Power/Performance Bits: May 26

Woven fabric electrodes

An international team including scientists from the University of Exeter pioneered a new technique to embed transparent, flexible graphene electrodes into fibers commonly associated with the textile industry.

Exeter Professor Monica Craciun, co-author of the research said: “This is a pivotal point in the future of wearable electronic devices. The potential has been there for a number of years, and transparent and flexible electrodes are already widely used in plastics and glass, for example. But this is the first example of a textile electrode being truly embedded in a yarn. The possibilities for its use are endless, including textile GPS systems, to biomedical monitoring, personal security or even communication tools for those who are sensory impaired. The only limits are really within our own imagination.”

(Source: University of Exeter/Scientific Reports)

This new research has identified that the exceptional electrical, mechanical and optical properties of ‘monolayer graphene’ makes it a highly attractive proposition as a transparent electrode for applications in wearable electronics. In this work graphene was created by chemical vapor deposition (CVD) onto copper foil.

The collaborative team established a technique to transfer graphene from the copper foils to a polypropylene fiber already commonly used in the textile industry.

Dr Ana Neves, associate research fellow from Exeter added: “We are surrounded by fabrics, the carpet floors in our homes or offices, the seats in our cars, and obviously all our garments and clothing accessories. The incorporation of electronic devices on fabrics would certainly be a game-changer in modern technology. All electronic devices need wiring, so the first issue to be addressed in this strategy is the development of conducting textile fibers while keeping the same aspect, comfort and lightness. The methodology that we have developed to prepare transparent and conductive textile fibers by coating them with graphene will now open way to the integration of electronic devices on these textile fibers.”

Boosting with boron

A microsupercapacitor designed by scientists at Rice University that may find its way into personal and wearable electronics is getting an upgrade. The laser-induced graphene device benefits greatly when boron becomes part of the mix.

The Rice lab of chemist James Tour uses commercial lasers to create thin, flexible supercapacitors by burning patterns into common polymers. The laser burns away everything but the carbon to a depth of 20 microns on the top layer, which becomes a foam-like matrix of interconnected graphene flakes.

By first infusing the polymer with boric acid, the researchers quadrupled the supercapacitor’s ability to store an electrical charge while greatly boosting its energy density. For the new work, the lab dissolved boric acid into polyamic acid and condensed it into a boron-infused polyimide sheet, which was then exposed to the laser.

The simple manufacturing process may also be suitable for making catalysts, field emission transistors and components for solar cells and lithium-ion batteries, the team said.

Rice University scientists made this supercapacitor with interlocked “fingers” using a laser and writing the pattern into a boron-infused sheet of polyimide. The device may be suitable for flexible, wearable electronics. (Source: Tour Group/Rice University)

The two-step process produces microsupercapacitors with four times the ability to store an electrical charge and five to 10 times the energy density of the earlier, boron-free version. The new devices proved highly stable over 12,000 charge-discharge cycles, retaining 90 percent of their capacitance. In stress tests, they handled 8,000 bending cycles with no loss of performance, the researchers reported.

Tour said the technique lends itself to industrial-scale, roll-to-roll production of microsupercapacitors. “What we’ve done shows that huge modulations and enhancements can be made by adding other elements and performing other chemistries within the polymer film prior to exposure to the laser.”

Large-scale fabrication

One of the barriers to using graphene at a commercial scale could be overcome using a method demonstrated by researchers at the Department of Energy’s Oak Ridge National Laboratory.

Now, using chemical vapor deposition, a team led by ORNL’s Ivan Vlassiouk fabricated polymer composites containing 2-inch-by-2-inch sheets of the one-atom thick hexagonally arranged carbon atoms.

ORNL’s ultrastrong graphene features layers of graphene and polymers and is an effective conductor of electricity. (Source: Oak Ridge National Laboratory)

“Before our work, superb mechanical properties of graphene were shown at a micro scale,” said Vlassiouk. “We have extended this to a larger scale, which considerably extends the potential applications and market for graphene.”

While most approaches for polymer nanocomposition construction employ tiny flakes of graphene or other carbon nanomaterials that are difficult to disperse in the polymer, Vlassiouk’s team used larger sheets of graphene. This eliminates the flake dispersion and agglomeration problems and allows the material to better conduct electricity with less actual graphene in the polymer.

“In our case, we were able to use chemical vapor deposition to make a nanocomposite laminate that is electrically conductive with graphene loading that is 50 times less compared to current state-of-the-art samples,” Vlassiouk said. This is a key to making the material competitive on the market.