Hydrogen storage in molecular compounds

Abstract

At low temperature (T) and high pressure (P), gas molecules can be held in ice cages to form crystalline molecular compounds that may have application for energy storage. We synthesized a hydrogen clathrate hydrate, H(2)(H(2)O)(2), that holds 50 g/liter hydrogen by volume or 5.3 wt %. The clathrate, synthesized at 200-300 MPa and 240-249 K, can be preserved to ambient P at 77 K. The stored hydrogen is released when the clathrate is warmed to 140 K at ambient P. Low T also stabilizes other molecular compounds containing large amounts of molecular hydrogen, although not to ambient P, e.g., the stability field for H(2)(H(2)O) filled ice (11.2 wt % molecular hydrogen) is extended from 2,300 MPa at 300 K to 600 MPa at 190 K, and that for (H(2))(4)CH(4) (33.4 wt % molecular hydrogen) is extended from 5,000 MPa at 300 K to 200 MPa at 77 K. These unique characteristics show the potential of developing low-T molecular crystalline compounds as a new means for hydrogen storage.

Fig. 1.

Photomicrographs of hydrogen and water in the circular gasket hole at 300 MPa as viewed through the diamond windows. (a) At 250 K before the formation of clathrate, the crescent-shaped water in region A was clearly separated from the hydrogen in region B. (b) Cooling down to 249 K, a reaction zone of clathrate formed between hydrogen and water. The residual water darkened as clathrate nucleated. (c) The clathrate further grew at the expense of the water and hydrogen. A wedge-shaped crack developed during the volume expansion. (d) The reaction was completed after 30 min at 249 K. At this point, all of the water had transformed completely into clathrate.

Fig. 2.

Hydrogen vibron region in Raman spectra of C₂ filled ice. Spectra are shown relative to frequency of the Q₁(1) peak in pure hydrogen at those conditions (indicated by the dashed line). Peaks at 0 relative Raman shift in the two spectra are due to unreacted hydrogen in sample. The peak at higher energy is due to C₂ filled ice.

Fig. 3.

Hydrogen vibron region in Raman spectra of H4M. Spectra are shown relative to frequency of the Q₁(1) peak in pure hydrogen at those conditions (indicated by the dashed line). Peaks at 0 relative Raman shift in the two spectra are due to unreacted hydrogen in sample. The top two spectra show the presence of H4M, indicated by the peak at lower energy, which formed at 159 K and was retained down to 200 MPa at liquid N₂ temperature.