Friday, January 10, 2014

Harvard's new battery offers renewable energy breakthrough

Harvard technology could economically store energy for use when the wind doesn’t blow and the sun doesn’t shine

A team of Harvard scientists and engineers has demonstrated a new type of battery that could fundamentally transform the way electricity is stored on the grid, making power from renewable energy sources such as wind and sun far more economical and reliable.
The novel battery technology is reported in a paper published in Nature on Jan. 9. Under the OPEN 2012 program, the Harvard team received funding from the U.S. Department of Energy’s Advanced Research Projects Agency — Energy (ARPA-E) to develop the grid-scale battery, and plans to work with the agency to catalyze further technological and market breakthroughs over the next several years.
The paper describes a metal-free flow battery that relies on the electrochemistry of naturally abundant, inexpensive, small organic (carbon-based) molecules called quinones, which are similar to molecules that store energy in plants and animals.
The mismatch between the availability of intermittent wind or sunshine and the variable demand is the biggest obstacle to using renewable sources for a large fraction of our electricity. A cost-effective means of storing large amounts of electrical energy could solve this problem.
The battery was designed, built, and tested in the laboratory of Michael J. Aziz, the Gene and Tracy Sykes Professor of Materials and Energy Technologies at the Harvard School of Engineering and Applied Sciences (SEAS). Roy G. Gordon, the Thomas Dudley Cabot Professor of Chemistry and Professor of Materials Science, led the work on the synthesis and chemical screening of molecules. Al├ín Aspuru-Guzik, professor of chemistry and chemical biology, used his pioneering high-throughput molecular screening methods to calculate the properties of more than 10,000 quinone molecules in search of the best candidates for the battery.
Flow batteries store energy in chemical fluids contained in external tanks, as with fuel cells, instead of within the battery container itself. The two main components — the electrochemical conversion hardware through which the fluids are flowed (which sets the peak power capacity) and the chemical storage tanks (which set the energy capacity) — may be independently sized. Thus the amount of energy that can be stored is limited only by the size of the tanks. The design permits larger amounts of energy to be stored at lower cost than with traditional batteries.
By contrast, in solid-electrode batteries, such as those commonly found in cars and mobile devices, the power conversion hardware and energy capacity are packaged together in one unit and cannot be decoupled. Consequently they maintain peak discharge power for less than an hour before they are drained, and are therefore ill-suited to store intermittent renewables.
“Our studies indicate that one to two days’ worth of storage is required for making solar and wind dispatchable through the electrical grid,” said Aziz.
To store 50 hours of energy from a 1-megawatt power capacity wind turbine (50 megawatt-hours), for example, a possible solution would be to buy traditional batteries with 50 megawatt-hours of energy storage, but they would come with 50 megawatts of power capacity. Paying for 50 megawatts of power capacity when only 1 megawatt is necessary makes little economic sense.
For this reason, a growing number of engineers have focused their attention on flow-battery technology. But until now, flow batteries have relied on chemicals that are expensive or hard to maintain, driving up the cost of storing energy.
The active components of electrolytes in most flow batteries have been metals. Vanadium is used in the most commercially advanced flow-battery technology now in development, but it sets a rather high floor on the cost per kilowatt-hour at any scale. Other flow batteries contain precious metal electrocatalysts, such as the platinum used in fuel cells.
The new flow battery developed by the Harvard team already performs as well as vanadium flow batteries, with chemicals that are significantly less expensive, and with no precious-metal electrocatalyst.
“The whole world of electricity storage has been using metal ions in various charge states, but there is a limited number that you can put into solution and use to store energy, and none of them can economically store massive amounts of renewable energy,” Gordon said. “With organic molecules, we introduce a vast new set of possibilities. Some of them will be terrible and some will be really good. With these quinones we have the first ones that look really good.”
Aspuru-Guzik noted that the project is very well aligned with the White House Materials Genome Initiative. “This project illustrates what the synergy of high-throughput quantum chemistry and experimental insight can do,” he said. “In a very quick time period, our team homed in to the right molecule. Computational screening, together with experimentation, can lead to discovery of new materials in many application domains.”
Quinones are abundant in crude oil as well as in green plants. The molecule the Harvard team used in its first quinone-based flow battery is almost identical to one found in rhubarb. The quinones are dissolved in water, which prevents them from catching fire.
To back up a commercial wind turbine, a large storage tank would be needed, possibly located in a below-grade basement, said co-lead author Michael Marshak, a postdoctoral fellow at SEAS and in the Department of Chemistry and Chemical Biology. With a whole field of turbines or a large solar farm, one could imagine a few very large storage tanks.
The same technology could also have applications at the consumer level, Marshak said. “Imagine a device the size of a home heating-oil tank sitting in your basement. It would store a day’s worth of sunshine from the solar panels on the roof of your house, potentially providing enough to power your household from late afternoon, through the night, into the next morning, without burning any fossil fuels.”
“The Harvard team’s results published in Nature demonstrate an early, yet important technical achievement that could be critical in furthering the development of grid-scale batteries,” said ARPA-E Program Director John Lemmon. “The project team’s result is an excellent example of how a small amount of catalytic funding from ARPA-E can help build the foundation to hopefully turn scientific discoveries into low-cost, early-stage energy technologies.”
Team leader Aziz said the next steps in the project will be to further test and optimize the system that has been demonstrated on the benchtop and bring it toward a commercial scale. “So far, we’ve seen no sign of degradation after more than 100 cycles, but commercial applications require thousands of cycles,” he said. He also expects to achieve significant improvements in the underlying chemistry of the battery system. “I think the chemistry we have right now might be the best that’s out there for stationary storage and quite possibly cheap enough to make it in the marketplace,” he said. “But we have ideas that could lead to huge improvements.”
By the end of the three-year development period, Connecticut-basedSustainable Innovations, LLC, a collaborator on the project, expects to deploy demonstration versions of the organic flow battery contained in a unit the size of a horse trailer. The portable, scaled-up storage system could be hooked up to solar panels on the roof of a commercial building, and electricity from the solar panels could either directly supply the needs of the building or go into storage and come out of storage when needed. Sustainable Innovations anticipates playing a key role in the product’s commercialization by leveraging its ultra-low-cost electrochemical cell design and system architecture already under development for energy storage applications.
“You could theoretically put this on any node on the grid,” Aziz said. “If the market price fluctuates enough, you could put a storage device there and buy electricity to store it when the price is low and then sell it back when the price is high. In addition, you might be able to avoid the permitting and gas-supply problems of having to build a gas-fired power plant just to meet the occasional needs of a growing peak demand.”
This technology could also provide very useful backup for off-grid rooftop solar panels — an important advantage considering some 20 percent of the world’s population does not have access to a power distribution network.
“The intermittent renewables storage problem is the biggest barrier to getting most of our power from the sun and the wind,” Aziz said. “A safe and economical flow battery could play a huge role in our transition off fossil fuels to renewable electricity. I’m excited that we have a good shot at it.”

