Activated carbon nanospheres as high-power and -energy supercapacitor electrodes

Supercapacitors have seen significant study as an additional electrochemical energy storage technology. Due in large part to their high specific power, they have seen growing application for rapid energy storage, or for the delivery of large bursts of power in a short amount of time. However, there is still a desire to enhance their specific energy without sacrificing that specific power. In this work, the synthesis and novel use of activated carbon spheres between 300 and 400 nm with an exceptionally high specific surface area of 2972 m2/g were used as electrode materials for supercapacitors. When this material was used in an aqueous symmetric device, specific energies of up to 10.59 Wh/kg were obtained at a high specific power of 5.15 kW/kg. This performance was achieved with less than 1% capacitance loss over 10,000 galvanostatic charge-discharge cycles. Finally, a novel and facile electrodeposition of manganese oxide using potassium permanganate solution was described. The technique was found to have a high degree of uniformity in deposition, and the ability to tune the thickness of the deposition layer. The manganese oxide deposited was characterised using a range of techniques and found to behave closely to that of birnessite-type manganese oxide. When deposited onto electrodes with activated carbon spheres, the manganese oxide layer conformed to the spherical surface well. These activated carbon sphere-manganese oxide core-shell electrodes were used in asymmetric two-electrode pseudocapacitors where a specific energy of 5.24 Wh/kg at a specific power of 819 W/kg was achieved.