Redefining Water Oxidation Selectivity Through Electrolyte Microenvironment Engineering.

The two-electron water oxidation reaction (2e- WOR) offers a sustainable route to in situ H2O2 production from water, distinct from the anthraquinone process and direct H2/O2 synthesis. Its central challenge is the competition between H2O2 formation and the four-electron oxygen evolution reaction (OER), governed by water polarization, hydroxyl-intermediate formation, O─O bond construction, peroxy-species desorption, and product stability. Because catalyst-site regulation alone rarely optimizes selectivity, stability, and productivity simultaneously, the electrolyte microenvironment becomes a critical reaction variable. Through specific ion adsorption, electric-double-layer reconstruction, local pH control, solvation and hydrogen-bond-network regulation, and interfacial electric fields, electrolytes reshape intermediate configurations, proton-electron transfer barriers, and H2O2 decomposition. This Review introduces the framework of "electrolyte-encoded reaction pathways" to describe how electrolyte-derived interfacial states differentially regulate competing H2O2-forming channels. It distinguishes direct surface-mediated 2e- WOR, electrolyte-assisted direct 2e- WOR, and indirect electrolyte-mediated anodic H2O2 synthesis, thereby separating pathway regulation from reaction-network reconstruction. By linking electrolyte descriptors and catalyst-electrolyte coupling to selectivity, H2O2 stability, and system-level performance, this framework redefines the electrolyte as an active variable in reaction-network design.