13 Jun 2016
A cross-disciplinary team at Harvard University has created a system that uses solar energy to split water molecules and hydrogen-eating bacteria to produce liquid fuels. The system can convert solar energy to biomass with 10 percent efficiency, far above the one percent seen in the fastest-growing plants.
The bionic leaf is one step closer to reality.
Daniel Nocera, a professor of energy science at Harvard who pioneered the use of artificial photosynthesis, says that he and his colleague Pamela Silver have devised a system that completes the process of making liquid fuel from sunlight, carbon dioxide, and water. And they’ve done it at an efficiency of 10 percent, using pure carbon dioxide—in other words, one-tenth of the energy in sunlight is captured and turned into fuel.
That is much higher than natural photosynthesis, which converts about 1 percent of solar energy into the carbohydrates used by plants, and it could be a milestone in the shift away from fossil fuels. The new system is described in a new paper in Science.
“Bill Gates has said that to solve our energy problems, someday we need to do what photosynthesis does, and that someday we might be able to do it even more efficiently than plants,” says Nocera. “That someday has arrived.”
In nature, plants use sunlight to make carbohydrates from carbon dioxide and water. Artificial photosynthesis seeks to use the same inputs—solar energy, water, and carbon dioxide—to produce energy-dense liquid fuels. Nocera and Silver’s system uses a pair of catalysts to split water into oxygen and hydrogen, and feeds the hydrogen to bacteria along with carbon dioxide.
The bacteria, a microörganism that has been bioengineered to specific characteristics, converts the carbon dioxide and hydrogen into liquid fuels.
Several companies, including Joule Unlimited and LanzaTech, are working to produce biofuels from carbon dioxide and hydrogen, but they use bacteria that consume carbon monoxide or carbon dioxide, rather than hydrogen. Nocera’s system, he says, can operate at lower temperatures, higher efficiency, and lower costs.
Nocera’s latest work “is really quite amazing,” says Peidong Yang of the University of California, Berkeley. Yang has developed a similar system with much lower efficiency. “The high performance of this system is unparalleled” in any other artificial photosynthesis system reported to date, he says.
The new system can use pure carbon dioxide in gas form, or carbon dioxide captured from the air—which means it could be carbon-neutral, introducing no additional greenhouse gases into the atmosphere. “The 10 percent number, that’s using pure CO2,” says Nocera. Allowing the bacteria themselves to capture carbon dioxide from the air, he adds, results in an efficiency of 3 to 4 percent—still significantly higher than natural photosynthesis.
“That’s the power of biology: these bioörganisms have natural CO2 concentration mechanisms.”
Nocera’s research is distinct from the work being carried out by the Joint Center for Artificial Photosynthesis, a U.S. Department of Energy-funded program that seeks to use inorganic catalysts, rather than bacteria, to convert hydrogen and carbon dioxide to liquid fuel.
According to Dick Co, who heads the Solar Fuels Institute at Northwestern University, the innovation of the new system lies not only in its superior performance but also in its fusing of two usually separate fields: inorganic chemistry (to split water) and biology (to convert hydrogen and carbon dioxide into fuel). “What’s really exciting is the hybrid approach” to artificial photosynthesis, says Co. “It’s exciting to see chemists pairing with biologists to advance the field.”
Commercializing the technology will likely take years. In any case, the prospect of turning sunlight into liquid fuel suddenly looks a lot closer.
| Solar energy is the world’s most plentiful and ubiquitous energy source, and researchers around the world are pursuing ways to convert sunlight into a useful form.
Most people are aware of solar photovoltaics that generate electricity and solar panels that produce hot water. But there is another thrust of solar research: turning sunlight into liquid fuels.
