100 times stronger than steel1 yet 4 times lighter.2 Conductive and biocompatible. Carbon nanotubes (CNTs) are fascinating materials that could channel a kaleidoscope of futuristic technologies. They’re often hailed as a key conduit for a speedy transition to a zero-carbon energy future. However, while these graphene-based tiny tubes have been overhyped as revolutionary for about 30 years, they haven’t made it to the market yet. But we’re seeing slow progress towards interesting use cases, like a recent advancement that could turn CNTs into hot stuff for generating clean electricity out of waste heat.3 Let’s revisit carbon nanotubes and see how things are shaping up.
The hollow grail of nanotech
About a year ago I covered how carbon nanotubes might be able to boost solar energy in another video, so I thought it was a good time to revisit the topic.4 And I’m not a scientist, but like to look at these technologies from a broader view, how they may impact our lives, and if they’re actually coming to market. I know you’ve probably heard a lot of promises on graphene and carbon nanotubes that haven’t been delivered yet, but researchers are unleashing new avenues to channel their phenomenal potential, as well as finding ways to improve their manufacturing. And it’s that last point that’s one of the biggest sticking points with CNTs.
Let’s refresh your memory and mine as well on what’s meant to be the hollow grail of the nanotechnology field. Although CNTs were first observed in the late 1950s5, it wasn’t until 1991 that they became famous. That was thanks to the experiment run by the Japanese physicist Sumio Iijima.6 Applying a direct current across two graphite electrodes immersed in an inert gas, he generated a spark of electricity between them. The so-called arc discharge7 vaporized the tip of the anode. After the carbon cloud settled on top of the cathode, Iijima looked at the soot under an electron microscope. That’s where he spotted some needle-like tubes. Each of those needles contained up to 50 tubes nested in a concentric manner. Not surprisingly his first discovery was named as multi-walled carbon nanotubes (MWCNTs). Put simply, these are nanometer-thick carbon sheets rolled up in a tubular form. Within each sheet or wall, which is essentially graphene8, you have hexagonal carbon atoms as building bricks. This honeycomb framework happens to be extremely strong and energy-efficient.9 2 years later, adding a metal catalyst to the graphite anode, the same scientist created single-walled carbon nanotubes (SWCNTs).10 Which, as you might have guessed, they have a single sheet of graphene. Besides SWCNTs and MWCNTs, you can also have double-walled carbon nanotubes (DWCNTs), formed by two layers of graphene. Not exactly catchy names, but very accurate.
Since Iijima brought them to life, CNTs have been associated with endless fanciful applications. Like a space elevator to name the most out there prediction.11 However, the astronomical promise has never been fulfilled. One reason for this is that when you try to spin CNTs into long fibers, their exceptional strength shrinks by 100 times.12 Now, I’m obviously dubious whether CNTs will ever elevate us into space, but they can certainly conduct electricity. That’s because each carbon atom has a spare electron which can’t wait to flow around. While MWCNTs conductivity isn’t affected by the material geometry13, SWCNTs can have a metallic or semiconducting behavior based on the way the graphene layer is twisted, a.k.a. the chiral angle.14 As a result, you can end up with a more or less conductive configuration by design, like armchair, zig-zag, and chiral nanotubes.15 16 Again, you’ve got to love the names … armchair? Anyway, CNTs with an armchair structure are similar to metals from the electric point of view. That’s why they’re the focus for making power cables.17 But the challenge is being able to make these nanowires as long as possible.18
Back in 2014, after years of lab work, researchers at Rice University knitted nanotubes into a fiber that carried 4 times more current than copper.19 When filling a cable, these armchair DWCNT wires can transfer power over longer distances while sustaining lower losses compared to copper. You may see why their scale-up could make our grid so much more efficient. But we have a macro problem. It’s very difficult to mass produce pure armchair CNTs, which I’ll get to in a minute.
CNTs are getting hot again
Other than potentially improving grids, researchers have recently opened up a new thread for CNTs: Making the most out of waste heat. Just think of sources like solar panels or power plants.
According to the US Energy Information Agency, over 60% of the energy used for generating electricity in America is lost as heat.20 Well, apparently CNTs may be a good fit for recovering this unused thermal power and upcycling it into renewable electricity. And there seems to be a common thread in the innovators here. Once again, Rice University. After improving power cables, Rice scientists used SWCNTs … wait, rice scientists? I think that’s a completely different thing … Rice University scientists used SWCNTs to make a waste heat-to-energy device. Basically, they designed a carbon nanotubes film containing cavities that trap the infrared heat from the sun and narrow its bandwidth. When doing so, they give off photons, which can be efficiently converted into electricity. Researchers claim that integrating their heat-recovery device within solar panels would boost their efficiency up to 80%.21 Compared to solar panels available on the market today, which don’t go above about 23%, that sounds dramatic.22 But you’d need to compare those to the efficiencies in other lab solar cell tests, which are around the 50% mark.23 So this claim still needs to be vetted and work its way out of the lab.
