Coordination environment engineering of single-atom catalysts for the oxygen reduction reaction

Benefiting from high efficiency and environmental friendliness, Zn-air batteries, fuel cells and electrochemical H2O2 production have attracted significant attention in the energy field. However, the oxygen reduction reaction (ORR), which takes place at the cathode and involves a multi-electron transfer process, has become a barrier to the widespread applications of these pollution-free systems. It is urgent to develop efficient catalysts for the ORR. Single-atom catalysts (SACs), particularly M-N-C SACs (M = non-precious metal atom), have emerged as attractive candidates with maximum metal atom utilization, uniform active centers, strong metal-support interaction and well-defined active sites. In this review, recent developments in improving the intrinsic activity of M-N-C SACs were summarized, emphasizing the impact of the surrounding environment on the ORR performance of single-atom sites as determined by experimental investigations and density functional theory (DFT) simulations. In addition, advanced characterization techniques, synthesis strategies and the applications of M-N-C SACs in Zn-air batteries, proton exchange membrane fuel cells (PEMFCs) and H2O2 production were also documented. This review may stimulate the intensive exploration of highly active M-N-C SACs for practical applications in the near future.