hydrogen storage#Chemical storage

File:Storage Density of Hydrogen.jpg

Compressed hydrogen is a storage form whereby hydrogen gas is kept under pressures to increase the storage density. Compressed hydrogen in hydrogen tanks at 350 bar (5,000 psi) and 700 bar (10,000 psi) are used for hydrogen tank systems in vehicles, based on type IV carbon-composite technology.{{clarify|date=September 2024|reason=what is "type IV carbon-composite technology" or where to find out about it?}}{{cite journal |title=Fuel cell electric vehicles and hydrogen infrastructure: status 2012 |journal=Energy & Environmental Science |volume=5 |issue=10 |pages=8780 |url=https://www.researchgate.net/publication/233987484 |last1=Eberle |first1=Ulrich |first2=Bernd |last2=Mueller |first3=Rittmar |last3=von Helmolt |access-date=2014-12-19 |doi=10.1039/C2EE22596D |year=2012 |bibcode=2012EnEnS...5.8780E |archive-date=2014-02-09 |archive-url=https://web.archive.org/web/20140209172012/http://www.researchgate.net/publication/233987484_Fuel_cell_electric_vehicles_and_hydrogen_infrastructure_status_2012?ev=prf_pub |url-status=live}} Car manufacturers including Honda{{cite web |url=http://world.honda.com/FCXClarity/package/index.html |website=Honda Worldwide |title=FCX Clarity |access-date=2012-01-08 |archive-date=2011-12-09 |archive-url=https://web.archive.org/web/20111209051037/http://world.honda.com/FCXClarity/package/index.html |url-status=live}} and Nissan{{cite web |url=http://www.nissan-global.com/EN/TECHNOLOGY/INTRODUCTION/DETAILS/XTRAIL-FCV/index.html |title=X-TRAIL FCV '03 model |archive-url=https://web.archive.org/web/20100917105718/http://www.nissan-global.com/EN/TECHNOLOGY/INTRODUCTION/DETAILS/XTRAIL-FCV/index.html |archive-date=2010-09-17 |website=Nissan-global.com |access-date=2012-01-08}} have been developing this solution.

Liquid hydrogen tanks for cars, producing for example the BMW Hydrogen 7. Japan has a liquid hydrogen (LH2) storage site in Kobe port.{{cite web |last1=Savvides |first1=Nick |title=Japan plans to use imported liquefied hydrogen to fuel Tokyo 2020 Olympics |url=https://fairplay.ihs.com/ship-construction/article/4280251/japan-plans-to-use-imported-liquefied-hydrogen-to-fuel-tokyo-2020-olympics |website=Fairplay |publisher=IHS Markit Maritime Portal |access-date=22 April 2018 |date=2017-01-11 |archive-date=2018-04-23 |archive-url=https://web.archive.org/web/20180423033549/https://fairplay.ihs.com/ship-construction/article/4280251/japan-plans-to-use-imported-liquefied-hydrogen-to-fuel-tokyo-2020-olympics |url-status=live}} Hydrogen is liquefied by reducing its temperature to −253 °C, similar to liquefied natural gas (LNG) which is stored at −162 °C. A potential efficiency loss of only 12.79% can be achieved, or 4.26 kW⋅h/kg out of 33.3 kW⋅h/kg.{{cite journal |last1=Sadaghiani |first1=Mirhadi S. |title=Introducing and energy analysis of a novel cryogenic hydrogen liquefaction process configuration |journal=International Journal of Hydrogen Energy |date=2 March 2017 |volume=42 |issue=9 |pages=6033–6050 |doi=10.1016/j.ijhydene.2017.01.136|bibcode=2017IJHE...42.6033S }}

[[File:DOE FCTO hydrogen storage materials capacity chart.png|thumb|upright=2|Hydrogen gravimetric capacity of proposed storage materials for hydrogen fuel as a function of hydrogen release temperature. The targets have since been lowered.{{cite web |title=Target Explanation Document: Onboard Hydrogen Storage forLight-Duty Fuel Cell Vehicles |url=https://www.energy.gov/sites/default/files/2017/05/f34/fcto_targets_onboard_hydro_storage_explanation.pdf |publisher=US Department of Energy |archive-url=https://web.archive.org/web/20210411153121/https://www.energy.gov/sites/default/files/2017/05/f34/fcto_targets_onboard_hydro_storage_explanation.pdf |archive-date=Apr 11, 2021|url-status=live}}

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Chemical storage could offer high storage performance due to the high storage densities. For example, supercritical hydrogen at 30 °C and 500 bar only has a density of 15.0 mol/L while methanol has a hydrogen density of 49.5 mol H₂/L methanol and saturated dimethyl ether at 30 °C and 7 bar has a density of 42.1 mol H₂/L dimethyl ether.{{Citation needed|date=March 2024}}

Regeneration of storage material is problematic. A large number of chemical storage systems have been investigated. H₂ release can be induced by hydrolysis reactions or catalyzed dehydrogenation reactions. Illustrative storage compounds are hydrocarbons, boron hydrides, ammonia, and alane etc.{{Cite journal |url=https://zenodo.org/record/1258818 |title=The U.S. Department of Energy's National Hydrogen Storage Project: Progress towards meeting hydrogen-powered vehicle requirements |last=Sunita |first=Satyapal |date=2007 |journal=Catalysis Today |doi=10.1016/j.cattod.2006.09.022 |volume=120 |issue=3–4 |pages=246–256 |access-date=2019-06-27 |archive-date=2019-10-21 |archive-url=https://web.archive.org/web/20191021020448/https://zenodo.org/record/1258818 |url-status=live}} A most promising chemical approach is electrochemical hydrogen storage, as the release of hydrogen can be controlled by the applied electricity.{{Cite journal |title=Electrochemical hydrogen storage: Opportunities for fuel storage, batteries, fuel cells, and supercapacitors |last1=Eftekhari |first1=Ali |last2=Baizeng |first2=Fang |date=2017 |journal=International Journal of Hydrogen Energy |doi=10.1016/j.ijhydene.2017.08.103 |volume=42 |issue=40 |pages=25143–25165|bibcode=2017IJHE...4225143E }} Most of the materials listed below can be directly used for electrochemical hydrogen storage.

Nanomaterials, particularly those produced by ball mill and severe plastic deformation, offer an alternative that overcomes the two major barriers of bulk materials, rate of sorption and activation.{{cite journal |last1=Edalati |first1=K. |last2=Akiba |first2=E. |last3=Botta |first3=W.J. |last4=Estrin |first4=Y. |last5=Floriano |first5=R. |last6=Fruchart |first6=D. |last7=Grosdidier |first7=T. |last8=Horita |first8=Z. |last9=Huot |first9=J. |last10=Li |first10=H.W. |last11=Lin |first11=H.J. |last12=Révész |first12=Á. |last13=Zehetbauer |first13=M.J. |title=Impact of severe plastic deformation on kinetics and thermodynamics of hydrogen storage in magnesium and its alloys |journal=Journal of Materials Science and Technology |date=May 2023 |volume=146 |pages=221–239 |doi=10.1016/j.jmst.2022.10.068 |s2cid=255120922 |arxiv=2301.05009 }} High-entropy alloy materials such as TiZrCrMnFeNi also show advantages of fast and reversible hydrogen storage at room temperature with good storage capacity for stationary applications.{{cite journal |last1=Edalati |first1=P. |last2=Floriano |first2=R. |last3=Mohammadi |first3=A. |last4=Li |first4=Y. | last5=Zepon |first5=G. |last6=Li |first6=H.W. |last7=Edalati |first7=K. |title=Reversible room temperature hydrogen storage in high-entropy alloy TiZrCrMnFeNi |journal=Scripta Materialia |date=March 2020 |volume=178 |pages=387–390 |doi=10.1016/j.scriptamat.2019.12.009 |s2cid=213782769 }}{{cite journal |last1=Dangwal |first1=S. |last2=Ikeda |first2=Y. |last3=Grabowski |first3=B. |last4=Edalati |first4=K. |title=Machine learning to design high-entropy alloys with desired enthalpy for room-temperature hydrogen storage: Comparison with density functional theory and experiments |journal=Chemical Engineering Journal |date=August 2024 |volume=493 |pages=152606 |doi=10.1016/j.cej.2024.152606 |s2cid=270086446 |arxiv=2405.19838 }}

Enhancement of sorption kinetics and storage capacity can be improved through nanomaterial-based catalyst doping, as shown in the work of the Clean Energy Research Center in the University of South Florida.{{cite journal |last1=Niemann |first1=Michael U. |last2=Srinivasan |first2=Sesha S. |last3=Phani |first3=Ayala R. |last4=Kumar |first4=Ashok |last5=Goswami |first5=D. Yogi |last6=Stefanakos |first6=Elias K. |title=Nanomaterials for hydrogen storage applications: a review |journal=Journal of Nanomaterials |date=2008 |volume=2008 |pages=1–9 |doi=10.1155/2008/950967|doi-access=free}} This research group studied LiBH₄ doped with nickel nanoparticles and analyzed the weight loss and release temperature of the different species. They observed that an increasing amount of nanocatalyst lowers the release temperature by approximately 20 °C and increases the weight loss of the material by 2-3%. The optimum amount of Ni particles was found to be 3 mol%, for which the temperature was within the limits established (around 100 °C) and the weight loss was notably greater than the undoped species.

The rate of hydrogen sorption improves at the nanoscale due to the short diffusion distance in comparison to bulk materials. They also have favorable surface-area-to-volume ratio.

The release temperature of a material is defined as the temperature at which the desorption process begins. The energy or temperature to induce release affects the cost of any chemical storage strategy. If the hydrogen is bound too weakly, the pressure needed for regeneration is high, thereby cancelling 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).[http://ec.europa.eu/research/energy/pdf/efchp_hydrogen3.pdf EU Hydrogen Storage] {{Webarchive|url=https://web.archive.org/web/20121025215906/http://ec.europa.eu/research/energy/pdf/efchp_hydrogen3.pdf |date=2012-10-25 |access-date=2012-01-08}}. (PDF) A modified van 't Hoff equation, relates temperature and partial pressure of hydrogen during the desorption process. The modifications to the standard equation are related to size effects at the nanoscale.

{{Equation box 1

|indent =:

|equation = $\backslash ln(p\_\backslash mathrm\{H\_\backslash mathrm\{2\}\})\; =\; \backslash frac\{\backslash Delta\; H(r)\}\{RT\}+\backslash frac\{3\; V\_\backslash mathrm\{m\}\; \backslash gamma\}\{rRT\}+\backslash frac\{\backslash Delta\; S(r)\}\{R\}$

|border colour = #50c870

|background colour = #d5f6d2}}

Where {{math|p_H2}} is the partial pressure of hydrogen, {{math|ΔH}} is the enthalpy of the sorption process (exothermic), {{math|ΔS}} is the change in entropy, {{math|R}} is the ideal gas constant, T is the temperature in Kelvin, {{math|V_m}} is the molar volume of the metal, {{math|r}} is the radius of the nanoparticle and {{math|γ}} is the surface free energy of the particle.

From the above relation we see that the enthalpy and entropy change of desorption processes depend on the radius of the nanoparticle. Moreover, a new term is included that takes into account the specific surface area of the particle and it can be mathematically proven that a decrease in particle radius leads to a decrease in the release temperature for a given partial pressure.{{cite journal |last1=Sunandana |first1=C.S. |date=2007 |title=Nanomaterials for hydrogen storage |journal=Resonance |volume=12 |issue=5 |pages=31–36 |doi=10.1007/s12045-007-0047-9 |s2cid=118701455}}

Hydrogenation of CO₂ to methanol has been evaluated for hydrogen storage. Barriers of CO₂ hydrogenation includes purification of captured CO₂, H₂ source from splitting water and energy inputs for hydrogenation. For industrial applications, CO₂ is often converted to methanol. Until now, much progress has been made for CO₂ to C1 molecules. However, CO₂ to high value molecules still face many roadblocks and the future of CO₂ hydrogenation depends on the advancement of catalytic technologies.{{Cite journal |last=WEATHERBEE |first=G |date=October 1982 |title=Hydrogenation of CO2 on group VIII metals II. Kinetics and mechanism of CO2 hydrogenation on nickel |url=http://dx.doi.org/10.1016/0021-9517(82)90186-5 |journal=Journal of Catalysis |volume=77 |issue=2 |pages=460–472 |doi=10.1016/0021-9517(82)90186-5 |issn=0021-9517 |access-date=2021-11-19 |archive-date=2022-10-16 |archive-url=https://web.archive.org/web/20221016173302/https://www.sciencedirect.com/science/article/abs/pii/0021951782901865?via%3Dihub |url-status=live|url-access=subscription }}

Image:Metal hydride hydrogen.storage.graph.gif

Metal hydrides, such as MgH₂, NaAlH₄, LiAlH₄, LiH, LaNi₅H₆, TiFeH₂, ammonia borane, and palladium hydride represent sources of stored hydrogen. There are three main classes of metal hydrides:{{Cite journal |last1=Schlapbach |first1=Louis |last2=Züttel |first2=Andreas |date=November 2001 |title=Hydrogen-storage materials for mobile applications |url=https://www.nature.com/articles/35104634 |journal=Nature |language=en |volume=414 |issue=6861 |pages=353–358 |doi=10.1038/35104634 |pmid=11713542 |bibcode=2001Natur.414..353S |issn=0028-0836|url-access=subscription }}{{Cite journal |last1=Orimo |first1=Shin-ichi |last2=Nakamori |first2=Yuko |last3=Eliseo |first3=Jennifer R. |last4=Züttel |first4=Andreas |last5=Jensen |first5=Craig M. |date=2007-10-01 |title=Complex Hydrides for Hydrogen Storage |url=https://pubs.acs.org/doi/10.1021/cr0501846 |journal=Chemical Reviews |language=en |volume=107 |issue=10 |pages=4111–4132 |doi=10.1021/cr0501846 |pmid=17848101 |issn=0009-2665|url-access=subscription }}

• Inter-metallic Hydrides: exhibit fast kinetics and moderate hydrogen capacities. Such as LaNi₅H₆, TiFeH₂.
• Complex Hydrides: capable of higher hydrogen storage capacities but require catalysts. Such as NaAlH₄, LiBH₄.
• Lightweight Hydrides: offer high gravimetric hydrogen storage but require high temperatures for desorption. Such as MgH₂, CaH₂.

