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Nanotechnology

Novel carbon materials can store and sieve hydrogen

Carbon-based nanostructures are potential H2 storage devices for automotive applications due to their light weight and useful mechanical and chemical properties.
4 May 2007, SPIE Newsroom. DOI: 10.1117/2.1200704.0714
Global warming and the shortage of fossil fuels provide motivation for the replacement of traditional combustion engines. The electrochemical oxidation of molecular hydrogen is an efficient, sustainable, and environmentally friendly alternative for automotive applications. When implemented with modern fuel cells, this technology offers high energy efficiency in an effectively pollution-free process. Given a hydrogen economy, the biggest challenge for this technology is H2 gas storage. At ambient conditions, a liter of hydrogen gas contains less than 0.1% of the energy in the same volume of n-octane or gasoline.
The current prototype H2 tanks are high-pressure (700bar) bottles and liquid hydrogen cryodevices. Both solutions are awkward for routine mobile applications, prompting an ongoing search for alternative storage materials.1,2 The most likely possibilities are materials in which the H2 is either chemisorbed or physisorbed. In chemisorption, H2 is chemically bound in a host material, such as a metal hydride. In physisorption the gas is bound to the host through weak van der Waals forces. Chemisorption is often hampered by slow loading and/or unloading due to activation barriers for the chemical reactions involved. Devices based on H2 physisorption have thus far demonstrated insufficient mechanical stability and persistence as well low volumetric storage capacities, although the latter is debated.2,3
We aim to better understand the guest-host interaction of carbon nanostructures with H2 and other gases, as such nanostructures are among the most promising physisorption-based, lightweight H2 storage materials. Our work predicts maximum theoretically possible storage capacities as well as suggests new materials that may be useful as H2 storage media.

Figure 1. The equlilibrium constant Keq for H2 physisorbed in a slit pore is plotted against the pore size for different temperatures.
Methodology
We employ multi-scale simulation techniques to investigate the H2-host system and its properties. Using an approximate density-functional scheme with van-der-Waals corrections,4,5 we determine the mechanical and structural properties of carbon nanostructures. Then, we model H2 interactions with the host using an additive Rutherford potential, with parameters fitted to high-level ab initio calculations (second order M⊘ller-Plesset perturbation theory, coupled-cluster with single and double perturbative triple excitations) on model systems. We solve the stationary Schrödinger equations for the motion of the H2 center of mass inside the nanostructure on a numerical grid, with periodic boundary conditions applied. We neglect the internal degrees of freedom of the hydrogen molecule and interactions between the molecules. The result is the the energy spectrum εi of bound H2, together with corresponding eigenstates Ψi, determined in the ideal gas approximation. The equilibrium constant Keq for hydrogen adsorption is then calculated as a function of temperature T using standard thermodynamic expressions. For details, we refer to the literature.6,7 Finally, to estimate storage capacities we partially correct for the neglected H2–H2 interactions using the experimental equation of state8 when computing the density of H2 inside the nanostructure at the internal pressure Pint=Keq,Pext.
H2 storage in carbon nanostructures
The strength of the H2–host interaction is proportional to the polarizability of the host material. In a graphene layer, our prototype structure, the H2–graphene interaction energy is 7kJ mol−1.9 Our simulation shows that graphene increases H2 density by only 60% (Keq=1.6) at ambient temperature, however: hardly useful for practical applications. In contrast, a graphene bilayer with an optimized interlayer distance exhibits remarkable hydrogen storage potential, having Keq≥50 at room temperature (see Figure 1). We found, in agreement with recent experiments,10 that optimal slit pores have a width of ≈0.6–0.7nm.6

Figure 2. Structure of C60 intercalated graphite.

Figure 3. Structure of carbon foam (honeycomb graphite).
A nanostructure with a pore size optimized to accomodate H2 is insufficient, however. The likely contaminant gases (N2, O2, CO2) are more polarizable, and are much more strongly bound to the nanostructure as compared to H2. For the most abundant atmospheric gas, N2, we calculate a graphene binding energy of 18 kJ mol−1.11 An optimal storage material design not only maximizes the H2 abundance, but also resists `poisoning’ by larger molecules present in the atmosphere.
One possible slit pore design is the intercalation of graphite, for example with C60 fullerenes (C60-intercalated graphite, or CIG). Gupta and coworkers recently reported such a material (see Figure 2).12 Our calculations confirm the experimentally observed structure and atomization energy of this new carbon phase. The fullerene intercalants simultaneously spread the graphene layers and enhance the H2–host interaction, leading to a reasonable potential hydrogen storage medium. Our simulations indicate that at moderate temperature (250K) and pressure (10MPa), this material could host enough hydrogen gas to approach the 2007 United States Department of Energy target for mobile applications (4.5% gravimetric, volumetric 0.039kg H2/l). Furthermore, given the pore sizes of this material, N2 contaminants should be trapped between the fullerene cages and thus penetrate into the material slowly.
Having demonstrated the potential to tune carbon nanostructures for H2 storage, we are presently investigating other candidates with well-defined pore sizes and high mechanical stability, such as honeycomb graphite (see Figure 3) and Mackay structures.13
Conclusion
Well-defined carbon nanostructures are suitable materials for hydrogen storage devices. In fact, they may be able to achieve the target storage capacities of the U.S. Department of Energy at moderate pressures and temperatures. The weak, attractive interaction of the host material with hydrogen gas allows reversible loading cycles with low heat exchange with the environment. Well-defined pore sizes avoid poisoning of the material with larger molecules present in the atmosphere. New carbon materials such as honeycomb graphite structures show very promising results so far. Due to their mechanical and structural properties, carbon nanostructures are excellent candidates for future automotive hydrogen storage applications.

Thomas Heine, Lyuben Zhechkov, Gotthard Seifert
Institute for Physical Chemistry,
Dresden University of Technology
Dresden, Germany
Serguei Patchkovskii
Steacie Institute for Molecular Sciences,
National Research Council Canada
Ottawa, Canada

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