Hydrogen storage

Hydrogen storage describes the methodologies for storing H₂ for subsequent use. The methodologies span many approaches, including high pressures and cryogenics, but usually focus on chemical compounds that reversibly release H₂ upon heating. Hydrogen storage is a topical goal in the development of a hydrogen economy. Most research into hydrogen storage is focused on storing hydrogen in a lightweight, compact manner for mobile applications.

Some attention has been given to the role of underground hydrogen storage to provide grid energy storage for unpredictable energy sources, like wind power.

Hydrocarbons are stored extensively at the point of use, be it in the gasoline tanks of automobiles or propane tanks hung on the side of barbecue grills. Hydrogen, in comparison, is quite difficult to store or transport with current technology. Hydrogen gas has good energy density by weight, but poor energy density by volume versus hydrocarbons, hence it requires a larger tank to store. A large hydrogen tank will be heavier than the small hydrocarbon tank used to store the same amount of energy, all other factors remaining equal. Increasing gas pressure would improve the energy density by volume, making for smaller, but not lighter container tanks (see pressure vessel). Compressed hydrogen will require energy to power the compressor. Higher compression will mean more energy lost to the compression step.

Alternatively, higher volumetric energy density liquid hydrogen or slush hydrogen may be used (as in the Space Shuttle). However liquid hydrogen requires cryogenic storage and boils around 20.268 K (–252.882 °C or -423.188 °F). Hence, its liquefaction imposes a large energy loss (as energy is needed to cool it down to that temperature). The tanks must also be well insulated to prevent boil off. Insulation for liquid hydrogen tanks is usually expensive and delicate. Assuming all of that is solvable, the density problem remains. Liquid hydrogen has worse energy density by volume than hydrocarbon fuels such as gasoline by approximately a factor of four. This highlights the density problem for pure hydrogen: there is actually about 64% more hydrogen in a liter of gasoline (116 grams hydrogen) than there is in a liter of pure liquid hydrogen (71 grams hydrogen). The carbon in the gasoline also contributes to the energy of combustion.

Contents 1 Mobile hydrogen storage 1.1 Established technologies 1.2 Proposals and research 1.2.1 Chemical storage 1.2.1.1 Metal hydrides 1.2.1.2 Synthesized hydrocarbons 1.2.1.3 Ammonia 1.2.1.4 Amine borane complexes 1.2.1.5 Formic acid 1.2.1.6 Imidazolium ionic liquids 1.2.1.7 Phosphonium borate 1.2.2 Physical storage 1.2.2.1 Carbonite substances 1.2.2.2 Carbon nanotubes 1.2.2.3 Metal-organic frameworks 1.2.2.4 Doped polymers 1.2.2.5 Glass Capillary Arrays 1.2.2.6 Glass microspheres 1.2.2.7 Keratine 2 Stationary hydrogen storage 2.1 Underground hydrogen storage 3 See also 4 References 5 External links

Mobile hydrogen storage

Targets were set by the FreedomCAR Partnership in January 2002 between the United States Council for Automotive Research (USCAR) and U.S. DOE (Targets assume a 5-kg H₂ storage system). The 2005 targets were not reached.^[1]

It is important to note that these targets are for the hydrogen storage system, not the hydrogen storage material. Thus while a material may store 6 wt% H₂, a working system using that material may only achieve 3 wt% when the weight of tanks, temperature and pressure control equipment, etc., is considered. System densities are often around half those of the working material.

Established technologies

• Compressed hydrogen (CGH₂) in a hydrogen tank
• Liquid hydrogen in a (LH₂) cryogenic hydrogen tank
• Slush hydrogen in a cryogenic hydrogen tank

Proposals and research

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

Metal hydrides

Metal hydrides, like NaAlH₄, LaNi₅H₆ and TiFeH₂, with varying degrees of efficiency, can be used as a storage medium for hydrogen, often reversibly^[2]. Some are easy-to-fuel liquids at ambient temperature and pressure, others are solids which could be turned into pellets. Proposed hydrides for use in a hydrogen economy include simple hydrides of magnesium or transition metals and complex metal hydrides, typically containing sodium, lithium, or calcium and aluminium or boron. These materials have good energy density by volume, although their energy density by weight is often worse than the leading hydrocarbon fuels. Furthermore, temperatures around 120 °C (248 °F) - 200 °C (392 °F) are often required to release their hydrogen content.

