The selection and engineering of specific advanced materials (AdMs) play an important role in the design of hydrogen aircraft. Particularly key is the efficient storage of hydrogen, which requires the selected material to have either a strong interaction with hydrogen, or no reaction at all. Six methods of reversible hydrogen storage with a high volumetric and gravimetric density have been identified, which vaguely centre around three storage types: high-pressure gas storage, cryogenic liquid storage, and absorbed storage, where hydrogen is absorbed into a material and then selectively released.
High-pressure (<20MPa) gas cylinders are currently the most ubiquitous method of hydrogen storage, with austenitic stainless steels – a form of stainless steel containing significant amounts of chromium and nickel – and aluminium alloys being the most popular to date, due to their very high tensile strengths and relatively low densities, as well as their high immunity to hydrogen effects (reaction and diffusion) at ambient temperatures. Lightweight fibre-reinforced composite structures have also been developed which, while not isotropic (equal in every direction) in strength, can be designed to withstand pressures up to 80MPa, for a significant volumetric density – a key factor in mobile hydrogen storage. However, a critical issue with high-pressure gas storage is the opposition of volumetric and gravimetric density, whereby increasing the pressure increases the former but decreases the latter, and vice-versa. While gas cylinders have been sufficient to date, new designs are needed for hydrogen aircraft.
One such highly promising alternative to gas-state hydrogen storage is liquid-state storage in cryogenic tanks (21.2K/-251.8°C) at ambient pressure. This would present a multitude of benefits, including improved safety as a result of reduced operating pressures and improved tank design flexibility as pressurized tanks can generally only be built in cylindrical geometries. There is however one fundamental issue with cryogenic liquid storage: cost. The Joule-Thompson/Linde cycle, the simplest hydrogen liquefaction method, is still complicated and thus expensive. Additionally, storage at cryogenic temperatures is complex, and boil-off losses can result from heat leaks. In optimal conditions (a double-walled, vacuum-insulated spherical dewar), a 100m³ tank would typically experience a 0.2% daily loss, although this will increase for non-optimal tank designs (e.g. non-spherical tanks) likely necessary for aircraft.
While less developed, storage by absorption is also possible. There are several propositions, including physisorption (attraction) of hydrogen molecules onto the surface of a solid. Large specific surface area (i.e. surface area-to-weight) materials, such as nanostructured or activated carbon, and carbon nanotubes (CNTs), are possible substrates. CNTs are of particular interest as the tube cavity, which has a width of less than a few molecular diameters, causing field overlap and increased attractive force between the carbon and the hydrogen. By comparison, the planar graphene sheets in graphite have less attraction but are easier to manufacture.
Physisorption for hydrogen storage has potential due to low operating pressure and material cost, as well as simple design architecture, but the small volumetric and gravimetric densities are significant drawbacks. Another method of solid hydrogen storage is the reaction with transition metals at elevated temperatures to form hydrides. Hydrogen reacts with many of the more electropositive elements (i.e. Sc, Ti, Va) and sits in the metallic crystal structure, without pressure changes in the system. This can result in extremely high volumetric hydrogen density, making metal hydrides a very effective method by which to safely and compactly store large amounts of hydrogen. The current achievable gravimetric density of about 3 mass% is however a limiting factor for aircraft, meaning the challenge to design a lightweight metal hydride system still remains.
A different system of complex hydrides can also be used: lightweight metals from groups 1, 2, and 3 (e.g. Li, Mg, B, Al, etc.), to give rise to a large variety of metal-hydrogen complexes. The primary difference between these and metallic hydrides is the transition to an ionic or covalent compound upon the absorption of hydrogen. These are very stable and decompose only at temperatures above the melting point of the complex. However, very high gravimetric densities at room temperature are possible: iBH4 has 18 mass% hydrogen – ideal for aircraft. Overall, materials science is a key piece in the hydrogen aircraft puzzle. New materials for absorbed hydrogen storage will be important in the transformation of hydrogen-propelled travel from prototypes to a scaled market solution in sustainable air travel.
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