Energy storage device could deliver more power than current versions of this technology.
Energy storage devices called supercapacitors have become a hot area of research, in part because they can be charged rapidly and deliver intense bursts of power. However, all supercapacitors currently use components made of carbon, which require high temperatures and harsh chemicals to produce.
Now researchers at MIT and elsewhere have for the first time developed a supercapacitor that uses no conductive carbon at all, and that could potentially produce more power than existing versions of this technology.
The team’s findings are being reported in the journal Nature Materials, in a paper by Mircea Dinc?, an MIT associate professor of chemistry; Yang Shao-Horn, the W.M. Keck Professor of Energy; and four others.
“We’ve found an entirely new class of materials for supercapacitors,” Dinc? says.
Dinc? and his team have been exploring for years a class of materials called metal-organic frameworks, or MOFs, which are extremely porous, sponge-like structures. These materials have an extraordinarily large surface area for their size, much greater than the carbon materials do. That is an essential characteristic for supercapacitors, whose performance depends on their surface area. But MOFs have a major drawback for such applications: They are not very electrically conductive, which is also an essential property for a material used in a capacitor.
“One of our long-term goals was to make these materials electrically conductive,” Dinc? says, even though doing so “was thought to be extremely difficult, if not impossible.” But the material did exhibit another needed characteristic for such electrodes, which is that it conducts ions (atoms or molecules that carry a net electric charge) very well.
“All double-layer supercapacitors today are made from carbon,” Dinc? says. “They use carbon nanotubes, graphene, activated carbon, all shapes and forms, but nothing else besides carbon. So this is the first noncarbon, electrical double-layer supercapacitor.”
One advantage of the material used in these experiments, technically known as Ni3(hexaiminotriphenylene)2, is that it can be made under much less harsh conditions than those needed for the carbon-based materials, which require very high temperatures above 800 degrees Celsius and strong reagent chemicals for pretreatment.
The team says supercapacitors, with their ability to store relatively large amounts of power, could play an important role in making renewable energy sources practical for widespread deployment. They could provide grid-scale storage that could help match usage times with generation times, for example, or be used in electric vehicles and other applications.
The new devices produced by the team, even without any optimization of their characteristics, already match or exceed the performance of existing carbon-based versions in key parameters, such as their ability to withstand large numbers of charge/discharge cycles. Tests showed they lost less than 10 percent of their performance after 10,000 cycles, which is comparable to existing commercial supercapacitors.
But that’s likely just the beginning, Dinc? says. MOFs are a large class of materials whose characteristics can be tuned to a great extent by varying their chemical structure. Work on optimizing their molecular configurations to provide the most desirable attributes for this specific application is likely to lead to variations that could outperform any existing materials. “We have a new material to work with, and we haven’t optimized it at all,” he says. “It’s completely tunable, and that’s what’s exciting.”
While there has been much research on MOFs, most of it has been directed at uses that take advantage of the materials’ record porosity, such as for storage of gases. “Our lab’s discovery of highly electrically conductive MOFs opened up a whole new category of applications,” Dinc? says. Besides the new supercapacitor uses, the conductive MOFs could be useful for making electrochromic windows, which can be darkened with the flip of a switch, and chemoresistive sensors, which could be useful for detecting trace amounts of chemicals for medical or security applications.
While the MOF material has advantages in the simplicity and potentially low cost of manufacturing, the materials used to make it are more expensive than conventional carbon-based materials, Dinc? says. “Carbon is dirt cheap. It’s hard to find anything cheaper.” But even if the material ends up being more expensive, if its performance is significantly better than that of carbon-based materials, it could find useful applications, he says.
This discovery is “very significant, from both a scientific and applications point of view,” says Alexandru Vlad, a professor of chemistry at the Catholic University of Louvain in Belgium, who was not involved in this research. He adds that “the supercapacitor field was (but will not be anymore) dominated by activated carbons,” because of their very high surface area and conductivity. But now, “here is the breakthrough provided by Dinca et al.: They could design a MOF with high surface area and high electrical conductivity, and thus completely challenge the supercapacitor value chain! There is essentially no more need of carbons for this highly demanded technology.”
And a key advantage of that, he explains, is that “this work shows only the tip of the iceberg. With carbons we know pretty much everything, and the developments over the past years were modest and slow. But the MOF used by Dinca is one of the lowest-surface-area MOFs known, and some of these materials can reach up to three times more [surface area] than carbons. The capacity would then be astonishingly high, probably close to that of batteries, but with the power performance [the ability to deliver high power output] of supercapacitors.”
