The objective of this project was to accelerate the application of ionic liquids and deep eutectic solvents (DES) in nonaqueous electroplating to address the Department of Defense (DoD) life-cycle cost and environmental issues related to weapon systems. Researchers have explored the use of task-specific ionic liquids and deep eutectic solvents (DESs) that could mitigate the issues of air/water-stability associated with current electroplating baths for aluminum or aluminum alloys. In addition, new methods of enhancing the conformal coating while mitigating the environmental issues have also been explored. Furthermore, the stability of these nonaqueous plating baths under various operational environments have been investigated to determine the long-term viability of these plating technologies.
To avoid using volatile and toxic organic solvents, chloroaluminate-based ionic liquids have been intensively investigated for use in electrodeposition of aluminum. These ionic liquids can be conveniently prepared by combining anhydrous AlCl3 with an organic chloride such as 1-ethyl-3-methyl imidazolium chloride (EMIm.Cl) and N-butylpyridinium chloride (N-BPCl). Unfortunately, these ionic liquids can be only used in an inert-gas atmosphere because of the hygroscopic nature of AlCl3 and the chloroaluminate. This has prevented practical application of these ionic liquids for electrodeposition of aluminum, especially for open filed military related applications.
To mitigate the above issues of AlCl3 and chloroaluminate, several approaches have been used to develop air- and water-stable ionic liquids and DESs for electrodeposition of aluminum in this project. The first approach was to synthesize stable aluminum salts to replace hygroscopic AlCl3 that could be used to prepare ionic liquids for electrodeposition of aluminum. Large ligands, such as O and N-containing ligands, were used to replace small Cl atom, which could be less moisture sensitive because of their bulkiness. The second approach was to develop new aluminum cation based ionic liquids using different neutral ligands to reduce the moisture sensitivity and improve the plating efficiency of the ionic liquids during electrodeposition of aluminum. This approach was similar to the first approach, i.e. partially replacing small Cl ligands with large neutral ligands. On the one hand, it could reduce moisture sensitivity of the ionic liquids; on the other hand, it could generate Al-containing cationic species, which were more active than anionic species of Al2Cl7- under electrodeposition condition. The third approach was to develop a convenient wrap-plating technique using polymer gel membranes to reduce the moisture sensitivity of the ionic liquids. The polymer gel electrolytes could function as blankets covering the substrates to be plated, not only providing electroactive ionic liquids for electrodeposition of aluminum but also preventing moisture and oxygen for the air to interfere with or in the electrodeposition of aluminum. The fourth approach was to seal the ionic liquids inside a chamber to obtain a portable aluminum deposition system (PADs), not only reducing the moisture sensitivity of the ionic liquids but also increasing the flexibility of aluminum electrodeposition. Parallel to the development of air and water stable ionic liquids for aluminum deposition, different parameters affecting aluminum deposition such as current density, co-solvent, current on/off duration of pulsed current plating method, anodization time, have also been systematically investigated in this project.
This effort presents the development of new ionic liquids for electrodeposition of aluminum. The first task of development of new ionic liquids was to synthesize new aluminum salts to replace AlCl3 because of the hygroscopic nature of the latter. Novel ionic liquids were developed via neutral ligand complexation to generate electroactive aluminum-containing cationic species. N, S, and P-containing ligands were used to synthesize these new ionic liquids. Next, the development of polymer gel-based baths for electrodeposition of aluminum, was explored. The interactions between the functional groups of the polymer and the components of the ionic liquids were evaluated. Electrodeposition of aluminum using an ionic liquid bath was then addressed. To optimize parameters, such as current density, co-solvent, current duration of pulsed current, anodization time etc., the eutectic mixture of 1-ethyl-3-methyl imidazolium chloride and AlCl3 at a molar ratio of 1:1.5 is used. Eelectrodeposition of aluminum using a portable aluminum deposition system (PADs) was the next topic. The effects of surface treatment of the substrate, deposition protocol (constant current vs. constant voltage) is presented. Finally, the utilization of common 12.5V D battery to power the PADs for electrodeposition of aluminum is discussed.
The research team has successfully developed new classes of ionic liquids based on the complexation of N, P and S-containing neutral ligands with AlCl3, which can be successfully used for electrodeposition of aluminum. It has been demonstrated that the molar ratio of AlCl3 is important for the electrochemical properties of the ionic liquids. To mitigate the moisture sensitive issues of the AlCl3 based ionic liquids, polymer gel membranes were also successfully developed for the first time, which also exhibited a good electrochemical behavior for the deposition/stripping of Al. It has shown that the selection of functional groups and solvent is crucial in the preparation process of the polymer gel membranes, which will ultimately affect the electrochemical properties of the membranes. It also suggests that the intrinsic ionic conductivity of the ionic liquid will play a key role in the performance of the final polymer gel membrane. Different parameters such as current density, selection of co-solvent, duration of different ton and toff for pulsed current technique, and anodization of the substrate have been optimized for electrodeposition of aluminum. In addition, a portable aluminum plating system (PADs) using an Al-based ionic liquid electrolyte was also successfully prepared for the first time. The process was successfully used to plate Al on Cu or carbon steel substrates. Both a potentiostat instrument and a 1.5V D cell battery can be used to operate the PADs for aluminum deposition. Al films can be produced after 5 minuntes of plating time, a dense specular film of Al is obtained on Cu or steel. The plating process can also be used to plate aluminum alloy, for example, Al-Mn alloy. Portable plating brush can be used for a wide variety of applications in the defense industry. However, one limitation of the PADs is that it can only be used for electrodeposition on the exposed surface, and can for internal (such as internal surface of a tube) or line of sites.