Here are the properties of some metal hydrides:{{Cite journal |last1=Li |first1=Hai-Wen |last2=Yan |first2=Yigang |last3=Orimo |first3=Shin-ichi |last4=Züttel |first4=Andreas |last5=Jensen |first5=Craig M. |date=2011-01-24 |title=Recent Progress in Metal Borohydrides for Hydrogen Storage |journal=Energies |language=en |volume=4 |issue=1 |pages=185–214 |doi=10.3390/en4010185 |doi-access=free |issn=1996-1073}}

Again the persistent problems are the % weight of H₂ that they carry and the reversibility of the storage process.[http://www1.eere.energy.gov/hydrogenandfuelcells/storage/metal_hydrides.html DOE Metal hydrides] {{Webarchive|url=https://web.archive.org/web/20080131232123/http://www1.eere.energy.gov/hydrogenandfuelcells/storage/metal_hydrides.html |date=2008-01-31 |access-date=2012-01-08}}. eere.energy.gov (2008-12-19) Some are easy-to-fuel liquids at ambient temperature and pressure, whereas others are solids which could be turned into pellets. These materials have good energy density, although their specific energy is often worse than the leading hydrocarbon fuels.

An alternative method for lowering dissociation temperatures is doping with activators. This strategy has been used for aluminium hydride, but the complex synthesis makes the approach unattractive.{{Cite web |last1=Graetz |first1=J. |last2=Reilly |first2=J. |last3=Sandrock |first3=G. |last4=Johnson |first4=J. |last5=Zhou |first5=W. M. |last6=Wegrzyn |first6=J. |doi=10.2172/899889 |title=Aluminum Hydride, A1H3, As a Hydrogen Storage Compound |year=2006 |url=https://digital.library.unt.edu/ark:/67531/metadc887034/ |access-date=2019-06-27 |archive-date=2019-10-21 |archive-url=https://web.archive.org/web/20191021222939/https://digital.library.unt.edu/ark:/67531/metadc887034/ |url-status=live}}

Proposed hydrides for use in a hydrogen economy include simple hydrides of magnesium[http://neel.cnrs.fr/spip.php?article1281 CNRS Institut Neel H2 Storage] {{Webarchive|url=https://web.archive.org/web/20160303200554/http://neel.cnrs.fr/spip.php?article1281 |date=2016-03-03 |access-date=2012-01-08}}. Neel.cnrs.fr 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. A French company McPhy Energy is developing the first industrial product, based on magnesium hydride, already sold to some major clients such as Iwatani and ENEL.{{cn|date=May 2025}}

Reversible hydrogen storage is exhibited by frustrated Lewis pairs.{{cite journal |title=Reversible, Metal-Free Hydrogen Activation |doi=10.1126/science.1134230 |year=2006 |last1=Welch |first1=G. C. |last2=Juan |first2=R. R. S. |last3=Masuda |first3=J. D. |last4=Stephan |first4=D. W. |journal=Science |volume=314 |issue=5802 |pages=1124–6 |pmid=17110572 |bibcode=2006Sci...314.1124W |s2cid=20333088}}Elizabeth Wilson [http://pubs.acs.org/cen/news/84/i47/8447notw8.html H2 Activation, Reversibly Metal-free compound readily breaks and makes hydrogen] {{Webarchive|url=https://web.archive.org/web/20061127181146/http://pubs.acs.org/cen/news/84/i47/8447notw8.html |date=2006-11-27}}, Chemical & Engineering News November 20, 2006Mes stands for a mesityl substituent and C₆F₅ for a pentafluorophenyl group, see also tris(pentafluorophenyl)boron

Image:Phosphinoboranehydrogenstorage 2.png

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%.

{{Main|Aluminium}}

Hydrogen is produced by hydrolysis of aluminium. It was previously believed that, to react with water, aluminium must be stripped of its natural oxide passivation layer,{{Cite web |url=https://phys.org/news/2007-05-hydrogen-aluminium-alloy-fuel-cells.html |title=New process generates hydrogen from aluminium alloy to run engines, fuel cells |website=phys.org}} or mixing with gallium (which produces aluminium nanoparticles that allow 90% of the aluminium to react).{{Cite web |last=Blain |first=Loz |date=2022-09-02 |title=Aluminum-gallium powder bubbles hydrogen out of dirty water |url=https://newatlas.com/energy/aluminum-gallium-hydrogen-powder/ |access-date=2022-09-04 |website=New Atlas |language=en-US |archive-date=2022-09-04 |archive-url=https://web.archive.org/web/20220904153555/https://newatlas.com/energy/aluminum-gallium-hydrogen-powder/ |url-status=live}} It has since been demonstrated that efficient reaction is possible by increasing the temperature and pressure of the reaction.{{Cite journal |last1=Trowell |first1=Keena A. |last2=Goroshin |first2=Sam |last3=Frost |first3=David L. |last4=Bergthorson |first4=Jeffrey M. |date=2020 |title=The use of supercritical water for the catalyst-free oxidation of coarse aluminum for hydrogen production |url=http://xlink.rsc.org/?DOI=D0SE00996B |journal=Sustainable Energy & Fuels |language=en |volume=4 |issue=11 |pages=5628–5635 |doi=10.1039/D0SE00996B |s2cid=225254629 |issn=2398-4902 |access-date=2022-09-06 |archive-date=2022-10-16 |archive-url=https://web.archive.org/web/20221016173320/https://pubs.rsc.org/en/content/articlelanding/2020/SE/D0SE00996B |url-status=live|url-access=subscription }} The byproduct of the reaction to create hydrogen is aluminium oxide, which can be recycled back into aluminium with the Hall–Héroult process, making the reaction theoretically renewable. Although this requires electrolysis, which consumes a large amount of energy, the energy is then stored in the aluminium (and released when the aluminium is reacted with water).

Traditional MgH₂ stores 7.6 wt% hydrogen, but its high desorption temperature (>300 °C) limits applications. Mg-Ti-V nanocomposites can lower the desorption temperature to below 200 °C. Carbon-coordinated MgH₂ exhibits 80% of improvement on cycling stability over 1000 cycles.{{Cite journal |last1=Yu |first1=Yuan-Hsiang |last2=Yeh |first2=Jui-Ming |last3=Liou |first3=Shir-Joe |last4=Chang |first4=Yen-Po |date=January 2004 |title=Organo-soluble polyimide (TBAPP–OPDA)/clay nanocomposite materials with advanced anticorrosive properties prepared from solution dispersion technique |url=https://linkinghub.elsevier.com/retrieve/pii/S1359645403005652 |journal=Acta Materialia |language=en |volume=52 |issue=2 |pages=475–486 |doi=10.1016/j.actamat.2003.09.031|bibcode=2004AcMat..52..475Y }}

LiBH₄ + MgH₂ composites stored about 11 wt% of hydrogen, one of the highest capacities reported. And ammonia borane (H₃NBH₃) releases 12 wt% hydrogen at moderate temperatures (~100–150 °C).

Mg-based hydrogen storage materials include pure Mg, Mg-based alloys, and Mg-based composites.{{Cite journal |last1=Zhao |first1=Dong-Liang |last2=Zhang |first2=Yang-Huan |date=2014-10-01 |title=Research progress in Mg-based hydrogen storage alloys |url=https://doi.org/10.1007/s12598-014-0398-9 |journal=Rare Metals |language=en |volume=33 |issue=5 |pages=499–510 |doi=10.1007/s12598-014-0398-9 |bibcode=2014RareM..33..499Z |s2cid=98790485 |issn=1867-7185 |access-date=2021-11-20 |archive-date=2022-10-16 |archive-url=https://web.archive.org/web/20221016173320/https://link.springer.com/article/10.1007/s12598-014-0398-9 |url-status=live|url-access=subscription }} Nonetheless, the inferior hydrogen absorption/desorption kinetics rooting in the overly undue thermodynamic stability of metal hydride make the Mg-based hydrogen storage alloys currently not appropriate for the real applications, and therefore, massive attempts have been dedicated to overcoming these shortages. Some sample preparation methods, such as smelting, powder sintering, diffusion, mechanical alloying, the hydriding combustion synthesis method, surface treatment, and heat treatment, etc., have been broadly employed for altering the dynamic performance and cycle life of Mg-based hydrogen storage alloys. Besides, some intrinsic modification strategies, including alloying,{{Cite journal |last1=Cui |first1=N. |last2=He |first2=P. |last3=Luo |first3=J. 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Q. |date=2006-02-01 |title=Composite structure and hydrogen storage properties in Mg-base alloys |url=https://www.sciencedirect.com/science/article/pii/S0360319905001357 |journal=International Journal of Hydrogen Energy |series=HTM 2004 |language=en |volume=31 |issue=2 |pages=251–257 |doi=10.1016/j.ijhydene.2005.04.030 |bibcode=2006IJHE...31..251Z |issn=0360-3199|url-access=subscription }}{{Cite journal |last1=Lu |first1=Chong |last2=Zou |first2=Jianxin |last3=Zeng |first3=Xiaoqin |last4=Ding |first4=Wenjiang |last5=Shao |first5=Huaiyu |date=2019-11-25 |title=Enhanced hydrogen sorption properties of core-shell like structured Mg@NaBH4/MgB2 composite |url=https://www.sciencedirect.com/science/article/pii/S0925838819329962 |journal=Journal of Alloys and Compounds |language=en |volume=810 |pages=151763 |doi=10.1016/j.jallcom.2019.151763 |s2cid=202212456 |issn=0925-8388|url-access=subscription }} etc., have been mainly explored for intrinsically boosting the performance of Mg-based hydrogen storage alloys.{{Cite journal |last1=Ouyang |first1=Liuzhang |last2=Liu |first2=Fen |last3=Wang |first3=Hui |last4=Liu |first4=Jiangwen |last5=Yang |first5=Xu-Sheng |last6=Sun |first6=Lixian |last7=Zhu |first7=Min |date=August 2020 |title=Magnesium-based hydrogen storage compounds: A review |url=https://linkinghub.elsevier.com/retrieve/pii/S0925838820312287 |journal=Journal of Alloys and Compounds |language=en |volume=832 |pages=154865 |doi=10.1016/j.jallcom.2020.154865 |hdl=10397/104153 |s2cid=216182360 |access-date=2021-11-20 |archive-date=2022-06-15 |archive-url=https://web.archive.org/web/20220615181804/https://linkinghub.elsevier.com/retrieve/pii/S0925838820312287 |url-status=live|hdl-access=free }} Like aluminium, magnesium also reacts with water to produce hydrogen.{{Cite journal |last1=Yabe |first1=T. |last2=Bagheri |first2=B. |last3=Ohkubo |first3=T. |last4=Uchida |first4=S. |last5=Yoshida |first5=K. |last6=Funatsu |first6=T. |last7=Oishi |first7=T. |last8=Daito |first8=K. |last9=Ishioka |first9=M. |last10=Yasunaga |first10=N. |last11=Sato |first11=Y. |last12=Baasandash |first12=C. |last13=Okamoto |first13=Y. |last14=Yanagitani |first14=K. |date=2008-10-15 |title=100 W-class solar pumped laser for sustainable magnesium-hydrogen energy cycle |url=http://aip.scitation.org/doi/10.1063/1.2998981 |journal=Journal of Applied Physics |language=en |volume=104 |issue=8 |pages=083104–083104–8 |doi=10.1063/1.2998981 |bibcode=2008JAP...104h3104Y |issn=0021-8979 |access-date=2022-09-06 |archive-date=2022-08-02 |archive-url=https://web.archive.org/web/20220802091956/https://aip.scitation.org/doi/10.1063/1.2998981 |url-status=live|url-access=subscription }}

Of the primary hydrogen storage alloys progressed formerly, Mg and Mg-based hydrogen storage materials are believed to provide the remarkable possibility of the practical application, on account of the advantages as following: 1) the resource of Mg is plentiful and economical. Mg element exists abundantly and accounts for ≈2.35% of the earth's crust with the rank of the eighth; 2) low density of merely 1.74 g cm-3; 3) superior hydrogen storage capacity. The theoretical hydrogen storage amounts of the pure Mg is 7.6 wt % (weight percent),{{Cite journal |last1=Li |first1=Jun-Jiao |last2=Wang |first2=Chong-Chen |last3=Guo |first3=Jie |last4=Cui |first4=Jing-Rui |last5=Wang |first5=Peng |last6=Zhao |first6=Chen |date=2018-01-08 |title=Three coordination compounds based on tris(1-imidazolyl)benzene: Hydrothermal synthesis, crystal structure and adsorption performances toward organic dyes |url=https://www.sciencedirect.com/science/article/pii/S0277538717306526 |journal=Polyhedron |language=en |volume=139 |pages=89–97 |doi=10.1016/j.poly.2017.10.011 |issn=0277-5387|url-access=subscription }}{{Cite web |title=Scopus preview - Scopus - Welcome to Scopus |url=https://www.scopus.com/home.uri |access-date=2021-11-19 |website=www.scopus.com |archive-date=2019-09-06 |archive-url=https://web.archive.org/web/20190906222238/https://www.scopus.com/home.uri |url-status=live}}{{Cite journal |last1=Shao |first1=Huaiyu |last2=Xin |first2=Gongbiao |last3=Zheng |first3=Jie |last4=Li |first4=Xingguo |last5=Akiba |first5=Etsuo |date=2012-07-01 |title=Nanotechnology in Mg-based materials for hydrogen storage |url=https://www.sciencedirect.com/science/article/pii/S2211285512001127 |journal=Nano Energy |language=en |volume=1 |issue=4 |pages=590–601 |doi=10.1016/j.nanoen.2012.05.005 |bibcode=2012NEne....1..590S |issn=2211-2855 |access-date=2021-11-20 |archive-date=2012-11-07 |archive-url=https://web.archive.org/web/20121107043425/http://www.sciencedirect.com/science/article/pii/S2211285512001127 |url-status=live|url-access=subscription }} and the Mg2Ni is 3.6 wt%, respectively.

