The field of energy storage has placed significant emphasis on the creation of high-performance batteries. Lithium-ion batteries, which offer a higher energy density than other rechargeable battery technologies, have garnered much attention for use in a wide variety of devices and vehicles. Despite this, there is continuous research into innovative materials with superior qualities above the state-of-the-art because of the societal expectations for lighter, thinner and greater capacity lithium-ion batteries.

Carbon nanotubes

Carbon nanotubes (CNTs) have a variety of desirable electrochemical and mechanical characteristics, making them a potential material for use in lithium-ion batteries. When used as a conductive addition, CNTs are more effective than traditional carbons like carbon black and graphite in creating an electrical percolation network, even when used at lower weight loadings. CNTs can also serve as a physical support for anode materials such as germanium or silicon, which are ultra-high-capacity materials or as an active lithium-ion storage material when constructed into free-standing electrodes (without a binder or current collector).

Single-walled CNTs

Single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) are the two primary categories of CNTs depending on their diameter and structure. SWCNTs feature a diameter of around 1 nm and are made up of a single layer of carbon atoms organized in a cylinder form. However, MWCNTs are composed of many layers of graphene sheets stacked in a concentrical fashion around a central axis.

Chemical vapor deposition (CVD) is used to create SWCNTs by heating a carbon source gas with a catalyst — typically iron, cobalt or nickel — in a high-temperature furnace. Due to the metal's catalytic properties, the carbon source gas is broken down on its surface to produce SWCNTs. SWCNTs and MWCNTs are very desirable for a variety of uses because of their remarkable mechanical, electrical and thermal capabilities. SWCNTs, offer some features that make them desirable for specific uses, and these structures are preferable to MWCNTs for use in electronics and energy storage because they are more conductive, have a bigger surface area and a smaller diameter.

Categories of SWCNTs

SWCNT can be defined by how the graphene sheet is coiled to create the tube. It may be broken down into two distinct types: armchair and zigzag. Armchair SWCNTs are fabricated by rolling up a graphene sheet so that its edge is parallel to the carbon-carbon links in the lattice. The resulting tube has a circular cross-section and is called an "armchair" because its ends are shaped like chair arms. The zigzag shape of SWCNTs is the result of a graphene sheet being rolled up so that its edge is perpendicular to the carbon-carbon bonds in the lattice. The resulting tube has a cross section that is polygonal, with each edge running parallel to the tube's axis.

The chiral angle may also be used to categorize SWCNTs as it determines their electrical and optical properties, making it an important factor in their application. Based on this, SWCNTS can be either metallic or semiconducting. A chiral angle (the angle between the vector joining two places on the graphene sheet and the tube axis) of 30° or less characterizes metallic SWCNTs, while a chiral angle of more than 30° characterizes semiconducting SWCNTs.

SWCNTs for energy storage

The huge surface area, high electrical conductivity, and compact cylindrical shape of SWCNTs make them ideal for energy storage, which needs higher energy density, higher power, and longer cycle life than the currently available variants. Some current applications of SWCNTs for energy storage include:

• Supercapacitors: These can swiftly store and discharge electrical energy and can incorporate SWCNTs as an electrode material, improving device performance.
• Batteries: The use of SWCNTs as an electrode material for batteries, especially lithium-ion batteries, has been the subject of much research. Including SWCNTs in the electrode improves its electrical conductivity, which in turn can boost the battery's overall efficiency.
• Solar cells: As a conductive material, SWCNTs have also been investigated for usage in solar cells to aid in the transfer of electrons produced. Researchers have shown that adding SWCNTs to a solar cell's structure boosts device efficiency.
• Hydrogen storage: In order to make clean energy more feasible and efficient, researchers have found that the quantity of hydrogen that can be supplied may be increased by employing SWCNTs as a hydrogen storage material.

Conclusion

SWCNTs have shown great potential in various energy-related applications, including supercapacitors, batteries, solar cells and hydrogen storage. Their unique properties, such as high electrical conductivity and large surface area, make them an attractive material for improving the performance and efficiency of energy devices. However, the use of SWCNTs in energy applications may also raise concerns about their potential environmental and health impacts, as well as the high cost of production and scalability. Continued research and development of SWCNTs in energy applications could lead to significant advancements in clean energy technology.