# Progress in Electrodeposition of Zinc and Zinc Nickel Alloys Using Ionic Liquids
## Influence of Additives
### Zinc  
Additives also function as levellers and brighteners when added to the electrodeposition baths. As a levelling agent, the additive molecules are adsorbed on the high points of the substrate surface (most active sites), and hinder the deposition of metal ions while favoring the metal deposition on low points. Such a deposit exhibits uniform thickness with a good coverage and throwing power. Brightness is related to the smoothness of a surface, which can reflect visible light. As a brightener, additive molecules influence the size of the metal deposited and result in the formation of crystals that have a size smaller than 400 nm. Such deposits produce a reflective surface under visible light conditions, indicating a good degree of brightness. In addition, they also play an important role in tailoring the crystal orientation. [PERSON] [48] studied the effect of additives--nicotinic acid, boric acid, and p-benzoquinone--on the deposition of Zn from ethaline (ChCl/EG = 1.2), and achieved a mirror-like finish on copper substrate with nicotinic acid as the additive. Boric acid and p-benzoquinone improved the brightness of the Zn deposit, demonstrating that they can be employed as brighteners. However, these additives resulted in a slight decrease in the conductivity of the ethaline electrolyte. [PERSON] et al. [49] studied the effects of dihydric alcohols--ethylene glycol, 1,3-propanediol, 1,2-butanediol, and 1,3-butanediol--on the electrodeposition of Zn from 1-ethyl-3-methylimidazoliumbromide/ZnBr, demonstrating that the addition of each of these species enhanced the cathodic current efficiency, obtaining a smooth Zn deposit in all cases. Ethylene glycol was reported to be the superior to other three alcohols in terms of brightness and smoothness of the deposit; cathodic current efficiency when the deposition of Zn is performed within the applied current density range between 50 and 250 A/m\({}^{2}\). Moreover, the addition of the alcohols \(>\)50 mol % concentration resulted in a grey deposit with the formation of dendritic deposits at the edges. [PERSON] group [50] studied the electrodeposition of zinc from ChCl-based DESs, ChCl/urea and ChCl/EG, employing a series of polar additives including ethylene diamine, acetonitrile, and ammonia. They found that the additives formed a complex with the chloride ions, evidencing the complexing nature of the additive. However, the reports suggested that the additives did not affect the speciation of the metal complex, but shifted the onset in reduction potential as shown in Table 2 and improved the brightness of the deposit. The addition of ethylene diamine and ammonia had a significant brightening effect, probably owing to their ability to inhibit the adsorption of chloride at the electrode surface. [PERSON] et al. [51] demonstrated the effect of acetonitrile and ethylene diamine on the deposit morphology and grain size of zinc coatings electrodeposited from ChCl/urea. They showed that the additives influenced the nature of the zinc deposit from ionic liquid by modifying the morphologies and grain sizes. [PERSON] et al. [52] reported on the influence of the tartrate ion on the electrodeposition of zinc from the ChCl/EG mixture, reporting that tartaric acid modified the nucleation mechanism of zinc deposition and the morphology of the resultant deposit. The same group extended the work further and studied the influence of organic additives--acetamide, dimethylacetamide, and dimethyl sulfoxide (DMSO) on Zn electrodeposition from choline chloride/ethylene glycol mixture. The addition of acetamide to the ionic liquid displayed an irregular deposit, whereas a uniform deposit was obtained in the presence of dimethylacetamide, and the addition of DMSO did not display a major change in the morphology of the deposit. On comparing the polarization data, Zn deposit with DMSO exhibited the best corrosion resistance.