Design for new electrode could boost supercapacitors’ performance – UCLA Researchers Design Super-efficient and Long-lasting electrode for Supercapacitors – 10X Efficiency

IMAGE: THE BRANCH-AND-LEAVES DESIGN IS MADE UP OF ARRAYS OF HOLLOW, CYLINDRICAL CARBON NANOTUBES (THE ‘BRANCHES’) AND SHARP-EDGED PETAL-LIKE STRUCTURES (THE ‘LEAVES’) MADE OF GRAPHENE. view more  CREDIT: UCLA ENGINEERING

Engineers from UCLA, 4 other universities produce nanoscale device that mimics the structure of tree branches

UCLA HENRY SAMUELI SCHOOL OF ENGINEERING OF APPLIED SCIENCE

Mechanical engineers from the UCLA Henry Samueli School of Engineering and Applied Science and four other institutions have designed a super-efficient and long-lasting electrode for supercapacitors. The device’s design was inspired by the structure and function of leaves on tree branches, and it is more than 10 times more efficient than other designs.

 

The branch-and-leaves design is made up of arrays of hollow, cylindrical carbon nanotubes (the “branches”) and sharp-edged petal-like structures (the “leaves”) made of graphene.

The electrode design provides the same amount of energy storage, and delivers as much power, as similar electrodes, despite being much smaller and lighter. In experiments it produced 30 percent better capacitance — a device’s ability to store an electric charge — for its mass compared to the best available electrode made from similar carbon materials, and 30 times better capacitance per area. It also produced 10 times more power than other designs and retained 95 percent of its initial capacitance after more than 10,000 charging cycles.

Their work is described in the journal Nature Communications.

Supercapacitors are rechargeable energy storage devices that deliver more power for their size than similar-sized batteries. They also recharge quickly, and they last for hundreds to thousands of recharging cycles. Today, they’re used in hybrid cars’ regenerative braking systems and for other applications. Advances in supercapacitor technology could make their use widespread as a complement to, or even replacement for, the more familiar batteries consumers buy every day for household electronics.

Engineers have known that supercapacitors could be made more powerful than today’s models, but one challenge has been producing more efficient and durable electrodes. Electrodes attract ions, which store energy, to the surface of the supercapacitor, where that energy becomes available to use. Ions in supercapacitors are stored in an electrolyte solution. An electrode’s ability to deliver stored power quickly is determined in large part by how many ions it can exchange with that solution: The more ions it can exchange, the faster it can deliver power.

Knowing that, the researchers designed their electrode to maximize its surface area, creating the most possible space for it to attract electrons. They drew inspiration from the structure of trees, which are able to absorb ample amounts of carbon dioxide for photosynthesis because of the surface area of their leaves.

“We often find inspiration in nature, and plants have discovered the best way to absorb chemicals such as carbon dioxide from their environment,” said Tim Fisher, the study’s principal investigator and a UCLA professor of mechanical and aerospace engineering. “In this case, we used that idea but at a much, much smaller scale — about one-millionth the size, in fact.”

To create the branch-and-leaves design, the researchers used two nanoscale structures composed of carbon atoms. The “branches” are arrays of hollow, cylindrical carbon nanotubes, about 20 to 30 nanometers in diameter; and the “leaves” are sharp-edged petal-like structures, about 100 nanometers wide, that are made of graphene — ultra thin sheets of carbon. The leaves are then arranged on the perimeter of the nanotube stems. The leaf-like graphene petals also give the electrode stability.

The engineers then formed the structures into tunnel-shaped arrays, which the ions that transport the stored energy flow through with much less resistance between the electrolyte and the surface to deliver energy than they would if the electrode surfaces were flat.

The electrode also performs well in acidic conditions and high temperatures, both environments in which supercapacitors could be used.

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Fisher directs UCLA’s Nanoscale Transport Research Group and is a member of the California NanoSystems Institute at UCLA. Lei Chen, a professor at Mississippi State, was the project’s other principal investigator. The first authors are Guoping Xiong of the University of Nevada, Reno, and Pingge He of Central South University. The research was supported by the Air Force Office of Scientific Research.

 

Tough Textile Batteries

With the launch of Google Glass and the Samsung Galaxy Gear wristwatch this year, wearable electronics have moved from abstract concepts to tangible products. To integrate these electronic devices seamlessly into clothing, watchbands, and backpacks, some engineers are developing flexible, powerful textile-based batteries. Now researchers in South Korea have built one of the most durable wearable batteries to date on polyester fabric (Nano Lett. 2013, DOI: 10.1021/nl403860k). The battery, which the researchers sewed into a shirt, can be folded 10,000 times without losing function.