Tuesday, June 11, 2013

Harvard SEAS: Beautiful "flowers" self-assemble in a beaker

"Spring is like a perhaps hand," wrote the poet E. E. Cummings: "carefully / moving a perhaps / fraction of flower here placing / an inch of air there... / without breaking anything."
With the hand of nature trained on a beaker of chemical fluid, the most delicate flower structures have been formed in a Harvard laboratory—and not at the scale of inches, but microns.
These minuscule sculptures, curved and delicate, don't resemble the cubic or jagged forms normally associated with crystals, though that's what they are. Rather, fields of carnations and marigolds seem to bloom from the surface of a submerged glass slide, assembling themselves a molecule at a time.
By simply manipulating chemical gradients in a beaker of fluid, Wim L. Noorduin, a postdoctoral fellow at the Harvard School of Engineering and Applied Sciences(SEAS) and lead author of a paper appearing on the cover of the May 17 issue ofScience, has found that he can control the growth behavior of these crystals to create precisely tailored structures.
"For at least 200 years, people have been intrigued by how complex shapes could have evolved in nature. This work helps to demonstrate what’s possible just through environmental, chemical changes," says Noorduin.
The precipitation of the crystals depends on a reaction of compounds that are diffusing through a liquid solution. The crystals grow toward or away from certain chemical gradients as the pH of the reaction shifts back and forth. The conditions of the reaction dictate whether the structure resembles broad, radiating leaves, a thin stem, or a rosette of petals.
It is not unusual for chemical gradients to influence growth in nature; for example, delicately curved marine shells form from calcium carbonate under water, and gradients of signaling molecules in a human embryo help set up the plan for the body. Similarly, Harvard biologist Howard Berg has shown that bacteria living in colonies can sense and react to plumes of chemicals from one another, which causes them to grow, as a colony, into intricate geometric patterns.
Replicating this type of effect in the laboratory was a matter of identifying a suitable chemical reaction and testing, again and again, how variables like the pH, temperature, and exposure to air might affect the nanoscale structures.