Just this August Rice University researchers revamped their tailoring skills and released their latest tubular collection … a CNT-containing textile that turns heat into electricity. This could set a new fashion trend in the green energy world. Donning their dressmaker’s hats, scientists have used a sewing machine to craft a smart cloth out of DWCNTs fibers. As it turns out, this tubular dress is a thermoelectric generator. Basically, it can convert sunlight and other heat sources into electricity. During a lab experiment, jointly set up by Tokyo Metropolitan University and Rice University Carbon Hub, this supercharged fabric used the heat from a hotplate to turn on an LED light24. It may not sound like a powerful achievement to you, but this new CNT clothing line hit a record-breaking power factor which is three times larger than the commercial standard.
But how does that work? Essentially, you generate electricity whenever there’s a temperature gradient across the material. Meaning that one side of it is hotter than the other. Funny as it sounds, individual nanotubes hold a giant power factor. This is the energy density you can get out of a material depending on a particular temperature difference. There are two components affecting the power factor of a material. Its electrical conductivity and the so-called Seebeck coefficient25, a.k.a. thermopower, which gives you an indication of how much electricity a material generates out of thermal variations. Imagine you join two wires made of different metals and you heat up one of them. When you do that, electrons will flow towards the cooler part of this circuit. Well, Seebeck was the first one to see that, which is why this phenomenon is called the Seebeck effect. And it’s the basic principle of how thermocouples work. So far, scientists have struggled to preserve the CNTs power factor when weaving them together into a larger framework like a fabric … until now. The Rice group … sorry, the Rice University group was the first at creating a large-size CNTs-based structure that retains the superior energy density of the individual tubes.
They managed to pack the fibers into a dense and highly aligned pattern. This led to an electrical conductivity over 10 times higher than those measured in the past for CNT macrostructures. On top of that, designers played around with the CNTs’ Fermi energy. This can be defined as the difference between the highest and the lowest kinetic energy level of electrons at 0 kelvin (ca. -460 F).26 At any other temperature, you refer to the Fermi level instead, which is often called electrochemical potential. Now, I’m not a scientist, so if I missed anything here, sound off in the comments. I know it all sounds complicated so let’s focus on a keyword: kinetic. Adjusting the Fermi energy, or level, will affect how fast electrons move throughout the material. In other words, the electric conductivity. A successful tuning of this parameter allowed researchers to tweak the fibers’ electronic properties. They doped the textile with some chlorine-containing chemicals and achieved a higher Seebeck coefficient, which means having a more efficient nanocouple.
Obtaining a very high power factor is crucial for energy harvesting applications because you maximize the electricity output you get when tapping into renewable sources like solar or industrial waste heat. For the sake of the experiment, scientists tested centimeters-long fibers. However, researchers obtained the same electronic properties after plying the smaller fabric pieces into longer CNTs threads, which means the product is scalable.
The major caveat here is that it’s only been proved in the lab, but it’s a very encouraging result as energy-efficient CNT macrostructures will be essential to develop real-world energy harvesting devices.27 And that’s what the research group is planning to develop further in the future. A possible application would be active cooling of electronics. Thanks to their high thermal conductivity, the nanotube fibers could draw heat out of sensitive optoelectronics that may otherwise fail when exposed to high temperatures.28 While they haven’t disclosed any funding from private investors, this research was supported by a few foundations as well as by the US Air Force and Department of Defense.
Why CNTs up-scale is still flushing down the tubes
Although these recent developments are heating up the vibes around CNTs, it’s taking ages to commercialize these revolutionary materials. So, what’s been obstructing the tube between the lab and the market?
The main challenge has been developing larger CNT-based structures that preserve the marvelous properties of each of their building blocks. Another major conundrum has been selectively growing a nanotube with a specific geometry. Whenever creating CNTs, you always end up having a cocktail of nano-straws with different shapes and properties. And, most importantly, this clumping mess doesn’t have the wondrous qualities of single CNTs.29 However, over the last year or two researchers have come up with different methods to disentangle the CNTs clustering issue.
