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

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.