GAS ADSORPTION IN GRAPHENE OXIDE FRAMEWORKS AND ENGINEERED CARBON NANOSPACES

 

Jacob Burress1, Jason Simmons1, Taner Yildirim1, Matthew Beckner2, Peter Pfeifer2

1)      NIST Center for Neutron Research

2)      ALL-CRAFT, Physics and Astronomy Department,

 

Vehicular storage of gaseous fuels is a key enabling technology for the two pillars of a non-petroleum based transportation economy as envisaged under various federal and state alternative fuel plans: 1) natural-gas vehicles (internal combustion engines) and 2) hydrogen fuel cell vehicle. Our research focuses on the development of high-capacity storage materials for natural gas (methane) and molecular hydrogen in on-board fuel tanks for next-generation clean vehicles.  Two different carbon-based materials are being investigated: lignocelluloses precursor (corn cob) activated carbons and graphene oxide frameworks (GOFs). Adsorbent-based storage materials are attractive due to their low operating pressure (relative to compressed gas), reversibility, ease of fueling, and absence of thermal management issues. Nanopores generate high storage capacities by high surface area to volume ratios, and by hosting deep potential wells through overlapping substrate potentials from opposite pore walls, with binding energies approximately twice the binding energy in wide pores.  Additional functionalization by boron is also being investigated.

 

The activated carbons are produced in a multi-step process using KOH activation using corncob as the precursor. Only the center of the cob is used for carbon synthesis, leaving the kernels for food or ethanol production.  These carbons have surface areas of up to 3500 m2/g, porosities of up to 0.8.  The activated carbons have exceptional storage capacities (~80 g H2/kg carbon, ~50 g H2/liter carbon, at 50 bar and 77 K and 115-119 g CH4/L carbon at 35 bar and ambient temperature).  Further, The H2 gas-carbon surface interface physics was explored using supercritical hydrogen isotherms [77 K and 293 K, < 100 bar].  Experimental case studies in which 40% of all surface sites reside in pores of width ~0.7 nm and binding energy ~9 kJ/mol, and 60% of sites in pores of width >1.0 nm and binding energy ~5 kJ/mol, are also presented. 

 

Additionally, there has been a recent resurgence in graphene oxide research as a potential route to large scale graphene synthesis. The dehydration reactions of boronic acids for the formation of covalent organic frameworks (COFs) and other new nanoporous materials have also driven recent material research. The GOFs are synthesized by linking the OH groups on graphene oxide with benzene-boronic acids.  Initial powder x-ray diffraction and prompt gamma neutron activation analysis studies indicate that the benzene-boronic acids are successfully incorporated into graphene-oxide (GO) layers expanding the interlayer spacing up to 11 Angstroms. Our research also found that the amorphous phases of bare dehydrated benzene-boronic acid polymers (amorphous borocarbons, ABCs) show unusual hydrogen adsorption behavior. Diffusion of hydrogen into the sample is thermally activated. The rate of hydrogen adsorption is extremely slow at 30 K and increases with temperature increase.  The apparent maximum of hydrogen storage on reasonable time scales takes place ~ 70 K.  Detailed high-pressure isotherms of H2/CO2/Methane at different temperatures of these interesting new GOF and ABC materials will be presented.