Nanotubes can be used as catalysts in various ways. One method is by passing an electric current through them, which allows them to donate electrons to molecules that come in contact with the reaction sites. This electron transfer process facilitates chemical reactions and speeds up reaction rates.
In terms of production processes, nanotubes can be synthesized using different methods. Traditional methods include laser ablation and arc discharge, but the most common commercial process today is chemical vapor deposition (CVD). Modified CVD methods involve using carbon monoxide as a feedstock. However, there is an emerging field that focuses on utilizing green or waste feedstocks for nanotube production. For example, carbon dioxide captured by electrolysis in molten salts can be used to produce nanotubes from green feedstocks. Methane pyrolysis, which is the direct thermal decomposition of methane into hydrogen and solid carbon black (including nanotubes), is another method that utilizes waste or by-product methane as a feedstock.
The choice of feedstock can also affect the synthesis process. Methane and ethylene require hydrogen during thermal conversion prior to doping into carbon nanotubes. On the other hand, hydrogen does not play a significant role in the synthesis of nanotubes via acetylene, except for its reducing effect on the catalyst. It has been observed that at relatively low hydrogen concentrations, hydrogen may promote the growth of carbon nanotubes synthesized through methane and ethylene by reducing the catalyst or participating in the thermal reaction. Additionally, the growth rate of nanotubes synthesized through ethylene is higher compared to those synthesized through acetylene, suggesting a "polymerization-like formation mechanism."
Maintaining an optimal residence time is crucial for achieving a relatively high growth rate of nanotubes. Too low of a residence time may result in an inability to accumulate a sufficient carbon source, leading to wastage. Conversely, too high of a residence time may limit carbon source replenishment and result in the accumulation of unwanted by-products.
Nanotubes also have significant potential in green technologies. They can be used in applications such as concrete, films, and electronics, where their unique properties offer environmentally friendly solutions. However, the flagship market for nanotubes in green technology is lithium-ion batteries. As the decarbonization efforts drive automotive electrification, nanotubes play a critical role as conductive additives in lithium-ion batteries. They are primarily used in the cathode as part of the conductive paste. Research is also exploring the use of nanotubes in next-generation batteries, such as lithium-air or lithium-sulfur batteries, as well as lithium metal anodes.
When evaluating the environmental impact of nanotubes, it is essential to compare them with alternative materials. In the case of carbon nanotubes as conductive additives, they can be compared to carbon black and graphene. Carbon black typically has higher CO2 emissions per kilogram compared to graphene and carbon nanotubes, as well as higher loading requirements in composites. Moreover, nanotube-reinforced tires have shown lower nanoparticle releases compared to other nanocarbons, according to a study by Michelin. Graphene, on the other hand, has its own challenges in terms of energy efficiency, water requirements, and the use of harsh chemicals in its production method, such as Hummer's method.
Overall, nanotubes as catalysts and their applications in various industries hold great promise for sustainable and green technologies.
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