Rare earth elements (REEs) are critical materials for many existing and emerging technologies. Electrified transportation, wind power generation, hard disk drives, and defense equipment rely on neodymium (Nd), praseodymium (Pr), and dysprosium (Dy) as the key components of permanent magnets that enable these technologies. Demand for REEs has drastically increased over the past few decades and is predicted to accelerate further in the coming decades because REE-based permanent magnets have become indispensable. Currently, the processing and refining of REEs involve environmentally hazardous and energy-intensive steps that are practiced in only a few countries. (1) Heavy dependence on a single source for critical materials access poses serious supply chain risks as well as national security threats. (2,3) Furthermore, developing innovative and sustainable methods for metal refining is crucial to mitigating the environmental impacts caused by REE production.
Presently, REE metal production involves “molten salt electrolysis (MSE)” to convert oxide feedstock into metal. The state-of-the-art method for Nd metal processing uses an oxyfluoride MSE process in which Nd2O3 is dissolved in a molten NdF3–LiF mixture at a temperature exceeding the melting point of Nd (1050–1100 °C). Utilizing a carbon anode, Nd2O3 electrolysis leads to the following overall cell reaction: (4)



This method has several drawbacks, which prohibit its widespread deployment. (5?8) The process necessitates the oxidative consumption of a carbon anode, producing carbon dioxide (CO2), a greenhouse gas (GHG). Annual global production of about 70,000 tonnes of Nd results in the emission of about 16 thousand tonnes of direct CO2. (9) Although this rate of CO2 emission is small in comparison to other metal production processes (e.g., Al or steel production), advancements in electric motors and the push toward green energy is predicted to increase Nd demand by at least fourfold by 2030 with steady growth thereafter. Another disadvantage of oxyfluoride electrolysis is that at high current densities, the fluoride ions produce perfluorocarbons (PFCs), such as CF4 and C2F6 at the anode, which are GHGs with very large global warming potentials (GWPs). Over time, the “consumable” carbon anode uncontrollably increases the interelectrode gap and thus the cell voltage, making process control and scale-up difficult, as well as increasing energy consumption. Finally, the consumable anode necessitates “batch” operation wherein significant energy is wasted in cell heat-up and cool-down cycles, lowering process energy efficiency. These drawbacks make oxyfluoride MSE unattractive for scale-up from a safety, cost, or sustainability point-of-view in geographical locations with stringent environmental regulations...