# Zinc Plating on Inkjet-Printed Ti3C2 Tx MXene: Effect of Electrolyte and PEG Additive
## 3 Results
The R\({}_{2,\,\rm{Eq.3}}\) is the charge transfer resistance calculated using Equation (3) and i\({}_{0}\) values reported in Table 1.  
reaction and nucleation overpotential, according to the equation (derived by the Butler-Volmer equation) [14; 49]:  
\[i=i_{0}\frac{zF}{RT}\eta,\ \ \ R_{ct}=\frac{RT}{zFi_{0}} \tag{3}\]  
where \(\eta\) is the applied overpotential, \(T\) is the temperature, \(R\) is the gas constant (8.314 J mol\({}^{-1}\) K\({}^{-1}\)), \(F\) is the Faraday constant (96,485 A s mol\({}^{-1}\)), \(z\) is the electron moles, and \(i_{0}\) is the exchange current density. The R\({}_{2,\,\text{Eq. }3}\), calculated from Equation (3), and the i\({}_{0}\) values reported in Table 1 were in good agreement with the R\({}_{2}\) obtained by the fitting of Nyquist plots. In summary, both R\({}_{2}\) and i\({}_{0}\) reflected the fact that in chloride-based solutions, the fast rate of charge transfer, i.e., the fast rate of reversible Zn\({}^{2+}\) de-solvation and adsorption to the Ti\({}_{3}\)C\({}_{2}\)T\({}_{\text{x}}\) surface, resulted in the lowest nucleation overpotential values, different from the zinc sulfate electrolytes. As a consequence, it was possible that the anions (Cl\({}^{-}\) or SO\({}_{4}\)\({}^{2-}\)) were adsorbed differently on the Ti\({}_{3}\)C\({}_{2}\)T\({}_{\text{x}}\) surface, affecting the charge transfer resistance, as already demonstrated for other species as saccharin anions [47]. Reasonably, PEG400 always increased the R\({}_{2}\) for the aforementioned mechanisms [48; 50; 51]. The electrochemical results were correlated to the morphological and microstructural properties by SEM and XRD analyses on samples prepared by Hull cell. Hull cell is a useful tool widely employed in the galvanic plating industry to reproduce the cathodic plating process. The trapezoidal shape of the Hull cell was designed to allow the investigation of the electroplating characteristic over a wide range of current densities in a single experiment. A picture of the employed Hull cell setup is reported in Figure S8. Figure 2 reports the SEM images of the Zn deposits plated on the Ti\({}_{3}\)C\({}_{2}\)T\({}_{\text{x}}\)-coated electrodes according to the specific electrolytes and local current density. Compared to the pristine Ti\({}_{3}\)C\({}_{2}\)T\({}_{\text{x}}\)-coated surface (Figure S9), Zn crystals always appeared at both low (1 mA cm\({}^{-2}\)) and higher (5 mA cm\({}^{-2}\)) current densities after 20 min of plating. PEG-free ZnCl\({}_{2}\) solutions (Figure 2a-c) produced a compact Zn deposition without a well-defined surface texture and irregular crystal morphology. Moreover, Zn crystal density was evidently higher in the case of 5 mA cm\({}^{-2}\), as a result of the higher current density, which produced a superior nuclei density. PEG-free ZnSO\({}_{4}\) electrolyte (Figure 2g,h) clearly improved the preferential orientation of the zinc HCP crystals, resulting in a more defined morphology of hexagonal-like Zn particles, particularly evident at 5 mA cm\({}^{-2}\) (Figure 2i). Individual Zn crystals seemed to grow vertically from the Ti\({}_{3}\)C\({}_{2}\)T\({}_{\text{x}}\) substrate rather than forming a very compact and dense layer. A few perfectly hexagonal Zn crystals were evidently growing from the surface of the MXene coating (Figure 2i), with an approximate size of 100 \(\upmu\)m. Comparing the PEG-free electrolytes, the electrolyte itself (specifically the counter-ions Cl\({}^{-}\) or SO\({}_{4}\)\({}^{2-}\)) seemed to play a major role in the growth of textured Zn rather than the Ti\({}_{3}\)C\({}_{2}\)T\({}_{\text{x}}\) layer itself. In fact, the Ti\({}_{3}\)C\({}_{2}\)T\({}_{\text{x}}\) coating should promote epitaxial-like growth of the plated Zn layer because of the good matching between the two crystal lattices [40].