ARTICLE pubs.acs.org/JPCC
Local Atomic Density of Microporous Carbons Wojtek Dmowski,*,† Cristian I. Contescu,‡ Anna Llobet,§ Nidia C. Gallego,‡ and Takeshi Egami†,‡,^ †
Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Lujan Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ^ Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, United States ‡
ABSTRACT: We investigated the structure of two disordered carbons: activated carbon fibers (ACF) and ultramicroporous carbon (UMC). These carbons have highly porous structure with large surface areas and consequently low macroscopic density that should enhance adsorption of hydrogen. We used the atomic pair distribution function to probe the local atomic arrangements. The results show that the carbons maintain an in-plane local atomic structure similar to regular graphite, but the stacking of graphitic layers is strongly disordered. Although the local atomic density of these carbons is lower than graphite, it is only ∼20% lower and is much higher than the macroscopic density due to the porosity of the structure. For this reason, the density of graphene sheets that have optimum separation for hydrogen adsorption is lower than anticipated.
’ INTRODUCTION A multitude of porous materials have been evaluated in search for an efficient adsorbent of hydrogen at near-ambient temperatures. Adsorptive storage of hydrogen has been studied in activated carbon and carbon fibers, carbon nanotubes, fullerenes, metalorganic frameworks, inorganic oxides, and porous polymers. However, so far none of these materials meet the desired target for storage of 5.5 wt %. Recently, carbons with porous structure enhanced through chemical activation have attracted attention. It was reported1,2 that a chemical activation with alkali hydroxides produced materials with well-defined pore sizes and with good hydrogen adsorption properties. The resulting carbon has highly porous structure with a very large surface area. According to studies,3 the strongest interaction between molecular hydrogen and carbon occurs in narrow micropores
