From Packed Beds to Porous Iron Structures: Improving Cycle Stability in Indirect Hydrogen Storage via the Steam–Iron Redox Cycle

The steam–iron redox cycle is a long-established yet currently reemerging concept for indirect hydrogen storage, whose practical deployment is limited by the degradation of iron and iron oxide particles and structures under repeated cycling. This work compares packed beds of micrometer-sized iron particles with macroporous iron structures manufactured via a replica process and links redox conditions and reaction pathways to structural evolution and cycle stability. Thermodynamic considerations of the Fe–O–H system, cycling experiments in a fixed-bed reactor, thermogravimetric analysis, and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy of surfaces and cross-sections were used to resolve the transition from direct oxidation to a two-step route via wüstite (FeO). For particle beds, two-step oxidation with steam increases the hydrogen yield but drives vacancy formation, pore coarsening, and agglomeration. Even when reduction conditions are optimized, the oxidation degree and hydrogen yield drop strongly within only a few cycles when full cycling between iron and magnetite (Fe ↔ Fe3O4) is targeted. Porous iron structures partially alleviate these limitations. They operate at oxidation degrees below full conversion that remain stable over several cycles and, in a literature context, approach the cyclic stability of the best pure iron systems while delivering roughly twice the usable hydrogen per cycle, purely by structural design. Previous studies show that additive-modified iron systems can enhance cycle stability; combining such modifications with porous architectures represents a promising route for future improvement of the stability–yield trade-off.