Nanospace engineering of porous carbon for gas storage
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] 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. High specific surface areas, porosities, and sub-nm/supra-nm pore volumes are quantitatively selected by controlling the degree of carbon consumption and metallic potassium intercalation into the carbon lattice during the activation process. Tunable bimodal pore-size distributions of sub-nm and supra-nm pores are established by subcritical nitrogen adsorption, small-angle x-ray scattering, and transmission electron microscopy. Optimal pore structures for gravimetric and volumetric gas storage, respectively, are presented. We present an experimental study of the prediction that substitution of surface carbon with boron (“boron doping”) increases the binding energy for hydrogen and, therefore, enhances hydrogen adsorption (electron donation from H2 to electron-deficient boron). Microscopic Fourier transform infrared spectroscopy establishes the presence of B-C bonds. A sample with 8 wt% boron exhibits a 30% increase in excess adsorption per surface area at 303 K and 200 bar, relative to the undoped parent material. Methane and hydrogen adsorption isotherms up to 250 bar on monolithic and powdered activated carbons are reported and validated, using several gravimetric and volumetric instruments. Current best gravimetric and volumetric storage capacities are: 256 g CH4/kg carbon and 132 g CH4/liter carbon at 293 K and 35 bar; 26, 44, and 107 g H2/kg carbon at 303, 194, and 77 K respectively and 100 bar. Adsorbed film density, specific surface area, and binding energy are analyzed separately using the Clausius-Clapeyron equation, Langmuir model, and lattice gas models. This material is based upon work supported by the U.S. Department of Energy under Award No. DE-FG36-08GO18142, U.S. Department of Defense under Award No. N00164-08-C-GS37, and California Energy Commission under Contract No. 500-08-022.
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