This article is a translated summary of an article in German science magazine Bild der Wissenschaft

Hydrogen is a central pillar of Power-to-X technologies, enabling renewable electricity to be converted into a versatile, storable energy carrier. Yet one of the biggest challenges in the hydrogen economy is how to store it safely, efficiently, and at scale. Two promising approaches – underground salt caverns and iron-based storage – are now showing their potential.

In May 2025, the pilot project H2Cast Etzel in northern Germany successfully stored 90 tonnes of hydrogen in underground salt caverns that had previously held natural gas and oil. This marks an important milestone for the large-scale hydrogen infrastructure needed to supply industries and transport sectors that cannot easily switch to direct electrification, such as steelmaking, glass production, shipping, and aviation. Salt caverns, when connected to an efficient pipeline network, offer a cost-effective way to store large hydrogen volumes close to demand centres.

In parallel, researchers at ETH Zurich are developing a completely different concept: storing hydrogen in iron. Using a reversible redox reaction, hydrogen is chemically bound to iron oxide at high temperatures, forming pure iron and water. When energy is needed, steam is passed over the iron to release the hydrogen again. This method offers high energy density, low raw material costs, and – crucially – safe handling, since the hydrogen is locked inside a stable solid material. The ETH team envisions seasonal storage systems where excess summer solar power produces hydrogen for iron-based storage, which can then supply electricity and heat in winter.

Both storage pathways can become crucial for balancing the fluctuations in wind and solar power production. They help bridge so-called “Dunkelflauten” – periods with little sun and wind – and increase energy security while reducing dependence on fossil imports.

As Power-to-X technologies scale up, integrating such storage solutions will be key to achieving a reliable, renewable-based energy system. Whether in the form of underground caverns or innovative iron storage, safe hydrogen depots are essential for a climate-neutral future.

We’re proud that (not only) one of the technologies has been and gets further developed at ETH Zurich.

Source: Bild der Wissenschaft, August 2025, “Sichere Depots für Wasserstoff”