Biohydrogen

Abstract

Hydrogen is being considered as a clean, renewable and ideal fuel of the future. Hydrogen has a high specific energy content (142kJg-1), and its combustion, which produces water, does not form greenhouse gases. Currently, the vast majority of hydrogen produced is from steam reforming of fossil fuels, hence implementation of a future hydrogen economy demands the development of renewable, sustainable hydrogen production. Biological hydrogen production could fulfill this mandate and a wide variety of microorganisms and metabolic pathways are available for biohydrogen production. Some organisms and pathways, chiefly green algae and cyanobacteria, can use captured solar energy to split water and generate hydrogen through either direct biophotolysis or indirect biophotolysis. Dark fermentation can use a variety of organic wastes to produce hydrogen and other by-products. Photosynthetic bacteria are also capable of using captured solar energy in a process called photofermentation where the captured energy is used as energy input to derive hydrogen from substrates, such as volatile fatty acids from dark fermentation effluents, which normally cannot be used to produce hydrogen. Finally, in an analogous manner, MECs (microbial electrolysis cells) use microbes to extract electrons from organic wastes at an anode, and additional energy (voltage) is used to augment the energy to a level where hydrogen can be evolved at the cathode. © 2019 Elsevier B.V. All rights reserved.