Hierarchical 3D NiFe₂O₄@MnO₂ core-shell nanosheet arrays on Ni foam for high-performance asymmetric supercapacitors

Hierarchical NiFe₂O₄@MnO₂ core-shell nanosheet arrays (NSAs) were synthesized on Ni foam as an integrated electrode for supercapacitors, using a facile two-step hydrothermal method followed by calcination treatment. The NiFe₂O₄ nanosheets were designed as the core and ultrathin MnO₂ nanoflakes as the shell, creating a unique three-dimensional (3D) hierarchical electrode on Ni foam. The composite electrode exhibited remarkable electrochemical performance with a high specific capacitance of 1391 F g^-1 at a current density of 2 mA cm^-2 and long cycling stability at a high current density of 10 mA cm^-2 (only 11.4% loss after 3000 cycles). Additionally, an asymmetric supercapacitor (ASC) device was fabricated with a NiFe₂O₄@MnO₂ composite as the positive electrode material and activated carbon (AC) as the negative one. The ASC device exhibited a high energy density (45.2 W h kg^-1) at a power density of 174 W kg^-1, and an excellent cycling stability over 3000 cycles with 92.5% capacitance retention. The remarkable electrochemical performance demonstrated its great potential as a promising candidate for high-performance supercapacitors.