Reversible H₂ Storage via High-Temperature Redox Cycling of Fe-Mo Lamellar Foams with High Structural Stability

Hydrogen storage using Fe-based redox materials represents a potentially inexpensive, environmentally friendly alternative to high pressure gas storage: hydrogen is stored through the reduction of Fe₃O₄, yielding steam and Fe, and hydrogen is formed via the reversible steam oxidation of Fe. At high temperatures (400-600°C) where the reaction kinetics are favorable, the large volumetric expansion and contraction of the Fe - Fe₃O₄ redox reaction (110%) drives sintering in Fe powder-bed systems, reducing and eventually choking gas access, and thus storage capacity. Unlike a packed bed of Fe powders, lamellar Fe foams, created by freeze-casting, provide free volume for the Fe lamellae to expand during oxidation without sintering to nearby material. However, upon oxidation, Kirkendall porosity develops within the lamellae due to the mismatch in diffusion between Fe and O. As Kirkendall pores coalesce inside lamellae, combined with fracture at lamellae tips from redox stresses, lamellar contact and densification persists. In the past, alloying with redox-inert elements (Ni, Co), has shown promise in stabilizing the Fe lamellae: as Fe is oxidized, a metallic core (Ni- or Co-rich) remains in each lamella, providing an interface that acts as a Kirkendall pore sink, and adhesion to the outer oxide layer to prevent fracture. Upon reduction, the lamellae re-homogenize due to extensive solid solution between Fe and Co/Ni, providing reversibility. Under accelerated cycling degradation studies at 800°C, repeated expansion and contraction of Fe-Ni and Fe-Co however triggers lamellar buckling, which leads to contacting and sintering of neighboring lamellae, and thus choking of the foams.<br/>Here, we develop a strategy to counteract lamellar buckling during redox cycling, by using Mo as an alloying element for Fe foams, with the goal of improving lamellar mechanical strength and preventing their sintering. Freeze-cast Fe-25 at% Mo lamellar foams demonstrate strong sintering inhibition with a stable hierarchical porous structure, having porosity within each lamella, providing further volume for the Fe to expand into upon oxidation. As opposed to the solid solution Fe-Ni and Fe-Co foams studied before, a three-phase mixture of αFe-Mo, µFeMo, and Fe₃Mo₃C is present, further limiting diffusion, sintering and plastic deformation. Mo, like Fe, is oxidized by steam, leading to a three-phase composition of Fe₃O₄-MoO₂-Fe₂Mo₃O₈ at full oxidation. While the expansion from oxidation leads to the densification of the initially porous lamellae, the shrinkage upon reduction restores the internal lamellar porosity, achieving a reversibly porous lamella. Hydrogen reduction yields α-Fe(Mo) and a nanocrystalline Fe₂Mo phase, which produces a high surface area morphology, further increasing the oxidation rate for subsequent cycles. Buckling from metallic regions present during partial oxidation or reduction proceeds significantly slower, due to the lack of a percolating metallic network. Fracture of ceramic oxides does not cause significant lamellar widening, due to internal porosity, leading to a near constant foam density, without sintering, over the first 10 cycles, as compared to a loss of up to 40% porosity in Fe and 20% in Fe-Ni/Co foams. While lamellar buckling builds up over time in the new Fe-Mo foams, large scale densification is eliminated, since contacting lamellae sinter very slowly. By modifying the lamellar width and length, the extent of buckling can be lowered, further slowing densification. Thus, Mo alloying represents a significant improvement in structural stabilization and reversibility as compared to other Fe-foams.