Electrodeposition of nickel-aluminum alloys from the aluminum chloride-1-methyl-3-ethylimidazolium chloride room temperature molten salt

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 degrees 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.