# Progress in Electrodeposition of Zinc and Zinc Nickel Alloys Using Ionic Liquids
## 1 Introduction
Though electroplating using aqueous baths remains a key industrial route for coating production, the limitations stated above prevent the technology from being applied to the deposition of certain metals such as Zn and Zn-Ni alloys. Further, hydrogen evolution reaction (HER) also influences the physical and structural properties of zinc electrodeposits owing to its effect on the nucleation and on the electrocrystallisation of zinc [6]. In addition to the metallic zinc deposition process and HER, zinc oxide deposition also competes owing to the induced precipitation, which increases the interfacial pH [7]. The formation of ZnO films is reported to be interesting in optical, photovoltaic, photocatalytic, and optoelectronics applications and displayed great interest in corrosion protection when deposited on different substrates. However, the formation of ZnO is by indirect electrochemical deposition and is significantly affected by the simultaneous hydrogen evolution, as well as the competition between the process of formation of zinc oxide and the formation of hydroxides or mixed salts. Nevertheless, the technological demand in industries such as automotive and aerospace fastener components for higher quality and longer lasting finishes with improved corrosion resistance has driven the development of efficient electrolytes for zinc and zinc alloy electrodeposition. Owing to the disadvantages associated with Zn/ZnO electrodeposition from aqueous medium, deposition in non-aqueous systems can be considered as one of the effective ways to design and optimize the existing processes in order to obtain pure and homogeneous electrodeposits. Therefore, the search for alternative electrolytes for zinc electrodeposition is of primary importance. In recent times, electrodeposition of metal from ionic liquids has gained significant attention owing to the fact that ionic liquids provide different environments/electrolytes for electrodeposition, wide electrochemical windows, high thermal stability, good ionic conductivity, and better mass transport. In addition, ionic liquids (ILs) can eliminate the mechanical stability issues that arise from metals produced in the presence of hydrogen evolution. Ionic liquids (ILs) consist of an organic cation and an anion (organic or inorganic); both serve as solvents and supporting electrolytes, thereby making them an attractive alternative to toxic solvents or aqueous solutions in the electroplating industry. Figure 1 shows the schematic arrangement of an IL representing a large, bulky asymmetric cationic component and a relatively small symmetric anion [8]. The highly asymmetric structure of the cation makes it difficult to stack with the anions close to each other, and reduces the lattice energy, thus effectively prohibiting IL's crystallization and lowering its melting point. ILs distinguish themselves from traditional molecular liquids in terms of their mass transport and charge transport. In general, five types of ILs have been employed in the electrodeposition of zinc and zinc-based alloys and is mentioned in Table 1. These ILs are differentiated in terms of their preparation, mechanism and limitations. A detailed discussion on the ILs and their preparations can be found in the literature [9; 10; 11]. Reviews by [PERSON] et al. [12] and [PERSON] et al. [13] provide an overview on the deep eutectic solvents and their practical applications. Deep eutectic solvents (DESs) are systems composed of a simple eutectic mixture of Lewis or Bronsted acids and bases and contain a variety of anions, with cations being the hydrogen bond donors. On the contrary, ILs are systems with one type of discrete anion and cation. The main advantage of ILs refers to its ability to tailor metal redox potentials and control deposit morphology. The simplest ionic liquids consist of a single cation and a single anion, whereas more complex systems are formed by combining several cations and/or anions, or complex anions, obtained in equilibrium processes.