Longitudinal unzipped carbon nanotubes with high specific surface area and trimodal pore structure

Abstract

This study reports unzipped carbon nanotubes (CNTs) with a trimodal (micro–meso–macro) pore structure using KOH as the activating agent. It is possible to unzip CNTs under severe conditions (in our study, CNT (C)/KOH = 1 : 10 (w/w) at 1000 °C) in contrast to the surface activation of CNTs under general conditions (in our study, C/KOH = 1 : 4 (w/w) at 900 °C). After severe alkali activation, various pores were initially formed on the surface. Subsequently, a longitudinally unzipped structure was obtained as the individual pores connected. In contrast with other methods used to prepare unzipped and porous CNTs, this method is economical and scalable because it enables a one-step synthesis of unzipped and porous CNTs. As per the non-localized density functional theory, the distribution of micro–meso pores provides evidence of unzipping because the peak for pore sizes <1 nm, measured from the partially opened tips of the pristine CNTs, was broadened. The perfectly opened tips observed after activation indicate that the micropores on the unzipped structure increased. In addition, the results indicated that the unzipped porous CNTs exhibited a trimodal pore structure. This structure resulted in increased specific surface area as well as energy storage and adsorption capacities. The maximum energy density of the unzipped porous CNTs in ultracapacitors based on an organic electrolyte was 50 W h kg⁻¹. Thus, the method is suitable for fabrication of unzipped porous CNTs, which demonstrate potential as electrode materials for ultracapacitors.