Lithium alanate (LiAlH₄) was synthesized for the first time in 1947 by dissolution of lithium hydride in an ether solution of aluminium chloride.{{Cite journal |last1=Fichtner |first1=Maximilian |last2=Engel |first2=Jens |last3=Fuhr |first3=Olaf |last4=Glöss |first4=Andreas |last5=Rubner |first5=Oliver |last6=Ahlrichs |first6=Reinhart |date=2003-10-01 |title=The Structure of Magnesium Alanate |url=http://dx.doi.org/10.1021/ic034160y |journal=Inorganic Chemistry |volume=42 |issue=22 |pages=7060–7066 |doi=10.1021/ic034160y |pmid=14577773 |issn=0020-1669|url-access=subscription }} LiAlH₄ has a theoretical gravimetric capacity of 10.5 wt %H₂ and dehydrogenates in the following three steps:{{Cite journal |last1=Dilts |first1=J. A. |last2=Ashby |first2=E. C. |date=June 1972 |title=Thermal decomposition of complex metal hydrides |url=http://dx.doi.org/10.1021/ic50112a015 |journal=Inorganic Chemistry |volume=11 |issue=6 |pages=1230–1236 |doi=10.1021/ic50112a015 |issn=0020-1669|url-access=subscription }}{{Cite journal |last1=RESAN |first1=M |last2=HAMPTON |first2=M |last3=LOMNESS |first3=J |last4=SLATTERY |first4=D |date=October 2005 |title=Effects of various catalysts on hydrogen release and uptake characteristics of LiAlH |url=http://dx.doi.org/10.1016/j.ijhydene.2004.12.009 |journal=International Journal of Hydrogen Energy |volume=30 |issue=13–14 |pages=1413–1416 |doi=10.1016/j.ijhydene.2004.12.009 |issn=0360-3199 |access-date=2021-11-20 |archive-date=2022-10-16 |archive-url=https://web.archive.org/web/20221016173326/https://www.sciencedirect.com/science/article/abs/pii/S0360319904004392?via%3Dihub |url-status=live|url-access=subscription }}{{Cite journal |last1=Ares |first1=J.R. |last2=Aguey-Zinsou |first2=K.-F. |last3=Porcu |first3=M. |last4=Sykes |first4=J.M. |last5=Dornheim |first5=M. |last6=Klassen |first6=T. |last7=Bormann |first7=R. |date=May 2008 |title=Thermal and mechanically activated decomposition of LiAlH₄ |url=http://dx.doi.org/10.1016/j.materresbull.2007.05.018 |journal=Materials Research Bulletin |volume=43 |issue=5 |pages=1263–1275 |doi=10.1016/j.materresbull.2007.05.018 |issn=0025-5408|url-access=subscription }} 3LiAlH₄ ↔ Li₃AlH₆ + 3H₂ + 2Al (423–448 K; 5.3 wt %H₂; ∆H = −10 kJ·mol−1 H₂); Li₃AlH₆ ↔ 3LiH + Al + 1.5H₂ (453–493 K; 2.6 wt %H₂; ∆H = 25 kJ·mol−1 H₂); 3LiH + 3Al ↔ 3LiAl + 3/2H₂ (>673 K; 2.6 wt %H₂; ∆H = 140 kJ·mol−1 H₂).{{Cite journal |last1=Milanese |first1=Chiara |last2=Garroni |first2=Sebastiano |last3=Gennari |first3=Fabiana |last4=Marini |first4=Amedeo |last5=Klassen |first5=Thomas |last6=Dornheim |first6=Martin |last7=Pistidda |first7=Claudio |date=2018-07-24 |title=Solid State Hydrogen Storage in Alanates and Alanate-Based Compounds: A Review |journal=Metals |volume=8 |issue=8 |pages=567 |doi=10.3390/met8080567 |issn=2075-4701|doi-access=free|hdl=11336/97224 |hdl-access=free}} 50px Text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License] {{Webarchive|url=https://web.archive.org/web/20171016050101/https://creativecommons.org/licenses/by/4.0/ |date=2017-10-16}}. The first two steps lead to a total amount of hydrogen released equal to 7.9 wt %, which could be attractive for practical applications, but the working temperatures and the desorption kinetics are still far from the practical targets. Several strategies have been applied in the last few years to overcome these limits, such as ball-milling and catalysts additions.{{Cite journal |last1=Balema |first1=V.P. |last2=Wiench |first2=J.W. |last3=Dennis |first3=K.W. |last4=Pruski |first4=M. |last5=Pecharsky |first5=V.K. |date=November 2001 |title=Titanium catalyzed solid-state transformations in LiAlH₄ during high-energy ball-milling |url=http://dx.doi.org/10.1016/s0925-8388(01)01570-5 |journal=Journal of Alloys and Compounds |volume=329 |issue=1–2 |pages=108–114 |doi=10.1016/s0925-8388(01)01570-5 |issn=0925-8388 |access-date=2021-11-20 |archive-date=2022-10-16 |archive-url=https://web.archive.org/web/20221016173326/https://www.sciencedirect.com/science/article/abs/pii/S0925838801015705?via%3Dihub |url-status=live|url-access=subscription }}{{Cite journal |last1=Ismail |first1=M. |last2=Zhao |first2=Y. |last3=Yu |first3=X.B. |last4=Ranjbar |first4=A. |last5=Dou |first5=S.X. |date=March 2011 |title=Improved hydrogen desorption in lithium alanate by addition of SWCNT–metallic catalyst composite |url=http://dx.doi.org/10.1016/j.ijhydene.2010.12.050 |journal=International Journal of Hydrogen Energy |volume=36 |issue=5 |pages=3593–3599 |doi=10.1016/j.ijhydene.2010.12.050 |bibcode=2011IJHE...36.3593I |issn=0360-3199 |access-date=2021-11-20 |archive-date=2022-10-16 |archive-url=https://web.archive.org/web/20221016173509/https://www.sciencedirect.com/science/article/abs/pii/S036031991002392X?via%3Dihub |url-status=live|url-access=subscription }}{{Cite journal |last1=Li |first1=Li |last2=Qiu |first2=Fangyuan |last3=Wang |first3=Yijing |last4=Xu |first4=Yanan |last5=An |first5=Cuihua |last6=Liu |first6=Guang |last7=Jiao |first7=Lifang |last8=Yuan |first8=Huatang |date=March 2013 |title=Enhanced hydrogen storage properties of TiN–LiAlH₄ composite |url=http://dx.doi.org/10.1016/j.ijhydene.2013.01.088 |journal=International Journal of Hydrogen Energy |volume=38 |issue=9 |pages=3695–3701 |doi=10.1016/j.ijhydene.2013.01.088 |issn=0360-3199 |access-date=2021-11-20 |archive-date=2022-10-16 |archive-url=https://web.archive.org/web/20221016173507/https://www.sciencedirect.com/science/article/abs/pii/S0360319913001973?via%3Dihub |url-status=live|url-access=subscription }}{{Cite journal |last1=Li |first1=Zhibao |last2=Liu |first2=Shusheng |last3=Si |first3=Xiaoliang |last4=Zhang |first4=Jian |last5=Jiao |first5=Chengli |last6=Wang |first6=Shuang |last7=Liu |first7=Shuang |last8=Zou |first8=Yong-Jin |last9=Sun |first9=Lixian |last10=Xu |first10=Fen |date=February 2012 |title=Significantly improved dehydrogenation of LiAlH₄ destabilized by K2TiF6 |url=http://dx.doi.org/10.1016/j.ijhydene.2011.10.038 |journal=International Journal of Hydrogen Energy |volume=37 |issue=4 |pages=3261–3267 |doi=10.1016/j.ijhydene.2011.10.038 |issn=0360-3199 |access-date=2021-11-20 |archive-date=2022-10-16 |archive-url=https://web.archive.org/web/20221016173508/https://www.sciencedirect.com/science/article/abs/pii/S0360319911023974?via%3Dihub |url-status=live|url-access=subscription }}{{Cite journal |last1=Tan |first1=Chia-Yen |last2=Tsai |first2=Wen-Ta |date=August 2015 |title=Catalytic and inhibitive effects of Pd and Pt decorated MWCNTs on the dehydrogenation behavior of LiAlH₄ |url=http://dx.doi.org/10.1016/j.ijhydene.2015.06.106 |journal=International Journal of Hydrogen Energy |volume=40 |issue=32 |pages=10185–10193 |doi=10.1016/j.ijhydene.2015.06.106 |issn=0360-3199 |access-date=2021-11-20 |archive-date=2022-10-16 |archive-url=https://web.archive.org/web/20221016173507/https://www.sciencedirect.com/science/article/abs/pii/S0360319915015918?via%3Dihub |url-status=live|url-access=subscription }}

Potassium Alanate (KAlH₄) was first prepared by Ashby et al.{{Cite journal |last1=Ashby |first1=E. C. |last2=Kobetz |first2=P. |date=September 1966 |title=The Direct Synthesis of Na₃AlH₆ |url=http://dx.doi.org/10.1021/ic50043a034 |journal=Inorganic Chemistry |volume=5 |issue=9 |pages=1615–1617 |doi=10.1021/ic50043a034 |issn=0020-1669|url-access=subscription }} by one-step synthesis in toluene, tetrahydrofuran, and diglyme. Concerning the hydrogen absorption and desorption properties, this alanate was only scarcely studied. Morioka et al.,{{Cite journal |last1=Morioka |first1=Hiroyuki |last2=Kakizaki |first2=Kenichi |last3=Chung |first3=Sai-Cheong |last4=Yamada |first4=Atsuo |date=April 2003 |title=Reversible hydrogen decomposition of KAlH₄ |url=http://dx.doi.org/10.1016/s0925-8388(02)01307-5 |journal=Journal of Alloys and Compounds |volume=353 |issue=1–2 |pages=310–314 |doi=10.1016/s0925-8388(02)01307-5 |issn=0925-8388 |access-date=2021-11-20 |archive-date=2022-10-16 |archive-url=https://web.archive.org/web/20221016173507/https://www.sciencedirect.com/science/article/abs/pii/S0925838802013075?via%3Dihub |url-status=live|url-access=subscription }} by temperature programmed desorption (TPD) analyses, proposed the following dehydrogenation mechanism: 3KAlH₄ →K₃AlH₆ + 2Al + 3H₂ (573 K, ∆H = 55 kJ·mol−1 H₂; 2.9 wt %H₂), K₃AlH₆ → 3KH + Al + 3/2H₂ (613 K, ∆H = 70 kJ·mol−1 H₂; 1.4 wt %H₂), 3KH → 3K + 3/2H₂ (703 K, 1.4 wt %H₂). These reactions were demonstrated reversible without catalysts addition at relatively low hydrogen pressure and temperatures. The addition of TiCl3 was found to decrease the working temperature of the first dehydrogenation step of 50 K,{{Cite journal |last1=Ares |first1=Jose R. |last2=Aguey-Zinsou |first2=Kondo-Francois |last3=Leardini |first3=Fabrice |last4=Ferrer |first4=Isabel Jímenez |last5=Fernandez |first5=Jose-Francisco |last6=Guo |first6=Zheng-Xiao |last7=Sánchez |first7=Carlos |date=2009-03-26 |title=Hydrogen Absorption/Desorption Mechanism in Potassium Alanate (KAlH₄) and Enhancement by TiCl₃ Doping |url=http://dx.doi.org/10.1021/jp807184v |journal=The Journal of Physical Chemistry C |volume=113 |issue=16 |pages=6845–6851 |doi=10.1021/jp807184v |s2cid=93043691 |issn=1932-7447|url-access=subscription }} but no variations were recorded for the last two reaction steps.

{{main|Liquid organic hydrogen carriers}}

File:Perhydro-N-Carbazol.PNG

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. Using LOHCs, relatively high gravimetric storage densities can be reached (about 6 wt-%) and the overall energy efficiency is higher than for other chemical storage options such as producing methane from the hydrogen.{{cite journal |doi=10.1002/cite.201100113 |volume=83 |issue=11 |title=Energiespeicherung mittels Methan und energietragenden Stoffen - ein thermodynamischer Vergleich |trans-title=Energy Storage by CO2 Methanization and Energy Carrying Compounds: A Thermodynamic Comparison |language=de |year=2011 |journal=Chemie Ingenieur Technik |pages=2002–2013 |last1=Müller |first1=Benjamin}} Both hydrogenation and dehydrogenation of LOHCs requires catalysts. It was demonstrated that replacing hydrocarbons by hetero-atoms, like N, O etc. improves reversible de/hydrogenation properties.

Cycloalkanes have relatively high hydrogen capacity (6-8 wt %).{{Cite journal |title=Liquid organic hydrogen carriers |journal=Journal of Energy Chemistry |date=2015-09-01 |pages=587–594 |volume=24 |issue=5 |doi=10.1016/j.jechem.2015.08.007 |first1=Teng |last1=He |first2=Qijun |last2=Pei |first3=Ping |last3=Chen |url=https://zenodo.org/record/895538 |doi-access=free |bibcode=2015JEnCh..24..587H |access-date=2019-11-29 |archive-date=2021-03-09 |archive-url=https://web.archive.org/web/20210309021000/https://zenodo.org/record/895538 |url-status=live}} Heterocyclic aromatic compounds (or N-Heterocycles) are also appropriate for this task.{{cite journal |last1=Teichmann |first1=Daniel |last2=Arlt |first2=Wolfgang |last3=Wasserscheid |first3=Peter |last4=Freymann |first4=Raymond |title=A future energy supply based on Liquid Organic Hydrogen Carriers (LOHC) |journal=Energy & Environmental Science |date=2011 |volume=4 |issue=8 |pages=2767–2773 |doi=10.1039/C1EE01454D|bibcode=2011EnEnS...4.2767T }} but many others do exist.{{cite patent |country=US |number=7351395 |status=patent |title= Hydrogen storage by reversible hydrogenation of pi-conjugated substrates}} [https://pubchem.ncbi.nlm.nih.gov/compound/2_3-Dibenzyltoluene Dibenzyltoluene], which is already used as a heat transfer fluid in industry, was identified as potential LOHC. With a wide liquid range between -39 °C (melting point) and 390 °C (boiling point) and a hydrogen storage density of 6.2 wt% dibenzyltoluene is ideally suited as LOHC material.{{cite journal |doi=10.1002/cssc.201300426 |pmid=23956191 |volume=7 |issue=1 |title=Evaluation of Industrially Applied Heat-Transfer Fluids as Liquid Organic Hydrogen Carrier Systems |year=2013 |journal=ChemSusChem |pages=229–235 |last1=Brückner |first1=Nicole}} Formic acid has been suggested as a promising hydrogen storage material with a 4.4wt% hydrogen capacity.{{Cite journal |title=Formic acid as a hydrogen source – recent developments and future trends |issue=8 |pages=8171–8181 |journal=Energy & Environmental Science |volume=5 |doi=10.1039/C2EE21928J |first1=Martin |last1=Grasemann |first2=Gábor |last2=Laurenczy |date=2012-07-18|bibcode=2012EnEnS...5.8171G }}

Cycloalkanes reported as LOHC include cyclohexane, methyl-cyclohexane and decalin. The dehydrogenation of cycloalkanes is highly endothermic (63-69 kJ/mol H₂), which means this process requires high temperature. Dehydrogenation of decalin is the most thermodynamically favored among the three cycloalkanes, and methyl-cyclohexane is second because of the presence of the methyl group.{{Cite journal |title=Kinetic modeling of pure hydrogen production from decalin |journal=Journal of Catalysis |date=2008-01-25 |pages=229–238 |volume=253 |issue=2 |doi=10.1016/j.jcat.2007.11.012 |first1=Bo |last1=Wang |first2=D. Wayne |last2=Goodman |first3=Gilbert F. |last3=Froment}} The dehydrogenation of cycloalkanes is a mature area. Nickel-, molybdenum-, andcat platinum-based catalysts are established. Coking remains a challenge.{{Cite journal |title=Efficient evolution of hydrogen from liquid cycloalkanes over Pt-containing catalysts supported on active carbons under "wet–dry multiphase conditions" |journal=Applied Catalysis A: General |date=2002-07-10 |pages=91–102 |volume=233 |issue=1–2 |doi=10.1016/S0926-860X(02)00139-4 |first1=Nobuko |last1=Kariya |first2=Atsushi |last2=Fukuoka |first3=Masaru |last3=Ichikawa|bibcode=2002AppCA.233...91K }}{{Cite journal |title=Ni/Al2O3 catalysts and their activity in dehydrogenation of methylcyclohexane for hydrogen production |journal=Catalysis Today |date=2008-11-01 |pages=198–202 |volume=138 |series=Selected papers from the EUROPACAT VIII Hydrogen Society Session, Turku, Finland, 26–31 August 2007 |issue=3–4 |doi=10.1016/j.cattod.2008.07.020 |first1=Sevim |last1=Yolcular |first2=Özden |last2=Olgun}}

