Surfactant headgroup charge density governs the interfacial microenvironment for selective H 2 O 2 electrosynthesis

Electrolyte engineering effectively steers reaction selectivity in electrochemical H 2 O 2 synthesis via the two‐electron oxygen reduction reaction (2e − ORR), yet rational additive design remains challenging due to limited understanding of how molecular structure governs interfacial microenvironments. Herein, we establish the headgroup charge density of anionic surfactants as a key descriptor for 2e − ORR selectivity. Systematic comparison of four dodecyl‐chain surfactants (sodium dodecyl sulfate, SDS; sodium dodecyl benzene sulfonate, SDBS; sodium dodecyl sulfonate, SDSO; sodium monododecyl phosphate, SMDP) reveals a selectivity order (SMDP > SDSO > SDBS > SDS) that correlates directly with headgroup charge density and K + binding strength. Multiscale investigations show that high‐charge‐density headgroups participate in K + solvation, displacing water and rigidifying the interfacial hydrogen‐bond network, creating a kinetic bottleneck for proton transfer that suppresses *OOH over‐reduction. Consequently, the SMDP‐modified electrolyte achieves 98% Faradaic efficiency at 200 mA cm −2 in a flow cell with stable 44 h operation.