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

Abstract

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 10 000 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 10 000 cycles). The highly efficient energy storage performance of this new class of heterostructures synthesized with earth-abundant materials enables commercial applications.