An enhanced hydrogen adsorption enthalpy for fluoride intercalated graphite compounds
An Enhanced Hydrogen Adsorption Enthalpy for Fluoride Intercalated Graphite Compounds
Hansong Cheng,*,† Xianwei Sha,† Liang Chen,‡ Alan C. Cooper,† Maw-Lin Foo,† Garret C. Lau,†
Wade H. Bailey III,† and Guido P. Pez†
Air Products and Chemicals, Inc., 7201 Hamilton BouleVard, Allentown, PennsylVania 18195, and Ningbo Instituteof Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, P. R. China
Received September 23, 2009; E-mail: Chengh@airproducts.com
It has been widely recognized that improved hydrogen storage
fluorination levels are accessible, and the nature of the C-F bonding
devices require new lightweight materials capable of interacting with
evolves from ionic to semi-ionic to covalent as the decreasing C:F
molecular hydrogen via either physisorption or chemisorption.1,2 This
interaction must occur with an appropriate strength to enable the capture
We conducted ab initio molecular dynamics (AIMD) simulations
of hydrogen to be reversible under practical operating conditions of
at room temperature on H2 in a GIC containing fluoride anions and
temperature and/or pressure. Porous hydrogen storage materials that
subsequently synthesized fluoride GICs for isotherm measurements.
store H2 via physical adsorption have a perceived advantage for system
The electronic energies were calculated with periodic density functional
heat removal during filling relative to most metal hydrides and chemical
theory (DFT) under the local density approximation (LDA) employing
hydrides due to the relatively low enthalpy associated with physisorp-
the Perdew-Zunger exchange-correlation functional11 coupled with
tion. Most hydrogen storage materials in this class rely on physical
a plane-wave basis set, while the AIMD simulation was performed in
adsorption due to van der Waals forces and/or electron density donation
the constant NVE canonic ensemble using the Nose´ thermostat.12
from the σ orbital of H2 to an electrophile (“Kubas-type binding”).3
Details of the computational method, material synthesis, and charac-
Here, we report a study of H2 adsorption in graphite intercalation
terization can be found in the Supporting Information. The selected
compounds (GICs) containing fluoride anions. In contrast to Kubas-
unit cell in our simulations contains 32 C and 8 F atoms. For maximum
type binding and alkali metal GICs,4 the dominant mechanism of
capacity, stage-1 GICs were studied with 1, 2, 12, and 24 H2 molecules
hydrogen binding in these materials is donation of electron density
per unit cell, corresponding to 0.37-8.22 wt % H2.
from a nucleophile to the σ* orbital of H2. We show that thesecompounds exhibit a higher heat of adsorption at near-ambienttemperatures than other porous H2 storage materials, such as activatedcarbons and metal organic framework (MOF) compounds, and thuscould potentially serve as practical H2 storage media.
In gas-phase calculations, “naked” fluoride anions5 and charge-
separated ammonium fluoride salts6 have been shown to interactstrongly with molecular hydrogen. Sweany et al. have reported anexperimental characterization of adducts of H2 with CsF ion pairs.7The normally infrared-silent H2 was perturbed by the salt, yieldingspectra interpreted as arising from H2 and CsF adducts, with as manyas 3 H2 molecules per CsF, where H2 interacts primarily with the F-anion. Based on frequency shifts showing a lengthening of the H-Hbond, the interaction was attributed to the donation of electron density
Figure 1. Fully optimized structures of the partially fluorinated GIC (a)
from the F- anion to the H2 antibonding σ* orbital.
and the GIC with 2 H2 (b), 12 H2 (c), and 24 H2 (d).
