Evolution of lamellar architecture and microstructure during redox cycling of Fe-Co and Fe-Cu foams

The effects of alloying Fe with 25 at% Co or 30 at% Cu are studied in freeze-cast lamellar foams subjected to redox cycling under H₂- and H₂O-rich atmospheres at 800 ºC, relevant to metal-air batteries. In unalloyed Fe foams, redox cycling causes irreversible Kirkendall porosity growth within lamellae, leading to fracture and buckling in the lamellar architecture which in turn leads to foam densification after a few cycles. By contrast, Fe-25Co lamellae develop, upon oxidation, a pure Co core and a Fe₃O₄ shell which decreases buckling and Kirkendall pore growth, thus slowing sintering and densification of the lamellar foams. After Fe₃O₄ reduction, the Fe-rich shells and Co-rich cores of the lamellae re-homogenize by diffusion to the original single-phase Fe-25Co alloy, achieving a reversible microstructure upon a full redox cycle. After 10 cycles, average channel porosity (between lamellae) undergoes only a small decrease (from 62 % to 46 %), with minimal Kirkendall pore coarsening in the lamellae, consistent with strong sintering resistance in the Fe-25Co foams. The Fe-30Cu foams also display lamellae with Cu core and a Fe₃O₄ shell structure after oxidation, since Cu, like Co, does not oxidize under steam. However, the lack of solubility of Cu in Fe prevents re-homogenization after Fe₃O₄ reduction, so the resulting Cu-core / Fe-shell lamellae undergo severe sintering and densification upon subsequent redox cycling, with channel porosity reducing from 61 % to 9 % after just 5 cycles. For both systems, operando x-ray diffraction during redox cycling reveals that Cu, unlike Co, doubles the Fe oxidation rate, as compared to pure Fe foams.