Designing Strategy of Microemulsion as a Bath for Electrodeposition of a Co-Ni-Cu Medium-Entropy Alloy

Medium- and high-entropy alloys are promising coating materials; however, electrodeposition of multicomponent alloys remains challenging because of differences in reduction potentials and anomalous codeposition. Here, we demonstrate the electrodeposition of Co-Ni-Cu medium-entropy alloy thin films using microemulsions. A microemulsion containing hexadecyltrimethylammonium bromide yielded smooth films with metallic luster without ultrasonic agitation. Using an optimized emulsion, the equiatomic Co-Ni-Cu composition was achieved. The deposits formed a single-phase face-centered cubic solid solution consisting mainly of metallic Ni and Cu with a minor fraction of cobalt hydroxide. Molecular dynamics simulations revealed nanoscale phase-separated structures, and analysis of structure factors indicated that smaller characteristic lengths correlate with improved compositional control. These results establish microemulsions as a practical and tunable platform for electrodeposition of multicomponent alloy thin films.