Chapter 4Hydrogen Storage Materials Department of Mechanical Engineering, Yuan Ze University 1 Developing safe, reliable, compact, and costeffective hydrogen storage technologies is one of the most technically challenging barriers to the widespread use of hydrogen as a form of energy. To be competitive with conventional vehicles, hydrogen-powered cars must be able to travel more than 300 mi between fills. This is a challenging goal because hydrogen has physical characteristics that make it difficult to store in large quantities without taking up a significant amount of space. Department of Mechanical Engineering, Yuan Ze University 2 Where and How Will Hydrogen be Stored? Hydrogen storage will be required onboard vehicles and at hydrogen production sites, hydrogen refueling stations, and stationary power sites. Possible approaches to storing hydrogen include: Physical storage of compressed hydrogen gas in high pressure tanks (up to 700 bar); Physical storage of cryogenic hydrogen (cooled to -253°C, at pressures of 6-350 bar) in insulated tanks; and Storage in advanced materials — within the structure or on the surface of certain materials, as well as in the form of chemical compounds that undergo a chemical reaction to release hydrogen. Department of Mechanical Engineering, Yuan Ze University 3 hydrogen associates with the surface of a material either as hydrogen molecules (H2) or hydrogen atoms (H). A light-duty fuel cell vehicle will carry approximately 4-10 kg of hydrogen on board (depending on the size and type of the vehicle) to allow a driving range of more than 300 mi. Drivers must also be able to refuel at a rate comparable to the rate of refueling today’s gasoline vehicles. along with low-cost. This makes hydrogen a challenge to store. Hydrogen can be stored on the surfaces of solids by adsorption. Department of Mechanical Engineering. but it has a very low energy content by volume (liquid hydrogen is about 4 times less than gasoline). placing a sufficient quantity of hydrogen onboard a vehicle to provide a 300-mile driving range would require a very large tank — larger than the trunk of a typical automobile. which could reduce fuel economy. In adsorption. This figure depicts hydrogen adsorption within MOF-74. there would also be the added weight of the tank(s). Low-cost materials and components for hydrogen storage systems are needed. Yuan Ze University 4 . which is generally regarded as the minimum for widespread public acceptance. Aside from the loss of cargo space.What Are the Challenges? Hydrogen has a very high energy content by weight (about 3 times more than gasoline). particularly within the size and weight constraints of a vehicle. Using currently available high-pressure tank storage technology. high-volume manufacturing methods for those materials and components. U. Department of Mechanical Engineering. Department of Energy (DOE). 2005. Yuan Ze University 5 .S. December.Roadmap on Manufacturing R&D for the Hydrogen Economy. and chemical compounds that reversibly release H2 upon heating. Department of Mechanical Engineering.Introduction Hydrogen storage describes the methods for storing H2 for subsequent use. The methods span many approaches. Yuan Ze University 6 . as well as providing a fuel for transportation. Underground hydrogen storage is useful to provide grid energy storage for intermittent energy sources. including high pressures. like wind power. compact energy carrier for mobile applications. cryogenics. Most research into hydrogen storage is focused on storing hydrogen as a lightweight. particularly for ships and airplanes. Utility scale underground liquid hydrogen storage 7 Department of Mechanical Engineering. Yuan Ze University . Yuan Ze University 8 . Information center Department of Mechanical Engineering.USDOE. EERE. Yuan Ze University 9 .882 °C or −423. Insulation by design for liquid hydrogen tanks is adding costs for this method. Liquid hydrogen has less energy density by volume than hydrocarbon fuels such as gasoline by approximately a factor of four. This highlights the density problem for pure hydrogen: there is actually about 64% more hydrogen in a liter of gasoline (116 grams hydrogen) than there is in a liter of pure liquid hydrogen (71 grams hydrogen). The carbon in the gasoline also contributes to the energy