biohydrogen

Biohydrogen refers to hydrogen produced by biological systems, including microorganisms and enzymatic reactions. As a potential renewable energy carrier, biohydrogen can be generated from water and organic substrates using sunlight or anaerobic microbial metabolism. It is distinct from hydrogen obtained through fossil fuel–driven processes or non-biological electrolysis.

Production pathways are commonly categorized as photobiological and fermentative. In photobiological hydrogen production, cyanobacteria or green algae use photosynthesis to split water under light. The released electrons are transferred to hydrogenases or nitrogenases, producing hydrogen gas; yields can be variable and are influenced by oxygen presence, light intensity, nutrients, and culture conditions. A related route is photofermentation, where photosynthetic bacteria convert organic acids produced by other microbes into additional hydrogen under illumination.

Dark fermentation is another major route, in which anaerobic bacteria such as Enterobacter, Clostridium, or other fermenters decompose carbohydrates or organic wastes to produce hydrogen, carbon dioxide, and organic acids. Yields are often limited by byproducts, substrate quality, and the need to separate hydrogen from other gases during processing.

Bioelectrochemical approaches, including microbial electrolysis cells, couple microbial metabolism with an applied voltage to enhance hydrogen production, potentially increasing yields from low-cost substrates. Integrated or sequential processes aim to improve overall efficiency and flexibility, but technical and economic barriers remain.

Advantages of biohydrogen include the potential for low-carbon energy and valorization of waste streams; challenges include relatively low volumetric productivity, sensitivity of key enzymes to oxygen, the need for specialized bioreactors and gas separation, and competition with established hydrogen production technologies. Ongoing research focuses on enzyme stability, reactor design, substrate selection, and scalable storage of biogenic hydrogen.