To achieve the goal of a sustainable society, high-energy storage devices are needed. They should be compact and light with longer lifespans and should offer good safety. Ultimately, they should surpass the present battery and supercapacitor technologies. To meet these demands, single-walled carbon nanotubes (SWCNTs) are being considered. They have shown great toughness in emerging as a potential technology for innovative energy solutions. Researchers discovered how twisted CNTs offer better energy storage than lithium batteries.
Aim of the Study: To show the ability of SWCNT to reversibly store nanomechanical energy.
Highlights
- To produce SWCNT ropes wrapped in thermoplastic polyurethane elastomers.
- SWCNT twisted rope has a good ability to reversibly store nanomechanical energy.
- Twisted ropes achieve a gravimetric density of up to 2.1 MJ kg−1.
- The twisted rope surpasses the energy storage capacity of mechanical steel springs by more than 4 orders of magnitude.
- It surpasses Li-ion batteries by a factor of 3.
- The experimented twisted SWCNT rope can keep stored energy safe in hostile environments.
- There is no energy depletion over time.
- It is accessible in temperatures ranging from -60° C to +100° C.
Twisted CNTs Offer Better Energy Storage than Lithium Batteries
Single-walled carbon nanotubes were discovered in 1993. Since then, they have continuously shown unique possibilities to develop high-performance energy conversion and storage devices. After fixing various technical limitations, scientists have used these nanotubes in batteries, solar cells, and supercapacitors1.
Present Energy Storage Mechanisms
The currently used reversible mechanisms include the following:
- Electrochemical potential energy in capacitors and batteries.
- Gravitational potential energy in elevated water reservoirs.
- Mechanical energy.
The present system can store large amounts of reversible energy with a retrieval efficiency of around ∼98% in superconducting magnets.
Drawbacks
- This approach has an extremely high refrigeration cost.
- Mechanical energy stored statically in conventional mechanical steel springs have a low gravimetric energy density (GED) of ∼1.4 × 10−4 MJ kg−1.
- Lithium-ion batteries have ≤0.72 MJ kg−1 GED values, which are 4 orders of magnitude higher than mechanical springs.
- The higher energy-storing capacities are followed by safety risks like catching fire.
Here is a comparison of current energy storage devices. Among these, only a few are suitable to store and deliver energy across a wide range of temperatures. That too, in remotely miniature sensors or medical implant devices.
As per a research cost-effective green hydrogen production with active MXenes.
Characterization of Mechanically Tough SWCNT Ropes
Researchers used commercially available materials containing SWCNTs. The diameter of the material was 1.5 nm and the length around 1 µm. By using different fabrication processes, SWCNT ropes were made.
Types of ropes generated were:
- y-rope: formed by Yarn method
- r-rope: formed by roll method
- d-rope: formed by dispersion method
To achieve the objective of the study, researchers first need to identify a reliable measurement technique. This figure below shows the differences in terms of max GED values and graphic perfections based on the fabrication method used. Here, yarn method shows the highest GED values with an average of 0.22 ± 0.05 MJ kg−1 at an average twist value (ε) = 0.95.
Without any further processing, the GED values of all types of ropes are low, possibly due to tube bundling.
Term: A situation when several SWCNTs come together in ropes, causing strain and lattice distortions, is known as tube bundling. It happens when tubes interact with each other, leading to defects and disorders resulting in lower GED.
Researchers understood the influence of bundle morphology on the performance of SWCNTs energy storage through SEM micrographs. Thus, the lowest GED was evidently visible in d-rope characterized by the smallest average bundle size.
This was supported by the analysis of linear density of the prepared ropes. It shows a decline in the GED as linear density of the ropes increases. On the other hand, with increase in bundle size of y-rope, its GED increased. Factors leading to low efficiency
- High tortuosity
- Tube bundling
- Low packing density
Due to these factors, nanotubes become inefficient in transferring loads between each other. This results in stiffness and lower strength, which causes a low GED value.
In another research by UNSW, it was discovered that bacteria nanowires are helpful to develop green electronics
Rope Reinforcement Processing: To Address the Limitations
This includes a polymer treatment to enhance inter-SWCNT load transfer. The important mechanical properties of individual nanotubes was preserved during the process. The stretching of individual SWCNTs speeds up through this method, making the ropes better for energy storage. Researchers used polymers like PSS, PVA, TPU, and PSL for improving the energy storage potential of y-rope materials.
The following figure shows the modification of SWCNT y-rope by the intercalation of polymers or decomposition of sulfur or carbon.