The project fits right in with the work of Joanna Aizenberg, an expert in biologically inspired materials science, biomineralization, and self-assembly, and principal investigator for this research.
Aizenberg is the Amy Smith Berylson Professor of Materials Science at Harvard SEAS, Professor of Chemistry and Chemical Biology in the Harvard Department of Chemistry and Chemical Biology, and a Core Faculty Member of the Wyss Institute for Biologically Inspired Engineering at Harvard.
Her recent work has included the invention of an extremely slippery material, inspired by the pitcher plant, and the discovery of how bacteria use their flagella to cling to the surfaces of medical implants.
"Our approach is to study biological systems, to think what they can do that we can’t, and then to use these approaches to optimize existing technologies or create new ones," says Aizenberg. "Our vision really is to build as organisms do."
To create the flower structures, Noorduin and his colleagues dissolve barium chloride (a salt) and sodium silicate (also known as waterglass) into a beaker of water. Carbon dioxide from air naturally dissolves in the water, setting off a reaction which precipitates barium carbonate crystals. As a byproduct, it also lowers the pH of the solution immediately surrounding the crystals, which then triggers a reaction with the dissolved waterglass. This second reaction adds a layer of silica to the growing structures, uses up the acid from the solution, and allows the formation of barium carbonate crystals to continue.
"You can really collaborate with the self-assembly process," says Noorduin. "The precipitation happens spontaneously, but if you want to change something then you can just manipulate the conditions of the reaction and sculpt the forms while they're growing."
Increasing the concentration of carbon dioxide, for instance, helps to create 'broad-leafed' structures. Reversing the pH gradient at the right moment can create curved, ruffled structures.
Noorduin and his colleagues have grown the crystals on glass slides and metal blades; they've even grown a field of flowers in front of President Lincoln's seat on a one-cent coin.
"When you look through the electron microscope, it really feels a bit like you’re diving in the ocean, seeing huge fields of coral and sponges," describes Noorduin. "Sometimes I forget to take images because it's so nice to explore."
In addition to her roles at Harvard SEAS, the Department of Chemistry and Chemical Biology, and the Wyss Institute, Joanna Aizenberg is Director of the Kavli Institute for Bionano Science and Technology at Harvard and Director of the Science Program at the Radcliffe Institute for Advanced Study.
Coauthors included Alison Grinthal, a research scientist at Harvard SEAS, and L. Mahadevan, who is the Lola England de Valpine Professor of Applied Mathematics at SEAS, Professor of Organismic and Evolutionary Biology and of Physics, and a Core Faculty Member at the Wyss Institute.
The project was supported by National Science Foundation grants to the Harvard Materials Research Science and Engineering Center (DMR-0820484) and the Harvard Center for Nanoscale Systems (ECS-0335765); and by the Netherlands Organization for Scientific Research.

Friday, September 28, 2012

Scientists Produce Hydrogen for Fuel Cells Using an Inexpensive Catalyst Under Real-World Conditions

Scientists at the University of Cambridge have produced hydrogen, H2, a renewable energy source, from water using an inexpensive catalyst under industrially relevant conditions (using pH neutral water, surrounded by atmospheric oxygen, O2, and at room temperature).

Lead author of the research, Dr Erwin Reisner, an EPSRC research fellow and head of the Christian Doppler Laboratory at the University of Cambridge, said: "A H2 evolution catalyst which is active under elevated O2 levels is crucial if we are to develop an industrial water splitting process -- a chemical reaction that separates the two elements which make up water. A real-world device will be exposed to atmospheric O2 and also produce O2 in situ as a result of water splitting."

Although H2 cannot be used as a 'direct' substitute for gasoline or ethanol, it can be used as a fuel in combination with fuel cells, which are already available in cars and buses. H2 is currently produced from fossil fuels and it produces the greenhouse gas CO2 as a by-product; it is therefore neither renewable nor clean. A green process such as sunlight-driven water splitting is therefore required to produce 'green and sustainable H2'.

