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.