Liquid Fuel From Bionic Leafs

Harvard researcher Daniel Nocera developed together with Pamela Silver a new bionic leaf for artificial photosynthesis (Source: Harv. Univ.)

Harvard researcher Daniel Nocera developed together with Pamela Silver a new bionic leaf for artificial photosynthesis (Source: Harvard U.)

The days of drilling into the ground in the search for fuel may be numbered, because if Daniel Nocera has his way, it’ll just be a matter of looking for sunny skies. Nocera, the Pat­terson Rockwood Professor of Energy at Harvard Uni­versity, and Pamela Silver, the Elliott T. and Onie H. Adams Professor of Bio­chemistry and Systems Biology at Harvard Medical School, have co-created a system that uses solar energy to split water molecules and hydrogen-eating bacteria to produce liquid fuels.

“This is a true artificial photo­synthesis system,” Nocera said. “Before, people were using artificial photo­synthesis for water-splitting, but this is a true A-to-Z system, and we’ve gone well over the efficiency of photo­synthesis in nature.” While the study shows the system can be used to generate usable fuels, its potential doesn’t end there, said Silver. “The beauty of biology is it’s the world’s greatest chemist,  biology can do chemistry we can’t do easily,” she said. “In principle, we have a platform that can make any downstream carbon-based molecule. So this has the potential to be in­credibly versatile.”

Dubbed “bionic leaf 2.0,” the new system builds on previous work by Nocera, Silver and others, which – though it was capable of using solar energy to make iso­propanol – faced a number of challenges. Chief among those challenges, Nocera said, was the fact that the catalyst used to produce hydrogen, a nickel-molybdenum-zinc alloy, also created reactive oxygen species, molecules that attacked and destroyed the bacteria’s DNA. To avoid that problem, researchers were forced to run the system at abnor­mally high voltages, resulting in reduced efficiency.

“We designed a new cobalt-phos­phorous alloy catalyst, which we showed does not make reactive oxygen species,” Nocera said. “That allowed us to lower the voltage, and that led to a dramatic increase in efficiency.” The system can now convert solar energy to biomass with ten percent efficiency, Nocera said, far above the one percent seen in the fastest growing plants. In addition to increasing the efficiency, Nocera and colleagues were able to expand the portfolio of the system to include iso­butanol and iso­pentanol. Researchers also used the system to create PHB, a bio-plastic precursor.

The new catalyst also came with another advantage. Its chemical design allows it to self-heal, meaning it wouldn’t leech material into solution. “This is the genius of Dan,” Silver said. “These catalysts are totally biolo­gically compatible.” Though there may yet be room for additional increases in efficiency, Nocera said the system is already effective enough to consider possible commercial appli­cations but within a different model for techno­logy trans­lation. “It’s an important discovery. It says we can do better than photo­synthesis,” Nocera said. “But I also want to bring this techno­logy to the developing world as well.”

Working in con­junction with the First 100 Watts program at Harvard, which helped fund the research, Nocera hopes to continue deve­loping the technology and its appli­cations in nations like India with the help of their scientists. In many ways, Nocera said, the new system marks the ful­fillment of the promise of his arti­ficial leaf, which used solar power to split water and make hydrogen fuel. “If you think about it, photo­synthesis is amazing,” he said. “It takes sunlight, water and air. And then look at a tree. That’s exactly what we did, but we do it signi­ficantly better, because we turn all that energy into a fuel.” (Source: Harvard U.)

Reference: C. Liu et al.: Water splitting–biosynthetic system with CO2 reduction efficiencies exceeding photosynthesis, Science 352, 1210 (2016); DOI: 10.1126/science.aaf5039

Link: Dept. Chemistry and Chemical Biology (D. Nocera), Harvard University, Cambridge, USA

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