The chemical compositions of the samples were characterized with X-ray photoelectron spectroscopy (XPS) depth profiling. Figure shows the atomic percentage of Cr inside the rGO films after exposure to the specific voltages. In cases of -1.8 and -2.3 V, the Cr content inside the rGO films is six times higher compared to -1.1 V. Figure and show the depth profiles of Cr 3+ /Cr ratio for different voltages and exposure times, respectively. Clearly, the higher voltages (-1.8 V and -2.3 V) promote increased Cr 3+ /Cr ratios inside the rGO structure. Figure reveals that the oxygen content in samples exposed to -1.1 V is below original GO.