Electrodeposition of a CoNiCu Medium-Entropy Alloy: Promotion of the Electrodeposition Rate and Composite Deposition with Carbon Nanotubes

High-entropy and medium-entropy alloys are attracting keen attention as new classes of alloys in materials science. These alloys exhibit an excellent balance of strength and toughness, in addition to high corrosion resistance. The electrodeposition of such alloys has remained difficult because the overpotentials for electrodeposition vary depending on the elements which constitute the target multicomponent alloy. Furthermore, anomalous codeposition is sometimes unavoidable when the alloy includes iron group elements; in such a case, the control of composition in electrodeposition becomes nearly impossible. We have reported that these bottlenecks are overcome using water-in-oil emulsions as electrodeposition baths, where water droplets encapsulate metal ions to control the alloy composition by tuning the metal ion concentrations within the droplet. This method, by contrast, exhibited a low deposition rate because of the discontinuous supply of droplets to the electrode surface. This paper presents several strategies to increase the deposition rate: The findings clarified that increasing the metal ion concentrations within water droplets is effective, but a threshold concentration exists. The addition of multi-walled carbon nanotubes further promoted electrodeposition because they function as nucleation sites.