Which has not been suggested as a reasonably practical way to store large amounts of hydrogen in relatively small spaces for its use as a fuel?

A. Liquefy hydrogen under pressure and store it much as we do with liquefied natural gas today.

B. Absorb hydrogen onto activated charcoal; then heat the mixture to release the hydrogen.

C. Store it in the form of ionic metal hydrides, such as LiH, which release hydrogen gas when they react with water.

D. Encapsulate hydrogen molecules in fullerene molecules (large, carbon-based molecules that can act like cages) that may be heated later to release the hydrogen.

physical storage where compressed hydrogen gas is stored under pressure or as a liquid; and
chemical storage, where the hydrogen is bonded with another material to form a hydride and released through a chemical reaction.

Physical storage solutions are commonly used technologies but are problematic when looking at using hydrogen to fuel vehicles. Compressed hydrogen gas needs to be stored under high pressure and requires large and heavy tanks. Also, liquid hydrogen boils at -253°C (-423°F) so it needs to be stored cryogenically with heavy insulation and actually contains less hydrogen compared with the same volume of gasoline.

Chemical storage methods allow hydrogen to be stored at much lower pressures and offer high storage performance due to the strong binding of hydrogen and the high storage densities. They also occupy relatively smaller spaces than either compressed hydrogen gas or liquified hydrogen. A large number of chemical storage systems are under investigation, which involve hydrolysis reactions, hydrogenation/dehydrogenation reactions, ammonia borane and other boron hydrides, ammonia, and alane etc.

Other practical storage methods being researched that focuses on storing hydrogen as a lightweight, compact energy carrier for mobile applications include;

Metal hydrides e.g. LiH

Nanostructured metal hydrides

Non-metal hydrides

Carbohydrates

Synthesized hydrocarbons

Aluminum

Liquid organic hydrogen carriers (LOHC)

Encapsulation , e.t.c.