Advances in coordination engineering of M-N-C single atom catalysts for superior oxygen reduction performance

The oxygen reduction reaction (ORR) is a cornerstone of sustainable energy conversion technologies, such as fuel cells, metal–air batteries, and green synthesis of H2O2. However, the widespread adoption of ORR is hindered by persistent challenges in terms of catalytic activity, selectivity, and durability of the catalysts. A transformative approach to overcome these limitations is the chemical engineering of metal‑nitrogen‑carbon single-atom catalysts (M-N-C SACs), which allows precise tuning of electronic structures and coordination environments to mimic the efficiency of natural metalloenzymes. The electronic structure of M-N-C SACs can be modulated by incorporation of heteroatoms (e.g., S, B), which alter the d-band structure to enhance O2 adsorption and OO bond cleavage, consequently reducing the overpotential for ORR. Atomic-scale engineering of bond lengths, coordination numbers, and electronic states in metal‑nitrogen‑carbon single-atom catalysts (M-N-C SACs) significantly improves their ORR performance. Specifically, the engineering of the first and higher coordination spheres through ligand design or hetero-element doping enhances charge transfer dynamics and selectivity of 4e- process, which is a key step in ORR. This review systematically evaluates the influence of coordination engineering in M-N-C SACs on benchmark ORR metrics, while highlighting breakthroughs in operando techniques and advanced electron microscopy that resolve active-site dynamics under working conditions. This study highlights the integration of density functional theory (DFT) predictions with experimental validation to demonstrate the synergy between tailored coordination environments and catalytic activity. Finally, the existing challenges, such as the scalability of defect-engineered SACs and their long-term stability in acidic media, are discussed in the context of emerging catalytic materials. In addition, the opportunities in machine learning-guided design and plasma-enhanced synthesis of hierarchical N-doped carbons for electrode engineering are discussed.