The ultimate green-energy source

IN HIS latest novel, “Solar”, Ian McEwan's protagonist is Michael Beard, a Nobel prize-winning physicist who has been resting on his laurels for two decades specialising in “after-dinner speeches and eulogies for retiring or about-to-be-cremated colleagues”. To reinvigorate his career, Dr Beard steals a postdoctoral scientist's discovery of artificial photosynthesis, described as “an engine running on nothing”, and tries to harness it.

That may be fiction, but it is not fantasy. Photosynthesis, the process by which plants convert energy from sunlight into chemical energy, is one of the most important chemical reactions on earth, so there are many efforts under way to try to replicate it. The process is, though, proving to be particularly difficult to emulate with any great efficiency.

During photosynthesis two “half-reactions” take place. First, sunlight is used to split water into hydrogen and oxygen. Plants do this with a complex molecular “photosystem” that uses the energy of sunlight to break apart water molecules, liberating electrons, hydrogen ions and oxygen. Then, in the second half-reaction, the electrons and hydrogen ions combine with carbon dioxide to create energy-rich carbohydrates, such as glucose, which plants use to grow.

It is the first half-reaction which provides the potential for an energy source. Sunlight can already be used to split water artificially with solar panels generating the electricity needed for a process called electrolysis. The hydrogen obtained from water can then be stored and used in, say, a fuel cell to produce power when the sun does not shine. But the process is not particularly productive, given the present output of solar panels, and it is costly.

Researchers have tried to mimic the process used by plants with experimental systems that use some of the components of photosynthesis, such as chlorophyll, which makes plants green and helps trap light, and catalysts which facilitate the process of converting carbon dioxide, water and sunlight. But some of the materials and catalysts have turned out to be unstable and inefficient in the laboratory.

Viral power

Now Angela Belcher of the Massachusetts Institute of Technology and her colleagues have succeeded in mimicking the first part of photosynthesis by using a genetically modified virus to help split water into hydrogen and oxygen. The researchers took a virus called M13, which normally infects bacteria and is harmless to humans, and engineered it to assemble on its coating both iridium oxide, one of the catalysts used by researchers, and light-sensitive biological pigments called zinc porphyrins, which act as an antenna to capture the light.

What the virus provides, says Dr Belcher, is a sort of scaffolding or frame around which the components used in artificial photosynthesis can attach themselves, and do so at the right distances from one another to trigger the water-splitting reaction. The arrangement provided by the virus greatly improves the efficiency of the reaction. In addition, the structure also assembles itself. There are problems. One is that viruses tend to clump together, which hampers their performance. To prevent this from happening, the team embedded the viruses into a microgel matrix, which preserved their light-collecting abilities.

In their research, recently published in Nature Nanotechnology, Dr Belcher and her colleagues concentrated on the more technically challenging production of oxygen, but they now hope to complete the process by recombining the hydrogen ions and harvesting hydrogen gas. This would provide a way of capturing sunlight, in much the same way as plants do, and then storing the energy in the form of hydrogen gas. The energy is then released when the hydrogen is burned or used in a fuel cell to make electricity.

The researchers are also seeking a catalyst that is cheaper than one based on iridium, which is relatively expensive. In a few years, Dr Belcher speculates, it may be possible to build a prototype machine to split water efficiently using sunlight. Meanwhile, they should watch out for any disgruntled physicists desperate to boost their careers.