NANO LETTERS
Molecular Simulation of Novel Carbonaceous Materials for Hydrogen Storage
2004 Vol. 4, No. 8 1489-1492
Dapeng Cao,† Pingyun Feng,‡ and Jianzhong Wu*,† Department of Chemical and EnVironmental Engineering, and Department of Chemistry, UniVersity of California, RiVerside, California 92521 Received June 4, 2004; Revised Manuscript Received July 1, 2004
ABSTRACT We predict by using molecular simulation that a novel class of carbonaceous materials called graphitic carbon inverse opal (GCIO) provides an excellent absorbent for hydrogen storage at room temperature. Simulation results based on well-calibrated force fields show that the gravimetric delivery amount of hydrogen at T ) 298 K and p ) 30.25 MPa achieves up to 5.9 wt % when the diameter of the spherical cavity in the GCIO materials is 1.78 nm. The corresponding volumetric delivery capacity of hydrogen reaches up to 50 kg/m3, very close to the target set by the Department of Energy (DOE). A major advantage of the GCIO materials is that their mass-production is technically accessible, which makes them a good candidate for inexpensive storage of hydrogen in future automobile vehicles.
Hydrogen as a futuristic energy resource free of air pollution and greenhouse effects has attracted a great deal of attention in recent years. Among a number of challenges in prevalent utilization of hydrogen energy, a pressing task is to store hydrogen efficiently. The problem is particularly critical for the development of a new generation of cost-effective hydrogen-fuelled automobiles.1 Despite numerous research efforts on hydrogen storage,2-8 efficient techniques and materials are yet to be developed to attain high storage capacity.9 Here we predict by using molecular simulation that a novel class of carbonaceous materials called graphitic carbon inverse opal (GCIO) is highly promising for hydrogen storage at room temperature. One major advantage of these graphitic materials is that they are economically very accessible for mass production.10-12 The novel GCIO materials were first fabricated by Zakhidov et al.10 using a synthesis route resembling the geological formation of natural opal. These porous materials consist of a three-dimensional periodic network of spherical cavities connected by microchannels. The inverse space of spherical cavities was filled with graphitic carbon. Because their original goal was to fabricate photonic crystals with optical wavelengths, Zakhidov et al.10 synthesized GCIO materials using spherical SiO2 particles of hundreds of nanometers in diameter as the template. However, a recent report13 shows that polysilsesquioxane colloids with the diameters of 7∼9 nm can also be used as the template for * Corresponding author. E-mail:
[email protected]. † Department of Chemical and Environmental Engineering. ‡ Department of Chemistry. 10.1021/nl0491475 CCC: $27.50 Published on Web 07/24/2004
© 2004 American Chemical Society
the preparation of GCIO materials. In this work, the adsorption of hydrogen in GCIO materials containing uniform spherical cavities was investigated by molecular simulation using well-established force fields. Grand canonical Monte Carlo (GCMC) simulations14 were used to calculate the adsorption of hydrogen in the GCIO materials. A unit cell of face-centered-cubic structure in the close-packed state was employed as the simulation box, which contains eight one-eighth-spheres and six hemispheres of cavities. Periodic boundary conditions were imposed in all three directions (i.e., x, y, z directions). Figure 1 shows a snapshot of the adsorbed hydrogen molecules and the novel GCIO material in the unit cell. This material has a porosity of 74% and the carbonaceous material occupies the remaining volume.15,16 In the simulations, the GCIO material is assumed to be perfectly rigid free of structure changes with pressure. The pair interaction between hydrogen molecules is represented by the standard cut-shifted Lennard-Jones (LJ) potential,17 which is able to reproduce the vapor-liquid phase behavior of hydrogen gas over a broad range of conditions.18 The interaction between a hydrogen molecule and the GCIO material was described by the summation of all the pair interactions between a hydrogen molecule and a carbon atom φHC )
∫σ5σ ∫0π ∫02πFcφLr2 sinθ dr dθ dφ HC
(1)
HC
where Fc ) 114 nm-3 is the number density of carbon atoms in the graphite,19,20 σHC ) (σH2 + σC)/2 is the collision diameter, and φL is the London dispersion attraction between
Table 1. Molecular Parameters Used in the GCMC Simulations H217 a
carbon19
ionization potential29
polarizability 29
σHH (nm)
HH/k (K)
σCC (nm)
CC/k (K)
IH (eV)
IC (eV)
R0H(4π0) m3
R0C(4π0) m3
0.296
34.2
0.34
28.0
13.5984
11.2603
0.804 × 10-30
1.76 × 10-30
a It has been shown that these parameters yield accurate P-V-T behavior of hydrogen over a broad range of temperatures and pressures.18 A slightly different set of parameters for hydrogen (σHH ) 0.297, HH ) 42.8 K) has been reported in the recent literature.30,31 A larger energy parameter would lead to even higher delivery amount of hydrogen.
Figure 1. Snapshot of hydrogen molecules adsorbed in the GCIO porous material. The light-blue network stands for the novel carbonaceous material, yellow spheres are hydrogen molecules, and the red framework is the unit cell to guide the eye.
a hydrogen molecule and a carbon atom. The dispersion potential is given by21
{
0
Case 1 (2)
Figure 2. Effect of pressure on the hydrogen gravimetric (a) and volumetric (b) delivery amounts at 298 K. From top to bottom, the diameters of spherical cavity are 1.78, 2.37, 2.96, 4.14, 5.92 nm, respectively.
