Ultrathin NiCo₂O₄ nanosheets grown on three-dimensional interwoven nitrogen-doped carbon nanotubes as binder-free electrodes for high-performance supercapacitors

A novel three-dimensional (3D) Ni foam/N-CNT/NiCo₂O₄ nanosheet electrode was synthesized by combining a chemical vapor deposition method and a facile electrochemical deposition method followed by a calcination process. The N-CNTs entangle with each other and construct a 3D highly conductive network, creating a structure which offers a skeleton for homogeneous electrodeposition of thin NiCo₂O₄ nanosheets. By taking advantage of the one-dimensional (1D) N-CNTs, the two-dimensional (2D) ultrathin nanosheets and the 3D hybrid structure, the 3D Ni foam/N-CNT/NiCo₂O₄ nanosheet electrode exhibits superior supercapacitive performances with high specific capacitance (1472 F g^-1 at 1 A g^-1), remarkable rate capability and excellent cycling stability (less than 1% loss after 3000 cycles). The outstanding supercapacitive performance is attributed to the highly conductive 3D Ni foam/N-CNT substrates and the ultrathin morphology of the NiCo₂O₄ nanosheets. The former offer a strong skeleton for uniform electrodeposition, endure the volume change, and provide a good electrical conducting pathway for ion and electron transport; meanwhile the latter possess numerous active sites and a short diffusion path. Moreover, the synthesis strategy can be extended to the preparation of other 3D electrode materials for supercapacitors and other energy-storage devices.