The first-ever studies of industrially produced membranes made from small-diameter carbon nanotubes have revealed that these materials are as good in terms of characteristics and performance as small-scale laboratory prototypes. The experiments also bring to light some important phenomena that could be put to good use in applications such as water desalination, air purification and separating industrial gases, to name but three.
“Our work is the first to test carbon nanotube (CNT) membranes made on the large scale by a start-up company, which is quite different to previous studies that looked at CNT membranes made in small batches exclusively for laboratory work,” explains team leader Benny Freeman of the University of Texas at Austin. “Having access to such materials in such large quantities, and at so low a cost, is an exciting development.”
Freeman and colleagues at the University of Connecticut and Mattershift, a New York City-based start-up, decided to study how gas and water transports through these membranes to find out if their characteristics and performance in this respect matched those of previously studied lab-scale prototypes. “We indeed found that they did, but we were also able to observe some important phenomena that had only been predicted for small-sized CNTs before now, but were not very often observed in experiments.”
Surface diffusion along the CNT inner wall
The inner diameters of the tubes, which are so-called arc discharge CNTs, were between just 0.67 and 1.27 nm. At such a small scale, researchers predict that there should be certain transport mechanisms at play, including surface diffusion along the nanotube inner wall. “The type of transport here is quite different to conventional Knudsen diffusion or viscous flow,” says Freeman. “In particular, it is much faster than Knudsen because absorption phenomena begin to become important here.
“For example, we found that propane diffuses through these membranes as fast as helium, even though propane is 11 times heavier and would normally be expected to flow through much more slowly. We believe that the fast transport rate comes from the affinity of hydrocarbons for the inner CNT wall.”
Absorption effects are important
The researchers also observed that water flows through the membranes 1000 times higher than predicted by Hagen-Poiseuille flow, a result that is in agreement with previous studies on lab-quality materials. Another important finding is that CO2 diffuses through the tiny tubes quicker than nitrogen gas in a mixed transport experiment. Once again, this behaviour is likely due to absorption effects, says Freeman.
“These are the phenomena that Mattershift would like to exploit to separate fuels and biofuels from dilute sources, such as water, using very little energy,” he tells nanotechweb.org. “Removing CO2 from air using these membranes and catalytically reducing it to ethanol and other liquid transportation fuels might also be a possibility.”
Indeed, Rob McGinnis, Mattershift founder and CEO says that this has already been done using conventional technologies but that it has been too expensive to be practical until now. “Using our tech, I think we’ll be able to produce carbon-zero gasoline, diesel and jet fuels that are cheaper than fossils,” he writes in a company press release.
Towards real-world applications
Mattershift also says that it is working on using these membranes to extract ethanol fuel from sources like corn, sugar cane and cellulosic fermentation broths in a way that will reduce fossil fuel use for such renewable fuels by as much as 90% – by replacing distillation with a technique called pervaporation.
“We are looking forward to finding out what this new class of membranes can do in such industrial gas and energy applications,” adds Freeman. “Our lab is a leader in these fields and having access to these commercial CNT membranes will hopefully lead to new real-world applications.”
The commercial CNT membrane characterization study is detailed in Science Advances DOI: 10.1126/sciadv.1700938.