However, the chief limitation of H₂ as fuel is the lack of a cost competitive technology for production and utilization. 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.

H₂ can be produced by several methods, including

(i) electrolysis of 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 referred to as biohydrogen. Biohydrogen is produced mainly by the following two routes:

(i) during anaerobic fermentation and

(ii) by photolysis 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 Hz generating system.