Ion intercalation into HOPG in supercapacitor electrolyte - An X-ray photoelectron spectroscopy study

Highly oriented pyrolytic graphite electrodes were electrochemically treated in electrochemical double layer capacitor electrolyte and studied by X-ray photoelectron spectroscopy (XPS). The used electrolyte was 1 M tetra-ethyl-ammonium tetra-fluoroborate (Et4NBF4) dissolved in propylene carbonate. Intercalation of the ions into the graphitic host structure results in core level shifts of the XP-spectra. The spectra of the host and the intercalating species shift towards higher values of binding energy for the negatively and towards lower values for the positively polarized electrodes, which is ascribed to the Fermi level shifts of the intercalation compound. The reversibility of ion insertion is quantified by deconvolution of the XP signals and is shown to be less for the Et4N+ cation than for the BF4− anion.