One of the many problems that scientists face is finding an efficient and inexpensive catalyst that can function under real-world conditions: in water, under air and at room temperature. Currently, highly efficient catalysts such as the noble metal platinum are too expensive and cheaper alternatives are typically inefficient. Very little progress was made so far with homogeneous catalyst systems that work in water and atmospheric O2.

However, Cambridge researchers found that a simple catalyst containing cobalt, a relatively inexpensive and abundant metal, operates as an active catalyst in pH neutral water and under atmospheric O2.

Dr Reisner said: "Until now, no inexpensive molecular catalyst was known to evolve H2 efficiently in water and under aerobic conditions. However, such conditions are essential for use in developing green hydrogen as a future energy source under industrially relevant conditions.

"Our research has shown that inexpensive materials such as cobalt are suitable to fulfill this challenging requirement. Of course, many hurdles such as the rather poor stability of the catalyst remain to be addressed, but our finding provides a first step to produce 'green hydrogen' under relevant conditions."

The results show that the catalyst works under air and the researchers are now working on a solar water splitting device, where a fuel H2 and the by-product O2 are produced simultaneously.

Fezile Lakadamyali and Masaru Kato, co-authors of the study, add: "We are excited about our results and we are optimistic that we will successfully assemble a sunlight-driven water splitting system soon."


Tuesday, August 21, 2012

Self-Charging Power Cell Converts and Stores Energy

Researchers have developed a self-charging power cell that directly converts mechanical energy to chemical energy, storing the power until it is released as electrical current. By eliminating the need to convert mechanical energy to electrical energy for charging a battery, the new hybrid generator-storage cell utilizes mechanical energy more efficiently than systems using separate generators and batteries.

At the heart of the self-charging power cell is a piezoelectric membrane that drives lithium ions from one side of the cell to the other when the membrane is deformed by mechanical stress. The lithium ions driven through the polarized membrane by the piezoelectric potential are directly stored as chemical energy using an electrochemical process.

By harnessing a compressive force, such as a shoe heel hitting the pavement from a person walking, the power cell generates enough current to power a small calculator. A hybrid power cell the size of a conventional coin battery can power small electronic devices -- and could have military applications for soldiers who might one day recharge battery-powered equipment as they walked.

"People are accustomed to considering electrical generation and storage as two separate operations done in two separate units," said Zhong Lin Wang, a Regents professor in the School of Materials Science and Engineering at the Georgia Institute of Technology. "We have put them together in a single hybrid unit to create a self-charging power cell, demonstrating a new technique for charge conversion and storage in one integrated unit."

The research was reported Aug. 9, 2012 in the journal Nano Letters. The research was supported by the Defense Advanced Research Projects Agency (DARPA), the U.S. Air Force, the U.S. Department of Energy, the National Science Foundation, and the Knowledge Innovation Program of the Chinese Academy of Sciences.

The power cell consists of a cathode made from lithium-cobalt oxide (LiCoO2) and an anode consisting of titanium dioxide (TiO2) nanotubes grown atop a titanium film. The two electrodes are separated by a membrane made from poly(vinylidene fluoride) (PVDF) film, which generates a piezoelectric charge when placed under strain. When the power cell is mechanically compressed, the PVDF film generates a piezoelectric potential that serves as a charge pump to drive the lithium ions from the cathode side to the anode side. The energy is then stored in the anode as lithium-titanium oxide.

Charging occurs in cycles with the compression of the power cell creating a piezopotential that drives the migration of lithium ions until a point at which the chemical equilibrium of the two electrodes are re-established and the distribution of lithium ions can balance the piezoelectric fields in the PVDF film. When the force applied to the power cell is released, the piezoelectric field in the PVDF disappears, and the lithium ions are kept at the anode through a chemical process.

The charging cycle is completed through an electrochemical process that oxidizes a small amount of lithium-cobalt oxide at the cathode to Li1-xCoO2 and reduces a small amount of titanium dioxide to LixTiO2 at the anode. Compressing the power cell again repeats the cycle.

When an electrical load is connected between the anode and cathode, electrons flow to the load, and the lithium ions within the cell flow back from the anode side to the cathode side.