In Case 1, the center of a hydrogen molecule is located in the spherical cavity; in Case 2 the center of a hydrogen molecule is located within the carbonaceous materials. In eq 2, 0 ) 8.854 × 10-12 C2/Jm is the permittivity of free space, IH and IC are the ionization potentials of a hydrogen molecule and a carbon atom, respectively, and RC0 and RH0 are the corresponding polarizabilities. Table 1 lists all the molecular parameters used in this work. Adsorption of hydrogen in a porous material is conventionally characterized by the gravimetric capacity defined as the mass of adsorbed hydrogen divided by the overall mass. The delivery amount is the mass of adsorbed hydrogen at the storage or working pressure minus that at the discharge pressure. Figure 2a shows the gravimetric delivery amount of hydrogen stored on the GCIO materials calculated from GCMC. Here the discharge pressure is assumed 0.1 MPa.
When the diameters of spherical cavities are 1.78 and 2.37 nm, respectively, the delivery amounts of hydrogen are 5.9 wt % and 5.6 wt % at 30.25 MPa and room temperature, very close to the target (6.0 wt %) set by the U.S. Department of Energy (DOE) for hydrogen-powered vehicles. For a better insight into the adsorption properties of the GCIO materials, the volumetric delivery capacity of hydrogen at room temperature is shown in Figure 2b. The volumetric capacity is defined as the mass of adsorbed hydrogen divided by the pore volume of the GCIO materials. It can be found that the volumetric delivery capacity of hydrogen reaches 50 kg/m3 at the diameter of the spherical cavity σ ) 1.78 nm and P ) 30.25 MPa, which is also close to the target (62 kg/m2) set by the DOE. Both the gravimetric and volumetric delivery capacities are much higher than those for similar carbonaceous material reported before.6,22
φL )
1490
1 IH‚IC Case 2 ‚ ‚ -1.5‚ (4π0) r6 (IH + IC) RH0
RC0
Nano Lett., Vol. 4, No. 8, 2004
Figure 3. Effect of the GCIO pore diameter on the amount of hydrogen delivery. Here the temperature is 298 K and the pressures are, from top to bottom, 30.25, 27.38, 24.75, 22.35, 20.16, 18.17 MPa, respectively.
typical activated carbon24 and carbon nanotube.25,26 If a binary colloid mixture of different size, rather than uniform particles, is used as the template for the fabrication of GCIO materials,27 larger specific surface areas would be attainable. That will lead to even higher hydrogen delivery amounts. As investigated by Frenkel and co-workers28 on the phase diagram of dissimilar colloid particles, a number of more complicated ordered structures are possible. These novel colloidal structures may be utilized as the templates for the fabrication of more efficient GCIO materials for hydrogen storage. In summary, this work suggests that a new class of carbonaceous materials possesses high delivery capacity for hydrogen storage at room temperature. The gravimetric and volumetric delivery capacities reach up to 5.9 wt % and 50 kg/m3 respectively; both are very close to the targets set by the DOE for hydrogen-powered vehicles. Since mass production of these materials is feasible from both technical and economical points of view, large-scale applications of these materials may provide a viable approach for inexpensive hydrogen storage. In the simulations, several simplifying assumptions are made and an idealized model is used for the GCIO materials. While all of these simplifications are very reasonable and have been extensively used in the computational literature, they could affect the accuracy of the predicted results. As a result, it will be extremely interesting to see the experimental test of the calculated results. Acknowledgment. This work was partially funded by the National Science Foundation (CTS0406100 and CTS0340948). References
Figure 4. Specific surface area of the GCIO porous materials declines monotonically with the cavity diameter (σ). Triangle points are specific surface areas of the five cases studied in this work and the solid line is to guide the eye.
Apparently, the diameter of the spherical cavity plays an important role in hydrogen storage, especially at the intermediate pressures (5∼10 MPa). At p ) 5 MPa, the GCIO material with a cavity diameter of σ ) 1.78 nm absorbs up to 3 wt % of hydrogen. However, the same material adsorbs only 1.7 wt % of hydrogen at σ ) 5.92 nm. As shown in Figure 3, the gravimetric delivery amount of hydrogen declines as the diameter of the spherical cavity increases, and the reduction follows approximately a linear relation. For the cavity diameter equal to 9 nm, the GCIO material can deliver 3.3 wt % of hydrogen at T ) 298 K and P ) 30 MPa. The high gravimetric capacity of the GCIO materials is closely related to their extraordinarily large specific surface area,23 defined as the total surface area divided by the mass of graphitic carbon. Figure 4 shows the specific surface area of the GCIO materials as a function of the cavity diameter. For the GCIO materials with the cavity diameters of σ ) 1.78, 2.37, and 2.96 nm, the specific surface areas are 4248, 3190, 2554 m2/g, respectively, much larger than that for a Nano Lett., Vol. 4, No. 8, 2004
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NL0491475
Nano Lett., Vol. 4, No. 8, 2004
