Small tubes come in large packages

Membrane systems have great potential as low energy alternatives in applications such as desalination, pharmaceutical recovery, purification and waste treatment.

Scientists at the Lawrence Livermore National Laboratory (LLNL) have created the largest defect-free membranes reported to date that take full advantage of the unique mass transport properties of carbon nanotubes as flow channels. This work was recently published online in Advanced science.

Chemically robust materials that selectively transport molecules at high speed are critical to the development of advanced membrane systems that outperform state-of-the-art products. Carbon nanotubes – all-carbon channels over 50,000 times thinner than a human hair – belong to this highly promising class of membrane building blocks. Unlike conventional porous materials, these tiny channels allow for exceptionally fast gas and liquid flow that gets even faster as the tubes shrink

To take full advantage of these extraordinary materials, maximizing the density of carbon nanotubes open across the membrane is critical. LLNL researchers developed single-walled high-density nanotubes on 4-inch silicon wafers and used them to create membranes with outstanding large-scale transport properties.

“Upgrading carbon nanotube membranes without introducing leaks is no small task,” said lead author and LLNL postdoctoral researcher Melinda Jue. “We rigorously test and characterize our membranes to ensure they are free from defects before using them to accurately measure the flow improvement.”

There are 10 times more conductive nanotubes in these large area membranes than previously.

“By using a high density of carbon nanotubes as transport pathways, we are able to create membranes that are 6,200 times more permeable to water than theorized and have up to 10 times the flow of water. commercially available membranes with similar pore sizes, ”said Francesco Fornasiero, the LLNL scientist leading the project.

Together with quantifying the transport improvement enabled by carbon nanotubes, the research team investigated the nanofiltration capacity of these large carbon nanotube membranes.

“You could imagine using these membranes to efficiently separate highly saline solutions where you want to remove the salt while concentrating the most valuable component,” said Jue.

The degradation of the performances due to the encrustations is an important obstacle in the widespread use of membrane technologies and to counter the effects it is necessary to treat the membranes with aggressive chemicals. The robust nature of the carbon nanotubes and polymer matrix also leads to excellent chemical stability, allowing for easy cleaning of the membranes. “So when the membrane gets dirty, you can clean it with bleach without having to worry about degradation like with typical polymer membranes,” Jue said.