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.
In the bionic leaf, the hydrogen gas is fed to a metabolically engineered version of a bacterium called Ralstonia eutropha. The bacteria combine the hydrogen with carbon dioxide as they divide to make more cells, and then—through a trick of bioengineering pioneered by Anthony Sinskey, professor of microbiology and of health sciences and technology at MIT—produce isopropanol (rubbing alcohol), which can be burned in an engine much like the gasoline additive ethanol.
“The advantage of interfacing the inorganic catalyst with biology is you have an unprecedented platform for chemical synthesis that you don’t have with inorganic catalysts alone,” says Brendan Colón, a graduate student in systems biology in the Silver lab and a coauthor of the Proceedings of the National Academy of Sciences paper (along with first authors Joseph Torella, a recent graduate of the department of systems biology, and Christopher Gagliardi, a postdoctoral fellow in FAS’s department of chemistry and chemical biology). “Life has evolved for billions of years to produce catalysts capable of making chemical modifications on complicated molecules with surgical precision, many times at room temperature,” Colón explains. “If you can use enzymes for building chemicals, you open the door to making many of the natural compounds we rely on every day,” such as antibiotics, pesticides, herbicides, fertilizer, and pharmaceuticals.