Cooperative Adsorption of Supercritical CH₄ in Single-Walled Carbon Nanohorns for Compensation of Nanopore Potential

Abstract

High-density CH₄ storage using adsorption techniques is an important issue in the use of CH₄ as a clean energy source. The CH₄ adsorption mechanism has to be understood to enable innovative improvements in CH₄ adsorption storage. Here, we describe the adsorption mechanism, based on CH₄ structure, and stabilities in the internal and external nanopores of single-walled carbon nanohorns, which have wide and narrow diameters, respectively. The adsorption of larger amounts of CH₄ in the narrow nanopores at pressures lower than 3 MPa was the result of strong adsorption potential fields; in contrast, the wider nanopores achieve higher-density adsorption above 3 MPa, despite the relatively weak adsorption potential fields. In the wider nanopores, CH₄ molecules were stabilized by trimer formation. Formation of CH₄ clusters therefore compensates for the weak potential fields in the wider nanopores and enables high-density adsorption and adsorption of large amounts of CH₄