High volumetric and energy densities of methane stored in nanoporous materials at ambient temperatures and moderate pressures

Experimental results for methane adsorption on two high-surface area carbons (TE7-20 and AX-21) and one metal-organic framework (MIL-101(Cr)) are presented, with isotherms obtained at temperatures ranging from 250 to 350 K and at pressures up to 15 MPa. The isotherms were analysed to determine if these materials could be viable alternatives for on-board solid-state storage of methane. The results show a very high adsorbate density in the pores of all materials, which for some can even exceed liquid methane density. At moderate pressures below 5 MPa, the calculated total energy densities are close to the energy density of methanol, and are almost 40 % of the energy density of gasoline (petrol). Compared with standard compression at the same conditions, the results show that adsorption can be a competitive storage alternative, as it can offer equal volumetric capacities at much lower pressures, hence reducing the energy penalty associated with compression. It is shown that the optimal conditions for adsorptive methane storage in these materials are at moderate pressure ranges, where the gains in amounts stored when using an adsorbent are more pronounced when compared to cylinders of compressed methane gas at the same operating conditions. Finally, a study on deliverable capacities for adsorbed methane was carried out, simulating two charging pressure scenarios of 3.5 and 6.5 MPa and discharge at 0.5 MPa. The results show that some of the tested materials have high working volumetric capacities, with some materials displaying more than 140 kg m-3 volumetric working capacity for charging at 6.5 MPa and delivery at 0.5 MPa.