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Hydrogen Storage
(Released June 2001)

 
  by Laura Becker  

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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 use.

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. [1]

MaterialH-atoms per cm3 (x 1022)% of weight that is hydrogen
H2 gas, 200 bar (2850 psi)0.99100
H2 liquid, 20 K (-253 C)4.2100
H2 solid, 4.2 K (-269 C) 5.3100
MgH2 6.57.6
Mg2NiH4 5.93.6
FeTiH2 6.01.89
LaNi5H6 5.51.37

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 controlled by adjusting temperature or pressure. [4]

The hydrogen storage alloys in common use occur in four different forms: AB5 (e.g., LaNi5), AB (e.g., FeTi), A2B (e.g., Mg2Ni) and AB2 (e.g., ZrV2). [8]

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. [7]

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 hydriding/dehydriding cycles.

The electrochemistry of the negative electrode can be represented as:

Alloy + xH2O + xe- <-->Alloy(Hx)+ xOH
Charging -> <- Discharging

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 and in 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 sensor-actuators, which 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.

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