Harvard researchers have announced a method to turn sunlight and air into isopropanol (C3H8O). While mainly used as a solvent and in pharmaceuticals, isopropanol can also be used as a liquid fuel for transportation, or converted into even more convenient fuels.
Plants have been transforming sunlight into things that we can use for fuel for 1.6 billion years. However, with a few exceptions, they are still only about 1% efficient. Solar cells do much better, but the energy produced must either be used immediately or stored in ways that can be problematic.
To address this, researchers are trying to find ways to turn the energy generated from sunlight into fuels that are easy to store. In 2011, Professor Daniel Nocera announced what he called the “artificial leaf,” a silicon strip coated with catalysts on each side. When placed in water and exposed to sunlight, the leaf splits the H2O to release oxygen on one side and hydrogen on the other.
Last year, Nocera boasted of achieving 7% efficiency, but a problem remains. The fuel that Nocera's leaf produces is hydrogen, which is very expensive, and sometimes dangerous, to store and transport. As he then admitted to The New York Times, “If I give you a canister of hydrogen that we got from the artificial leaf, you can’t use it right away.” Nocera's hope was that progress would be made in the quest for fuel cells that turn hydrogen back into energy in a controlled way.
Now, however, Nocera is part of a project taking a very different approach. In the Proceedings of the National Academy of Sciences, Nocera and Professor Pamela Silver announce what they call a bionic leaf, one that combines Nocera's catalysts with bacteria that turn the hydrogen into something easier to handle.
"The catalysts I made are extremely well adapted and compatible with the growth conditions you need for living organisms like a bacterium," says Nocera. Silver added the bacterium Ralstonia eutropha, which uses the hydrogen and carbon dioxide from the atmosphere as food to multipy.
MIT Professor Anthony Sinskey has previously demonstrated that R. eutropha can be engineered to make isopropanol as it grows, generating a fuel we can use. When the isopropanol is burnt, it will release CO2 into the atmosphere, but no more than is removed by the bacteria in the growth process.
So far, however, less than a seventh of the hydrogen produced gets turned into fuel by the bacteria. Silver is not arguing that the product is ready for commercialization, saying, "This is a proof of concept that you can have a way of harvesting solar energy and storing it in the form of a liquid fuel.” Nocera is even more upbeat. "We're almost at a 1% efficiency rate of converting sunlight into isopropanol," he says. "There have been 2.6 billion years of evolution, and Pam and I working together a year and a half have already achieved the efficiency of photosynthesis."