High capacity molecular hydrogen storage in novel crystalline solids

A number of hydrogen-rich van der Waals compounds have recently been discovered and hold promise for improving hydrogen storage technology, but the structure and nature of the intermolecular interactions in many of these materials is unknown. We have determined the previously unknown structure of CH4(H2)4 using a novel combined theoretical and experimental approach. This material contains the largest amount of hydrogen of any known compound (33.4 wt%). CH4(H2)4 was found to have an orthorhombic methane substructure with two different sites for H2 molecules which interact strongly as single entities with the two nearest methane molecules. Interactions of the hydrogen molecules with the methane sublattice in CH4(H2)4 allows the hydrogen molecules to be packed with higher density than solid hydrogen at its normal freezing point, indicating a novel increased interaction in CH4(H2)4. This finding provides insight into understanding intermolecular interactions in hydrogen-dense environments has implications for designing high capacity hydrogen storage materials. Introduction Replacing fossil fuels with hydrogen as an energy carrier has potential for significantly reducing greenhouse gas emissions from transportation applications at the point of use. One of the major hurdles to realizing a hydrogen economy is finding a practical on-board hydrogen storage material. The material must satisfy a number of requirements which include (but are not limited to): high hydrogen content (by mass and volume), near ambient synthesis, storage, and release conditions, environmentally friendly by-products, reasonable cost, etc. Major effort has been devoted to finding a suitable solution, and a wide variety of methods and materials have been investigated. However, none of the current approaches meets all the requirements mentioned above. The objective of our exploratory proposal was to study novel crystalline phases with a high capacity for molecular hydrogen storage over varying pressures and temperatures using both an experimental and theoretical approach. Synthesizing novel materials in extreme environments (e.g. pressure, temperature, radiation fluxes) may seem like an impractical approach for finding materials which will be used at ambient conditions, but nature already provides us with an excellent example, diamond, whose stability field is at high pressure and temperature, but whose metastable field extends down to the ambient conditions. Knowledge about this high pressure phase of carbon led to the development of synthesis techniques to mimic its creation (high pressure-temperature methods) which drive the billion dollar diamond abrasives industry. More ingenious, understanding the sp bonding of diamond enabled the design of a new synthesis pathway which uses sp bonded carbon in a methane plasma as a starting material for chemical vapor deposition growth of diamond in a near vacuum environment; high pressure is no longer required.