In order to use hydrogen on a large scale, safe, practical storage systems must be developed, especially for automobiles. Although hydrogen can be stored as a liquid, it is a difficult process because the hydrogen must be cooled to -253oC. Refrigerating hydrogen to this temperature uses the equivalent of 25% to 30% of its energy content, and requires special materials and handling. To cool one kilogram of hydrogen requires 10 kWh of electrical energy. (reference)
Hydrogen may also be stored as a gas, which uses less energy than making liquid hydrogen. Because hydrogen is a gas, it must be pressurized to store any appreciable amount. For large-scale use, pressurized Hydrogen gas could be stored in caverns, gas fields, and mines. The hydrogen gas could then be piped into individual homes in the same way as natural gas. Though this means of storage is feasible for heating, it is not practical for transportation because the pressurized metal tanks used for storing hydrogen gas for transportation are very expensive.
A potentially more efficient method of storing hydrogen is in hydrides. Hydrides are chemical compounds of hydrogen and other materials. Research is currently being conducted on magnesium hydrides. Certain metal alloys such as magnesium nickel, magnesium copper, and iron titanium compounds, absorb hydrogen and release it when heated. Hydrides, however, store little energy per unit weight. Current research aims to produce a compound that will carry a significant amount of hydrogen with a high energy density, release the hydrogen as a fuel, react quickly, and be cost-effective.