Smog in Beijing

Yi Cui of Stanford and his students have used a material commonly used in surgical gloves into a low-cost, highly efficient air filter that could be used to improve facemasks, window screens, and could even reduce the exhaust from power plants.

Finding himself choked by smog from produced by automobiles and coal power plants on trips to China, Yi Cui, an associate professor of materials science and engineering at Stanford, and his graduate students set to work designing an inexpensive, efficient air filter that could ease the breathing for people in polluted cities.

Using surgical-glove material in an air filter

Cui’s team works on batteries, not air filters, so they were able to think differently. They started by looking for polymers that would have a strong attraction to the main components of smog — especially, particle matters that are smaller than 2.5 microns, known as PM2.5. These pose the greatest risk to the human respiratory system and overall health; current filtration systems that can remove them from the air are very energy-intensive.

It turned out that polyacrylonitrile (PAN), a material commonly used to make surgical gloves, met these requirements. Using a technique called electrospinning, the researchers converted liquid PAN into spider-web-like fibers that are just 200 nanometers in diameter (about a thousandth the diameter of a human hair).

They applied a high voltage to the tip of a syringe containing a polymer solution and grounded the fibers on a metallic screen. By a simple static-electricity principle, the resulting electrical force pulled the polymer solution into the polymer nanofibers that lie across the mesh holes, forming an effective network for air filtration.

Built into a window

As a bonus. the final material also allows about 70 percent transparency and yet collects 99 percent of the particles. “The fiber just keeps accumulating particles, and can collect 10 times its own weight,” said Chong Liu, lead author on the paper and a graduate student in Cui’s lab. “The lifespan of its effectiveness depends on application, but in its current form, our tests* suggest it collects particles for probably a week.”

The first two immediate applications, Cui said, would probably be simple passive systems, such as personal masks and window screens, or possibly hospital air filtration systems.

“The transparency and distance between the fibers means that light and air can pass through very efficiently, which makes it a very good application for windows,” Cui said. “It might be the first time in years that people in Beijing can open their window and let in a fresh breeze. We think we could use this material for personal masks, window shades, and maybe automobiles and industrial waste. It works really well, and it might be a game-changer.”

The material might also be useful for filtering exhaust from cars, or from the smoke stacks of power plants and industrial complexes. These applications, Cui said, would require additional testing of the material to ensure that it is robust enough to withstand other acidic or toxic compounds in these types of exhaust.

The work is published in the current issue of the journal Nature Communications.

Abstract of Transparent air filter for high-efficiency PM2.5 capture

Particulate matter (PM) pollution has raised serious concerns for public health. Although outdoor individual protection could be achieved by facial masks, indoor air usually relies on expensive and energy-intensive air-filtering devices. Here, we introduce a transparent air filter for indoor air protection through windows that uses natural passive ventilation to effectively protect the indoor air quality. By controlling the surface chemistry to enable strong PM adhesion and also the microstructure of the air filters to increase the capture possibilities, we achieve transparent, high air flow and highly effective air filters of ~90% transparency with >95.00% removal of PM2.5 under extreme hazardous air-quality conditions (PM2.5 mass concentration >250 μg m−3). A field test in Beijing shows that the polyacrylonitrile transparent air filter has the best PM2.5 removal efficiency of 98.69% at high transmittance of ~77% during haze occurrence.

Image credit:  David Barrie | Flickr
Via Kurzweilai