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Nanoscale Fasteners Strengthen Cost-Effective Membranes in Fuel Cells

The role of nanotechnology in next-generation fuel cells has a bit of a shaky past. There were initial thrills nearly a decade ago, when carbon nanotubes appeared to store hydrogen at an enormous ratio of 50 percent of their total weight, which, unfortunately, was later reduced to the rather sobering ratio of 1%wt in practical applications. And, of course, lest we forget, there was NEC’s promise, over a decade-and-a-half ago, of a fuel-cell-powered laptop enabled by nanomaterials. It’s not a spoiler to say it never came to fruition.

Since then, the hype around nanomaterials has largely moved on from improvements in the fuel cell itself, to efforts to achieve artificial photosynthesis for producing hydrogen.

Now researchers at the Korean Advanced Institute of Science and Technology (KAIST) have bucked that trend and gone back to trying to exploit nanotechnology to make a better fuel cell. The Korean researchers have devised a way to make the more cost-effective hydrocarbon membranes used in today’s proton exchange membrane (PEM) fuel cells even stronger and longer lasting.

PEM fuel cells are attractive for use in vehicles because they operate at low temperatures. However, they do have weaknesses. The KAIST researchers made the breakthrough while attempting to address one of these weaknesses. In the PEM fuel cell, the the permeable catalyst membrane that separates the two chemical compartments serves as a kind of electrolyte. One side of it is bonded to the positive electrode, and the other side, to a negative one.

The problem is that the membrane can be made from either a perfluorosulfonic acid–based polymer, which is very expensive, or from a hydrocarbon that is less expensive but doesn’t last very long.

The KAIST researchers set out to strengthen the less expensive hydrocarbon membranes. To that end, the KAIST researchers have developed nanoscale fasteners that bond the membrane to the electrodes mechanically rather than chemically.

“This physically fastened bond is almost five times stronger and harder to separate than current bonds between the same layers,” said Professor Hee-Tak Kim of KAIST, in a press release.

The nanoscale fasteners were fabricated by creating a mold for tiny pillars on the surface of the hydrocarbon membrane. When the interface between the membrane and the electrodes begins to heat up, the pillars begin to push into the softened surface of the electrode. When the interface cools and absorbs water, the connection between the pillars and the electrodes begin to set and the mechanical bond is established.

While this production method provides a much sought after way to fabricate fuel cells that are less expensive, more efficient, and easier to manufacture, it may have applications beyond fuel cells. The researchers envision this approach being useful for any application that employs hydrocarbon membranes, such as rechargeable “redox flow” batteries.

Hydrogen Treatment of Graphene Makes for Super Li-ion Batteries

There is a growing litany of research efforts aimed at improving the ubiquitous lithium-ion (Li-ion) battery with nanomaterials, and graphene is increasingly taking up the Li-ion’s share of those efforts.

However, one of the issues with exotic nanomaterials trying to take the place of graphite as the storage material for these batteries’ electrodes is cost. Whatever benefit may be derived from using nanomaterials seems to be offset by their rather steep comparative cost.

Now researchers at Lawrence Livermore National Laboratory (LLNL) have discovered that if they use a graphene produced in a low-temperature process that is full of defects, they can still make it a highly effective electrode material simply by treating it with hydrogen.

In research published in the journal Nature Scientific Reports, the LLNL researchers found that the hydrogen interacts with defects in the graphene in a way that opens up gaps that make it easier for the lithium to penetrate the material and thereby improves its transport. Further, the hydrogen goes to the edges of the electrodes; this improves the lithium binding in these areas and ends up boosting storage capacity.

The positive role of hydrogen in this research is a bit unusual since it usually is regarded as an unwanted byproduct of the chemical production of graphene.

“We found a drastically improved rate capacity in graphene nanofoam electrodes after hydrogen treatment,” said LLNL scientist Brandon Wood, one of the co-authors of the paper, in a press release. “By combining the experimental results with detailed simulations, we were able to trace the improvements to subtle interactions between defects and dissociated hydrogen. This results in some small changes to the graphene chemistry and morphology that turn out to have a surprisingly huge effect on performance.”

The researchers believe this research shows that controlled hydrogen treatment could be a way forward to optimize lithium transport as well as improve the storage capacity in other graphene-based anode materials.

“The performance improvement we’ve seen in the electrodes is a breakthrough that has real world applications,” said Jianchao Ye, the lead author of the paper, in the press release.

Graphene Paper Transforms Into Tiny Origami Robots

A tiny sheet of graphene “paper” smaller than a human fingernail can behave like an origami robot that folds and walks on command. The inspired work by Chinese researchers could pave the way for such self-folding devices as tiny robots and artificial muscles, or even help with biological tissue engineering on the smallest scales.

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Researchers Develop Fastest and Most Flexible Silicon Phototransistor Ever

Researchers at the University of Wisconsin-Madison (UW-Madison) have developed a flexible phototransistor based on single-crystalline silicon nanomembranes (Si NM). They claim that this phototransistor is the fastest and most flexible one ever produced. 

