This can be done by a number of biological, biochemical, and synthetic methods.

In a biological approach, living organisms are used to decompose water into hydrogen and oxygen; thus the term biophotolysis. However, the liberation of molecular oxygen generally inhibits the activity of hydrogenases, the biological enzymes that produce molecular hydrogen. Thus, an important task for research in this area is to find species or mutations with hydrogenases that are effective in the presence of oxygen. One option is to produce the hydrogen and oxygen separately, as has been done with cultures of a blue-green alga. However, in all known cases of biophotolysis the rate of hydrogen production is extremely small, and great progress will be required before practical conversion schemes can be designed.

In a biochemical approach, enzyme systems would be obtained from biological organisms and then combined in an appropriate reaction cell to perform all the steps involved in collecting energy and driving the water-decomposing reactions. Production of hydrogen, at least at low rates for short periods of time, has been demonstrated, but more basic research on the biochemical mechanisms of photosynthesis, and inventive ideas for incorporating molecular components into systems, will be required for this technique ever to become practical.

In a synthetic approach, a complete chemical or electrochemical system for photolysis would be designed and synthesized without using any components taken from plants or algae. This has a great potential advantage in that problems of instability of biological components would be avoided. There are some promising ideas about the form such chemical systems might take, and some electrochemical systems have been demonstrated in the laboratory, but this must be considered a long-term research problem.

The achievement of a practical technology for photochemical conversion to produce fuels will depend on significant advances in our fundamental understanding of primary photochemical processes and the subsequent processes involved in the transfer of the energy of electronic excitation to stable chemical products. Experiments performed thus far on biochemical systems have suffered from very low efficiencies (often less than 0.1 percent) and from instability of the reactant systems. Some electrochemical systems have shown higher efficiencies, at least for short periods of operation. The difficulties to be overcome in the development of a practical technology of photochemical conversion are formidable. However, the potential rewards are great.

Theoretical considerations indicate that efficient photochemical processes should be possible. The attainable conversion efficiency might be on the order of 20–30 percent, based on incident solar energy.4041 With this efficiency, which is more than 10 times the efficiency probably to be



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