Iron reacts with hydrogen. Heat iron in hydrogen and it absorbs the gas, forming iron hydride. Heat the hydride in the absence of hydrogen and it releases the gas. This reversible reaction stores hydrogen at high density — 42 grams per kilogram of iron — without the pressure vessels or cryogenics required for gas or liquid storage. The temperature window (650–800°C) is accessible with concentrated solar or waste industrial heat. The chemistry works. The engineering doesn't, because of sintering.
Each absorption-desorption cycle involves a volume change as the crystal structure accommodates and releases hydrogen. At operating temperatures, adjacent iron particles fuse at their contact points — sintering — reducing the porosity that allows hydrogen to penetrate the powder bed. After tens of cycles, the bed densifies into a solid mass with poor gas access. Capacity drops. The fundamental chemistry is unchanged; the geometry defeats it.
Qi and Dunand (arXiv 2602.22807, February 2026) solve the problem by alloying iron with 25 atomic percent tungsten. The tungsten undergoes gas-phase transport during the redox cycling: at high temperature, tungsten forms volatile oxide species that migrate through the powder bed and re-deposit on particle surfaces. This chemical vapor transport increases porosity — the material foams itself. The foaming counteracts the sintering. Each cycle that would densify a pure iron bed instead maintains or increases the porosity in the iron-tungsten bed.
The mechanism is self-correcting. More sintering produces more contact area between particles, which accelerates the gas-phase tungsten transport that breaks those contacts apart. The two processes — sintering that densifies and foaming that opens — reach a dynamic balance that stabilizes the powder bed structure across many cycles.
A kilogram-scale demonstration maintained 96% capacity utilization over 30 full redox cycles, and survived 90 partial-capacity cycles without significant degradation. The tungsten doesn't participate in the hydrogen chemistry — it's a structural additive that maintains the architecture while iron does the chemical work.
The material repairs itself every time it breaks. The degradation mechanism is its own antidote.