Unraveling the catalytic mechanisms of defective carbons for selective 2e-/4e- oxygen reduction: from intrinsic defects to synergies with heteroatoms and single-metal atoms.

Selective oxygen reduction via either the two-electron pathway for distributed H2O2 electrosynthesis or the four-electron route for high-efficiency energy conversion represents a cornerstone of sustainable electrocatalysis, where defect engineering offers unprecedented opportunities for atomic-level site manipulation. This review comprehensively summarizes defect-centric functional active sites, encompassing intrinsic carbon defects, synergistic structures between heteroatom dopants and defects, and synergistic motifs combining metal single-atom sites with surrounding defect configurations. We systematically detail state-of-the-art synthetic methodologies, preparation protocols, and structural regulation strategies for these tailored active configurations. Concurrently, we deep-dive into the fundamental mechanisms by which these defect-engineered structures modulate the local electronic environment of carbon catalytic materials, alter the adsorption energetics of key oxygenated intermediates, and ultimately dictate ORR pathway selectivity. Beyond intrinsic catalytic properties, we further discuss how these atomic-scale design principles translate into zinc-air batteries, H2O2 electrosynthesis, environmental remediation, and hybrid electrochemical systems. Finally, we highlight current challenges and outline future prospects, emphasizing how integrating operando characterization techniques, data-driven computational modeling, and precise synthesis can bridge active-site identification, theoretical prediction, targeted construction, and device-level benchmarking toward the rational design of selective ORR electrocatalysts.