Solid hydride storage is a leading contender for automotive storage. A hydride tank is about three times larger and four times heavier than a gasoline tank holding the same energy. For a standard car, that's about 45 US gallons (0.17 m³) of space and 600 pounds (270 kg) versus 15 US gallons (0.057 m³) and 150 pounds (70 kg). A standard gasoline tank weighs a few dozen pounds (tens of kilograms) and is made of steel costing less than a dollar a pound ($2.20/kg). Lithium, the primary constituent by weight of a hydride storage vessel, currently costs over $40 a pound ($90/kg). Any hydride will need to be recycled or recharged with hydrogen, either on board the automobile or at a recycling plant. A metal-oxide fuel cell, (i.e. zinc-air fuel cell or lithium-air fuel cell), may provide a better use for the added weight, than a hydrogen fuel cell with a metal hydride storage tank.

Often hydrides react by combusting rather violently upon exposure to moist air, and are quite toxic to humans in contact with the skin or eyes, hence cumbersome to handle (see borane, lithium aluminium hydride). For this reason, such fuels, despite being proposed and vigorously researched by the space launch industry, have never been used in any actual launch vehicle.

Few hydrides provide low reactivity (high safety) and high hydrogen storage densities (above 10% by weight). Leading candidates are sodium borohydride, lithium aluminium hydride and ammonia borane. Sodium borohydride and ammonia borane can be stored as a liquid when mixed with water, but must be stored at very high concentrations to produce desirable hydrogen densities, thus requiring complicated water recycling systems in a fuel cell. As a liquid, sodium borohydride provides the advantage of being able to react directly in a fuel cell, allowing the production of cheaper, more efficient and more powerful fuels cells that do not need platinum catalysts. Recycling sodium borohydride is energy expensive and would require recycling plants. More energy efficient means of recycling sodium borohydride are still experimental. Recycling ammonia borane by any means is still experimental.

New Scientist ^[3] stated that Arizona State University is investigating using a borohydride solution to store hydrogen, which is released when the solution flows over a catalyst made of ruthenium.

Hydrogen produced for metal hydride storage must be of a high purity. Contaminants alter the nascent hydride surface and prevent absorption. This limits contaminants to at most 10 ppm oxygen in the hydrogen stream, with carbon monoxide, hydrocarbons and water at very low levels.

Synthesized hydrocarbons

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.

Direct methanol fuel cells do not require a reformer, but provide a lower energy density compared to conventional fuel cells, although this could be counter balanced with the much better energy densities of ethanol and methanol over hydrogen. Alcohol fuel is a renewable resource.

Solid-oxide fuel cells can operate on light hydrocarbons such as propane and methane without a reformer, or can run on higher hydrocarbons with only partial reforming, but the high temperature and slow startup time of these fuel cells are problematic for automotive applications.

Ammonia

Ammonia (NH₃) releases H₂ in an appropriate catalytic reformer. Ammonia provides high hydrogen storage densities as a liquid with mild pressurization and cryogenic constraints: It can also be stored as a liquid at room temperature and pressure when mixed with water. Ammonia is the second most commonly produced chemical in the world and a large infrastructure for making, transporting, and distributing ammonia exists. Ammonia can be reformed to produce hydrogen with no harmful waste, or can mix with existing fuels and under the right conditions burn efficiently. Pure ammonia burns poorly at the atmospheric pressures found in natural gas fired water heaters and stoves. Under compression in an automobile engine it is a suitable fuel for slightly modified gasoline engines. Ammonia is a toxic gas at normal temperature and pressure and has a potent odor^[4].

In September 2005 chemists from the Technical University of Denmark announced a method of storing hydrogen in the form of ammonia saturated into a salt tablet. They claim it will be an inexpensive and safe storage method. ^[5]

Amine borane complexes

Prior to 1980, several compounds were investigated for hydrogen storage including complex borohydrides, or aluminohydrides, and ammonium salts. These hydrides have an upper theoretical hydrogen yield limited to about 8.5% by weight. Amongst the compounds that contain only B, N, and H (both positive and negative ions), representative examples include: amine boranes, boron hydride ammoniates, hydrazine-borane complexes, and ammonium octahydrotriborates or tetrahydroborates. Of these, amine boranes (and especially ammonia borane) have been extensively investigated as hydrogen carriers. During 1970's and 80's, the U.S. Army and Navy funded efforts aimed at developing hydrogen/deuterium gas-generating compounds for use in the HF/DF and HCl chemical lasers, and gas dynamic lasers. Earlier hydrogen gas-generating formulations used amine boranes and their derivatives. Ignition of the amine borane(s) forms boron nitride (BN) and hydrogen gas. In addition to ammonia borane (H₃BNH₃), other gas-generators include diborane diammoniate, H₂B(NH₃)₂BH₄.

Formic acid

In 2006 researchers of EPFL, Switzerland, reported the use of formic acid as a hydrogen storage material^[6]. 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^[7]. This catalytic system overcomes the limitations of other catalysts (e.g. poor stability, limited catalytic lifetimes, formation of CO) for the decomposition of formic acid making it a viable hydrogen storage material^[8]. And the co-product of this decomposition, carbon dioxide, can be used as hydrogen vector by hydrogenating it back to formic acid in a second step. The catalytic hydrogenation of CO₂ has long been studied and efficient procedures have been developed^[9][10]. 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.

