ZIF-67 derived nitrogen doped CNTs decorated with sulfur and Ni(OH)2 as potential electrode material for high-performance supercapacitors

Abstract

Herein we report the synthesis of nitrogen-doped carbon nanotubes (NC) based sulfide (S) and nickel hydroxide (NH) supported on nickel foam as a novel electrode material for the supercapacitors (abbreviated as Co/NC/S@NH). A class of zeolitic imidazolate frameworks, ZIF-67 (Co), is initially grown to synthesize nanotubes directly on the nickel foam (NF). The anion exchange reactions are performed for the growth of sulfide and nickel hydroxide on ZIF-67 (Co) derived carbon nanotubes. In the structural analysis, X-ray diffraction confirmed the successful in-situ growth of carbon nanotubes. Transmission electron microscopy was employed to confirm the formation of NC-based sulfide and nickel hydroxide. The Co/NC/S@NH, hierarchical hybrid electrode, exhibited improved capacitance of 1636 F/g at 1 A/g (982 C/g or 273 mA/h) with exceptional cyclic stability. The Co/NC/S@NH showed capacity retention of 94% over 5,000 cycles. The enhanced electrochemical performance of the synthesized electrode is due to the improved redox reactions and direct growth of nitrogen-doped carbon nanotubes on 3D nickel foam which act as "superhighways" for electron transportation. Moreover, the sulfur ions could hinder the collapse of a structure by the intercalation among layers, whereas the extremely hydroxylated surface of Ni(OH)2 nanosheets could promote accessibility of ions to the electrolyte which decreases the resistance of electrodes which agreed with the finding from electrochemical impedance spectroscopy.