# Progress in Electrodeposition of Zinc and Zinc Nickel Alloys Using Ionic Liquids
## Zinc Deposition in Ionic Liquids
Zn deposited from ChCl/2 urea displayed a rice-grain like morphology, while the ChCl/2 EG-based liquid resulted in the formation of very thin platelet like structures with the planar face perpendicular to the electrode surface. Nucleation is a process that determines how often a random arrangement of incoming atoms will spontaneously order themselves into a crystal, whereas growth is how quickly these nucleated regions spread outwards. The two different systems displayed a contrasting behaviour in terms of the nucleation and growth rates. Nucleation in the ChCl/2 urea-based liquid is fast, but bulk growth is slow, whereas nucleation in the ChCl/2 EG-based liquid is slow, but bulk growth is relatively fast. Generally, ionic liquids are more viscous than the molecular ones owing to stronger Van der Waals forces between cations, and influence the rate of mass transportation of the metal ion species, metal speciation, and interfacial structure. Cyclic voltammogram (CV) results from the works reported by [PERSON] et al. [22] showed that the magnitude of currents in ChCl/2 urea is smaller than in ChCl/2 EG during Zn deposition. However, it is worth noting that the difference in magnitude of the currents is a consequence of the high viscosity of the ChCl/2 urea system, which affects the mass transport of species, alters the deposition characteristics, and displays different morphology, indicative of a different reduction mechanism and deposition rates. [PERSON] and [PERSON] et al. [26; 27] also investigated the zinc electrodeposition from a mixture comprising 1:2 molar ratio of ChCl/ethylene glycol (EG) on a glassy carbon (GC) substrate and compared the voltammetric response of zinc with a choline-free electrolyte containing only the ZnCl\({}_{2}\) salt and EG or sodium ethoxide (EtONa). They demonstrated the role of ChCl and observed an unusual cyclic voltammetry response, suggesting that choline ions blocked the surface of the electrode during the forward cathodic potential sweep and that zinc is deposited during the backward scan. However, the morphology of the obtained Zn deposit from the two liquids, (i) ChCl/2 urea and (ii) ChCl/2 EG, reported by [PERSON] group [21; 22], appeared to be different, indicative of surface blocking on the electrode owing to the specific adsorption of chloride species on certain crystal faces. The possible reactions at the electrode for the Zn deposition on copper (Cu) substrate employing the ChCl/urea mixture as proposed by [PERSON] et al. [28] are as follows:  
\[\text{Dissolution reaction: ZnO}+\text{Cl}^{-}+\text{urea}\ \ \ \rightarrow\ \ \ \text{[ZnO.Cl.urea]}^{-} \tag{1}\]  
\[\text{Anodic reaction: [ChCl.O.urea]}^{2-}\ \ \ \rightarrow\ \ \text{Ch}^{+}+\text{Cl}^{-}+2\text{e}^{-}+0.5\text{ O}_{2} \tag{2}\]  
\[\text{Cathodic reaction: }\text{Ch}^{+}+\text{[ZnO.Cl.urea]}^{-}+2\text{e}^{-}\ \ \ \rightarrow\ \ \text{Zn (s)}+\text{[ChCl.O.urea]}^{2-} \tag{3}\]  
\[\text{Overall reaction: [ZnO.Cl.urea]}^{-}\ \ \ \rightarrow\ \ \text{Zn (s)}+0.5\text{ O}_{2}+\text{Cl}^{-}+\text{urea} \tag{4}\]  
[PERSON] et al. [29] investigated it further by extending the mechanistic studies on the electrodeposition of zinc from a ChCl/EG deep eutectic solvent containing the Lewis base and zinc precursor (ZnCl\({}_{2}\)). The results confirmed that adsorption of choline and EG species shall induce the blockage of active sites on the glassy carbon electrodes at low potentials, which gradually desorb at potentials \(>\)\(-\)0.4 V (vs. Zn/Zn\({}^{2+}\)). It is worth noting that these ILs were formulated at a temperature of 70 \({}^{\circ}\)C, and the Zn deposition from ILs containing zinc chloride (ZnCl\({}_{2}\)) was carried out at lower temperatures (40 \({}^{\circ}\)C). These conditions signify the technical and commercial viability of the zinc deposition process using ILs and do not require initial drying of the metal salt (ZnCl\({}_{2}\)) before the process.