Supercapacitors, also called as ult

Supercapacitors, also called as ultracapacitors, are electrochemical energy storage devices that combine the high energy-storage-capability of conventional batteries with the high power-delivery-capability of conventional capacitors 1, 2, 3, 4. Being able to achieve higher power and longer cycle life than conventional dielectric capacitors and batteries, supercapacitors have been developed to provide power pulses for a large variety of applications, ranging from consumer electronics through hybrid electric vehicles (HEVs) to industrial electric utilities [5]. Therefore, supercapacitors play an important role in achieving better fuel economy, decreasing harmful emissions, and reducing the reliance on petroleum sources. The world market for supercapacitors has been growing steadily and rapidly [1]. To improve the performance of state-of-the-art supercapacitors to meet the stringent requirements for the applications mentioned above, and many other advanced applications not discussed (e.g. portable, transparent and wearable electronics), new electrode materials with superior properties over those of current activated carbon electrodes are needed and new device structures (e.g. all-solid state supercapacitors 6, 7, optically transparent 8, 9, mechanically flexible and stretchable 10, 11, 12, and even fiber-shaped 13, 14, 15 supercapacitors) are highly desirable.