Enhanced Hydrogen Adsorption in Boron Substituted Carbon Nanospaces ͒ L

THE JOURNAL OF CHEMICAL PHYSICS 131, 164702 ͑2009͒ Enhanced hydrogen adsorption in boron substituted carbon nanospaces ͒ L. Firlej,1,2 Sz. Roszak,3 B. Kuchta,2,4,a P. Pfeifer,2 and Carlos Wexler2 1LCVN, University of Montpellier 2, 34095 Montpellier, France 2Department of Physics and Astronomy, University of Missouri, Columbia, Missouri 65211, USA 3Institut of Physical and Theoretical Chemistry, Wroclaw University of Technology, 50370 Wroclaw, Poland 4Labortoire Chimie Provence, University of Aix-Marseille 1, 13397 Marseille, France ͑Received 14 July 2009; accepted 30 September 2009; published online 23 October 2009͒ Activated carbons are one of promising groups of materials for reversible storage of hydrogen by physisorption. However, the heat of hydrogen adsorption in such materials is relatively low, in the range of about 4–8 kJ/mol, which limits the total amount of hydrogen adsorbed at P=100 bar to ϳ2wt% at room temperature and ϳ8wt% at 77 K. To improve the sorption characteristics the adsorbing surfaces must be modified either by substitution of some atoms in the all-carbon skeleton by other elements, or by doping/intercalation with other species. In this letter we present ab initio calculations and Monte Carlo simulations showing that substitution of 5%–10% of atoms in a nanoporous carbon by boron atoms results in significant increases in the adsorption energy ͑up to 10–13.5 kJ/mol͒ and storage capacity ͑ϳ5wt% at 298 K, 100 bar͒ with a 97% delivery rate. © 2009 American Institute of Physics. ͓doi:10.1063/1.3251788͔ Hydrogen is widely considered an essential part of our presented ͑to the best of our knowledge͒. Specifically, a ma- energy future despite the substantial difficulties derived from jor question is to what extent a single boron atom, substituted its low volumetric energy density. The development of a suit- in graphitic carbon surface, creates high binding energies not able material for reversible storage of hydrogen still remains only locally at the substitution site but also on nearest- a “grand challenge,”1,2 in particular for vehicular neighbor and perhaps even next-nearest-neighbor carbon at- applications.1 Although some materials are potentially attrac- oms. tive, even the most promising candidates, nanoporous acti- This paper, for the first time, presents general/generic vated carbons,3–6 have yet to meet the U.S. Department of evidence—from first-principles ͑ab initio͒ computations of ͑ ͒ ͑ / Energy DOE 2010 targets 0.045 kg H2 kg system, 28 kg adsorption potentials of boron-substituted carbons and grand / 3 7͒ H2 m system at room temperature for light-duty vehicles . canonical Monte Carlo ͑GCMC͒ simulations of hydrogen ad- Recent achievement in engineering carbons nanospaces re- sorption equilibria in the resulting potentials—that boron sulted in preparation of very high surface area materials doping massively increases storage capacities, with as little ͑Ͼ3000 m2 /g͒ performing exceedingly well at cryogenic as a 5%–10% boron concentration. The results make a strong ͑ϳ / 6,8͒ temperature 0.1 kg H2 kg system at P=100 bar . case that it is only a matter of time until superior hydrogen However, the low heat of hydrogen physisorption on carbons storage capacities on boron-doped carbons will be observed ͑4–8 kJ/mol͒ results in low storage capacities at room tem- experimentally. ͑ϳ / ͒ 5,6 perature 0.02 kg H2 kg system at P=100 bar . The The interaction of H2 with modified carbon nanostruc- challenge is, thus, to find ways to increase the interaction of tures is to a large extent controlled by dispersion terms and hydrogen and a carbon substrate. its evaluation requires the proper treatment of the correlation Boron-doped carbons are widely conjectured to be the energy. It can be achieved using one of two available basic suitable materials for hydrogen storage. It is believed that methods, Møller–Plesset13 or coupled cluster14 approaches boron doping raises the binding energy to levels that would ͑the latter is usually considered to be of higher precision͒.In enable room temperature storage at moderate ͑Ͻ100 bar͒ the present study we used the Møller–Plesset approach for pressures. This increase in the binding energy appears to be two reasons. First, the available data15,16 indicate that the caused by boron acting as a p-type dopant, introducing elec- adsorption energy calculated within both approaches differs tron deficiency in the graphite layer planes, thus lowering the by a few percent only while the geometry remains practically 9,10 Fermi level and increasing the surface polarizability. It is unchanged. Second, the Møller–Plesset calculations are less also possible that a partial charge transfer from the occupied computational time consuming than the coupled clusters ␴ orbital of H2 to the empty pz orbital of B increases the method while retaining sufficient precision for our proce- interaction energy