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Biohydrogen
Hydrogen (H2) has several attractive features as a fuel: (i) it has a high energy to mass ratio, (ii) its combustion yields only water vapour and no pollutant, (iii) it can be stored as a metal hydride which makes it safe, portable and usable as a transport fuel, and (iv) it can be produced using renewable energy sources similar to solar electricity and photosynthesis. However, the chief limitation of H2 as a fuel is the back of a cost-competitive technology for production and utilization of H2. In addition, this is the only gas which can escape from the atmosphere into the space and be lost from the earth. At present it may be regarded as an objective of long-term research and development, although H2 fuel cells may be approaching the commercial stage.

H2 can be produced by several methods, including (i) electrolysis if water, (ii) gasification of biomass (reaction of biomass with steam) and (iii) by biological agents, e.g. bacteria and algae. When H2 is generated by biological means it is, for convenience, referred to as biohydrogen. Biohydrogen is produced mainly by the following two routes: (i) during anaerobic fermentation and (ii) by protolysis of water. In addition, (iii) an in vitro system created by coupling together the photosynthetic unit (say, from algae/green plants) and hydrogenase (from bacteria like Clostridium sp.) is a potent H2 generating system.

Anaerobic bacteria

Anaerobic bacteria oxidise the substrate by reducing NAD+ to NADH. But for a continued substrate oxidation it is essential to remove NADH from the cell environment and regenerate NAD+. The electrons from NADH are transferred to H+ ions to produce H2 gas, thereby regenerating NAD+; the reaction is catalyzed by the enzyme hydrogenase. If H2-producing bacteria are grown in the absence of H2-utilizing species of bacteria, H2 accumulates and can be collected. The substrate for such a digestion can be any biodegradable organic material including cellulose that has first been hydrolysed (enzymatically or chemically).



Some microscopic algae also produce H2 when exposed to, particularly low levels of sunlight. The photosynthetic apparatus splits water molecules into H2 and O2 most likely as follows. The photosystem I produces reduced ferredoxin, the ferredoxin is then reoxidized and protons (H+) act as electron acceptors to produce H2. The efficiency of H2 production is reasonable at low light intensities (about 15%) of the energy is stored as chemical energy in the form of H2). But a higher and more realistic light intensities, the efficiency is much lower. It is imperative that this problem is suitably solved for making this route of H2 production of commercial interest.

In vitro Photosynthetic-Hydrogenase System

The photosynthetic apparatus of higher plants, i.e. chloroplast, and the hydrogenase produced by H2-producing bacteria have been combined in an in vitro system to produce H2 from water using solar energy. The photosystem I generates H+, e- and O2 from H2O while hydrogenase combines e- and H+ to yield H2. This system functions and produces H2 but is unstable due partly to the sensitivity of hydrogenase to the O2 produced the photo-system I component of the arrangement. Further research is expected to resolve the instability problems; if this does happen, this H2 production system may develop into an economically attractive process.

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