Researchers at Harvard have created a hybrid water splitting–biosynthetic system based on a biocompatible Earth-abundant inorganic catalyst system to split water into molecular hydrogen and oxygen (H2 and O2) at low driving voltages.
Grown in contact with these catalysts, the bacterium Ralstonia eutropha then consumes the produced H2 to synthesize biomass and fuels or chemical products from low CO2 concentration in the presence of O2. The scalable system has a CO2 reduction energy efficiency of ~50% when producing bacterial biomass and liquid fuel alcohols, scrubbing 180 grams of CO2 per kWh of electricity. Coupling this hybrid device to existing photovoltaic systems would yield a CO2 reduction energy efficiency of ~10%, exceeding that of natural photosynthetic systems, the researchers said in their paper published in the journal Science.
The research was led by Daniel 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.
Dubbed “bionic leaf 2.0,” the new system builds on previous work by Nocera, Silver and others (earlier post), 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.
For this paper, we designed a new cobalt-phosphorous alloy catalyst, which we showed does not make reactive oxygen species. That allowed us to lower the voltage, and that led to a dramatic increase in efficiency.—Daniel Nocera
The system can now convert solar energy to biomass with 10% efficiency, Nocera said, far above the one percent seen in the fastest growing plants.
This is a true artificial photosynthesis system. 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.—Daniel Nocera
While the study shows the system can be used to generate usable fuels, its potential doesn’t end there, said Silver, who is also a Founding Core Member of the Wyss Institute at Harvard University.
The beauty of biology is it’s the world’s greatest chemist—biology can do chemistry we can’t do easily. In principle, we have a platform that can make any downstream carbon-based molecule. So this has the potential to be incredibly versatile.—Pamela Silver
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, a process first demonstrated by MIT professor Anthony Sinskey.
The chemical design of the new catalyst allows it to “self-heal”—meaning it wouldn’t leech material into solution.
This is the genius of Dan. These catalysts are totally biologically compatible.—Pamela Silver
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. But I also want to bring this technology to the developing world as well.—Daniel Nocera
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 such as India with the help of their scientists.
If you think about it, photosynthesis is amazing. 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.—Daniel Nocera
This work was supported by Office of Naval Research Multidisciplinary University Research Initiative Award (N00014-11-1-0725), Air Force Office of Scientific Research Grant (FA9550-09-1-0689), and the Wyss Institute for Biologically Inspired Engineering. The Harvard University Climate Change Solutions Fund is supporting ongoing research into the "bionic leaf" platform. The work is a direct result of the First 100 W Program established at Harvard University.
Chong Liu, Brendan C. Colón, Marika Ziesack, Pamela A. Silver, Daniel G. Nocera (2016) “Water splitting–biosynthetic system with CO2 reduction efficiencies exceeding photosynthesis” Science doi: 10.1126/science.aaf5039