Recent Advances in Heteroatom-Doped Metal-Free Electrocatalysts for Highly Efficient Oxygen Reduction Reaction

Heteroatom-doped metal-free electrocatalysts for oxygen reduction reaction (ORR) represent one of the most prominent families of electrocatalysts for fuel cells. While nitrogen (N)-doped carbon electrocatalysts toward ORR have experienced great progress throughout the past decades and yielded promising material concepts, also other heteroatom-doped catalysts have gained the researchers' tremendous interest recently. Boron (B)-doping on carbon has been extensively studied, and due to the contrary electronic properties between N and B, a synergetic effect between the codoped N and B on carbon has been found for ORR. The carbons doped with sulfur (S), phosphorus (P), silicon (Si), and halogen (fluorine (F), chlorine (Cl), bromine (Br), iodine (I)) have also been studied as metal-free electrocatalysts for ORR in both experimental and theoretical ways. It has been known that the difference in electronegativity and size between the heteroatoms (N, B, S, P, Si, Cl, Br, I) and carbon can polarize adjacent carbon atoms to facilitate the oxygen reduction process. Especially, our research group reported the first F-doped or N,F-codoped carbon black as highly efficient ORR electrocatalysts which possess one of the best price/performance ratio ever. In this feature article, we review the recent research progress in the development of heteroatom-doped carbon-based metal-free electrocatalysts for ORR.