Imidazolium ionic liquids

In 2007 Dupont and others reported hydrogen-storage materials based on imidazolium ionic liquids. Simple alkyl(aryl)-3-methylimidazolium N-bis(trifluoromethanesulfonyl)imidate salts that possess very low vapour pressure, high density, and thermal stability and are not inflammable can add reversibly 6-12 hydrogen atoms in the presence of classical Pd/C or Ir0 nanoparticle catalysts and can be used as alternative materials for on-board hydrogen-storage devices. These salts can hold up to 30 g L^-1 of hydrogen at atmospheric pressure. ^[11]

Phosphonium borate

In 2006 researchers of University of Windsor reported on reversible hydrogen storage in a non-metal phosphonium borate frustrated Lewis pair^[12][13][14]:

The phosphino-borane on the left accepts one equivalent of hydrogen at one atmosphere and 25°C and expels it again by heating to 100°C. The storage capacity is 0.25 wt% still rather below the 6 to 9 wt% required for practical use.

Physical storage

Carbonite substances

Research has proven that graphene can store hydrogen efficiently. After taking up hydrogen, the substance becomes graphane. After tests, conducted by dr André Geim at the University of Manchester, it was shown that not only can graphene store hydrogen easily, it can also release the hydrogen again, after heating to 450°C. ^[15][16]

Carbon nanotubes

Hydrogen carriers based on nanostructured carbon (such as carbon buckyballs and nanotubes) have been proposed. Despite initial claims of greater than 50 wt% hydrogen storage, it has generally come to be accepted that less than 1 wt% is practical.^[17]

Metal-organic frameworks

Metal-organic frameworks represent another class of synthetic porous materials that store hydrogen. In 2006, chemists at UCLA and the University of Michigan have achieved hydrogen storage concentrations of up to 7.5% weight in MOF-74^[18]. However, the storage was achieved at the low temperature of 77 K. ^[19]. In 2009 researchers of the University of Nottingham reached 10 wt% at 77 bar (1,117 psi) and 77 K with MOF NOTT-112^[20].

Doped polymers

In 2006, a team of Korean researchers led by Professor Ihm Jisoon at Department of Physics and Astronomy of Seoul National University proposed a new material with the hydrogen storage efficiency of 7.6 percent based on first-principles electronic structure calculations for hydrogen binding to metal-decorated polymers of many different kinds. According to these researchers, hydrogen can be stored in a solid material at ambient temperatures and pressures by attaching a titanium atom to a polyacetylene. [1][2]

Glass Capillary Arrays

A team of Russian, Israeli and German scientists have collaboratively developed an innovative technology based on glass capillary arrays for the safe infusion, storage and controlled release of hydrogen in mobile applications[3]. The C.En [4]technology has achieved the US Department of Energy (DOE) 2010 targets for on-board hydrogen storage systems^[21].

Glass microspheres

Hollow glass microspheres (HGM) can be utilized for controlled storage and release of hydrogen.^[22][23]

Keratine

Keratine, a compound found in bird feathers, has been found to be useful to increase the interior surface (and thus the hydrogen storage capacity) of tanks. The research stated that the use of carbonized chicken feather fibres would result in far lower manufacturing costs than other common hydrogen tanks on the market. The research was conducted by Richard Wool and Erman Senoz. ^[24]

Stationary hydrogen storage

Unlike mobile applications, hydrogen density is not a huge problem for stationary applications. As for mobile applications, stationary applications can use established technology:

• Compressed hydrogen (CGH₂) in a hydrogen tank
• Liquid hydrogen in a (LH₂) cryogenic hydrogen tank
• Slush hydrogen in a cryogenic hydrogen tank

Underground hydrogen storage

Underground hydrogen storage is the practice of hydrogen storage in underground caverns, salt domes and depleted oil and gas fields. Large quantities of gaseous hydrogen are stored in underground caverns by ICI for many years without any difficulties^[25]. The storage of large quantities of hydrogen underground can function as grid energy storage which is essential for the hydrogen economy.

See also

Wikimedia Commons has media related to: Hydrogen storage

• Lithium borohydride
• Cascade storage system
• Complex hydride
• Cryo-adsorption
• Hydrogenography
• Hydrogen tank
• Hydrogen molecular technologies
• Hydrogen energy plant in Denmark

References

External links

• EU Storhy
• Nesshy
• Hycones
• United States Department of Energy Planned program activities for 2003-2010
• Ammonia Borane (NhxBHx)
• C.En
• Hyweb (1996)
• Research into metal-organic framework or Nano Cages [5][6]
• Hydrogen Storage Technical Data

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