Studies on the electrochemical reduction of oxygen catalyzed by reduced graphene sheets in neutral media

Reduced graphene sheets (RGSs) were prepared via chemical reduction of graphite oxide and their morphology was characterized by atomic force microscopy. The electrochemical reduction of oxygen (O₂) with RGSs was studied by cyclic, rotating disk electrode, and rotating ring-disk electrode voltammetry using the RGSs-modified glassy carbon (RGSs/GC) electrode in 3.5% NaCl solution. The results show that O₂ reduction undergoes three steps at the RGSs/GC electrode: electrochemical reduction of O₂ to H₂O₂ mediated by quinone-like groups on the RGSs surface, a direct 2-electron reduction of O₂, and reduction of the H₂O₂ produced to H₂O. The modification of RGSs results in an obvious positive shift of the peak potential and a larger current density. The kinetics study shows that the number of electrons transferred for O₂ reduction can reach to 3.0 at potentials of the first reduction step, indicating RGSs can effectively catalyze the disproportionation of H₂O₂. Such catalytic activity of RGSs enables a 4-electron reduction of O₂ at a relatively low overpotential in neutral media. RGSs are a potential electrode material for microbial fuel cells.