Using a mechanical compressive force with a frequency of 2.3 Hertz, the researchers increased the voltage in the power cell from 327 to 395 millivolts in just four minutes. The device was then discharged back to its original voltage with a current of one milliamp for about two minutes. The researchers estimated the stored electric capacity of the power cell to be approximately 0.036 milliamp-hours.

So far, Wang and his research team -- which included Xinyu Xue, Sihong Wang, Wenxi Guo and Yan Zhang -- have built and tested more than 500 of the power cells. Wang estimates that the generator-storage cell will be as much as five times more efficient at converting mechanical energy to chemical energy for as a two-cell generator-storage system.

Much of the mechanical energy applied to the cells is now consumed in deforming the stainless steel case the researchers are using to house their power cell. Wang believes the power storage could be boosted by using an improved case.

"When we improve the packaging materials, we anticipate improving the overall efficiency," he said. "The amount of energy actually going into the cell is relatively small at this stage because so much of it is consumed by the shell."

Beyond the efficiencies that come from directly converting mechanical energy to chemical energy, the power cell could also reduce weight and space required by separate generators and batteries. The mechanical energy could come from walking, the tires of a vehicle hitting the pavement, or by harnessing ocean waves or mechanical vibrations.

"One day we could have a power package ready to use that takes advantage of this hybrid approach," Wang said. "Almost anything that involves mechanical action could provide the strain needed for charging. People walking could be generating electricity as they move."


Monday, August 6, 2012

Scientists Use Microbes to Make 'Clean' Methane

Microbes that convert electricity into methane gas could become an important source of renewable energy, according to scientists from Stanford and Pennsylvania State universities.

Researchers at both campuses are raising colonies of microorganisms, called methanogens, which have the remarkable ability to turn electrical energy into pure methane -- the key ingredient in natural gas. The scientists' goal is to create large microbial factories that will transform clean electricity from solar, wind or nuclear power into renewable methane fuel and other valuable chemical compounds for industry.

"Most of today's methane is derived from natural gas, a fossil fuel," said Alfred Spormann, a professor of chemical engineering and of civil and environmental engineering at Stanford. "And many important organic molecules used in industry are made from petroleum. Our microbial approach would eliminate the need for using these fossil resources."

While methane itself is a formidable greenhouse gas, 20 times more potent than CO2, the microbial methane would be safely captured and stored, thus minimizing leakage into the atmosphere, Spormann said.

"The whole microbial process is carbon neutral," he explained. "All of the CO2 released during combustion is derived from the atmosphere, and all of the electrical energy comes from renewables or nuclear power, which are also CO2-free."

Methane-producing microbes, he added, could help solve one of the biggest challenges for large-scale renewable energy: What to do with surplus electricity generated by photovoltaic power stations and wind farms.

"Right now there is no good way to store electricity," Spormann said. "However, we know that some methanogens can produce methane directly from an electrical current. In other words, they metabolize electrical energy into chemical energy in the form of methane, which can be stored. Understanding how this metabolic process works is the focus of our research. If we can engineer methanogens to produce methane at scale, it will be a game changer."

'Green' methane

Burning natural gas accelerates global warming by releasing carbon dioxide that's been trapped underground for millennia. The Stanford and Penn State team is taking a "greener" approach to methane production. Instead of drilling rigs and pumps, the scientists envision large bioreactors filled with methanogens -- single-cell organisms that resemble bacteria but belong to a genetically distinct group of microbes called archaea.

By human standards, a methanogen's lifestyle is extreme. It cannot grow in the presence of oxygen. Instead, it regularly dines on atmospheric carbon dioxideand electrons borrowed from hydrogen gas. The byproduct of this microbial meal is pure methane, which methanogens excrete into the atmosphere.

The researchers plan to use this methane to fuel airplanes, ships and vehicles. In the ideal scenario, cultures of methanogens would be fed a constant supply of electrons generated from emissions-free power sources, such as solar cells, wind turbines and nuclear reactors. The microbes would use these clean electrons to metabolize carbon dioxide into methane, which can then be stockpiled and distributed via existing natural gas facilities and pipelines when needed.

When the microbial methane is burnt as fuel, carbon dioxide would be recycled back into the atmosphere where it originated from -- unlike conventional natural gas combustion, which contributes to global warming.

