Understand the Role of Water in Water-in-Salt Electrolytes for Efficient Iron Production

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Electrochemical production of iron is emerging as a promising low-cost, modular alternative to the traditional blast furnace. Among the various electrochemical approaches, 1-3 aqueous electrochemistry remains the most cost-effective alternative. However, parasitic hydrogen evolution side reaction (HER) significantly lowers the efficiency of iron electrodeposition in an aqueous electrolyte. 4 One way to suppress HER is to utilize water-in-salt electrolytes (WiSE), which were initially developed for intercalated lithium-ion batteries because of their ability to reduce proton activity and broaden the practical electrochemical stability window of an aqueous electrolyte. 5 In the present work, a new lithium chloride based WiSE was identified to suppress HER and promote iron plating, achieving a high plating efficiency (>90%) at industrially relevant current densities (250 mA/cm 2 ). Electrochemical analysis of the partial current densities of iron plating and hydrogen evolution revealed that water reduction is kinetically suppressed in this WiSE, resulting in the measured increase in plating efficiency. Furthermore, a detailed investigation of the iron and water complexation in WiSE was performed using 1 H NMR and Raman spectroscopies, which revealed that Fe 2+ complexes change in structure and stability as the lithium chloride concentration increases. Overall, this presentation will outline the mechanisms underlying efficient iron plating in water-in-salt electrolytes that kinetically suppresses HER while not sacrificing electrolyte stability. This work was supported by the U.S. Department of Energy, Office of Science Energy Earthshot Initiative, as part of the Center for Steel Electrification by Electrosynthesis (C-STEEL). [1] A. Allanore, L. Yin, and D. Sadoway, A new anode material for oxygen evolution in molten oxide electrolysis. Nature 2013, 497, 353–356. DOI:10.1038/nature12134 [2] B. Noble, A. Konovalova, L. Moutarlier, V. Brogden, and P. Kempler, Electrochemical chlor-iron process for iron production from iron oxide and salt water. Joule 2024, 8 (3), 714-727. DOI: 10.1016/j.joule.2024.01.001. [3] S. Koutsoupa, S. Koutalidi, E. Balomenos, and D. Panias, ΣIDERWIN—A New Route for Iron Production. Materials Proceedings 2021, 5 (1), 58 DOI: 10.3390/materproc2021005058. [4] B. Beverskog and I. Puigdomenech, Revised pourbaix diagrams for iron at 25–300 °C. Corrosion Science 1996, 38 (12), 2121-2135. DOI: 10.1016/S0010-938X(96)00067-4. [5] L. Sup, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, C. Luo, C. Wang, and K. Xu, “Water-in-Salt” electrolyte enables high-voltage aqueous lithium-ion chemistries. Science 2015, 350 (6263), 938-943. DOI: 10.1126/science.aab1595.

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