Application of additives for the electrodeposition of metals and alloys from ionic liquids

Ionic liquids (ILs) are a new class of green electrolyte systems owing to their unique characteristics of no proton interference, wide electrochemical window, low vapor pressure, good electrical conductivity, and low melting point. Electrodeposition is a promising high-precision coating method for the tunable preparation of high-performance metal-based film materials. Remarkably, electrodeposition can be easily tuned by tailoring the operating parameters, such as current density, type of current control, potential applied and the deposition model, electrolyte composition and pH, temperature, and the choice of additives, thus garnering various industrial applications. Recently, preparing metal and alloy coatings electrodeposited from ILs has been widely used in materials synthesis, catalysis, electrochemical energy storage, and other fields. Metal and alloy electrodeposition from ILs can address the drawbacks of conventional aqueous systems, such as limited electrochemical windows and easy interference from hydrogen evolution side reactions. Unlike high-temperature molten salts, ILs can be used for the electrodeposition of active metals (such as aluminum, titanium, and rare earth metals) and their alloys at low temperatures and under relatively mild conditions. Nevertheless, ILs have high viscosity, leading to a low ion migration rate and easy-to-produce concentration polarization during the electrodeposition process, thus impacting the quality and performance of the deposited products and restricting large-scale preparation. Introducing efficient additives can optimize the solvent environment, modify the electrochemical reduction potential of the active species in the IL electrodeposition system, influence the electrochemical crystallization process of the electrodeposited grains, and significantly enhance the microstructure and performance of the deposited layer. However, an effective screening approach is still required, and the action mechanism of additives should be intensively studied. Examining highly efficient and functional additives appropriate for IL electrodeposition systems is crucial to advancing their industrial applications; however, they still face challenges. In this review, recent research progress on additives for the electrodeposition of active, transition, and noble metals and their alloys from ILs is summarized. Moreover, the mechanism, utility, and limitations of the existing additives during the electrodeposition process are systematically analyzed, and the future research directions of these additives for metal and alloy electrodeposition from ILs are prospected. This review can provide researchers with a comprehensive and systematic understanding and stimulate research breakthroughs to accelerate the large-scale application of additive-assisted electrodeposition technology from ILs.