'Synthetic' Leaves: The Energy Plants of the Future?

Artist's conception of bacteria — An artist's conception of bacteria wrapping themselves around nanowires to feed on their electrons in the University of California, Berkeley, natural-synthetic photosynthesis system.

(Image credit: University of California, Berkeley)

Alan Brown, writer and blogger for The Kavli Foundation contributed this article to Live Science's Expert Voices: Op-Ed & Insights.

Imagine using plants to grow the natural gas that heats homes and the gasoline that powers cars. People could store it this form of solar energy in cars' fuel tanks, distribute it through pipelines, and buy it in gas stations. And everyone could use it without adding a single molecule of the greenhouse gas carbon dioxide (CO2) to the atmosphere.

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By marrying nanoscience and biology, Yang and his colleagues created a biologically inspired, but completely artificial, system that converts the sun's rays into fuel and chemicals. The system uses long, nanoscale filaments to turn sunlight into electrons, which bacteria use to convert carbon dioxide and water into butanol fuel and more complex molecules such as acetate, a chemical building block, and amorphadiene, which is used to make antimalarial drugs.

Thomas Moore: That's a good question. In order to create sustainable, solar-driven societies, we need a way to store solar energy. With solar cells, we can make electricity efficiently, but we cannot conveniently store that electricity to use when it is cloudy — or at night. If we want to stockpile large quantities of energy, we have to store it as chemical energy, the way it is locked up in coal, oil, natural gas, hydrogen and biomass.

[Atmospheric] CO2 levels surpassed 400 parts per million this year. If they reach 500 or 600 parts per million, the environmental impact is going to be severe. We will need some form of carbon capture and storage. This leads right into Peidong's system, because it could remove copious amounts of CO2 from the atmosphere, use some for fuel, and make carbon rocks out of the excess. In that way, it could reduce atmospheric CO2 to pre-industrial levels.

We used semiconductors to capture solar energy and generate current. We used the current to energize two catalysts — materials that speed up chemical reactions without actually taking part in them. One catalyst reduced, or added electrons to, CO2, and the second oxidized [took electrons from] water to produce oxygen, which is what happens in natural photosynthesis. The synthetic CO2 catalysts were the problem, because they were simply not very efficient.

T.S.: Peidong's right to put the focus on exactly this question: What can biology teach us about making fuels? His model system makes it possible to explore some really important physics and chemistry. This is not about mimicking nature directly or literally. Instead, it is about learning nature's guidelines, its rules on how to make a compellingly efficient and selective catalyst, and then using these insights to create better-engineered solutions.

T.S.: Nature has figured out efficient CO2-to-liquid-fuel catalysts. We have not yet managed to do that. In particular, as Peidong noted, we need high selectivity to make the product we want without undesired side products. We also need catalysts that convert chemicals quickly, and without making us pay an energy penalty for their high throughput. Finally, nature builds catalysts using abundant materials. On all these fronts, nature has us beat. But it is also exciting, because nature proves it's possible. This is a problem that has been solved before.

P.Y.: That relates to the one key requirement of the original design: We want transfer electrons from our semiconductors to our S. ovata bacteria, which act as our CO2 catalysts. To do that, we want the highest possible surface area, so that we put more bacteria in contact with the semiconductors and reduce more CO2. Nanowires do that because they extend upwards, like trees. They create a forest, and you can squeeze a lot more bacteria into a three-dimensional forest than onto a two-dimensional flat surface.

T.M.: Sure. The ways in which we can use synthetic biology to reengineer things is almost unimaginable. Already, Pete Schultz at Scripps Research Institute has bacteria that run on 21 amino acids, one of which is completely new. The bacteria have been programed with all the genetic material and information necessary to copy this unnatural amino acid and include it as part of its metabolism. And right there at Berkeley, you've got Jay Keasling. He has bacteria that can make almost anything from acetate.

It is a tool that brings together different researchers who frankly have a hard time talking with each other. In a room of people who study enzymes — proteins that serve at nature's catalysts — and people who research synthetic heterogeneous catalysts, the systems are so different, it can be hard to know where to start the conversation. Computational material science helps us learn from each other about how nature's catalysts differ from the ones we build artificially.

We also need to learn how to make light harvesting systems from materials that are not costly, toxic or energy-intensive to make. Nature synthesizes those materials at room temperature, with very low energy costs, and they use coherence effects to move energy efficiently over long distances to centers where reactions take place. I'm very excited to work on robust, biologically inspired energy transport.

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