High-surface-area biocarbon for reversible on-board storage of natural gas and hydrogen

An overview is given of the development of advanced nanoporous carbons as storage materials for natural-gas (methane) and molecular hydrogen in on-board fuel tanks for next-generation clean automobiles.' The carbons are produced in a multi-step process from corncob, have surface areas of up to 3500 m(2)/g, porosities of up to 0.8, and reversibly store, by physisorption, record amounts of methane and hydrogen. Current best gravimetric and volumetric storage capacities are: 250 g CH4/kg carbon and 130 g CH4/liter carbon (199 V/V) at 35 bar and 293 K; and 80 g H-2/kg carbon and 47 g H-2/liter carbon at 47 bar and 77 K. This is the first time the DOE methane storage target of 180 V/V at 35 bar and ambient temperature has been reached and exceeded. The hydrogen values compare favorably with the 2010 DOE targets for hydrogen, excluding cryogenic components. A prototype adsorbed natural gas (ANG) tank, loaded with carbon monoliths produced accordingly and currently undergoing a road test in Kansas City, is described. A preliminary analysis of the surface and pore structure is given that may shed light on the mechanisms leading to the extraordinary storage capacities of these materials. analysis includes pore-size distributions from nitrogen adsorption isotherms; spatial organization of pores across the entire solid from small-angle x-ray scattering (SAXS); pore entrances from scanning electron microscopy (SEM) and transmission electron microscopy (TEM); H-2 binding energies from temperature-programmed desorption (TPD); and analysis of surface defects from Raman spectra. For future materials, expected to have higher H-2 binding energies via appropriate surface functionalization, preliminary projections of H-2 storage capacities based on molecular dynamics simulations of adsorption of H-2 on graphite, are reported.