Cost-effective H₂O₂ electrosynthesis in seawater for ballast water treatment

H₂O₂ produced from electrochemical oxygen reduction in seawater offers a sustainable approach for ballast water treatment. However, efficient H₂O₂ electrosynthesis in seawater remains a great challenge due to Cl⁻ adsorption and unsuitable interfacial microenvironment. Here we report cost-effective H₂O₂ electrosynthesis in seawater for microbial inactivation of ballast water via enhancing chlorine resistance, active hydrogen (*H) generation and O₂ adsorption on F-doped CNTs (F-CNTs). The F-CNTs show impressive performance for H₂O₂ electrosynthesis with Faradaic efficiencies of 94.1-96.3 % and H₂O₂ concentration of 3.8 wt.% (0.86 min) in simulated and natural seawaters at 1000 mA cm^-2. The estimated cost for H₂O₂ production on F-CNTs is much lower than the market price. The H₂O₂ in situ generated on F-CNTs (600 mA cm^-2, 1 min) is highly efficient for the inactivation of bacteria (10⁷-10⁸ cfu mL^-1) and algae (2.5-7.5 × 10⁶ cells mL^-1) in ballast water. The structures of bacteria and algae are destructed in <1 min. The superior performance was attributed to that the negatively charged F-CNTs is efficient for Cl⁻ repulsion, while simultaneously boosting *H generation from water dissociation and O₂ adsorption as well as creating alkaline microenvironment for H₂O₂ electrosynthesis.