Carbon Nanotubes Coated with NiOOH-Ni Converted from Ni(HCO₃)₂-Ni Nanoflakes for Electrochemical Energy Storage

A functionalized carbon nanotube (FCNT)/Ni(HCO3)2-Ni nanostructures were synthesized by decorating nanoflaky and mesoporous Ni(HCO3)2 onto FCNT along with in situ formed reduced metallic Ni for the purpose of constructing an energy-storage system with both high specific capacity and cycling stability. Such flaky nanostructures can significantly increase the contact surface areas between the mesoporous and nanoflaky Ni(HCO3)2-Ni and electrolyte, thus immensely reducing the electrolyte-ion diffusion paths and accelerating the electron-transport rates through the use of a FCNT. The in situ reduced metallic Ni atoms are directly contacted with the electrolytes and quickly converted into NiOOH in the alkaline environment. For this reason, the capacity retention of battery-type electrochemical materials can be significantly enhanced, especially at the beginning of cycles. The obtained flaky FCNT/Ni(HCO3)2-Ni nanomaterials exhibit excellent electrochemical performances with a high specific capacity of 203 mAh·g–1 at 3 A·g–1 current density for supercapacitor energy-storage systems. An aqueous asymmetric supercapacitor made of FCNT/Ni(HCO3)2-Ni//graphene oxide achieves a 132 Wh·kg–1 energy density at 2880 W·kg–1 power density and an excellent cycling stability with 84% after 6000 cycles.