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Bionic leaf 2.0

Bionic leaf 2.0: An artificial photosynthesis system (credit: Jessica Polka)

Harvard scientists have created a system a system that uses solar energy plus hydrogen-eating bacteria to produce liquid fuels with 10 percent efficiency, compared to the 1 percent seen in the fastest-growing plants.

The system, co-created 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, uses solar energy to split water molecules into hydrogen and oxygen molecules.

A paper on the research was published June 3 in Science.

“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.”

“What we’ve invented is an artificial leaf. You just drop it in water and sunlight hits it, and out one side comes hydrogen and out the other side comes oxygen.” — Daniel Nocera

“The beauty of biology is it’s the world’s greatest chemist: Biology can do chemistry we can’t do easily,” said Silver, who is also a founding core member of the Wyss Institute at Harvard University. “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 faced a number of challenges. Mainly, the catalyst they 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.

Ready for commercial applications, with a new model

“For this paper, we designed a new cobalt-phosphorus 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.”

Nocera and colleagues were also able to expand the portfolio of the system to include isobutanol (a solvent) and isopentanol (used in geothermal power production to drive turbines), along with PHB, a bioplastic precursor.

“Instead of having a gas station, the Sun is hitting your house, you have the artificial leaf, you could be generating your own fuel.” — Daniel Nocera (credit: Rose Lincoln/Harvard Staff Photographer)

The new catalyst’s chemical design also allows it to “self-heal,” meaning it won’t leach material into solution — it’s biologically compatible.

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… [that] can do better than photosynthesis,” Nocera said. “But I also want to bring this technology to the developing world.”

Working in conjunction with the First 100 Watts Project 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 that country’s scientists.

In many ways, Nocera said, the new system marks fulfillment of the promise of his “artificial leaf,” which used solar power to split water and make hydrogen fuel (see ‘Artificial leaf’ harnesses sunlight for efficient, safe hydrogen fuel production).

“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.”

The work, a direct result of the First 100 Watts Project established at Harvard University, was was supported by Office of Naval Research Multidisciplinary University, Research Initiative Award, Air Force Office of Scientific Research Grant, and the Wyss Institute for Biologically Inspired Engineering. The Harvard University Climate Change Solutions Fund is supporting ongoing research into the “bionic leaf” platform.


Harvard University | Bionic Leaf Turns Sunlight Into Liquid Fuel

Abstract of Water splitting–biosynthetic system with CO2 reduction efficiencies exceeding photosynthesis

Artificial photosynthetic systems can store solar energy and chemically reduce CO2. We developed 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. When grown in contact with these catalysts, Ralstonia eutropha consumed the produced H2 to synthesize biomass and fuels or chemical products from low CO2 concentration in the presence of O2. This scalable system has a CO2 reduction energy efficiency of ~50% when producing bacterial biomass and liquid fusel alcohols, scrubbing 180 grams of CO2 per kilowatt-hour 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.

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A new nontoxic way to generate portable power

In this time-lapse series of photos, progressing from top to bottom, a coating of sucrose (ordinary sugar) over a wire made of carbon nanotubes is lit at the left end, and burns from one end to the other. As it heats the wire, it drives a wave of electrons along with it, thus converting the heat into electricity. (credit: MIT)

Here’s a new idea for a nontoxic battery: light fuel-coated carbon nanotubes on fire (like a fuse) to generate electricity.

Sounds crazy but it works, according to inventor Michael Strano, the Carbon P. Dubbs Professor in Chemical Engineering at MIT. Plus it avoids toxic materials such as lithium, which can be difficult to dispose of and that have limited global supplies),

The new approach is based on a discovery announced in 2010 by Strano and his co-workers: A wire made from carbon nanotubes can produce an electrical current when it is progressively heated from one end to the other — for example, by coating it with a combustible material and then lighting one end to let it burn like a fuse.

Basically, the effect arises as a pulse of heat pushes electrons through the bundle of carbon nanotubes, carrying the electrons with it like a bunch of surfers riding a wave.

Experiments at the time produced only a minuscule amount of current in a simple laboratory setup. But now, Strano and his team have increased the efficiency of the process more than a thousandfold and have produced devices that can put out power that is, pound for pound, in the same ballpark as what can be produced by today’s best batteries. The researchers caution, however, that it could take several years to develop the concept into a commercializable product.

