Jacob Burress

Hydrogen storage in engineered carbon nanospaces

It is shown how appropriately engineered nanoporous carbons provide materials for reversible hydrogen storage, based on physisorption, with exceptional storage capacities (approximately 80 g H2/kg carbon, approximately 50 g H2/liter carbon, at 50 bar and 77 K). Nanopores generate high storage capacities (a) by having high surface area to volume ratios, and (b) by hosting deep potential wells through overlapping substrate potentials from opposite pore walls, giving rise to a binding energy nearly twice the binding energy in wide pores. Experimental case studies are presented with surface areas as high as 3100 m(2) g(-1), in which 40% of all surface sites reside in pores of width approximately 0.7 nm and binding energy approximately 9 kJ mol(-1), and 60% of sites in pores of width>1.0 nm and binding energy approximately 5 kJ mol(-1). The findings, including the prevalence of just two distinct binding energies, are in excellent agreement with results from molecular dynamics simulations. It is also shown, from statistical mechanical models, that one can experimentally distinguish between the situation in which molecules do (mobile adsorption) and do not (localized adsorption) move parallel to the surface, how such lateral dynamics affects the hydrogen storage capacity, and how the two situations are controlled by the vibrational frequencies of adsorbed hydrogen molecules parallel and perpendicular to the surface: in the samples presented, adsorption is mobile at 293 K, and localized at 77 K. These findings make a strong case for it being possible to significantly increase hydrogen storage capacities in nanoporous carbons by suitable engineering of the nanopore space.

HIGH-SURFACE-AREA BIOCARBONS 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 nextgeneration 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 H2/kg carbon and 47 g H2/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. The 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); H2 binding energies from temperature-programmed desorption (TPD); and analysis of surface defects from Raman spectra. For future materials, expected to have higher H2 binding energies via appropriate surface functionalization, preliminary projections of H2 storage capacities based on molecular dynamics simulations of adsorption of H2 on graphite, are reported.

Complex pore spaces create record-breaking methane storage system for natural-gas vehicles

P. Pfeifer, L. Aston, M. Banks, S. Barker, J. Burress, S. Carter, J. Coleman, S. Crockett, C. Faulhaber, J. Flavin, M. Gordon, L. Hardcastle, Z. Kallenborn, M. Kemiki, C. Lapilli, J. Pobst, R. Schott, P. Shah, S. Spellerberg, G. Suppes, D. Taylor, A. Tekeei, C. Wexler, and M. Wood University of Missouri, Columbia, Missouri 65211, USA P. Buckley, T. Breier, J. Downing, S. Eastman, P. Freeze, S. Graham, S. Grinter, A. Howard, J. Martinez, D. Radke, and T. Vassalli Midwest Research Institute, Kansas City, Missouri 64110, USA J. Ilavsky Argonne National Laboratory, Argonne, Illinois 60439, USA Received 27 August 2007; published online 27 December 2007 DOI: 10.1063/1.2786007

Combination volumetric and gravimetric sorption instrument for high accuracy measurements of methane adsorption

The accurate measurement of adsorbed gas up to high pressures (∼100 bars) is critical for the development of new materials for adsorbed gas storage. The typical Sievert-type volumetric method introduces accumulating errors that can become large at maximum pressures. Alternatively, gravimetric methods employing microbalances require careful buoyancy corrections. In this paper, we present a combination gravimetric and volumetric system for methane sorption measurements on samples between ∼0.5 and 1 g. The gravimetric method described requires no buoyancy corrections. The tandem use of the gravimetric method allows for a check on the highest uncertainty volumetric measurements. The sources and proper calculation of uncertainties are discussed. Results from methane measurements on activated carbon MSC-30 and metal-organic framework HKUST-1 are compared across methods and within the literature.