Electrodeposition of cobalt-aluminum alloys from room temperature chloroaluminate molten salt

The electrochemical reduction of cobalt(II) in the 1.5:1.0 AlCl3:EMIC (EMIC = 1-ethyl-3-methylimidazolium chloride) room temperature molten salt leads to cobalt metal at potentials positive of +0.4 V and to CoAlx alloys when the deposition potential is negative of +0.4 V. The value of x in CoAlx gradually increases up to a value of 2 as the electrodeposition potential (E(deposition)) decreases, however, plots of x vs. E(deposition) exhibit sloping plateaus and indicate preferential formation of alloys having integral compositions of CoAl1 and CoAl2. The formation of these alloys can be interpreted as an under-potential deposition-based process or as a free energy of alloy formation. Evaluation of the chronoamperometric transient behavior during electrodeposition shows that pure Co deposition proceeds via 3D progressive nucleation with diffusion-controlled growth. However, as E(deposition) crosses from Co metal into the alloy-forming range, th nucleation process exhibits characteristics indicative of kinetic control.