# Fabrication of Eco-Friendly Graphene Nanoplatelet Electrode for Electropolishing and Its Properties
## 3 Results and Discussion
With this highly oriented structure, the effective contact area was maximized compared to the case of vacuum-filtered GNP films (89.54 \(\pm\) 17.49 S/cm, n = 9); the liquid crystal-based M-GNP film exhibited a 1150% enhanced electrical conductivity (1032.89 \(\pm\) 26.41 S/cm, n = 6) (Figure 2d) [19]. Compared to M-GNP films, the GNP film is fragile and has less than 1/10 of the electrical conductivity that GNP films could be directly applied as cathode electrodes for electropolishing. The M-GNP film was directly applied as cathode electrode for electropolishing of 304 stainless steel through an electrolyte (sulfuric acid: phosphoric acid = 2:8) [20; 21; 22] in an electrolytic cell (temperature: 45 \({}^{\circ}\)C, current density: 20 ASD, sample size: 10 cm \(\times\) 10 cm, experiment time: 10 min). Figure 3 provides a comparison of 304 stainless steel electropolished by the conventional Cu electrode and by the M-GNP electrode. Before the electropolishing process, the microstructure of the 304 stainless steel specimen showed clear grain boundaries with surface roughness (R\({}^{0}\)\({}_{a}\)) of 5.25 \(\pm\) 1.15 \(\upmu\)m (n = 5). However, after the electropolishing process, most of the grain boundaries faded out, leading to low surface roughness (R\({}^{l}\)\({}_{a}\)) of both the Cu (0.62 \(\pm\) 0.12) \(\upmu\)m (n = 5), Figure 3b) and M-GNP (0.67 \(\pm\) 1.8) \(\upmu\)m (n = 5), Figure 3c) electrodes [23; 24]. The polishing percentage was calculated in terms of decreasing percentages of the arithmetic mean roughness value; both Cu (88%) and M-GNP (86%) electrodes had polishing values of over 85%. The XPS spectra of the binding energy regions of Fe2p (714.68 eV), Ni2p (858.1 eV), and Cr2p (580.32 eV) electrons were obtained from the polished 304 stainless steel [25; 26]. The composition and thickness of the surface were calculated using theoretical aug  
Figure 1: X-ray Photoelectron Spectroscopy (XPS) of GNP and M-GNP. (**a**) Survey and (**b**) high resolution C1s peak regions of GNP and (**c**) M-GNP. (**d**) The Raman spectroscopy of GNP and M-GNP. sis [27]. At the surface and in the internal areas, the pristine 304 stainless steel exhibited a composition of Fe 26.56%, Ni 7.36%, and Cr 2.37% (Figure 3d). After electropolishing with Cu electrodes, surface Fe decreased to 12.65% and surface Cr increased to 16.65% (Figure 3e). After electropolishing with M-GNP electrodes, surface Fe decreased to 17.64% and Cr increased to 16.28% (Figure 3f). The intent of passivation of stainless steel is to remove free iron or other foreign matter from the surface and to create a Cr-rich surface that is resistant to corrosion [28,29]. Therefore, as well as surface polishing, M-GNP showed similar levels of surface passivation of 304 stainless steels. Figure 3: Microstructure and chemical composition of 304 stainless steel after electropolishing. (**a**) SEM image of pristine 304 stainless steel and stainless steel after (**b**) electropolishing at the Cu electrode and (**c**) M-GNP electrode. Auger depth profiling analysis of (**d**) pristine 304 stainless steel and stainless steel after (**e**) electropolishing at the Cu electrode and (**f**) M-GNP electrode. Figure 2: Microstructure and properties of M-GNP electrodes. (**a**) Polarized optical microscopy (POM) image of 20 mg/mL of M-GNP. (**b**) SEM image and photograph of M-GNP electrode (5 cm \(\times\) 2 cm). (**c**) Tensile properties of GNP and M-GNP and (**d**) their electrical conductivity. After 400 cycles of electropolishing, elution of cathode electrode was observed by ICP analysis. Figure 4 shows the amount of eluted Cu electrodes and M-GNP electrodes. After 100 cycles of electropolishing, the Cu electrode eluted by 14 ppm (Figure 4a). Until 400 cycles, the eluted Cu constantly increased as the electropolishing process continued. After 400 cycles, eluted Cu was over 41 ppm. This degradation of the electrode can be directly observed in the SEM image (Figure 4b).