Hydrogen, the most abundant element in
the universe, has great potential as an energy source. Unlike petroleum,
it can be easily generated from renewable energy sources. It is also
nonpolluting, and forms water as a harmless byproduct during use. Yet it
is so difficult to store that its use as a fuel has been limited.
A gram of hydrogen gas occupies about 11 liters (2.9 gallons) of space
at atmospheric pressure, so for convenience the gas must be intensely
pressurized to several hundred atmospheres and stored in a pressure
vessel. In liquid form, hydrogen can only be stored under
cryogenic temperatures. These options are not practical for everyday
The solution to these difficulties is
storage of hydrogen in hydride form. This method uses an alloy that can absorb and hold large amounts of hydrogen by bonding with hydrogen and forming hydrides. A hydrogen storage alloy is capable of absorbing and releasing hydrogen without compromising its own structure. [1,7]
Some alloys (in boldface in the table below) store hydrogen at a
higher density than pure hydrogen. 
cm3 (x 1022)||% of weight that is hydrogen|
|H2 gas, 200 bar (2850
|H2 liquid, 20 K (-253
|H2 solid, 4.2 K (-269 C)
Qualities that make these alloys useful include their
ability to absorb
and release large amounts of hydrogen gas many times without
deteriorating, and their selectivity (they absorb only hydrogen). In
addition, their absorption and release rates can be
adjusting temperature or pressure. 
The hydrogen storage alloys in common use occur in four different
forms: AB5 (e.g.,
LaNi5), AB (e.g., FeTi),
(e.g., Mg2Ni) and AB2
(e.g., ZrV2). 
Research is being conducted to modify the compositions
of these base
alloys by alloying with various other elements. Such modifications can
enhance their stability during cycles of charging and discharging, allow
them to undergo cycles at ambient pressure and temperature, increase
their hydrogen storage capacity, and increase their hydrogen
absorption/desorption rate. 
Other research is exploring ways to synthesize alloys. Currently mechanical alloying is the process of choice,
overcoming difficulties associated with arc-melting. This powder
metallurgy method causes alloying to take
place between powder metals
during their milling (pulverizing). Mechanical alloying enables the
formation of crystalline, amorphous, or nanostructured materials, allows a variety of
elements to be added for easy alloying, and helps to prepare the alloy
surface for the reactions it will undergo. [7,8]
Batteries are the most common application for hydrogen storage alloys.
These hydride-forming alloys are the M in Ni-MH (nickel-metal hydride)
batteries, the negative electrode in the battery cell. Once a negative
electrode is fabricated, it must be activated, or charged, with
hydrogen. Then, during the battery's lifetime, it proceeds through many
The electrochemistry of the negative electrode can be represented
xH2O + xe-
|Charging -> ||
where x represents the number of molecules.
This hydrogen cycling, repeatedly forming and
breaking bonds, may have detrimental effects on the alloy, causing a type
of degradation called decrepitation which weakens the alloy's solid
structure and breaks it down, eventually ending the battery's life. [5,10]
Metal hydride batteries are similar to traditional nickel-cadmium cells, but use a hydrogen
storage alloy instead of a cadmium-based electrode. They are able to last
over 500 cycles at 1C charge and over 700 cycles at 0.2C charge (the rated
cell capacity, C, is the discharge rate that
fully depletes the cell in
five hours). Ni-MH batteries have up to a 40% higher electrical capacity
than Ni-Cd and are not associated with environmental toxicity concerns, as
is the toxic heavy metal, cadmium. Replacement of Ni-Cd batteries with
Ni-MH normally does not involve major design changes. [2,9]
Ni-MH batteries are typically used in portable computers
electronics, cell phones and power tools. They are also being used in the
newly emerging hybrid
vehicles. These batteries both supply energy to and draw energy from
the gas-powered motor, eliminating the need for a separate recharging.
Hydrogen storage alloys are used for other applications besides
batteries. Naturally, since these alloys have higher storage densities
than pure hydrogen, they can be used to store hydrogen in vessels. Also,
because they absorb the three isotopes of
hydrogen (protium, deuterium and tritium) at different rates, they can be
used to purify hydrogen.
Another property of hydrogen storage alloys is that they release heat
when absorbing hydrogen and absorb heat when releasing hydrogen. This
property allows their use in heat pumps, heaters and air conditioners. In
a heat pump, two hydride beds containing the same type of alloy are
connected by a compressor. The compressor
drives the hydrogen from one bed to the second bed, causing cooling at the
first bed and heating at the second bed. These alloys can also be used to
power compressors. Compressors make use of the reverse activity that heat
pumps do: changing the alloy's temperature creates a change in pressure as
it absorbs/releases hydrogen, creating enough force to do work.
This is also the theory behind temperature
utilize the force generated by a release in pressure when the temperature
increases. When the hydrogen storage alloys in these mechanisms are
activated by a temperature increase, they generate enough force to open or
close valves, as demonstrated by a fire sprinkler that turns water on when
it detects a fire, and off again as it cools. [3,6]
This new source of energy, hydrogen, is being applied to items in
common use: batteries in cell phones, power tools, portable electronics,
and even automobiles. As hydrogen storage alloys and their batteries
improve in quality, we can expect to find them in more applications, and
to see hydrogen gain acceptance as a mainstream energy source.
- Technical University of Denmark (http://www.materiale.kemi.dtu.dk/hydrid).
- Energizer (http://data.energizer.com/batteryinfo/application_manuals/nickel_metal_hydride.htm).
- Ergenics, Inc. (http://www.hydrides.com/).
- Westinghouse Savannah River Company (http://www.srs.gov/general/sci-tech/technologies/hydrogen/dsmh.html).
- Japan Institute of Metals (http://www.sendai.kopas.co.jp/METAL/PUBS/thesis_j/j_abst/63-05/601-604.html).
- JMC (USA), Inc. (http://www.jmcusa.com/mh1.html).
- Ricardo B. Schwartz, Los Alamos National Laboratory (http://www.education.lanl.gov/resources/h2/schwarz/education.html).
- Ricardo B. Schwartz, Los Alamos National Laboratory (http://itri.loyola.edu/nano/US.Review/04_06.htm).
- Suppo Battery Company (http://www.suppo.com/frproduct.htm).
- Materials for rechargeable batteries and clean hydrogen energy sources (International Materials Reviews, vol. 46, no. 1, pp.50, 2001)
- For more information, visit (href=http://www.howstuffworks.com/hybrid-car.htm).