A few years ago, Daniel Nocera pioneered an "artificial leaf" that—just like the real thing—uses only the sun and water to produce energy. He touted the silicon cell as a breakthrough that could allow every home to become its own power station.

His compelling invention, a cheap wafer-thin device, attracted lots of publicity but hasn't quite taken off. The leaf works well, Nocera says, but there's a key flaw.

"The problem with the artificial leaf," Nocera says, is that "it makes hydrogen. You guys don't have an infrastructure to use hydrogen." (See related profile: "Daniel Nocera: Maverick Inventor of the Artificial Leaf.")

By "you guys," Nocera means the world outside the lab. Although Toyota and others companies are making cars built to run on hydrogen, emitting only water vapor, filling up is a problem: Most gas stations are set up to serve liquid fuel.

Storing the Sun

Enter Nocera's latest creation, a collaboration with biologists at Harvard University and detailed in the Proceedings of the National Academy of Sciences Monday. The researchers created a specially engineered bacteria that can convert hydrogen (from the artificial leaf or another source) into alcohol-based fuel.

The Harvard researchers are aiming to solve a problem known to any electric utility: Capturing energy from the sun has come a long way, but how can it be stored for times when there's no sunlight? Going a step further, how can that stored energy be used for purposes other than electricity?

In natural photosynthesis, biomass is produced when sunlight meets with water and carbon dioxide. Another step is typically required to turn that biomass into fuel—breaking down corn to make ethanol, for example. (Take the quiz: "What You Don't Know About Biofuel.")

Instead, the researchers made a genetically modified bacterium that could bypass the biomass step and go straight to producing liquid fuel. Using the artificial leaf, they split water into oxygen and hydrogen. The special bacterium absorbed the hydrogen, combining it with carbon dioxide to produce isopropanol: an alcohol fuel comparable to ethanol.

The resulting system would look like an algae farm, Nocera says, except that the bacteria wouldn't need the continuous light or maintenance that algae require.

"Shocking" Finding

Nocera says his team also solved a problem that long bedeviled researchers working with bacteria and solar energy: The bacteria die. Keeping them alive requires high-voltage current, making the process far less efficient.

The culprit was known to be a type of molecule called reactive oxygen species, but the surprise lay in where they were coming from. When water split, the reactive oxygen species were coming out of the hydrogen side of the water splitting, not the oxygen side.

"We were shocked," Nocera says. "That confused us for a while." By pinpointing that problem, the Harvard researchers were able to produce fuel much more efficiently.

Despite the streamlined process, Nocera's system has quite a way to go before it's filling gas tanks.

John Turner, a research fellow who works on hydrogen energy at the National Renewable Energy Laboratory, says the paper is "some very excellent science," but cautions that the Harvard researchers "are a long, long way from showing any commercial viability."

Aside from the energy needed to grow the microbes and eventually extract the fuel, Turner says, any system that needs carbon dioxide must get it from the atmosphere to be sustainable. "That," he says, "will be a very energy-intensive process."

Nocera acknowledges that his system needs to become more efficient. To start running it as an industry, he says, "we'd still have to do more science."

Still, Nocera says, the paper contains advances that apply to work other scientists are doing: "There's a lot of neat science in here that people will now be able to build on."

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