Learn more: New kind of supercapacitor made without carbon
Nanomaterial combines attributes of both batteries and supercapacitors
A powerful new material developed by Northwestern University chemist William Dichtel and his research team could one day speed up the charging process of electric cars and help increase their driving range.
An electric car currently relies on a complex interplay of both batteries and supercapacitors to provide the energy it needs to go places, but that could change.
“Our material combines the best of both worlds — the ability to store large amounts of electrical energy or charge, like a battery, and the ability to charge and discharge rapidly, like a supercapacitor,” said Dichtel, a pioneer in the young research field of covalent organic frameworks (COFs).
Dichtel and his research team have combined a COF — a strong, stiff polymer with an abundance of tiny pores suitable for storing energy — with a very conductive material to create the first modified redox-active COF that closes the gap with other older porous carbon-based electrodes.
“COFs are beautiful structures with a lot of promise, but their conductivity is limited,” Dichtel said. “That’s the problem we are addressing here. By modifying them — by adding the attribute they lack — we can start to use COFs in a practical way.”
And modified COFs are commercially attractive: COFs are made of inexpensive, readily available materials, while carbon-based materials are expensive to process and mass-produce.
Dichtel, the Robert L. Letsinger Professor of Chemistry at the Weinberg College of Arts and Sciences, is presenting his team’s findings today (Aug. 24) at the American Chemical Society (ACS) National Meeting in Philadelphia. Also today, a paper by Dichtel and co-authors from Northwestern and Cornell University was published by the journal ACS Central Science.
To demonstrate the new material’s capabilities, the researchers built a coin-cell battery prototype device capable of powering a light-emitting diode for 30 seconds.
The material has outstanding stability, capable of 10,000 charge/discharge cycles, the researchers report. They also performed extensive additional experiments to understand how the COF and the conducting polymer, called poly(3,4-ethylenedioxythiophene) or PEDOT, work together to store electrical energy.
Dichtel and his team made the material on an electrode surface. Two organic molecules self-assembled and condensed into a honeycomb-like grid, one 2-D layer stacked on top of the other. Into the grid’s holes, or pores, the researchers deposited the conducting polymer.
Each pore is only 2.3 nanometers wide, but the COF is full of these useful pores, creating a lot of surface area in a very small space. A small amount of the fluffy COF powder, just enough to fill a shot glass and weighing the same as a dollar bill, has the surface area of an Olympic swimming pool.
The modified COF showed a dramatic improvement in its ability to both store energy and to rapidly charge and discharge the device. The material can store roughly 10 times more electrical energy than the unmodified COF, and it can get the electrical charge in and out of the device 10 to 15 times faster.
“It was pretty amazing to see this performance gain,” Dichtel said. “This research will guide us as we investigate other modified COFs and work to find the best materials for creating new electrical energy storage devices.”
Plasma etching makes biochar activation faster
The ability to absorb and discharge energy quickly make supercapacitors an integral part of energy harvesting systems, such as the regenerative braking systems of hybrid vehicles, according to explainthatstuff.com. However, supercapacitors are expensive.
About half the materials cost comes from the use of activated carbon to coat the electrodes, according to Materials Today. Supercapacitor-grade activated carbon can cost $15 per kilogram.
Two South Dakota State University engineering researchers are using biochar, an inexpensive carbon-rich material and a new method of creating the porous surface needed to capture electricity to reduce the cost of supercapacitors.
Associate professor Qi Hua Fan of electrical engineering and computer Science uses plasma etching to active the biochar. Associate professor Zhengrong Gu of agricultural and biosystems engineering uses the activated biochar to make supercapacitors. Biochar is a byproduct of the pyrolysis process that turns plant materials into biofuel.
“Raw biochar needs activation to create the porous structure needed to trap ions,” explained Fan. Traditional chemical activation requires a high temperature, in the range of 1,700 Fahrenheit for two hours, and a chemical catalyst, followed by chemical washing and prolonged drying. This makes it an energy-intensive, time-consuming process.
The charcoal-like biochar can be made from crop residue, such as corn stover, wood or even dried distillers grain with solubles, known as DDGS. However, for this research, Fan used commercially available biochar made from yellow pine.