The temperature required for hydrogenation and dehydrogenation drops significantly for heterocycles vs simple carbocycles.{{Cite journal |title=Computational structure–activity relationships in H2 storage: how placement of N atoms affects release temperatures in organic liquid storage materials |issue=22 |pages=2231–2233 |journal=Chemical Communications |volume= |doi=10.1039/B705037B |pmid=17534500 |first1=Eric |last1=Clot |first2=Odile |last2=Eisenstein |first3=Robert H. |last3=Crabtree |date=2007-05-30}} Among all the N-heterocycles, the saturated-unsaturated pair of dodecahydro-N-ethylcarbazole (12H-NEC) and NEC has been considered as a promising candidate for hydrogen storage with a fairly large hydrogen content (5.8wt%).{{Cite journal |title=Comparison of catalytic performance of supported ruthenium and rhodium for hydrogenation of 9-ethylcarbazole for hydrogen storage applications |doi=10.1039/C2EE22066K |first1=Katarzyna Morawa |last1=Eblagon |first2=Kin |last2=Tam |first3=Shik Chi Edman |last3=Tsang |volume=5 |issue=9 |journal=Energy & Environmental Science |pages=8621 |year=2012|bibcode=2012EnEnS...5.8621E }} The figure on the top right shows dehydrogenation and hydrogenation of the 12H-NEC and NEC pair. The standard catalyst for NEC to 12H-NEC is Ru and Rh based. The selectivity of hydrogenation can reach 97% at 7 MPa and 130 °C-150 °C. Although N-Heterocyles can optimize the unfavorable thermodynamic properties of cycloalkanes, a lot of issues remain unsolved, such as high cost, high toxicity and kinetic barriers etc.

The imidazolium ionic liquids such alkyl(aryl)-3-methylimidazolium N-bis(trifluoromethanesulfonyl)imidate salts can reversibly add 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.{{cite journal |doi=10.1021/ef060481t |title=Hydrogen-Storage Materials Based on Imidazolium Ionic Liquids |year=2007 |last1=Stracke |first1=Marcelo P. |last2=Ebeling |first2=Günter |last3=Cataluña |first3=Renato |last4=Dupont |first4=Jairton |journal=Energy & Fuels |volume=21 |issue=3 |pages=1695–1698}}

Formic acid is a highly effective hydrogen storage material, although its H₂density is low. 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.{{cite journal |title=A Viable Hydrogen-Storage System Based On Selective Formic Acid Decomposition with a Ruthenium Catalyst |pmid=18393267 |year=2008 |last1=Fellay |first1=C |last2=Dyson |first2=PJ |last3=Laurenczy |first3=G |volume=47 |issue=21 |pages=3966–8 |doi=10.1002/anie.200800320 |journal=Angewandte Chemie International Edition in English}} 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.{{cite journal |author=F. Joó |title=Breakthroughs in Hydrogen Storage – Formic Acid as a Sustainable Storage Material for Hydrogen |pmid=18781551 |year=2008 |volume=1 |issue=10 |pages=805–8 |doi=10.1002/cssc.200800133 |journal=ChemSusChem|bibcode=2008ChSCh...1..805J}} 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.P. G. Jessop, in Handbook of Homogeneous Hydrogenation (Eds.: J. G. de Vries, C. J. Elsevier), Wiley-VCH, Weinheim, Germany, 2007, pp. 489–511.{{cite journal |author1=P. G. Jessop |author2=F. Joó |author3=C.-C. Tai |title=Recent advances in the homogeneous hydrogenation of carbon dioxide |doi=10.1016/j.ccr.2004.05.019 |year=2004 |journal=Coordination Chemistry Reviews |volume=248 |issue=21–24 |pages=2425}} 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 flammable.

{{Further|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. Since there is no carbon in ammonia, no carbon by-products are produced; thereby making this possibility a "carbon neutral" option for the future. 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 suitable alternative fuel because it has 18.6 MJ/kg energy density at NTP and carbon-free combustion byproducts.{{Cite journal |last=AVERY |first=W |date=1988 |title=A role for ammonia in the hydrogen economy |journal=International Journal of Hydrogen Energy |volume=13 |issue=12 |pages=761–773 |doi=10.1016/0360-3199(88)90037-7 |bibcode=1988IJHE...13..761A |issn=0360-3199}}

Ammonia has several challenges to widespread adaption as a hydrogen storage material. Ammonia is a toxic gas with a potent odor at standard temperature and pressure.[http://www.memagazine.org/contents/current/webonly/webex710.html The ammonia economy] {{webarchive|url=https://web.archive.org/web/20080513030624/http://www.memagazine.org/contents/current/webonly/webex710.html|date=2008-05-13 |access-date=2012-01-08}}. Memagazine.org (2003-07-10) Additionally, advances in the efficiency and scalability of ammonia decomposition are needed for commercial viability, as fuel cell membranes are highly sensitive to residual ammonia and current decomposition techniques have low yield rates.{{Cite journal |last1=Lamb |first1=Krystina E. |last2=Dolan |first2=Michael D. |last3=Kennedy |first3=Danielle F. |date=2019-02-05 |title=Ammonia for hydrogen storage; A review of catalytic ammonia decomposition and hydrogen separation and purification |url=http://www.sciencedirect.com/science/article/pii/S0360319918339272 |journal=International Journal of Hydrogen Energy |language=en |volume=44 |issue=7 |pages=3580–3593 |doi=10.1016/j.ijhydene.2018.12.024 |bibcode=2019IJHE...44.3580L |s2cid=104446684 |issn=0360-3199|url-access=subscription }} A variety of transition metals can be used to catalyze the ammonia decomposition reaction, the most effective being ruthenium. This catalysis works through chemisorption, where the adsorption energy of N₂ is less than the reaction energy of dissociation.{{Cite journal |last1=Bligaard |first1=T. |last2=Nørskov |first2=J. K. |last3=Dahl |first3=S. |last4=Matthiesen |first4=J. |last5=Christensen |first5=C. H. |last6=Sehested |first6=J. |date=2004-05-15 |title=The Brønsted–Evans–Polanyi relation and the volcano curve in heterogeneous catalysis |url=http://www.sciencedirect.com/science/article/pii/S0021951704001010 |journal=Journal of Catalysis |language=en |volume=224 |issue=1 |pages=206–217 |doi=10.1016/j.jcat.2004.02.034 |issn=0021-9517 |access-date=2020-11-18 |archive-date=2020-07-16 |archive-url=https://web.archive.org/web/20200716200107/https://www.sciencedirect.com/science/article/pii/S0021951704001010 |url-status=live|url-access=subscription }} Hydrogen purification can be achieved in several ways. Hydrogen can be separated from unreacted ammonia using a permeable, hydrogen-selective membrane.{{Cite journal |last1=Dolan |first1=Michael D. |last2=Viano |first2=David M. |last3=Langley |first3=Matthew J. |last4=Lamb |first4=Krystina E. |date=2018-03-01 |title=Tubular vanadium membranes for hydrogen purification |url=http://www.sciencedirect.com/science/article/pii/S037673881733185X |journal=Journal of Membrane Science |language=en |volume=549 |pages=306–311 |doi=10.1016/j.memsci.2017.12.031 |issn=0376-7388|url-access=subscription }} It can also be purified through the adsorption of ammonia, which can be selectively trapped due to its polarity.{{Cite journal |last1=Park |first1=Soo-Jin |last2=Kim |first2=Byung-Joo |date=2005-11-15 |title=Ammonia removal of activated carbon fibers produced by oxyfluorination |url=http://www.sciencedirect.com/science/article/pii/S0021979705005291 |journal=Journal of Colloid and Interface Science |language=en |volume=291 |issue=2 |pages=597–599 |doi=10.1016/j.jcis.2005.05.012 |pmid=15975585 |bibcode=2005JCIS..291..597P |issn=0021-9797|url-access=subscription }}

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.[http://www.netpublikationer.dk/um/6567/html/chapter12.htm Focus Denmark] {{Webarchive|url=https://web.archive.org/web/20070927235915/http://www.netpublikationer.dk/um/6567/html/chapter12.htm |date=2007-09-27 |access-date=2012-01-08}}. Netpublikationer.dk (2006-06-13){{Update inline|date=September 2023|reason=Its been almost 20 years}}

• High theoretical energy density
• Wide spread availability
• Large scale commercial production
• Benign decomposition pathway to H₂ and N₂

• Toxicity
• Corrosive
• High decomposition temperature leading to efficiency loss

Hydrazine breaks down in the cell to form nitrogen and hydrogen/{{cite web |url=http://www.theengineer.co.uk/Articles/303939/Liquid+asset.htm |archive-url=https://archive.today/20121209053251/http://www.theengineer.co.uk/Articles/303939/Liquid+asset.htm |url-status=dead |archive-date=2012-12-09 |title=Liquid asset |work=The Engineer |date=2008-01-15 |access-date=2015-01-09}} Silicon hydrides and germanium hydrides are also candidates of hydrogen storage materials, as they can subject to energetically favored reaction to form covalently bonded dimers with loss of a hydrogen molecule.Zong, J., J. T. Mague, and R. A. Pascal, Jr., Exceptional Steric Congestion in an in,in-Bis(hydrosilane), J. Am. Chem. Soc. 2013, 135, 13235-13237.{{cite journal |last1=Echeverría |first1=Jorge |last2=Aullón |first2=Gabriel |last3=Alvarez |first3=Santiago |year=2017 |title=Intermolecular interactions in group 14 hydrides: Beyond C-H··· H-C contacts |journal=International Journal of Quantum Chemistry |volume=117 |issue=21 |page=e25432 |doi=10.1002/qua.25432}}

{{Main|Amine borane complex}}

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 the 1970s and 1980s, 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.{{Citation needed|date=March 2024}}

In this case hydrogen remains in physical forms, i.e., as gas, supercritical fluid, adsorbate, or molecular inclusions. Theoretical limitations and experimental results are consideredCompendium of Hydrogen Energy.Volume 2:hydrogen Storage, Transportation and Infrastructure. A volume in Woodhead Publishing Series in Energy 2016, Chapter 8 – Other methods for the physical storage of hydrogen {{doi|10.1016/B978-1-78242-362-1.00008-0}} concerning the volumetric and gravimetric capacity of glass microvessels, microporous, and nanoporous media, as well as safety and refilling-time demands. Because hydrogen is the smallest molecule, it easily escapes from containers and during transfer from container to container. While it does not directly contribute to radiative forcing, hydrogen is estimated to have an effective 100-year global warming potential of {{Plusminus|11.6|2.8}} due to its impact on processes such as atmospheric methane oxidation and tropospheric ozone production.Bjørnæs, Christian. [https://cicero.oslo.no/en/hydrogen-leaks-add-to-global-warming "Global warming potential of hydrogen estimated"], Centre for International Climate and Environmental Research, June 7, 2023. Retrieved June 15, 2023.{{Cite journal |last1=Sand |first1=Maria |last2=Skeie |first2=Ragnhild Bieltvedt |last3=Sandstad |first3=Marit |last4=Krishnan |first4=Srinath |last5=Myhre |first5=Gunnar |last6=Bryant |first6=Hannah |last7=Derwent |first7=Richard |last8=Hauglustaine |first8=Didier |last9=Paulot |first9=Fabien |last10=Prather |first10=Michael |last11=Stevenson |first11=David |title=A multi-model assessment of the Global Warming Potential of hydrogen |journal=Communications Earth & Environment |volume=4 |pages=203 |year=2023 |issue=1 |doi=10.1038/s43247-023-00857-8 |doi-access=free |bibcode=2023ComEE...4..203S |hdl=11250/3120918 |hdl-access=free }}

Zeolites are microporous and highly crystalline aluminosilicate materials. As they exhibit cage and tunnel structures, they offer the potential for the encapsulation of non-polar gases such as H₂. In this system, hydrogen is physisorbed on the surface of the zeolite pores through a mechanism of adsorption that involves hydrogen being forced into the pores under pressure and low temperature.{{Cite journal |last1=Dong |first1=Jinxiang |last2=Wang |first2=Xiaoyan |last3=Xu |first3=Hong |last4=Zhao |first4=Qiang |last5=Li |first5=Jinping |date=2007-12-01 |title=Hydrogen storage in several microporous zeolites |url=https://www.sciencedirect.com/science/article/pii/S0360319907004843 |journal=International Journal of Hydrogen Energy |language=en |volume=32 |issue=18 |pages=4998–5004 |doi=10.1016/j.ijhydene.2007.08.009 |bibcode=2007IJHE...32.4998D |issn=0360-3199|url-access=subscription }} Therefore, similar to other porous materials, its hydrogen storage capacity depends on the BET surface area, pore volume, the interaction of molecular hydrogen with the internal surfaces of the micropores, and working conditions such as pressure and temperature.{{Cite journal |last1=Ren |first1=Jianwei |last2=Musyoka |first2=Nicholas M. |last3=Langmi |first3=Henrietta W. |last4=Mathe |first4=Mkhulu |last5=Liao |first5=Shijun |date=2017-01-05 |title=Current research trends and perspectives on materials-based hydrogen storage solutions: A critical review |url=https://www.sciencedirect.com/science/article/pii/S0360319916335285 |journal=International Journal of Hydrogen Energy |language=en |volume=42 |issue=1 |pages=289–311 |doi=10.1016/j.ijhydene.2016.11.195 |bibcode=2017IJHE...42..289R |issn=0360-3199|url-access=subscription }}

Channel diameter is also one of the parameters determining this capacity, especially at high pressure. In this case, an effective material should exhibit a large pore volume and a channel diameter close to the kinetic diameter of the hydrogen molecule (d_H=2.89 Å).

