Hydrogen can easily be generated from renewable energy sources making it a primary focus in the area of alternative energy research. Hydrogen is the most abundant element in the universe and is produced from various sources such as fossil fuels, water and renewables.

Hydrogen is nonpolluting and forms water as a harmless byproduct during use. The challenges associated with the use of hydrogen as a formof energy include developing safe, compact, reliable, and cost-effective hydrogen storage and delivery technologies. Currently, hydrogen can be stored in these three forms: Compressed Hydrogen, Liquid Hydrogen and Chemical Storage.

Compressed Hydrogen

Compressed hydrogen storage method is the physical storage of compressed hydrogen gas in high pressure tanks (up to 10,000 pounds per square inch). This method is beneficial for fuel purposes because in this form it can be stored in a smaller space while retaining its energy effectiveness. When gas pressure is increased, energy density by volume is improved.

Liquid Hydrogen

Liquid hydrogen, also known as slush hydrogen, is non-corrosive and colorless. Liquid hydrogen, which requires cryogenic storage, is often used as a concentrated form of hydrogen storage. Liquid hydrogen tanks can store 0.070 kg/L of liquid hydrogen compared to 0.030 kg/L for 10,000-psi compressed gas tanks. Although this method of hydrogen storage offers improved energy density, it has many challenges as well. The issues with liquid hydrogen tanks are hydrogen boil-off, high hydrogen liquefaction causing large energy loss, and the volume, weight and tank cost.

Chemical Storage

Chemical storage uses technologies in which hydrogen is generated through a chemical reaction. Storage within advanced materials or on the surface of certain materials or in the form of chemical precursors undergo a chemical reaction to release hydrogen. Typical reactions involve chemical hydrides with water or alcohols.

Currently scientists are investigating several different kinds of materials, including carbohydrates, ammonia, borane complexes, glass capillary, clathrate hydrates, doped polymers, formic acid, metal hydrides, carbon-based materials, and chemical hydrides as well as identifying new materials with potential for storing hydrogen.

Carbohydrate (Polymeric C6H10O5 ) is the most abundant renewable bioresource available. Carbohydrate presents high hydrogen storage densities as a liquid with lower pressurization and cryogenic constraints. It can also be stored as a solid power. Biochemical engineers from the Virginia Polytechnic Institute and State University and chemists and biologists from the Oak Ridge National Laboratory announced in 2007, a method of producing high-yield pure hydrogen from starch and water. The emerging cell-free synthetic biology technology (cell-free synthetic pathway biotransformation) has allowed hydrogen production by use of renewable carbohydrate as a high-density hydrogen carrier and energy source. Renewable carbohydrate can be isolated from plant biomass or could be produced from a combination of solar electricity/hydrogen and carbon dioxide fixation mediated by high-efficiency artificial photosynthesis mediated by cell-free synthetic pathway biotransformation (SyPaB).

Ammonia as a hydrogen carrier in contrast to other hydrogen storage materials has the advantages of high hydrogen density, a well-developed technology for synthesis and distribution, and easy catalytic decomposition. Compared to alcohols and hydrocarbons, ammonia has the benefit that there is no CO2 emission at the end user, it produces no harmful waste and can mix with existing fuels and under the right conditions burn efficiently. The disadvantages are primarily the toxicity of liquid ammonia and the issues related to trace amounts of ammonia in the hydrogen after decomposition. Ammonia is also a toxic gas at normal temperature and pressure and has a strong odor.

Amine boranes, especially ammonia borane, have been studied comprehensively as hydrogen carriers. Several other compounds were investigated prior to 1980 including ammonium salts and complex borohydrides or aluminohydrides. Examples of compounds that contain only B, N, and H (both positive and negative ions) consist of amine boranes, boron hydride ammoniates, hydrazine-borane complexes, and ammonium octahydrotriborates or tetrahydroborates.

