THERE IS MORE than enough energy in sunlight to break apart molecules of water. But it's hard to get sufficient light energy into water molecules quickly enough to split hydrogen from oxygen. Nature's answer is photosynthesis -- what green plants have been doing for millennia.
Not only does photosynthesis supply oxygen by splitting water, it also provides energy for the plant. It does this by trapping the energy contained in sunlight and using it to convert carbon dioxide and water into oxygen gas and energy-rich substances called carbohydrates. "If we fully understood photosynthesis," says James Norris, a chemist at Argonne National Laboratory in Illinois, "it might be possible to build solar chemical factories to make food and fuel faster and with higher overall quality than nature can."
One promising approach for achieving highly efficient solar conversion at a low cost is to imitate photosynthesis using synthetic molecular materials that absorb sunlight, separate electrical charge and speed up fuel-forming reactions. Enough is now known about charge transport and light absorption to tackle the synthesis of rugged, efficient molecular assemblies for solar conversion to electricity or to fuels.
To break down water molecules, the cells in a plant (or an artificial cell imitating the process) must capture the sun's light long enough to put it to work. Water itself is largely transparent to sunlight. The energy passes straight through it and very little is absorbed. Plants use green chlorophyll as their energy-trapping pigment.
Researchers working on artificial photosynthesis favor pigments that are called porphyrins, synthetic cousins of chlorophyll and hemoglobin (the pigment that gives red blood cells their color). When sunlight strikes porphyrins, the pigments release electrons. To keep these freed electrons from falling back into place among the pigment's atoms, special electron-attracting molecules shunt them away. In effect, this mechanism generates an electric current which provides the energy for certain chemical reactions.
One such reaction is the production of hydrogen gas from water. In the presence of a suitable catalyst responsible for making the reaction go quickly, water is split into charged fragments of hydrogen and oxygen. The addition of electrons to the positively charged hydrogen fragments produces hydrogen gas.
Much research into artificial photosynthesis is focused on devising better catalysts. The most commonly used -- all based on platinum -- are expensive and difficult to prepare; but scientists have recently come up with experimental non-platinum-based alternatives.