Green hydrogen, by definition, must be produced with renewable energy sources without using fossil fuels. To transform the energy system, we need a fully sustainable production of green and renewable energy as well as the introduction of such "solar fuels" to tackle the chemical storage aspect of renewable energies. Conventional electrolysis of water splitting into oxygen and hydrogen gases is a clean and nonfossil method, but the use of massive noble-metal electrodes makes it expensive. Direct photocatalytic hydrogen evolution in water is an ideal approach, but an industrial scale is not available yet. In this paper, we intend to introduce flavins as metal-free organic photosensitizers for photoinduced reduction processes. Specifically, a flavin photosensitizer was employed for the photocatalytic evolution of hydrogen gas in aqueous media. The ratio of photosensitizer to cocatalyst concentration has been found to affect the efficiency of the hydrogen evolution reaction. Since flavins are nature-inspired molecules (like vitamin B2) with easily tunable properties through structure modification, this family of compounds opens the door for new possibilities in sustainable green hydrogen production.

Department of Advanced Materials Hannam University 70 Hannamro Daedeok Gu Daejeon 34430 Republic of Korea

Faculty of Chemistry Brno University of Technology Purkyňova 118 CZ 612 00 Brno Czech Republic

Linz Institute for Organic Solar Cells Institute of Physical Chemistry Johannes Kepler University Linz Altenberger Straße 69 4040 Linz Austria

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