Artificial leaves have been shown to harness solar energy and turn it into hydrogen for use in fuel cells. But cars and other vehicles still predominantly rely on liquid fuels. With this in mind, researchers have designed a bionic leaf that not only uses solar energy to produce oxygen and hydrogen, it feeds this hydrogen to bacteria that is then engineered to make isopropanol.
Isopropanol is an alcohol molecule that can be used as fuel, similar to ethanol or gasoline, and can be separated from water using salt. The technology was created by a team from Harvard University’s Faculty of Arts and Sciences, Harvard Medical School and the Wyss Institute for Biologically Inspired Engineering at Harvard University.
This research builds on the work of Professor of Energy Daniel Nocera and his ‘artificial leaf’ first demonstrated in 2011. Professor Nocera’s ‘leaf’ was made from a silicon strip coated with catalysts on each side. When placed in water and exposed to sunlight, the strip split the water molecules to release and oxygen and hydrogen. ‘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,’ said researcher Pamela Silver. ‘[Professor Nocera’s] formidable discovery of the catalyst really set this off, and we had a mission of wanting to interface some kinds of organisms with the harvesting of solar energy.
‘It was a perfect match.’ The Harvard leaf produces oxygen and hydrogen in the same way as Professor Nocera’s, but the hydrogen is then channelled through a chamber filled with a bacterium called Ralstonia eutropha. An enzyme in this bacteria takes the hydrogen back to protons and electrons, and these are combined with carbon dioxide within the same chamber. The researchers then extract this bacteria, with the protons, electrons and carbon dioxide and metabolically engineers it to make isopropanol. This engineering method was discovered by Anthony Sinskey, professor of microbiology and of health sciences and technology at MIT. ‘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,’ said Brendan Colón, a graduate student in systems biology and co-author of the paper.