|Research in solar-derived liquid fuels, or solar fuels, aims to make a range of products that are compatible with our energy infrastructure today, such as gasoline, jet fuel and hydrogen. The goal is to store sunlight in liquid form, conveniently overcoming the transient nature of sunlight. I am among the growing number of researchers focused on this field.|
|How can this be done? And what scientific challenges remain before one can fill up a car on solar-generated fuel?|
|Packing heat: concentrating sunlight into a reactor to split H2O and CO2 – a step toward making liquid fuels. (Courtesy of Professor David Hahn, University of Florida, Author provided)|
|Speeding up nature|
|The production of solar fuels is particularly attractive because it addresses both the conversion and storage problem endemic to sunlight; namely, the sun is available for only one-third of the day. This is a distinct advantage compared to other solar conversion technologies. Solar photovoltaic panels, for instance, must be coupled to a complex distribution and storage network, such as batteries, when production of electric power doesn’t equal demand.|
|The term “solar fuel” is a bit of a misnomer. In fact, all fossil fuels are technically solar-derived. Solar energy drives photosynthesis to form plant matter through the reaction of carbon dioxide (CO2) and water (H2O) and, over millions of years, the decay of plant matter creates the hydrocarbons we use to power our society.|
|One thermochemical approach strips oxygen from steam and carbon dioxide gas using the sun’s heat. Then, the resulting gases are combined chemically in a separate process to make liquid fuels. (Image: Jonathan Scheffe, Author provided)|
|The downside of this process is that nature’s efficiency at producing hydrocarbons is excruciatingly low, and humankind’s hunger for energy has never been greater. The result is that the current rate of fossil fuel consumption is much larger than the rate they are produced by nature alone, which provides the motivation to increase nature’s efficiencies and speed up the process of solar fuel production through artificial means.|
|This is the true meaning of “solar fuel” as it is used today, but the ultimate goal is the same: namely, the conversion of solar energy, CO2 and H2O to chemical forms such as gasoline.|
|To the lab|
|The first step in creating manmade solar fuels is to break down CO2 and/or H2O molecules, often to carbon monoxide (CO) or carbon and hydrogen (H2). This is no easy feat, as both of these molecules are very stable (H2 does not form spontaneously from H2O!) and therefore this step requires a substantial amount of energy supplied from sunlight, either directly in the form of photons or indirectly as electricity or heat.|
|This step is often the most crucial component of the process and represents the greatest roadblock to commercialization of solar fuel technologies today, as it largely defines the efficiency of the overall fuel production process and therefore the cost.|
|Downstream of this step, the resulting molecules – in this case, a mixture of CO and hydrogen called synthesis gas – may be converted through a variety of existing technologies depending on the final product desired. This step of converting hydrocarbon gases to liquid form is already performed at an industrial scale, thanks to large corporations such as Shell Global Solutions and Sasol that use these technologies to leverage to low cost of today’s natural gas to make more valuable liquid fuels.|
|Recently, a European Union-sponsored project called SOLAR-JET (Solar chemical reactor demonstration and Optimization for Long-term Availability of Renewable JET fuel) demonstrated the first-ever conversion of solar energy to jet fuel, or kerosene. Researchers coupled the solar-driven production of synthesis gas, also called syngas, from CO2 and H2O with a downstream gas-to-liquids reactor – in this case a Fischer-Tropsch reactor at Shell’s Headquarters in Amsterdam.|
|The production of liquid fuels is especially important for the aviation industry that relies on energy-dense fuels and represents another important advantage of solar fuel production compared to solar electricity.|
|The SOLAR-JET project, which I worked on with several other researchers, utilized a process called solar thermochemical fuel production, in which solar energy is concentrated using optics – mirrors and lenses – much the way a magnifying glass can start a fire. The resulting heat is then absorbed in a chamber that acts as a chemical reactor. The absorbed heat is then used to dissociate H2O and/or CO2 through a catalytic-type process – one of the most technically challenging steps for all solar fuel conversion processes. The resulting products (hydrogen or synthesis gas) can then be captured and further converted to liquid fuels downstream.|
|There are numerous other strategies to drive these reactions needed for the first step of solar fuel production, including those that utilize light – photons – directly or indirectly in the form of electricity.|
|For example, so-called artificial photosynthesis utilizes photons directly in a catalytic process, rather than absorbing them as heat, to break down H2O and CO2 molecules.|
|Former Energy Secretary Steven Chu visiting the Joint Center for Artificial Photosynthesis, which received an additional US$75 million in funding earlier this year. The lab is pursuing converting light (not heat) directly into fuels. (Image: Lawrence Berkeley National Laboratory)|
|Electrochemical approaches utilize electricity that could be generated from a photovoltaic cell to drive the separation of H2O and CO2 through a process known as electrolysis.|
|To date, the key barriers to commercialization of all of these technologies are primarily related to their low efficiencies – that is related to the amount of energy needed to produce a liquid fuel – and overall robustness. For example, the efficiency of the SOLAR-JET thermochemical conversion project discussed above is still less than 2%, but for this technology to become commercially viable, efficiencies greater than 10% will need to be achieved.|
|A team working at the University of Florida funded by research agency ARPA-E is working toward these efficiency goals using another thermochemical process that uses optics to generate heat. Yet robustness because of extreme temperatures (greater than 1200 Celsius or over 2000 Farenheit) is still a major concern that is being addressed.|
|Furthermore, for solar fuel production to truly reduce greenhouse gas levels, it must be coupled with methods to capture CO2 from the air. This is still a relatively immature technology, but companies such as Climeworks are working to make this a reality.|
|Add in the complexity of integrating a temporally varying energy input (the sun) with a chemical reactor and the overall scope of the challenge can appear large. Nevertheless, advances are being made daily that give hope that solar fuels at higher efficiencies will soon be a reality.|
|Source: By Jonathan Scheffe, Associate Professor Department of Mechanical and Aerospace Enginering at University of Florida, via The Conversation|
This transmission electron microscope image shows a graphene quantum dot with zigzag edges. The quantum dots can be created in bulk from carbon fiber through a chemical process discovered at Rice University.
A Rice University laboratory has found a way to turn common carbon fiber into graphene quantum dots, tiny specks of matter with properties expected to prove useful in electronic, optical and biomedical applications.
The Rice lab of materials scientist Pulickel Ajayan, in collaboration with colleagues in China, India, Japan and the Texas Medical Center, discovered a one-step chemical process that is markedly simpler than established techniques for making graphene quantum dots. The results were published online this month in the American Chemical Society’s journal Nano Letters.
