Supercapacitors get supercharged with a new class of ionic liquids

Illustration of detergent-like ionic liquids on an electrode surface Xianwen Mao/Massachusetts Institute of Technology

An international team of scientists, including a professor of chemistry from the University of Bristol, has worked out a way to improve energy storage devices called supercapacitors, by designing a new class of detergents chemically related to laxatives.

Their paper, published today in the journal Nature Materials, explains why these detergents, called ionic liquids, are better electrolytes than current materials and can improve supercapacitors.

Currently, aqueous and organic electrolytes are used, but more recently, researchers and manufacturers have been testing ionic liquids instead to boost performance.

Although ionic liquids are salts, at room temperature they are surprisingly not crystalline solids – as their name suggests they are in fact liquids.

This gives ionic liquids numerous advantages over conventional electrolytes because they are stable, non-flammable, and often much more environmentally friendly.

To explore the exciting potential offered by ionic liquids for emerging electrochemical technologies the authors designed a new set of highly efficient detergent-like ionic liquid electrolytes and explained how they work at electrode surfaces.

Understanding how they operate will help design even more efficient devices for storing electrical energy.

Professor Julian Eastoe, from the University of Bristol’s School of Chemistry, is a co-author of the study. He said: “To make this discovery required a team of scientists with a very diverse skill set, spanning chemical synthesis, advanced structural, microscopy and electrical techniques as well as computational methods.

“This work demonstrates the power of scientific research ‘without borders’, the groups from different nations contributed their own expertise to make ‘the whole greater than the sum of parts’.”

Co-author, Xianwen Mao, from the Massachusetts Institute of Technology (MIT), added: “We engineered a new class of ionic liquids that can store energy more efficiently.

“These detergent-like ionic liquids can self-assemble into sandwich-like bilayer structures on electrode surfaces. And that is very reason why they give better energy storage performance.”

Typically, for electrolytes in contact with a charged electrode, the distribution of ions is dominated by electrostatic Coulombic interactions.

However, this distribution can be controlled by making the ionic liquids soap-like, or amphiphilic, so that the molecules now have separate polar and non-polar domains, exactly like common detergents.

These soap-like electrolytes then spontaneously form bilayer structures on the electrode surfaces, leading to much improved energy storage capabilities. The researchers found that temperature and applied voltage also affect the energy storage performance.

This new class of electrolytes may be suitable for challenging operations, such as oil drilling and space exploration, but they may also pave the way to new and improved supercapacitors in hybrid cars.

These devices are essential components in modern hybrid cars and can outperform batteries in terms of higher power and better efficiency.

This is particularly the case during regenerative braking where mechanical work is turned into electrical energy, which can be stored quickly in supercapacitors ready to be released.

This reduces energy consumption and is much more environmentally friendly. More importantly, using the new electrolytes such as developed in this study, future supercapacitors may even be able to store more energy than batteries, potentially replacing batteries in applications such as electrical vehicles, personal electronics, and grid-level energy storage facilities.

Learn more: Supercapacitors turbocharged by laxatives

 

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Wearable electronic components can now be incorporated directly into fabrics

via University of Cambridge

Washable, wearable ‘batteries’: based on cheap, safe and environmentally-friendly inks and woven directly into fabrics, have been developed by researchers at the University of Cambridge.

Turning textiles into functional energy storage elements can open up an entirely new set of applications

Felice Torrisi

Wearable electronic components incorporated directly into fabrics have been developed by researchers at the University of Cambridge. The devices could be used for flexible circuits, healthcare monitoring, energy conversion, and other applications.

The Cambridge researchers, working in collaboration with colleagues at Jiangnan University in China, have shown how graphene – a two-dimensional form of carbon – and other related materials can be directly incorporated into fabrics to produce charge storage elements such as capacitors, paving the way to textile-based power supplies which are washable, flexible and comfortable to wear.

The research, published in the journal Nanoscale, demonstrates that graphene inks can be used in textiles able to store electrical charge and release it when required. The new textile electronic devices are based on low-cost, sustainable and scalable dyeing of polyester fabric. The inks are produced by standard solution processing techniques.

Building on previous work by the same team, the researchers designed inks which can be directly coated onto a polyester fabric in a simple dyeing process. The versatility of the process allows various types of electronic components to be incorporated into the fabric.

Most other wearable electronics rely on rigid electronic components mounted on plastic or textiles. These offer limited compatibility with the skin in many circumstances, are damaged when washed and are uncomfortable to wear because they are not breathable.

