New system generates clean energy on the small scale
PENNY JENNINGS/UCLA CHEMISTRY & BIOCHEMISTRY
Chong Liu, 30
For Chong Liu, asking a scientific question is something like placing a bet: You throw all your energy into tackling a big and challenging problem with no guarantee of a reward. As a student, he bet that he could create a contraption that photosynthesizes like a leaf on a tree — but better. For the now 30-year-old chemist, the gamble is paying off.“He opened up a new field,” says Peidong Yang, a chemist at the University of California, Berkeley who was Liu’s Ph.D. adviser. Liu was among the first to combine bacteria with metals or other inorganic materials to replicate the energy-generating chemical reactions of photosynthesis, Yang says. Liu’s approach to artificial photosynthesis may one day be especially useful in places without extensive energy infrastructure.
Liu first became interested in chemistry during high school, and majored in the subject at Fudan University in Shanghai. He recalls feeling frustrated in school when he would ask questions and be told that the answer was beyond the scope of what he needed to know. Research was a chance to seek out answers on his own. And the problem of artificial photosynthesis seemed like something substantial to throw himself into — challenging enough “so [I] wouldn’t be jobless in 10 or 15 years,” he jokes.
Photosynthesis is a simple but powerful process: Sunlight helps transform carbon dioxide and water into chemical energy stored in the chemical bonds of sugar molecules. But in nature, the process isn’t particularly efficient, converting just 1 percent of solar energy into chemical energy. Liu thought he could do better with a hybrid system.
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Artificial “leaves” designed by Chong Liu and colleagues collect solar energy to generate electric current. The current splits water molecules into oxygen and hydrogen, and bacteria in the water transform carbon dioxide and hydrogen into fuels or other useful chemicals.
The efficiency of natural photosynthesis is limited by light-absorbing pigments in plants or bacteria, he says. People have designed materials that absorb light far more efficiently. But when it comes to transforming that light energy into fuel, bacteria shine.
“By taking a hybrid approach, you leverage what each side is better at,” says Dick Co, managing director of the Solar Fuels Institute at Northwestern University in Evanston, Ill.
Liu’s early inspiration was an Apollo-era attempt at a life-support system for manned space missions. The idea was to use inorganic materials with specialized bacteria to turn astronauts’ exhaled carbon dioxide into food. But early attempts never went anywhere.
“The efficiency was terribly low, way worse than you’d expect from plants,” Liu says. And the bacteria kept dying — probably because other parts of the system were producing molecules that were toxic to the bacteria.
As a graduate student, Liu decided to use his understanding of inorganic chemistry to build a system that would work alongside the bacteria, not against them. He first designed a system that uses nanowires coated with bacteria. The nanowires collect sunlight, much like the light-absorbing layer on a solar panel, and the bacteria use the energy from that sunlight to carry out chemical reactions that turn carbon dioxide into a liquid fuel such as isopropanol.
As a postdoctoral fellow in the lab of Harvard University chemist Daniel Nocera, Liu collaborated on a different approach. Nocera had been working on a “bionic leaf” in which solar panels provide the energy to split water into hydrogen and oxygen gases. Then, Ralstonia eutropha bacteria consume the hydrogen gas and pull in carbon dioxide from the air. The microbes are genetically engineered to transform the ingredients into isopropanol or another liquid fuel. But the project faced many of the same problems as other bacteria-based artificial photosynthesis attempts: low efficiency and lots of dead bacteria.
The bionic leaf doesn’t resemble something you’d find on a tree. Here, wires carry electric current into bottles filled with water and microbes. The electricity splits the water molecules, and then microbes transform the resulting hydrogen into fuel.
“Chong figured out how to make the system extremely efficient,” Nocera says. “He invented biocompatible catalysts” that jump-start the chemical reactions inside the system without killing off the fuel-generating bacteria. That advance required sifting through countless scientific papers for clues to how different materials might interact with the bacteria, and then testing many different options in the lab. In the end, Liu replaced the original system’s problem catalysts — which made a microbe-killing, highly reactive type of oxygen molecule — with cobalt-phosphorus, which didn’t bother the bacteria.
Chong is “very skilled and open-minded,” Nocera says. “His ability to integrate different fields was a big asset.”
The team published the results in Science in 2016, reporting that the device was about 10 times as efficient as plants at removing carbon dioxide from the air. With 1 kilowatt-hour of energy powering the system, Liu calculated, it could recycle all the carbon dioxide in more than 85,000 liters of air into other molecules that could be turned into fuel. Using different bacteria but the same overall setup, the researchers later turned nitrogen gas into ammonia for fertilizer, which could offer a more sustainable approach to the energy-guzzling method used for fertilizer production today.
Soil bacteria carry out similar reactions, turning atmospheric nitrogen into forms that are usable by plants. Now at UCLA, Liu is launching his own lab to study the way the inorganic components of soil influence bacteria’s ability to run these and other important chemical reactions. He wants to understand the relationship between soil and microbes — not as crazy a leap as it seems, he says. The stuff you might dig out of your garden is, like his approach to artificial photosynthesis, “inorganic materials plus biological stuff,” he says. “It’s a mixture.”
Liu is ready to place a new bet — this time on re-creating the reactions in soil the same way he’s mimicked the reactions in a leaf.
C. Liu et al. A fully integrated nanosystem of semiconductor nanowires for direct solar water splitting. Nano Letters. Vol. 13, May 6, 2013, p. 2989. doi: 10.1021/nl401615t.
C. Liu et al. Nanowire-bacteria hybrids for unassisted solar carbon dioxide fixation to value-added chemicals. Nano Letters. Vol. 15, April 7, 2015. doi:10.1021/acs.nanolett.5b01254.
C. Liu et al. Water splitting-biosynthetic system with CO2 reduction efficiencies exceeding photosynthesis. Science. Vol. 352, June 3, 2016. P. 1210. doi:10.1126/science.aaf5039.
C. Liu et al. Ambient nitrogen reduction cycle using a hybrid inorganic-biological system. Proceedings of the National Academy of Sciences. May 2, 2017. doi:10.1073/pnas.1706371114.