TY - JOUR
T1 - Hierarchical Structural Changes during Redox Cycling of Fe-Based Lamellar Foams Containing YSZ, CeO2, or ZrO2
AU - Wilke, Stephen K.
AU - Lundberg, Robert A.
AU - Dunand, David C.
N1 - Funding Information:
This research was supported by the U.S. National Science Foundation (NSF) via grant CMMI-1562941. S.K.W. also received partial support from the Institute for Sustainability and Energy at Northwestern. Tomography experiments were performed at Sector 2-BM of the Advanced Photon Source (APS), operated by Argonne National Laboratory (DOE DE-AC02-06CH11357). Experiments and characterization made use of several Northwestern University (NU) resources: the Materials Characterization and Imaging Facility, the NUANCE Center (supported by SHyNE under NSF ECCS-1542205, MRSEC under NSF DMR-1720139, the International Institute for Nanotechnology, the Keck Foundation, and the State of Illinois), and the Center for Advanced Molecular Imaging (supported by NCI CCSG P30 CA060553). The authors gratefully acknowledge Prof. Sossina Haile (NU) for her contribution to the discussion of ion-transport pathways. They also thank Dr. Xianghui Xiao (APS), Pavel Shevchenko (APS), Dr. Christoph Kenel (NU), and Jennifer Glerum (NU) for their help in collecting and processing the tomography data, and they gratefully acknowledge Kristen Scotti (NU) for numerous insightful discussions.
Funding Information:
This research was supported by the U.S. National Science Foundation (NSF) via grant CMMI-1562941. S.K.W. also received partial support from the Institute for Sustainability and Energy at Northwestern. Tomography experiments were performed at Sector 2-BM of the Advanced Photon Source (APS), operated by Argonne National Laboratory (DOE DE-AC02-06CH11357). Experiments and characterization made use of several Northwestern University (NU) resources: the Materials Characterization and Imaging Facility, the NUANCE Center (supported by SHyNE under NSF ECCS-1542205, MRSEC under NSF DMR-1720139, the International Institute for Nanotechnology, the Keck Foundation, and the State of Illinois) and the Center for Advanced Molecular Imaging (supported by NCI CCSG P30 CA060553). The authors gratefully acknowledge Prof. Sossina Haile (NU) for her contribution to the discussion of ion-transport pathways. They also thank Dr. Xianghui Xiao (APS), Pavel Shevchenko (APS), Dr. Christoph Kenel (NU), and Jennifer Glerum (NU) for their help in collecting and processing the tomography data, and they gratefully acknowledge Kristen Scotti (NU) for numerous insightful discussions.
Publisher Copyright:
Copyright © 2020 American Chemical Society.
PY - 2020/6/17
Y1 - 2020/6/17
N2 - 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 Y2O3-stabilized ZrO2 (YSZ), CeO2, or ZrO2 - at 0, 5, 10, or 15% of the solid volume. We characterize these foams before, during, and after redox cycling (Fe/FeO/Fe3O4, via H2O and H2) at 800 °C using operando synchrotron X-ray microtomography, metallography, and scanning electron microscopy. Shrinkage of the foam volume and formation of a gas-blocking shell surrounding the foam are reduced as the SI fraction increases. Volumetric shrinkage after the first five 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 five 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/Fe3O4 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.
AB - 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 Y2O3-stabilized ZrO2 (YSZ), CeO2, or ZrO2 - at 0, 5, 10, or 15% of the solid volume. We characterize these foams before, during, and after redox cycling (Fe/FeO/Fe3O4, via H2O and H2) at 800 °C using operando synchrotron X-ray microtomography, metallography, and scanning electron microscopy. Shrinkage of the foam volume and formation of a gas-blocking shell surrounding the foam are reduced as the SI fraction increases. Volumetric shrinkage after the first five 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 five 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/Fe3O4 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.
KW - Kirkendall effect
KW - metal-ceramic composite foams
KW - operando X-ray tomography
KW - redox cycling
KW - sintering inhibitors
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U2 - 10.1021/acsami.0c05107
DO - 10.1021/acsami.0c05107
M3 - Article
C2 - 32434318
AN - SCOPUS:85086681565
SN - 1944-8244
VL - 12
SP - 27190
EP - 27201
JO - ACS Applied Materials and Interfaces
JF - ACS Applied Materials and Interfaces
IS - 24
ER -