Hydrogen gas provides exceptional value as an energy carrier and industrial feedstock, but currently is produced entirely by reforming fossil fuels. Biological hydrogen production (BioH2), which offers the possibility of being renewable and carbon neutral, can be achieved by photosynthesis, fermentation, and microbial electrolysis cells. This review introduces the principles, advantages and challenges of each approach to BioH2. Photosynthetic BioH2 is the ultimate renewable source, since it directly uses inexhaustible resources: sunlight energy and electrons from H2O. However, it presents major technical challenges, particularly due to oxygen sensitivity. Fermentative BioH2 offers a high production rate, but poor conversion efficiency from the organic substrate to H2. The microbial electrolysis cell can achieve high conversion efficiency, but is an emerging technology.
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