Reverse Cycle: Inspired by Leaves, a New Invention Turns Sunlight and Water into Fuel

By Chelsea Leu

For the past ten years, Peidong Yang has been trying to make like a tree. Yang, a professor in UC Berkeley’s Department of Chemistry, researches artificial photosynthesis, a process that mimics a leaf’s ability to convert sun, water, and carbon dioxide into fuel. But in his case, the fuel isn’t glucose—it’s gasoline.

Last winter, Yang and his colleagues took a major step forward in achieving that goal. Their invention looks more like a small patch of moss than a leaf: It’s a papery green disk slightly bigger than a quarter. Yang and his colleagues describe it as a nanowire mesh made up of a network of nanometers-thick semiconductor wires that use the sun’s energy to break the chemical bonds in water. Dip one of these circles into a sunlit beaker of water, and hydrogen and oxygen come bubbling out.

Yang hopes these circles will one day use carbon dioxide emitted by automobiles and water from the atmosphere to create fuels such as methane, methanol, and butanol. The result would be a fuel system that’s both carbon neutral and self-sustaining—a vast improvement on our current system, which involves digging carbon out of the earth in the form of oil, coal, or gas, and releasing it as exhaust into the atmosphere, where it contributes to climate change. “We’re trying to reverse the chemical cycle,” Yang says. “If you could capture the CO2 and water and use this sort of artificial photosynthetic device, you could convert them back into gasoline.”

That’s the idea. Putting it into practice is a little trickier, partly because turning carbon dioxide and water into methanol is an “uphill” reaction (one that requires energy input to get going). Yang and his lab have spent countless hours figuring out how to harness the sun’s unlimited energy to power it. With his “moss,” Yang is getting close.

However, this latest breakthrough is only half of the solution Yang envisions because it only produces hydrogen. Yang and his team must still find an efficient way to split carbon dioxide molecules. It’s a crucial missing piece, since carbon dioxide is the source of carbon molecules that make up fuels like methanol and butanol.

Until that time comes, the hydrogen produced by Yang’s moss is still a viable fuel. Though hydrogen cell–fueled cars boast no harmful emissions, hydrogen gas is currently extracted through a process called methane reforming, which burns natural gas, making it far from carbon neutral. The hydrogen produced by Yang’s solar-powered device, by contrast, is emissions-free and relatively cheap to manufacture.

Still, when asked if he’s proud of the work his lab has accomplished, Yang demurs. “We’re making progress,” he says. “But we’re not there yet.”

From the Spring 2015 Dropouts and Drop-ins issue of California.
Filed under: Innovation
Image source: Courtesy of Peidong Yang
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Converting CO2 emissions back into fuels and creating a carbon-neutral system would be good, but why not just take that CO2 out of the atmosphere for good—and use Yang’s process to create hydrogen fuel? Then, we wouldn’t have to use methane reforming (which is not clean) to create hydrogen gas. Virginia Tech researchers Percival Zhang and Joe Rollin just announced a clean method for creating hydrogen fuels, but Yang’s disk method seems just as promising.

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