Functionalized and tip-open carbon nanotubes for high-performance symmetric supercapacitors

Abstract

Carbon nanotubes (CNTs) have been widely studied for use in supercapacitor electrodes because of their excellent conductivity, high aspect ratio, excellent mechanical properties, chemical stability, and large specific surface area. However, the electrochemical performance of CNTs is usually limited by their closed tips and fewer active sites. Therefore, a facile and efficient chemical-acid-etching method was employed to open the tips of CNTs and introduce functional groups. Different types of ions (Li⁺, Na⁺, and Mg²⁺) in aqueous electrolytes were investigated using the functionalized and tip-open CNTs (FTO-CNTs), and the Li⁺-based electrolyte has the best electrochemical performance. The areal capacitance when using FTO-CNTs as positive and negative electrodes could reach 542 mF cm⁻² and 410 mF cm⁻², respectively, at a scan rate of 10 mV s⁻¹, and the positive electrode reached the highest areal capacitance of 903 mF cm⁻² at a current density of 1 mA cm⁻². The symmetric supercapacitor-based FTO-CNTs electrode delivered a superior areal energy density of 39 μW h cm⁻² and an areal power density of 10.2 mW cm⁻², with remarkable cycling stability.