of combustion. The tanks must also be well insulated to prevent boil off. Liquid hydrogen or slush hydrogen may be used.188 °F). Hence. its liquefaction imposes a large energy loss (as energy is needed to cool it down to that temperature).268 K (−252. However liquid hydrogen requires cryogenic storage and boils around 20. Department of Mechanical Engineering. as in the Space Shuttle. Compressed hydrogen storage can exhibit very low permeation. Higher compression without energy recovery will mean more energy lost to the compression step. Yuan Ze University 10 . Compressed hydrogen will require 2. in comparison. is quite different to store. all other factors remaining equal.1% of the energy content to power the compressor. hence it requires a larger tank to store. Increasing gas pressure would improve the energy density by volume. A large hydrogen tank will be heavier than the small hydrocarbon tank used to store the same amount of energy. but not lighter container tanks (see hydrogen tank). making for smaller. Hydrogen gas has good energy density by weight. Compressed hydrogen. Department of Mechanical Engineering. but poor energy density by volume versus hydrocarbons. On Board Hydrogen Storage Targets were set by the FreedomCAR Partnership in January 2002 between the United States Council for Automotive Research (USCAR) and U. only two storage technologies were identified as being susceptible to meet DOE targets: MOF-177 exceeds 2010 target for volumetric capacity. 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. The ultimate goal for volumetric storage is still above the theoretical density of liquid hydrogen. It is important to note that these targets are for the hydrogen storage system. The targets were revised in 2009 to reflect new data on system efficiencies obtained from fleets of test cars. Department of Mechanical Engineering. The 2005 targets were not reached in 2005. etc. In 2010. while cryo-compressed H2 exceeds more restrictive 2015 targets for both gravimetric and volumetric capacity). is considered. System densities are often around half those of the working material. not the hydrogen storage material. DOE (Targets assume a 5-kg H2 storage system). Yuan Ze University 11 .S.. hanwha.jsp 12 Department of Mechanical Engineering.http://hcc.kr/english/pro/ren_hsto_idx. Yuan Ze University .co. Targets for On-Board Hydrogen Storage Department of Mechanical Engineering. Yuan Ze University 13 . right filler neck for gasoline. 14 Department of Mechanical Engineering. Compressed hydrogen in hydrogen tanks at 350 bar (5. Car manufacturers have been developing this solution. producing for example the BMW Hydrogen 7.000 psi) is used for in hydrogen vehicles. Established Compressed technologies Compressed hydrogen hydrogen is the gaseous state of the element hydrogen which is kept under pressure. Yuan Ze University .000 psi) and 700 bar (10. has been working on liquid tank for cars. such as Honda or Nissan. Liquid hydrogen BMW BMW Hydrogen 7 12-cylinder hydrogen engine of BMW Hydrogen 7 left: filler neck of a BMW for hydrogen. December.Roadmap on Manufacturing R&D for the Hydrogen Economy. 2005. Department of Energy Department of Mechanical Engineering.S. 15 . U. Yuan Ze University (DOE). Proposals Hydrogen and research storage technologies can be divided into physical storage. where hydrogen molecules are stored (including pure hydrogen storage via compression and liquefication). Department of Mechanical Engineering. Yuan Ze University 16 . and chemical storage. where hydrides are stored. the BMW Group has started a thorough component and system level validation of cryo-compressed vehicle storage on its way to a commercial product.12 per mile (including cost of fuel and every associated other cost). the main difference is that. the tank is allowed to go to pressures much higher (up to 350 bars versus a couple of bars for liquid storage). it takes more time before the hydrogen has to vent.07 per mile. As a consequence.05 and $0. Research is still on its way in order to study and demonstrate the full potential of the technology.Proposals and research . when the hydrogen would warm-up due to heat transfer with the environment ("boil off").Physical storage Cryo-compressed (denoted as "CcH2" ) Cryo-compressed storage of hydrogen is the only technology