Caged hydrogen

When simple gases meet water at ambient conditions, little happens. But at high pressure and low temperature, hydrogen-bond networks within water can build polyhedron cages around guest molecules like methane, nitrogen, or argon to form hydrogen clathrates. Wendy Mao (University of Chicago) and her father Ho-kwang Mao (Carnegie Institution of Washington) have developed a way to store large concentrations of hydrogen within such clathrate cages. Using a diamond anvil cell, they compressed hydrogen gas and water to 250 MPa and noticed the mixture nucleate at about 250 K to form H₂(H₂O)₂, a clathrate cage with enclosed hydrogen gas clusters. Provided that it’s stored at temperatures below about 140 K, the compound remains stable even when the pressure drops to ambient levels. At warmer temperatures, the clathrate decomposes to release its stored gas. The density of hydrogen stored within the hydrate is 50 g/L, significantly close to that of pure liquid hydrogen, about 70 g/L. The Maos argue that the search for other novel clathrates could lead to an optimized hydrogen-storage compound. Meanwhile, their study is helping researchers understand a very different puzzle: how icy planets managed to incorporate large quantities of hydrogen during their evolution. (W. L. Mao, H.-k. Mao, Proc. Natl. Acad. Sci. USA 101, 708, 2004.http://dx.doi.org/10.1073/pnas.0307449100 )