Hierarchical Co3O4@Ni-Co-O supercapacitor electrodes with ultrahigh specific capacitance per area

High specific capacitance per area is a critical requirement for a practical supercapacitor electrode, and needs a combination of high mass-loading of the electrochemically active material per area, and high utilization efficiency of this material. However, pursuing high mass-loading on conventional electrodes usually leads to an increase in “dead” material which is not accessible to the electrolyte in the supercapacitor, and thus prevents high utilization efficiencies of the material being realized. Here we show that this antagonism can be overcome by incorporating the electrochemically active material in a mesoporous hierarchical architecture. Fabrication of ternary ordered hierarchical Co3O4@Ni-Co-O nanosheet-nanorod arrays—involving the growth of densely aligned slim Ni-Co-O nanorods (diameter <20 nm) on Co3O4 microsheets which had been previously loaded on macroporous nickel foam—gives a material with excellent electrochemical performance as a supercapacitor electrode. At a current density of 5 mA/cm2, the electrodes have both high mass loading per area (12 mg/cm2) and high efficiency of 2098 F/g, giving specific capacitances per area as high as ∼25 F/cm2. When the current density was increased from 5 to 30 mA/cm2, 72% of the specific capacitance was retained and, furthermore, no significant decrease in capacitance was observed over 1000 charge/discharge cycles. The combination of these merits makes the composite material an excellent candidate for practical application as a supercapacitor electrode and, more generally, highlights the increased efficacies of materials which can result from fabricating mesoporous hierarchical structures at the nanoscale.