LAUSR

Abstract Recent development on alternative route to bypass the bottle-neck of water oxidation, two-electron oxidation of water catalyzed by earth-abundant metalloporphyrins to form hydrogen peroxide, has been briefly reviewed. The three pioneering reports by Fujishima (Honda-Fujishima effect), Meyer (chemical oxidation of water), and Lehn (photochemical reduction of CO2) triggered the extensive studies on artificial photosynthesis to have been forming the modern history of artificial photosynthesis. The current situation, especially for artificial photosynthesis by molecular catalysts, however, has faced with bottleneck subject of photon-flux-density problem of sun light radiation. In this review article, three milestones in the modern history of artificial photosynthesis, photon-flux-density problem as the bottleneck subject in water oxidation, crucial viewpoint of renewable energy factor (REF) in designing artificial photosynthesis even in fundamental stage of studies are explained. Several approaches to minimize the energy cost in REF for molecular catalysts such as utilization of earth-abundant elements and synthesis of molecular catalyst in water at ambient temperature were also introduced. Recent development of two-electron oxidation of water to form hydrogen peroxide, which would uncover the blind point of the bottleneck subject, would induce a game change in the methodologies of artificial photosynthesis.