Open carbon nanotube materials with hierarchical porosity and N-doping are prepared from polyaniline nanotubes via a combination method of pre-carbonization and post-KOH activation. The morphology, pore texture and surface properties of the carbon materials are investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), N 2 adsorption, X-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The prepared carbon materials have a typical hierarchical pore texture and very high specific surface area up to 3253 m 2 g À1 . The electrochemical capacitive performance of the prepared carbons was systematically investigated in the 6 M KOH electrolyte. HPCT-4 exhibits high charge storage capacity with a specific capacitance of 365.9 F g À1 at a current density of 0.1 A g À1 , good rate capability of 60% in the range of 0.1-10 A g À1 , and excellent stability over 10 000 cycles. The high capacitive performance could be due to the hierarchical porosity combined with high effective surface area and heteroatom doping effects, resulting in both electrochemical double layer and Faradaic capacitance contributions.

Ultramicroporous carbon materials with uniform pore size accurately adjusted to the dimension of electrolyte ions or CO2 molecule are highly desirable for maximizing specific capacitance and CO2 uptake. However, efficient ways to fine‐tuning ultramicropore size at angstrom level are scarce. A completely new approach to precisely tuning carbon ultramicropore size at sub‐angstrom level is proposed herein. Due to the varying activating strength and size of the alkali ions, the ultramicropore size can be finely tuned in the range of 0.60–0.76 nm as the activation ion varies from Li+ to Cs+. The carbons prepared by direct pyrolysis of alkali salts of carboxylic phenolic resins yield ultrahigh capacitances of up to 223 F g‐1 (205 F cm‐3) in ionic liquid electrolyte, and superior CO2 uptake of 5.20 mmol g‐1 at 1.0 bar and 25 °C. Such outstanding performance of the finely tuned carbons lies in its adjustable pore size perfectly adapted to the electrolyte ions and CO2 molecule. This work paves the way for a new route to finely tuning ultramicropore size at the sub‐angstrom level in carbon materials.

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