“There have been several attempts to make graphene-based quantum dots with specific electronic and luminescent properties using chemical breakdown or e-beam lithography of graphene layers,” said Ajayan, Rice’s Benjamin M. and Mary Greenwood Anderson Professor of Mechanical Engineering and Materials Science and of chemistry. “We thought that as these nanodomains of graphitized carbons already exist in carbon fibers, which are cheap and plenty, why not use them as the precursor?”
Quantum dots, discovered in the 1980s, are semiconductors that contain a size- and shape-dependent band gap. These have been promising structures for applications that range from computers, LEDs, solar cells and lasers to medical imaging devices. The sub-5 nanometer carbon-based quantum dots produced in bulk through the wet chemical process discovered at Rice are highly soluble, and their size can be controlled via the temperature at which they’re created.
The Rice researchers were attempting another experiment when they came across the technique. “We tried to selectively oxidize carbon fiber, and we found that was really hard,” said Wei Gao, a Rice graduate student who worked on the project with lead author Juan Peng, a visiting student from
Green-fluorescing graphene quantum dots created at Rice University surround a blue-stained nucleus in a human breast cancer cell. Cells were placed in a solution with the quantum dots for four hours. The dots, each smaller than 5 nanometers, easily passed through the cell membranes, showing their potential value for bio-imaging.
Nanjing University who studied in Ajayan’s lab last year. “We ended up with a solution and decided to look at a few drops with a transmission electron microscope.”
The specks they saw were bits of graphene or, more precisely, oxidized nanodomains of graphene extracted via chemical treatment of carbon fiber. “That was a complete surprise,” Gao said. “We call them quantum dots, but they’re two-dimensional, so what we really have here are graphene quantum discs.”
Gao said other techniques are expensive and take weeks to make small batches of graphene quantum dots. “Our starting material is cheap, commercially available carbon fiber. In a one-step treatment, we get a large amount of quantum dots. I think that’s the biggest advantage of our work,” she said.Further experimentation revealed interesting bits of information: The size of the dots, and thus their photoluminescent properties, could be controlled through processing at relatively low temperatures, from 80 to 120 degrees Celsius. “At 120, 100 and 80 degrees, we got blue, green and yellow luminescing dots,” she said.
They also found the dots’ edges tended to prefer the form known as zigzag. The edge of a sheet of graphene — the single-atom-thick form of carbon — determines its electrical characteristics, and zigzags are semiconducting.
Their luminescent properties give graphene quantum dots potential for imaging, protein analysis, cell tracking and other biomedical applications, Gao said. Tests at Houston’s MD Anderson Cancer Center and Baylor College of Medicine on two human breast cancer lines showed the dots easily found their way into the cytoplasm and did not interfere with the cells’ proliferation.
“The green quantum dots yielded a very good image,” said co-author Rebeca Romero Aburto, a graduate student in the Ajayan lab who also studies at MD Anderson. “The advantage of graphene dots over fluorophores is that their fluorescence is more stable and they don’t photobleach. They don’t lose their fluorescence as easily. They have a depth limit, so they may be good for in vitro and in vivo (small animal) studies, but perhaps not optimal for deep tissues in humans.
Dark spots on a transmission electron microscope grid are graphene quantum dots made through a wet chemical process at Rice University. The inset is a close-up of one dot. Graphene quantum dots may find use in electronic, optical and biomedical applications.
“But everything has to start in the lab, and these could be an interesting approach to further explore for bioimaging,” Romero Alburto said. “In the future, these graphene quantum dots could have high impact because they can be conjugated with other entities for sensing applications, too.”
Co-authors include Angel Martí, a professor of chemistry and bioengineering, postdoctoral research associates Zheng Liu and Liehui Ge, senior research scientist Lawrence Alemany and graduate student Xiaobo Zhan, all of Rice; Rice alumnus Li Song of Shinshu University, Japan; Bipin Kumar Gupta of the National Physical Laboratory, New Delhi, who worked at the Ajayan lab on an Indo-US Science and Technology Forum fellowship; Guanhui Gao of the Ocean University of China; Sajna Antony Vithayathil, a research technician, and Benny Abraham Kaipparettu, a postdoctoral researcher, both at Baylor College of Medicine; Takuya Hayashi, an associate professor of engineering at Shinshu University, Japan; and Jun-Jie Zhu, a professor of chemistry at Nanjing University.
The research was supported by Nanoholdings, the Office of Naval Research MURI program on graphene, the Natural Science Foundation of China, the National Basic Research Program of China, the Indo-US Science and Technology Forum and the Welch Foundation.
See Also from Rice University:
Quantum Dots from Coal + Graphene Could Dramatically Cut the Cost of Energy from Fuel Cells
Rice University’s cheap hybrid outperforms rare metal as fuel-cell catalyst
Graphene quantum dots created at Rice University grab onto graphene platelets like barnacles attach themselves to the hull of a boat. But these dots enhance the properties of the mothership, making them better than platinum catalysts for certain reactions within fuel cells.
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