Electrochemical Insights into Copper Electrodeposition on Nd2Fe14B Grains: A Proof-of- Concept Study

This study explores Cu electrodeposition from a near-neutral sulphate bath onto Nd-Fe-B bulk and powder electrodes. The former
served for the preliminary electrochemical tests, while the latter was used for Cu coating of the corrosion-sensitive powdery raw
material. Cyclic voltammetry established the potential intervals for Cu deposition (at least −0.1 V and below) and the Nd-Fe-B
oxidation (above −0.5 V). Cu electrodepositions were performed on both electrodes in potentiostatic mode for 30 s. Scanning
electron microscopy/energy-dispersive spectroscopy showed that Cu deposited at high overpotentials (−1.05 and −0.5 V) had a
dendritic structure mainly due to mass transport limitations. A chronoamperometric study on Nd2Fe14B powder electrodes at
−0.25 V resulted in a positive current, indicating the Nd-Fe-B oxidation dominance. At −0.5 V, the current remained negative, but
showed diffusion limitations. The latter was improved by using ultrasonic agitation, which resulted in a higher total negative charge
and more uniform Cu deposits on Nd2Fe14B grains. Cu-coated Nd₂Fe₁₄B grains showed a mass magnetization decrease from 137 to
127 emu g−1, corresponding to a ∼9% Cu mass increase determined via gravimetry. The study demonstrates successful Cu
electrochemical deposition with no magnetization loss beyond the paramagnetic Cu phase, paving the way for grain-boundary
engineering of novel Nd-Fe-B magnets.