Challenges in developing biohydrogen as a sustainable energy source: implications for a research agenda

The U.S. Department of Energy's Hydrogen Program aims to develop hydrogen as an energy carrier to decrease emissions of greenhouse gases and other air pollutants and reduce the use of fossil fuels. However, current hydrogen production technologies are not sustainable as they rely heavily on fossil fuels, either directly or indirectly through electricity generation. Production of hydrogen by microorganisms, biohydrogen, has potential as a renewable alternative to current technologies. The state-of-the-art for four different biohydrogen production mechanisms is reviewed, including biophotolysis, indirect biophotolysis, photofermentation, and dark fermentation. Future research challenges are outlined for bioreactor design, optimization of bioreactor conditions, and metabolic engineering. Development of biohydrogen technologies is still in the early stages, although some fermentation systems have demonstrated efficiencies reasonable for implementation. To enhance the likelihood of biohydrogen as a feasible system to meet future hydrogen demands sustainably, directed investment in a strategic research agenda will be necessary.