Several research groups had analyzed the specific capacitance and performance of this type of biochar, he explained, “so we had a baseline.” In addition, a company could supply the quantities of biochar necessary to make sure that test results were repeatable.
To do the plasma etching, oxygen was used and excited by radio frequency through a dielectric barrier discharge. Fan then gave the activated biochar to Gu, who made the supercapacitors. The research was supported by a five-month, proof-of-concept grant from the North Central Regional Sun Grant Center. Two graduate students worked on the project.
Increasing capacitance, improving efficiency
When the researchers compared capacitor performance, they found that those made using plasma treatment had 1.7 times higher specific capacitance, 171.4 Farads, compared to 99.5 Farads using chemical activation. “That’s a big improvement,” Fan pointed out.
The process took only five minutes with no external heating or chemicals needed. “It is very fast and consumes very little energy,” he noted. “The energy required to activate biochar is equivalent to what we use for a light bulb.”
In a paper published in the Journal of Power Sources, Fan, Gu and assistant physics professor Parashu Kharel explain, “oxygen plasma was capable of creating various pore sizes that would allow easy access for the electrolyte ions to the porous surface, leading to a higher capacitance than the chemically activated biochar.”
In addition, oxygen plasma-activated capacitors had lower estimated resistance, 3.3 ohms, as opposed to 14.5 ohms for chemically treated capacitors. This was attributed to the ions having easier access to the micropores and mesopores created by plasma processing.
And, Fan added, “Yellow pine is not the best biochar for supercapacitors.” He expects a similar improvement in performance using biochar derived from other types of biomass.
However, he pointed out, the process must be optimized for each type of structure. “Activation depends on what kind of plasma, what conditions are used and how long we treat the material.”
Fan has filed a patent application for the plasma activation process he developed. The next step will be to apply for funding to expand this promising processing technique for other types of biochar.
“No matter what kind of parameters we eventually end up with, this will be very efficient,” he added.
Tour is convinced the day is coming when supercapacitors replace batteries entirely, as energy storage systems will charge in minutes rather than hours.
“We’re not quite there yet, but we’re getting closer all the time,” he said. “In the interim, they’re able to supplement batteries with high power. What we have now is as good as some commercial supercapacitors. And they’re just plastic.”
Nanotechnology offers new approach to increasing storage ability of dielectric capacitors
For Back to the Future fans, this week marked a milestone that took three decades to reach.
Oct. 21, 2015, was the day that Doc Brown and Marty McFly landed in the future in their DeLorean, with time travel made possible by a “flux capacitor.”
While the flux capacitor still conjures sci-fi images, capacitors are now key components of portable electronics, computing systems, and electric vehicles.
In contrast to batteries, which offer high storage capacity but slow delivery of energy, capacitors provide fast delivery but poor storage capacity.
A great deal of effort has been devoted to improving this feature — known as energy density — of dielectric capacitors, which comprise an insulating material sandwiched between two conducting metal plates.
The work is reported in a paper, “Dielectric Capacitors with Three-Dimensional Nanoscale Interdigital Electrodes for Energy Storage,” published in Science Advances, the first open-access, online-only journal of AAAS.
“With our approach, we achieved an energy density of about two watts per kilogram, which is significantly higher than that of other dielectric capacitor structures reported in the literature,” says Bingqing Wei, professor of mechanical engineering at UD.
“To our knowledge, this is the first time that 3D nanoscale interdigital electrodes have been realized in practice,” he adds. “With their high surface area relative to their size, carbon nanotubes embedded in uniquely designed and structured 3D architectures have enabled us to address the low ability of dielectric capacitors to store energy.”
Read more: Capacitor breakthrough
An international team of scientists has developed what may be the first one-step process for making seamless carbon-based nanomaterials that possess superior thermal, electrical and mechanical properties in three dimensions.
The research holds potential for increased energy storage in high efficiency batteries and supercapacitors, increasing the efficiency of energy conversion in solar cells, for lightweight thermal coatings and more. The study is published today (Sept. 4) in the online journal Science Advances.
In early testing, a three-dimensional (3D) fiber-like supercapacitor made with the uninterrupted fibers of carbon nanotubes and graphene matched or bettered–by a factor of four–the reported record-high capacities for this type of device.
Used as a counter electrode in a dye-sensitized solar cell, the material enabled the cell to convert power with up to 6.8 percent efficiency and more than doubled the performance of an identical cell that instead used an expensive platinum wire counter electrode.