Redox Cycling of Metallic Foams for Iron-Air Batteries: Preventing Degradation and Promoting Self-Healing
Solid-oxide iron-air battery and chemical-looping combustion both rely on cyclical oxidation/reduction between metallic and oxidized iron at elevated temperature. However, the lifetime of current Fe-based redox materials is limited by microstructural degradation arising from (i) sintering, accelerated by phase transformation volume changes and stresses, and (ii) microstructural damage, such as Kirkendall porosity and cracking. We performed X-ray microtomography during redox cycling of Fe foams created by freeze casting to study the evolution of the foam structure, and we combine these operando observations with post-cycling microanalysis and a finite-element model.
Two approaches are studied to improve redox stability of Fe foams. First, addition of fine oxide particles (ZrO2, CeO2) as sintering inhibitor creates Fe-composite foams which better resist densification during redox cycles. Second, addition of nickel results in Fe-Ni foams which, upon oxidation, exhibit lamellae with Ni-rich metallic cores and Fe3O4 surface shells. The ductile core provides structural support and prevents delamination at the metal/oxide interfaces; upon reduction, newly formed Fe at the metal/oxide interface diffuses back into the Ni-rich core, reversing the outward flux from oxidation and re-homogenizing the alloy. The resulting redox-reversible, self-healing microstructure enables Fe-Ni foams to maintain, much longer than pure Fe foams, open interlamellar channels (needed for gas in/outlet) and minimal microporosity within the lamellae (responsible for damage).