Microstructural evolution of lamellar freeze-cast Fe-Cu-Ni foams during oxidation/reduction cycling

Abstract

A novel iron-based alloy, Fe-15Cu-10Ni (wt%), has been introduced as a candidate for storage energy of iron air batteries. Cyclical experiments involving steam oxidation and hydrogen reduction were conducted on the Fe-15Cu-10Ni (wt%), lamellar foams fabricated by freeze-casting method, and were compared with the Fe-25Cu and Fe-25Ni alloys with the same foam structure. The experiments were carried out at a high temperature of 800 °C, which is relevant to the energy storage in iron-air batteries. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction techniques are employed to analyze the microstructure and chemical evolution of the cycled foams. Upon oxidation, Fe-15Cu-10Ni lamellae form a core rich in Ni and Cu (since Cu and Ni do not oxidize under steam), surrounded by an Fe3O4 shell. This reduces fractures and slows down the growth of Kirkendall pores, thereby delaying the sintering and densification of the foam structure. Upon reducing Fe3O4, the Fe-rich shells and the Cu-rich and Ni-rich cores homogenize, creating a reversible microstructure after a complete redox cycle. The Fe-15Cu-10Ni composition presents notable advantages compared to Fe-25Ni, exhibiting a higher oxidation rate, increased energy density, and lower cost. As a result, Fe-15Cu-10Ni emerges as the optimal composition within the scope of this study, showcasing superior properties.