The cathodes were also subjected to X-ray photoelectron spectroscopy (XPS) analysis, to shed further light on the surface chemical states and composition of the various species. The XPS survey spectrum (Figure ) revealed the presence of S, C, N, and O at binding energies of 163 eV, 285 eV, 400 eV, and 533 eV, respectively. Curve fitting of the C 1s spectrum (Figure ) revealed the presence of two chemical species at binding energies of 284.6 and 286.2 eV, corresponding to C-C/C-H and C-O functional groups, respectively. Based on their binding energy, the formed material was identified as graphene oxide-type product, as already reported in the literature. The deconvoluted S 2p spectrum (Figure ) showed the presence of multiple peaks at different binding energies. The doublet at 163.6 and 165.0 eV can be attributed to S8 molecules, whereas the peaks appearing at 168.9 and 171.2 eV can be associated with highly oxidized SO4 groups.