Self-foaming sintering-resistant iron-tungsten powders enable high-cycle thermochemical hydrogen storage

H₂-H₂O redox cycling of iron powder beds at 650-800 °C offers a compact, safe, economical hydrogen storage method, but sintering-induced capacity loss has stalled its scalability for decades. Here, we show that adding redox-active tungsten to Fe powders solves this problem in static powder beds: Fe-19W (at%) alloyed powder self-foams during redox cycling via W gas-phase transport, increasing porosity and preserving capacity. In a custom automated reactor, a kilogram-scale powder bed reversibly stores 43.8 g H₂ and sustains 93 ± 3% capacity utilization over 30 redox cycles. Temperature-resolved in-situ X-ray diffraction reveals a chemical-vapor-transport-mediated self-foaming mechanism that redistributes W to refine the microstructure, complemented by a contact-barrier stabilization mechanism during high-temperature holds. Partial-capacity cycling up to 90 cycles further confirms sintering resistance under incomplete redox conditions. These results establish Fe-W powder beds as a robust, scalable, and compact platform for safe, stationary hydrogen storage.