S-doped M-N-C catalysts for the oxygen reduction reaction: Syntheticstrategies, characterization, and mechanism

Due to the strong electronegativity of the N atom, the adsorption energy of the transition metal-nitrogen activesite for the ORR intermediate is too high to facilitate the subsequent elementary reactions. It is worth notingthat introducing sulfur atoms in transition metal-nitrogen-carbon catalysts can optimize the charge distribu-tion of nitrogen atoms around metal sites to improve the electronic structure and structural defects of the catal-ysis, so as to boost the adsorption and transfer of oxygen-containing intermediates. Meanwhile, sulfur-dopingcan also inhibit metal nanoparticle formation and promote hierarchical porous carbon formation. This reviewwill afford the S-doped M-N-C catalysts rational design principle and strategies and introduce the correspond-ing ORR electrochemical properties. We will combine advanced characterization techniques and DFT calcula-tion to discuss the coordination environment of active sites and the ORR mechanism of S-doped M-N-Ccatalysts. Finally, a general perspective on the challenges and opportunities for S-doped M-N-C catalysts willbe proposed to facilitate further development.