Cation‐Mediated Interface Engineering Directs Electrogenerated Reactive Oxygen Species Pathways

Persistent organic contaminants are increasingly accumulating in saline waters, where high ionic strength and complex electrolytes hinder oxidative remediation. Electrogenerated reactive oxygen species (ROS) offer a promising route for pollutant degradation, yet the role of electrolyte cations in controlling ROS pathways remains poorly understood. Although singlet oxygen ( 1 O 2 ) is more persistent and matrix‐tolerant than hydroxyl radicals (•OH), whether cation‐induced changes in interfacial water structure govern ROS selectivity remains unclear. Here, using atomically dispersed Cu on Fe 2 O 3 ‐carbon nanotubes (Cu 1 /Fe 2 O 3 ‐CNTs) as a dual‐ROS electrocatalyst, we show that alkali‐metal cations exert distinct effects on ROS generation. •OH production follows the expected cation‐size‐dependent trend, whereas 1 O 2 generation exhibits an anomalous maximum in Na + ‐containing electrolytes, leading to superior contaminant degradation. Quantitative ROS measurements, in situ electrochemical analysis, spectroscopy, and theoretical calculations show that this behavior is primarily governed by selective stabilization of the *OOH intermediate rather than enhanced O 2 adsorption. Na + promotes a highly interconnected interfacial hydrogen‐bond network that stabilizes *OOH and favors 1 O 2 formation. In natural seawater, this regulation enables >98.2% contaminant removal within 8 h at an estimated energy demand of 0.779 kWh m −3 . These findings identify electrolyte cations as important regulators of ROS selectivity and provide a basis for efficient saline‐water decontamination.