Glass capillary arrays are installed in larger pressure resistant vessel made from steel. This technology was developed by a team of Russian, German and Israeli scientists to provide storage and controlled release of hydrogen in mobile applications and has achieved the US Department of Energy 2010 targets for on-board hydrogen storage systems. The glass capillaries are closed by melting at one end and closing the other end with an alloy after adding hydrogen to the steel vessel. The set-up is pressure resistant up to pressures of more than 1500 bar. After the set-up is evacuated, hydrogen is filled into the vessel until the storage pressure is reached.

Issues with using this technology include defects in the glass structures like bubbles, cracks or grooves will strongly increase stress peaks and therefore decrease the pressure resistance.

Clathrate Hydrate of hydrogen exhibiting two different sized cages meeting storage requirements was first reported in 2002. Although storage requirements are met using this technology, the extreme pressures required for stability make it impractical. In 2004, researchers from Colorado School of Mines and Delft University of Technology showed solid H2-containing hydrates could be formed at ambient temperature and 10s of bar by adding small amounts of promoting substances such as Tetrahydrofuran (THF). These clathrates have a theoretical maximum hydrogen densities of around 5 wt% and 40 kg/m3.

A new material for hydrogen binding to doped (metal-decorated) polymers was reported in 2006 by Jisson Ihm and a team of Korean researchers at Department of Physicis and Astronomy of Seoul National University. Using this material, hydrogen storage efficiency is 7.6 percent based on the first-principles electronic structure calculations for hydrogen binding to metal-decorated polymers of many different kinds. By attaching a titanium atom to polyacetylene, hydrogen can be stored in a solid material at ambient temperature and pressures, according to these researchers.

Formic Acid has been used as a hydrogen storage material since 2006 when researchers of EPFL, Switzerland first confirmed that the catalytic decomposition of formic acid provides efficient release of hydrogen. Its decomposition yields CO-free hydrogen while the co-produced carbon dioxide can be hydrogenated back to formic acid which makes hydrogen suitable for fuel cell applications. Carbon monoxide free hydrogen has been generated in a very wide pressure range (1-600 bar). A homogeneous catalytic system based on water soluble ruthenium catalysts selectively decompose HCOOH into H2 and CO2 in aqueous solution. Formic acid contains 53 g L-1 hydrogen at room temperature and atmospheric pressure. By weight, pure formic acid stores 4.3 wt% hydrogen. Pure formic acid is a liquid with a flash point 69 °C (cf. gasoline -40 °C, ethanol 13 °C). 85% formic acid is not inflammable.

Microporous metal-organic frameworks (MOFs), with exceptionally chemically-tunable structures and high surface areas, have recently emerged as some of the most promising hydrogen storage materials. MOFs also provide unique systems with large overall pore volumes and adjustable pore sizes. Research interests on hydrogen storage in MOFs have been growing since 2003 when the first MOF-based hydrogen storage was introduced. In 2006, chemists at the University of Michigan and UCLA attained hydrogen storage concentrations of up to 7.5 wt% in MOF-74 at a low temperature of 77K. In 2009, researchers at University of Nottingham reached 10 wt% at 77 bar (1,117 psi) and 77 K with MOF NOTT-112. Typically, research about hydrogen storage in MOFs state hydrogen uptake capacity at a pressure of 1 bar and a temperature of 77K, but this condition is commonly available and the binding energy between hydrogen and MOF is frequently compared to the thermal vibration energy which allows high hydrogen uptake ability.

