Comparative study of various preparation methods of metal-free N and S Co-doped porous graphene as an ORR catalyst in alkaline solution

In this work, various methods (chemical vapor deposition (CVD), hydrothermal, and pyrolysis) have been applied for preparing sulfur–nitrogen co-doped porous graphene. Afterwards, the electrochemical behavior of the samples prepared through different techniques was investigated for oxygen reduction reaction (ORR). The prepared heteroatoms-doped electro-catalysts were thoroughly characterized by X-ray diffraction, Raman spectroscopy, N2 adsorption-desorption, field emission scanning electron microscopy, and X-ray photoelectron spectroscopy techniques. The results showed that co-doping of S and N into porous graphene significantly enhanced the ORR performance. The obtained results showed that the efficiency trend of preparation methods on the ORR performance followed as pyrolysis > CVD > hydrothermal. In addition, the electron transfer numbers in these methods at optimal conditions were 4.1, 3.4, and 2.6, respectively. Among different preparation techniques, pyrolysis as a post-synthesis doping technique has exhibited the most efficient ORR process, in which electron transfer follows the 4e pathway. Moreover, the catalyst prepared by the pyrolysis method has an onset potential of 0.93V which is very close to the conventional Pt/C 20 wt.% (0.99 V) electro-catalysts because of surface doping and better control over the final product. Finally, the microbial fuel cell test showed a high peak power density of 30.94 mW m−2 for the optimal sample, which is close to the Pt/C 20 wt.% (38.63 mW m−2) cathodes, because of the synergistic effect of N and S co-doped carbon structure. Graphical abstractSynthesis and characterization of heteroatom co-doped graphenes using CVD, Pyrolysis and hydrothermal techniques are reported and their electrochemical behaviors evaluated. The results represent the high dependence of ORR activity of the prepared electrocatalysts to the synthesis method and the electrocatalyst via pyrolysis method shows high selectivity for the 4e- reduction.