that meets 2015 DOE targets for volumetric and gravimetric efficiency. Department of Mechanical Engineering. However. another study has shown that cryo-compressed exhibits interesting cost advantages: ownership cost (price per mile) and storage system cost (price per vehicle) are actually the lowest when compared to any other technology. Yuan Ze University 17 . As of 2010.3 K and slightly above) in order to reach a high energy density. a cryocompressed hydrogen system would cost $0. enough hydrogen is used by the car to keep the pressure well below the venting limit.050 km) were driven with a full tank mounted on an hydrogen-fueled engine of Toyota Prius. For example. Consequently. it has been demonstrated that a high driving range could be achieved with a cryo-compressed tank : more than 650 miles (1. and in most driving situations. cryo-compressed uses cold hydrogen (20. Alike liquid storage. while conventional gasoline vehicles cost between $0. Furthermore. htm June 7.uk/pages/greener_cars/5_greener_cars_PM2. Yuan Ze University .org. https://news. "Spillover" Mechanism in Carbon Nanotube Hydrogen Storage http://www.slac. it was later accepted that a realistic number is less than 1 wt%. Despite initial claims of greater than 50 wt% hydrogen storage.stanford. Carbon nanotubes Hydrogen carriers based on nanostructured carbon (such as carbon buckyballs and nanotubes) have been proposed.edu/image/spillover-mechanismcarbon-nanotube-hydrogen-storage 18 Department of Mechanical Engineering. 2011 Schematic of the "spillover" mechanism by which platinum nanoparticles (tan) helps make it possible to store hydrogen (purple) in single-walled carbon nanotubes (gray).greenerindustry. When guest molecules (solvent) occupying the pores are removed during solvent exchange and heating under vacuum. catenation. MOFs have very high number of pores and surface area which allow higher hydrogen uptake in a given volume. researchers at University of Nottingham reached 10 wt% at 77 bar (1.117 psi) and 77 K with MOF NOTT-112. Yuan Ze University 19 . research interests on hydrogen storage in MOFs have been growing since 2003 when the first MOF-based hydrogen storage was introduced.5 wt% in MOF-74 at a low temperature of 77 K. The. and sample purity can result different amount of hydrogen uptake in MOFs. Metal-organic frameworks (MOFs) Metal-organic frameworks represent another class of synthetic porous materials that store hydrogen and energy at the molecular level. Most articles about hydrogen storage in MOFs report hydrogen uptake capacity at a temperature of 77K and a pressure of 1 bar because such condition is commonly available and the binding energy between hydrogen and MOF is large compare to the thermal vibration energy which will allow high hydrogen uptake capacity. Varying several factors such as surface area. MOFs are highly crystalline inorganic-organic hybrid structures that contain metal clusters or ions (secondary building units) as nodes and organic ligands as linkers. In 2009. Since there are infinite geometric and chemical variations of MOFs based on different combinations of SBUs and linkers. Compared to traditional zeolites and porous carbon materials. In 2006. many researches explore what combination will provide the maximum hydrogen uptake by varying materials of metal ions and linkers. chemists at UCLA and the University of Michigan have achieved hydrogen storage concentrations of up to 7. Department of Mechanical Engineering. pore size. spillover. ligand structure. 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. Yuan Ze University 20 .Common ligands in MOFs Department of Mechanical Engineering. USA) Department of Mechanical Engineering.MOF-177 On-Board Hydrogen Storage System (Argonne National Laboratory. Yuan Ze University 21 . researchers from Delft University of Technology and Colorado School of Mines showed solid H2-containing hydrates could be formed at ambient temperature and 10s of bar by adding small amounts of promoting substances such as THF. Yuan Ze University 22 Glass capillary arrays Glass microspheres . In 2004. storage and controlled release of hydrogen in mobile applications. These clathrates have a theoretical maximum hydrogen densities of around 5 wt% and 40 kg/m3. Israeli and German scientists have collaboratively developed an