One of scientists’ main goals has been to separate metallic and semiconducting tubes. To achieve this, they developed different kinds of polymers to dissolve and wash away one type while leaving the other one behind.30 Another team of researchers used cresol, an ingredient of commercial cleaning products, to purify the CNTs jumble. In addition to avoiding harsh chemicals that could reduce CNTs conductivity, using cresol gave the isolated tubes a sticky twist. As researchers increased the amount of cresol, their purified product changed consistency from toothpaste-like to gel to a sort of charcoal dough that basically behaves like plastic.31 This is still in the lab phase, so more testing and development is happening, but their polymer-like creation could bring massive practical advantages. It’d be much easier to handle and carry around compared to fluffy carbon powders.32 And they’d be more suitable for composite industrial production.
After 30 years, CNTs pioneer Sumio Iijima hasn’t lost his early enthusiasm and is still trying to accelerate carbon nanotubes mass production. Apparently, he filed a patent for a new method that would tidy up the CNT mess you normally get.33 All these contributions could make life easier for the few companies who are working hard to commercialize them. Like Huntsman Corp, which has been able to increase production from a gram an hour to being able to create a kilogram an hour. That’s a 1000 times increase in a few years and they’re continuing to scale it up.34
The tubular path to the future of nanotech
The hype on carbon nanotubes isn’t as high today as it was 30 years ago, which is a good thing. I think the expectations are more realistic now. But they’re still some of the most promising materials for the future of green technologies. While manufacturing CNTs at scale has worked out to be far more complex and time-consuming than expected, researchers have made significant, albeit slow, progress. Clearly, they still can’t promise us a space elevator, but harvesting heat for energy and making our grid and solar panels more efficient sounds like an electrifying, yet down-to-Earth, achievement. It’s important to keep our eye on the progress … and expectations in check.
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- “Properties of Carbon Nanotubes – UnderstandingNano.”↩
- “Woven nanotube fibers turn heat energy into electrical energy.” 16 Aug. 2021↩
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- “The History and Development of Carbon Nanotubes – Azonano.” 17 Jun. 2004↩
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- “Arc Evaporation – an overview | ScienceDirect Topics.”↩
- “Graphene as the building block for many other carbon allotropes.”↩
- “Development of efficient and cost-effective spacecraft structures ….”↩
- “Single-shell carbon nanotubes of 1-nm diameter | Nature.” 17 Jun. 1993↩
- “Long, Stretchy Carbon Nanotubes Could Make Space Elevators ….” 23 Jan. 2009↩
- “Carbon nanotubes too weak to get a space elevator off the ground.” 13 Jun. 2016↩
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- “Properties of Semiconducting and Metallic Carbon Nanotubes.”↩
- “Single-walled Carbon Nanotubes: Structure, Properties, Applications.” 13 May. 2021↩
- “Electrical properties of carbon nanotubes – UnderstandingNano.”↩
- “Carbon Nanotube Fibers Outperform Traditional Copper Cables.” 14 Feb. 2014↩
- “High-Ampacity Power Cables of Tightly-Packed and Aligned Carbon ….”↩
- “Scientists Aim To Improve Solar Cells With Nanomaterials.” 9 Oct. 2021↩
- “Device channels heat into light: Carbon nanotube films enable ….” 12 Jul. 2019↩
- “Most efficient solar panels: solar panel cell efficiency explained.” 15 Oct. 2021↩
- “Sunny superpower: solar cells close in on 50% efficiency.” 15 Apr. 2021↩
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- “Thermoelectric Seebeck Coefficient – Materials Research Institute.”↩
- “Can we relate the Electrochemical potential and Fermi level in ….” 16 May. 2017↩
- “Portable and wearable self-powered systems based on emerging ….” 17 Mar. 2021↩
- “Macroscopic weavable fibers of carbon nanotubes with giant ….” 13 Aug. 2021↩
- “Chemingineering | Carbon Nanotubes – The unfulfilled Promise.” 16 Jul. 2019↩
- “Researchers resolve a problem that has been holding back a ….” 16 Aug. 2016↩
- “Important carbon nanotube breakthrough found to have surprising ….” 16 May. 2018↩
- “Mass Production of Carbon Nanotubes made Easy – TEAM-TRADE ….” 19 Jun. 2019↩
- “The Discovery and Future of Carbon Nanotubes Sumio Iijima – NEC ….” 5 Nov. 2020↩
- “Rice scientists unveil secrets of carbon nanotubes – Houston Chronicle.” 27 Aug. 2021↩