Hydrogen Storage

One of the primary impediments to the widespread implementation of hydrogen fuel cells is the storage of hydrogen. At standard temperature and pressure, hydrogen is a gas that is highly explosive. It can be stored either as a compressed gas or as a liquid. These are the current means by which hydrogen is stored, but are highly energy intensive and not practical for use in vehicles.

The only alternative to storing hydrogen as a liquid or compressed gas is to store it as a chemical compound. In other words, to store it reversibly bound to a liquid or solid.

Compressed Hydrogen

Manufacturers such as Honda and Nissan have been experimenting with compressed hydrogen gas. In order to store adequate volumes of hydrogen as a gas, it must be compressed to somewhere between 5000 pounds per square inch and 10,000 pounds per square inch.

Hydrogen storage tanks must be larger than hydrocarbon storage tanks. Increasing gas pressure would improve the energy density by volume therefor allowing smaller storage tanks. However, this does not translate into lighter storage tanks because tanks capable of handling higher pressures must necessarily be made of stronger and denser material.

Liquid Hydrogen

BMW is one of the leader producers of liquid hydrogen storage tanks for cars. The BMW Hydrogen 7, of which 100 units were produced, was the first and only limited production vehicle to utilize a liquid storage tank for hydrogen.

Liquid hydrogen is occasionally referred to as slush hydrogen and is used to power the Space Shuttle. The problem with liquid hydrogen is that it requires exceptionally low temperatures because its boiling point is -252° C or -423° F. To create liquid hydrogen requires a great deal of energy and the tanks must be well insulated to prevent the liquid from becoming gas at standard temperatures.

Liquid hydrogen has a low energy density compared to hydrocarbon fuels by a factor of four. In other words, for the same volume of fuel, hydrogen produces four times less energy than does hydrocarbon fuel. Interestingly, there is about 64% more hydrogen per litre of gasoline than there is per litre of pure hydrogen.

Metal Hydrides

Metal hydrides include magnesium hydride, lithium hydride, and several others. They can be used to reversibly bind hydrogen gas for storage at ambient temperature and pressure. Hydrogen stored by these mechanisms has a good energy density by volume but still has a lower energy density by weight than leading hydrocarbon fuels.

Developing metal hydrides for hydrogen storage is a tradeoff between the pressure needed to cause the hydrogen to bind and the temperature required to release the hydrogen for utilization. Most bile hydrides bind quite strongly with hydrogen and require temperatures around 120° C in order to release the gas for use. This temperature can be reduced by combining the metals into alloys that form weaker bonds with the hydrogen. If the interaction is too weak however, storage and absorption of hydrogen will require higher pressures. The current target for on board hydrogen fuel systems is less than 100° C for release in less than 10,000 pounds per square inch.

The most promising hydrides include the lithium, boron, and aluminum based compounds. The benefits of using hydrides are high safety levels due to the low risk of flammability of absorbed hydrogen and high hydrogen storage densities. Lithium aluminum hydride and sodium borohydride are the leading contenders for wide scale commercial use.

Compressed Liquid Hydrogen (Cryo-compressed)

This mechanism of hydrogen storage is the only technology that currently meets the 2015 Department Of Energy targets for volumetric and gravimetric efficiency. Studies have shown that there are cost advantages to storing hydrogen as a compressed liquid. While conventional gasoline vehicles cost between 5¢ and 7¢ per mile to operate a cryo-compressed hydrogen system would cost 12¢ per mile, making compressed liquid hydrogen the most economically feasible means of storing and transporting hydrogen.

The BMW Group has been investigating compressed liquid hydrogen storage systems since 2010. They have found that even though the system still use very cold hydrogen, the boil off temperature can be greatly increased by allowing the tank to reach pressures as high as 5000 pounds per square inch. The end result is that it takes the hydrogen gas a significantly longer amount of time to vent and enough of it is used in standard driving conditions to keep pressures well below the venting limit. The overall outcome is that less hydrogen is lost to venting and therefore more is available for use.

Metal-Organic Frameworks (MOFs)

MOFs are highly crystalline inorganic-organic hybrid structures that have exceptionally high surface areas due to their porous nature. In 2006, chemists were able to achieve hydrogen storage concentrations as high as 7.5% by weight using MOFs. In other words, 7.5% of the total weight of the storage system was hydrogen. This compares favorably to metal hydride storage systems where the percent by weight is roughly two. The current major impediment to such systems is the difficulty of producing them.

Conclusion

There are several other hydrogen storage solutions currently under investigation. The possibilities include carbon nanotubes, doped polymers, glass capillary arrays, and glass microspheres. Keratine, a compound found in bird feathers, has been found to be used for increasing surface area of hydrogen storage tanks and lower manufacturing costs than other mechanisms.