An advanced sandwich-type architecture of MnCo₂O₄@N-C@MnO₂ as an efficient electrode material for a high-energy density hybrid asymmetric solid-state supercapacitor

The design and development of innovative heterostructures with multifunctional properties are technically very important for efficient practical energy storage and conversion applications. Herein, we report the synthesis of a nitrogen-doped carbon (N-C) layer sandwiched between MnCo₂O₄ and MnO₂ (MnCo₂O₄@N-C@MnO₂) as a core@sandwich@shell type heterostructure on Ni foam. The thin layer of sandwiched N-C acts as a "superhighway" for good electron/ion transport and protects the MnCo₂O₄ and MnO₂ from destructive morphological changes during repeated charge-discharge processes. The MnCo₂O₄@N-C@MnO₂ material is well characterized by standard techniques, and its energy storage performance is studied in a three-electrode system and solid-state asymmetric capacitor device. The resultant electrochemical performance is compared with those of MnCo₂O₄ and MnCo₂O₄@N-C. The MnCo₂O₄@N-C@MnO₂ electrode exhibits an excellent areal/gravimetric capacity of 0.75 mA h cm^-2/312 mA h g^-1 at 3 mA cm^-2 with ca. 89.6% capacitance retention after 10000 cycles. A solid-state asymmetric supercapacitor device assembled with MnCo₂O₄@N-C@MnO₂ as a cathode and nitrogen-doped graphene hydrogel as an anode exhibits a high energy density of 68.2 W h kg^-1 at 749.2 W kg^-1 power density without compromising long cycle life (ca. 91.1% retention after 10000 cycles). The highly efficient energy storage performance of this new class of heterostructures synthesized with earth-abundant materials enables commercial applications.