Orientated Co₃O₄ nanocrystals on MWCNTs as superior battery-type positive electrode material for a hybrid capacitor

Using a simple hydrothermal process, cobalt oxide (Co₃O₄) with preferred orientation along (220) planes is in situ prepared and coated on MWCNTs. The Co₃O₄/MWCNTs nanohybrid shows superior electrochemical performance as positive electrode material in 1 M KOH solution. Its redox peaks retain the well-defined shapes even when the scan rate is up to 200 mV/s, and the specific capacity is high, 92 mAh/g at 10 A/g and still 85 mAh/g even at 100 A/g within the potential range from 0 to 0.5 V (vs. SCE). There is no capacity fading after 9000 full cycles. When assembled into a hybrid capacitor by using activated carbon as the negative electrode, the hybrid capacitor achieves excellent cycling performance between 0 and 1.8 V with energy density of 31 Wh/kg and power density of 3 kW/kg, which is superior to that of the reported Co₃O₄. The main reason is ascribed to the unique nanohybrid structure, which provides good electronic conductivity, short diffusion distance and good buffering capacity to volume change during the charge and discharge processes.