Electrodeposition of Nickel‐Aluminum Alloys from the Aluminum Chloride‐1‐methyl‐3‐ethylimidazolium Chloride Room Temperature Molten Salt

Abstract

The electrodeposition of nickel and nickel‐aluminum alloys on glassy carbon was investigated in the 66.7–33.3 mole percent (m/o) aluminum chloride‐1‐methyl‐3‐ethylimidazolium chloride molten salt containing electrogenerated nickel(II) at 40°C. The electrodeposition of nickel on glassy carbon involves three‐dimensional progressive nucleation on a finite number of active sites with hemispherical diffusion‐controlled growth of the nuclei. At potentials slightly more negative than those needed to induce the reduction of nickel(II) to the metal, aluminum is codeposited with nickel to produce Ni‐Al alloys. Controlled‐potential and controlled‐current experiments revealed that it is possible to produce alloy deposits containing up to approximately 40 atomic percent (a/o) aluminum under conditions that circumvent the bulk deposition of aluminum. The aluminum content of the Ni‐Al deposit was found to vary linearly with the deposition potential but nonlinearly with the current density. The electrodeposited Ni‐Al alloys are thermodynamically unstable with respect to nickel(II), i.e., immersion of the alloy deposit in melt containing nickel(II) under open‐circuit conditions leads to a reduction in the aluminum content of the alloy. The mechanism of alloy formation appears to involve underpotential deposition of aluminum on the developing nickel deposit; however, alloy formation must be kinetically hindered because the aluminum content is always less than predicted from theoretical considerations. Ni‐Al alloys produced at 0.30 V [vs. Al/Al(III) in pure 66.7–33.3 m/o melt] in melt containing nickel(II) and 20% (w/w) benzene as a cosolvent contained about 15 a/o nickel and were of high quality with a disordered fee structure, but alloys produced at more negative potentials had the visual appearance of a loosely adherent, finely divided, black powder and were heavily contaminated with chloride, probably as a result of the occlusion of the molten salt solvent by the dendritic alloy deposit during deposit growth.