On the one hand, hydrogen's great asset as a renewable energy carrier is that it is storable and transportable. On the other hand, its very low natural density requires storage volumes that are impractical for vehicles and many other uses. Current practice is to compress the gas in pressurized tanks, but this still provides only limited driving range for vehicles and is bulkier than desirable for other uses as well. Liquefying the hydrogen more than doubles the fuel density, but uses up substantial amounts of energy to lower the temperature sufficiently (-253°C at atmospheric pressure), requires expensive insulated tanks to maintain that temperature, and still falls short of desired driving range. One possible way to store hydrogen at higher density is in the spaces within the crystalline structure of metal hydrides. Heat then releases the hydrogen for use. Thus far, however, densities are still not high enough and costs are high. Another possibility is chemically storing hydrogen in compounds that readily release their hydrogen. The reverse reactions, however, could not likely be performed on board or at the filling station, so the reaction byproduct would have to be retrieved from the vehicle and returned to the production plant for regeneration.
NREL's hydrogen-storage research focuses primarily on carbon-nanotube hydrogen storage. In just the last 20 years, researchers have been able to create nano-scale (one-billionth of a meter) spheres, cylinders, and other geometric shapes of simple carbon atoms. Not much bigger than hydrogen molecules, with large surface-to-volume ratios, NREL researchers have found that these nanostuctures have an affinity for adsorbing hydrogen on their surfaces. By manipulating the characteristics of these nanostructures, chiefly single-wall nanotubes, they are working to increase this hydrogen storage capacity. The work is still very preliminary, but holds great promise. See NREL's Hydrogen Storage Research webpage.