“Other techniques to incorporate electronic components directly into textiles are expensive to produce and usually require toxic solvents, which makes them unsuitable to be worn,” said Dr Felice Torrisi from the Cambridge Graphene Centre, and the paper’s corresponding author. “Our inks are cheap, safe and environmentally-friendly, and can be combined to create electronic circuits by simply overlaying different fabrics made of two-dimensional materials on the fabric.”

The researchers suspended individual graphene sheets in a low boiling point solvent, which is easily removed after deposition on the fabric, resulting in a thin and uniform conducting network made up of multiple graphene sheets. The subsequent overlay of several graphene and hexagonal boron nitride (h-BN) fabrics creates an active region, which enables charge storage. This sort of ‘battery’ on fabric is bendable and can withstand washing cycles in a normal washing machine.

“Textile dyeing has been around for centuries using simple pigments, but our result demonstrates for the first time that inks based on graphene and related materials can be used to produce textiles that could store and release energy,” said co-author Professor Chaoxia Wang from Jiangnan University in China. “Our process is scalable and there are no fundamental obstacles to the technological development of wearable electronic devices both in terms of their complexity and performance.”

The work done by the Cambridge researchers opens a number of commercial opportunities for ink based on two-dimensional materials, ranging from personal health and well-being technology, to wearable energy and data storage, military garments, wearable computing and fashion.

“Turning textiles into functional energy storage elements can open up an entirely new set of applications, from body-energy harvesting and storage to the Internet of Things,” said Torrisi “In the future our clothes could incorporate these textile-based charge storage elements and power wearable textile devices.”

Learn more: Washable, wearable battery-like devices could be woven directly into clothes

 

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If you need more energy storage just print it with MXene ink

The development of highly conductive MXene ink allows for inkjet printing of energy storage devices like micro-supercapacitors.

Researchers from Drexel University and Trinity College in Ireland, have created ink for an inkjet printer from a highly conductive type of two-dimensional material called MXene. Recent findings, published in Nature Communications, suggest that the ink can be used to print flexible energy storage components, such as supercapacitors, in any size or shape.

Conductive inks have been around for nearly a decade and they represent a multi-hundred million-dollar market that is expected to grow rapidly into the next decade. It’s already being used to make the radiofrequency identification tags used in highway toll transponders, circuit boards in portable electronics and it lines car windows as embedded radio antennas and to aid defrosting. But for the technology to see broader use, conductive inks need to become more conductive and more easily applied to a range of surfaces.

 

Yury Gogotsi, PhD, Distinguished University and Bach professor in Drexel’s College of Engineering, Department of Materials Science and Engineering, who studies the applications of new materials in technology, suggests that the ink created in Drexel’s Nanomaterials Institute is a significant advancement on both of these fronts.

“So far only limited success has been achieved with conductive inks in both fine-resolution printing and high charge storage devices,” Gogotsi said. “But our findings show that all-MXene printed micro-supercapacitors, made with an advanced inkjet printer, are an order of magnitude greater than existing energy storage devices made from other conductive inks.”

While researchers are steadily figuring out ways to make inks from new, more conductive materials, like nanoparticle silver, graphene and gallium, the challenge remains incorporating them seamlessly into manufacturing processes. Most of these inks can’t be used in a one-step process, according to Babak Anasori, PhD, a research assistant professor in Drexel’s department of Materials Science and Engineering and co-author of the MXene ink research.

 

“For most other nano inks, an additive is required to hold the particles together and allow for high-quality printing. Because of this, after printing, an additional step is required — usually a thermal or chemical treatment — to remove that additive,” Anasori said. “For MXene printing, we only use MXene in water or MXene in an organic solution to make the ink. This means it can dry without any additional steps.”

MXenes are a type of carbon-based, two-dimensional layered materials, created at Drexel in 2011, that have the unique ability to mix with liquids, like water and other  organic solvents, while retaining their conductive properties. Because of this, Drexel researchers have produced and tested it in a variety of forms, from conductive clay to a coating for electromagnetic interference shielding to a near-invisible wireless antenna.

Adjusting the concentration to create ink for use in a commercial printer was a matter of time and iteration. The solvent and MXene concentration in the ink can be adjusted to suit different kinds of printers.

“If we really want to take advantage of any technology at a large scale and have it ready for public use, it has to become very simple and done in one step,” Anasori said. “An inkjet printer can be found in just about every house, so we knew if we could make the proper ink, it would be feasible that anyone could make future electronics and devices.”

