Tungsten Strongly Inhibits Sintering of Porous Iron During High-Temperature Redox Cycling

Area of Science:

• Materials Science
• Chemical Engineering
• Solid State Chemistry

Background:

• Iron-based materials are candidates for thermochemical energy storage and chemical looping applications.
• Redox cycling in steam-hydrogen environments can lead to material degradation, impacting long-term performance.
• Tungsten addition and specific microstructures are explored to enhance material stability.

Purpose of the Study:

• To investigate the stability and performance of freeze-cast Fe-25 at% W lamellar foams during prolonged steam-hydrogen redox cycling.
• To understand the role of tungsten and the freeze-cast architecture in preventing degradation.
• To elucidate the underlying mechanisms responsible for the observed stability and reaction kinetics.

Main Methods:

• Fabrication of freeze-cast Fe-25 at% W lamellar foams.
• Steam-hydrogen redox cycling at 800 °C for up to 100 cycles.
• Microstructural characterization using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS).
• In situ X-ray Diffraction (XRD) to monitor phase evolution during cycling.
• Comparison with tapped Fe-25 at% W powder beds.

Main Results:

• Freeze-cast Fe-25 at% W foams exhibit excellent resistance to degradation over 100 redox cycles at 800 °C.
• Fast reaction kinetics and full iron utilization are maintained throughout the cycling.
• Tungsten addition inhibits sintering, contributing to material stability.
• A hierarchical porosity (macroscopic channels, micro-scale pores, submicron pores) is formed, influenced by the freeze-cast architecture and chemical vapor transport (CVT).
• Comparison with powder beds indicates that macroscopic freeze-cast features are not critical for performance.

Conclusions:

• Freeze-cast Fe-25 at% W lamellar foams offer exceptional stability for redox applications.
• The combination of tungsten's sintering inhibition and the inherent porosity is key to performance.
• The freeze-cast architecture, while creating hierarchical porosity, is less critical than the material composition and sintering resistance for achieving high cycle stability.