"Microbial methane is much more ecofriendly than ethanol and other biofuels," Spormann said. "Corn ethanol, for example, requires acres of cropland, as well as fertilizers, pesticides, irrigation and fermentation. Methanogens are much more efficient, because they metabolize methane in just a few quick steps."

Microbial communities

For this new technology to become commercially viable, a number of fundamental challenges must be addressed.

"While conceptually simple, there are significant hurdles to overcome before electricity-to-methane technology can be deployed at a large scale," said Bruce Logan, a professor of civil and environmental engineering at Penn State. "That's because the underlying science of how these organisms convert electrons into chemical energy is poorly understood."

In 2009, Logan's lab was the first to demonstrate that a methanogen strain known as Methanobacterium palustre could convert an electrical current directly into methane. For the experiment, Logan and his Penn State colleagues built a reverse battery with positive and negative electrodes placed in a beaker of nutrient-enriched water.

The researchers spread a biofilm mixture of M. palustre and other microbial species onto the cathode. When an electrical current was applied, the M. palustre began churning out methane gas.

"The microbes were about 80 percent efficient in converting electricity to methane," Logan said.

The rate of methane production remained high as long as the mixed microbial community was intact. But when a previously isolated strain of pure M. palustre was placed on the cathode alone, the rate plummeted, suggesting that methanogens separated from other microbial species are less efficient than those living in a natural community.

"Microbial communities are complex," Spormann added. "For example, oxygen-consuming bacteria can help stabilize the community by preventing the build-up of oxygen gas, which methanogens cannot tolerate. Other microbes compete with methanogens for electrons. We want to identify the composition of different communities and see how they evolve together over time."

Microbial zoo

To accomplish that goal, Spormann has been feeding electricity to laboratory cultures consisting of mixed strains of archaea and bacteria. This microbial zoo includes bacterial species that compete with methanogensfor carbon dioxide, which the bacteria use to make acetate -- an important ingredient in vinegar, textiles and a variety of industrial chemicals.

"There might be organisms that are perfect for making acetate or methane but haven't been identified yet," Spormann said. "We need to tap into the unknown, novel organisms that are out there."

At Penn State, Logan's lab is designing and testing advanced cathode technologies that will encourage the growth of methanogens and maximize methane production. The Penn State team is also studying new materials for electrodes, including a carbon-mesh fabric that could eliminate the need for platinum and other precious metal catalysts.

"Many of these materials have only been studied in bacterial systems but not in communities with methanogens or other archaea," Logan said. "Our ultimate goal is to create a cost-effective system that reliably and robustly produces methane from clean electrical energy. It's high-risk, high-reward research, but new approaches are needed for energy storage and for making useful organic molecules without fossil fuels."

The Stanford-Penn State research effort is funded by a three-year grant from the Global Climate and Energy Project at Stanford.


Wednesday, July 25, 2012

Clothing the Body Electric: Cotton T-Shirt Fabric Can Store Electricity, Maybe Keep Your Cell Phone Charged

Over the years, the telephone has gone mobile, from the house to the car to the pocket. The University of South Carolina's Xiaodong Li envisions even further integration of the cell phone -- and just about every electronic gadget, for that matter -- into our lives.
He sees a future where electronics are part of our wardrobe.

"We wear fabric every day," said Li, a professor of mechanical engineering at USC. "One day our cotton T-shirts could have more functions; for example, a flexible energy storage device that could charge your cell phone or your iPad."

Li is helping make the vision a reality. He and post-doctoral associate Lihong Bao have just reported in the journal Advanced Materials how to turn the material in a cotton T-shirt into a source of electrical power.

Starting with a T-shirt from a local discount store, Li's team soaked it in a solution of fluoride, dried it and baked it at high temperature. They excluded oxygen in the oven to prevent the material from charring or simply combusting.

The surfaces of the resulting fibers in the fabric were shown by infrared spectroscopy to have been converted from cellulose to activated carbon. Yet the material retained flexibility; it could be folded without breaking.

"We will soon see roll-up cell phones and laptop computers on the market," Li said. "But a flexible energy storage device is needed to make this possible."