The new results were published in the journal Energy & Environmental Science, in a paper by Strano, doctoral students Sayalee Mahajan PhD ’15 and Albert Liu, and five others.


MPC-MIT | Experimenting With Thermopower Waves

Virtually indefinite shelf life

The improvements in efficiency, he says, “brings [the technology] from a laboratory curiosity to being within striking distance of other portable energy technologies,” such as lithium-ion batteries or fuel cells. In their latest version, the device is more than 1 percent efficient in converting heat energy to electrical energy, the team reports, which is about 10,000 times greater than that reported in the original discovery paper.

“It took lithium-ion technology 25 years to get where they are” in terms of efficiency, Strano points out, whereas this technology has had only about a fifth of that development time. And lithium is extremely flammable if the material ever gets exposed to the open air — unlike the fuel used in the new device, which is much safer and also a renewable resource.

Already, the device is powerful enough to show that it can power simple electronic devices such as an LED light. And unlike batteries that can gradually lose power if they are stored for long periods, the new system should have a virtually indefinite shelf life, Liu says. That could make it suitable for uses such as a deep-space probe that remains dormant for many years as it travels to a distant planet and then needs a quick burst of power to send back data when it reaches its destination.

In addition, the new system is very scalable for use wearable devices. Batteries and fuel cells have limitations that make it difficult to shrink them to tiny sizes, Mahajan says, whereas this system “can scale down to very small limits. The scale of this is unique.”

This work is “an important demonstration of increasing the energy and lifetime of thermopower wave-based systems,” says Kourosh Kalantar-Zadeh, a professor of electrical and computer engineering at RMIT University in Australia, who was not involved in this research. “I believe that we are still far from the upper limit that the thermopower wave devices can potentially reach,” he says. “However, this step makes the technology more attractive for real applications.”

He adds that with this technology, “We can obtain phenomenal bursts of power, which is not possible from batteries. For instance, the thermopower wave systems can be used for powering long-distance transmission units in micro- and nano-telecommunication hubs.”

The work was supported by the Air Force Office of Scientific Research and the Office of Naval Research.

Abstract of Sustainable power sources based on high efficiency thermopower wave devices

There is a pressing need to find alternatives to conventional batteries such as Li-ion, which contain toxic metals, present recycling difficulties due to harmful inorganic components, and rely on elements in finite global supply. Thermopower wave (TPW) devices, which convert chemical to electrical energy by means of self-propagating reaction waves guided along nanostructured thermal conduits, have the potential to address this demand. Herein, we demonstrate orders of magnitude higher chemical-to-electrical conversion efficiency of thermopower wave devices, in excess of 1%, with sustainable fuels such as sucrose and NaN3 for the first time, that produce energy densities on par with Li-ion batteries operating at 80% efficiency (0.2 MJ L−1 versus 0.8 MJ L−1). We show that efficiency can be increased significantly by selecting fuels such as sodium azide or sucrose with potassium nitrate to offset the inherent penalty in chemical potential imposed by strongly p-doping fuels, a validation of the predictions of Excess Thermopower theory. Such TPW devices can be scaled to lengths greater than 10 cm and durations longer than 10 s, an over 5-fold improvement over the highest reported values, and they are capable of powering a commercial LED device. Lastly, a mathematical model of wave propagation, coupling thermal and electron transport with energy losses, is presented to describe the dynamics of power generation, explaining why both unipolar and bipolar waveforms can be observed. These results represent a significant advancement toward realizing TPW devices as new portable, high power density energy sources that are metal-free.

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Experiments show magnetic chips could dramatically increase computing’s energy efficiency

Magnetic microscope image of three nanomagnetic computer bits. Each bit is a tiny bar magnet only 90 nanometers long. The image hows a bright spot at the “North” end and a dark spot at the “South” end of the magnet. The “H” arrow shows the direction of magnetic field applied to switch the direction of the magnets. (credit: Jeongmin Hong et al./Science Advances)

UC Berkeley engineers have shown for the first time that magnetic chips can actually operate at the lowest fundamental energy dissipation theoretically possible under the laws of thermodynamics. That means dramatic reductions in power consumption are possible — down to as little as one-millionth the amount of energy per operation used by transistors in modern computers.

The findings were published Mar. 11 an open-access paper in the peer-reviewed journal Science Advances.

