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Improving Hydrogen Storage
October 31, 2013   
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Researchers at the Institute of Physical Chemistry of the Polish Academy of Sciences in Warsaw are working to improve methods for the storage of hydrogen, which is widely seen as a versatile fuel of the future.

The researchers have applied a user-friendly electrochemical method to study hydrogen diffusion in highly reactive metals.

Though the potential of hydrogen as a fuel is widely recognized, it is rarely found in a free state. Therefore, it must be first generated (for example, through the electrolysis of water) and then stored before it can finally be used—ideally in fuel cells transforming chemical energy directly into electricity.

However, hydrogen storage is a challenge. The drawbacks of conventional storage tanks for gaseous and liquid hydrogen force researchers to look for other solutions. One promising method for hydrogen storage makes use of the capability of some metals and alloys to easily absorb this element. The development of efficient hydrogen storage systems requires detailed study of how hydrogen diffuses in metals.

Hydrogen permeation through metals can be conveniently studied with electrochemical methods. These methods are ineffective, however, in the case of metals where the diffusion of hydrogen is relatively slow, and also in cases where metals strongly react with aqueous electrolyte solutions. The problem relates in particular to magnesium and magnesium alloys that are considered the most promising materials for hydrogen storage.

“We have managed to overcome this obstacle,” says Prof. Tadeusz Zakroczymski, whose team at the Institute of Physical Chemistry of the Polish Academy of Sciences in Warsaw has for many years been carrying out comprehensive research into hydrogen permeation, diffusion and uptake in metals.

The information on how hydrogen diffuses in metals is usually obtained from electrochemical measurements of the rate of hydrogen permeation through a sample—usually a membrane separating two independent electrolytic cells. On one side, the membrane is charged with hydrogen produced cathodically in an aqueous solution. Hydrogen atoms enter the membrane, diffuse through it and subsequently leave the membrane on the other side. Here, in appropriate conditions, they do not recombine but are immediately electrochemically oxidized to protons.

This electrochemical detection of hydrogen is extremely sensitive. An easy-to-measure current density of one microampere per square centimeter corresponds to a stream of about six trillion single hydrogen atoms per second per square centimeter.

Zakroczymski’s team constructed a membrane that makes it possible to electrochemically insert hydrogen into highly reactive metals and subsequently detect it. The membrane has a multilayer structure. The main layer, which forms the structural basis of the membrane, is made of iron. This metal was selected because hydrogen atoms move exceptionally fast in the iron crystal lattice: their rate of diffusion at room temperature is comparable to that of hydrogen ions in aqueous solutions. Therefore, the iron layer has a relatively small effect on how fast hydrogen permeates through the entire membrane.

Both sides of the iron membrane are coated electrochemically with a thin palladium film. Then they are coated with magnesium and (for protection purposes) again with palladium. Both elements were deposited in cooperation with Prof. Wen-Ta Tsai’s laboratory from the National Cheng Kung University in Tainan, Taiwan.

“The measured rate of hydrogen permeation through a multilayer membrane depends on hydrogen diffusion in each membrane layer,” Zakroczymski says. “Because hydrogen diffusion in iron and palladium has been extensively studied, the diffusion coefficient of hydrogen in the magnesium layer can be deduced if we know the thickness of each layer.”
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