A facile one-step hydrothermal approach to synthesize hierarchical core–shell NiFe₂O₄@NiFe₂O₄ nanosheet arrays on Ni foam with large specific capacitance for supercapacitors

In this contribution, NiFe₂O₄@NiFe₂O₄ nanosheet arrays (NSAs) with three-dimensional (3D) hierarchical core-shell structures were synthesized by a facile one-step hydrothermal method and they were used as electrode materials for supercapacitors (SCs). The NiFe₂O₄@NiFe₂O₄ composite electrode showed a high specific capacitance of 1452.6 F g^-1 (5 mA cm^-2). It also exhibited a superior cycling stability (93% retention after 3000 cycles). Moreover, an asymmetric supercapacitor (ASC) was constructed utilizing NiFe₂O₄@NiFe₂O₄ NSAs and activated carbon (AC) as the positive and negative electrode, respectively. The optimized ASC shows extraordinary performances with a high energy density of 33.6 W h kg^-1 at a power density of 367.3 W kg^-1 and an excellent cycling stability of 95.3% capacitance retention over 3000 cycles. Therefore, NiFe₂O₄@NiFe₂O₄ NSAs have excellent pseudocapacitance properties and are good electrode materials for high energy density.