Hydrogen is a gas at ambient conditions, so the volumetric energy density is very poor compared to liquid fuels like petrol and diesel. There are, however, several options to store hydrogen:

 - pressurised gas
 - liquid
 - metal hydrid
 - chemically bonded
 - carbon structures 

Gaseous hydrogen can be compressed to higher pressures in order to increase the energy density. By doubling the tank pressure, one can roughly say that the stored amount of energy is doubled as well. Today, storage at 200-350 bar is available technology. However, the volumetric energy density is still almost ten times lower than for petrol. For most mobile applications, this gives a very limited operating range. An exception is city buses which can refuel frequently. Prototype hydrogen tanks with up to 700 bar pressure have been presented by GM/Opel, and several car manufacturers apply pressurised hydrogen tanks in their fuel-cell cars. The 700 bar system was developed in collaboration with GM's strategic fuel-cell partner, Quantum Fuel Systems Technology Worldwide, Inc.

Liquid hydrogen has to be kept at approximately -250°C to avoid boiling. The energy density is much higher than the gaseous form, but a lot of energy is required to liquefy and keep the hydrogen at such a low temperature. Despite the fact that the high complexity of the storage and refilling systems makes it more suitable for large amounts and/or long distances, automobile producers like BMW and GM are pursuing liquid storage technology in their hydrogen cars, internal combustion engines and fuel cells, respectively. A large part of the liquid hydrogen research has been done in conjunction with space rocket activities, where hydrogen is used as a fuel.

Hydrogen stored in metal hydrides (MH) offers an even higher volumetric energy density than liquid hydrogen. Metal alloys (based on e.g. magnesium, aluminium or rare earth metals) absorb hydrogen in the molecular structure, so the gas molecules are closely packed together. Heat is released during charging and required during discharging. The major disadvantages are the high mass and costs of the materials. In addition, the slow refilling is also a problem. The gravimetric hydrogen density is less than 5 wt%, and MH is therefore more suitable where volume rather than weight is important. For instance in some portable electronic devices, and special applications like forklifts and submarines.

Chemically bonded hydrogen (chemical hydrides) is somewhat similar to metal hydrides. As the name says, hydrogen is chemically bonded in(to) a compound. By mixing the chemical hydrides with water, they will produce hydrogen. For instance sodium borohydride, NaBH₄:

NaBH₄ + 2H₂O 4H₂ + NaBO₂

The reaction is reversible and the produced sodium borooxide (NaBO₂) can be “recharged” back to sodium borohydride. NaH and LiBH₄ are other examples of compounds which are commonly used. Although the energy density of the chemical hydride is high, there are still problems with complete systems, both technically and concerning size.

Carbon structures may also be used to store hydrogen. Some extremely promising results were presented in the 1990’s on carbon nano-structures. Unfortunately, they could not be reproduced and were most probably incorrect. Another possible option is the absorption of hydrogen in coal powders.

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