This is the SEM micrographs of the SWCNT rope before processing and TPU modifications. It shows how the overall morphology changed upon modification. Microwave irradiation displays important differences between the interstitial site and surface of the TPU-wrapped SWCNT strand (before and after).
After irradiation, molten TPU diffused through the interstitial sites. This decorated the exterior of the SWCNT and acts like a potential linker for the adjacent tubes and strands. Thus, the close packing of the y-rope also increased. Intertube connected ropes when closely packed show uniform transfer and retain the mechanical properties of the nanoscale SWCNT rope samples. This further leads to higher GED.
Raman Spectra: Morphological Changes in SWCNT
Researcher’s interpretation about the morphological changes in SWCNT ropes is supported by the Raman spectra, as shown in the image below. The y-rope shows the highest upward shift in the G mode. Its G/D ratio is 84.8, which is much larger than carbon or sulfur deposited y-rope but slightly less than normal y-rope.
Polymer Modifications Impact on Y-rope’s Mechanical Properties
This enhanced the mechanical properties of y-ropes, which their stress-strain curves confirmed. Particularly, y-ropes (TPU) have the largest values of σB and εb along with the larger E than the y-rope (general). This further increases in high mechanical energy storage.
The deformation of SWCNT ropes under pressure was monitored by in situ Raman spectroscopy. Twist cycling enhanced the alignment of the SWCNT within the rope. It optimized the load transfer between the tubes.
It is amazing to find that Tohoku University Researchers using disordered rocksalt oxides for rechargeable magnesium batteries
Not All Chemical Treatments Were Equally Effective
Yes, GED was not increased equally in all treatments.
- With increase in the number of carbon deposition cycles, the GED of y-ropes (C) also increased.
- Since the toughness of the ropes shows strong correlation with the GED, adding more sulfur to y-ropes shows minimal effect on increasing energy storage capacity.
- Similarly, when comparing y-ropes modified by PSL and PSS, a similar torsional strain limit with lower max GED is shown in comparison to the ones made of TPU.
- These conditions show that deposited carbon and sulfur improved intertube coupling. However, they were not as effective as TPU in enhancing the GED.
Energy Output and Conversion of Twisted y-rope
Output of direct energy was investigated by the rotation of a load (eye-hook+paddle) attached to it. This device was 4 by 104 times heavier than the rope samples.
Process of preparing rope for output test
First, the rope was twisted 10, 20, and 30 rotations with a motor at 110 rotations per minute (RPM). Then, it was allowed to untwist with the load attached. Particularly, after 10 rotations, the rope untwisted back to around 90% of the original untwisted configuration.
It shows that a residual twist remains in the rope even after it was untwisted. Its presence suggests the occurrence of some energy dissipation caused by air resistance and internal friction. The periodic motion of the system can decay due to this.
In another research, KIST researchers develop safe and economical aqueous rechargeable battery.
Other Observations
- Researchers observed that Rr (recovery) values exceed 100% during the reverse rotation of TPU y-rope samples. This was evident in ropes that were twisted 20 and 30 times in the forward direction.
- Moreover, TPU acting as a link in y-ropes, allowed for a strain energy recovery of around 90 ± 2% in just 1.1 seconds.
- The untwisting time was very short, which means a higher power density of around ≤1.85 ± 0.43 MW kg-1.
- There was a drop in energy efficiency to 65 ± 5% without TPU.
- For over 20 hours, there was up to a 20% decline in energy recovery efficiency due to structural changes in the rope, caused by self-discharge.
- The presence of TPU polymers efficiently reduced the above-mentioned energy loss.
Potential Applications of SWCNT Ropes
As per the observations, around 3 times more energy can be stored in twisted SWCNT rope in comparison to LIBs. For further applications, a system can be designed by using composite pulleys. Or seams using CNTs can also be used with sewing machines can also be used. Both methods could allow for storing large nanomechanical energy in a compact.
Conclusions
The twisted CNTs offer better energy storage than lithium batteries as they work similarly to steel coil springs, but they can store much more energy than them. This nanomechanical technology shows several advantages over present technologies in terms of high energy density and reliable energy retention. Also, they can be charged and discharged multiple times without any risk of security attached to them.
Which is a great contrast if compared to other systems. Also, SWCNT ropes are capable of delivering steady temperatures over a wide temperature range and are ideal for biocompatible medical use. Moreover, this technology can power small devices like artificial organs, for longer periods of time without any requirement of surgical replacement. All in all, SWCNT ropes can solve numerous problems in a sustainable way.
Source: Giant nanomechanical energy storage capacity in twisted single-walled carbon nanotube ropes
