Hydrogen storage

Several methods exist for storing hydrogen. These include mechanical approaches such as using high pressures and low temperatures, or employing chemical compounds that release H2 upon demand. While large amounts of hydrogen are produced by various industries, it is mostly consumed at the site of production, notably for the synthesis of ammonia. For many years hydrogen has been stored as compressed gas or cryogenic liquid, and transported as such in cylinders, tubes, and cryogenic tanks for use in industry or as propellant in space programs. The overarching challenge is the very low boiling point of H2: it boils around 20.268 K (−252.882 °C or −423.188 °F). Achieving such low temperatures requires expending significant energy. Although molecular hydrogen has very high energy density on a mass basis, partly because of its low molecular weight, as a gas at ambient conditions it has very low energy density by volume. If it is to be used as fuel stored on board a vehicle, pure hydrogen gas must be stored in an energy-dense form to provide sufficient driving range. Because hydrogen is the smallest molecule, it easily escapes from containers. Its effective 100-year global warming potential (GWP100) is estimated to be 11.6 ± 2.8.

Established technologies

Compressed hydrogen 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. Car manufacturers including Honda and Nissan have been developing this solution.

Liquefied hydrogen Liquid hydrogen tanks for cars, producing for example the BMW Hydrogen 7. Japan has a liquid hydrogen (LH2) storage site in Kobe port. 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.

Chemical storage

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. 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. A most promising chemical approach is electrochemical hydrogen storage, as the release of hydrogen can be controlled by the applied electricity. 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. 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. 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. 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). 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.

Where pH2 is the partial pressure of hydrogen, ΔH is the enthalpy of the sorption process (exothermic), ΔS is the change in entropy, R is the ideal gas constant, T is the temperature in Kelvin, Vm is the molar volume of the metal, r is the radius of the nanoparticle and 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.

Hydrogenation of CO2 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.

Metal hydrides

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:

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