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Hydrogen storage

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Main article: Hydrogen economy

Hydrogen storage describes the methods for storing H2 for subsequent use. The methods 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 as a lightweight, compact energy carrier for mobile applications.

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 by design for liquid hydrogen tanks is adding costs for this method. Liquid hydrogen has less 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. 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.

Compressed hydrogen, in comparison, is quite different to store. 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 hydrogen tank). Compressed hydrogen will require 2.1% of the energy content[1] to power the compressor. Higher compression without energy recovery will mean more energy lost to the compression step. Compressed hydrogen storage is worldwide accepted as a safe established technology[2].

Some researchers have investigated underground hydrogen storage to provide grid energy storage for intermittent energy sources, like wind power.

Mobile hydrogen storage

Timeline Targets assume a 5-kg H2 storage system
Timeline Targets assume a 5-kg H2 storage system

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.[3] The targets were revised in 2009 to reflect potentially more attainable goals, where the ultimate goal for volumetric storage is still above the theoretical density of liquid hydrogen. [4]


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

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 hydride hydrogen storage

Metal hydrides, like LiH, NaAlH4, LiAlH4, LaNi5H6 and TiFeH2, with varying degrees of efficiency, can be used as a storage medium for hydrogen, often reversibly[5]. 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. Any hydride will need to be recharged with hydrogen[6]. Hydrogen stations are becoming available in some parts of the world. Home hydrogen generators can also be used to charge the vehicle's hydrogen tanks. The generator creates hydrogen from a water supply and an energy source, such as a solar panel, wind turbine, or electrical outlet.

Most metal hydrides bind with hydrogen very strongly. As a result high temperatures around 120 °C (248 °F) - 200 °C (392 °F) are required to release their hydrogen content. This energy cost can be reduced by using alloys which consists of a strong hydride former and a weak one such as in LiNH2, NaBH4 and LiBH4. These are able to form weaker bonds, thereby requiring less input to release stored hydrogen. However if the interaction is too weak, the pressure needed for rehydriding is high, thereby eliminating any energy savings. The target for onboard hydrogen fuel systems is roughly <100°C for release and <700 bar for recharge (20-60 kJ/mol H2 )[7][8].

In addition to this requirement, FreedomCAR and Vehicle Technologies (FCVT)[9] has established a goal of 9 wt. % hydrogen for 2015 onboard storage systems. Currently the only hydrides which are capable of achieving this goal are limited to lithium, boron and aluminum based compounds; at least one of the first-row elements or Al must be added. Research is being done to determine new compounds which can be used to meet these requirements.

Proposed hydrides for use in a hydrogen economy include simple hydrides of magnesium[10] or transition metals and complex metal hydrides, typically containing sodium, lithium, or calcium and aluminium or boron. Hydrides chosen for storage applications provide low reactivity (high safety) and high hydrogen storage densities. Leading candidates are Lithium hydride, sodium borohydride, lithium aluminium hydride and ammonia borane.

New Scientist [11] 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.

Carbohydrates

Carbohydrates (Polymeric C6H10O5) releases H2 in an bioreformer mediated by the enzyme cocktail -- cell-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.

In May 2007 biochemical engineers from the Virginia Tech University and biologists and chemists from the Oak Ridge National Laboratory announced a method of producing high-yield pure hydrogen from starch and water [12]. In 2009, they demonstrated to produce nearly 12 moles of hydrogen per glucose unit from cellulosic materials and water [13]. Thanks to complete conversion and modest reaction conditions, they propose to use carbohydrate as a high energy density hydrogen carrier with a density of 14.8 wt %. [14]

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
Further information: Ammonia production

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[15].

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. [16]

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[17]. 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[18]. 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[19]. 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[20][21]. 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. [22]

Phosphonium borate

In 2006 researchers of University of Windsor reported on reversible hydrogen storage in a non-metal phosphonium borate frustrated Lewis pair[23][24][25]:

Phosphino borane hydrogenstorage
Phosphino borane hydrogenstorage

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. [26][27]

Carbon nanotubes
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.[28]

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[29]. However, the storage was achieved at the low temperature of 77 K. [30]. In 2009 researchers of the University of Nottingham reached 10 wt% at 77 bar (1,117 psi) and 77 K with MOF NOTT-112[31].

