Electrochemical Reactivity and Stability of the Fe Electrode in Alkaline Electrolyte

Abstract

Fe anodes are emerging as cost‐effective components in long duration storage applications, with a benefit being the high natural abundance of Fe. Additive and electrode design have advanced the performance of Fe electrodes, but a more precise understanding of the electrode formation process and failure mechanisms is important for continued optimization. These interfacial electrochemical processes, which involve short‐lived intermediate species, require analysis with high spatial and temporal resolution to provide a full picture. This study therefore explores the behavior of the Fe electrode in alkaline electrolyte using electrochemical liquid cell transmission electron microscopy, extending the technique toward high pH (10–13) conditions. By combining identical location imaging and diffraction, in situ imaging, and benchtop experiments, this study shows distinct microstructural changes on cycling as a function of pH, in particular the appearance of multiple electrodeposited Fe species and a passivation layer. The dependence on electrochemical parameters is discussed, showing that the observations can be related to stability predictions from the Pourbaix diagram. However, it is also necessary to consider kinetic effects, such as the solubility and diffusion of soluble species. Strategies to control these material transformations are discussed as a function of potential, along with opportunities for further optimizing the Fe electrode.