Hydrogen gas production & storage cycle with organic hydrides based on hydrogen fixing and release

Hydrogen gas is a promising clean energy source alternative to fossil fuels. However, hydrogen gas has low volumetric energy density and explosiveness, which pose challenges for storage and transport, limiting its practical use as an energy carrier. Organic hydrides, which are liquid (or solid) organic compounds that store hydrogen via chemical bonding under ambient conditions, have been researched as a promising hydrogen storage material for safer hydrogen storage and transport. Recently, electrolytic hydrogenation has emerged as a novel approach to hydrogen production and storage. This method allows hydrogen to be produced from water and stored directly in organic hydrides simultaneously, and can simplify multiple conventional energy-consuming processes (e.g., hydrogen gas production and purification). This paper reviews new hydrogen storage methods based on electrochemistry, photochemistry, and biochemistry to enable direct hydrogen storage in organic hydrides from water. Furthermore, this paper outlines hydrogen production and storage cycles, combining these hydrogen storage methods and hydrogen release methods. First, electrolytic hydrogenation, which stores hydrogen in organic hydrides applying water electrolysis, is discussed along with research to improve its efficiency. Next, transfer hydrogenation reaction applying water photolysis, which enables direct hydrogen storage in organic hydrides from water, is outlined. Finally, an innovative hydrogen storage method using alcoholic fermentation with yeasts requiring no metal catalysts and external energy is introduced. Overall, direct hydrogen storage methods from water to organic hydrides summarized in this paper have the potential to provide technologically effective and economically promising hydrogen production, storage, and transportation in a hydrogen energy society. • Direct hydrogen storage methods from water into organic hydrides were reviewed • Hydrogen production and storage cycles with organic hydrides were outlined • Hydrogen production methods via hydrogen fixing from water and release were proposed