Superstoichiometric Alloying of H and Close‐Packed Fe‐Ni Metal Under High Pressures: Implications for Hydrogen Storage in Planetary Core

Although high pressure enables alloying between hydrogen and iron, hydrogen‐to‐iron molar ratio (H/Fe) so far found in experiments is mostly limited to 1 in the close‐packed iron metal under high pressure. We report a H/(Fe + Ni) ratio of 1.8 ± 0.1 from (Fe,Ni)Hx (or x ≥ 1.8) quenched from liquid, exceeding the amounts so far reported for densely packed Fe alloys. From the metastable behavior of the frozen (Fe,Ni)Hx liquid during decompression, we infer that the amount is a lower bound and therefore even a greater amount of H can be dissolved in the liquid part of Fe‐rich cores of planets. The significant H storage capacity of liquid Fe‐Ni alloy is important to consider for potential storage of H in the interiors of low‐density planets as well as rocky planets. Plain Language Summary Our new high‐pressure experiments show that much more hydrogen can dissolve in the iron‐nickel alloy liquid than what is known for solid iron‐nickel alloy at high pressures and high temperatures. These experimental observations open up the possibility for large internal hydrogen storage in the metallic cores for a range of planet types. Key Points Iron‐nickel alloy quenched from liquid at high pressures contains more than 3.1 wt% hydrogen Liquid iron‐nickel alloy can contain at least 80% more H than the solid Hydrogen storage capacity of the liquid part of the core could be the largest among the internal layers of planets