Metal-free heteroatom-doped carbon-based catalysts for ORR: A critical assessment about the role of heteroatoms

Metal-free carbon-based catalysts have gained much attention during last years because of their interesting properties towards oxygen reduction reaction. Intrinsic parameters of carbon materials such as porosity, structural order, conductivity and defects have proved to have a strong influence in the catalytic activity of these materials. However, the highest differences in catalytic activity are obtained via doping with heteroatoms, being nitrogen the most remarkable in terms of activity and selectivity. One of the most challenging goals of the scientific community is to unravel the role of the functional groups in order to design an optimized material. However, the complexity of isolating one specific functionality, the difficult unambiguous characterization of the species and the influence of the intrinsic properties of the carbon materials, make the identification of the active sites a complex and controversial issue. This review presents a critical assessment about the role of heteroatoms on ORR from the analysis of the literature that combine both experimental work and computational modelling.