Thinking Green

The article below is from New Scientist Magazine, Nov. 11, 2006. I have always thought this was the way to go since several millions of years of evolution have already done the research on the best way to trap sunlight for the production of energy. In fact, I don't see why we need to find synthetic mimics of the real thing. We should be able to use the actual product - deep sea algae for instance - grown in a medium such as agar agar. The trick would be in finding a way to convert the plant's energy into electricity or other forms of energy with producing greenhouse gases. We already use this green energy producer by way of burning wood or coal to produce heat which we then convert into electricity. Perhaps a simple combustion system which burns compacted algae so completely that very little particulate waste is produced and carbon dioxide could be reused to "fertilize" the algae would be possible.

Enlarge image

Solar fuel cell

IN JUST one hour enough solar energy reaches Earth's surface to meet all of our energy needs for an entire year. If we could come up with a way to harness this sunlight efficiently, it would solve all our energy problems at a stroke.

The trouble is: we are barely even beginning to exploit the full potential of solar energy. Solar cells only utilise a narrow range of frequencies, which means that even the most efficient and expensive cells typically convert only 17 per cent of the energy falling on them into electricity. Cheaper cells made of polymer coated with a thin film of titanium dioxide fare even worse, with an efficiency of just 10 per cent.

There is one notable exception to this inefficiency, however, a light-capturing technology that has the potential to revolutionise the rest of the field: plants. By developing synthetic versions of the pigment structures plants use to exploit even the faintest light levels, researchers believe they can make solar cells more efficient.

Some deep water algae, for example, have adapted to the tiny amount of sunlight they receive by growing rod-like structures called chlorosomes that contain thousands of light-harvesting pigment molecules. "They trap up to 97 per cent of the available photons," says Teodor Silviu Balaban of the Institute of Nanotechnology at the University of Karlsruhe in Germany.

In fact, all plants use similar "antennae" comprising stacks of pigment molecules, although most green leaves are more like 30 to 40 per cent efficient. Now Balaban and others are building artificial versions of these light-harvesting antennae. They plan to incorporate these into a new kind of solar cell, to develop photovoltaics that can absorb a greater amount of the available sunlight. Although they are unlikely to achieve the 97 per cent absorption rate of deep-water algae, the researchers hope to significantly improve on existing solar cells.

As a first step, a team led by Max Crossley at the University of Sydney in Australia has developed an antenna made up of synthetic porphyrins, a class of pigment molecules. The synthetic porphyrins absorb light across a broad range of frequencies, and more than 100 of the molecules can be assembled around a branching scaffold to mimic plant antennae, says Crossley. However, at the moment this process involves painstakingly constructing the antennae bond by bond, which is a slow process.

Much better, says Balaban, would be to find a way to make the antennae assemble themselves. One option for this is to use a technique called DNA origami to manipulate the molecules. This exploits the way that the complementary base pairs of DNA stick together, which is how the two sides of the double helix zip together. "It's been demonstrated that you can build 3D structures using DNA," says Rudy Diaz at the Arizona State University in Tempe, whose team has just received $1.1 million from the US National Science Foundation to develop the technique. By binding synthetic porphyrin molecules to unpaired DNA strands, and combining different strands, the team hopes to assemble 3D scaffolds containing large numbers of pigment molecules.

Balaban's group, in contrast, has gone a step further in mimicking plants, by uncovering the way antennae are constructed in nature. "I have copied nature's design for self-assembly," he says. The team studied natural plant antennae to identify how the pigment molecules bind together to create a stack. They were looking for the recognition groups - molecules that are attracted to porphyrins and so act like glue between the pigment molecules to bind them together. They identified a number of groups, including zinc, that form strong bonds with the pigment molecules, and are the key to antenna self-assembly. "They are like keys searching for their locks," says Balaban.

When the group began trying to construct artificial antennae by exposing the porphyrins to these molecules, they had no idea if the technique would work. "It was a gamble," he says. But it paid off: sure enough, the molecules bound together to form antennae. What's more, these antennae fluoresce when exposed to light, demonstrating that they are absorbing photons.

The cigar-shaped stacks can be tuned to harness specific frequency ranges of light by adjusting their size, says Balaban. To absorb light across the visible spectrum, the stacks typically need to be about 100 nanometres long, he says.

Balaban is now attempting to attach his antennae to a film of titanium dioxide, a vital step if the structures are to form the basis for low-cost thin-film solar cells. He also hopes to incorporate the light-harvesting structures in other existing types of solar cell, by coupling them to different semiconductor materials.

So how would the pigment molecules supply useful energy? In plants the light energy is transferred to a specialised chlorophyll molecule called P680, which releases a high-energy electron that can be used to reduce carbon dioxide to sugars. In a solar cell, the porphyrin molecules would transfer the photons to the semiconductor, where they would each knock loose an electron. Then, just as in a conventional photovoltaic device, these electrons would be corralled to generate a current.

However, improving the efficiency of solar cells by adding molecular antennae will not on its own turn solar power into a major energy source. What is also needed is a way to store solar energy so that it can be used at night or transported on demand, says Daniel Nocera at the Massachusetts Institute of Technology.

Nocera is working with Nathan Lewis at Stanford University in California to address this issue. They too are building photon-absorbing antennae, but rather than generating electricity, they plan to use them to produce hydrogen, which is more easily stored. In their version, the electrons liberated by the photons are used along with a catalyst to split water into hydrogen and oxygen (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0603395103). The hydrogen would then be stored for later use in fuel cells, says Nocera. The entire process would be closer to the way photosynthesis splits water into oxygen and hydrogen ions.

The idea of using renewable energy sources to produce hydrogen is not new, but by combining the two steps in a single device, the team believe they can not only increase the efficiency of the solar cells, but also eliminate the losses caused by transmitting the electricity to a separate device for splitting water. "This is integrating storage into photovoltaics," says Nocera.

Bringing the two steps together will take time, he admits, but if the team can marry plants' light-gathering ability with their talent for using this energy to produce chemical fuel, it will be worth the wait.

From issue 2577 of New Scientist magazine, 11 November 2006, page 30-31