# Zinc Plating on Inkjet-Printed Ti3C2 Tx MXene: Effect of Electrolyte and PEG Additive
## 3 Results
Contrarily, in zinc sulfate-based electrolytes, mass transfer limitations to the reduction process appeared, starting from 25 mV s\({}^{-1}\) and 10 mV s\({}^{-1}\) for the ZnSO\({}_{4}\) electrolytes with and without PEG, respectively, as the transition from a sloped branch to a well-defined cathodic peak. Comparing the ionic conductivity and nucleation overpotentials, a coherent trend could be pointed out. The exchange current density (i\({}_{0}\)) values reported in Table 1, obtained by the fitting of linear branches in Tafel plots (Figure 1c), showed that the highest i\({}_{0}\) was reported for the ZnCl\({}_{2}\) electrolyte, representative of a faster intrinsic rate of the redox reaction at the equilibrium. In PEG-free zinc sulfate electrolyte, the i\({}_{0}\) decreased to 2.95 mA cm\({}^{-2}\) and i\({}_{0}\) values lowered by the addition of PEG400, both in ZnCl\({}_{2}\) and ZnSO\({}_{4}\) electrolytes, consistent with the hypothesis of the Zn-oligomer adsorption, or more generally speaking of the PEG400 macromolecules, which can increase the de-solvation energy of zinc ions [48]. Nyquist plots (Figure 1d) clearly showed a right shift on the Z' axis with respect to the electrolyte composition as well as an increase in charge transfer resistance. In particular, the electrolyte ionic conductivity was related to the R\({}_{1}\) value, while the charge transfer resistance was the diameter of the semicircle and was noted as R\({}_{2}\). Both R\({}_{1}\) and R\({}_{2}\) values are reported in Table 3.  
It can be easily demonstrated that R\({}_{1}\) is representative of the ohmic resistance of the electrolyte since the ratio of ionic conductivities with respect to the 2M ZnCl\({}_{2}\) solution corresponded to the ratio of R\({}_{1}\) values with respect to the one of 2M ZnCl\({}_{2}\) (\(\
icefrac{{\sigma}}{{\sigma_{ZnCl_{2}}}}=\
icefrac{{R_{1}}}{{R_{1}}}/\
icefrac{{ R_{1,\,\,Z_{\rm{Eq.3}}}}}\)). On the other hand, the charge-transfer resistance (R\({}_{2}\)) depends on the energy of de-solvation of ions from the surface, and it is directly related to the rate of the redox \begin{table}
\begin{tabular}{c c c c c} \hline  & **2M ZnCl\({}_{2}\)** & **2M ZnCl\({}_{2}\) + 10 wt.\% PEG400** & **2M ZnSO\({}_{4}\)** & **2M ZnSO\({}_{4}\) + 10 wt.\% PEG400** \\ \hline \(\
icefrac{{1}}{{\rm{i}}_{\rm{p,plat}}}\) 1(A cm\({}^{-2}\)) & 0.037 & 0.024 & 0.014 & 0.007 \\ Q\({}_{\rm{plat}}\) (C) & 5.96 & 3.70 & 1.81 & 0.84 \\ I\({}_{\rm{p,strip}}\) (A cm\({}^{-2}\)) & 0.044 & 0.027 & 0.018 & 0.008 \\ Q\({}_{\rm{strip}}\) (C) & 5.86 & 3.70 & 1.75 & 0.78 \\ \(\eta\) (mV) & 62 & 71 & 80 & 121 \\ i\({}_{0}\) (mA cm\({}^{-2}\)) & 10.94 & 6.66 & 2.95 & 1.47 \\ \hline \end{tabular}
\end{table} 
Table 1: Electrochemical parameters extracted by cyclic voltammetries at 2 mV s\({}^{-1}\) according to different electrolytes, i.e., 2M ZnCl\({}_{2}\), 2M ZnCl\({}_{2}\) + 10 wt.\% PEG400, 2M ZnSO\({}_{4}\) and 2M ZnSO\({}_{4}\) + 10 wt.% PEG400. \begin{table}
\begin{tabular}{c c c c c} \hline  & **2M ZnCl\({}_{2}\)** & **2M ZnCl\({}_{2}\) + 10 wt.\% PEG400** & **2M ZnSO\({}_{4}\)** & **2M ZnSO\({}_{4}\) + 10 wt.\% PEG400** \\ \hline \(\sigma\) (mS cm\({}^{-1}\)) & 100.2 & 68.1 & 54 & 29.1 \\ \hline \end{tabular}
\end{table} 
Table 2: Ionic conductivities of the different electrolytes, as measured by a conductivity meter probe at room temperature. \begin{table}
\begin{tabular}{c c c c c} \hline  & **2M ZnCl\({}_{2}\)** & **2M ZnCl\({}_{2}\) + 10 wt.\% PEG400** & **2M ZnSO\({}_{4}\)** & **2M ZnSO\({}_{4}\) + 10 wt.\% PEG400** \\ \hline R\({}_{1}\) & 4.98 & 7.2 & 15.71 & 23.27 \\ R\({}_{2}\) & 5.55 & 5.85 & 10.55 & 14.94 \\ R\({}_{2,\,\rm{Eq.3}}\) & 4.42 & 7.33 & 10.69 & 16.45 \\ \hline \end{tabular}
\end{table} 
Table 3: R\({}_{1}\) and R\({}_{2}\) values obtained from fitting the semicircle in Nyquist plots with the equivalent circuit reported in Figure 1d.