Metal-free, carbon-based catalysts for oxygen reduction reactions

Developing metal-free, carbon-based catalysts to replace platinum-based catalysts for oxygen reduction reactions (ORRs) is an emerging area of research. In recent years, different carbon structures including carbon doped with IIIA-VIIA heteroatoms (C-M site-based, where M represents the doped heteroatom) and polynitrogen (PN) compounds encapsulated in carbon nanotubes (CNTs) (N-N site-based) have been synthesized. Compared to metallic catalysts, these materials are highly active, stable, inexpensive, and environmentally friendly. This review discusses the development of these materials, their ORR performances and the mechanisms for how the incorporation of heteroatoms enhances the ORR activity. Strategies for tailoring the structures of the carbon substrates to improve ORR performance are also discussed. Future studies in this area will need to include optimizing synthetic strategies to control the type, amount and distribution of the incorporated heteroatoms, as well as better understanding the ORR mechanisms in these catalysts.