Rational design of asymmetric supercapacitors via a hierarchical core-shell nanocomposite cathode and biochar anode.

Hierarchical MnO 2 nanosheets attached on hollow NiO microspheres have been designed by a facile hydrothermal process. The core-shell structure is achieved by decorating an MnO 2 nanosheet shell on a hollow NiO sphere core. The highly hollow and porous structure exhibits a high surface area, shortened ion diffusion length, outstanding electrochemical properties (558 F g -1 at a current density of 5 mA cm -2 ), and excellent cycling stability (83% retention after 5000 cycles). To further evaluate the NiO/MnO 2 core-shell composite electrode for real applications, three asymmetric supercapacitors (NiO/MnO 2 //pomelo peel (PPC), NiO/MnO 2 //buckwheat hull (BHC), and NiO/MnO 2 //activated carbon (AC)) are assembled. The results demonstrated that NiO/MnO 2 //BHC delivered a substantial energy density (20.37 W h kg -1 at a power density of 133.3 W kg -1 ) and high cycling stability (88% retention after 5000 cycles) within a broad operating potential window of 1.6 V.