 

As part of the study, the Drexel team, working with researchers at Trinity College, who are experts in printing, put the MXene ink to the test in a series of printouts, including a simple circuit, a mirco-supercapacitor and some text, on substrates ranging from paper to plastic to glass. In doing so, they found that they could print lines of consistent thickness and that the ink’s ability to pass an electric current varied with its thickness — both important factors in manufacturing electronics components. And the printouts maintained their superior electric conductivity, which is the highest among all carbon-based conductive inks, including carbon nanotubes and graphene.

This all amounts to a very versatile product for making the tiny components that perform important, but often overlooked functions in our electronics devices — jobs like keeping the power on when the battery dies, preventing damaging electrical surges, or speeding the charging process. Providing a higher-performing material and a new way to build things with it could lead not only to improvements to our current devices, but also the creation of entirely new technologies.

“Compared to conventional manufacturing protocols, direct ink printing techniques, such as inkjet printing and extrusion printing, allow digital and additive patterning, customization, reduction in material waste, scalability and rapid production,” Anasori said. “Now that we have produced a MXene ink that can be applied via this technique, we’re looking at a world of new opportunities to use it.”

Learn more: Need More Energy Storage? Just Hit ‘Print’

 

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Breakthrough: Reliable and relatively inexpensive renewable energy storage is just over the liquid silicon horizon

MIT researchers propose a concept for a renewable storage system, pictured here, that would store solar and wind energy in the form of white-hot liquid silicon, stored in heavily insulated tanks.
Image: Duncan MacGruer

Design for system that provides solar- or wind-generated power on demand should be cheaper than other leading options.

MIT engineers have come up with a conceptual design for a system to store renewable energy, such as solar and wind power, and deliver that energy back into an electric grid on demand. The system may be designed to power a small city not just when the sun is up or the wind is high, but around the clock.

The new design stores heat generated by excess electricity from solar or wind power in large tanks of white-hot molten silicon, and then converts the light from the glowing metal back into electricity when it’s needed. The researchers estimate that such a system would be vastly more affordable than lithium-ion batteries, which have been proposed as a viable, though expensive, method to store renewable energy. They also estimate that the system would cost about half as much as pumped hydroelectric storage — the cheapest form of grid-scale energy storage to date.

“Even if we wanted to run the grid on renewables right now we couldn’t, because you’d need fossil-fueled turbines to make up for the fact that the renewable supply cannot be dispatched on demand,” says Asegun Henry, the Robert N. Noyce Career Development Associate Professor in the Department of Mechanical Engineering. “We’re developing a new technology that, if successful, would solve this most important and critical problem in energy and climate change, namely, the storage problem.”

Henry and his colleagues have published their design today in the journal Energy and Environmental Science.

Record temps

The new storage system stems from a project in which the researchers looked for ways to increase the efficiency of a form of renewable energy known as concentrated solar power. Unlike conventional solar plants that use solar panels to convert light directly into electricity, concentrated solar power requires vast fields of huge mirrors that concentrate sunlight onto a central tower, where the light is converted into heat that is eventually turned into electricity.

“The reason that technology is interesting is, once you do this process of focusing the light to get heat, you can store heat much more cheaply than you can store electricity,” Henry notes.

Concentrated solar plants store solar heat in large tanks filled with molten salt, which is heated to high temperatures of about 1,000 degrees Fahrenheit. When electricity is needed, the hot salt is pumped through a heat exchanger, which transfers the salt’s heat into steam. A turbine then turns that steam into electricity.

“This technology has been around for a while, but the thinking has been that its cost will never get low enough to compete with natural gas,” Henry says. “So there was a push to operate at much higher temperatures, so you could use a more efficient heat engine and get the cost down.”

However, if operators were to heat the salt much beyond current temperatures, the salt would corrode the stainless steel tanks in which it’s stored. So Henry’s team looked for a medium other than salt that might store heat at much higher temperatures. They initially proposed a liquid metal and eventually settled on silicon — the most abundant metal on Earth, which can withstand incredibly high temperatures of over 4,000 degrees Fahrenheit.

Last year, the team developed a pump that could withstand such blistering heat, and could conceivably pump liquid silicon through a renewable storage system. The pump has the highest heat tolerance on record — a feat that is noted in “The Guiness Book of World Records.” Since that development, the team has been designing an energy storage system that could incorporate such a high-temperature pump.

