The Effect of pH on the Reduction of Graphene Oxide in Regards to the Structure and ORR Capabilities in Alkaline Media

The hydrothermal reduction of many compounds including graphene oxide usually takes place in a basic aqueous environment when using many reducing agents. In this study, graphene oxide was reduced by sodium borohydride in aqueous solutions among a wide spectrum of pH values ranging from 1 to 13. X-ray powder diffraction (XRD), thermogravimetric analysis, and infrared spectroscopy (FTIR) confirmed the reduction of the graphene oxide at each respective pH value. With increasing pH values, Raman and X-Ray photoelectron spectroscopy (XPS) spectra of the resultant graphenes showed an increase in the repair in the carbon sp2 lattice while XRD showed a larger overall particle size. Electrochemical testing in basic media showed higher oxygen reduction capabilities in the catalysts that were reduced at higher pH values. The onset potential of the oxygen reduction reaction (ORR) as well as the electron transfer number also increased as the pH of the solution increased overall. The graphene oxide catalyst reduced at pH 1, however, showed superior limiting current densities in the linear sweep voltammetries even though the onset potential was the lowest. This increased ORR activity is most likely due to the better repaired graphitic domain of the catalysts reduced at higher pH values even though the overall particle size is bigger.