Why gold-palladium alloys are better than palladium for hydrogen storage

phys.org | 7/10/2018 | Staff
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Materials that absorb hydrogen are used for hydrogen storage and purification, thus serving as clean energy carriers. The best-known hydrogen absorber, palladium, can be improved by alloying it with gold.

New research led by The University of Tokyo Institute of Industrial Science explains for the first time how gold makes such a difference, which will be valuable for fine-tuning further improvements.

Step - Hydrogen - Storage - Chemisorption - H2

The first step in hydrogen storage is chemisorption, wherein gaseous H2 collides with palladium and adsorbs (sticks) to the surface. Secondly, the chemisorbed H atoms diffuse into the sub-surface, several nanometers deep. A recent article published in Proceedings of the National Academy of Sciences (PNAS) reports that the group focused on this slow second step, which is the bottleneck to the overall process.

In pure palladium, only around 1 in 1,000 of the H2 molecules that collide with the metal actually absorb into the interior. Hence, only these can be stored as energy carriers. However, when the palladium surface is alloyed with gold, absorption is over 40 times faster.

Amount - Gold - Right—hydrogen - Absorption - Number

It is vital to get the amount of gold just right—hydrogen absorption is maximized when the number of gold atoms is slightly less than half (0.4) of a single monolayer of palladium, according to the study. This was discovered by thermal desorption spectroscopy, and by depth-measurement of the H atoms using gamma-ray emissions.

"We wanted to know what role gold plays," study first author Kazuhiro Namba says. "The gold atoms are mostly at the alloy surface. However, our results showed that hydrogen storage is improved even below this depth, in pure palladium. Therefore,...
(Excerpt) Read more at: phys.org
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