Molecular Insights into Curvature Effects on the Capacitance of Electrical Double Layers in Tricationic Ionic Liquids with Carbon Nanotube Electrodes.

ABSTRACT

Ionic liquid (IL) electrolytes and carbon nanotube (CNT) electrodes have exhibited promising electrochemical performance in supercapacitors. Nevertheless, the adaptability of tricationic ILs (TILs) in CNT-based supercapacitors remains unknown. Herein, the performance of supercapacitors with (6,6), (8,8), (12,12), and (15,15) CNT electrodes in the TIL [C6(mim)3](Tf2N)3 was assessed via molecular dynamics simulations, paying attention to the electric double-layer (EDL) structures and the relations between the CNT curvature and capacitance. The results disclose that counterion and co-ion number densities near CNT electrodes have a marked reduction, compared with that of the graphene electrode. The capacitance of the EDL in the TIL increases significantly as the CNT curvature increases and the capacitance of the TIL/CNT systems is higher than that of the TIL/graphene system. Moreover, different EDL structures in the TIL and the monocationic IL (MIL) [C6mim][Tf2N] near CNT electrodes were revealed, showing higher-concentration anions [Tf2N]- at the CNT surfaces in the TIL. It is also verified that the TIL has a greater energy-storage ability under high potentials. Furthermore, the almost flat or weakly camel-like capacitance-voltage (C-V) curve of EDLs in the TIL turns into a bell shape in the MIL, because of the ion accumulation at the CNT surfaces and the associations between ions.