QENS STUDIES OF THE HYDROGEN STORAGE CAPACITY OF NOVEL SEPIOLITE-DERIVED
CARBONACEOUS MATERIALS

Giselle Sandí^1^, Chang Keun Back^2^, Humberto Joachin^2^, Nicolas de
Souza^3^, and Jai Prakash^2^

^1^Chemistry Division and ^3^Intense Pulsed Neutron Source, Argonne
National Laboratory, 9700 South Cass Ave., Argonne, IL 60439-4803;
^2^Department of Chemical and Environmental Engineering, Illinois
Institute of Technology, 10 W. 33rd Street, Chicago, IL 60616

INTRODUCTION

Gas-on-solid adsorption is an inherently safe and potentially high
energy density hydrogen storage method that could be extremely energy
efficient. Consequently, the hydrogen storage properties of high surface
area activated carbons have been extensively studied (1-3). However,
activated carbons are ineffective in storing hydrogen because only a
small fraction of the pores in the typically wide pore size distribution
are small enough to interact strongly with hydrogen molecules at room
temperatures and moderate pressures. Many new carbon nanostructured
adsorbents have been produced, including graphite nanofibers, carbon
multi-wall, and single-wall nanotubes (1-3). However, most of these
materials are synthesized using very long and costly procedures. In our
laboratories we have recently used a particular clay called sepiolite as
an inorganic template to load organic precursors and obtain carbonaceous
materials. Sepiolite has fibers of 0.2 to 2 µm in length, 100 x 300 Å in
width and a thickness of 50 to 100 Å (4). Channels are present and
oriented in the longitudinal direction of the fibers. The channels are
suitable for controlling the structure of organic compounds.

EXPERIMENTAL

Details about the synthesis of the carbon derived from sepiolite can be
found in (4). In brief, ethylene or propylene were loaded and pyrolyzed
in the gas phase in one step using a three-zone furnace. Quartz boats
containing sepiolite were placed within a quartz tube. The temperature
of the oven was gradually increased from room temperature to 700 °C. The
oven was then held at that target temperature for 4 hours. The clay from
the loaded/pyrolyzed sepiolite sample was removed using HF, rinsed to
neutral pH and refluxed with concentrated HCl for 2 hours. The sample
was washed with distilled water until the pH was \> 5 to ensure that
there was no acid left. The resultant carbon was oven dried overnight at
120 °C. The hydrogen storage set up has been described in details in
(5).

Hydrogen absorption in the carbon nanofiber material was studied in
situ, by inelastic neutron scattering, on the Quasi-Elastic Neutron
Spectrometer at the Intense Pulsed Neutron Source, Argonne National
Laboratory. Two nanofiber carbon samples were investigated, as
synthesized from raw sepiolite (A), and from the second fraction of the
sepiolite (B) (See Figure 1). Both samples were loaded by applying a
hydrogen pressure of 100 bar, to the sample at 77 K for 18 h, and then
further cooling in stages to 20-25 K, before isolating the hydrogen
cylinder. The inelastic neutron spectra were recorded at 12 K; to
determine the scattering from the loaded hydrogen alone, the signal from
the samples before loading was subtracted from the signal of the loaded
samples.

RESULTS

The hydrogen storage capacity of carbons derived from sepiolite is
summarized in (5). From those results, it is inferred that the samples
derived from fraction two are considered the best candidates for
practical applications. Quasi-elastic neutron spectroscopy spectra of
the carbons derived from raw material and fraction two are shown in
Figure 1A (blue curve) and 1B (red curve). The peaks around 14.5 meV
evidence that the hydrogen absorbed in both samples stayed in molecular
form. Apart from this, the significant differences are indicative of
hydrogen being in its liquid state in sample A (blue curve), whereas the
feature around 5 meV in sample B (red curve) indicates that hydrogen is
in its solid state in sample B. To address the possibility that a
significant amount of hydrogen condensed in the sample by the loading
procedure outlined above, the hydrogen pressure in sample A was released
at 25 K, before re-measuring at 12 K. The result (black curve) shows
that there is some hydrogen left in the sample, in the form of adsorbed
layer(s).

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REFERENCES

1\. Lee, S. M.;.Park, K S.; Choi, Y. C.; Park, Y. S.; Bok, J. M.; Bae,
D. J.; Nahm, K. S.; Choi, Y. G.;Yu, S.C.; Kim, N. G.; Frauenheim, T.;
Lee, Y. H. Synthetic Metals 113, 3, 209 (2000).

2\. Wu, X. B.; Chen, P.; Lin, J.; Tan, K. L. International Journal of
Hydrogen Energy 25, 3, 261 (2000).

3\. Lee, S. M.; Lee, Y. H. Applied Physics Letters 76, 20, 2877 (2000).

4\. G. Sandí, R. E. Winans, K. Carrado-Gregar. USA Patent 6,110,621.

5\. G. Sandí, H. Yang, H. Joachin, K. A. Carrado, R. E. Winans, and J.
Prakash. J. New Materials for Electrochemical Systems 6, 169-173 (2003).

ACKNOWLEDGMENTS

This work was performed under the auspices of the Office of Basic Energy
Sciences, Division of Chemical Sciences, Geosciences, and Biosciences,
U.S. Department of Energy, under contract number W-31-109-ENG-38.