Strong Electric Fields on Water Microdroplets Enable Near-Unity Selectivity in H₂O₂ Photosynthesis

Selective conversion of solar energy to chemical bonds remains a grand challenge in artificial photosynthesis. Though H₂O₂ production via photocatalytic two-electron oxygen reduction (2e^--ORR) offers a sustainable alternative to the energy-intensive anthraquinone process, competing hydrogen evolution reaction (HER) severely limits both efficiency and selectivity. Here, we reveal that the strong electric fields on water microdroplet surfaces serve as powerful selectivity switches, directing photogenerated electrons exclusively toward H₂O₂ synthesis while completely suppressing hydrogen evolution. This interfacial electric field control mechanism transforms ZnIn₂S₄-based photocatalysts─commonly dominated by HER─into H₂O₂ producers with near-unity selectivity and production rates 2 orders of magnitude higher than bulk reactions. Through spatially resolved spectroscopy characterizations and theoretical calculations, we elucidate that the high electric fields on water microdroplets simultaneously enhance charge carrier separation, lower energy barriers for 2e^--ORR, and erect kinetic barriers against HER. Beyond providing an energy-efficient route to selective H₂O₂ photosynthesis, this study offers valuable insights into selectivity control in other solar-to-chemical transformations without the need for catalyst modification or system engineering.