Hydrogen is the lightest gas, but it has a knack for sneaking into metals. If you pump hydrogen into a steel cylinder at high pressure, some atoms will squeeze between the metal atoms. The metal can become as brittle as a stale cracker. But you can also turn this property on its head — store hydrogen inside a metal sponge.
In a new study, researchers took two alloys made of aluminum, cobalt, chromium, iron, and nickel. The first alloy had little aluminum—0.3 parts per formula unit. The second had ten times more — three parts. Both alloys were compressed in a hydrogen atmosphere. The first started absorbing hydrogen at a pressure of 3 gigapascals (that's like 30,000 atmospheres). The second didn't absorb a drop even at 50 gigapascals and with heating.
Calculations showed that aluminum makes hydrogen "parking" energetically unfavorable. Hydrogen has to pay more energy to take a spot in the lattice. So even under enormous pressure, it stays outside. Interestingly, the key role is played not only by the amount of aluminum but also by how it orders the atoms in the alloy.
So, by adding aluminum, you can control how much hydrogen the alloy will absorb. This could be useful for safe hydrogen storage and for protecting parts from hydrogen embrittlement.
The pressure of 3 gigapascals at which the first alloy began absorbing hydrogen is three times greater than the pressure at the bottom of the Mariana Trench.