Researcher at TU Graz demonstrates in Nature Materials that it is possible to combine the high-energy density of batteries with the high-power output of super capacitors in a single system – thanks to liquid energy storage materials.
Batteries and super capacitors are electrochemical energy storage media, but they are as different as night and day. Both are capable of energy storage and targeted energy release – and yet there are major differences between the two. Batteries store very large amounts of energy that is released slowly but constantly. By contrast, super capacitors can only store small amounts of energy, but they release this energy much faster and more powerfully with large short-term peak currents.
Stefan Freunberger at TU Graz together with a group of researchers from Université de Montpellier in Southern France had a sudden flash of insight. Why not exploit the benefits of batteries and super capacitors simultaneously and combine them in some kind of energy hybrid, they asked themselves. In the current issue of renowned scientific journal Nature Materials the group introduces its approach, describing a liquid energy storage material for the first time in a European Research Council (ERC) sponsored study. While the energy density of this material is comparable to that of a battery, its power output equals that of a super capacitor.
Ions with an urge to move
“Batteries release energy so slowly and take so long to charge because their energy storage materials are solid. This make it difficult for the ions to move. But as the ions in a super capacitor move in a liquid, they are much more mobile than in a solid body,” explains Stefan Freunberger from the Institute of Chemistry and Technology of Materials at TU Graz. The novel redox active ionic liquid developed by Freunberger in co-operation with the French colleagues consists of an organic salt that is liquid at a temperature of just below 30 °C – only slightly above room temperature. Similar to a solid storage medium this liquid can store many ions, but allows them to be much more mobile.
The sudden flash of insight of Freunberger and colleagues culminated in a first approach to create an integrated energy supply system that offers a constant energy supply with high-power output. In some cases we are still faced with an either/or decision. Automatic doors, for example in trams or trains, are typical candidates for super capacitors. Energy is only needed for a very short time but when it is, a high-power output is of the essence. In other cases batteries are clearly the first choice. “But our principle of an energy hybrid can offer enormous advantages, for example when applied in electric vehicles. So far, electric vehicles often carry a combination of different battery types or battery systems together with super capacitors. If we had a single system that combines the benefits of both energy storage types, we could save considerable space and resources,” remarks Freunberger.
Learn more: Energy hybrid: Battery meets supercapacitor
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
Dr David Eisenberg and Prof. Gadi Rothenberg of the University of Amsterdam’s Van ‘t Hoff Institute for Molecular Sciences have invented a new type of supercapacitor material with a host of potential applications in electronics, transportation and energy storage devices. The UvA has filed a patent application on this invention.
Eisenberg and Rothenberg discovered the supercapacitor material during sideline experiments as part of the Fuel Cells project of the Research Priority Area Sustainable Chemistry. Originally, the materials were developed as solid catalytic electrodes for fuel cells. By modifying the surface of these materials the scientists created a highly porous yet well-structured compound, with ample sites for fast redox reactions, inspiring the successful testing for supercapacitance.
The new material combines several practical advantages: It is light, cheap, and non-toxic, and it can be prepared easily on a large scale. This last aspect is crucial for industrial applications, according to Eisenberg: ‘Companies making electronic devices look for low-cost, highly reproducible materials with a low environmental impact. The literature abounds with reports of high-performance electronic materials, but these will only be applied if they can be made cheaply in large quantities’.
Graphene and metal nitrides improve the performance and stability of energy storage devices
Supercapacitors can be charged and discharged tens of thousands of times, but their relatively low energy density compared to conventional batteries limits their application for energy storage. Now, A*STAR researchers have developed an ‘asymmetric’ supercapacitor based on metal nitrides and graphene that could be a viable energy storage solution.