Fluorous MOF containing covalent C-F bonds exhibited a large
Table 1. H2 Gravimetric Density, the Calculated Lattice Spacing,
2 uptake at 77 K, but unusually high H2 adsorption enthalpies were
the Average Bader Charge on F, and the Average H
not reported.8 Thus, the conceptual challenge is to devise solid-state
materials which incorporate both the strong hydrogen interactions of“naked” fluoride and sufficient porosity. Chemical intercalation of
graphite has been demonstrated to increase the H
by creating porosity through intercalation and separation of the
graphene layers as well as increasing the hydrogen affinity of the
graphite through increased electron density. For example, the second-
stage graphite intercalation complex KC24 has been shown to adsorb2H2/K at 77 K corresponding to 1.2 wt % H2.9 Electron transfer fromK to the graphene layers in this “donor type” GIC increases the heat
Upon F- intercalation, significant lattice expansion (∼2.3 Å) occurs,
of adsorption from -4 kJ/mol in native graphite to ca. -10 kJ/mol in
based upon the DFT structural optimization, and the intercalated F
KC24. To date, there have been no examples of “acceptor type” GICs
atoms form long C-F bonds with the graphene sheets. Compared to
with a demonstrated appreciable hydrogen adsorption. Graphite
the typical C-F bond lengths in organofluorine compounds and
fluorides represent a well-studied subset of acceptor GICs. A range of
perfluorinated graphite (∼1.35 Å), the C-F bonds in the partiallyfluorinated graphite GIC are considerably longer, ranging from 1.417
to 1.533 Å. The significantly elongated, semi-ionic bonds arise from
bond angle distortions resulting from the partial fluorination as only
17732 9 J. AM. CHEM. SOC. 2009, 131, 17732–17733 10.1021/ja907232y 2009 American Chemical Society C O M M U N I C A T I O N S
some of the C atoms change their electronic configuration from sp2 to
of the GIC at near-ambient temperatures and the heat of adsorption
sp3. As shown in Table 1, Bader population analysis indicates that the
are comparable to an MOF incorporating coordinatively unsaturated
average charge (QF) on the F atoms ranges from -0.655 to -0.659.
transition metals18 despite the extremely large difference in surface
At low H2 loadings, the spacing between adjacent graphene layers
shrinks slightly since the H2 molecules interact more strongly with
Besides the well-known LDA overbinding,19 the gap between the
the semi-ionic F atoms (Table 1). At higher H2 loadings, however,
calculated energies at low loading and the experimental heat of
the interlayer distance increases substantially to accommodate the large
adsorption is likely due to the presence of residual BF4 in the
number of H2 molecules. Figure 1 displays the fully equilibrated
as-prepared GIC, which dilutes the gravimetric hydrogen density and
structures of partially fluorinated GIC and GIC with 2, 12, and 24 H2
adsorption enthalpy. After measuring isotherms on a large number of
molecules in the unit cell. A detailed analysis of the AIMD trajectories
CxFy(BF4)1-y samples, we found the isosteric heat of adsorption to be
indicates that H2 molecules in the lattice interact with the fluoride ions
very sensitive to both the C:F ratio and the amount of residual BF4.
but are highly mobile in the interlayer spaces at 300 K. The H2
These variables likely affect the semi-ionic/covalent nature of the C-F
molecules are confined to the interlayer spaces between F atoms and
bonds and accessibility of H2 to the intercalated fluoride ions. It is
are rarely in close contact with graphene sheets. No dissociative
apparent from the surface areas and H2 uptake that only a fraction of
chemisorption of H2 was observed in the entire course of AIMD
the fluoride ions in the samples are accessible to H2.
simulations. Our results suggest that the calculated average H2
In summary, we propose a novel class of “acceptor type” GICs,
adsorption energy decreases as H2 loading increases. At high H2
which based on theoretical predictions and experimental evidence, can
H2 repulsion is largely responsible for the decrease
exhibit significantly higher isosteric heats of adsorption for H2 than
of H2 adsorption energy. At lower H2 loadings, however, the simulation
previously demonstrated for commonly available, porous carbon-based
suggests that the average adsorption energy is substantially higher than
materials. The unusually strong interaction with H2 arises from the
those observed in most porous carbons.
semi-ionic nature of the C-F bonds. Although high H2 storage capacity(>4 wt %) at near-ambient temperatures may not be feasible due todiminished heats of adsorption at very high H2 densities, enhancedstorage properties can be envisaged by doping the graphitic host withappropriate species (e.g., nitrogen) to promote higher levels of chargetransfer from graphene to F- anions. Synthetic strategies for increasingaccessibility of hydrogen to the intercalated anions are also underdevelopment. Acknowledgment. The authors gratefully acknowledge funding
for this work provided by the U.S. DOE’s Office of Energy Efficiencyand Renewable Energy via the Hydrogen Sorption Center of Excellence(Contract DE-FC-05GO15074). Note Added after ASAP Publication. Reference numbering was Supporting Information Available: Computational and experimental Figure 2. Hydrogen isotherms measured on C22F0.2(BF4)0.8 at 273, 298,
details and the MD trajectory for 12 H2 molecules in the partially fluorinated
and 313 K and the derived isosteric heat of adsorption.
GIC lattice in AVI format. This material is available free of charge via
To ascertain if partially fluorinated graphites are capable of
the Internet at http://pubs.acs.org.
adsorbing H2, we synthesized a number of first- and second-stage
fluoride GIC samples and subsequently measured H2 isotherms andderived the isosteric heats of adsorption. Graphite fluoroborates, C
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J. AM. CHEM. SOC. 9 VOL. 131, NO. 49, 2009
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