innovative technology based on glass capillary arrays for the safe infusion. The C. but requires very high pressures to be stable. Clathrate hydrates H2 caged in a clathrate hydrate was first reported in 2002. Hollow glass microspheres (HGM) can be utilized for controlled storage and release of hydrogen. A team of Russian.En technology has achieved the United States Department of Energy (DOE) 2010 targets for on-board hydrogen storage systems. Department of Mechanical Engineering. % gravimetric goal for 2015 (see chart above) are limited to lithium. often reversibly. with varying degrees of efficiency. others are solids which could be turned into pellets. TiFeH2 and palladium hydride. lithium aluminium hydride and ammonia borane. LaNi5H6. thereby eliminating any energy savings. However if the interaction is too weak. The target for onboard hydrogen fuel systems is roughly <100 °C for release and <700 bar for recharge (20-60 kJ/mol H2). Currently the only hydrides which are capable of achieving the 9 wt. lithium. Most metal hydrides bind with hydrogen very strongly. although their energy density by weight is often worse than the leading hydrocarbon fuels. or calcium and aluminum or boron. Hydrides chosen for storage applications provide low reactivity (high safety) and high hydrogen storage densities. Yuan Ze University 23 . can be used as a storage medium for hydrogen. based on magnesium hydrate. Some are easy-to-fuel liquids at ambient temperature and pressure. LiAlH4. thereby requiring less input to release stored hydrogen. These materials have good energy density by volume. LiH.Chemical storage Metal hydrides Metal hydrides. Proposed hydrides for use in a hydrogen economy include simple hydrides of magnesium or transition metals and complex metal hydrides. A French company McPhy Energy is developing the first industrial product. NaAlH4. typically containing sodium. such as MgH2. NaBH4 and LiBH4. already sold to some major clients such as Iwatani and ENEL. which is released when the solution flows over a catalyst made of ruthenium. sodium borohydride. As a result high temperatures around 120 °C (248 °F) – 200 °C (392 °F) are required to release their hydrogen content. the pressure needed for rehydriding is high. Research is being done to determine new compounds which can be used to meet these requirements.Proposals and research . boron and aluminium based compounds. These are able to form weaker bonds. at least one of the firstrow elements or Al must be added. This energy cost can be reduced by using alloys which consists of a strong hydride former and a weak one such as in LiNH2. New Scientist stated that Arizona State University is investigating using a borohydride solution to store hydrogen. Department of Mechanical Engineering. Leading candidates are lithium hydride. Yuan Ze University 24 .Metal hydride hydrogen storage Department of Mechanical Engineering. they demonstrated to produce nearly 12 moles of hydrogen per glucose unit from cellulosic materials and water.8 wt %. Department of Mechanical Engineering. Carbohydrate is the most abundant renewable bioresource in the world. In 2009. In May 2007 biochemical engineers from the Virginia Polytechnic Institute and State University and biologists and chemists from the Oak Ridge National Laboratory announced a method of producing high-yield pure hydrogen from starch and water. Thanks to complete conversion and modest reaction conditions. Carbohydrates Carbohydrates (polymeric C6H10O5) releases H2 in an bioreformer mediated by the enzyme cocktail—cell-free synthetic pathway biotransformation. they propose to use carbohydrate as a high energy density hydrogen carrier with a density of 14. Carbohydrate provides high hydrogen storage densities as a liquid with mild pressurization and cryogenic constraints: It can also be stored as a solid power. Yuan Ze University 25 . Then a small hydrogen reformer would extract the hydrogen as needed by the fuel cell. Synthesized hydrocarbons An alternative to hydrides is to use regular hydrocarbon fuels as the hydrogen carrier. However. although this could be counterbalanced with the much better energy densities of ethanol and methanol over hydrogen. Solid-oxide fuel cells can operate on light hydrocarbons such as propane and methane without a reformer. Department of Mechanical