As the world searches for cleaner sources for energy, hydrogen (H) fuel cell technology stands out as a popular route to a ‘zero-emission’ future, and industry is searching for ways to store hydrogen efficiently on a large scale for commercial use.
Currently, the best-known hydrogen absorber is palladium (Pd), but parts of the process are slow and its capability to store hydrogen is limited. But researchers at the University of Tokyo of Industrial Science, Japan, have now found a way to improve this process by alloying Pd with gold (Au).
During the first step of hydrogen storage – chemisorption – gaseous H2 collides with Pd and sticks to its surface, then diffuses into the sub-surface, which is several nanometres thick. Only one in 1,000 H atoms absorb into the sub-surface in pure Pd, but by alloying Pd with Au absorption of H atoms is 40 times faster.
‘We wanted to know what role Au plays,’ said first author Kazuhiro Namba. ‘The Au atoms are mostly at the alloy surface. However, our results showed that hydrogen storage is improved even below this depth, in pure Pd. Therefore, Au must be accelerating the diffusion of hydrogen into the sub-surface, rather than improving its solubility.’
In order to maximise the efficiency of absorption, the number of Au atoms of should be 40% of a single monolayer of Pd, the team found. The rate at which H atoms are absorbed is determined by the energy barrier of the chemical reaction – Au reduces this barrier by destabilising the chemisorbed hydrogen so that they have more energy, making absorption more successful in Au-Pd alloys.
‘Our study reveals, at the electronic level, how Au alloying controls hydrogen absorption,’ said co-author Shohei Ogura. ‘This will help us to design better hydrogen storage materials, which will play a role in carbon-neutral energy transport, as well as solid catalysts for chemical reactions, which often depend on surface-bound hydrogen.’
By Georgina Hines