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Storage Metal Hydrides

Solid storage of hydrogen is possible with metal hydrides. Metal hydrides are chemical compounds of hydrogen and other material such as magnesium, nickel, copper, iron or titanium. Basically, hydrogen bonds easily with more than 80 metallic compounds, forming a weak attraction that stores hydrogen until heated. These so-called metal hydride systems can either be at low (< 150ēC) or high temperature (300ēC).

Hydrogen can be stored in the form of hydrides at higher densities than by simple compression. However they still store little energy per unit weight. On the other hand, since heat is required to release the hydrogen, this method reduces safety concerns surrounding leakage that can be a problem with compressed hydrogen and LH2. However, as metal hydride material may react spontaneously when exposed to air or water, other specific safety issues need to be addressed.

Metal hydrides begin as intermetallic compounds produced in much the same way as any other metal alloy. They exhibit one important difference. When metal hydrides are exposed to hydrogen at certain pressures and temperatures absorb large quantities of the gas and form metal hydride compounds.(PalcanMetalHydrides)

When molecular hydrogen from the hydrogen gas comes into contact with the surface of a hydrogen storage metal hydride material, it dissociates into atomic hydrogen and distributes compactly throughout the metal lattice. Metal hydrides literally trap hydrogen within the alloy, much like a sponge absorbs water. When heat is applied, the gas is released.

Absorption process

Hydrogen gas molecules (H2) stick to the metal surface and break down into hydrogen atoms (H). The hydrogen atoms* then penetrate into the interior of the metal crystal to form a new solid substance called a "metal hydride". The metal atoms are usually stretched apart to accommodate the hydrogen atoms. The physical arrangement (structure) of the metal atoms may also change as the hydride forms.

Desorption process

Hydrogen atoms* migrate to the surface of the metal hydride, combine into hydrogen molecules (H2) and flow away as hydrogen gas. The metal atoms contract to form the original metal crystal structure.

  • Note: It is not exactly correct to say "hydrogen atoms migrate". A hydrogen atom consists of a proton and an electron. As metallic substances absorb and release hydrogen, protons move among the metal atoms through a "sea of electrons" that include electrons from the metal and from hydrogen. If the proton is not closely associated with any particular electron it is not, strictly speaking, a "hydrogen atom".

Metal hydride compounds are thus formed, allowing for the absorption of hydrogen in the materials, while heat is simultaneously released in the process. Conversely, hydrogen is released (desorbed) when heat is applied to the materials. Hydrides can desorb the hydrogen at roughly the same pressure required for storage. In fact, the key to practical use of metal hydrides is their ability to both absorb and release the same quantity of hydrogen many times without deterioration.

In chemical shorthand, a typical reaction for such reversible metal hydrides can be expressed as shown below where M represents the metal, H2 is hydrogen and MH is the metal hydride.

M + H2 -> MH + Heat Out

M + H2 -> MH + Heat In

This reaction is reversible. By changing conditions, the reaction can be made to go in either the forward or reverse direction. Its direction is determined by the pressure of the hydrogen gas. If the pressure is above a certain level (the equilibrium pressure or “plateau pressure”), the reaction proceeds to the right to form a metal hydride (the metal absorbs hydrogen to form a metal hydride); if below the equilibrium pressure, hydrogen is liberated and the metal returns to its original state. The equilibrium pressure depends upon temperature as it increases with increasing temperature.

The storage in metal hydrides requires an absorption and a desorption step, in which heat must be taken out from the material or fed to the material from the environment. The heat on the right-hand side indicates that heat or energy is released when the metal hydride is formed, and thus, heat must be put in to release hydrogen from the metal hydride phase. The heat is the enthalpy (heat of formation) of the reaction and is an indication of the strength of the metal-hydrogen bond in the metal hydride phase.

The hydrogen absorbing behaviour of metal hydride alloys is characterized using equilibrium pressure-temperature-composition (PTC) data. This data is determined by keeping an alloy sample at constant temperature while precisely measuring the quantity of hydrogen absorbed and the pressure at which sorption occurs. The quantity of hydrogen absorbed is expressed in terms of alloy composition, either as an atomic ratio of hydrogen atoms to the number of atoms in the base metal alloy, or as the capacity of hydrogen in the alloy on a weight percent basis. Hydride alloys can be engineered to operate at different temperatures and pressures by modifying alloy composition and production techniques.

Hydride absorption is accompanied by a heat of formation that is exothermic. In order to continuously absorb hydrogen to an alloy's maximum capacity, heat must be removed from an alloy bed. The rate at which a hydride alloy can absorb or release hydrogen is dependent upon the rate at which heat can be transferred into or out of the alloy. Increasing the heat transfer rate allows the processing of higher flow rates.

To improve the performance of the storage systems based on metal hydrides, researchers must find ways to increase the proportion of hydrogen in the hydrides, whilst maintaining the reversibility of the reaction within a reasonable temperature and pressure range. Many alloys form hydrides with up to 9% Hydrogen but will release the gas only at extreme temperatures. [Nickel Institute]

Today one class of metal hydride material is used in practical applications, the conventional low temperature hydrides. Other classes have been developed or are being developed: the high temperature Magnesium hydrides and the medium temperature Alanates.

Low temperature hydrides release hydrogen at near ambient temperature and pressure (different groups of materials exist, based on Ti, Zr, V, Rare Earth or other compounds). Their reversible hydrogen storage density is usually between 1.5 to 1.8wt%.

The high temperature metal hydrides are mainly based on Mg. These materials need operating temperatures above 230ēC (260-280ēC) to release the hydrogen. They are capable of theoretically storing about 7wt% with about 5-6 wt% being reached at lab scale today.

A new class of metal hydrides, the so called medium temperature materials and particular the Alanates (e.g. NaAlH4, or LiAlH4) are currently being investigated with high expectations. Based on the use of light metals such as Mg and Al storage densities of 4.5 to 5.0 wt% at 130ēC have been shown with the theoretical maximum being 5.5 wt% or 4.5 system wt%. However, reactivity of this medium to air, water or other fluids is a safety concern that remains to be addressed.

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Page last modified on March 01, 2009, at 10:09 PM