Table below shows the hydrogen uptake of several zeolites at liquid nitrogen temperature (77K):

Activated carbons are highly porous amorphous carbon materials with high apparent surface area. Hydrogen physisorption can be increased in these materials by increasing the apparent surface area and optimizing pore diameter to around 7 Å.{{Cite journal |last1=Sevilla |first1=Marta |last2=Mokaya |first2=Robert |date=2014-03-21 |title=Energy storage applications of activated carbons: supercapacitors and hydrogen storage |journal=Energy Environ. Sci. |language=en |volume=7 |issue=4 |pages=1250–1280 |doi=10.1039/c3ee43525c |bibcode=2014EnEnS...7.1250S |issn=1754-5706 |hdl=10261/140713|hdl-access=free}} These materials are of particular interest due to the fact that they can be made from waste materials, such as cigarette butts which have shown great potential as precursor materials for high-capacity hydrogen storage materials.{{Cite journal |last1=Blankenship II |first1=Troy Scott |last2=Balahmar |first2=Norah |last3=Mokaya |first3=Robert |date=2017-11-16 |title=Oxygen-rich microporous carbons with exceptional hydrogen storage capacity |journal=Nature Communications |language=En |volume=8 |issue=1 |pages=1545 |doi=10.1038/s41467-017-01633-x |pmid=29146978 |pmc=5691040 |issn=2041-1723 |bibcode=2017NatCo...8.1545B}}{{Cite journal |last1=Blankenship |first1=Troy Scott |last2=Mokaya |first2=Robert |date=2017-12-06 |title=Cigarette butt-derived carbons have ultra-high surface area and unprecedented hydrogen storage capacity |journal=Energy & Environmental Science |language=en |volume=10 |issue=12 |pages=2552–2562 |doi=10.1039/c7ee02616a |bibcode=2017EnEnS..10.2552B |s2cid=104050734 |issn=1754-5706 |url=http://eprints.nottingham.ac.uk/47622/1/RMokaya%20Cigarette%20butt-derived%20carbons.pdf |access-date=2019-06-27 |archive-date=2019-04-28 |archive-url=https://web.archive.org/web/20190428090647/http://eprints.nottingham.ac.uk/47622/1/RMokaya%20Cigarette%20butt-derived%20carbons.pdf |url-status=live}}

Graphene can store hydrogen efficiently. The H₂ adds to the double bonds giving graphane. The hydrogen is released upon heating to 450 °C.[http://physicsworld.com/cws/article/news/22689 Graphene as suitable hydrogen storage substance] {{Webarchive|url=https://web.archive.org/web/20081205164029/http://physicsworld.com/cws/article/news/22689 |date=2008-12-05 |access-date=2012-01-08}}. Physicsworld.com[http://www.rsc.org/chemistryworld/News/2009/January/29010902.asp Graphene to graphane] {{Webarchive|url=https://web.archive.org/web/20110608231130/http://www.rsc.org/chemistryworld/News/2009/January/29010902.asp |date=2011-06-08 |access-date=2012-01-08}}. Rsc.org. January 2009

Image:Carbon nanotubes.jpg

Hydrogen carriers based on nanostructured carbon (such as carbon buckyballs and nanotubes) have been proposed. However, hydrogen content amounts up to ≈3.0-7.0 wt% at 77K which is far from the value set by US Department of Energy (6 wt% at nearly ambient conditions).{{Citation needed|date=March 2024}}

To realize carbon materials as effective hydrogen storage technologies, carbon nanotubes (CNTs) have been doped with MgH₂. The metal hydride has proven to have a theoretical storage capacity (7.6 wt%) that fulfills the United States Department of Energy requirement of 6 wt%, but has limited practical applications due to its high release temperature. The proposed mechanism involves the creation of fast diffusion channels by CNTs within the MgH₂ lattice. Fullerene substances are other carbonaceous nanomaterials that have been tested for hydrogen storage in this center. Fullerene molecules are composed of a C₆₀ close-caged structure, that allows for hydrogenation of the double bonded carbons leading to a theoretical C₆₀H₆₀ isomer with a hydrogen content of 7.7 wt%. However, the release temperature in these systems is high (600 °C).

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. When guest molecules (solvent) occupying the pores are removed during solvent exchange and heating under vacuum, porous structure of MOFs can be achieved without destabilizing the frame and hydrogen molecules will be adsorbed onto the surface of the pores by physisorption. Compared to traditional zeolites and porous carbon materials, MOFs have very high number of pores and surface area which allow higher hydrogen uptake in a given volume.{{cn|date=May 2025}}

Temperature, pressure and composition of MOFs can influence their hydrogen storage ability. The adsorption capacity of MOFs is lower at higher temperature and higher at lower temperatures. With the rising of temperature, physisorption decreases and chemisorption increases.{{Cite journal |last1=Shet |first1=Sachin P. |last2=Shanmuga Priya |first2=S. |last3=Sudhakar |first3=K. |last4=Tahir |first4=Muhammad |date=March 2021 |title=A review on current trends in potential use of metal-organic framework for hydrogen storage |url=http://dx.doi.org/10.1016/j.ijhydene.2021.01.020 |journal=International Journal of Hydrogen Energy |volume=46 |issue=21 |pages=11782–11803 |doi=10.1016/j.ijhydene.2021.01.020 |bibcode=2021IJHE...4611782S |s2cid=233623695 |issn=0360-3199 |access-date=2021-11-21 |archive-date=2022-10-16 |archive-url=https://web.archive.org/web/20221016173508/https://www.sciencedirect.com/science/article/abs/pii/S0360319921000331?via%3Dihub |url-status=live|url-access=subscription }} For MOF-519 and MOF-520, the isosteric heat of adsorption decreased with pressure increase.{{Cite journal |last1=Xia |first1=Liangzhi |last2=Liu |first2=Qing |date=January 2017 |title=Adsorption of H2 on aluminum-based metal-organic frameworks: A computational study |url=http://dx.doi.org/10.1016/j.commatsci.2016.09.039 |journal=Computational Materials Science |volume=126 |pages=176–181 |doi=10.1016/j.commatsci.2016.09.039 |issn=0927-0256 |access-date=2021-11-21 |archive-date=2022-10-16 |archive-url=https://web.archive.org/web/20221016173518/https://www.sciencedirect.com/science/article/abs/pii/S0927025616304888?via%3Dihub |url-status=live|url-access=subscription }} For MOF-5, both gravimetric and volumetric hydrogen uptake increased with increase in pressure. The total capacity may not be consistent with the usable capacity under pressure swing conditions. For instance, MOF-5 and IRMOF-20, which have the highest total volumetric capacity, show the least usable volumetric capacity.{{Cite journal |last1=Ahmed |first1=Alauddin |last2=Seth |first2=Saona |last3=Purewal |first3=Justin |last4=Wong-Foy |first4=Antek G. |last5=Veenstra |first5=Mike |last6=Matzger |first6=Adam J. |last7=Siegel |first7=Donald J. |date=2019-04-05 |title=Exceptional hydrogen storage achieved by screening nearly half a million metal-organic frameworks |url=http://dx.doi.org/10.1038/s41467-019-09365-w |journal=Nature Communications |volume=10 |issue=1 |page=1568 |doi=10.1038/s41467-019-09365-w |pmid=30952862 |pmc=6450936 |bibcode=2019NatCo..10.1568A |issn=2041-1723 |access-date=2021-11-21 |archive-date=2022-10-16 |archive-url=https://web.archive.org/web/20221016173519/https://www.nature.com/articles/s41467-019-09365-w |url-status=live}} Absorption capacity can be increased by modification of structure. For example, the hydrogen uptake of PCN-68 is higher than PCN-61.{{Cite journal |last1=Liu |first1=Jia |last2=Zou |first2=Ruqiang |last3=Zhao |first3=Yanli |date=November 2016 |title=Recent developments in porous materials for H2 and CH4 storage |url=http://dx.doi.org/10.1016/j.tetlet.2016.09.085 |journal=Tetrahedron Letters |volume=57 |issue=44 |pages=4873–4881 |doi=10.1016/j.tetlet.2016.09.085 |issn=0040-4039 |access-date=2021-11-21 |archive-date=2022-10-16 |archive-url=https://web.archive.org/web/20221016173520/https://www.sciencedirect.com/science/article/abs/pii/S0040403916312680?via%3Dihub |url-status=live|url-access=subscription }} Porous aromatic frameworks (PAF-1), which is known as a high surface area material, can achieve a higher surface area by doping.{{Cite journal |last1=Rochat |first1=Sébastien |last2=Polak-Kraśna |first2=Katarzyna |last3=Tian |first3=Mi |last4=Holyfield |first4=Leighton T. |last5=Mays |first5=Timothy J. |last6=Bowen |first6=Christopher R. |last7=Burrows |first7=Andrew D. |date=2017 |title=Hydrogen storage in polymer-based processable microporous composites |journal=Journal of Materials Chemistry A |volume=5 |issue=35 |pages=18752–18761 |doi=10.1039/c7ta05232d |s2cid=104093990 |issn=2050-7488 |doi-access=free |hdl=1983/0c7e3254-3967-4318-83a1-91e9696b8dae |hdl-access=free}}

There are many different ways to modify MOFs, such as MOF catalysts, MOF hybrids, MOF with metal centers and doping. MOF catalysts have high surface area, porosity and hydrogen storage capacity. However, the active metal centers are low. MOF hybrids have enhanced surface area, porosity, loading capacity and hydrogen storage capacity. Nevertheless, they are not stable and lack active centers. Doping in MOFs can increase hydrogen storage capacity, but there might be steric effect and inert metals have inadequate stability. There might be formation of interconnected pores and low corrosion resistance in MOFs with metal centers, while they might have good binding energy and enhanced stability. These advantages and disadvantages for different kinds of modified MOFs show that MOF hybrids are more promising because of the good controllability in selection of materials for high surface area, porosity and stability.

In 2006, chemists achieved hydrogen storage concentrations of up to 7.5 wt% in MOF-74 at a low temperature of 77 K.[https://www.nist.gov/public_affairs/techbeat/tb2008_0401.htm#hydrogen MOF-74 – A Potential Hydrogen-Storage Compound] {{Webarchive|url=https://web.archive.org/web/20120620053430/http://www.nist.gov/public_affairs/techbeat/tb2008_0401.htm#hydrogen |date=2012-06-20 |access-date=2012-01-08}}. Nist.gov[http://www.greencarcongress.com/2006/03/researchers_dem.html Researchers Demonstrate 7.5 wt% Hydrogen Storage in MOFs] {{Webarchive|url=https://web.archive.org/web/20070228150332/http://www.greencarcongress.com/2006/03/researchers_dem.html |date=2007-02-28 |access-date=2012-01-08}}. Green Car Congress (2006-03-06) MOF NOTT-112 exhibit 10 wt% at 77 bar (1,117 psi) and 77 K with .[http://www.greencarcongress.com/2009/02/new-mof-materia.html New MOF Material With hydrogen Uptake Of Up To 10 wt%] {{Webarchive|url=https://web.archive.org/web/20100507022510/http://www.greencarcongress.com/2009/02/new-mof-materia.html |date=2010-05-07}}. 22 February 2009 Most articles about hydrogen storage in MOFs report hydrogen subitptake capacity at a temperature of 77K and a pressure of 1 bar because these conditions are commonly available and the binding energy between hydrogen and the MOF at this temperature is large compared to the thermal vibration energy. Varying several factors such as surface area, pore size, catenation, ligand structure, and sample purity can result in different amounts of hydrogen uptake in MOFs.

NU-1501-Al, an ultraporous metal–organic framework (MOF) has a hydrogen delivery capacity of 14.0% w/w, 46.2 g/litre.{{cite web |author1=Matt McGrath |title=Climate change: 'Bath sponge' breakthrough could boost cleaner cars |url=https://www.bbc.co.uk/news/science-environment-52328786 |website=BBC News |access-date=19 April 2020 |date=18 April 2020 |archive-date=19 April 2020 |archive-url=https://web.archive.org/web/20200419015602/https://www.bbc.co.uk/news/science-environment-52328786 |url-status=live}}{{cite journal |url=https://www.science.org/doi/10.1126/science.aaz8881 |author1=Zhijie Chen |title=Balancing volumetric and gravimetric uptake in highly porous materials for clean energy |journal=Science |year=2020 |volume=368 |issue=6488 |pages=297–303 |doi=10.1126/science.aaz8881 |pmid=32299950 |bibcode=2020Sci...368..297C |s2cid=215789994 |access-date=19 April 2020 |archive-date=25 February 2022 |archive-url=https://web.archive.org/web/20220225210910/https://www.science.org/doi/10.1126/science.aaz8881 |url-status=live|url-access=subscription }}

Cryo-compressed storage of hydrogen is the only technology that meets 2015 DOE targets for volumetric and gravimetric efficiency (see "CcH2" on slide 6 inR. K. Ahluwalia, T. Q. Hua, J. K. Peng and R. Kumar [http://www.hydrogen.energy.gov/pdfs/review10/st001_ahluwalia_2010_o_web.pdf System Level Analysis of Hydrogen Storage Options] {{Webarchive|url=https://web.archive.org/web/20110719105238/http://www.hydrogen.energy.gov/pdfs/review10/st001_ahluwalia_2010_o_web.pdf |date=2011-07-19}}. 2010 DOE Hydrogen Program Review, Washington, DC, June 8–11, 2010).

Furthermore, another study has shown that cryo-compression 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 (see third row in slide 13 ofStephen Lasher [http://www.hydrogen.energy.gov/pdfs/review10/st002_lasher_2010_o_web.pdf Analyses of Hydrogen Storage Materials and On-Board Systems] {{Webarchive|url=https://web.archive.org/web/20110929012222/http://www.hydrogen.energy.gov/pdfs/review10/st002_lasher_2010_o_web.pdf |date=2011-09-29}}. DOE Annual Merit Review June 7–11, 2010).

