A Tubular Sandwich-Structured CNT@Ni@Ni₂(CO₃)(OH)₂ with High Stability and Superior\nCapacity as Hybrid Supercapacitor

Abstract

Development\nof highly stabile battery-type electrode\nmaterials\nwith superior capacity has been a critical challenge for hybrid supercapacitors.\nWe report a high-performance electrode material, tubular sandwich-structured\nCNT@Ni@Ni₂(CO₃)­(OH)₂, synthesized\nvia a scalable, dynamic, controlled in situ reduction–chemical\ndeposition process. Applied as a battery-type electrode material,\nthis novel nanostructure exhibits excellent electrochemical stability,\nmajorly attributed to the Ni midshell serving a dual role as “capacity\nsupplement” and “electron highway”, which, to\nour knowledge, was incorporated into the nanocomposite electrodes\nfor the first time. Also benefiting from the high conductivity of\ncarbon nanotubes (CNTs) and the high capacity of the amorphous NiOOH\nultrathin film [converted from the Ni₂(CO₃)­(OH)₂ outer shell], the resulting CNT@Ni@Ni₂(CO₃)­(OH)₂ material as a battery-type electrode achieves\na superior capacity of 221 mAh·g^–1 at 5 A·g^–1 with 76% capacity retention at 50 A·g^–1 and maintains 81% capacity after 9000 cycles at 5 A·g^–1. An advanced aqueous hybrid supercapacitor using activated carbon\nand CNT@Ni@Ni₂(CO₃)­(OH)₂ nanocomposite\nas the negative and positive electrodes, respectively, delivers a\nhigh energy density of 179 Wh·kg^–1 at a power\ndensity of 2880 W·kg^–1 with capacitance retention\nin excess of 85% over 5500 cycles. The outstanding performance demonstrates\nits practical potential in advanced hybrid supercapacitors.