Optimization of pore-opening condition in single-walled carbon nanohorns to achieve high capacity in double layer capacitor at high charge-discharge rate: Critical effect of their hierarchical pore structures

We firstly reveal the unique tendency in the capacitance of an electric double layer capacitor (EDLC) made of single-walled carbon nanohorns (SWCNHs), where there is an optimized value of specific surface area (SSA) to achieve the high capacitance value at a high charge-discharge rate. As a result, in the experimental observation, SWCNHs treated by oxidation in air at 550 °C can exhibit 963.4 m2/g of SSA and 286.6 F/g of EDLC capacitance, whereas the ones treated at 500 °C can exhibit 797.5 m2/g and 316.4 F/g at a high scan rate of 100 mV/s, although the SSA-capacitance relation seems ordinal at 5 mV/s. This result is against a common sense that the higher the SSA, the higher the EDLC capacitance. This unique relationship between the SSA of SWCNHs and the capacitance can be explained by consideration of the hierarchical pore structures of SWCNHs, including the internal mesopore zone and inner-core micropore zone, and the diffusion limitation of ionic species in these different structures. These pores are interlinked via the nanowindows opened by the oxidation treatment, and the nanowindow-link can be enhanced by temperature elevation from 500 to 550 °C, while the superficial mesopore can be partially destroyed at 550 °C.