NiCo₂S₄ nanosheets grown on nitrogen-doped carbon foams as an advanced electrode for supercapacitors

To push the energy density limit of supercapacitors, a new class of electrode materials with favorable architectures is strongly needed. Binary metal sulfides hold great promise as an electrode material for high-performance energy storage devices because they offer higher electrochemical activity and higher capacity than mono-metal sulfides. Here, the rational design and fabrication of NiCo₂S₄ nanosheets supported on nitrogen-doped carbon foams (NCF) is presented as a novel flexible electrode for supercapacitors. A facile two-step method is developed for growth of NiCo₂S₄ nanosheets on NCF with robust adhesion, involving the growth of Ni-Co precursor and subsequent conversion into NiCo₂S₄ nanosheets through sulfidation process. Benefiting from the compositional features and 3D electrode architectures, the NiCo₂S₄/NCF electrode exhibits greatly improved electrochemical performance with ultrahigh capacitance (877 F g^-1 at 20 A g^-1) and excellent cycling stability. Moreover, a binder-free asymmetric supercapacitor device is also fabricated by using NiCo₂S₄/NCF as the positive electrode and ordered mesoporous carbon (OMC)/NCF as the negative electrode; this demonstrates high energy density (≈45.5 Wh kg^-1 at 512 W kg^-1). A general paradigm for fabricating 3D electrode and high-performance asymmetric supercapacitors is demonstrated via the use of a highly conductive 3D nitrogen-doped carbon foam (NCF) based composite electrode design. This provides efficient and rapid pathways for ion and electron transport, which achieve a simultaneous improvement in the form factor flexibility, light weight, excellent specific energy, and power performance.