Decouple H ₂ O ₂ Electrosynthesis From Microenvironmental Decomposition via Atomic Site Density Engineering

Electrocatalytic H 2 O 2 production through two‐electron oxygen reduction reaction (2e − ‐ORR) offers a promising route to decentralised chemical synthesis and water purification. However, Faradaic efficiency typically falls short of intrinsic selectivity due to competing H 2 O 2 reduction reaction (HPRR) and disproportionation reaction (HDR). Here, we show that active atomic density engineering governs this microenvironmental loss and can secure the net H 2 O 2 output. Using a series of defect‐rich Co‐N 2 O 2 single‐atom catalysts (SACs), closely spaced sites induce inter‐site electronic coupling and broaden the d‐band, which promotes further reaction with the freshly produced H 2 O 2 toward reduction and disproportionation. Isolating the sites at an optimal Co loading of 0.84 wt.% (Co/NOC‐0.8) effectively suppresses side reactions by increasing the thermodynamic barrier to OH*/O* formation, protecting H 2 O 2 from secondary dissociation. The resulting catalyst reduces secondary H 2 O 2 consumption by up to 79% and closes the gap between intrinsic selectivity (86.5%) and practical Faradaic efficiency (78.7%). When integrated into a flow‐through bilayer electrified membrane reactor, the optimised catalyst combines convection‐enhanced mass transport with rapid product release at the isolated sites, achieving over 90% single‐pass removal of aniline for more than 72 h. These results define a site‐density principle for balancing product formation and product preservation in single‐atom electrocatalysis.