Redox Cycling of Fe/FeO Microstructures

Stephen Wilke, Jacob Mack, Pedro Javier Lloreda Juardo, Teakyung Um, *Robert Lundberg, and *Amelia Plunk


Redox cycling of iron/iron oxide has many potential applications for gas-phase reactions and electrochemical devices, including intermediate temperature (550-800°C) iron-air batteries [2,3]. The major limitation to such technologies is the pulverization and sintering of the iron substrate with repeated phase transformation-induced volume changes and consequent mechanical stresses. To address this obstacle, we propose using freeze casting [4] as an economical and scalable route to fabricate highly porous iron scaffolds designed to withstand these cyclical stresses.


The Dunand group has demonstrated successful freeze casting preparation of iron foams with highly elongated pores and bulk porosities greater than 80% [5,6]. By varying the freezing parameters we can tune the pore wall spacing and thickness, as well as the density of connection struts between the pore walls. With the ability to tailor these microstructures and experimentally cycle them through oxidation (to iron oxide) and reduction (again to metallic iron), we seek to optimize the structure for improved longevity in redox cycling. Our characterization techniques include metallography, SEM, and X-ray computed tomography [1]. As part of this study, we are also developing a finite element model to describe the phase change kinetics, volume change, and microstructural evolution in response to the induced stresses.




Fig. 1. Phase diagram of water, showing the process steps of freeze casting [4].

Fig. 2. Freeze-cast iron foams comprising dense, iron lamellae separated by macropores. (a) A cylindrical volume imaged using X-ray microtomography, (b) optical micrograph of cross-section perpendicular to freezing direction, and (c) electron micrograph at higher magnification.

Fig. 3. Iron foam structural evolution during five oxidation/reduction cycles at 800 °C, via H2O and H2, reconstructed from in operando X-ray microtomography. (Reproduced from [1].).

Related Publications

  1. S.K. Wilke, D.C. Dunand, In Operando Tomography Reveals Degradation Mechanisms in Lamellar Iron Foams during Redox Cycling at 800oC, J. Power Sources. 448 (2020) 227463.
  2. C.M. Berger, O. Tokariev, P. Orzessek, A. Hospach, Q. Fang, M. Bram, W.J. Quadakkers, N.H. Menzler, H.P. Buchkremer, Development of storage materials for high-temperature rechargeable oxide batteries, J. Energy Storage. 1 (2015) 54-64. doi:10.1016/j.est.2014.12.001.
  3. Q. Fang, C.M. Berger, N.H. Menzler, M. Bram, L. Blum, Electrochemical characterization of Fe-air rechargeable oxide battery in planar solid oxide cell stacks, J. Power Sources. 336 (2016) 91-98. doi:10.1016/j.jpowsour.2016.10.059.
  4. S. Deville, Freeze-casting of porous ceramics: A review of current achievements and issues, Adv. Eng. Mater. 10 (2008) 155-169. doi:10.1002/adem.200700270.
  5. A.A. Plunk, D.C. Dunand, Iron foams created by directional freeze casting of iron oxide, reduction and sintering, Mater. Lett. 191 (2017) 112-115. doi:10.1016/j.matlet.2016.12.104.
  6. S.K. Wilke, D.C. Dunand, Structural evolution of directionally freeze-cast iron foams during oxidation/reduction cycles, Acta Mater. 162 (2019) 90-102. doi:10.1016/j.actamat.2018.09.054.

Funding support

  1. NSF
  2. ISEN Cluster Fellowship