Nanocarbon based supercapacitors with reduced internal resistance

Supercapacitors, also known as electrical double layer capacitors, are promising candidates to meet the increasing power demands upon energy storage systems. They have an important role in complementing or replacing batteries in the energy storage field, possessing advantages such as high power density, rapid charge/discharge (few seconds), >100 000 cycle life, and intrinsic safety (they contain no heavy metals and have a reduced likelihood of catastrophic failure). The performance of a supercapacitor depends on charge capacitance, operating voltage and internal resistance. In this paper, particular attention is given to methods of decreasing the internal resistance of supercapacitors because for intended use in high current applications, high internal resistances cause unacceptable power loss and voltage drop. We describe two steps to decrease the equivalent series resistance (ESR) of nanocarbon based supercapacitors. First, a thin, conductive carbon layer was coated on a copper current collector to decrease the contact resistance between the active electrodes and the copper. The carbon layer formed on the copper has a rough surface, which increases both the contact area and adhesion ability between the electrode material and the copper surface. In addition, the carbon layer prevents oxidation and corrosion of the metal layer, and therefore prevents ESR increase after extended cycling, thus prolonging the supercapacitor life. Second, multiwall carbon nanotubes (CNTs) were added to the nanocarbon to further decrease the ESR of the device. CNTs have good electrical conductivity and a readily accessible surface area. Scanning electron microscope images show that the added CNTs cover the surface of the nanocarbon particles and bridge the gap between the particles. After surface treatment of the copper and adding CNTs to the active electrodes, the supercapacitor have a significantly reduced ESR and highly enhanced capacitance. Evaluation of capacitor performance by different techniques, such as voltammetry, charge/discharge characteristics is also discussed.