Engineering. or can run on higher hydrocarbons with only partial reforming. Alcohol fuel is a renewable resource. Direct methanol fuel cells do not require a reformer. Yuan Ze University 26 . but provide a lower energy density compared to conventional fuel cells. but the high temperature and slow startup time of these fuel cells are problematic for automotive applications. these reformers are slow to react to changes in demand and add a large incremental cost to the vehicle powertrain. Ammonia can be reformed to produce hydrogen with no harmful waste. They claim it will be an inexpensive and safe storage method. Department of Mechanical Engineering. and distributing ammonia exists. Under compression in an automobile engine it is a suitable fuel for slightly modified gasoline engines. or can mix with existing fuels and under the right conditions burn efficiently. Ammonia Ammonia (NH3) releases H2 in an appropriate catalytic reformer. Ammonia is the second most commonly produced chemical in the world and a large infrastructure for making. 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. transporting. Yuan Ze University 27 . Ammonia is a toxic gas at normal temperature and pressure and has a potent odor. 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. Pure ammonia burns poorly at the atmospheric pressures found in natural gas fired water heaters and stoves. several compounds were investigated for hydrogen storage including complex borohydrides. Ignition of the amine borane(s) forms boron nitride (BN) and hydrogen gas. and H (both positive and negative ions). or aluminohydrides. Earlier hydrogen gas-generating formulations used amine boranes and their derivatives. boron hydride ammoniates. Amongst the compounds that contain only B. the U. H2B(NH3)2BH4. amine boranes (and especially ammonia borane) have been extensively investigated as hydrogen carriers. representative examples include: amine boranes. and ammonium octahydrotriborates or tetrahydroborates. These hydrides have an upper theoretical hydrogen yield limited to about 8. Army and Navy funded efforts aimed at developing hydrogen/deuterium gas-generating compounds for use in the HF/DF and HCl chemical lasers. Yuan Ze University 28 . other gas-generators include diborane diammoniate.5% by weight. In addition to ammonia borane (H3BNH3). hydrazine-borane complexes. and gas dynamic lasers. Of these.S. During the 1970s and 1980s. N. Department of Mechanical Engineering. Amine borane complexes Prior to 1980. and ammonium salts. pure formic acid stores 4. The catalytic hydrogenation of CO2 has long been studied and efficient procedures have been developed. 85% formic acid is not flammable. Formic acid In 2006 researchers of EPFL. And the co-product of this decomposition. Formic acid contains 53 g L−1 hydrogen at room temperature and atmospheric pressure. and thermal stability and are not inflammable can add reversibly 6–12 hydrogen atoms in the presence of classical Pd/C or Ir0 nanoparticle catalysts and can be used as alternative materials for on-board hydrogen-storage devices. Department of Mechanical Engineering. Yuan Ze University 29 Imidazolium ionic liquids .g. reported the use of formic acid as a hydrogen storage material. Switzerland. poor stability. Simple alkyl(aryl)-3-methylimidazolium N-bis(trifluoromethanesulfonyl)imidate salts that possess very low vapour pressure. Pure formic acid is a liquid with a flash point 69 °C (cf. ethanol 13 °C). These salts can hold up to 30 g L−1 of hydrogen at atmospheric pressure. This catalytic system overcomes the limitations of other catalysts (e. In 2007 Dupont and others reported hydrogen-storage materials based on imidazolium ionic liquids. A homogeneous catalytic system based on water soluble ruthenium catalysts selectively decompose HCOOH into H2 and CO2 in aqueous solution. By weight. formation of CO) for the decomposition of formic acid making it a viable hydrogen storage material. Carbon monoxide free hydrogen has been generated in a very wide pressure range (1–600 bar). high density. limited catalytic lifetimes. can be used as hydrogen vector by hydrogenating it back to formic acid in a second step.3 wt% hydrogen. carbon dioxide. gasoline −40 °C. 25 wt% still rather below the 6 to 9 wt% required for practical use. After tests. it can also