# Zinc Plating on Inkjet-Printed Ti3C2 Tx MXene: Effect of Electrolyte and PEG Additive
## 3 Results
However, SEM analyses supported the electrochemical results, suggesting that ion mobility and/or a different anion adsorption at the electrolyte/Ti\({}_{3}\)C\({}_{2}\)T\({}_{\text{x}}\) interface mostly ruled the surface overpotential and the resulting Zn electro crystallization. Increasing surface overpotential was, in fact, demonstrated to be the key feature enabling the smooth growth of compact Zn. Specifically, higher surface overpotential (or higher current density) produces more spatially uniform current distribution and faster growth rate of the [002] planes with respect to the [001] one [14; 46; 52; 53; 54]. Actually, the higher nucleation overpotential and charge-transfer resistance in ZnSO\({}_{4}\) electrolytes, contrary to the fast deposition kinetic in ZnCl\({}_{2}\) resulting from the electrochemical analyses, were coherent with this view and the actual plated-Zn morphology. A 10% wt. PEG400 addition to the electrolyte greatly affected the plated-Zn morphology but, in the opposite way, when added to the ZnCl\({}_{2}\) or ZnSO\({}_{4}\) solutions. In ZnCl\({}_{2}\) + 10% wt. PEG400, the coating homogeneity was macroscopically greatly improved compared to the PEG-free chloride-based electrolyte. In fact, polyethylene glycols are known to act as a leveling agent; PEG macromolecules adsorbed at the Ti\({}_{3}\)C\({}_{2}\)T\({}_{\text{x}}\) surface hinder the lateral 2D diffusion of zinc ions and the restricted surface migration, resulting in a higher density of small nuclei and more homogeneous Zn. However, SEMimages (Figure 2d-f) didn't show any evident difference at the microscopic scale with respect to PEG-free ZnCl\({}_{2}\) in the surface morphology of Zn grains. Oppositely, in ZnSO\({}_{4}\) + 10% wt. PEG400 (Figure 2g-j), lamellar structures formed, as typical of zinc hydroxide byproducts (e.g., zinc hydroxide sulfate). Zn crystals were scarcely visible and appeared in the whole current density range as irregular, small particles with undefined morphology beneath and within the zinc hydroxide lamellar network. Specifically, the lamellas were more clearly visible at 5 mA cm\({}^{-2}\), suggesting that the ZnSO\({}_{4}\) + 10% wt. PEG400 electrolyte was characterized by a low Zn deposition faradaic efficiency. The local pH rise due to the side hydrogen evolution reaction likely induced the precipitation of the zinc hydroxide structures. The highest charge transfer resistance and limited ionic conductivities detected for the ZnSO\({}_{4}\) + 10% wt. PEG400 solution corroborated this hypothesis [50]. XRD analyses, reported in Figure 3, showed the highest intensity [002] peak associated with the Ti\({}_{3}\)C\({}_{2}\)T\({}_{x}\) MXene coating at 7.5\({}^{\circ}\), while the diffraction peaks of metallic Zn were centered at 36.2\({}^{\circ}\), 38.9\({}^{\circ}\), and 43.2\({}^{\circ}\), corresponding to the [002], [001], and [101] planes, respectively (JCPDS card # 03-065-5973). Insets show the magnified portion of the XRD patterns, where the significant Zn peaks are present. The [002] preferential orientation is  
Figure 2: SEM images (25 KX) of the Zn structures grown on Ti\({}_{3}\)C\({}_{2}\)T\({}_{x}\) substrate by Hull cell preparation in: (**a–c**) 2M ZnCl\({}_{2}\), (**d–f**) 2M ZnCl\({}_{2}\) + 10 wt.% PEG400, (**g–i**) 2M ZnSO\({}_{4}\) and (**j–l**) 2M ZnSO\({}_{4}\) + 10 wt.% PEG400 electrolytes. The corresponding current density value is indicated at the top of each column. widely recognized to be the most favorable to achieve a low-porosity, dendrite-free metallic Zn coating for highly-reversible plating and stripping in Zn-ion battery applications [53, 54, 55]. The ratio between the intensity of the [002] peak and the [101] one (I\({}_{002}\)/I\({}_{101}\)) was considered to quantify the preferential surface texture [53, 54].