It would be nice if we could store hydrogen as a liquid; however, it has a very low boiling point (−252.9 °C), which makes this rather inefficient. Due to the strong tendency to evaporate at room temperature, significant energy must actually be expended just to keep hydrogen in its liquid phase. To store hydrogen gas, one possibility is to just compress it inside a metallic container similar to what is typically done with other gases. There have been several other approaches used, however, to attempt to store hydrogen for use as a fuel. These include both chemical and physical storage methods. Some of the chemical storage systems that have been investigated include metal hydrides (like NaAlH4, LiAlH4, or TiFeH2), aqueous carbohydrate solutions (which release H2 via an enzymatic reaction), synthesized hydrocarbons, ammonia, formic acid, ionic liquids, carbonite compounds, and others as well. These methods generally rely on chemical reactions to make H2 available for use as an energy source. Physical storage methods include cryogenic compression (involving a combination of low temperatures and high pressures) and a variety of materials, such as metal-organic frameworks, carbon nan-otubes, clathrate hydrates, capillary arrays, and others as well. Unfortunately, few of these physical storage methods have thus far been able to demonstrate strongly promising results in working toward a practically useful method of storing hydrogen as a fuel.
