Ammonia Storage

Hydrogen is often claimed to be the best alternative fuel for automotive purposes if not for the complications involving its safe and efficient storage. The technology developed by STFC, utilizes compounds having the general formula M1_x(BH₄)_y(NH₂(R₂))_n to safely and efficiently store and when required release hydrogen. This technology has multiple advantages such as the ability to modify desorption temperatures, low energy production methods and high gravimetric densities of hydrogen.

DESCRIPTION

Hydrogen is widely regarded as a very useful fuel, when produced from renewable resources, such as biological production or electrolysis of water, which offers near zero emissions on combustion. One of the major hurdles to the exploitation of hydrogen is its storage.

Traditional storage of hydrogen is in the form of liquefied or compressed gas cylinders. These methods require high energy input as well as have associated safety risks due to the high pressure and liquid H₂ boil-off.

The chemical storage of hydrogen using Ammonia as a carrier has been explored in detail due to its particularly suitable chemical and physical properties. For instance, Ammonia has a high gravimetric density of hydrogen (17.6%) and is easily available. This high gravimetric density allows greater storage of hydrogen in small volumes (less than 45 Litres when stored in Mg(NH₃)₆Cl₂ form) compared to 125 litres required in pressurized storage on automobiles to allow a 300 mile range.

This technology utilizes compounds having the general formula M1_x(BH₄)_y(NH₂(R₂))_n wherein M1 comprises one or more of Li, Na, K, Rb, Cs, etc. and in particular LiBH₄(NH₃)_n due to their advantages in storing and producing ammonia and/or hydrogen. These compounds have high gravimetric hydrogen storage densities and also allow release of hydrogen under suitably low temperature and pressure conditions for use in applications like fuel cells. This temperature can be adjusted by choice of M1. Additionally, the compounds have high absorption and desorption kinetics which also allow a reversibility of the reaction.

These compounds can be synthesised via a number of routes; one of which would include the passage of excess gaseous ammonia over dry M1_x(BH₄)_y in an inert environment at room temperature implying low cost of production. However, the variations of the process may be carried out in a wide temperature range of 0^oC to 400^oC, depending on the starting composition of the starting compound and the metal hydrides used. In conclusion these compounds may be made cheaply from readily available materials using low energy preparation methods and are resistant to poisoning by trace impurities.

INNOVATIONS

• The technology allows for release of hydrogen at temperatures as low as -20^oC
• Reversible absorption and desorption of hydrogen and/ or ammonia with high kinetics.

ADVANTAGES

• High gravimetric density of hydrogen.
• Easy and safe storage of large quantities of hydrogen especially when compared to cryogenic or pressurised options.
• Large temperature range for desorption of hydrogen (-20^oC to 400^oC).
• Low energy requirements for production of the storage compounds.
• Easy control of hydrogen desorption through choice of metal hydride in compound.
• Starting materials are cheap and easily available and immune to poisoning by trace amounts.

DOMAIN OF APPLICATION

• Automotive Fuel