Electrodeposition of a Rare‐Earth Iron Alloy from an Ionic‐Liquid Electrolyte

Rare earth element (REE)‐based metals and alloys are generally synthesized by molten‐salt electrolysis which is an energy‐intensive approach. Previous attempts to deposit alloys of rare earths from solutions at mild temperatures have met with little success. Excitingly, in this investigation we were able to electrodeposit Nd−Fe from the 1‐ethyl‐3‐methylimidizolium dicyanamide ([EMIM][DCA]) ionic liquid (IL) at 110 °C. We observed that NdIII cannot be reduced independently, although it can be co‐deposited inductively on a Cu substrate with the addition of FeII. The transmission electron microscopy (TEM) analysis combined with electron‐energy‐loss spectroscopy (EELS) verified that NdIII is reduced to Nd0 during the electrodeposition process. The TEM/EELS was also able to confirm that the deposition of the Nd−Fe starts with the sole deposition of Fe, followed by the co‐deposition of Nd−Fe. This is in agreement with transition‐state theory, which has the iron initially reduced to an activated state (Fe*), where it is able to catalyse the reduction of the rare earth from NdIII to Nd0. This new insight into the electrodeposition process brings us a very important step closer to being able to recycle rare earths efficiently and even to realise electrodeposited rare‐earth‐based permanent‐magnet thin films at a mild temperature, thus giving us a sustainable, green‐chemistry approach that provides a genuine alternative to high‐temperature molten‐salt electrolysis. Induced co‐deposition mechanism of rare earth element (REE) ‐transition metal (TM) describes the role of the activated state of TM which catalyzes the reduction of REEIII to REE0. The results highlight the theoretical model for REE‐TM co‐deposition and provide a novel mechanistic insight. The realization of electrodeposited Nd−Fe‐based thin films at a mild temperature exhibits a sustainable, green‐chemistry approach compared to molten‐salt electrolysis.