Graphene storage method involves storing hydrogen between layers of graphite. Research has proven that graphene can store hydrogen more efficiently than other materials, such as carbon nanotubes, because it is cheap, safe to use and simply prepared. Carbon nanotubes and hydrogen clathrate hydrate compounds have previously been the focus of hydrogen-storage research; however, these materials only work in fuel cells at high pressures or low temperatures. In contrast, graphene store hydrogen easily allowing hydrogen to release again, after heating to 450°C. According to John Tse (now at the University of Saskatchewan) and colleagues at the Steacie Institute for Molecular Sciences in Canada and the Technical University of Dresden in Germany, thin layers of graphite or graphene (two-dimensional sheets of carbon atoms) spaced between 6 and 7 Angstroms apart can store hydrogen at room temperature and moderate pressures of just 10 MPa. Furthermore, the amount of hydrogen stored comes close to the 62 kilograms per cubic meter goal set by the US Department of Energy.

Metal hydrides can be used as a storage means with reversal potential for hydrogen. With varying degrees of effectiveness, NaAlH4, LiAlH4, MgH2, LaNi5H6, LiH, and TiFeH2 can be used. Some are solids which could be turned into pellets, others are easy-to-fuel liquids at ambient temperature and pressure. These materials have good energy density by volume, though their energy density by weight is often inferior to the top hydrocarbon fuels.

Based on rare-earth metals, the hydrogen storage alloys, Ti, Zr, Fe, et. al, have been studied at length. However, only rare-earth based AB5-type and transition metal based AB2-type alloys have reached the stage of mass production and commercialization. Simultaneously, as a reversible gas storage material, only AB5-type alloys can operate at moderate temperatures (from -20°C up to +60°C), while the AB2-type ones require further heating.

Metal hydrides tend to bind strongly with hydrogen creating a need for high temperatures around 120 °C (248 °F) - 200 °C (392 °F) to release their hydrogen content. By using alloys which consist of a strong hydride former and a weak one such as LiNH2, NaBH4 and LiBH4, energy cost can be reduced. The target for onboard hydrogen fuel systems is roughly <100°C for release and <700 bar for recharge (20-60 kJ/mol H2 ).

Lithium, boron and aluminum based compounds are currently the only hydrides which are able to accomplish the 9 wt. % gravimetric goal for 2015. Proposed hydrides for use in a hydrogen economy include complex metal hydrides, generally containing sodium, lithium, or calcium and aluminium or boron, and simple hydrides of magnesium or transition metals. Hydrides chosen for storage applications provide low reactivity (high safety) and high hydrogen storage densities. Primary candidates are Lithium hydride, sodium borohydride, lithium aluminium hydride and ammonia borane.

Arizona State University is reportedly investigating the use of a borohydride solution to store hydrogen, which is released when the solution flows over a catalyst made of ruthenium.

Some metal amides M(NH2)x such as LiNH2, NaNH2, Mg(NH2)2 and Ca(NH2)2 play important roles for crafting a new group of metal-N-H hydrogen storage systems. Metal imides (Li(2)NH, CaNH), a metal amide (LiNH(2)) and metal hydrides (LiH, CaH(2)) were produced by ball milling of their particular metal nitrides (Li(3)N, Ca(3)N(2)) in a H(2) atmosphere at 1 MPa and at room temperature with the H2 content of ball-milled metal nitrides was 0.2-5.0 wt.%.

As hydrogen storage materials, rare earth alloy as the hydrogen carrier, hydrogen is stored on the gap between the metal atoms. Rare earth hydrogen storage alloy can be used in batteries, air conditioners, heat pumps etc. The hydrogen storage alloys, based on rare-earth metals are comprised of Ti, Zr, Fe, et. al. The only rare-earth based supposed AB5-type and transition metal based AB2-type alloys has reached the point of mass production and commercialization. Simultaneously, as a reversible gas storage material, only AB5-type alloys can function at moderate temperatures (from -20°C up to +60°C), while the AB2-type ones require additional heating.

Alanates are compounds that contain aluminum, (stored) hydrogen, and a metal like sodium or lithium.The addition of transition metals, such as Ti, to sodium alanate (NaAlH4) enables reversible hydrogen absorption at moderate temperatures. Additionally, the kinetics of hydrogen reabsorption is increased by various orders of magnitude.