Carbon-based metal-free oxygen reduction reaction electrocatalysts: past, present and future

In recent years, metal-free carbon materials have been the subject of much research concerning their potential use in replacing high-cost Pt-based oxygen reduction reaction (ORR) electrocatalysts. Myriads of research papers in this field have been dedicated to the preparation and characterization of various metal-free nanocarbon materials, as well as to their practical applications. Non-metal heteroatom doping and the introduction of edge defects are typical nanocarbon modification methods, which can significantly reduce the overpotential of the ORR in alkaline and acidic electrolytes. In order to have good activity in actual devices such as fuel cells, it is necessary to increase the ORR intrinsic activity of nanocarbons. Despite many studies of the subject, the intrinsic relationship between nanocarbon composition, structure regulation and catalytic activity is still not clear and needs further exploration. This review details the various nanocarbons used for the ORR as well as their reaction mechanisms in an attempt to propose scientific and specific structural modification strategies. The development of carbon-based metal-free electrocatalysts in the field of oxygen reduction catalysis in recent years is summarized, with a view to providing relevant knowledge for the future design, synthesis and applications of these carbon-based non-metallic catalysts for the ORR.