(333e) Electrochemical Reduction Pathways from Goethite to Green Iron in Alkaline Solution with Silicate Additive

Climate change induced by CO₂ emissions has become a major environmental concern. The Intergovernmental Panel on Climate Change (IPCC) concluded that to achieve the goal of limiting global warming to 1.5°C by 2050 require immediate actions by reducing the CO₂ footprints. Around 8% of global CO₂ emissions are contributed by the steelmaking industry. The industrial process is carried out in a blast furnace where iron and coke are heated to around 1600°C to produce iron which is then converted into steel. Thus, decarbonizing the steelmaking industry will help us take a big step towards achieving the 2050 goal.

Producing iron without CO₂-emission (the so-called “green iron”) can be achieved by reducing the iron ore using hydrogen or electrical energy. Hydrogen Direct Reduction of Iron ore (HDRI) has been investigated as a low-emission steelmaking technology, but the availability of green hydrogen and its requirement of high equilibrium H₂ concentration and high-grade iron ore increases the complexity of the process. Molten oxide electrolysis (MOE) is a high-temperature process, typically operating around 1600 °C, in which molten iron ore is directly reduced to liquid iron with simultaneous oxygen evolution. A key challenge in this process is the development of a low-cost, stable inert anode material that can withstand the extreme conditions while continuously facilitating oxygen evolution without degradation. Electrochemical reduction of iron ore in alkaline solutions operating at room temperature is a viable alternative for green iron production.

Electrowinning of iron ore is usually carried out in strong alkaline conditions where the iron ore is dissolved into the electrolyte which is then electrodeposited as metallic iron at the cathode. The low solubility of iron oxides and uncertainty in the reduction mechanism are the major barriers in the advancement of this technology. An all-solid-state electrochemical reduction without any dissolution step is a promising alternative. This requires the use of weak alkaline solutions (pH around 12) as iron dissolves into soluble species at higher pH based on the Fe-H₂O Pourbaix diagram. However, the lack of structural evidence hinders the understanding of the reduction process. The accumulation of electrochemically inert Fe₃O₄ and parasitic hydrogen evolution reaction (HER) are the main challenges for the complete reduction of Fe(III) to Fe. In this work, we used a weak NaOH electrolyte (pH=12) with silicate additive (5000 ppm) to improve the electrochemical conversion from goethite (FeOOH) to iron. Operando X-ray diffraction and X-ray absorption spectroscopy reveal a FeOOH→Fe₃O₄→Fe(OH)₂→Fe reduction pathway. Silicate suppresses HER and Fe₃O₄ accumulation by slowing down the water and ions transport as supported by atomistic simulations. A poorly crystalline or amorphous Fe(OH)₂ was formed when silicate was added which facilitates the solid-state reduction of Fe₃O₄→Fe(OH)₂→Fe due to its disordered structure. Our work provides valuable insights into understanding the electrochemical reduction mechanism and production of green iron using a low-concentration silicate additive.