Microstructural evolution of lamellar Fe-25Ni foams during steam-hydrogen redox cycling

Cyclical steam oxidation and hydrogen reduction, relevant to iron-air batteries, is performed on freeze-cast Fe-25Ni (at.%) foams consisting of colonies of parallel lamellae separated by channels, both ∼20 µm thick and millimeters in length. This structure is designed to accommodate volumetric changes associated with the cyclical oxidation and reduction of Fe. Metallographic imaging performed at various redox stages, together with time-resolved in situ X-ray diffraction during redox cycling, detail the reaction kinetics and phase evolution of the foam, the evolution of its lamellar microstructure, and the eventual degradation of its internal architecture. As Fe preferentially oxidizes over Ni, each lamella develops an outer Fe-oxide scale, with metallic Ni rejected to the cores of the lamellae which develops an interconnected network of Fe-oxide veins. The Ni-rich metallic core limits the accumulation of Kirkendall pores and provides adhesion to the Fe-oxide scale, thus preventing lamellar fracture observed in unalloyed Fe foams. While the oxidation rate is slowed by the presence of Ni, the reduction rate is accelerated, as Ni acts as a catalyst and as the network of oxidized veins reduces quickly and become open microchannels, thereby providing rapid hydrogen access (driving reduction) and steam egress to the lamellar interior. After complete reduction, the Fe-rich shell and the Ni-rich core interdiffuse and homogenize, which helps eliminate both Kirkendall pores and microchannels from the lamellae. Furthermore, the ductile Ni-rich core limits lamellar buckling (another densification mechanism active in Fe foams). Architectural changes also affect resistance to internal damage, with smaller lamellar colonies exhibiting better resistance to buckling. These combined effects provided by Ni alloying allow the Fe-25Ni foams to maintain a high channel porosity (>40% porous), and thus high active surface area, after 10 redox cycles, as compared to a near complete loss of open channel porosity reported in unalloyed Fe foams.