Fe–Ni foams self-heal during redox cycling via reversible formation/homogenization of a ductile Ni scaffold

Abstract

We investigate the degradation mechanisms during redox cycling of directionally freeze-cast, lamellar Fe foams containing 0, 7, 19, or 25 at% Ni, relevant to applications in solid-oxide Fe–air batteries and chemical looping processes. In pure Fe, the oxidation/reduction cycle causes a net outward Fe mass flux. This imbalance, left untreated, leads to irreversible microstructural changes: growth of internal microporosity in Fe lamellae and formation of a dense, gas-blocking shell at the foam exterior surface. We propose and demonstrate a novel strategy of alloying Fe with Ni to make reversible the oxidation and reduction pathways, creating a self-healing effect for redox cycling by H₂O/H₂ at 800 °C (representative of a solid-oxide Fe–air battery). During oxidation, each Fe–Ni lamella transforms, in situ, into a composite structure with a Ni-rich alloy core and FeO/Fe₃O₄ surfaces. The internal Ni-rich scaffold imparts mechanical stability against oxide fracture and spallation typical of Fe redox materials. During subsequent reduction, the Ni-rich scaffold maintains adhesion to the FeO/Fe₃O₄ surfaces, and Ni catalyzes Fe reduction at the metal/oxide interface. From this interface, Fe diffuses inward to the Ni-rich core, reversing the diffusive flux of the oxidation half-cycle, and Fe–Ni re-homogenization eliminates the microporosity formed within the lamellae during oxidation. Foams are redox-cycled and their microstructural changes are examined by metallography, SEM/EDS, and X-ray diffraction. In pure–Fe foams, the channel porosity necessary for gas flow decreases from 63 to 27 vol% after 5 redox cycles, while the deleterious microporosity increases from 3.6 to 13.7% of the lamellae volume. In contrast, Fe–25Ni foams maintain 54 vol% channel porosity and develop microporosity of only 6.7% of the lamellae volume, with mechanically stable microstructures for at least 20 redox cycles.