S-Doped hierarchical graphene decorated with Co-porphyrins as an efficient electrocatalyst for zinc–air batteries

Abstract

The oxygen reduction reaction (ORR) is an important cathode reaction of fuel cells, metal–air batteries, water-splitting and other devices; however, the reaction kinetics is very slow, and thus the development of high-performance electrocatalysts has been one of the key research directions. In this work, we doped graphene with S element and the results of EDS mapping and XPS showed that the S-doping was successful. In addition, the doping process also faciltated graphene changing from the original chip to a thinner hierarchical structure. Combined with the macrocyclic porphyrin with rich Co–N₄ active sites, the highly dispersed cobalt porphyrin greatly improved the catalytic performance. The high-performance ORR catalyst Por/S/rGO was prepared by S-doping and compositing with cobalt porphyrin, and was applied as a catalyst to the zinc–air battery. Electrochemical tests showed that Por/S/rGO had high ORR activity, which was comparable to commercial Pt/C. The onset potential and half-wave potential of Por/S/rGO were 0.91 and 0.812 V, respectively. Moreover, Por/S/rGO showed better long-term stability and methanol tolerance. Compared with a Pt/C-based Zn–air battery, the Por/S/rGO-based Zn–air battery displayed a higher galvanostatic discharge capacity as well as a desirable stability in 6 M KOH and 0.2 M Zn(OAc)₂ alkaline electrolyte. The enhanced ORR activity of Por/S/rGO was attributed to the rich and highly dispersed Co-N₄ active sites derived from the macrocyclic porphyrin and S-doping strategy.