Like liquid storage, cryo-compressed uses cold hydrogen (20.3 K and slightly above) in order to reach a high energy density. However, the main difference is that, when the hydrogen would warm-up due to heat transfer with the environment ("boil off"), the tank is allowed to go to pressures much higher (up to 350 bars versus a couple of bars for liquid storage). As a consequence, it takes more time before the hydrogen has to vent, and in most driving situations, enough hydrogen is used by the car to keep the pressure well below the venting limit.{{Citation needed|date=March 2024}}

Consequently, it has been demonstrated that a high driving range could be achieved with a cryo-compressed tank : more than {{convert|650|mi|km}} were driven with a full tank mounted on a hydrogen-fueled engine of Toyota Prius.[https://www.llnl.gov/str/June07/Aceves.html S&TR | Setting a World Driving Record with Hydrogen] {{Webarchive|url=https://web.archive.org/web/20081203082104/https://www.llnl.gov/str/June07/Aceves.html |date=2008-12-03 |access-date=2012-01-08}}. Llnl.gov (2007-06-12)[http://www.hydrogen.energy.gov/pdfs/review10/st003_berry_2010_o_web.pdf Compact (L)H2 Storage with Extended Dormancy in Cryogenic Pressure Vessels] {{Webarchive|url=https://web.archive.org/web/20110929012401/http://www.hydrogen.energy.gov/pdfs/review10/st003_berry_2010_o_web.pdf |date=2011-09-29}}. Lawrence Livermore National Laboratory June 8, 2010

As of 2010, the BMW Group has started a thorough component and system level validation of cryo-compressed vehicle storage on its way to a commercial product.[http://www.fisita2010.com/programme/programme/F2010A018 Technical Sessions] {{Webarchive|url=https://web.archive.org/web/20110711004618/http://www.fisita2010.com/programme/programme/F2010A018 |date=2011-07-11 |access-date=2012-01-08}}. FISITA 2010

H₂ caged in a clathrate hydrate are stable at very high pressures. Some solid H₂-containing hydrates form at ambient temperature and tens of bars in the presence of THF.{{Cite journal |doi=10.1039/C8CS00989A |title=Gas hydrates in sustainable chemistry |year=2020 |last1=Hassanpouryouzband |first1=Aliakbar |last2=Joonaki |first2=Edris |last3=Vasheghani Farahani |first3=Mehrdad |last4=Takeya |first4=Satoshi |last5=Ruppel |first5=Carolyn |last6=Yang |first6=Jinhai |last7=J. English |first7=Niall |last8=M. Schicks |first8=Judith |last9=Edlmann |first9=Katriona|last10 = Mehrabian|first10 = Hadi |last11=M. Aman |first11=Zachary |last12=Tohidi |first12=Bahman |journal=Chemical Society Reviews |volume=49 |issue=15 |pages=5225–5309 |pmid=32567615 |s2cid=219971360|doi-access=free |bibcode=2020CSRev..49.5225H |hdl=1912/26136 |hdl-access=free}}{{cite journal |doi=10.1126/science.1102076 |title=Stable Low-Pressure Hydrogen Clusters Stored in a Binary Clathrate Hydrate |year=2004 |last1=Florusse |first1=L. J. |journal=Science |volume=306 |issue=5695 |pages=469–71 |pmid=15486295 |last2=Peters |first2=CJ |last3=Schoonman |first3=J |last4=Hester |first4=KC |last5=Koh |first5=CA |last6=Dec |first6=SF |last7=Marsh |first7=KN |last8=Sloan |first8=ED |bibcode=2004Sci...306..469F |s2cid=38107525}} These clathrates have a theoretical maximum hydrogen densities of around 5 wt% and 40 kg/m³.

Glass capillary arrays show potential for the safe infusion, storage and controlled release of hydrogen in mobile applications.{{cite journal |doi=10.1016/j.enconman.2006.11.017 |title=Hydrogen storage in capillary arrays |year=2007 |last1=Zhevago |first1=N.K. |last2=Glebov |first2=V.I. |journal=Energy Conversion and Management |volume=48 |issue=5 |pages=1554–1559|bibcode=2007ECM....48.1554Z }}{{cite journal |doi=10.1016/j.ijhydene.2009.10.011 |title=Experimental investigation of hydrogen storage in capillary arrays |year=2010 |last1=Zhevago |first1=N.K. |last2=Denisov |first2=E.I. |last3=Glebov |first3=V.I. |journal=International Journal of Hydrogen Energy |volume=35 |issue=1 |pages=169–175|bibcode=2010IJHE...35..169Z }} The C.En technology has achieved the United States Department of Energy (DOE) 2010 targets for on-board hydrogen storage systems.Dan Eliezer et al. [https://web.archive.org/web/20110708131441/http://www.cenh2go.com/PDF/CEnPoster_small.pdf A New Technology for Hydrogen Storage in Capillary Arrays]. C.En & BAM

DOE 2015 targets can be achieved using flexible glass capillaries and cryo-compressed method of hydrogen storage.{{Cite journal |last1=Zhevago |first1=N. K. |last2=Chabak |first2=A. F. |last3=Denisov |first3=E. I. |last4=Glebov |first4=V. I. |last5=Korobtsev |first5=S. V. |title=Storage of cryo-compressed hydrogen in flexible glass capillaries |doi=10.1016/j.ijhydene.2013.03.107 |journal=International Journal of Hydrogen Energy |volume=38 |issue=16 |pages=6694–6703 |year=2013|bibcode=2013IJHE...38.6694Z }}

Hollow glass microspheres (HGM) can be utilized for controlled storage and release of hydrogen. HGMs with a diameter of 1 to 100 μm, a density of 1.0 to 2.0 gm/cc and a porous wall with openings of 10 to 1000 angstroms are considered for hydrogen storage. The advantages of HGMs for hydrogen storage are that they are nontoxic, light, cheap, recyclable, reversible, easily handled at atmospheric conditions, capable of being stored in a tank, and the hydrogen within is non-explosive.{{cite journal |last1=Dalai |first1=Sridhar |last2=Savithri |first2=Vijayalakshmi |title=Investigating the effect of cobalt loading on thermal conductivity and hydrogen storage capacity of hollow glass microspheres (HGMs) |journal=MaterialsToday: Proceedings |date=26 October 2017 |volume=4 |issue=11 |pages=11608–11616 |doi=10.1016/j.matpr.2017.09.072 |url=https://www.sciencedirect.com/science/article/pii/S2214785317318059 |access-date=16 November 2020|url-access=subscription }} Each of these HGMs is capable of containing hydrogen up to 150 MPa without the heaviness and bulk of a large pressurized tank. All of these qualities are favorable in vehicular applications. Beyond these advantages, HGMs are seen as a possible hydrogen solution due to hydrogen diffusivity having a large temperature dependence. At room temperature, the diffusivity is very low, and the hydrogen is trapped in the HGM. The disadvantage of HGMs is that to fill and outgas hydrogen effectively the temperature must be at least 300 °C which significantly increases the operational cost of HGM in hydrogen storage.{{cite journal |last1=Qi |first1=Xiaobo |last2=Gao |first2=Cong |last3=Zhang |first3=Zhanwen |last4=Chen |first4=Sufen |last5=Li |first5=Bo |last6=Wei |first6=Sheng |title=Production and characterization of hollow glass microspheres with high diffusivity for hydrogen storage |journal=International Journal of Hydrogen Energy |date=January 2012 |volume=37 |issue=2 |pages=1518–1530 |doi=10.1016/j.ijhydene.2011.10.034 |bibcode=2012IJHE...37.1518Q |url=https://www.sciencedirect.com/science/article/pii/S0360319911023937 |access-date=16 November 2020|url-access=subscription }} The high temperature can be partly attributed to glass being an insulator and having a low thermal conductivity; this hinders hydrogen diffusivity, and subsequently a higher temperature is required to achieve the desired storage capacity.

To make this technology more economically viable for commercial use, research is being done to increase the efficiency of hydrogen diffusion through the HGMs. One study done by Dalai et al. sought to increase the thermal conductivity of the HGM through doping the glass with cobalt. In doing so they increased the thermal conductivity from 0.0072 to 0.198 W/m-K at 10 wt% Co. Increases in hydrogen adsorption though were only seen up to 2 wt% Co (0.103 W/m-K) as the metal oxide began to cover pores in the glass shell. This study concluded with a hydrogen storage capacity of 3.31 wt% with 2 wt% Co at 200 °C and 10 bar.

A study done by Rapp and Shelby sought to increase the hydrogen release rate through photo-induced outgassing in doped HGMs in comparison to conventional heating methods. The glass was doped with optically active metals to interact with the high-intensity infrared light. The study found that 0.5 wt% Fe₃O₄ doped 7070 borosilicate glass had hydrogen release increase proportionally to the infrared lamp intensity. In addition to the improvements to diffusivity by infrared alone, reactions between the hydrogen and iron-doped glass increased the Fe²⁺/Fe³⁺ ratio which increased infrared absorption therefore further increasing the hydrogen yield.{{cite journal |last1=Rapp |first1=Douglas |last2=Shelby |first2=James |title=Photo-induced hydrogen outgassing of glass |journal=Journal of Non-Crystalline Solids |date=1 December 2004 |volume=349 |pages=254–259 |doi=10.1016/j.jnoncrysol.2004.08.151 |bibcode=2004JNCS..349..254R |url=https://www.sciencedirect.com/science/article/pii/S0022309304007549 |access-date=16 November 2020|url-access=subscription }}

As of 2020, the progress made in studying HGMs has increased its efficiency but it still falls short of Department of Energy targets for this technology. The operation temperatures for both hydrogen adsorption and release are the largest barrier to commercialization.{{cite journal |last1=Zarezadeh Mehrizi |first1=Majid |last2=Abdi |first2=Jafar |last3=Rezakazemi |first3=Mashallah |last4=Salehi |first4=Ehsan |title=A review on recent advances in hollow spheres for hydrogen storage |journal=International Journal of Hydrogen Energy |date=10 July 2020 |volume=45 |issue=35 |pages=17583–17604 |doi=10.1016/j.ijhydene.2020.04.201 |bibcode=2020IJHE...4517583Z |s2cid=225544099 |url=https://www.sciencedirect.com/science/article/pii/S0360319920316104 |access-date=16 November 2020|url-access=subscription }}

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{{Cite web |url=http://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/ihfpv_zheng2.pdf |title=R&D of large stationary hydrogen/CNG/HCNG storage vessels |access-date=2012-07-14 |archive-date=2016-03-03 |archive-url=https://web.archive.org/web/20160303195423/http://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/ihfpv_zheng2.pdf |url-status=live}}
• Liquid hydrogen in a (LH₂) cryogenic hydrogen tank
• Slush hydrogen in a cryogenic hydrogen tank

File:Available storage technologies, their capacity and discharge time.jpg

Underground hydrogen storage{{Cite journal |doi=10.1021/acsenergylett.1c00845 |title=Offshore Geological Storage of Hydrogen: Is This Our Best Option to Achieve Net-Zero? |year=2021 |last1=Hassanpouryouzband |first1=Aliakbar |last2=Joonaki |first2=Edris |last3=Edlmann |first3=Katriona |last4=Haszeldine |first4=R. Stuart |journal=ACS Energy Lett. |volume=6 |issue=6 |pages=2181–2186 |s2cid=236299486 |url=https://www.research.ed.ac.uk/en/publications/4de280c0-20f2-40be-bdeb-31ef68929826 |access-date=2022-06-27 |archive-date=2022-10-16 |archive-url=https://web.archive.org/web/20221016173553/https://www.research.ed.ac.uk/en/publications/offshore-geological-storage-of-hydrogen-is-this-our-best-option-t |url-status=live |doi-access=free |hdl=20.500.11820/4de280c0-20f2-40be-bdeb-31ef68929826 |hdl-access=free}} is the practice of hydrogen storage in caverns, salt domes and depleted oil and gas fields. Large quantities of gaseous hydrogen have been stored in caverns by ICI for many years without any difficulties.[http://www.hyweb.de/Knowledge/Ecn-h2a.html 1994 – ECN abstract] {{Webarchive|url=https://web.archive.org/web/20040102122446/http://www.hyweb.de/Knowledge/Ecn-h2a.html |date=2004-01-02 |access-date=2012-01-08}}. Hyweb.de The storage of large quantities of liquid hydrogen underground can function as grid energy storage. The round-trip efficiency is approximately 40% (vs. 75–80% for pumped-hydro (PHES)), and the cost is slightly higher than pumped hydro, if only a limited number of hours of storage is required.{{cite web |url=http://www.europarl.europa.eu/document/activities/cont/201202/20120208ATT37544/20120208ATT37544EN.pdf |title=European Renewable Energy Network Study |date=January 2012 |pages=86, 188 |publisher=European Union |location=Brussels |access-date=2012-09-02 |archive-date=2019-07-17 |archive-url=https://web.archive.org/web/20190717012026/http://www.europarl.europa.eu/document/activities/cont/201202/20120208ATT37544/20120208ATT37544EN.pdf |url-status=live}} Another study referenced by a European staff working paper found that for large scale storage, the cheapest option is hydrogen at €140/MWh for 2,000 hours of storage using an electrolyser, salt cavern storage and combined-cycle power plant.{{cite web |title=COMMISSION STAFF WORKING DOCUMENT: Energy storage – the role of electricity |url=https://ec.europa.eu/energy/sites/ener/files/documents/swd2017_61_document_travail_service_part1_v6.pdf |date=1 Feb 2017 |publisher=European Commission |access-date=22 April 2018 |archive-date=8 November 2020 |archive-url=https://web.archive.org/web/20201108140909/https://ec.europa.eu/energy/sites/ener/files/documents/swd2017_61_document_travail_service_part1_v6.pdf |url-status=live}}{{rp|15}} The European project Hyunder{{cite web |title=Why storing large scale intermittent renewable energies with hydrogen? |url=http://www.hyunder.eu/ |website=Hyunder |access-date=2018-11-25 |archive-date=2013-11-11 |archive-url=https://web.archive.org/web/20131111061655/http://www.hyunder.eu/ |url-status=live}} indicated in 2013 that for the storage of wind and solar energy an additional 85 caverns are required as it cannot be covered by PHES and CAES systems.{{Cite web |url=http://www.hyunder.eu/images/Presentations%20EUSEW/2%20HyUnder%20EUSEW%20workshop%20Luis%20Correas.pdf |title=Storing renewable energy: Is hydrogen a viable solution?}} {{dead link|date=January 2023 |bot=InternetArchiveBot |fix-attempted=yes}} A German case study on storage of hydrogen in salt caverns found that if the German power surplus (7% of total variable renewable generation by 2025 and 20% by 2050) would be converted to hydrogen and stored underground, these quantities would require some 15 caverns of 500,000 cubic metres each by 2025 and some 60 caverns by 2050 – corresponding to approximately one third of the number of gas caverns currently operated in Germany.{{cite web |title=Bringing North Sea Energy Ashore Efficiently |url=https://www.worldenergy.org/wp-content/uploads/2018/01/WEC-brochure_Online-offshore.pdf |publisher=World Energy Council Netherlands |access-date=22 April 2018 |archive-date=23 April 2018 |archive-url=https://web.archive.org/web/20180423034341/https://www.worldenergy.org/wp-content/uploads/2018/01/WEC-brochure_Online-offshore.pdf |url-status=live}} In the US, Sandia Labs are conducting research into the storage of hydrogen in depleted oil and gas fields, which could easily absorb large amounts of renewably produced hydrogen as there are some 2.7 million depleted wells in existence.{{cite web |last1=Gerdes |first1=Justin |title=Enlisting Abandoned Oil and Gas Wells as 'Electron Reserves' |url=https://www.greentechmedia.com/articles/read/enlisting-abandoned-oil-and-gas-wells-as-electron-reserves#gs.ByNEEjY |website=Greentech Media |access-date=22 April 2018 |date=2018-04-10 |archive-date=2018-04-23 |archive-url=https://web.archive.org/web/20180423035131/https://www.greentechmedia.com/articles/read/enlisting-abandoned-oil-and-gas-wells-as-electron-reserves#gs.ByNEEjY |url-status=live}}

