Synthesis and Characterization of Nitrogen-Doped Reduced Graphene Oxides for the Oxygen Reduction Reaction in the Zn-Air Batteries

The electron transfer number of the oxygen reduction reaction (ORR) on the nitrogen-doped reduced graphene oxide (N-rGO) has been controlled using the experimental design strategy which includes the studies of the fractional factorial design and the paths of the steepest ascent/descent. For the applications in the zinc-air batteries and alkaline fuel cells, the electron transfer number approaching 4 is desirable; i.e., rendering the ORR to undergo the four-electron transfer mechanism. The maximum electron transfer number of the ORR is 3.9 on our N-rGO prepared in this work. The structure and distribution of nitrogen doped onto the rGO have been measured by the x-ray photoelectron spectroscopic analysis. The N-rGO with the electron transfer number of the ORR close to 4 is employed in the aqueous Zn-air batteries. The typical discharge behavior of the zinc-air battery with an air cathode using N-rGO in 6 M KOH at 2, 10, and 20 mA cm^-2 was examined in this work. The corresponding discharge curves of the Zn anode and air cathode were measured to investigate their individual performances. The discharge capacity of the above full cell test is above 775 mAh g^-1 (based on the zinc mass) which is very close to the theoretical value (820 mAh g^-1).