Hierarchical Structural Changes during Redox Cycling of Fe-Based Lamellar Foams containing YSZ, CeO 2 , or ZrO 2

• Published in: ACS applied materials & interfaces Vol. 12; no. 24; pp. 27190 - 27201
• Main Authors: Wilke, Stephen K, Lundberg, Robert A, Dunand, David C
• Format: Journal Article
• Language: English
• Published: United States 17.06.2020
• ISSN: 1944-8244; 1944-8252
• DOI: 10.1021/acsami.0c05107

Several high-temperature energy conversion and storage technologies rely on redox cycling of Fe-based materials, including storage materials in solid-oxide Fe-air batteries and oxygen carriers in chemical looping combustion. The materials' macroporosity necessary for gas flow is, however, irreversibly diminished during redox cycling due to (i) large volume changes during the redox transformations, (ii) foam sintering at elevated operating temperature (550-900 °C), and (iii) formation and growth of Kirkendall microporosity. To address these challenges, we use directional freeze casting to create highly porous, lamellar, Fe-composite foams containing uniformly distributed sintering inhibitor (SI) particles - either Y O -stabilized ZrO (YSZ), CeO , or ZrO - at 0, 5, 10, or 15 % of the solid volume. We characterize these foams before, during, and after redox cycling (Fe/FeO/Fe O , via H O and H ) at 800 °C using in operando synchrotron X-ray microtomography, metallography, and scanning electron microscopy. Foam densification and formation of a gas-blocking shell around the foam exterior are reduced as the SI fraction increases. Volumetric shrinkage after the first 5 redox cycles is decreased from 66% (for pure-Fe foams) to 45% (for all Fe-composites containing 5 vol. % SI). Foams containing 15 vol. % YSZ show no volumetric shrinkage after 5 cycles, although, after 20 cycles, they have shrunk 53%. Post-cycling analysis reveals segregation of the SI particles to the cores of individual lamellae, surrounded by thick layers of sintered Fe on the lamellae surfaces. This segregation occurs due to Fe diffusion through FeO to the lamellae surfaces during oxidation, leaving behind the SI particles, which are then pushed into clusters by FeO/Fe O contraction during reduction. The SI is thus rendered ineffective, which explains why foam densification is delayed (compared with pure-Fe foams), rather than fully prevented, after repeated cycling.