Photocatalytic H ₂ O ₂ Production from Seawater: Mechanisms, Challenges and Design Strategies

Photocatalytic H 2 O 2 production using solar energy has emerged as a sustainable alternative to the energy–intensive anthraquinone process. Compared with freshwater systems, seawater represents an abundant and geographically accessible resource, accounting for approximately 97% of the Earth's water reserves. However, its complex ionic composition poses significant challenges, including chloride–induced side reactions, catalyst corrosion, H 2 O 2 decomposition, interference from dissolved organic matter, and mass–transfer limitations. This review systematically summarizes recent advances in photocatalytic H 2 O 2 synthesis from seawater, with particular emphasis on the interfacial mechanisms governing catalyst–seawater interactions and corresponding material design strategies. The effects of major seawater constituents, including alkali and alkaline–earth metal cations, chloride ions, trace transition metals, and dissolved organic matter, are discussed in terms of charge separation, oxygen activation, reaction selectivity, and product stability. Representative photocatalytic systems, including metal–based semiconductors, carbon–based photocatalysts, MOFs, COFs, and emerging single–atom and plasmonic catalysts, are comparatively analyzed. In addition, advanced engineering approaches, such as heterojunction construction, ion modulation, defect engineering, hydrophobic microenvironment design, surface functionalization, and catalyst–electrolyte synergy, are highlighted. Finally, current challenges and future perspectives are outlined to facilitate the practical implementation of solar–driven H 2 O 2 production from natural seawater.