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Carbon Nanofiber Supercapacitor Produced Without a Flammable Electrolyte Solution

Drexel University Researchers have developed a fabric-like material electrode capable of making energy storage devices, such as supercapacitors and batteries, faster and less susceptible to disastrous meltdowns and leaks.

Their design for a new supercapacitor, which appears to be something like a furry sponge infused with gelatin, provides a unique alternative to the flammable electrolyte solution that is a common component present in these devices.

To build their freestanding, solid-state supercapacitor, Drexel researchers electrospin a mat of carbon nanofibers and coat them with an ion-rich gel. This eliminates the need for a flammable electrolyte solution, which has been the cause of dangerous leaks and meltdowns in the batteries of mobile devices. Credit: Drexel University

The electrolyte fluid present inside both supercapacitors and batteries can be toxic or corrosive and is almost always flammable. To keep up with the progressing mobile technology, energy storage devices have been subject to material shrinking in the design process, which has indeed left them vulnerable to short circuiting — as can be seen in the latest cases with Samsung’s Galaxy Note devices — which, when compounded with the presence of a flammable electrolyte liquid, can cause an explosive situation.

Thus, instead of a flammable electrolyte solution, the device designed by Vibha Kalra, PhD, a Professor in Drexel’s College of Engineering, and her team, employed a thick ion-rich gel electrolyte absorbed in a freestanding mat of porous carbon nanofibers in order to develop a liquid-free device.

The group, which included Kalra’s Doctoral Assistant Sila Simotwo and Temple Researchers Stephanie L.Wunder, PhD, and Parameswara Chinnam, PhD, published its fresh design for a “solvent-free solid-state supercapacitor” in the recent edition of American Chemical Society journal Applied Materials and Interfaces.

We have completely eliminated the component that can catch fire in these devices. And, in doing so, we have also created an electrode that could enable energy storage devices to become lighter and better.

Professor Vibha Kalra, PhD, College of Engineering, Drexel University

Supercapacitors are known to be another type of energy storage device. They are similar to batteries, in that they electrostatically hold and discharge energy, but in the existing technology — mobile devices, electric cars, laptops, — they tend to assist as a power backup as they can distribute their stored energy in a rapid spurt, unlike batteries that do so over prolonged usage. However, like batteries, supercapacitors use a flammable electrolyte solution, and thus they are vulnerable to fires and leakage.

The group’s supercapacitor is solvent-free , meaning it does not comprise of flammable liquid, and the supercapacitor’s compact design is also more durable and its charge-discharge lifespan and energy storage capacity are better than comparable devices presently being used. It is also capable of operating at temperatures as high as 300 oC, meaning it would help make mobile devices a lot more durable and less likely to become a fire hazard owing to abuse.

To allow industrially relevant electrode thickness and loading, we have developed a cloth-like electrode composed of nanofibers that provides a well-defined three-dimensional open pore structure for easy infusion of the solid electrolyte precursor. The open-pore electrode is also free of binding agents that act as insulators and diminish performance.

Professor Vibha Kalra, PhD, College of Engineering, Drexel University

A fiber-like electrode framework, developed by the team through a process known as electrospinning, is the key to producing this durable device. The process deposits a carbon precursor polymer solution in the form of a fibrous mat by extruding it via a rotating electric field — a process that looks something like making cotton candy at the microscopic level.

This is followed by the ionogel being absorbed in the carbon fiber mat in order to develop a complete electrode-electrolyte network. Its exceptional performance characteristics are also tied to this unique method of merging electrolyte and electrode solutions. This is because they are making contact over a huge surface area.

If one thinks of an energy storage device as a bowl of corn flakes, then the place where energy storage takes place is roughly where the flakes meet the milk — Scientists refer to this as the “electrical double layer.” It is where the conductive electrode that stores electricity comes in contact with the electrolyte solution that is in fact carrying the electric charge. In a cereal bowl, the milk would make its way via all the flakes in order to get just the perfect coating on each —not too soggy and not too crunchy.

However, sometimes the cereal gets piled up and the milk — or the electrolyte solution, in the case of this comparison — fails to make its way all the way through, so the flakes on top are dry, while the flakes on the bottom get saturated. This is not a good bowl of cereal, and its electrochemical equivalent — an electron traffic jam en route to activation sites in the electrode — is not perfect for energy storage.

Kalra’s solid-state supercapacitor is similar to putting shredded wheat in the bowl, instead of cornflakes. The open architecture allows the milk to soak and coat the cereal, much like how the ionogel permeates the carbon fiber mat in Kalra’s solid-state supercapacitor. The mat provides a better surface area for ions from the ionogel in order to access the electrode, which increases the capacity and enhances the performance of the energy storage device. It also eradicates the requirement for many of the scaffolding materials that are vital parts of developing the physical electrode, but do not play a role in the energy storage process and contribute a major portion bit to the overall weight of the device.

State of the art electrodes are composed of fine powders that need to be blended with binding agents and made into a slurry, which is then applied into the device. These binders add dead weight to the device, as they are not conductive materials, and they actually hinder its performance. Our electrodes are freestanding, thus eliminating the need for binders, whose processing can account for as much as 20 percent of the cost of manufacturing an electrode.

Professor Vibha Kalra, PhD, College of Engineering, Drexel University

Going forward, Kalra’s team will be employing this technique for the production of solid-state batteries and also for exploring its application for smart fabrics.


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