Biochemist Daniel Nocera has been working for decades on an “artificial leaf” — a silicon wafer that can create energy from sunlight and water the way real leaves do.
HARVARD SCIENTISTS have created a “bionic leaf” that converts solar energy into a liquid fuel. The work—a proof of concept in an exciting new field that might be termed biomanufacturing—is the fruit of a collaboration between the laboratories of Adams professor of biochemistry and systems biology Pamela Silver at Harvard Medical School (HMS) and Patterson Rockwood professor of energy Daniel Nocera in the Faculty of Arts and Sciences (FAS). The pair, who began collaborating two years ago (Nocera came to Harvard from MIT in 2012), share an interest in developing energy sources that might someday have practical application in remote locales in the developing world. Silver dubbed the system “bionic” because it joins a biological system to a clever piece of inorganic chemistry previously developed by Nocera: that invention, widely known as the artificial leaf, converts solar energy into hydrogen fuel.
Nocera’s artificial leaf, which serves as the fuel source in the bionic leaf, works by sandwiching a photovoltaic cell between two thin metal oxide catalysts. When submersed in a glass of water at room temperature and normal atmospheric pressure, the artificial leaf mimics photosynthesis. Current from the silicon solar wafer is fed to the catalysts, which split water molecules: oxygen bubbles off the catalyst on one side of the wafer, while hydrogen rises from the catalyst on the wafer’s other side. Nocera has been perfecting the artificial leaf since he first demonstrated it in 2011; today, it is far more efficient than a field-grown plant, which captures only 1 percent of sunlight’s energy. He says he can reach efficiencies of 70 percent to 80 percent of the underlying solar-wafer technology, which is improving constantly.
The hydrogen it produces is a versatile fuel from a chemical standpoint, Nocera reports, and could easily become the basis of a fuel cell, but it has not been widely adopted, in part because it is a gas. Liquid fuels are much easier to handle and store, hence the new bionic leaf’s importance
Here’s how the leaf works:
You stick silicon wafer in a quart of water, and it splits the hydrogen and oxygen. The hydrogen is collected and stored in a fuel cell. A single leaf in a quart of water can provide 100 watts of energy, 24 hours a day.
Using an electro-chemical process similar to etching, Nocera and colleagues have developed a system of patterning that works in just minutes, as opposed to the weeks other techniques need.
Dubbed reactive interface patterning promoted by lithographic electrochemistry, or RIPPLE, the process can be so tightly controlled that researchers can build photonic structures that control the light hitting the device and greatly increase its efficiency. The new system is described in two papers that appeared in recent weeks in the Journal of the American Chemical Society and the Proceedings of the National Academy of Sciences.
But Nocera’s new leaf will work in dirty water, too. It’s self-healing, meaning it prevents bacteria from forming a foothold on its surface.
The project was one of seven research efforts supported in the inaugural year of Harvard President Drew Faust’s Climate Change Solutions Fund.
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