Electric double layer capacitors based on a composite electrode of activated mesophase pitch and carbon nanotubes

A novel composite of KOH activated mesophase pitch (aMP) and carbon nanotubes (CNTs) shows outstanding performance as an electrode for electric double-layer formation in 2 M H2SO4. The aMP powder is highly porous and the KOH activation may produce pores that are populated with graphitic edges. The resulting aMP electrode has a capacitance value of 295 F g−1 at 0.125 A g−1 discharge and decreases to 180 F g−1 at 100 A g−1. With particle milling, the pore diffusion resistance of the aMP electrode decreases significantly because of the elimination of a hindered diffusion mode from the particle interior. CNT addition provides inter-particle spacing and bridging media for the milled aMP and reduces the Warburg diffusion and electrical resistance. The composite of milled aMP and CNTs has capacitance values of 305 F g−1 at 0.125 A g−1 and 214 F g−1 at 100 A g−1. With a small potential window of 1 V, the resulting symmetric cells can deliver an energy level of 8.2 Wh kg−1 at a high power of 10 000 W kg−1. These cells show superior stability, with no decay of specific capacitance after 10 000 cycles of galvanostatic charge and discharge.