James Weifu Lee and Elias Greenbaum[3]

Chemical Technology Division, Oak Ridge National Laboratory,
Oak Ridge, TN 37831-6194

Present energy systems are heavily dependent on fossil fuels. This will eventually lead to the foreseeable depletion of fossil energy resources and, according to some reports, global climate changes due to the emission of carbon dioxide. In principle, hydrogen production by biophotolysis of water can be an ideal solar energy conversion system for sustainable development of human activities in harmony with the global environment. In photosynthetic hydrogen production research, there are currently three main efforts: (1) direct photoevolution of hydrogen and oxygen by photosynthetic water splitting using the ferredoxin/hydrogenase pathway, (2) dark hydrogen production by fermentation of organic reserves such as starch that are generated by photosynthesis during the light period, and (3) Two-stage hydrogen production in a combined fermentative and light-driven algae/bacteria system. In this chapter, the advantages and challenges of these approaches for hydrogen production are discussed in relation to a new opportunity brought by our recent discovery of a new photosynthetic water-splitting reaction [Nature, 373, 438-441 (1995); Science, 273, 364-367 (1996)], which, theoretically, has twice the energy efficiency of conventional water splitting via the two-light-reaction Z-scheme of photosynthesis.

Hydrogen is a versatile, clean, and environmentally acceptable energy carrier. It can be produced by photolysis of water, an inexpensive and inexhaustible raw material. Photolysis can be performed using either inorganic systems such as semiconductors or living organisms such as cyanobacteria or green microalgae. It is now clear that green algae are probably better for H2 production than cyanobacteria, since the latter use the more energy-intensive enzyme, adenosine triphosphate (ATP)-requiring nitrogenase, for production of H2. Based on a recent feasibility analysis (7), H2 production by green algae can be more cost-effective than semiconductor photovoltaic electronics. The discussion in this article is focused on H2 production by photosynthetic water splitting.