We have become dependent upon fossil fuels, but hydrogen appears to be a good alternative. Indeed, it can be used to store a great quantity of energy over long periods of time. Hydrogen can then be used for mobile or stationary applications with fuel cells or via direct combustion. Depending on its production, its carbon footprint can also be very interesting. However, storage capabilities strongly impact hydrogen applications and are currently a crucial challenge, particularly for mobility. It is therefore of upmost importance to design lightweight, compact, safe and low-cost storage tanks.
Liquid or gas storage
Liquid hydrogen is highly energetic and has a density of 71kg per cubic metre at atmospheric pressure. However, liquifying hydrogen has a major drawback: its energy cost, because hydrogen only becomes liquid at ‑253°C. In addition, liquid hydrogen must be stored in cryogenic tanks. Most of these are made of stainless steel and have storage capacities ranging from a few litres to several thousands of cubic meters. However, the heat insulation of these storage tanks is not perfect. A fraction of the gas usually boils off due to external sources of heat (caused by insulation problems) and the size and shape of tanks.
In gaseous state, hydrogen is the lightest element. It occupies a substantial volume of 11 m3 per kilogram in normal conditions of temperature and pressure (at 0 °C, under 1,013 bar). It is therefore absolutely necessary to reduce this volume in order to store and transport hydrogen efficiently. To that end, pressure is a good alternative. Compressing and storing gaseous hydrogen in steel cylinders filled at 200 or 250 bars, is standard practice. However, two main disadvantages remain with this method of storage: volume and mass. These problems have considerably decreased with the development of reservoirs called types III and IV, whose reinforcing structures are made of composite materials. The composites are made from glass, aramid or carbon fibres embedded in resin. They make it possible to work at higher pressures while reducing the mass and increasing the resistance to fatigue failure due to external aggression. Thanks to these composite structures, standard pressures have increased to 350 and 700 bars.
Solid storage: an alternative
Solid hydrogen storage involves sequestering the gas within a solid material. This sequestration can be chemical or physical depending on the type of material. Chemical storage through absorption rests on a metal hydride resulting from the reversible chemical combination between metallic bonds of hydrogen with atoms of a great variety of metals. In contrast, physical storage is characterized by an increase in gas density at the surface of the solid material due to the molecular interactions between the adsorbate (gas) and the adsorbent (solid). This surface phenomenon, which is completely reversible, is only possible with solid materials of large specific surface area. These materials are both very porous (tiny pores in the nanometre range) and very divided, in the form of a fine powder.
Ultimately, what is the best solution?
Despite the steady research progress for on-board hydrogen storage, none of the techniques mentioned above meet the specifications set by the American Department of Energy (DOE) in terms of physical performance (massive storage, volumetric storage, temperature, pressure, leakage rate), material constraints (mass and volume of the system) and economic constraints.
Liquid hydrogen offers the best value for storage quantity/volume. However, the weak boil-off of the liquid, due to the unavoidable thermal loss (as small as it may be), causes a permanent release of hydrogen, and thus a loss of mass. It means that no hydrogen-powered vehicle can be left in a confined area. For instance, the BMW Hydrogen7 was equipped with this storage technology but production was discontinued because of this problem. Nevertheless, this type of storage is well developed to transport gas, especially in North America, where it represents more than 90% of volumes transported by road.
Storage of compressed hydrogen in composite reservoirs makes it possible to reach a satisfying density at 350 bars, but the bulk density is far too weak. It is therefore necessary to increase pressure to 700 bars. At this pressure, the density of hydrogen is 42kg per cubic meter. This type of storage is used by many car manufacturers for vehicles with a range of 400 to 500kg (around 5kg of stored hydrogen). It should be noted that storing 5kg of hydrogen at 700 bars demands a volume of 125 litres. In economic terms, even if a cryogenic tank is less expensive than a pressure tank, the cost of gas liquefaction is much higher than that of compression, even at 700 bars.
Storing hydrogen in the form of metal hydrides or by adsorption offers a storage quantity/volume value which is 3 times higher than that of compressed gas. However, due to the high mass of metal hydrides, the weight percentage of stored hydrogen is far too low. Furthermore, restoring the gas requires heat whereas hydriding (formation of hydride) is exothermic and only involves slow kinetics. This type of storage is more suited to stationary applications. Finally, hydrogen storage through adsorption in porous materials is a physical process and as such, performance is optimal only at low temperatures (in the range of ‑196°C) and at a pressure of around 100 bars. Even though research on this subject has made significant progress in recent years, storage mass capacities at ambient temperature are still too low. Further progress is necessary before considering mobile applications.