Hydrogen is the most widely available element on earth. It can be easily produced and the byproduct of using hydrogen as a fuel is water. When hydrogen is burned, it burns clean and doesn’t create additional pollution. However, the ability to use hydrogen as an energy source depends on the ability of storing it safely. Hydrogen, while plentiful, is not an easy element to control. The most stable state of hydrogen is at normal atmospheric conditions; however, it must be stored in manageable containers in order for hydrogen to be used as a fuel source.
Hydrogen is extremely reactive. Many people have seen images of the Hindenburg explosion and have a natural fear of hydrogen. Hydrogen has the potential to spontaneously combust and can be very flammable. This is a concern when considering hydrogen as a fuel for passenger vehicles. Road conditions are often unpredictable and road debris is frequently thrown up at the undercarriage of a vehicle. The location of a fuel tank in a car powered by hydrogen would need to be both protected and stable. Currently the fuel tank is mounted to the bottom of the car, between the back tires. However, a hydrogen fuel tank would need to be more protected, which may require the fuel tank to be located within the passenger part of the vehicle.
Hydrogen storage technologies can be divided into physical storage, where hydrogen molecules are stored (including pure hydrogen storage via compression and liquefication), and chemical storage, where hydrides are stored.
Chemical storage contain metal hydrides, Carbohydrates, Synthesized hydrocarbons, Liquid organic hydrogen carriers (LOHC), Ammonia, Amine borane complexes, Formic acid, Imidazolium ionic liquids, Phosphonium borate. Metal hydrides, such as MgH2, NaAlH4, LiAlH4, LiH, LaNi5H6, TiFeH2 and palladium hydride, with varying degrees of efficiency, can be used as a storage medium for hydrogen, often reversibly. Some are easy-to-fuel liquids at ambient temperature and pressure, others are solids which could be turned into pellets. These materials have good energy density by volume, although their energy density by weight is often worse than the leading hydrocarbon fuels.
Carbohydrates (polymeric C6H10O5) releases H2 in a bioreformer mediated by the enzyme cocktailâcell-free synthetic pathway biotransformation. Carbohydrate provides high hydrogen storage densities as a liquid with mild pressurization and cryogenic constraints: It can also be stored as a solid power. Carbohydrate is the most abundant renewable bioresource in the world.
An alternative to hydrides is to use regular hydrocarbon fuels as the hydrogen carrier. Then a small hydrogen reformer would extract the hydrogen as needed by the fuel cell. However, these reformers are slow to react to changes in demand and add a large incremental cost to the vehicle powertrain.
Unsaturated organic compounds can store huge amounts of hydrogen. These Liquid Organic Hydrogen Carriers (LOHC) are hydrogenated for storage and dehydrogenated again when the energy/hydrogen is needed. Heterocyclic aromatic compounds are most appropriate for this task.
In 2006 researchers of EPFL, Switzerland, reported the use of formic acid as a hydrogen storage material. Carbon monoxide free hydrogen has been generated in a very wide pressure range (1â600 bar). A homogeneous catalytic system based on water soluble ruthenium catalysts selectively decompose HCOOH into H2 and CO2 in aqueous solution.
Physical storage contain Cryo-compressed, Carbon nanotubes, Metal-organic frameworks, Clathrate hydrates, Glass capillary arrays and Glass microspheres methods. Cryo-compressed storage of hydrogen is the only technology that meets 2015 DOE targets for volumetric and gravimetric efficiency. Furthermore, another study has shown that cryo-compressed exhibits interesting cost advantages: ownership cost (price per mile) and storage system cost (price per vehicle) are actually the lowest when compared to any other technology.Â
In addition to being able to store electrical energy, there has been some research in using carbon nanotubes to store hydrogen to be used as a fuel source. By taking advantage of the capillary effects of the small carbon nanotubes, it is possible to condense gases in high density inside single-walled nanotubes. This allows for gases, most notably hydrogen (H2), to be stored at high densities without being condensed into a liquid. Potentially, this storage method could be used on vehicles in place of gas fuel tanks for a hydrogen-powered car. A current issue regarding hydrogen-powered vehicles is the onboard storage of the fuel. Current storage methods involve cooling and condensing the H2 gas to a liquid state for storage which causes a loss of potential energy (25â45%) when compared to the energy associated with the gaseous state. Storage using SWNTs would allow one to keep the H2 in its gaseous state, thereby increasing the storage effciency. This method allows for a volume to energy ratio slightly smaller to that of current gas powered vehicles, allowing for a slightly lower but comparable range.
Metal-organic frameworks represent another class of synthetic porous materials that store hydrogen and energy at the molecular level. MOFs are highly crystalline inorganic-organic hybrid structures that contain metal clusters or ions (secondary building units) as nodes and organic ligands as linkers.
Chemical storage methods mostly simple in principle, but when it release hydrogen, often cost extra energy or catalyst. In the contrary, physical storage methods do not need extra energy, and have been widely used. Great efforts have been paid to storage hydrogen research on Special materials.