System Bits: May 21

Washable, wearable energy devices for clothing
Researchers at the University of Cambridge collaborated with colleagues at China’s Jiangnan University to develop wearable electronic components that could be woven into fabrics for clothing, suitable for energy conversion, flexible circuits, health-care monitoring, and other applications.

Graphene and other materials can be directly incorporated into fabrics to create capacitors and other charge-storage elements – essentially battery-like devices that are washable, wearable, flexible, and perhaps even fashionable.

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 of 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. “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.”

A graphite lithium-ion battery prototype
A team formed by the University of Maryland and the U.S. Army Research Laboratory worked together on developing new technology for lithium-ion batteries. While lithium-ion batteries can outperform lead-acid batteries, scientists continue to look for ways to improve the performance and safety of lithium-ion batteries.

To further improve battery performance, researchers Chongyin Yang and Ji Chen with co-workers under the direction of Chunsheng Wang at the University of Maryland developed a novel graphite lithium ion battery that utilizes high-density helper ion packing and a unique water-in-salt electrolyte to achieve a potential of more than 4 volts in aqueous batteries. The aqueous nature of their battery is also an advantage, because as the researchers highlight in their report, the intercalation of the helper ions within a water environment “comes with intrinsic safety and environment insensitivity.”

Graphite, stacked layers of the two-dimensional nanomaterial graphene, excels as a battery anode material, particularly in lithium-ion batteries where ion packing directly correlates to battery performance. Graphite has a high capacity of 372 mAhg-1 for lithium ions in between its graphene layers. Polyhalogen ions can also insert themselves into the graphite.

The university and the Army Research Lab have collaborated on water-in-salt electrolyte batteries for some time. As a result, Yang and Chen were able to use graphite’s advantages along with helper halide ions to achieve a “densely packed stage-I graphite intercalation compound, C3.5[Br0.5Cl0.5].” Specifically, they designed an electrode containing lithium and the helper halide ions. When exposed to the aqueous electrolyte solution and charged, the halide ions give up electrons and lithium ions travel through the battery to the cathode, a favorable reaction that generates a useful current. The helper halides then intercalate into the graphite. This insertion stabilizes the halogens and makes the entire process energetically favorable.

A key component of this consistency and excellent performance is the reversibility of the helper ion intercalation. By using extensive Raman spectroscopy, Yang and Chen show that the helper halide ions pack into the graphite instead of absorbing onto the outside graphite surface. This allows more ions to intercalate, meaning more lithium ions are free to move across the cell and generate a useful current. Upon charging, lithium ions move back across the cell and recombine with the helper halide ions, as released from the graphite.

Additional X-ray diffraction and absorption data show optimal close-packing within graphite occurs when the chloride and bromide ions alternate, which was also confirmed using density functional theory calculations. This implies both halide helpers are required to make the lithium ion movement across the battery favorable. Additionally, without the graphite present to stabilize the ions post-electron loss, the halides may gas off.

Yang and Chen hope this novel battery design will eliminate previous flammability issues with lithium ion batteries while also offering “an energy-dense concept for a future battery that is cost-effective, safe, and flexible.”

Catalyst can split water to produce hydrogen
The Technical University of Munich reports the creation of an efficient water-splitting catalyst, made up of a double-helix semiconductor structure encased in carbon nitride. TUM chemists worked with Canada’s University of Alberta to create the stable and flexible semiconductor which enables the economical and sustainable production of hydrogen.

An inorganic double-helix compound comprising the elements tin, iodine, and phosphorus (SnIP) forms the core of the structure. It is synthesized in a simple process at temperatures around 400 degrees Celsius. The SnIP fibers are flexible and, at the same time, robust as steel.

“The material combines the mechanical properties of a polymer with the potential of a semiconductor,” says Tom Nilges, Professor of Synthesis and Characterization of Innovative Materials at the Technical University of Munich. “From this, we can manufacture flexible semiconductor components in a further technical step.”

The use as a water-splitting catalyst is the first application for the unusual material. The chemists prepared nanoparticles from each of the starting substances and mixed the suspensions of these two nanoparticles with each other. The result was a structure with a hard but flexible SnIP-core and a soft carbon nitride shell.

Measurements show that the resulting heterogeneous structure is not only significantly more stable than either of the initial materials. It also splits water four times more efficiently than was previously possible, making it interesting as a material for producing cheap hydrogen or to chemically store surplus electricity from wind farms.

Knowing that the catalyst’s great efficiency stems primarily from its large surface, the chemists increased the surface area by splitting the SnIP fibers into thinner strands. A mixture of 30% SnIP and 70% carbon nitride turned out to be the most effective.

The thinnest fibers comprise several double-helix strands and are merely a few nanometers thick. The material is in principle one-dimensional. Wrapping it in carbon nitride allows the material to retain its high reactivity while becoming more durable – thereby making it more suitable as a catalyst.

But the one-dimensional SnIP double-helices also open the door to very different kinds of applications. The researchers would be particularly keen on obtaining single strands of SnIP. These would then be right- or left-handed – with their own respective very special optical properties. This makes SnIP a highly attractive material for optoelectronics.

“We were able to show theoretically that many other compounds of this kind are possible. Currently we are working on the synthesis of these materials,” says Nilges. “Flexible, inorganic, nanometer-sized, 1D semiconductors might create as much hype as 2D layered materials like graphene, phosphors, or molybdenum disulfide do today.”