Photobiohydrogen Production and Strategies for H2 Yield Improvements in Cyanobacteria.

Hydrogen gas (H₂) is one of the potential future sustainable and clean energy carriers that may substitute the use of fossil resources including fuels since it has a high energy content (heating value of 141.65 MJ/kg) when compared to traditional hydrocarbon fuels [1]. Water is a primary product of combustion being a most significant advantage of H₂ being environmentally friendly with the capacity to reduce global greenhouse gas emissions. H₂ is used in various applications. It generates electricity in fuel cells, including applications in transportation, and can be applied as fuel in rocket engines [2]. Moreover, H₂ is an important gas and raw material in many industrial applications. However, the high cost of the H₂ production processes requiring the use of other energy sources is a significant disadvantage. At present, H₂ can be prepared in many conventional ways, such as steam reforming, electrolysis, and biohydrogen production processes. Steam reforming uses high-temperature steam to produce hydrogen gas from fossil resources including natural gas. Electrolysis is an electrolytic process to decompose water molecules into O₂ and H₂. However, both these two methods are energy-intensive and producing hydrogen from natural gas, which is mostly methane (CH₄) and in steam reforming generates CO₂ and pollutants as by-products. On the other hand, biological hydrogen production is more environmentally sustainable and less energy intensive than thermochemical and electrochemical processes [3], but most concepts are not yet developed to production scale.