Protective In Situ Oxide Layer Formation at a Nonconsumable Iron Anode for Molten Salt Carbon Dioxide Electrolysis

Abstract

In this work, the apparent stability of an iron anode undergoing oxygen evolution in a ternary molten carbonate salt eutectic ((Li/Na/K)₂CO₃) is investigated at 600 °C. In this system, carbon is produced at the cathode for the overall transformation of carbon dioxide to high-value carbon materials. The high variability in the initial iron electrode electrochemical oxidation response at low potentials is shown to originate from the electrochemical sensitivity toward surface iron oxides, which form at the iron electrode surface under the operational conditions. These result from the interaction with small concentrations of oxygen in the cell gas environment (<3 vol %), chemical reaction of oxides, and iron with the carbonate salts present with both direct and indirect (vapor phase) contact, and electrochemical surface oxidation. Despite the demonstrated high susceptibility of iron toward oxidative corrosion under these conditions, when the partially oxidized iron electrode is polarized to undergo oxygen gas evolution, apparent stability is observed. The corrosion rate for iron electrodes operating under a current density of 300 mA·cm^–2 is estimated to be 3.18–7.68 mm·year^–1, an acceptable range for nonconsumable anodes. The unexpected stability of the iron electrode is ascribed to the formation of specific lithium ferrite species. LiFe₅O₈ is specifically confirmed to be formed, which is suggested here to result from the interaction of evolved oxygen gas (O₂) with oxide (O^2–) present in the carbonate melt to form a highly oxidizing species, peroxide (O₂^2–). LiFe₅O₈ is suggested here to form from the interaction of peroxide, lithium salts, and iron species, demonstrating a novel low-temperature pathway for the formation of this species. The intrinsic oxide species activity and reasonable electrical conductivity of lithium ferrite result in a stable and active final anode surface, which does not appear to rely on careful pretreatment prior to sustained oxygen evolution.