Here, we use in situ X-ray photoelectron spectroscopy (XPS) to investigate electrochemical processes at the graphene surface. As a model system, we design a sandwich-type electrochemical cell consisting of working graphene electrode separated from back by a fluorinated graphene (FG) film impregnated with a phosphoric acid electrolyte. Migration of the ions to the graphene electrode under applied potential causes a change of the observed concentration of carbon and phosphorus in the electrochemical cell as compared to the non-charged state. This fact allows revealing the process of electrical double layer formation at the graphene/FG-separator interface. The observed changes in the binding energies of elements composing electrolyte and graphene electrode are related with a shift of the Fermi level due to the charge transfer from the adsorbed ions.