Underground hydrogen storage is the practice of hydrogen storage in caverns,[http://www.osti.gov/scitech/biblio/6536941 1979 - Underground hydrogen storage. Final report.][http://www.praxair.com/praxair.nsf/AllContent/3A0AB529A089B473852571F0006398A3?OpenDocument&URLMenuBranch=19B4D941A5B182EE852571FC005510B2 hydrogen storage cavern system] salt domes and depleted oil/gas fields.{{Cite journal|doi=10.1021/acsenergylett.1c00845|title=Offshore Geological Storage of Hydrogen: Is This Our Best Option to Achieve Net-Zero?|year=2021|last1=Hassanpouryouzband|first1=Aliakbar|last2 = Joonaki|first2 = Edris|last3 = Edlmann|first3 = Katriona|last4 = Haszeldine|first4 = R. Stuart|journal=ACS Energy Lett.|volume=6|issue=6|pages=2181–2186|s2cid=236299486|doi-access=free|hdl=20.500.11820/4de280c0-20f2-40be-bdeb-31ef68929826|hdl-access=free}}[http://www.ecn.nl/docs/library/report/2012/l12013.pdf Energy storage 2012] Large quantities of gaseous hydrogen have been stored in caverns for many years.[http://www.hyweb.de/Knowledge/Ecn-h2a.html 1994 - ECN abstract] {{Failed verification|date=January 2022|talk=Article needs serious work}} {{Unreliable source?|date=January 2022|reason=Site is a pro-hydrogen blog}} The storage of large quantities of hydrogen underground in solution-mined salt domes,{{Cite web |url=http://energy.ruc.dk/HstoreMerida07.pdf |title=2006-Underground hydrogen storage in geological formations |access-date=2024-02-27 |archive-date=2007-06-13 |archive-url=https://web.archive.org/web/20070613041759/http://energy.ruc.dk/HstoreMerida07.pdf |url-status=dead }} aquifers,[http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=6536941 Brookhaven National Lab -Final report] excavated rock caverns, or mines can function as grid energy storage,[http://juwel.fz-juelich.de:8080/dspace/bitstream/2128/4071/1/HS1_8_Crotogino_rev0426.pdf Large-scale hydrogen underground storage for securing future energy supplies] {{Webarchive|url=https://web.archive.org/web/20140728194442/http://juwel.fz-juelich.de:8080/dspace/bitstream/2128/4071/1/HS1_8_Crotogino_rev0426.pdf |date=2014-07-28}} essential for the hydrogen economy.[http://cat.inist.fr/?aModele=afficheN&cpsidt=8443716 LINDBLOM U.E.; A conceptual design for compressed hydrogen storage in mined caverns] By using a turboexpander the electricity needs for compressed storage on 200 bar amounts to 2.1% of the energy content.[https://www.iea.org/reports/prospects-for-hydrogen-and-fuel-cells Energy technology analysis: Prospects for Hydrogen and Fuel Cells (International Energy Agency 2005) p.70]

The Chevron Phillips Clemens Terminal in Texas has stored hydrogen since the 1980s in a solution-mined salt cavern. The cavern roof is about {{convert|2800|ft}} underground. The cavern is a cylinder with a diameter of {{convert|160|ft}}, a height of {{convert|1000|ft}}, and a usable hydrogen capacity of {{convert|1066|Mcuft}}, or {{convert|2520|MT}}.[http://www.ornl.gov/~webworks/cppr/y2001/rpt/125102.pdf ORNL-Pag.20] {{Webarchive|url=https://web.archive.org/web/20081206092054/http://www.ornl.gov/~webworks/cppr/y2001/rpt/125102.pdf |date=2008-12-06}}

Salt caverns are artificially created by injecting water from the surface into a well in the rock salt, where rock salt is a polycrystalline material made of NaCl, halite. Locations such as salt domes or bedded salt are usually picked for salt caverns' creation. Salt caverns can reach a maximum depth of 2000 m and a maximum volume capacity of 1,000,000 m3. The frequency of injection and withdrawal cycles ranges between 10 and 12 cycles per year. The leak rate is around 1%.{{Cite journal |last=Tarkowski |first=Radoslaw |date=2019-05-01 |title=Underground hydrogen storage: Characteristics and prospects |url=https://www.sciencedirect.com/science/article/pii/S1364032119300528 |journal=Renewable and Sustainable Energy Reviews |volume=105 |pages=86–94 |doi=10.1016/j.rser.2019.01.051 |bibcode=2019RSERv.105...86T |s2cid=115848429 |issn=1364-0321|url-access=subscription }}{{Cite journal |last1=Thiyagarajan |first1=Sugan Raj |last2=Emadi |first2=Hossein |last3=Hussain |first3=Athar |last4=Patange |first4=Prathamesh |last5=Watson |first5=Marshall |date=2022-07-01 |title=A comprehensive review of the mechanisms and efficiency of underground hydrogen storage |url=https://www.sciencedirect.com/science/article/pii/S2352152X22005114 |journal=Journal of Energy Storage |volume=51 |pages=104490 |doi=10.1016/j.est.2022.104490 |bibcode=2022JEnSt..5104490T |s2cid=247822881 |issn=2352-152X|url-access=subscription }}

Due to the physiochemical properties of the rock salt, salt caverns exhibit multiple advantages. Key characteristics are low water content, low porosity and permeability, and its chemical inertia towards hydrogen.{{Cite journal |last1=Małachowska |first1=Aleksandra |last2=Łukasik |first2=Natalia |last3=Mioduska |first3=Joanna |last4=Gębicki |first4=Jacek |date=January 2022 |title=Hydrogen Storage in Geological Formations—The Potential of Salt Caverns |journal=Energies |language=en |volume=15 |issue=14 |pages=5038 |doi=10.3390/en15145038 |issn=1996-1073|doi-access=free}} Permeability is a key parameter in underground hydrogen storage, which affects its ability to seal. Though studies have found dilatancy and extensional fracture can cause significant permeability increase, rock salt crystal's recrystallization, which is a grain boundaries healing process, may contribute to its mechanical stiffness and permeability recovery.{{Cite journal |last1=Grgic |first1=D. |last2=Al Sahyouni |first2=F. |last3=Golfier |first3=F. |last4=Moumni |first4=M. |last5=Schoumacker |first5=L. |date=2022-02-01 |title=Evolution of Gas Permeability of Rock Salt Under Different Loading Conditions and Implications on the Underground Hydrogen Storage in Salt Caverns |url=https://doi.org/10.1007/s00603-021-02681-y |journal=Rock Mechanics and Rock Engineering |language=en |volume=55 |issue=2 |pages=691–714 |doi=10.1007/s00603-021-02681-y |bibcode=2022RMRE...55..691G |s2cid=240290598 |issn=1434-453X|url-access=subscription }} Its plastic properties prevent the formation and spread of fractures and protect it from losing its tightness, which is particularly important for hydrogen storage. Some of the disadvantages of salt caverns include lower storage capacity, large amount of water needed, and the effect of corrosion. Cushion gas is needed to avoid creep due to pressure drop when withdrawing gas from the reservoir. Though the need for cushion gas is relatively small, around 20%, the operational cost can still add up when working with a larger storage capacity. Cost is another big concern, where the cost of construction and operation are still high.{{Cite journal |last1=Lankof |first1=Leszek |last2=Tarkowski |first2=Radosław |date=2020-07-31 |title=Assessment of the potential for underground hydrogen storage in bedded salt formation |url=https://www.sciencedirect.com/science/article/pii/S0360319920317523 |journal=International Journal of Hydrogen Energy |volume=45 |issue=38 |pages=19479–19492 |doi=10.1016/j.ijhydene.2020.05.024 |bibcode=2020IJHE...4519479L |s2cid=225452215 |issn=0360-3199|url-access=subscription }}

Though people have experience with storing natural gas, storing hydrogen is a lot more complex. Factors such as hydrogen diffusivity in solids cause restrictions in salt cavern storage. Microbial activity is under extensive research worldwide because of its impact on hydrogen loss. As a result of methanogenic bacteria's bacterial metabolism, carbon dioxide and hydrogen are consumed and methane is produced, which leads to the loss of hydrogen stored in the salt caverns.{{Cite journal |last=Panfilov |first=Mikhail |date=December 2010 |title=Underground Storage of Hydrogen: In Situ Self-Organisation and Methane Generation |url=http://link.springer.com/10.1007/s11242-010-9595-7 |journal=Transport in Porous Media |language=en |volume=85 |issue=3 |pages=841–865 |doi=10.1007/s11242-010-9595-7 |bibcode=2010TPMed..85..841P |s2cid=121951492 |issn=0169-3913|url-access=subscription }}

• Sandia National Laboratories released in 2011 a life-cycle cost analysis framework for geologic storage of hydrogen.[http://prod.sandia.gov/techlib/access-control.cgi/2011/116221.pdf a life-cycle cost analysis framework for geologic storage of hydrogen]
• The European project Hyunder[http://www.hyunder.eu/ Hyunder] indicated in 2013 that for the storage of wind and solar energy an additional 85 caverns are required as it cannot be covered by pumped-storage hydroelectricity and compressed air energy storage systems.[http://www.hyunder.eu/images/Presentations%20EUSEW/2%20HyUnder%20EUSEW%20workshop%20Luis%20Correas.pdf Storing renewable energy: Is hydrogen a viable solution?] {{dead link|date=July 2018 |bot=InternetArchiveBot |fix-attempted=no}}
• ETI released in 2015 a report The role of hydrogen storage in a clean responsive power system noting that the UK has sufficient salt bed resources to provide tens of GWe.[http://www.eti.co.uk/wp-content/uploads/2015/05/3380-ETI-Hydrogen-Insights-paper.pdf The role of hydrogen storage in a clean responsive power system]
• RAG Austria AG finished a hydrogen storage project in a depleted oil and gas field in Austria in 2017, and is conducting its second project "Underground Sun Conversion".{{Cite web |url=https://www.underground-sun-storage.at/pressepublikationen/publikationen.html |title=Underground Sun Storage - Publikationen - Presse/Publikationen |access-date=2019-04-16 |archive-date=2019-04-16 |archive-url=https://web.archive.org/web/20190416093912/https://www.underground-sun-storage.at/pressepublikationen/publikationen.html |url-status=dead}}

A cavern sized 800 m tall and 50 m diameter can hold hydrogen equivalent to 150 GWh.{{cite web |last1=Hornyak |first1=Tim |title=An $11 trillion global hydrogen energy boom is coming. Here's what could trigger it |url=https://www.cnbc.com/2020/11/01/how-salt-caverns-may-trigger-11-trillion-hydrogen-energy-boom-.html |website=CNBC |archive-url= https://web.archive.org/web/20210520155551/https://www.cnbc.com/2020/11/01/how-salt-caverns-may-trigger-11-trillion-hydrogen-energy-boom-.html |archive-date=20 May 2021 |language=en |date=1 November 2020 |url-status=live}}{{cite journal |last1=Cyran |first1=Katarzyna |title=Insight into a Shape of Salt Storage Caverns |journal=Archives of Mining Sciences |date=June 2020 |volume=65(2):363-398 |page=384 |doi=10.24425/ams.2020.133198 |publisher=AGH University of Science and Technology in Kraków|doi-access=free}}

Power to gas is a technology which converts electrical power to a gas fuel. There are two methods: the first is to use the electricity for water splitting and inject the resulting hydrogen into the natural gas grid; the second, less efficient method is used to convert carbon dioxide and hydrogen to methane, (see natural gas) using electrolysis and the Sabatier reaction. A third option is to combine the hydrogen via electrolysis with a source of carbon (either carbon dioxide or carbon monoxide from biogas, from industrial processes or via direct air-captured carbon dioxide) via biomethanation,{{cite web |last1=Rathi |first1=Akshat |title=Batteries can't solve the world's biggest energy-storage problem. One startup has a solution. |url=https://qz.com/1133123/batteries-cant-solve-the-worlds-biggest-energy-storage-problem-one-startup-has-a-solution/ |website=qz.com |date=11 December 2017 |publisher=Quartz |access-date=22 April 2018 |archive-date=23 April 2018 |archive-url=https://web.archive.org/web/20180423034110/https://qz.com/1133123/batteries-cant-solve-the-worlds-biggest-energy-storage-problem-one-startup-has-a-solution/ |url-status=live}}{{cite web |title=Munich-based clean-tech startup Electrochaea and Hungarian utility MVM establish power-to-gas joint venture | work=MVM |url=http://mvm.hu/uncategorized/munich-based-clean-tech-startup-electrochaea-hungarian-utility-mvm-establish-power-gas-joint-venture/?lang=en |publisher=MVM Group |access-date=22 April 2018 |date=24 October 2016 |archive-date=23 April 2018 |archive-url=https://web.archive.org/web/20180423033613/http://mvm.hu/uncategorized/munich-based-clean-tech-startup-electrochaea-hungarian-utility-mvm-establish-power-gas-joint-venture/?lang=en |url-status=live}} where biomethanogens (archaea) consume carbon dioxide and hydrogen and produce methane within an anaerobic environment. This process is highly efficient, as the archaea are self-replicating and only require low-grade (60 °C) heat to perform the reaction.

Another process has also been achieved by SoCalGas to convert the carbon dioxide in raw biogas to methane in a single electrochemical step, representing a simpler method of converting excess renewable electricity into storable natural gas.{{cite web |title=SoCalGas and Opus 12 Successfully Demonstrate Technology That Simplifies Conversion of Carbon Dioxide into Storable Renewable Energy |url=https://www.prnewswire.com/news-releases/socalgas-and-opus-12-successfully-demonstrate-technology-that-simplifies-conversion-of-carbon-dioxide-into-storable-renewable-energy-300633313.html |website=prnewswire.com |publisher=prnewswire |access-date=22 April 2018 |archive-date=23 April 2018 |archive-url=https://web.archive.org/web/20180423033853/https://www.prnewswire.com/news-releases/socalgas-and-opus-12-successfully-demonstrate-technology-that-simplifies-conversion-of-carbon-dioxide-into-storable-renewable-energy-300633313.html |url-status=live}}

The UK has completed surveys and is preparing to start injecting hydrogen into the gas grid as the grid previously carried 'town gas' which is a 50% hydrogen-methane gas formed from coal. Auditors KPMG found that converting the UK to hydrogen gas could be £150bn to £200bn cheaper than rewiring British homes to use electric heating powered by lower-carbon sources.{{cite news |last1=Ambrose |first1=Jillian |title=Energy networks prepare to blend hydrogen into the gas grid for the first time |url=https://www.telegraph.co.uk/business/2018/01/06/hydrogen/ |newspaper=The Telegraph |access-date=22 April 2018 |date=2018-01-06 |archive-date=2018-04-23 |archive-url=https://web.archive.org/web/20180423100848/https://www.telegraph.co.uk/business/2018/01/06/hydrogen/ |url-status=live}}

Excess power or off peak power generated by wind generators or solar arrays can then be used for load balancing in the energy grid. Using the existing natural gas system for hydrogen, Fuel cell maker Hydrogenics and natural gas distributor Enbridge have teamed up to develop such a power to gas system in Canada.{{cite news |last=Anscombe |first=Nadya |title=Energy storage: Could hydrogen be the answer? |url=http://www.solarnovus.com/energy-storage-could-hydrogen-be-the-answer_N5028.html |access-date=3 November 2012 |newspaper=Solar Novus Today |date=4 June 2012 |archive-date=19 August 2013 |archive-url=https://web.archive.org/web/20130819022518/http://www.solarnovus.com/energy-storage-could-hydrogen-be-the-answer_N5028.html |url-status=live}}

Pipeline storage of hydrogen where a natural gas network is used for the storage of hydrogen. Before switching to natural gas, the German gas networks were operated using towngas, which for the most part (60-65%) consisted of hydrogen. The storage capacity of the German natural gas network is more than 200,000 GW·h which is enough for several months of energy requirement. By comparison, the capacity of all German pumped storage power plants amounts to only about 40 GW·h. The transport of energy through a gas network is done with much less loss (<0.1%) than in a power network (8%). The use of the existing natural gas pipelines for hydrogen was studied by NaturalHy.{{Cite web |url=http://www.naturalhy.net/docs/Naturalhy_Brochure.pdf |archiveurl=https://web.archive.org/web/20120118102741/http://www.naturalhy.net/docs/Naturalhy_Brochure.pdf|url-status=dead |title=naturalhy.net |archivedate=January 18, 2012 |website=www.naturalhy.net}}

Portability is one of the biggest challenges in the automotive industry, where high density storage systems are problematic due to safety concerns.

