Multiwalled Carbon Nanotubes with Tuned Surface Functionalities for Electrochemical Energy Storage

The surface functionalities effects on the electrochemical energy storage properties of the multiwalled nanotubes (MWNTs) were studied by employing as-received MWNTs (denoted as MWNTs-As) and two other MWNTs modified with ether and carboxylic acid groups (denoted as MWNTs-EC), and ester groups (denoted as MWNTs-E). The introduced functionalities were confirmed by Raman spectra and caused a slightly increased bandgap (0.97, 1.09, and 1.02 eV for the MWNTs-As, MWNTs-EC, and MWNTs-E, respectively) from the UV-vis spectra. The cyclic voltammetry (CV) results reveal that the capacitance of MWNTs-EC (31.7 F/g), and MWNTs-E (50.0 F/g), was increased by 41.5% and 123.2%, respectively, when compared to the as-received MWNTs (22.4 F/g) at a scan rate of 20 mV/s. An enhancement of 36.1 and 130.0% in the energy density is also observed for the former (3.81Wh/Kg) and the latter (6.44Wh/Kg), respectively, than that of the MWNTs-As (2.80Wh/Kg) at a current density of 5 A/g in the charge-discharge measurement. The increased capacitance and energy density originate from the enhanced surface wettability and larger pseudocapacitance. An equivalent circuit was proposed to simulate the electrochemical impedance spectroscopy (EIS) data. A long term cycling stability was observed with a retained 100.0% of the charge/discharge capacity even after 1000 charge-discharge cycles. (C) 2013 The Electrochemical Society. All rights reserved.