Considerations for Aqueous Electrochemistry Towards Sustainable Iron Production: From Dilute to Highly Concentrated Electrolytes
Authors: Sinclair Nicholas Scott , Martin Benjamin , Akolkar Rohan 
Published: December 2024

Iron (Fe) metal is used to make steel, which is a critical component in manufacturing and transportation in the industrialized world. Close to 1.2 billion tons of ‘primary’ Fe is produced globally using carbothermic reduction of iron ore to Fe metal in a blast furnace. This process is a major source of greenhouse gas (GHG) emissions. During carbothermic reduction, about 0.6 kg of CO2 is emitted per kg of Fe, corresponding to 0.7 billion tons of CO2 emitted annually by iron producers around the world. Furthermore, the steelmaking industry is highly energy intensive, accounting for about 2% of the global energy consumption.(1) Due to the scale of this environmental burden, energy-efficient and sustainable (GHG-free) Fe metal production that eliminates CO2 emission has been the holy grail of the steelmaking industry for decades.

Recently, electrochemical methods for iron production have been of particular interest for their ability to convert iron oxides to oxygen and iron metal in a low temperature carbon free process.(2, 3). However, electrochemical production of iron from aqueous systems poses several challenges. First the deposition potential of iron is -0.447 V vs SHE meaning that at any potential in which iron could be plated, hydrogen evolution will also occur as a parasitic side reaction. Minimizing or eliminating the hydrogen side reaction is critical to efficient deposition of iron and several pathways to eliminating this challenge have been investigated.(4) Literature suggest that that water-in-salt electrolytes at a wide variety of salt composition and concentrations have the ability to enhance or suppress hydrogen evolution rates depending on the choice of salt, concentration and electrode material.(5, 6)

While hydrogen evolution as a parasitic side reaction can be a major drawback when developing aqueous based electrochemical metal production methods, minimizing this effect is but one of many considerations that must be addressed in the development of a viable process. In this work, we investigate several convoluting factors that affect the energy requirements and rate capability of iron electrodeposition such as conductivity, solubility, deposition kinetics, and efficiency as a function of the electrolyte pH and salt composition from relatively dilute systems such as 0.1 M NaCl to highly concentrated salt systems such as 7 M choline chloride that approach water-in-salt type electrolytes. We use careful measurement techniques such IR-free RDE polarizations, gas collection and impedance spectroscopy to quantify these changes on parasitic side reaction rates on an iron surface at potentials negative of the iron plating potential to ensure a stable substrate and to characterize typical operating potentials for an iron plating process. We then discuss the tradeoffs between this Coulombic efficiency improvement and other critical parameters to gain a holistic view of the capability of aqueous electrolytes to enable sustainable iron production.

This work was supported as part of the Center for Steel Electrification by Electrosynthesis (C-STEEL), an Energy Earthshot Research Center funded by the U.S. Department of Energy, Office of Science.

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