Pore structures and electrochemical performances of mass-produced metal-free single-walled carbon nanotubes (mf-SWCNTs) heated in either CO2 gas or dry air were investigated. Pore structures, defects, and electrochemical performances of mf-SWCNTs were analyzed mainly using the N2 adsorption method, apparent density measurement, Raman spectroscopy, and cyclic voltammogram. mf-SWCNT sheets heated in CO2 gas showed a remarkable increase of the specific surface area up to 1900 m2 g−1, but sheets heated in air showed an increase of only up to 1400 m2 g−1. Propan-2-ol, 4-methyl-1,3-dioxolan-2-one, or ionic liquid was impregnated in the inner space of mf-SWCNTs to confirm whether the inner space of mf-SWCNTs is filled with each liquid. mf-SWCNTs heated in air were completely filled, but mf-SWCNTs heated in CO2 were partially filled. From these results, it is estimated that holes of the size of 0.56 nm or larger are opened on the side wall of mf-SWCNTs. A combination of two heat treatments maximized the effective...