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

Abstract

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.