Ultrathin MnO2 nanoflakes grown on N-doped carbon nanoboxes for high-energy asymmetric supercapacitors

We demonstrate the synthesis of ultrathin MnO2 nanoflakes grown on N-doped carbon nanoboxes, forming an impressive hierarchical MnO2/C nanobox hybrid with an average size of 500 nm, which exhibits an excellent electrochemical performance due to the unique structure, N-doping and strong synergistic effects between them. In addition, we also assembled a green asymmetric supercapacitor (ASC) using the as-synthesized MnO2/C nanoboxes as a positive electrode and the corresponding N-doped carbon nanoboxes as a negative electrode in a neutral aqueous electrolyte, aiming to further enhance its energy density by extending the operating potential. More significantly, our ASC device is able to reversibly cycle within a wide operating voltage of 2.0 V and delivers a maximum energy density of 39.5 W h kg(-1) with superior cycling stability (similar to 90.2% capacitance retention after 5000 cycles). These intriguing results show that hollow nanostructures will be promising electrode materials for advanced supercapacitors.