Optimization of MnO<inf>2</inf>/CNW composite electrodes for energy storage application

Abstract

Nanostructured MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin films were electrodeposited on carbon nanowalls (CNWs), which were grown first by microwave plasma enhanced chemical vapor deposition (MPECVD) on three-dimensional nickel foam substrates. The optimization theme for producing composite MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /CNW on large area electrodes for electrochemical supercapacitors is presented. The MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /CNW nanocomposite electrodes were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy. The control of the growth time of CNW is found to be key point for the optimization of the MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> electrodeposition process in view to enhance the supercapacitive behavior of MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /CNW nanocomposite electrodes. The capacitive behavior and morphology of MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> were strongly affected by the incorporation of CNWs. The MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /CNW nanocomposite electrodes showed better rate capability than MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> electrode. The MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /CNW nanocomposite electrode with CNW deposition time, 18 sec, showed the optimum capacitive behaviour. A specific capacitance of 851 F/g at a current density of 1 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , equivalent series resistance of 3.19 Ω, and charge transfer resistance of 1.02 Ω are obtained for MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /CNW (18 sec) electrode. This electrode also retained a stable capacitance, as its loss is only 8 % over 2000 cycles by charging and discharging at 3 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , indicative of long term electrochemical cycling stability which suggests its possible choice as a promising electrode for supercapacitors.