Multi-porous Co₃O₄ nanoflakes @ sponge-like few-layer partially reduced graphene oxide hybrids: Towards highly stable asymmetric supercapacitors

The controlled growth of metal oxide nanostructures within hierarchically porous conductive carbon-based frameworks is critically important to achieving high volumetric performance and appropriate channel size for energy storage applications. Herein, we grow cobalt oxide (Co₃O₄) nanoflakes, using a sequential-electrodeposition process, into spherically porous sponge-like few-layer partially reduced graphene oxide (SrGO) synthesized by template-directed ordered assembly. Maximum specific/volumetric capacitances of 1112 F g_Co3O4 ^-1 (at 3.3 A g_Co3O4 ^-1), 178 F cm^-3 (at 2.6 A cm^-2), and 406 F g_total ^-1 (at 1 A g_total ^-1) and sensible rate capability (80% retention by increasing the charge/discharge current from 1 A g^-1 to 16 A g^-1) are obtained for the Co₃O₄ nanoflakes@SrGO hybrid electrodes. Besides, an asymmetric supercapacitor is made with the Co₃O₄[63%]@SrGO[37%] hybrid and activated carbon as a positive and a negative electrode, respectively. Electrochemical results indicate an energy density of 23.3 W h kg^-1 at a high power density of 2300 W kg^-1 (discharge time of about 42 s) and 62% retention even at a remarkable power density of 36 600 W kg^-1 (discharge time of 1.6 s). Moreover, it shows excellent cycling stability with no decay after 20 000 charge/discharge cycles. This performance is attributed to the unique pore-sizes for an ion to channel into the porous structures, conductivity, and mechanical stability of the SrGO framework, which makes it promising for next-generation supercapacitors.