Decouple H₂O₂ Electrosynthesis From Microenvironmental Decomposition via Atomic Site Density Engineering

Electrocatalytic H₂O₂ 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 H₂O₂ reduction reaction (HPRR) and disproportionation reaction (HDR). Here, we show that active atomic density engineering governs this microenvironmental loss and can secure the net H₂O₂ output. Using a series of defect-rich Co-N₂O₂ 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 H₂O₂ 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 H₂O₂ from secondary dissociation. The resulting catalyst reduces secondary H₂O₂ 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.