Doped polymers

In 2006, a team of Korean researchers led by Professor Jisoon Ihm 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. [5][6]

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[7][8]. The C.En [9] technology has achieved the US Department of Energy (DOE) 2010 targets for on-board hydrogen storage systems[32].

Glass microspheres

Hollow glass microspheres (HGM) can be utilized for controlled storage and release of hydrogen.[33][34]

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. [35]

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:

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 have been stored in underground caverns by ICI for many years without any difficulties[36]. 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.

References

  1. ^ Energy technology analysis.Pag.70
  2. ^ Modeling of dispersion following hydrogen permeation for safety engineering and risk assessment
  3. ^ http://www.uscar.org/commands/files_download.php?files_id=82
  4. ^ http://www1.eere.energy.gov/hydrogenandfuelcells/storage/current_technology.html
  5. ^ DOE Metal hydrides
  6. ^ Storage by Mc-Phy
  7. ^ [1]
  8. ^ EU Hydrogen Storage
  9. ^ DOE FreedomCAR and Vehicle Technologies
  10. ^ CNRS Institut Neel H2 Storage
  11. ^ "New type of hydrogen fuel cell powers up". newscientist. Retrieved 2006-09-16.
  12. ^ [2]
  13. ^ [3]
  14. ^ [4]
  15. ^ The ammonia economy
  16. ^ Focus Denmark
  17. ^ G. Laurenczy, C. Fellay, P. J. Dyson, Hydrogen production from formic acid. PCT Int. Appl. (2008), 36pp. CODEN: PIXXD2 WO 2008047312 A1 20080424 AN 2008:502691
  18. ^ C. Fellay, P. J. Dyson, G. Laurenczy, A Viable Hydrogen-Storage System Based On Selective Formic Acid Decomposition with a Ruthenium Catalyst, Angew. Chem. Int. Ed., 2008, 47, 3966–3970.
  19. ^ F. Joó, Breakthroughs in Hydrogen Storage – Formic Acid as a Sustainable Storage Material for Hydrogen, ChemSusChem 2008, 1, 805–808.
  20. ^ P. G. Jessop, in Handbook of Homogeneous Hydrogenation (Eds.: J. G. de Vries, C. J. Elsevier), Wiley-VCH, Weinheim, Germany, 2007, pp. 489–511.
  21. ^ P. G. Jessop, F. Joó, C.-C. Tai, Recent advances in the homogeneous hydrogenation of carbon dioxide, Coord. Chem. Rev., 2004, 248, 2425–2442.
  22. ^ Stracke, M. P. ; Ebeling, G. ; Cataluña, R. ; Dupont, J. Energy & Fuels 2007, 21, 1695-1698. doi:10.1021/ef060481t
  23. ^ Reversible, Metal-Free Hydrogen Activation Gregory C. Welch, Ronan R. San Juan, Jason D. Masuda, Douglas W. Stephan Science (journal) 17 November 2006: Vol. 314. no. 5802, pp. 1124 - 1126 doi:10.1126/science.1134230
  24. ^ H2 Activation, Reversibly Metal-free compound readily breaks and makes hydrogen Elizabeth Wilson Chemical & Engineering News November 20, 2006 Link
  25. ^ Mes stands for a mesityl substituent and C6F5 for a pentafluorophenyl group, see also tris(pentafluorophenyl)boron
  26. ^ Graphene as suitable hydrogen storage substance
  27. ^ Graphene to graphane
  28. ^ http://dx.doi.org/10.1016/j.jallcom.2006.11.192
  29. ^ MOF-74 - A Potential Hydrogen-Storage Compound
  30. ^ Green Car Congress: Researchers Demonstrate 7.5 wt% Hydrogen Storage in MOFs
  31. ^ New MOF Material With Hydrogen Uptake Of Up To 10 wt%
  32. ^ http://www.cenh2go.com/PDF/CEnPoster_small.pdf
  33. ^ Glass microsphere diffusion
  34. ^ Porous Wall-Hollow glass microspheres
  35. ^ Carbonized chicken feather fibres to increase hydrogen storage capacity of tanks
  36. ^ 1994 - ECN abstract
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