“Sun in a box”

Now, the researchers have outlined their concept for a new renewable energy storage system, which they call TEGS-MPV, for Thermal Energy Grid Storage-Multi-Junction Photovoltaics. Instead of using fields of mirrors and a central tower to concentrate heat, they propose converting electricity generated by any renewable source, such as sunlight or wind, into thermal energy, via joule heating — a process by which an electric current passes through a heating element.

The system could be paired with existing renewable energy systems, such as solar cells, to capture excess electricity during the day and store it for later use. Consider, for instance, a small town in Arizona that gets a portion of its electricity from a solar plant.

“Say everybody’s going home from work, turning on their air conditioners, and the sun is going down, but it’s still hot,” Henry says. “At that point, the photovoltaics are not going to have much output, so you’d have to have stored some of the energy from earlier in the day, like when the sun was at noon. That excess electricity could be routed to the storage system we’ve invented here.”

The system would consist of a large, heavily insulated, 10-meter-wide tank made from graphite and filled with liquid silicon, kept at a “cold” temperature of almost 3,500 degrees Fahrenheit. A bank of tubes, exposed to heating elements, then connects this cold tank to a second, “hot” tank. When electricity from the town’s solar cells comes into the system, this energy is converted to heat in the heating elements. Meanwhile, liquid silicon is pumped out of the cold tank and further heats up as it passes through the bank of tubes exposed to the heating elements, and into the hot tank, where the thermal energy is now stored at a much higher temperature of about 4,300 F.

When electricity is needed, say, after the sun has set, the hot liquid silicon — so hot that it’s glowing white — is pumped through an array of tubes that emit that light. Specialized solar cells, known as multijunction photovoltaics, then turn that light into electricity, which can be supplied to the town’s grid. The now-cooled silicon can be pumped back into the cold tank until the next round of storage — acting effectively as a large rechargeable battery.

“One of the affectionate names people have started calling our concept, is ‘sun in a box,’ which was coined by my colleague Shannon Yee at Georgia Tech,” Henry says.  “It’s basically an extremely intense light source that’s all contained in a box that traps the heat.”

A storage key

Henry says the system would require tanks thick and strong enough to insulate the molten liquid within.

“The stuff is glowing white hot on the inside, but what you touch on the outside should be room temperature,” Henry says.

He has proposed that the tanks be made out of graphite. But there are concerns that silicon, at such high temperatures, would react with graphite to produce silicon carbide, which could corrode the tank.

To test this possibility, the team fabricated a miniature graphite tank and filled it with liquid silicon. When the liquid was kept at 3,600 F for about 60 minutes, silicon carbide did form, but instead of corroding the tank, it created a thin, protective liner.

“It sticks to the graphite and forms a protective layer, preventing further reaction,” Henry says. “So you can build this tank out of graphite and it won’t get corroded by the silicon.”

The group also found a way around another challenge: As the system’s tanks would have to be very large, it would be impossible to build them from a single piece of graphite. If they were instead made from multiple pieces, these would have to be sealed in such a way to prevent the molten liquid from leaking out. In their paper, the researchers demonstrated that they could prevent any leaks by screwing pieces of graphite together with carbon fiber bolts and sealing them with grafoil — flexible graphite that acts as a high-temperature sealant.

The researchers estimate that a single storage system could enable a small city of about 100,000 homes to be powered entirely by renewable energy.

“Innovation in energy storage is having a moment right now,” says Addison Stark, associate director for energy innovation at the Bipartisan Policy Center, and staff director for the American Energy Innovation Council. “Energy technologists recognize the imperative to have low-cost, high-efficiency storage options available to balance out nondispatchable generation technologies on the grid. As such, there are many great ideas coming to the fore right now. In this case, the development of a solid-state power block coupled with incredibly high storage temperatures pushes the boundaries of what’s possible.”

Henry emphasizes that the system’s design is geographically unlimited, meaning that it can be sited anywhere, regardless of a location’s landscape. This is in contrast to pumped hydroelectric — currently the cheapest form of energy storage, which requires locations that can accommodate large waterfalls and dams, in order to store energy from falling water.

“This is geographically unlimited, and is cheaper than pumped hydro, which is very exciting,” Henry says. “In theory, this is the linchpin to enabling renewable energy to power the entire grid.”

Learn more: “Sun in a box” would store renewable energy for the grid

 

 

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Paper supercapacitors for energy storage

via Phys.org

Potential uses of new technology range from medical devices and wearables to smart packaging

 

 

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