MnO 2 Nanoflowers and Reduced Graphene Oxide 3D Composites for Ultrahigh‐Energy‐Density Asymmetric Supercapacitors

Uniform MnO 2 nanoflowers and a reduced graphene oxide (rGO) 3D nanostructure composite are synthesized on Ni foam using a two‐step electrodeposition and hydrothermal method. The surface morphology and composition of the 3D nanostructure hybrid are intensively analyzed. MnO 2 /rGO/Ni is tested using a typical three‐electrode system for potential use as a supercapacitor electrode. The hybrid electrode has a high areal specific capacitance. An asymmetric supercapacitor (ASC) device is assembled and characterized using MnO 2 /rGO/Ni as the positive electrode and rGO/Ni as the negative electrode to further evaluate the electrochemical capability of the hybrids. Electrochemical characterizations indicate that the ASC device can be cycled reversibly between 0 V and 2.0 V in 1  m Na 2 SO 4 aqueous electrolyte. The maximum areal specific capacitance of 760.1 mF cm −2 (1266.83 F g −1 ) and the maximum ultrahigh energy density of 150.45 Wh kg −1 are obtained, demonstrating the potential for use as binder‐free electrodes in pseudocapacitors.