Hydrogen is the lightest and most abundant element in the universe. It forms the basis of all organic compounds, living things and is present in the essence of life - water.
There are three main ways that hydrogen can be stored - as a compressed gas, in liquid form, in metal hydrides and in carbon nanotubes.
Compressed Hydrogen Gas
Hydrogen gas can be compressed and stored in storage tanks that can hold it at the required pressure. The tanks can either be made with steel, aluminium or copper alloys that may be encased in fibreglass The steel tanks are most often used for static applications where weight is not a hindrance as the steel tanks tend to be heavy. High pressure tanks also appear in some test automobiles. but safety and space remain significant concerns. The tanks tend to be big and bulky with the ongoing concern regarding a leak.
Hydrogen can be stored as a liquid (LH2) at 21.2 K (-251.95° C) at ambient pressure in cryogenic tanks. Liquid hydrogen has long being the fuel of choice for rocket applications. Once liquified it can be maintained as a liquid in cooled and pressurised containers which have to be quite large since (LH2) has a very low desnity. The cooling and compressing process requires energy, resulting in a net loss of about 30% of the energy stored in the liquid hydrogen. Automotive giant BMW has been researching the use of liquid hydrogen as a fuel for automobiles. They are currently working with test models that store hydrogen as a liquid in super-cooled tanks.
Hydrogen stored in the form of Ammonia
Ammonia (NH3) is the second most commonly produced chemical in the world used in the production of fertilizers and sports a comprehensive infrastructure for making, transporting and distributing the substance. It can also be used to store chemically store hydrogen with the ability to release it in a catalytic reformer. Ammonia also provides very high hydrogen storage densities and can be stored either as a liquid or in solid form.
At high temperatures and under the right pressure, hydrogen reacts with many transition metals and their alloys to form hydrides. A Hydride is a compound that contains hydrogen and one or more other elements. It is stable compared to native hydrogen and can be stored, transported or used in applications. A second reaction to release the hydrogen is required when the fuel is used in say a fuel cell. The metal hydride storage system for hydrogen is considered safe in catastrophic events, like a collision, because a loss of pressure in the storage tank cools down the metal hydride and stops the release of hydrogen.
Hydrogen storage in carbon nanotubes
Carbon nanotubes are (microscopic) cylindrical carbon molecules that can be used in a wide range of applications. One such application is the ability to store hydrogen within the microscopic tube structures. The US Department of Energy has established a standard whereby carbon materials need to have a storage capacity of 6.5% of their own body weight if they are to be used for transportation use. Research continues on the use of carbon nanotubes for hydrogen storage. One of the drawbacks with nanotubes though is their extremely high cost. Two pounds of the material can sell for approximately $50,000.
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