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

Abstract

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.