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Metals, such as magnesium, chemically bond with hydrogen gas to be transported as a metal hydride.


Technology

What is it?

Metals, such as magnesium, chemically bond with hydrogen gas to be transported as a metal hydride. When the hydrogen is required, heat is applied to release it from the metal. Intermetallic hydrides are a variation in which transition metals are present instead of main group metals.

Why is it important?

Metal hydrides offer storage at moderate pressure, retrieval at safe temperatures, and a higher hydrogen storage density than pressurised or liquefied hydrogen.

Characteristics

Characteristics – high temperature hydrides

  • Volumetric hydrogen density: High (> 100 L/Kg)
  • Gravimetric hydrogen density: Moderate to high (7 to 10 wt%)
  • Hydrogenation and extraction conditions: ~100 to 500°C, or above to achieve hydrogen reversibility
  • Storage conditions: Reversibility in some cases occurs only under high temperature and pressure conditions, e.g. Mg/MgH2 is 20 bar and 300°C but AlH3 is >100 C and > 1000 bar H2 pressure
  • Hydrogenation/dehydrogenation energy efficiency: 80%

 Characteristics – low temperature hydrides

  • Volumetric hydrogen density: High (> 100 L/Kg)
  • Gravimetric hydrogen density: Low (< 2wt%)
  • Hydrogenation and extraction conditions: -10 to 50°C
  • Storage conditions: Reversible at room temperature and reasonable hydrogen pressure conditions (e.g. TiFe and LaNi5 can absorb hydrogen at 30 bar and 25°C)
  • Hydrogenation/dehydrogenation energy efficiency: 90%

Benefits

  • Hydrogen release is endothermic and self-regulated, reducing risk of accidental explosion
  • Lower pressures than pressurised gas and more moderate temperatures than liquefied hydrogen, leading to increased safety
  • Higher hydrogen-storage density than pressurised or liquefied hydrogen156
  • Low temperature hydrides have near-ambient operating pressure and temperature
  • For room temperature hydrides, extremely long cycle life, up to 20,000 cycles in some cases
  • Negligible self-discharge
  • 90% round trip efficiency for room temperature hydrides
  • Can be readily scaled to very large capacities for grid storage
  • Suitable for hydrogen transport to fuelling stations

Limitations

  • For high temperature hydrides, hydrogenation and hydrogen release occur at elevated temperatures, ranging from ~100-500°C. Light metal simple hydrides require high temperatures for rapid hydrogen release
  • Note: Hydrides of very high hydrogen storage capacity (e.g. aluminium hydride, AlH3) are low TRL as they are not reversible for hydrogen storage. Room temperature hydrides of lower hydrogen capacity (< 2 wt% H2) are high TRL

RD&D priorities

  • For high temperature hydrides, improve hydrogen sorption kinetics for fast and effective dissociation of hydrogen molecules3
  • Improve energy density, cycle life and operating temperature
  • Improve gravimetric hydrogen density

Known active organisations

  • The Australian National University
  • CSIRO
  • Curtin University
  • Deakin University
  • Griffith University
  • The University of New South Wales
  • The University of Queensland
  • The University of Technology Sydney

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