The flexible phototransistor could be incorporated into a wide range of applications. In a digital camera, for example, it could result in a thinner lens that would capture images faster and yield higher quality still photos and videos.

In research published in the journal Advanced Optical Materials, the silicon nanomembrane is used as the top layer of the phototransistor; it enables full exposure of the active region of the device to any light. The researchers used a technique known as  “flip-transfer” in which they essentially flip the nanomembrane onto a reflective metal layer.

This arrangement allowed the researchers to boost the light absorption capabilities of the phototransistor without the need of an external amplifier. They simply placed electrodes under the nanomembrane layer; both the electrodes and the metal layer serve as reflectors.

“In this structure—unlike other photodetectors—light absorption in an ultrathin silicon layer can be much more efficient because light is not blocked by any metal layers or other materials,” said Zhenqiang “Jack” Ma, a professor at UW-Madison, in a press release.

It is the combination of the device’s high sensitivity and flexibility that are unique in a phototransistor.

“This demonstration shows great potential in high-performance and flexible photodetection systems,” said Ma, in the press release. “It shows the capabilities of high-sensitivity photodetection and stable performance under bending conditions, which have never been achieved at the same time.”

The upshot, say the researchers is that the flexibility allows the photodetector to better mimic mammalian vision by curving to fit the  shape of the camera’s optical system. “Currently, there's no easy way to do that,” says Ma.

Simple Nano Trick Purifies Silicon for Energy Applications

Silicon is the second most abundant element on earth. Unfortunately, it’s mostly found in impure forms, not the refined elemental stuff needed for integrated circuits and solar cells.

The existing technology for making pure silicon is mature but it’s expensive and dirty, and it’s optimized for making electronics—not battery electrodes, thermoelectrics, and solar cells, which have different requirements, says Nanjing University’s Jia Zhu. He worked with Yi Cui, a materials scientist at Stanford University, to develop a simple and cheap nanotech trick to get 99.999 percent pure silicon from cheap bulk ferrosilicon.

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Graphene-based Magnetoresistance Sensor 200 Times as Sensitive as Silicon

Most of the sensor chips that turn home appliances such as refrigerators and washing machines into smart devices do so by detecting changes in electrical resistance brought on by the presence of magnetic field—also known as magnetoresistance (MR). These sensor chips, sometimes referred to as MR sensors, have traditionally been fabricated from silicon.

Now researchers at the National University of Singapore (NUS) have produced these MR sensor chips out of graphene and boron nitride. Their version is 200 times as sensitive to electrical resistance as its silicon counterpart. 

In research published last month in the journal Nature Communications, the NUS researchers used boron nitride as a substrate for graphene sheets; the resulting chip forms an interface that allows electrons to pass through the material very quickly.

“These electrons can thus respond to magnetic fields with greater sensitivity,” said Associate Professor Yang Hyunsoo in a local report on the research.

The chip possesses high sensitivity to both high and low intensity magnetic fields, and neither its tunability nor its resistance changes substantially in varying temperatures.

Silicon sensor chips’ properties begin to change as their temperatures approach 127 degrees Celsius—the maximum temperature at which most electronics operate—causing their sensitivity to wane. Conversely, the graphene-based sensors’ sensitivity actually improves as the temperature rises. As they reach 127 °C, their ability to sense resistance changes is as much as eight times greater than it is at room temperature.

What this could mean, according to Yang, is that the costly temperature correction mechanisms that these sensors now need when they’re used in automotive applications can be eliminated.

As far as tunability, the researchers discovered that they could alter the mobility of the graphene multilayers simply by tuning the voltage across the sensor, which enables the optimization of the sensor’s properties.

The researchers, who predict that this new sensor will make a big splash in the billion-dollar magnetic sensor market—which includes not only MR sensors but also superconducting quantum interference devices, or SQUIDs—have applied for a patent to protect their technology.

As Yang told the sci-tech news service Phys.org:

Our sensor is perfectly poised to pose a serious challenge in the magnetoresistance market by filling the performance gaps of existing sensors, and finding applications as thermal switches, hard drives and magnetic field sensors. Our technology can even be applied to flexible applications.

Li-ion Batteries Keep Improving, But for What Application?

This blog has long chronicled the efforts to change the electrodes of lithium ion (Li-ion) batteries from graphite to silicon. If you replace graphite with silicon on the anodes of a Li-ion battery, you can increase the charge by as much as a factor of ten.

Sounds great, but after a few charge-discharge cycles the silicon cracks and becomes inoperable from the expansion and contraction of the material. This is why researchers have been hard at work trying to create a nanostructured silicon that provides most of the attractive charge capacity but doesn’t crack after a few charge/discharge cycles.

The latest attempt at creating a silicon material that won’t crack under the demands of serving as an anode material comes out of the University of Waterloo in Canada where researchers have created a hybrid material made from silicon, sulfur and graphene that promises a 40 to 60 percent increase in energy density over today’s Li-ion batteries.

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The New Wrinkle in Graphene Is Wrinkles

One of the holy grails of graphene research has been a method for achieving wafer-scale growth of wrinkle-free single-crystal monolayer graphene on a silicon wafer.

