Metal hydrides are a special type of metallic alloys. They can absorb and desorb hydrogen reversibly, while releasing and absorbing thermal energy, respectively. Since the entropy of hydride is lower in comparison to metal and gaseous hydrogen phase at ambient and elevated temperatures, hydride formation is exothermic, while the reverse reaction (hydrogen release) is endothermic. The reaction equilibrium can be described by the Van’t Hoff expression as shown in Figure 1. When pressurised, most metals bind strongly with hydrogen, resulting in stable metal hydrides that can be used to store hydrogen conveniently on board vehicles. Examples of metal hydrides are LaNi5H6, MgH2, and NaAlH4. Metal hydrides can be liquids or powders that are usually stored in tanks at approximately 1 MPa (10 bar). As the pressure is reduced or the temperature is increased (between 120 °C and 200 °C), hydrogen is released. The metal hydride can be recharged without the use of a high pressure compressed gas or cryogenic liquid. When designing efficient metal hydride systems, the critical material properties to manipulate are thermal conductivity, heat of reaction and activation energy.
Metal hydride storage has a low risk of accidental leaks since the hydrogen is stored within the metal hydride crystal and requires energy to be released. In addition, metal hydride storage has an energy density (kWh/m3) that is about three times higher than compressed storage and cryogenic storage. According to the historical Hydride Information Center database of DOE, 2722 metal hydrides are reported by 2014.