release the hydrogen again. it was shown that not only can graphene store hydrogen easily. conducted by dr André Geim at the University of Manchester. the substance becomes graphene. after heating to 450 °C. Phosphonium borate In 2006 researchers of University of Windsor reported on reversible hydrogen storage in a non-metal phosphonium borate frustrated Lewis pair: 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. Department of Mechanical Engineering. Yuan Ze University 30 . After taking up hydrogen. Carbonite substances Research has proven that graphene can store hydrogen efficiently. Stationary hydrogen storage Unlike mobile applications. the German gas networks were operated using towngas. The use of the existing natural gas system for hydrogen was studied by NaturalHy. A natural gas network is suitable for the storage of hydrogen. Yuan Ze University 31 Pipeline storage . Large quantities of gaseous hydrogen have been stored in underground caverns by ICI for many years without any difficulties. Before switching to natural gas. By comparison. hydrogen density is not a huge problem for stationary applications. As for mobile applications. which for the most part consisted of hydrogen. stationary applications can use established technology: Underground hydrogen storage Underground hydrogen storage is the practice of hydrogen storage in underground caverns. The storage of large quantities of liquid hydrogen underground can function as grid energy storage. The storage capacity of the German natural gas network is more than 200. salt domes and depleted oil and gas fields. Department of Mechanical Engineering. The transport of energy through a gas network is done with much less loss (<0.1%) than in a power network (8%).000 GWh which is enough for several months of energy requirement. the capacity of all German pumped storage power plants amounts to only about 40 GWh. the hydrogen absorption kinetics is accelerated by the nano-size materials and they may change the thermodynamic stability of the materials.html nano-composite materials having high H2 capacity. The nano-composite materials for hydrogen storage encompass a catalyst and composite chemical hydrides at the nanometer scale. The goal of this research is to establish the principle of the basic technology in order to control the kinetics and thermodynamics of the http://hydro-star. The catalyst increases reaction rate.jp/english/group/hydrides/index. the high work temperature (thermodynamic stability) and the slow reaction rate (high activation energy) limit the practical application of the chemical hydride systems. Metal hydrides with light elements (chemical hydride: binary hydrides and complex hydrides) have large gravimetric H2 densities. Those properties can be improved by the nano-composite materials.kek. Yuan Ze University 32 . In addition. Department of Mechanical Engineering. However. The thermodynamic stability of the nanocomposite materials can be controlled by the composite chemical hydrides having protide (hydride) (Hδ-) and proton (Hδ+). ciw.gl. Yuan Ze University 33 .edu/content/molecular-hydrogen-storage-light-element-compounds Department of Mechanical Engineering.https://efree. Hydrogen storage density cost graph (USDOE) Department of Mechanical Engineering. Yuan Ze University 34 . July. Yuan Ze University 35 .S. Department of Mechanical Engineering.Manufacturing Research & Development of Onboard Hydrogen Storage Systems for Transportation Applications. 2005. Department of Energy (DOE). U. Yuan Ze University 36 . December.slac.References http://en.kek.S. Department of Energy (DOE).uk/pages/greener_cars/5_greener_cars_PM2.jsp http://hydro-star. Department of Energy (DOE).org.kr/english/pro/ren_hsto_idx.edu/content/molecular-hydrogen-storage-light-elementcompounds http://www.co. 2005.greenerindustry.gl. Roadmap on Manufacturing R&D for the Hydrogen Economy. July.edu/image/spillover-mechanism-carbonnanotube-hydrogen-storage USDOE. Manufacturing Research & Development of Onboard Hydrogen Storage Systems for Transportation Applications.stanford. U.html https://efree.wikipedia. Department of Mechanical Engineering.hanwha.S.jp/english/group/hydrides/index.ciw. Information center. Energy Efficiency and Renewable Energy (EERE). U.htm https://news. 2005.org/wiki/Hydrogen_storage http://hcc.