This is critical at two ends of the size scale: for mobile devices, which demand powerful processors that can run for a day or more on small, lightweight batteries; and on an industrial scale, as computing increasingly moves into “the cloud,” where the electricity demands of the giant cloud data centers are multiplying, collectively taking an increasing share of the country’s — and world’s — electrical grid.

“The biggest challenge in designing computers and, in fact, all our electronics today is reducing their energy consumption,” aid senior author Jeffrey Bokor, a UC Berkeley professor of electrical engineering and computer sciences and a faculty scientist at the Lawrence Berkeley National Laboratory.

Lowering energy use is a relatively recent shift in focus in chip manufacturing after decades of emphasis on packing greater numbers of increasingly tiny and faster transistors onto chips to keep up with Moore’s law.

“Making transistors go faster was requiring too much energy,” said Bokor, who is also the deputy director the Center for Energy Efficient Electronics Science, a Science and Technology Center at UC Berkeley funded by the National Science Foundation. “The chips were getting so hot they’d just melt.”

So researchers have been turning to alternatives to conventional transistors, which currently rely upon the movement of electrons to switch between 0s and 1s. Partly because of electrical resistance, it takes a fair amount of energy to ensure that the signal between the two 0 and 1 states is clear and reliably distinguishable, and this results in excess heat.

Nanomagnetic computing: how low can you get?

The UC Berkeley team used an innovative technique to measure the tiny amount of energy dissipation that resulted when they flipped a nanomagnetic bit. The researchers used a laser probe to carefully follow the direction that the magnet was pointing as an external magnetic field was used to rotate the magnet from “up” to “down” or vice versa.

They determined that it only took 15 millielectron volts of energy — the equivalent of 3 zeptojoules — to flip a magnetic bit at room temperature, effectively demonstrating the Landauer limit (the lowest limit of energy required for a computer operation). *

This is the first time that a practical memory bit could be manipulated and observed under conditions that would allow the Landauer limit to be reached, the authors said. Bokor and his team published a paper in 2011 that said this could theoretically be done, but it had not been demonstrated until now.

While this paper is a proof of principle, he noted that putting such chips into practical production will take more time. But the authors noted in the paper that “the significance of this result is that today’s computers are far from the fundamental limit and that future dramatic reductions in power consumption are possible.”

The National Science Foundation and the U.S. Department of Energy supported this research.

* The Landauer limit was named after IBM Research Lab’s Rolf Landauer, who in 1961 found that in any computer, each single bit operation must expend an absolute minimum amount of energy. Landauer’s discovery is based on the second law of thermodynamics, which states that as any physical system is transformed, going from a state of higher concentration to lower concentration, it gets increasingly disordered. That loss of order is called entropy, and it comes off as waste heat. Landauer developed a formula to calculate this lowest limit of energy required for a computer operation. The result depends on the temperature of the computer; at room temperature, the limit amounts to about 3 zeptojoules, or one-hundredth the energy given up by a single atom when it emits one photon of light.

Abstract of Experimental test of Landauer’s principle in single-bit operations on nanomagnetic memory bits

Minimizing energy dissipation has emerged as the key challenge in continuing to scale the performance of digital computers. The question of whether there exists a fundamental lower limit to the energy required for digital operations is therefore of great interest. A well-known theoretical result put forward by Landauer states that any irreversible single-bit operation on a physical memory element in contact with a heat bath at a temperature Trequires at least kBT ln(2) of heat be dissipated from the memory into the environment, where kB is the Boltzmann constant. We report an experimental investigation of the intrinsic energy loss of an adiabatic single-bit reset operation using nanoscale magnetic memory bits, by far the most ubiquitous digital storage technology in use today. Through sensitive, high-precision magnetometry measurements, we observed that the amount of dissipated energy in this process is consistent (within 2 SDs of experimental uncertainty) with the Landauer limit. This result reinforces the connection between “information thermodynamics” and physical systems and also provides a foundation for the development of practical information processing technologies that approach the fundamental limit of energy dissipation. The significance of the result includes insightful direction for future development of information technology.

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Converting atmospheric carbon dioxide into carbon nanotubes for use in batteries

The Solar Thermal Electrochemical Process (STEP) converts atmospheric carbon dioxide into carbon nanotubes that can be used in advanced batteries. (credit: Julie Turner, Vanderbilt University)

The electric vehicle of the future will be carbon negative (reducing the amount of atmospheric carbon dioxide) not just carbon neutral (not adding CO2 to the atmosphere), say researchers at Vanderbilt University and George Washington University (GWU).

