Biological splitting

Several biotechnologies can produce green H2 from organic biomass:.
1. Dark fermentation (DF) uses organic matter as the substrate. It relies on the biological conversion of organic compounds (waste/effluents/biomass) into H2 and other valuable green biobased molecules. Contrary to other biological processes, DF is light-independent. The DF process presents the simplest reactor design and the easiest way of operation which reduce drastically the costs of production. DF is carried out by strict or facultative anaerobic bacteria, that release H2 during the fermentation of organic substrates. Although DF can be applied on most of the waste or effluents, carbohydrate-rich waste presents the highest performances and can be more easily used as feedstocks. Nonetheless, H2 yields in dark fermentation are limited by thermodynamics. It is now well known that microbial interactions and dynamics in dark fermentation can strongly impact process performances and better understanding of these interactions is a key factor to improve and stabilise fermentative hydrogen production.
2. Water-splitting photosynthetic processes: under certain conditions, green algae and cyanobacteria can be used to generate molecular hydrogen. Two type of processes exists: direct biophotolysis, when light is irradiated during hydrogen evolution, or indirect biophotolysis when light is not irradiated during hydrogen evolution. Direct biophotolysis is an attractive process since solar energy is used to convert a readily available substrate, water, to oxygen and hydrogen. Microalgae, such as green algae and Cyanobacteria (blue-green algae), containing hydrogenases, have the ability to produce hydrogen. The main drawback of direct biophotolysis is that the process is limited because of the strong inhibition of hydrogenase by the oxygen produced (it is necessary to maintain the oxygen content below 0.1%, which is very difficult without additional energy demand and cost). In indirect biophotolysis, cyanobacteria and microalgae produce hydrogen through photosynthesis, with oxygen and hydrogen production occurring at different times. This avoids enzyme deactivation and explosive gas mixtures, but also makes hydrogen purification easier. Biophotolysis, is an immature technology, applied only at laboratory scale.
3. Bioelectrochemical systems for hydrogen production. At lab-scale, microbial electrolysis cells can be divided into single and double chamber depending on the presence of an ion exchange membrane (IEM) that separates the anodic and cathodic compartments. Electroactive microorganisms grow as a biofilm on the anode whereas abiotic hydrogen evolution takes place at the cathode. These microorganisms can degrade organic matter and transfer electrons extracellularly, which travel from the anode to the cathode through the electric circuit and reduce protons on the cathode, thereby forming hydrogen.