Hydrides: metal

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