Hierarchical nanosheets-assembled hollow NiCo2O4 nanoboxes for high-performance asymmetric supercapacitors

The conductivity of NiCo2O4 is at least 2 orders of magnitude higher than that of NiO or Co3O4, which makes it exhibit better electrochemical performance when it is used for supercapacitor applications. Morphology engineering is a crucial way to provide easy access for the electrolyte ions to the surface of the electrode to generate more surface redox reactions. Here, we use a sacrificial template method to prepare hollow NiCo2O4 nanoboxes and investigate their potential application for supercapacitors. SEM and TEM reveal that the unique hierarchical hollow NiCo2O4 nanoboxes were constructed by nanosheets with a large specific surface area and favorable pore structure, which provide fast electron and ion transport between the electrode and electrolyte and bring about abundant electrochemically active sites for redox reactions. Three-electrode electrochemical measurements show that it can deliver a high specific capacitance of 930 F g-1 and excellent cycling stability with 88 % retention after 2000 cycles. Moreover, an asymmetric supercapacitor (ASC) was constructed using hollow NiCo2O4 nanoboxes as the positive electrode and activated carbon (AC) as negative electrode. The device displays a maximum energy density of 51.7 Wh kg-1 at a power density of 800 W kg-1, and excellent performance stability with a capacity retention of 85.9 % after 10,000 cycles. These results demonstrate that the as-prepared hollow NiCo2O4 nanoboxes hold great potential as electrode materials for high performance supercapacitors.