Now researchers at the RIKEN research institute in Japan have discovered that the wrinkles in graphene may be their most attractive feature.

In research published in the journal Nature Communications, the RIKEN scientists discovered that the wrinkles found in graphene create unique electronic qualities, specifically a one-dimensional electron confinement. This restriction of electron movement results in a junction-like structure that changes from a zero-gap conductor to a semiconductor and back to zero-gap conductor.

The other revelation yielded by this research is that it’s possible to manipulate the wrinkles to change graphene’s band gap using mechanical methods rather than chemical techniques.

“Up until now, efforts to manipulate the electronic properties of graphene have principally been done through chemical means, but the downside of this is that it can lead to degraded electronic properties due to chemical defects,” said Yousoo Kim, who led the RIKEN team, in a press release. “Here we have shown that the electronic properties can be manipulated merely by changing the shape of the carbon structure. It will be exciting to see if this could lead to ways to find new uses for graphene.”

The discovery that it was possible to produce graphene semiconductors without the need to chemically dope the carbon sheets was the result of trying to produce graphene films using chemical vapor deposition (CVD). They were attempting to use CVD to grow graphene on a nickel substrate; they were examining how they could control the process with changes in temperature.

“We were attempting to grow graphene on a single crystalline nickel substrate, but in many cases we ended up creating a compound of nickel and carbon, Ni2C, rather than graphene,” explained Hyunseob Lim, the paper’s lead author, in a press release. “In order to resolve the problem, we tried quickly cooling the sample after the dosing with acetylene, and during that process we accidentally found small nanowrinkles, just five nanometers wide, in the sample.”

When examining the wrinkles with a scanning tunneling microscope, the researchers discovered that there were band gaps within them, which meant that they could act as semiconductors.

Much Ado About Carbon Nanotubes...Or Not

Researchers at the University of Paris-Saclay in France have discovered that fluid samples taken from the airways of 64 asthmatic children contained carbon nanotubes (CNTs). In addition, the France-based researchers determined that five other children studied also had CNTs in macrophages found in their lungs.

While this will no doubt add fuel to the fury of NGOs bent on shutting down research into nanotechnology immediately, there is little in this research that breaks new ground—at least qualitatively.

“From past studies, the conditions in combustion engines seem to favor the production of at least some CNTs (especially where there are trace metals in lubricants that can act as catalysts for CNT growth),” explained Andrew Maynard Director, Risk Innovation Lab and Professor, School for the Future of Innovation in Society at Arizona State University, in an e-mail interview. Says Maynard:

What, to my knowledge, is still not known, is the relative concentrations of CNT in ambient air that may be inhaled, the precise nature of these CNT in terms of physical and chemical structure, and the range of sources that may lead to ambient CNT. This is important, as the potential for fibrous particles to cause lung damage depends on characteristics such as their length—and many of the fibers shown in the paper appear too short to raise substantial concerns.

It’s not even clear from the research whether the nanoparticles in question are in fact carbon nanotubes. At this point, they are best described as carbon nanotube-like fibers.

Nonetheless, Maynard praises the research for establishing that these carbon nanotube-like fibers are part of the urban aerosol and therefore end up in the lungs of anyone who breathes it in. However, he cautions that the findings don’t provide information on the potential health risks associated with these exposures.

“Because of this,” Maynard told IEEE Spectrum, “it would be highly premature to draw any conclusions on health risk from the study.” He added that, “It would be appropriate to conduct further study into whether there is an association between these unusual carbon-based fibers and ill health.”

At least some of the coverage of the research has made the misleading point that “nanotubes have shown great potential in areas such as computing, clothing and healthcare technology” with the obvious implication being that the CNTs used in these applications are the ones found in the lungs of children.

While the research doesn’t draw a distinction between manufactured CNTs and the natural and incidental varieties produced by, say, car exhausts, there is little to suggest they are anything other than particles that have been around with us since the introduction of the internal combustion engine. Meanwhile, there has been little evidence showing that a manufactured CNT, once embedded in the matrix of a material, can ever be separated from that matrix so that it’s free to float around in the air.

“Some studies have indicated that occasionally single nanotubes might be released from abraded materials,” Maynard admits. “But it looks like the release rates are extremely low.  This is what would be expected given how tightly carbon nanotubes bind to polymers used in composite materials, and the amount of energy that would be required to release them.”

Maynard points that it may be possible in principle to create fingerprints for different types of CNTs based on their source, allowing scientists to determine definitively whether a sample of CNTs is from car exhaust, or tennis racquets and bicycles. He adds:

For instance, CNTs that are lab generated will often be associated with trace amounts of catalyst materials such as nickel or iron.  However, I suspect that such fingerprinting will require a level of characterization rarely used on such materials.

Such characterization may also be a moot point from a health perspective. Maynard notes that while ambient carbon nanotubes may be analytically difficult to distinguish from engineered carbon nanotubes, it’s reasonable to assume that our lungs will also find it hard to make the distinction.

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Nanoclast

IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

 
Editor
Dexter Johnson
Madrid, Spain
 
Contributor
Rachel Courtland
Associate Editor, IEEE Spectrum
New York, NY
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