# Zinc Plating on Inkjet-Printed Ti3C2 Tx MXene: Effect of Electrolyte and PEG Additive
## 3 Results
In particular, the nucleation process strongly depends on the nucleation overpotential (\(\eta\)), as defined in the following equation [46; 47]:  
\[\eta=\eta_{onset}-\eta_{cross} \tag{1}\]  
where \(\eta_{cross}\) is the crossover potential corresponding to the equilibrium growth regime, and \(\eta_{onset}\) is the onset potential of the reduction process [46]. Both \(\eta_{onset}\) and \(\eta_{cross}\) were extracted by cyclic voltammetries, as shown in Figure 1b. Values of the nucleation overpotential are reported in Table 1, and \(\eta\) coherently increased when shifting from ZnCl\({}_{2}\) to ZnSO\({}_{4}\)-based electrolytes as well as when adding PEG400. More specifically, the \(\eta\) values increased to different extents when PEG400 was added to the ZnCl\({}_{2}\) or ZnSO\({}_{4}\) solutions. In 2M ZnCl\({}_{2}\) + 10 wt.% PEG400, the overpotential value increased just slightly from 62 mV to 71 mV, while in the case of 2M ZnSO\({}_{4}\) + 10 wt.% PEG400, it consistently rose from 80 mV to 121 mV. This effect can be explained considering the different Zn-ion oligomer formations with PEG400 molecules. In fact, previous work demonstrated that the ZnPEG oligomer strength strictly depends on the counter-ions because of the different Zn ion solvation environments in zinc sulfate solutions with respect to zinc halide (Cl, I, Br) ones [48]. Contrary to zinc halide electrolytes, in ZnSO4 solution, zinc ions are strongly shielded by H2O molecules that firmly bind to PEG through H-bondings, resulting in stronger Zn-ion oligomer formation and, as a consequence, an overpotential increase. The nucleation overpotential is known to have an impact on the Zn nuclei radius, which is given by the following equation [14; 46]:  
\[r_{n}=2\frac{\gamma V_{m}}{F|\eta|} \tag{2}\]  
where \(\gamma\) is the surface energy of the interface between Zn and electrolyte, \(V_{m}\) is the molar volume of Zn, and \(F\) is the Faraday constant. As a consequence, a higher \(\eta\) should result in a higher density of smaller nuclei. That was demonstrated to be a favorable condition for dendrite-free growth of Zn. Moreover, the variation of \(\eta\) suggested that the nucleation process on the Ti3C2 Tx surface was strictly affected by the electrolyte composition. It is noteworthy to recall that nucleation and growth processes are affected both by the rate of charge transfer and mass transport, and the substrate mostly plays a role in the nucleation through the surface energy regulation in Equation (2). Mass transport is instead ruled by the ionic conductivity of the electrolyte, and, as reported in Table 2, the ionic conductivity decreased 2-fold from 2M ZnCl2 to 2M ZnSO4, and it further dropped when PEG400 was included in the electrolyte. The ionic conductivity reduction in PEG400 was reasonable considering the increase in solution viscosity as well as the complexing ability of PEG macromolecules with Zn2+, forming bulky zinc-ion oligomers [48]. Figure 1: (**a**) Cyclic voltammetries of Ti3C2 Tx-coated electrode in different electrolytes recorded at a scan rate of 2 mV s\({}^{-1}\), (**b**) magnified part of the cyclic voltammetries, highlighting the cross potential and onset potential (an example is pointed out for the 2M ZnSO4 solution), (**c**) Tafel plots obtained from the cathodic scan of CVs in (**a**), (**d**) Nyquist plots of the Ti3C2 Tx electrodes in different electrolytes (inset: equivalent circuit used for the fitting). Considering the cyclic voltammetries in a wide scan-rate range (2-100 mV s\({}^{-1}\), Figure S7), it is evident that the mass transport limitation regime strictly depended on the electrolyte composition and its ionic conductivity. In fact, at all scan rates, the reduction curves in chloride-based solutions remained sloped, as is typical of a charge-transfer kinetically controlled process.