Influence of nitrogen doping in graphene based materials for oxygen reduction reaction: role of active site exposure, porosity, and defects in enhancing the four electron transfer process

Abstract

Graphene, a 2D material of a honeycomb carbon lattice from sp2 hybridization possesses many exceptional properties, but is difficult to produce as a truly single layer or even a few-layered material. On the other hand, graphene oxide (GO), its oxided form, can be produced through viable routes, e.g. by modified Hummers method, invovling oxidation steps that introduce oxygen functional groups at the carbon edges and defected areas into a GO structure. The exfoliation of graphite layers into few-layered GOs was succcessfuly achieved by continuous sonication. Further strucutral characterisations were carried out using FTIR, XPS, SEM, and Raman. The formation of C-O bonds through the exfoliation of graphite introduce defects in the GO structure. The reduction of GO to rGO (Reduced GO) offers flexilbity of controlling the C:O ratio in the strucutre, which can be implimented by different treatment methods, including hydrothemal, chemical, and high-temperature processes. The rGO structure recovers the level of continuity of the sp2 aromatic structure and hence conductivity, while offers scope for further modification to enhance functionalities. The undoped rGO usually shows low catalytic performance towards ORR. In this work, we doped the rGO with nitrogen-containing compounds and other hetero-atoms, to boost the ORR performance. The nitrogen functionalities are shown to have profound effects on ORR kinetics, leading to a four-electron transfer process. After the introduction of these precursors materials to the GOx, there was a positive increase in current density and onset potentials which were comparable to the standard commercial ORR catalyst, Pt/C.