High-pressure tanks weigh much more than the hydrogen they can hold. For example, in the 2014 Toyota Mirai, a full tank contains only 5.7% hydrogen, the rest of the weight being the tank.{{cite web |url=http://www.greencarcongress.com/2014/11/20141118-mirai.html |title=Toyota FCV Mirai launches in LA; initial TFCS specs; $57,500 or $499 lease; leaning on Prius analogy |author=Mike Millikin |publisher=Green Car Congress |date=2014-11-18 |access-date=2014-11-23 |archive-date=2014-11-21 |archive-url=https://web.archive.org/web/20141121125909/http://www.greencarcongress.com/2014/11/20141118-mirai.html |url-status=live}}

System densities are often around half those of the working 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.{{Citation needed|date=March 2024}}

Due to its clean-burning characteristics, hydrogen is a clean fuel alternative for the automotive industry. Hydrogen-based fuel could significantly reduce the emissions of greenhouse gases such as CO₂, SO₂ and NO_x. Three problems for the use of hydrogen fuel cells (HFC) are efficiency, size, and safe onboard storage of the gas. Other major disadvantages of this emerging technology involve cost, operability and durability issues, which still need to be improved from the existing systems. To address these challenges, the use of nanomaterials has been proposed as an alternative option to the traditional hydrogen storage systems. The use of nanomaterials could provide a higher density system and increase the driving range towards the target set by the DOE at 300 miles. Carbonaceous materials such as carbon nanotube and metal hydrides are the main focus of research. They are currently being considered for onboard storage systems due to their versatility, multi-functionality, mechanical properties and low cost with respect to other alternatives.{{cite journal |last1=Hussein |first1=A.K. |date=2015 |title=Applications of nanotechnology in renewable energies—A comprehensive overview and understanding |journal=Renewable and Sustainable Energy Reviews |volume=42 |pages=460–476 |doi=10.1016/j.rser.2014.10.027|bibcode=2015RSERv..42..460H }}

The introduction of nanomaterials in onboard hydrogen storage systems may be a major turning point in the automotive industry. However, storage is not the only aspect of the fuel cell to which nanomaterials may contribute. Different studies have shown that the transport and catalytic properties of Nafion membranes used in HFCs can be enhanced with TiO₂/SnO₂ nanoparticles. The increased performance is caused by an improvement in hydrogen splitting kinetics due to catalytic activity of the nanoparticles. Furthermore, this system exhibits faster transport of protons across the cell which makes HFCs with nanoparticle composite membranes a promising alternative.

Another application of nanomaterials in water splitting has been introduced by a research group at Manchester Metropolitan University in the UK using screen-printed electrodes consisting of a graphene-like material.{{cite news |last=Evans |first=Scarlett |date=August 20, 2018 |title=Researchers to create hydrogen energy source using nanotechnology |url=https://www.power-technology.com/news/researchers-create-hydrogen-energy-source-using-nanotechnology/ |location=United Kingdom |access-date=December 14, 2018 |archive-date=December 16, 2018 |archive-url=https://web.archive.org/web/20181216031625/https://www.power-technology.com/news/researchers-create-hydrogen-energy-source-using-nanotechnology/ |url-status=live}} Similar systems have been developed using photoelectrochemical techniques.

{{See also|Compressed hydrogen}}

Increasing gas pressure improves the energy density by volume making for smaller container tanks. The standard material for holding pressurised hydrogen in tube trailers is steel (there is no hydrogen embrittlement problem with hydrogen gas). Tanks made of carbon and glass fibres reinforcing plastic as fitted in Toyota Mirai and Kenworth trucks are required to meet safety standards. Few materials are suitable for tanks as hydrogen being a small molecule tends to diffuse through many polymeric materials. The most common on board hydrogen storage in 2020 vehicles was hydrogen at pressure 700bar = 70MPa. The energy cost of compressing hydrogen to this pressure is significant.{{Citation needed|date=March 2024}}

Pressurized gas pipelines are always made of steel and operate at much lower pressures than tube trailers.

{{See also||Liquid hydrogen}}

Alternatively, higher volumetric energy density liquid hydrogen or slush hydrogen may be used. However, liquid hydrogen is cryogenic and boils at 20.268 K (−252.882 °C or −423.188 °F). Cryogenic storage cuts weight but requires large liquification energies. The liquefaction process, involving pressurizing and cooling steps, is energy intensive.{{cite book |last=Zubrin |first=Robert |author-link=Robert Zubrin |url=https://archive.org/details/energyvictorywin00zubr |title=Energy Victory |publisher=Prometheus Books |year=2007 |isbn=978-1-59102-591-7 |location=Amherst, New York |pages=[https://archive.org/details/energyvictorywin00zubr/page/n120 117]–118 |quote=The situation is much worse than this, however, because before the hydrogen can be transported anywhere, it needs to be either compressed or liquefied. To liquefy it, it must be refrigerated down to a temperature of −253 °C (20 degrees above absolute zero). At these temperatures, fundamental laws of thermodynamics make refrigerators extremely inefficient. As a result, about 40 percent of the energy in the hydrogen must be spent to liquefy it. This reduces the actual net energy content of our product fuel to 792 kcal. In addition, because it is a cryogenic liquid, still more energy could be expected to be lost as the hydrogen boils away as it is warmed by heat leaking in from the outside environment during transport and storage. |url-access=limited}} The liquefied hydrogen has lower energy density by volume than gasoline by approximately a factor of four, because of the low density of liquid hydrogen – there are actually more oxidizable hydrogen atoms in a litre of gasoline (116 grams) than there are in a litre of pure liquid hydrogen (71 grams). Like any other liquid at cryogenic temperatures, the liquid hydrogen storage tanks must also be well insulated to minimize boil off.

Japan has a liquid hydrogen (LH2) storage facility at a terminal in Kobe, and was expected to receive the first shipment of liquid hydrogen via LH2 carrier in 2020.{{cite web |last1=Savvides |first1=Nick |date=2017-01-11 |title=Japan plans to use imported liquefied hydrogen to fuel Tokyo 2020 Olympics |url=https://fairplay.ihs.com/ship-construction/article/4280251/japan-plans-to-use-imported-liquefied-hydrogen-to-fuel-tokyo-2020-olympics |url-status=dead |archive-url=https://web.archive.org/web/20180423033549/https://fairplay.ihs.com/ship-construction/article/4280251/japan-plans-to-use-imported-liquefied-hydrogen-to-fuel-tokyo-2020-olympics |archive-date=2018-04-23 |access-date=22 April 2018 |website=Safety At Sea |publisher=IHS Markit Maritime Portal}} Hydrogen is liquified by reducing its temperature to −253 °C, similar to liquified natural gas (LNG) which is stored at −162 °C. A potential efficiency loss of 12.79% can be achieved, or 4.26 kWh/kg out of 33.3 kWh/kg.{{cite journal |last1=S.Sadaghiani |first1=Mirhadi |date=2 March 2017 |title=Introducing and energy analysis of a novel cryogenic hydrogen liquefaction process configuration |journal=International Journal of Hydrogen Energy |volume=42 |issue=9|page=6033 |doi=10.1016/j.ijhydene.2017.01.136 |bibcode=2017IJHE...42.6033S }}

{{main|Liquid organic hydrogen carriers}}

The Hydrogen Storage Materials research field is vast, having tens of thousands of published papers.{{Cite web |last=Chanchetti |first=Lucas Faccioni |date=2014-09-18 |title=Cientometria aplicada a materiais para armazenamento de hidrogênio |url=https://repositorio.ufscar.br/handle/ufscar/935 |access-date=2021-11-19 |archive-date=2021-05-01 |archive-url=https://web.archive.org/web/20210501030748/https://repositorio.ufscar.br/handle/ufscar/935 |url-status=live}} According to Papers in the 2000 to 2015 period collected from Web of Science and processed in VantagePoint bibliometric software, a scientometric review of research in hydrogen storage materials was constituted. According to the literature, hydrogen energy went through a hype-cycle type of development in the 2000s. Research in Hydrogen Storage Materials grew at increasing rates from 2000 to 2010. Afterwards, growth continued but at decreasing rates, and a plateau was reached in 2015. Looking at individual country output, there is a division between countries that after 2010 inflected to a constant or slightly declining production, such as the European Union countries, the US and Japan, and those whose production continued growing until 2015, such as China and South Korea. The countries with most publications were China, the EU and the United States, followed by Japan. China kept the leading position throughout the entire period, and had a higher share of hydrogen storage materials publications in its total research output.{{Cite journal |last1=Chanchetti |first1=Lucas Faccioni |last2=Leiva |first2=Daniel Rodrigo |last3=Lopes de Faria |first3=Leandro Innocentini |last4=Ishikawa |first4=Tomaz Toshimi |date=2020-02-14 |title=A scientometric review of research in hydrogen storage materials |url=https://www.sciencedirect.com/science/article/pii/S0360319919323432 |journal=International Journal of Hydrogen Energy |series=22nd World Hydrogen Energy Conference |language=en |volume=45 |issue=8 |pages=5356–5366 |doi=10.1016/j.ijhydene.2019.06.093 |bibcode=2020IJHE...45.5356C |s2cid=199075995 |issn=0360-3199|url-access=subscription }}

Among materials classes, Metal-Organic Frameworks were the most researched materials, followed by Simple Hydrides. Three typical behaviors were identified:

1. New materials, researched mainly after 2004, such as MOFs and Borohydrides;
2. Classic materials, present through the entire period with growing number of papers, such as Simple Hydrides, and
3. Materials with stagnant or declining research through the end of the period, such as AB5 alloys and Carbon Nanotubes.

However, current physisorption technologies are still far from being commercialized. The experimental studies are executed for small samples less than 100 g.{{Citation |last1=Jain |first1=Ankur |title=Nitrogen-Based Hydrogen Storage Systems: A Detailed Overview |date=2018-08-06 |url=http://dx.doi.org/10.1002/9781119460572.ch2 |work=Hydrogen Storage Technologies |pages=39–88 |place=Hoboken, NJ, USA |publisher=John Wiley & Sons, Inc.|access-date=2021-11-19 |last2=Ichikawa |first2=Takayuki |last3=Ichikawa |first3=Takayuki |last4=Agarwal |first4=Shivani |doi=10.1002/9781119460572.ch2 |isbn=9781119460572 |s2cid=104929670|url-access=subscription }} The described technologies require high pressure and/or low temperatures as a rule. Therefore, at their current state of the art these techniques are not considered as a separate novel technology but as a type of valuable add-on to current compression and liquefaction methods.

Physisorption processes are reversible since no activation energy is involved and the interaction energy is very low. In materials such as metal–organic frameworks, porous carbons, zeolites, clathrates, and organic polymers, hydrogen is physisorbed on the surface of the pores. In these classes of materials, the hydrogen storage capacity mainly depends on the surface area and pore volume. The main limitation of use of these sorbents as H₂storage materials is weak van der Waals interaction energy between hydrogen and the surface of the sorbents. Therefore, many of the physisorption based materials have high storage capacities at liquid nitrogen temperature and high pressures, but their capacities become very low at ambient temperature and pressure.{{Citation needed|date=March 2024}}

LOHC, liquid organic hydrogen storage systems is a promising technique for future hydrogen storage. LOHC are organic compounds that can absorb and release hydrogen through chemical reactions. These compounds are characterized by the fact that they can be loaded and un-loaded with considerable amounts of hydrogen in a cyclic process. In principle, every unsaturated compound (organic molecules with C-C double or triple bonds) can take up hydrogen during hydrogenation. This technique ensures that the release of compounds into the atmosphere are entirely avoided in hydrogen storage. Therefore, LOHCs is an attractive way to provide wind and solar energy for mobility applications in the form of liquid energy carrying molecules of similar energy storage densities and manageability as today's fossil fuels.{{Cite journal |last1=Teichmann |first1=Daniel |last2=Arlt |first2=Wolfgang |last3=Wasserscheid |first3=Peter |last4=Freymann |first4=Raymond |date=2011 |title=A future energy supply based on Liquid Organic Hydrogen Carriers (LOHC) |url=http://dx.doi.org/10.1039/c1ee01454d |journal=Energy & Environmental Science |volume=4 |issue=8 |pages=2767 |doi=10.1039/c1ee01454d |bibcode=2011EnEnS...4.2767T |issn=1754-5692 |access-date=2021-11-19 |archive-date=2022-10-16 |archive-url=https://web.archive.org/web/20221016175543/https://pubs.rsc.org/en/content/articlelanding/2011/EE/c1ee01454d |url-status=live|url-access=subscription }}{{Obsolete source|date=February 2024}}

{{Portal|Energy}}

• Cascade storage system
• Cryo-adsorption
• Electrochemical hydrogen compressor
• Hydrogenography
• Hydrogen energy plant in Denmark
• Industrial gas
• Tunable nanoporous carbon
• Combined cycle hydrogen power plant
• Grid energy storage
• Hydrogen infrastructure
• Hydrogen economy
• Hydrogen turboexpander-generator
• Power-to-gas
• Timeline of hydrogen technologies

{{reflist|colwidth=30em}}

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{{Commons}}

• [http://www.mahytec.com/ MaHyTec Hydrogen Tanks]
• {{usurped|1=[https://web.archive.org/web/20080220094420/http://www.nesshy.net/ Nesshy]}}
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