The trick: replace graphite electrodes in lithium-ion batteries (used in electric vehicles) with carbon nanotubes and carbon nanofibers recovered from carbon dioxide in the atmosphere. The new technology could also be used in sodium-ion batteries, currently under development for large-scale applications, such as the electric grid.

How to convert CO2 to carbon nanotubes

As described in an open-access paper in the Mar. 2 issue of the journal ACS Central Science, the project builds on a solar thermal electrochemical process (STEP) that can create carbon nanofibers from ambient carbon dioxide (see “‘Diamonds from the sky’ approach to turn CO2 into valuable carbon nanofibers“).

STEP uses solar energy to provide both the electrical and thermal energy needed to break down carbon dioxide into carbon and oxygen and to produce carbon nanotubes, which are stable, flexible, conductive and stronger than steel.

In lithium-ion batteries, the nanotubes replace the carbon anode used in commercial batteries. The team demonstrated that the carbon nanotubes gave a small boost to the performance, which was amplified when the battery was charged quickly.

In sodium-ion batteries, the researchers found that small defects in the carbon, which can be tuned using STEP, can unlock stable storage performance more than 3.5 times above that of sodium-ion batteries with graphite electrodes.

Both carbon-nanotube batteries were exposed to about 2.5 months of continuous charging and discharging and showed no sign of fatigue.

Depending on the specifications, making one of the two electrodes out of carbon nanotubes means that up to 40 percent of a battery could be made out of recycled CO2, according to Vanderbilt Assistant Professor of Mechanical Engineering Cary Pint, not including packaging (which could also be replaced in the future).

Cost benefits

This approach also reduces end-user battery cost, unlike most efforts to reuse CO2 aimed at low-valued fuels, like methanol, which “cannot justify the cost required to produce them,” Pint said.

“Other applications for the carbon nanotubes include carbon composites for strong, lightweight construction materials, sports equipment and car, truck and airplane bodies,” said GWU Professor of Chemistry Stuart Licht.

The researchers estimate that with a battery cost of $325 per kWh (the average cost of lithium-ion batteries reported by the Department of Energy in 2013), a kilogram of carbon dioxide has a value of about $18 as a battery material — six times more than when it is first converted to methanol — a number that increases when moving from large batteries used in electric vehicles to the smaller batteries used in electronics.

And unlike methanol, combining batteries with solar cells provides renewable power with zero greenhouse emissions.

Comparison of conventional natural-gas plant (A), which has CO2 as an exhaust, with carbon nanofiber/carbon nanotube-based natural-gas plant (B) (steam turbine cooling/electricity-generating process (pink), common to both, omitted) (credit: Stuart Licht et al./ACS Central Science)

Licht also proposed that the STEP process could be coupled to a natural gas-powered electrical generator. The generator would provide electricity, heat, and a concentrated source of carbon dioxide that would boost the performance of the STEP process.

At the same time, the oxygen released in the process could be piped back to the generator, where it would boost the generator’s combustion efficiency to compensate for the amount of electricity that the STEP process consumes. The end result could be a fossil fuel electrical power plant with net-zero CO2 emissions.

The research was partially supported by National Science Foundation and NSF Graduate Research Fellowship grants.

Abstract of Carbon Nanotubes Produced from Ambient Carbon Dioxide for Environmentally Sustainable Lithium-Ion and Sodium-Ion Battery Anodes

The cost and practicality of greenhouse gas removal processes, which are critical for environmental sustainability, pivot on high-value secondary applications derived from carbon capture and conversion techniques. Using the solar thermal electrochemical process (STEP), ambient CO2 captured in molten lithiated carbonates leads to the production of carbon nanofibers (CNFs) and carbon nanotubes (CNTs) at high yield through electrolysis using inexpensive steel electrodes. These low-cost CO2-derived CNTs and CNFs are demonstrated as high performance energy storage materials in both lithium-ion and sodium-ion batteries. Owing to synthetic control of sp3 content in the synthesized nanostructures, optimized storage capacities are measured over 370 mAh g–1 (lithium) and 130 mAh g–1 (sodium) with no capacity fade under durability tests up to 200 and 600 cycles, respectively. This work demonstrates that ambient CO2, considered as an environmental pollutant, can be attributed economic value in grid-scale and portable energy storage systems with STEP scale-up practicality in the context of combined cycle natural gas electric power generation.