Hydride Storage and Handling

Metal hydride systems have been proposed and demonstrated as potential high-energy-density hydrogen storage solutions. Metals that form hydrides can reversibly absorb and release hydrogen when appropriate temperatures and pressures are applied. In general, the reaction expression can be described as

M + (x/2) H2 ⇔ MHx

where M is the metal or metal alloy. Traditionally, metal hydrides have been classified into two main categories: alloys and complexes. Intermetallic alloys, including AB5, AB2, AB, and A2B materials form hydride compounds resulting from combining a stronger hydride-forming element with a weaker hydride-forming element to attain appropriate thermodynamic characteristics.

Due to the requirements for high-energy-density hydrogen storage solutions, complex hydrides comprised of light metal elements are of specific interest - including, but not limited to, lithium (Li), sodium (Na), magnesium (Mg), calcium (Ca), boron (B), nitrogen (N) and aluminum (Al). Hydrides and complexes synthesized from these elements are typically found in the form of finely divided powders that may be pyrophoric and/or water-reactive. Additionally, the oxidation or hydrolysis reaction products may also present hazards. In the case of alkali-metal based materials, the oxidation products include, but are not limited to, hydroxides, oxides, peroxides, and possibly superoxides. Additionally, those compounds composed of nitrogen or boron under certain circumstances may form trace amounts of ammonia diborane gas. Some of these materials may present a health hazard and form hydrates in a humid environment that can decompose rapidly as a result of friction or heat.

The following controls should be considered when handling reactive metal hydrides:

  • Laboratory safety signage should include "Use Dry Powder Agent Fire Extinguisher Only, No Water".
  • Work should be completed in an inert gas environment of nitrogen or argon. This includes material preparation (including mechanical milling if possible), material installation, and material removal. Gloves and seals should be inspected on a regular basis to verify integrity.
  • The environment should be kept pure by purging the glove box and the use of an isolatable antechamber. The antechamber should be kept at vacuum while not in use. All items brought into the box should be purged and evacuated at least 3 times to remove the threat of contaminants. If a user has the antechamber in a purged state and then leaves the laboratory, they should not presume it is still in a purged state when returning, since others may have opened the outer door.
  • The inert gas pressure within the glove box should be kept slightly above atmosphere (~3mbar) at all times. The source of this gas should be checked for adequate supply on a daily basis.
  • Moisture/oxygen sensors in place and monitored continuously.
    Moisture level maintained at <1 ppm
    Oxygen level maintained at <10 ppm.
    An alarm should be set to notify the user if limits are reached.
  • Experimental vessels should be designed with a safety margin to withstand the pressure of fully dehydrogenated material samples and/or be equipped with appropriate pressure relief devices.
  • All materials should be sealed in secondary containment within glove box.
  • Material within sealed experimental vessels should be stored at ~1bar overpressure inert gas when not in use.
  • Experimental vessels should be locally transported in secondary containment to avoid accidental exposure to the environment or rupture.
  • Material to be shipped over public roadways must be properly contained and marked according to DOT regulations.
  • A working database of materials should be maintained and reviewed on a regular basis.
  • No protic solvents should be used or opened in glove box. (These are ROH compounds such as 1-propanol CH3CH2CH2-OH or methanol CH3OH.)
  • Proper care should be taken when using flammable solvents as cleaning fluids to avoid flammable gas cloud mixtures.
  • Use only hydrophobic wipes for cleaning crucibles and hardware within the glove box to minimize water vapor content within the box.
  • Use plastic or ceramic utensils as much as possible during scraping or cleaning of containment vessels to minimize localized heating during use.
  • All experimental containment vessels should be leak-checked with high-pressure helium prior to evacuating in an air environment.
  • Waste materials should be sealed under inert gas and disposed of as hazardous waste material.
  • Materials neutralization procedures should be identified and instituted before materials synthesis begins.
  • Any material that has been exposed to air under any condition should be properly disposed of as hazardous waste immediately.

*Note: This section was prepared by an international team of experts led by Sandia National Laboratory (Livermore).