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.