The once-cotton T-shirt proved to be a repository for electricity. By using small swatches of the fabric as an electrode, the researchers showed that the flexible material, which Li's team terms activated carbon textile, acts as a capacitor. Capacitors are components of nearly every electronic device on the market, and they have the ability to store electrical charge.

Moreover, Li reports that activated carbon textile acts like double-layer capacitors, which are also called a supercapacitors because they can have particularly high energy storage densities.

But Li and Bao took the material even further than that. They then coated the individual fibers in the activated carbon textile with "nanoflowers" of manganese oxide. Just a nanometer thick, this layer of manganese oxide greatly enhanced the electrode performance of the fabric. "This created a stable, high-performing supercapacitor," said Li.

This hybrid fabric, in which the activated carbon textile fibers are coated with nanostructured manganese oxide, improved the energy storage capability beyond the activated carbon textile alone. The hybrid supercapacitors were resilient: even after thousands of charge-discharge cycles, performance didn't diminish more than 5 percent.

"By stacking these supercapacitors up, we should be able to charge portable electronic devices such as cell phones," Li said.

Li is particularly pleased to have improved on the means by which activated carbon fibers are usually obtained. "Previous methods used oil or environmentally unfriendly chemicals as starting materials," he said. "Those processes are complicated and produce harmful side products. Our method is a very inexpensive, green process."


Highly Transparent Solar Cells for Windows That Generate Electricity

UCLA researchers have developed a new transparent solar cell that is an advance toward giving windows in homes and other buildings the ability to generate electricity while still allowing people to see outside. Their study appears in the journal ACS Nano.

The UCLA team describes a new kind of polymer solar cell (PSC) that produces energy by absorbing mainly infrared light, not visible light, making the cells nearly 70% transparent to the human eye. They made the device from a photoactive plastic that converts infrared light into an electrical current.

"These results open the potential for visibly transparent polymer solar cells as add-on components of portable electronics, smart windows and building-integrated photovoltaics and in other applications," said study leader Yang Yang, a UCLA professor of materials science and engineering, who also is director of the Nano Renewable Energy Center at California NanoSystems Institute (CNSI).

Yang added that there has been intense world-wide interest in so-called polymer solar cells. "Our new PSCs are made from plastic-like materials and are lightweight and flexible," he said. "More importantly, they can be produced in high volume at low cost."

Polymer solar cells have attracted great attention due to their advantages over competing solar cell technologies. Scientists have also been intensely investigating PSCs for their potential in making unique advances for broader applications. Several such applications would be enabled by high-performance visibly transparent photovoltaic (PV) devices, including building-integrated photovoltaics and integrated PV chargers for portable electronics.

Previously, many attempts have been made toward demonstrating visibly transparent or semitransparent PSCs. However, these demonstrations often result in low visible light transparency and/or low device efficiency because suitable polymeric PV materials and efficient transparent conductors were not well deployed in device design and fabrication.

A team of UCLA researchers from the California NanoSystems Institute, the UCLA Henry Samueli School of Engineering and Applied Science and UCLA's Department of Chemistry and Biochemistry have demonstrated high-performance, solution-processed, visibly transparent polymer solar cells through the incorporation of near-infrared light-sensitive polymer and using silver nanowire composite films as the top transparent electrode. The near-infrared photoactive polymer absorbs more near-infrared light but is less sensitive to visible light, balancing solar cell performance and transparency in the visible wavelength region.

Another breakthrough is the transparent conductor made of a mixture of silver nanowire and titanium dioxide nanoparticles, which was able to replace the opaque metal electrode used in the past. This composite electrode also allows the solar cells to be fabricated economically by solution processing. With this combination, 4% power-conversion efficiency for solution-processed and visibly transparent polymer solar cells has been achieved.

"We are excited by this new invention on transparent solar cells, which applied our recent advances in transparent conducting windows (also published in ACS Nano) to fabricate these devices," said Paul S.Weiss, CNSI director and Fred Kavli Chair in NanoSystems Sciences.

Study authors also include Weiss; materials science and engineering postdoctoral researcher Rui Zhu; Ph.D. candidates Chun-Chao Chen, Letian Dou, Choong-Heui Chung, Tze-Bin Song and Steve Hawks; Gang Li, who is former vice president of engineering for Solarmer Energy, Inc., a startup from UCLA; and CNSI postdoctoral researcher Yue Bing Zheng.