Electrodeposition of Reactive Aluminum-Nickel Coatings in an AlCl3:[Emim]Cl Ionic Liquid Containing Nickel Nanoparticles

Abstract

The electrodeposition of reactive aluminum-nickel dispersion coatings was performed by pulsed direct current (PDC) onto copper substrates in the ionic liquid 1.5:1 AlCl3:[EMIm]Cl (1-Ethyl-3-methylimidazolium chloride). A cathodic current density of 15 mA/cm2 was used with two different frequency (f)/duty cycle (r c) combinations, i.e., f= 0.33 Hz/ r c= 0.33 (t on= 1 s, t off= 2 s) and f= 0.05 Hz/ r c= 0.5 (t on= 10 s, t off= 10 s). Several electrochemical techniques like Cyclic Voltammetry, Electrochemical Impedance Spectroscopy and Open Circuit Potential measurements were used besides X-Ray Diffraction, Confocal Microscopy, Scanning Electron Microscopy, Atomic Absorption Spectroscopy and 27Al/1H Nuclear Magnetic Resonance in order to shed more light on the mechanism of Ni particle incorporation into the Al metal matrix. We could show that particle incorporation at the beginning of the deposition mainly takes place via particle adsorption at the substrate. As the thickness of the coating increases, it seems that the main mechanism for particle incorporation is via the reduction of Al2Cl7 - ions adsorbed at the particles surface. Three different Ni NPs concentration were tested in this work: 5, 10 and 20 g/L. Although a considerable high incorporation of Ni NPs has been achieved from the IL electrolyte containing the highest concentration of Ni NPs (i.e. ~ 33 wt.% from a 20 g/L of Ni NPs bath), a high concentration of NPs in the ionic liquid resulted having a negative effect in terms of quality of the coatings, due to local cathodic passivation, or solidification of the electrolyte in a poorly conductive compound. This phenomenon has been related to the increase in viscosity originating from the decrease in rotational mobility of the [EMIm]+ cation, as detected by 1H NMR. Despite this fact, almost equivalent amounts of Ni and Al (Ni ~ 45 wt.% and Al ~ 44 wt.%) have been detected by energy-dispersive X-ray spectroscopy mapping in some areas of the coatings prepared with longer pulse off-time (f = 0.05 Hz, t off= 10 s). Such a layer composition would be convenient for the targeted application. Nevertheless, more studies related to the homogeneity of the deposits, as well as ignition tests, must be performed to evaluate the suitability of the coatings to trigger a self-propagating reaction.