Most attempts to make textile batteries have had limited success, says materials scientist Jang Wook Choi of the Korea Advanced Institute of Science and Technology (KAIST).

 

Fashionable Batteries            

            South Korean researchers fabricated lithium ion batteries on polyester cloth and then sewed them into a hoodie (left) and a watch wristband (right). The bottom cartoons show the shape of the batteries used in the shirt (left) and wristband (right).

The problem has been finding battery materials that can retain high function while being bent repeatedly. For example, batteries with metal foils as electrodes can bend only a few times before breaking. Electrodes made by dipping cloth in nanoparticle inks, such as solutions of carbon nanotubes, are more durable than the foils, but the electrical resistance of these cloth electrodes is relatively high, which limits the size of the batteries and the total amount of energy they can store.

Polyester Electrode            

            In a new textile battery, researchers fabricated electrodes by electroplating nickel onto polyester fabric (top, center). After adding the nickel layer, they completed the electrode by coating the fabric with a lithium electrode composite using a polyurethane binder (top, right). The nickel coated the individual fibers of polyester yarn, allowing the fabric to retain most of its mechanical properties (bottom, right). The electrode composite then coated each strand of yarn in the fabric. (below)

To solve these challenges, Choi rethought the entire design of textile batteries, starting with the electrode. He turned to nickel, because it is a fantastic conductor. To make a flexible, but still highly conductive metal electrode, Choi came up with the idea of electroplating nickel onto polyester fabric. The process is simple, and the nickel-coated textile retains the mechanical properties of the fabric. The electrodes had a very low electrical resistance, about 0.35 ohms per square, comparable to that of a pure nickel metal foil.

The other critical component is the polymer used to bind the anode and cathode materials onto the electrodes in the battery. If this binder material fails, the battery will peel apart and stop functioning. Choi found that polyurethane had the right mechanical properties. To complete the battery, Choi’s group used conventional lithium-ion battery materials for the anodes and cathodes.

Choi’s group put the polyester-based batteries through their paces. Other groups have demonstrated bending and flexing of batteries, but the KAIST team thought the real test of mechanical durability would be to fold the device with firm creases. They powered an array of light-emitting diodes with the battery and folded it repeatedly. After 10,000 folding and unfolding cycles, the textile battery still worked. Batteries built with aluminum foil electrodes broke after three cycles and stopped working altogether after 100 cycles.

The KAIST group showed that their textile batteries can be sewn into a sweatshirt and a watchband. They also integrated the batteries with flexible solar cells so the batteries could recharge without needing to be removed from the clothing. “It’s quite comfortable to wear,” Choi says, adding that the battery is sealed so people could wash the fabric with the battery still attached.

“I’m really impressed,” says Yi Cui, a battery researcher at Stanford University. The KAIST group has successfully put their batteries through much harsher mechanical tests than others have been able to, he says.

The next step, Cui says, is to use battery materials that can store more energy to further improve the performance. So far, the KAIST team has used lithium iron phosphate for the cathode and lithium titanium oxide for the anode. Cui says that using a carbon anode material in the textile battery would increase the battery’s voltage, which determines how much power the device can deliver and how fast it can recharge. The voltage of the textile battery is about 2.5 V, and Choi says it should be about 3.8 V for practical applications.

Indeed, Choi’s group is experimenting with other materials, in collaboration with an unnamed South Korean battery maker that is interested in scaling up production of the wearable batteries.

Chemical & Engineering News

ISSN 0009-2347

Copyright © 2013 American Chemical Society

New colloidal films boost lithium-ion battery performance

1 November, 2012 Isaac Leung

 

SCIENTISTS from Cornell University have developed a way to fabricate carbon-free and polymer-free lightweight colloidal films for lithium-ion battery electrodes.

The films, which allow for additive-free electrodes that maintain high conductivity, could greatly improve battery performance, and reduce their weight and volume.

The technology is based on colloidal nanoparticles, combined with electrophoretic deposition.

Earlier techniques using colloidal nanoparticles for the electrodes required them to be combined with carbon-based conductive materials for enhancing charge transport. Polymeric binders were then used to stick the particles together and to the electrode substrate.

This process added extra weight to the battery and made it difficult to model the movement of Li-ions and electrons through the mixture.

The critical processing technique was electrophoretic deposition, which binds the metal nanoparticles to the surface of the electrode substrate to each other in an assembly, creating strong electrical contacts between the particles and current collector.

Once attached, the particles are no longer soluble and are mechanically robust. As a result, the film is more mechanically stable than those fabricated by conventional battery-making methods with binders.

This research has led to the first cobalt-oxide nanoparticle-film battery electrode made without using binders and carbon black additives, and they show high gravimetric and volumetric capacities, even after 50 cycles.

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