How do structural changes in electrodes during electrochemical cycling correlate to pH?
Shu Fen Tan, Frances M. Ross, and their colleagues at MIT, along with their collaborators at Form Energy, have used the Hummingbird Scientific Gen V liquid flow TEM sample holder to observe microstructural changes in Fe electrodes during potential cycling in alkaline environments. They identified the formation of multiple Fe species and a passivation layer that developed over repeated cycling, which was confirmed through in situ imaging and selected area electron diffraction.
a) In situ cyclic voltammetry (CV) measurements of Fe electrode at pH = 10 in 0.1 mM KOH electrolyte scanning from 0 V to −0.9 V to +0.9 V at 10 mV s−1 b) TEM time sequence of electrochemical scan. The top row, outlined in blue, shows reduction, and the bottom row outlined in black, shows oxidation. Red arrows indicate the formation of a thin layer upon switching from reduction to oxidation. c) TEM image showing newly-deposited Fe species after CV cycle shown in (a). d) Overview TEM image showing the newly-deposited Fe species after potential cycling. Dotted red lines show a blocking layer formed across the electrodes. Copyright © 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH
By comparing identical-location imaging before and after cycling, they demonstrated how electrolyte pH influences the deposition, dissolution, and transformation of Fe phases at the nanoscale. At lower pH (~10), Fe tends to form passivating layers like Fe(OH)₂, Fe₃O₄, and Fe₂O₃, which reduce electrochemical activity, while at higher pH (~12.7–13), Fe shows more reversible redox behavior, but risks entering a corrosion regime. This understanding of the mechanisms that govern the reactivity, stability, and transformation of Fe electrodes under repeated electrochemical cycling is important to the optimization of Fe as an electrode material, and is enabled by the bulk reference and counter electrodes of the Generation V system.
Shu Fen Tan, Hanglong Wu, Joseph S. Manser, Duhan Zhang, Maria Ronchi, Sylvia Smullin, Yet-Ming Chiang, Frances M. Ross. Adv. Funct. Mater. 2025, 35, 2407561. DOI: 10.1002/adfm.202407561
Full paper Copyright © 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH
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