Hydrides: complex

Technology

What is it?

Complex metal hydrides are salts in which the anion contains hydrides (negatively charged hydrogen atoms). Hydrogen chemically bonds with complex molecules such as LiBH₄ and LiAlH₄, allowing the complex hydride to act as a storage mechanism for the hydrogen. When the hydrogen is required after transport, the hydrogen can be released via application of heat.

Why is it important?

Complex hydrides offer storage at moderate pressure, retrieval at safe temperatures, and a higher hydrogen storage density than pressurised or liquefied hydrogen. Shipping and handling a powdered solid-state complex hydride is advantageous compared to liquids and gases.

Characteristics

• Volumetric hydrogen density: 80 to 150 kg/m³
• Gravimetric hydrogen density: 8 to 18.5 wt.% H₂
• Hydrogenation conditions: 100°C to 800°C
• Storage conditions: Room temperature, sealed container due to air sensitivity. Pressure at ~1-100 bar
• Extraction conditions: 100°C to 800°C
• Roundtrip energy efficiency: Varies widely depending on complex hydride material and if the system is reversible (i.e. temperature and pressure needed to achieve reversible hydrogen cycling)

Benefits

• Typically have higher gravimetric capacities and volumetric densities than metallic or alloy hydrides
• Higher hydrogen-storage density than pressurised or liquefied hydrogen156
• Solid state powders for ease of transport
• 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
• Negligible self-discharge

Limitations

• Kinetics and thermodynamics of complex metal hydrides mean that hydrogen storage and release must occur at elevated temperatures, ranging from 100°C to 800°C
• Hydrogen release is often not reversible under moderate conditions
• Developing a cost-effective complex hydride system is challenging
• Achieving the reversible uptake and release of hydrogen with high efficiency close to the ambient and under moderate hydrogen pressure is challenging
• Sensitive to air

RD&D priorities

• Develop new catalysts to improve kinetics for storage/retrieval of hydrogen
• Develop fundamental understanding of the reaction of complex hydrides with hydrogen and hydrogen diffusion in complex hydrides
• Develop novel approaches to control the thermodynamics for hydrogen release/uptake to achieve full hydrogen reversibly under moderate conditions of temperature and pressure
• Develop understanding of the integration/translation of the hydrides into full vessels/tanks delivering hydrogen against delivery/uptake targets
• Achieve long cycling life > 10,000 hydrogen uptake and release cycles
• Improve off-board regeneration of complex hydrides (re-hydrogenation

Known active organisations

• Curtin University
• Griffith University
• Monash University
• The University of Newcastle
• The University of New South Wales
• The University of Technology Sydney