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

Due to the strong electronegativity of the N atom, the adsorption energy of the transition metal –nitrogen active site for the ORR intermediate is too high to facilitate the subsequent elementary reactions. It is worth noting that introducing sulfur atoms in transition metal–nitrogen–carbon catalysts can optimize the charge distribution of nitrogen atoms around metal sites to improve the electronic structure and structural defects of the catalysis, so as to boost the adsorption and transfer of oxygen-containing intermediates. Meanwhile, sulfur-doping can also inhibit metal nanoparticle formation and promote hierarchical porous carbon formation. This review will afford the S-doped M-N-C catalysts rational design principle and strategies and introduce the corresponding ORR electrochemical properties. We will combine advanced characterization techniques and DFT calculation to discuss the coordination environment of active sites and the ORR mechanism of S-doped M-N-C catalysts. Finally, a general perspective on the challenges and opportunities for S-doped M-N-C catalysts will be proposed to facilitate further development.