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

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.