Effect of the size and position of ion-accessible nanoholes on the specific capacitance of single-walled carbon nanohorns for supercapacitor applications

We explore the importance of the size and position of nanoholes on the electrochemical performance of single-walled carbon nanohorn (SWCNH)-based supercapacitors using an ionic liquid electrolyte. The oxidized sample at 673 K showed a low specific capacitance per unit of internal specific surface area (4.0 μF cm^-2), as the nanoholes created on the tips of SWCNHs via a selective chemical attack are too small to introduce electrolyte ions. For a sample oxidized at 723 K, the enlarged diameter of the nanoholes on the tips allows electrolyte ions to penetrate into the internal spaces of the SWCNHs, thereby resulting in a 2-fold capacitance improvement (8.6 μF cm^-2). However, the abrupt decrease in the capacitance of the oxidized SWCNHs at 823 K (3.8 μF cm^-2) can be explained by the selective formation of nanoholes on the sidewalls of the SWCNHs, where the small interstitial pores restrict ion diffusion to deeply positioned nanoholes on the sidewalls of the SWCNHs. Our study clearly reveals that the size and position of nanoholes with regard to ion accessibility are crucial factors to improve the capacitive performance of SWCNH-based supercapacitors.