the general chemical formula  2þ 3þ Xþ M1X MX ðOHÞ2 ðAn Þx  mH2 O, where M21 and n M31 denote the divalent and trivalent metal cations respectively, and An represents almost any inorganic and organic anion.14 For the past few years, LDHs have Contributing Editor: Scott T. Misture a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.227

sparked a great interest in academic and industrial fields for its extensive applications, including catalysis, separation, biology, and electrochemistry.15–18 In the case of electrochemistry, LDHs special structures not only determine their anion exchange property and capacity to intercalate anions, but also provide a large specific surface area for the formation of electric double layer capacitance. At the same time, the variable anions can offer abundant electrochemically active sites fo