between H2 and boron substituted carbon dure, where a reduction in precision is unavoidable when the 11,12 surface. So far, however, neither solid computational evi- potential energy surfaces are being converted into inter- dence ͑i.e., from first principles, beyond Ref. 11 for B-doped atomic potentials for Monte Carlo simulations. fullerenes͒ nor clear experimental evidence for enhanced hy- In view of these considerations, first ab initio calcula- drogen storage capacities of boron-doped carbons have been tions were performed, to get the energy of interactions between H2 and boron atoms substituted in the graphene ma- ͒ a Electronic mail: [email protected]. trix. The interactions of hydrogen molecule with boron 0021-9606/2009/131͑16͒/164702/4/$25.00131, 164702-1 © 2009 American Institute of Physics Downloaded 04 Jun 2010 to 128.206.162.204. Redistribution subject to AIP license or copyright; see http://jcp.aip.org/jcp/copyright.jsp 164702-2 Firlej et al. J. Chem. Phys. 131, 164702 ͑2009͒ modified graphene surface were modeled through interactions of H2 with pyrene or its B-derivative. The external carbon frame of pyrene was kept frozen in B-derivative to better represent the real modified graphite material. The optimized boron position preserves the planar structure. The ab initio studies were performed applying the second order Møller–Plesset level of theory utilizing restricted open Hartree–Fock wave function. The present calculations were performed using the effective core potential SBJKJC VDZ basis set17 embedded by polarization functions d for boron and carbon and p function for hydrogen.18 All the calcula- FIG. 1. ͑Left panel͒ Energy landscape of graphite with central carbon sub- 19 ⌬ / tions were carried out using the GAUSSIAN 03 suite of codes. stituted by boron. Isoenergetic lines are separated by E kB =10 K and go / ͑ ͒ Ϫ ͑ ͒ ͑ ͒ from E kB =−580 K pink gray to 940 K navy . right panel Energy of The H2 molecule forms two perpendicular complexes with adsorption Emin and corresponding distance from the substrate zmin, along the pyrene surface. The minimum energy structures were ͑x,0͒ line. determined for hydrogen interacting with B-center ͓⌬ / ͑ ͒ ͔ E=7.8 kJ mol, R B–H2 =3.12 Å and C-center but it contributes to a dramatic enhancement of the adsorp- ͑⌬ / ͑ ͒ ͒ E=5.6 kJ mol, R C–H2 =3.24 Å . tion over the neighboring hexagons of the graphite lattice ͑in The ab initio energies were used to parametrize the force slightly off-center position͒. As the boron atom is larger than field describing interaction energy between H and the pore ͑␴ ␴ ͒ 2 carbon B =3.54 Å versus C =3.34 Å , there is a local walls. The distribution of adsorption energy in pores was weak maximum of the energy at the boron, and it is located then calculated for various boron contents and the pore farther from the surface ͑Fig. 1, right panel͒. Second, modi- width. The resulting 3D energy grids were implemented in fication of the energy landscape extends far beyond the C␤ GCMC code to perform large scale simulation of hydrogen carbons and is still perceptible ϳ7 Å from the substituted adsorption isotherms. In what follows, we show that the sub- boron. It suggests that the adsorption properties of the whole stitution of just 5% of carbons by boron atoms almost graphene-based surface may be strongly modified by even doubles the strength of hydrogen interaction with the substi- small amounts of substitutional boron atoms. tuted pore wall and may bring the storage capacity of substi- To explore this possibility, we prepared graphene sur- tuted carbon close to the limits required for practical appli- faces containing 1%, 2.5%, 5%, and 10% of boron. The sites cations. to be substituted were chosen at random, but observing a It has been previously suggested that substituting carbon separation of at least three bonds between them. This topo- by boron causes a local distortion of the perfectly planar logical constraint allowed us to use the ab initio results ob- 10 structure of graphite. No such deviation from planarity has tained for an isolated substituted boron in an additive way. been observed in the present ab intio calculations. Hence, in For low substitution ratio the strong adsorption regions the present work we modeled the substituted pore walls as around the boron atoms remain well separated ͑Fig. 2, left flat sheets of graphenelike structures. The hydrogen mol- panel͒. However, already at substitution ratio of 5% these ecules have been modeled by structureless superatoms inter- regions significantly overlap and the average adsorption en- acting via Lennard-Jones ͑LJ͒͑6–12͒ potential. The interac- ergy increases with respect to “pristine” graphite ͑from / ϳ / ϳ ͒ tion hydrogen-adsorbing surface was calculated assuming E kB 600 K to E kB 900 K ; the energy of the strongest the additive LJ-type interaction of H2 super atom with all adsorption site even doubles ͑Fig.

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