In operando tomography reveals degradation mechanisms in lamellar iron foams during redox cycling at 800 °C

• Published in: Journal of power sources Vol. 448; p. 227463
• Main Authors: Wilke, Stephen K., Dunand, David C.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.02.2020
• Subjects: In operando tomography; Iron-air battery; Kirkendall effect; Metal foam; Redox cycling; Kirkendall effect; In operando tomography; Redox cycling; Iron-air battery; Metal foam
• ISSN: 0378-7753; 1873-2755
• DOI: 10.1016/j.jpowsour.2019.227463

The lifetime of iron-based active materials in high-temperature redox applications (e.g., solid-oxide iron-air batteries, chemical looping) is limited by sintering, accelerated by oxidation/reduction volume changes. Here, in operando X-ray microtomography is used to reveal the evolution of structure, porosity, and extent of reaction in freeze-cast, lamellar iron foams during five redox cycles between Fe and FeO/Fe3O4 (via H2 and H2O) at 800 °C. Foam porosity decreases during oxidations but only partially recovers during reductions, with a net decrease from 77 to 52%. A gas-blocking surface layer, or shell, forms around the foam exterior. By correlating SEM microanalysis with in operando data, the Kirkendall effect is identified as the primary mechanism underlying these degradations. These insights suggest that material lifetime can be improved by measures that prevent the Kirkendall effect, such as combining iron with elements (in solid solution or as a second, redox-inactive phase) that lessen the imbalance of diffusional fluxes. [Display omitted] •Iron foams are redox cycled at 800 °C to simulate a solid-oxide iron-air battery.•Foam structural evolution is investigated with in operando X-ray tomography.•In operando method reveals novel mechanistic insights on material degradation.•Microporosity formed via the Kirkendall effect is a major degradation mechanism.