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The Chemistry Behind Metal Hydrides for Fuel Cells

Metal hydrides are chemical compounds formed when hydrogen gas reacts with metals. The most useful metal hydrides react near room temperature at hydrogen pressures a few times greater than the earth's atmosphere (e.g., 5 bar, 73 psia). Metal hydrides are certainly the safest way to store flammable hydrogen gas. If you would like to know more about metal hydrides, continue down this page. If not, just click here to go back to the metal hydrides category page to order yours today!

Typical metal hydrides are powders whose particles are only a few millionths of a meter (microns) across. When these metal powders absorb hydrogen to form hydrides, heat is released. Conversely, when hydrogen is released from a hydride, heat is absorbed. The process is illustrated below.

The upper portion of the illustration shows the 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 to form a hydride.

The lower portion of the illustration shows the desorption process. Hydrogen atoms* (H1) 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 metals bind hydrogen metallically, 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". Anyway, you get the idea.

The Battery Analogy

The absorption and release of hydrogen from a metal hydride is similar to the storage and withdrawal of electricity from a battery. If 14 volts are applied to a 12 volt battery, it will be charged with electricity until it is full. Applying 15 volts will charge it faster, but no more electrical energy will be stored. If an electrical load is applied to the battery, the voltage will fall to 12 volts or less and the stored electricity can be recovered. The faster the energy is withdrawn, the lower the voltage will fall.


Charging and Discharging a 12 Volt, 10 Amp-Hour Battery

If 6 atmospheres of hydrogen pressure is applied to a hydride whose room temperature equilibrium pressure is 5 atmospheres, it will be charged with hydrogen until it is full. Applying 7 atmospheres of hydrogen pressure will charge the hydride faster but not much fuller. If hydrogen is withdrawn from the hydride, the pressure will fall below 5 atmospheres and the stored hydrogen can be recovered. Withdrawing hydrogen faster will cause the pressure to fall further below 5 atmospheres.


Charging and Discharging a 5 Atmosphere, 10 Liter Hydride Container

Pressure-Temperature Sensitivity

The equilibrium voltage of an electric storage battery is relatively stable over a wide range of temperatures. Metal hydride equilibrium pressure is much more sensitive to temperature changes. If a metal hydride has an equilibrium pressure of 5 atmospheres at 20 Celcius, the pressure will fall to half of that (2.5 atmospheres) at about 5 Celcius and rise to about 10 atmospheres at 35 Celcius. The equilibrium pressure of a typical hydride doubles or halves in about 15-20 degrees Celcius of temperature increase or decrease near room temperature.

Metal Hydride Alloy Choices

If you want to see a pressure-composition isotherm and a pressure-temperature Van't Hoff plot for each alloy click here for Alloy A or here for Alloys L, M, H.  Alloy A contains iron and titanium while Alloys L, M and H contain rare earth metals and nickel.  Alloy A is less expensive than Alloys L, M or H.  This makes no difference in prices of smaller containers because assembly labor cost dominates production costs.  Larger containers, cost extra mostly due to the higher alloy costs.

Pressure Regulator

If your application requires low pressure hydrogen you will need a pressure regulator, like the one shown below.

Pressure Regulator, PR-50, sets the outlet pressure from 0-50 psig (0-3.4 bar gauge). Maximum inlet pressure is 300 psig (20 bar gauge). The outlet connection on the regulator has 1/8 NPT female pipe threads.  You can purchase the PR-50 here.

 

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