To achieve the shift to renewable energies, efficient energy storage is of the upmost importance. Hydrogen as a chemical energy storage represents a promising technology due to its high gravimetric energy density. However, the most efficient form of hydrogen storage still remains an open question. Absorption-based storage of hydrogen in metal hydrides offers high volumetric energy densities as well as safety advantages. In this work technical,. To achieve the shift to renewable energies, efficient energy storage is of the upmost importance. Hydrogen as a chemical energy storage represents a promising technology due to its high gravimetric energy density. However, the most efficient form of hydrogen storage still remains an open question. Absorption-based storage of hydrogen in metal hydrides offers high volumetric energy densities as well as safety advantages. In this work technical, economic and environmental aspects of different metal hydride materials are investigated. An overview of the material properties, production methods as well as possibilities for enhancement of properties are presented. Furthermore, impacts on material costs, abundance of raw materials and dependency on imports are discussed. Advantages and disadvantages of selected materials are derived and may serve as a decision basis for material selection based on application. Further research on enhancement of material properties as well as on the system level is required for widespread application of metal hydrides.••••A broad and recent review of different metal hydride materials for storing hydrogen is provided.••Application-based technical requirements of metal hydride storage are discussed.••An in-depth review of production, handling and enhancement methods of six selected metal hydride materials is provided.••Economic and environmental aspects of storing hydrogen in metal hydrides are investigated.CGH2compressed gaseous hydrogenLH2liquid hydrogenLHVlower heating valueLOHCliquid organic hydrogen carrierMHmetal hydrideMOFSustainable hydrogen represents the global solution to the economic, environmental, social and health threats of climate change. By replacing the currently predominant fossil fuels with emission-free primary sources and secondary energy carriers such as green electricity, green hydrogen and biomass, the decarbonisation of our energy system can be achieved.As seen in Fig. 1, the electrical energy from renewables is used to produce green hydrogen by electrolysis. The produced hydrogen can be used for mobility, heating, power or industrial applications. However, due to the volatile nature of renewable energies, the energy availability and energy demand are not in sync. Therefore, efficient storage of hydrogen for mobile (road-bound, railroad, waterborne etc.) as well as stationary (from large-scale power plants to industrial housing) applications in the energy and industry sector is essential. Although hydrogen has the highest energy density per unit mass of any fuel, its low volumetric mass density at ambient temperature and pressure correspondingly results in a rather low energy density per unit volume. Several technologies for storing hydrogen are displayed in Fig. 1. Today, hydrogen is stored either gaseous at high pressures (CGH2) or in its liquid form (LH2) at temperatures of approx. -253 °C. However, CGH2 requires energy for the compression of hydrogen, whereas enormous energy is needed for liquefactio. 2.1. Material propertiesBefore the various metal hydride materials can be evaluated regarding suitability for different applications, the relevant material properties must be discussed. The first key parameters when comparing different storage technologies are usually the gravimetric storage capacity and volumetric energy density. For metal hydrides the gravimetric capacity can be calculated as the quotient of the maximum absorbed hydrogen mass and the mass of the hydride material and has the unit weight percent (wt%). The volumetric energy density is typically specified as:(1)ved=mH2∙LHVVMHkWh/dm3where mH2 is the maximum absorbed hydrogen mass, LHV the lower heating value of hydrogen (at approx. 120 MJ/kg) and VMH the volume of the hydride material. [3,6]In general, the gravimetric storage capacities of metal hydride materials from the interstitial hydrides group range from 1 to 2 wt%. Significantly higher gravimetric capacities could be achieved with complex hydrides. LiBH4 is known as the MH material with the highest theoretical gravimetric capacity of 18.5 wt%. The storage capacities and volumetric energy densities of some metal hydride materials as well as gaseous and liquid hydrogen storage can be seen in Table 1. The values presented are for the pure substance. For the system (tank) level a weight increase of approximately 50 % and a volume inc.
